Gas Turbines: Definition, Applications, Working, Components, Types, Design, Advantages

Written by Anu Sharma in Mechanical,Process

A gas turbine is a rotary machine in which the chemical energy of the fuel is converted into mechanical energy or kinetic energy in terms of shaft power. In other words, it is a mechanical power or thrust-delivering machine. It uses a gaseous working fluid for this purpose. The generated mechanical power can be used by industrial devices. There is a continuous flow of the working fluid in a gas turbine. Power generation gas turbines are the ones that produce shaft power. To propel an aircraft, gas turbines are used that convert fuel energy into kinetic energy for the generation of thrust. Fig. 1 below shows a typical representation of a Gas turbine.

**Fig. 1: Representation of a gas turbine**

Applications of a gas turbine

Major applications of gas turbines are found in:

As the direct and mechanical drive for various industries
Aviation
Electrical Power generation
Oil and gas industry
Marine propulsion
Turbo generators
Turbo-compressor
Automotive sector

Working principle of a gas turbine

The working of a gas turbine is based on the thermodynamic Brayton Cycle. The Brayton cycle consists of two adiabatic work transfers and two constant pressure heat transfer heat processes (Fig 2). The gas undergoes an isentropic, adiabatic compression in State 1 to State 2. This process increases the temperature, pressure, and density of the gas. Next, heat is added at constant pressure in State 2 to State 3. For a gas turbine, a combustion process adds heat. During State 3 to State 4, the gas passes through an adiabatic isentropic turbine that decreases the temperature and pressure of the gas. In the case of a closed gas turbine Brayton cycle, heat is removed from the gas between State 4 and State 1 via a heat exchanger.

Let’s understand the basic operating principle of a gas turbine with the following example:

Imagine there is a rocket in which fuel is going to burn thereby creating high-pressure exhaust gas. According to energy conservation law, in high-pressure exhaust gas, the chemical energy of the fuel is converted into mechanical energy. The thrust of the exhaust gas tries to move the rocket forward when the rocket is fired. Now the question is if one fixes the rocket body with a mechanical structure in order to prevent its movement. What will happen?

In such a case, the high-pressure exhaust gas releases but in a backward direction. Now another case is that what if we add a set of turbine blades to this back-fired exhaust gas?

The released mechanical energy which is in the linear backward direction will transform into rotational movement of the turbine shaft which is a big success. This means the chemical energy of the fuel gas is transformed into rotational mechanical energy of the turbine shaft as shown in Fig. 3.

**Fig. 3: Gas turbine working principle**

In simple words, in a gas turbine, hot gases move through a multistage gas turbine. It has both stationary and moving blades just like a steam turbine. The stationary blades adjust their velocity and guide the moving gases to the rotor blades. The turbine’s shaft is coupled to a generator.

The working principle of a gas turbine in a power plant is as follows:

In a gas turbine power plant, there is a generator known as an electrical machine and this generator needs a prime mover which is a gas turbine in order to generate electricity as shown in Fig. 4.

**Fig. 4: Gas turbine as a prime mover**

It transforms the fuel’s chemical energy into mechanical energy or in other words converting natural gas into mechanical energy. The generated mechanical energy is then transferred to the generator’s shaft through a gearbox. Now the turbine can create electrical energy as shown in Fig. 5.

**Fig. 5: Mechanical energy to Chemical Energy in gas turbine system**

This prime form of electrical energy usually has a low or medium level of voltage. In order to manage power loss in transmission lines, step-up transformers are used to increase this voltage and the increased voltage is provided to the electrical energy which in turn is transmitted through the transmission lines and delivered to the grid as shown in the below Fig. 6.

**Fig. 6: Transmission lines in power generation**

Working principle of a gas turbine in the oil and gas industry

A few points mentioned below should be kept in mind like:

For the oil and gas production process, the turbine is coupled to a compressor or a pump instead of coupling with a turbine.
An arrangement similar to a steam turbine is considered when one uses a gas turbine to drive a compressor.
The header tanks and lube oil are required in the auxiliary piping system.
The exhaust system should be considered which has ducting to a few heat recovery systems, that is, a process heater or a steam raising plant.
There should be a provision for the maintenance and operation of all machinery.
Outside the compressor house, combustion air must be taken to the turbine burner from a safe location. The most likely required items are an inlet silencer and filter.

Components of a gas turbine

Gas turbines have three main parts as mentioned below in Fig. 7:

Air compressor
Combustion chamber
Turbine

Air Compressor:

With the combustion chamber between the air compressor and turbine, both the air compressor and turbine are mounted on either end on a common shaft. Gas turbines require a starting motor as they are not self-starting. The use of an air compressor is to suck the air and compress it thereby increasing its pressure. Axial design type compressors (multi-stage) are preferred for the most advanced and large gas turbines.

**Fig. 8: Typical high-performance compressor assembly**

Combustion chamber:

Here the compressed air is combined with fuel and the resulting fuel-air mixture is burnt and delivers the combustion products to the gas turbine. With the high pressure of air, the fuel mixture burns quite well. Nowadays liquid fuel, gaseous fuel, or natural gas is used in gas turbines. Generally, three types of combustion chambers are used:

annular combustor chambers
can (multi-can) combustor chambers
can-annular combustor chambers

Fuel is injected at the upstream end of the burner in the form of a highly atomized spray. Fuel nozzles may be a simplex type or dual fuel type. Some gas turbines are “bi-fuel” which means they have to ability to burn a mixture of gas and liquid fuel.

Turbine:

There is a multistage gas turbine from where hot gases move and the kinetic energy is transformed into shaft horsepower. A gas turbine has both stationary and moving blades just like a steam turbine. The purpose of stationary blades is to guide the moving gases to the rotor blades and then adjust their velocity. The turbine’s shaft is coupled to a generator.

Exhaust Module:

The hot gases from a gas turbine exit through the exhaust section. The exhaust case consists of an inner and outer housing.

Other gas turbine parts are

Cooling system
Bearing and Lubrication system
Fuel system, etc

Types of gas turbine

The below listed are types of gas turbines:

Open-cycle gas turbine
Closed-cycle gas turbine
Aero derivative gas turbine
Scale jet engines
Auxiliary power unit
Jet engines

Open-cycle gas turbine

Open cycle gas turbine consists of three parts mainly a combustion chamber, turbine, and compressor. The compressor raises the pressure by taking in the ambient air. Fuel is burnt to add heat to the air in the combustion chamber thereby raising its temperature. The heated gases from the combustion chamber are then passed to the turbine where it does its mechanical work while expanding. The below figure (Fig. 10) shows an image of an open-cycle gas turbine.

Closed cycle gas turbine:

The working fluid used in a closed-cycle gas turbine is air or any other suitable medium that comes out from a compressor and is heated in a heater by some external source at a relatively constant pressure. The heated high-pressure and high-temperature air is then passed to the turbine. The fluid from the turbine is then cooled to its original temperature by some external cooling agent and then passed to the compressor. This way the working fluid is constantly used in the system and the required heat is given to the fluid by the heat exchanger without significant change in its phase. The below figure (Fig. 11) shows an image of a closed-cycle gas turbine.

Aero derivatives gas turbine:

These types of gas turbines are used in electrical power generation because of their ability to handle load changes more quickly and the ability to shut down better than industrial machines.
These are also used in the marine industry in order to reduce weight.

Scale jet engines:

These are also known as miniature gas turbines.
The scale jet engines have the ability to produce up to 22 Newton’s of thrust and can be easily built by most of minded mechanical engineers with basic engineering tools such as a metal lathe.

Auxiliary gas turbine:

These are smaller types of gas turbines used to supply auxiliary power to aircraft. Auxiliary gas turbines are used to supply air conditioning and ventilation. They supply compressed air power to jet engines. They also supply mechanical power to the gearbox to start larger jet engines or to drive shafted accessories.

Design of a gas turbine

The factors that limit the size and efficiency of a gas turbine are the firing temperature, compression ratio, mass flow, and centrifugal stresses.
The most critical areas in the gas turbine design that determines the engine
efficiency and life are the hot gas path, i.e., the combustion chambers and the turbine first stage stationary nozzles, and rotating buckets. The components in these areas represent roughly 2% of the total cost of the gas turbine, but they control the gas turbine output and efficiency.

Nickel superalloys are normally used for gas turbine nozzles and buckets. These are coated under a vacuum with special metals (platinum-chromium-aluminide) to protect against the hot corrosion occurring at high temperatures in presence of contaminants like sodium, vanadium, and potassium.

The operating performance of gas turbines can be increased by the continuing improvements in firing temperatures and compression ratios. Air-cooled nozzles and buckets using bleed air from the compressor are major advancements to increase the firing temperature. This limits the metal temperatures of the nozzles and buckets to withstand hot corrosion and creep.

To provide additional turbine power output by increasing the Final compressor pressure, additional compressor stages can be added to the compressor rotor assembly to give a higher compression ratio.

Natural gas, diesel oil, residual or crude oil can be used as gas turbine fuel.
With an increase in ambient temperature and altitude, the air density reduces. This causes a significant reduction in the power output and efficiency of the gas turbine. Ambient air temperature and elevation changes do not affect steam plants and diesel.

Codes and Standards

Frequently used codes and standards that govern the design, construction, testing, etc of a gas turbine are

API 616
ASME PTC 22
ISO 2314
ISO 3977
ISO 11086
ISO 7919
ISO 10494
ISO 11042
ISO 21789
ASME 133
NFPA 37
IEC 60034
ISO 19859

Gas turbine performance

The factors that impact the performance of a gas turbine are

Inlet air density
Ambient air temperature
Altitude and ambient pressure
Humidity
Inlet and exhaust pressure losses
Number of shafts

Advantages of a gas turbine

Fuel storage requires less area and handling is easy.
The maintenance cost is less.
Construction is quite simple.
Compared to steam power plants, it does not require a condenser, boiler, or other accessories.
Fuels such as kerosene, benzene, paraffin, and powdered coal can be used which are cheaper than other petrol and diesel.
In the areas of water scarcity, gas turbines can be used.
It creates less pollution.
It requires less amount of water.

Disadvantages of a gas turbine

Most of the developed power is used to drive the compressor.
This is the reason that a gas turbine has low thermal efficiency.
High-frequency noise comes from the compressor which is again questionable.
For various parts of the turbine, special metals and alloys are used because the running speed of the turbine is 40000 to 100000 rpm and the operating temperature is 1100 to 1260 degrees Celsius.

Gas turbine manufacturers

The major share of gas turbine manufacturing is controlled by the following organizations:

General Electric (US)
Siemens (Germany)
Mitsubishi Hitachi Power Systems (Japan)
Ansaldo STS (Italy)
Solar Turbines (U.S.)
Kawasaki Heavy Industries, Ltd (Japan)
Doosan Heavy Industries & Construction (South Korea)
Bharat Heavy Electrical Limited (India)
OPRA Turbines (The Netherlands)
Vericor Power Systems LLC (U.S.)
Rolls-Royce (U.K)

The first three companies in the above list combinedly control more than 80% of the gas turbine market share.

Gas turbine vs steam turbine

The main differences between a gas turbine and a steam turbine are listed in the following table:

Parameter	Gas turbine	Steam turbine
Working fluid	The gas turbine uses air or gas as the working fluid	Steam is the working fluid in the steam turbine
Thermodynamic Cycle	Brayton Cycle	Rankine Cycle
Power generation	Gas turbines are powered by the combustion reaction	Expanding steam provides power to the steam turbine
Efficiency	Comparatively higher than steam turbines	Lower than gas turbines
Operating temperature	Much higher	Lower
Installation Space requirement	Lower	Higher
Output	Torque or thrust	Torque
Cost	The maintenance and installation cost of a gas turbine is comparatively less	Higher
Startup and Control	Easy and quick	Difficult and time taking
Main components	Compressor, combustion chamber, turbine.	Steam turbine, boilers, pumps, heat exchangers, condensers.
Versatility	More versatile with respect to input fuel and application	Less versatile.

Gas Turbine vs Steam Turbine

Online Video Courses on Gas Turbines

To learn more about gas turbines click on the below-mentioned subjects, review the course and enroll:

Pour Point: Definition, Significance, Features, Measurements, Factors

Written by Anup Kumar Dey in Mechanical,Pipeline,Piping Design Basics,Piping Interface,Process

The pour point of a liquid (crude oil or a petroleum fraction) is the temperature below which the liquid becomes plastic and loses its flow characteristics. So pour point is the demarcation point of a fluid’s flowability and an important parameter of liquids at low temperatures. Above the pour point temperature, the liquid will flow without stirring, under standard conditions. This is the lowest temperature at which the oil will flow under gravity. The pour point of a liquid depends on its molecular structure and the presence of waxes in the liquid.

Significance and Features of Pour Point

The pour point indicates a liquid’s lower temperature properties.
If the surrounding temperature is less than the pour point, it cannot be transferred through a pipeline.
The pour point is more significant for Lubricating oils.
Pour points provide the lowest temperature for that fluid at which it can transfer by pouring.
The high value of the pour point means it can become semi-solid at that temperature which may cause jamming of the machine during operation.
For Lubricating oils, the pour points determine the liquid’s suitability be used as a lubricant at sub-zero temperatures. Also, the pour point indicates the dissolved wax concentration in the oil.
The pour point is used to allow process dimensioning and pumping calculations and in preventive actions and process improvement.

Pour Point Measurement

There are two methods for measuring the pour point of a liquid; the Manual method and the Automatic method.

Manual Method of Pour Point Measurement:

ASTM D97 (ISO 3016 or IP 15) standard provides the standard test methods for determining the pour point of Crude oil. As per these standards, the Crude oil specimen is cooled inside a cooling bath. Paraffin wax crystals are formed upon cooling. At about 9°C above the expected pour point, and for every subsequent 3°C, the test jar is removed from the cooling bath and tilted to check if the crude oil surface is moving. When the crude oil specimen ceases to flow when tilted, the jar is held horizontally for 5 sec. If the liquid does not flow, the pour point of crude oil is determined by adding 3°C to the result. So Pour Point=Temperature at which the liquid does not flow+3°C.

Automatic Method of Pour Point Measurement:

ASTM D5949 provides the automatic method for pour point measurement of petroleum products. This method is known as the Automatic Pressure Pulsing Method. Under ASTM D5949, the test sample is heated and then cooled by a Peltier device at a rate of 1.5±0.1 °C/min. A pressurized pulse of compressed gas is imparted onto the surface of the sample at either 1 °C or 3 °C intervals. The liquid sample is continuously monitored for movement by multiple optical detectors. The lowest temperature at which surface movement is detected on the sample is indicated to be the pour point.
The pour point of crude oils generally relates to their paraffin content. With an increase in the paraffin content, the pour point of crude oil increases.

Typical Pour Point Values

The pour point for Crude oils ranges from 32 °C to below −57 °C (90 °F to below −70 °F). Some typical values of the Pour Point are provided below in the table:

Liquid	Pour Point
Multi-grade engine oil	-35 Deg. C
Monograde engine oil	-23 Deg. C
Turbine Oil	-18 Deg. C
Synthetic Polyol ester	-32 Deg. C
Castor Oil	-33 Deg. C
Coconut Oil	21 Deg. C
Groundnut Oil	3 Deg. C
Mustard Oil	-18 Deg. C
Sunflower Oil	-18 Deg. C
Olive Oil	-9 Deg. C
Kerosene	-69 Deg. C

Table: Typical Pour Point Values for Oils

Factors Affecting Pour Point of Crude Oil

Factors that directly affect the pour point of crude oil are

Temperature differential
Paraffin wax content
Flow rate
Surface Properties
Viscosity

Due to the presence of high content of high molecular weight components, such as waxes, asphaltenes, and resins, Heavy and extra-heavy crude oils normally have higher pour points. The pour point of a liquid can be improved using depressants like polymethacrylates, alkylated wax phenol, Alkylated wax naphthalene, etc. Such depressants modify the interface between the oil and wax present.

Steam Turbines: Basics, Types, Selection, Components, Construction, Codes, Manufacturers

Written by Ahmed Shafik in Mechanical

A Steam Turbine is an engine that converts heat energy from pressurized steam into mechanical energy where the steam is expanded in the turbine in multiple stages to generate the required work. Steam turbine engines are used to produce electricity and drive countless machines worldwide (used as the prime mover for pumps, compressors, and other shaft-driven equipment). The capacity of steam turbines can vary from a few kilowatts to several hundred megawatts. Sir Charles A. Parsons developed the first modern steam turbine in 1884.

The Power in a steam turbine is generated by the rate of change of momentum of a high-velocity jet of steam impinging on a curved blade that is free to rotate. Fig. 1 below shows a schematic representation of a steam turbine with its associated auxiliaries.

**Fig. 1: Schematic of Steam Turbine Power Generation**

Working Principle of Steam Turbines

The heat energy of steam is converted into mechanical work while expanding in the turbine. Steam is generated inside a boiler. The expansion of steam takes place through a series of fixed blades (nozzles) and moving blades. The working of a steam turbine is based on the thermodynamic cycle called the “Rankine cycle”.

Rankine Cycle:

The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work while undergoing a phase change. The concept is developed by William John Macquorn Rankine, a Scottish polymath and Glasgow University professor. It is an idealized cycle in which friction losses in each of the four components are neglected. The heat from an external source is supplied to a closed-loop, which normally uses water as the working fluid. Refer to Fig. 2

Fig. 2 above represents the Rankine Cycle and the Temperature and Entropy are plotted in the curve. The processes can be described as follows:

Process 1–2 Isentropic compression – Adiabatic Pumping: The working fluid is pumped from low to high pressure. The fluid, being liquid at this stage, the pump requires little input energy.
Process 2–3 Constant pressure heat addition in a boiler – Isobaric Heat Supply: The high-pressure liquid enters a boiler, where it is heated at constant pressure by an external heat source to become a dry saturated vapor (steam). The required energy input can easily be calculated graphically, using an enthalpy–entropy chart (Mollier diagram or h-s chart), or numerically, using steam tables.
Process 3–4 Isentropic expansion – Adiabatic Expansion: The dry saturated vapor expands through a turbine, generating power. The temperature and pressure of the vapor are reduced causing some condensation.
Process 4–1 Constant pressure heat rejection in condenser – Isobaric Heat Rejection: The wet vapor then enters a condenser, where it is condensed at a constant pressure to become a saturated liquid.

Types of Steam Turbines

Steam Turbine Types can be classified based on various parameters as listed below:

According to the action of steam
- Impulse Turbine and
- Reaction Turbine
According to the number of pressure stages or impellers
- Single-stage Turbine and
- Multistage Turbine
According to the type of blade row
- Rateau type(Pressure compounded stages)
- Curtis type (Velocity compounded stage)
- Reaction stage type
According to the type of steam flow
- Axial flow turbine and
- Radial flow turbine
According to the number of shafts
- Single-Shaft
- Multi-shaft.
According to the direction of Shaft
- Transverse type
- Vertical type
According to the method of governing
- Throttling
- Nozzles
- Bypass governing.
According to the heat drop process:
- generators
- one or more intermediate-stage extraction
- back pressure
- topping.
According to steam conditions
- Low-Pressure Steam Turbine
- Medium Pressure Steam Turbine
- High-Pressure Steam Turbine
- Very High-Pressure Steam Turbine
- Supercritical Pressure Steam turbine
According to Exhaust conditions
- Condensing Turbine
- Backpressure Turbine
- Extraction Turbine
According to the Exhaust flow
- Single-flow exhaust type
- Multi-flow Exhaust type
According to Exhaust Direction
- Down exhaust
- Top exhaust
- Axial exhaust
According to speed
- Fixed speed
- Variable speed
  - Low Speed (≤ 3000 rpm)
  - High Speed(≥ 3000 rpm)
According to the quantity of the inlet valves
- Single valve type
- Multi-valve type
According to the existence of reducer
- Direct-drive type
- Reduction type

The most basic steam turbine types are Impulse Turbine and Reaction turbine.

Impulse Steam Turbine

The basic idea of an impulse steam turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. So the impulse force of high-velocity steam exerts a force on the blade to turn the rotor. The kinetic energy of the steam is transferred to the rotating wheel by momentum transfer within the blades. Pelton Wheel, Banki Turbine, etc are typical examples of Impulse turbines.

Reaction Steam Turbine

In the reaction steam turbine, a jet of steam flows from a nozzle on the rotor (the moving blades) by fixed blades designed to expand the steam. The rotor gets its rotational force from the steam as it leaves the blades. Roughly 50% of the output power is generated by the impact force and the other 50% from the reaction force by the steam expansion. Francis Turbine, Kaplan Propeller turbine, Deriaz turbine, etc are examples of reaction turbines.

The main difference between impulse and reaction turbines lies in the way in which steam is expanded while it moves through them such that:

In the impulse-type steam turbine, the steam expands in the nozzle and its pressure doesn’t change as it moves over the blades.
In the reaction type, the steam expands continuously as it passes over the blades and thus there is a gradual fall in pressure during expansion.

**Fig. 3: Impulse Turbine vs Reaction Turbine**

The major differences between an impulse turbine and a reaction turbine are tabulated below:

Impulse Steam Turbine	Reaction Steam Turbine
In an impulse turbine, The steam flows through the nozzle first and then strikes the moving blade.	In a reaction turbine, Steam flows through the guide mechanism first and then through the moving blades.
Blades rotate only by impact force	Blades rotate by impact force and reaction force generated by steam expansion.
Relative fluid velocity remains constant across the blades	The relative fluid velocity increases across the blade.
The number of stages is less for impulse steam turbines for the same heat input.	Reaction steam turbine has more stages under the same heat supply.
More efficiency	Comparatively less efficient
Less Maintenance requirement	More Maintenance requirements.
The casing does not perform any hydraulic function but is required to prevent fluid splashing.	The casing is a must to contain the pressure.
Fluid Flow is tangential to the turbine wheel	Fluid flow is radial or axial to the turbine wheel.
Suitable for small power generation.	Ideal for higher power generation requirements.
High operating speed	Low Operating Speed.

Impulse Turbine vs Reaction Turbine

**Fig. 4: Difference Between Impulse and Reaction Turbine**

Selection of Steam Turbines

The following table from JIS B0127 provides typical guidelines for the general features and selection criteria based on the type of steam turbines. Depending on the purpose, use, required output, location, arrangement, and circumstances appropriate type of steam turbine should be selected:

Type of Steam Turbine	General Features	Selection Criteria	Use for
Condensing Steam Turbine	The turbines which work fully inflated to low pressure by obtaining a high vacuum in order to condense turbine exhaust steam in the condenser.	(1)When only electricity or power is required, (2) When cooling water to condensate exhaust steam is available.	Machine drive, geothermal power, heat recovery
Regenerative Steam Turbine	The condensing turbine which is obtained high efficiency of the turbine cycle by utilizing extracted steam from the middle stage for heating boiler feed-water by the feed-water heater.	(1)When small and middle output and high efficiency are required, (2) When cooling water to condensate exhaust steam is available.	Power generation for cement, ironworks, mine
Reheat Steam turbine	The turbines extract steam from the middle expansion stage, re-heat, back to the turbine and expand further.	(1)When large output and high efficiency are required, (2)In general, it is used as a regenerate and reheat turbine.	Large-scale power generation
Back-pressure Steam Turbine	The turbine’s exhaust pressure is above atmospheric pressure and utilizes steam as utility steam. In some cases, it may be operated exhaust released into the atmosphere.	(1)When a large amount of utility steam of a type is required, (2)It is necessary to operate electricity and steam in parallel due to excess and deficiency between generated electricity and the power demand of the plant.	Private power generation, machine drive, cogeneration
Extraction Condensing Steam Turbine	The turbines extract steam from the middle stage of condensing turbine after adjusting pressure by the control valve and utilize steam and exhaust steam as utility steam for works and others.	(1)When a large amount of utility steam of one or more types is required, (2)When the power demand is less than the amount of utility steam.	Private power generation, machine drive, cogeneration
Extraction back pressure Steam turbine	The turbines extract steam from the middle stage of the back pressure turbine after adjusting pressure by the control valve and utilize steam and exhaust steam as utility steam for works and others.	(1)When a large amount of utility steam of more than two types is required, (2)It is necessary to operate electricity and steam in parallel due to excess and deficiency between generated electricity and the power demand of the plant.	Private power generation, machine drive
Mixed-pressure Steam Turbine	The turbines which are made work into same turbine different pressure steam.	(1)When only electricity or driving power is required. (2)When it is necessary to collect low-pressure steam. (3)When it is capable to get cooling water for condensation of exhaust steam.	Private power generation by heat recovery, machine drive

Selection Criteria for Steam turbines

Components of a Steam Turbine

The major components that constitute a steam turbine are:

Casing: The Casing should withstand all normal and emergency service loads and allowable piping forces and moments caused by temperature change. The design of turbine casing design shall be such that thermal stresses are minimized. Adequate support must be provided to the steam turbine casing to maintain good alignment with the rotor.
Nozzles.
Rotor: This is the main component in a steam turbine that carries the blades to convert thermal energy.
Blades: Blades absorb the energy of high steam velocity and convey it to the rotor. The shape of the blades significantly affects the turbine performance of turbine and requires high reliability.
Governor for speed control.
Servo Mechanism.
Oil Pump for lubrication.

Fig. 5 shows these components.

Steam turbine Components — **Fig. 5: Steam Turbine Components**

Construction of a Steam Turbine

Often, Turbines are described by the number of stages that are grouped into different sections of the turbine. Depending on the pressure levels, the sections are known as the high pressure (HP) section, intermediate pressure (IP) section, or low pressure (LP) section. These turbine sections can be constructed in different types as mentioned below:

can be packaged into separate sections in a single turbine casing,
can be arranged into separate casings for each section, or
can be constructed in combination (HP/IP turbines in one casing and LP turbines in another).

Also, two turbines may be connected together in the same casing but in opposing directions to balance the thrust loads. Flow to these turbines is through the center of the casing and exits from each end of the turbine. These are referred to as turbines with double flows (i.e., opposing flow paths on the same shaft). However, the steam turbine MW rating is not indicative of section or casing numbers. Normally, less number of stages and casing will result in larger size blading and high loading for the same steam condition. The following Fig. shows a typical plot of the number of turbine casings as a function of steam turbine size

**Fig. 6: Number of Turbine Casing vs Steam Turbine Size**

Losses in a Steam Turbine

Residual velocity loss.
Losses in regulating valves.
Loss due to steam friction in the nozzle.
Loss due to leakage.
Loss due to mechanical friction.
Loss due to wetness of steam.
Radiation loss.

Techniques to Improve Steam Turbine Efficiency

Various techniques are employed to maximize steam turbine efficiency, each designed to attack a specific loss mechanism. For example:

the number of stages utilized can range from the fewest possible to develop the load reliably to the thermodynamically optimum selection.
Spill bands can be utilized to minimize throttling losses.
High-efficiency nozzle/bucket profiles are available to reduce friction losses.
To reduce the pressure within the exhaust casing, exhaust flow guides are available.
The specific features employed on a given application are usually based on the trade-off between capital investment and the cost to produce steam over the life of the turbine –SIMPLY, IT IS AN OPTIMIZATION APPROACH.

Process Surveillance – Why we should monitor closely?

Accurate measurement and tracking of parameters like temperature, pressure, and flow are important to plant safety and performance. Information collected at specific measuring points can be used to:

Avoid Metallurgical Failures: Temperatures need to be maintained below components’ melting points in order to avoid metallurgical failure. Too-high temperatures can also lead to creep deformation in the rotating blades.
Determine Efficiency and Performance: Calculate the efficiency of the turbine by knowing the inlet and exit temperatures, as well as the flow rate at the nozzle. When a turbine exhaust is used as heat input to a steam cycle, engineers can also estimate the performance of the heat recovery steam generator (HRSG) by using the temperature and flow measurement of the turbine exhaust.
Detect Inefficiencies: High exhaust temperatures and flow changes can be symptoms of an upset mode of turbine operation. If a flow measurement device picks up irregularities, the plant operator can perform a diagnostic to identify the underlying causes.
Calculate Residual Life: Tracking temperatures over time allows one To calculate how much life the component has left and to plan maintenance and replacements.

Process Surveillance – What & Where?

Barometric pressure.
Steam and steam condensate’s flow rate, temperature, and pressure on:
- The cold reheat.
- The high-pressure throttle.
- The hot reheat.
- Low-pressure induction sections.
Exhaust pressure.

Steam Turbine Codes and Standards

API 611 (ISO 10436) 4th Edition – General purpose steam turbines for refinery service (non-critical):

General purpose turbines are horizontal or vertical turbines used to drive equipment that is usually spared, is relatively small in size (power), or is in non-critical service.
They are used where steam conditions will not exceed a pressure of 48 bar and a temperature of 400°C or where speed will not exceed 6000 rpm.

API 612 (ISO 10437) 6th Edition – Special purpose steam turbine for refinery service (critical):

The purchaser’s approval is required for built-up rotors when blade tip velocities exceed 250 m/s at maximum continuous speed or when stage inlet steam temperatures exceed 440 °C.
Over Speed shutdown system:
- I. Electronic Overspeed detection system.
- II. Electro-hydraulic solenoid valves.
- III. Emergency trip valve(s) / combined trip and throttle valve(s).
If specified a turbine with an exhaust pressure less than atmospheric pressure shall be provided with an exhaust vacuum breaker actuated by the shutdown system.
Details of such a system shall be agreed upon by the purchaser and the turbine vendor.

Other codes and standards that are referred to for steam turbine applications are

ASME/NEMA SM23
NEMA SM24
IEC 60045
IEEE 122
IEC 60953
IEC 60962
IEC/TS 61370
BS EN60045-1
ASME PTC 6/6A
ASME/ANSI PTC 20.1/20.2
ISO 14661
IS 14205
JESC T0003
JEAC 3703
JIS B8101
DIN 4304
DIN 4305
DIN 4312
DIN EN 45510-5-1

Steam Turbine Manufacturer

Steam turbines are highly complex and sensitive pieces of machinery, and only a few manufacturers produce them worldwide. The majority of the steam turbines are manufactured by the following companies:

Harbin Electric
Shanghai Electric
Dongfang Electric
General Electric
Siemens
Alstom
Bharat Heavy Electricals Limited
Doosan
Mitsubishi Heavy Industries
Weir Allen
Elliot Group
and Toshiba.

Online Steam Turbine Courses

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Types of Maintenance

Written by Anup Kumar Dey in Maintenance,Mechanical,Piping Interface

In the world of engineering, manufacturing, and facility management, maintenance plays a crucial role in ensuring the longevity and efficiency of equipment and systems. From machinery in a manufacturing plant to oil and gas systems or HVAC systems in a commercial building, effective maintenance strategies are essential for minimizing downtime, reducing costs, and enhancing safety.

What is Maintenance?

Maintenance is the effort required to undertake for maintaining equipment performance similar to new ones. Unscheduled downtime due to equipment malfunction is one of the major concerns for industries, which calls for billions of dollars of losses each year. Maintenance is an important aspect of increasing productivity by reducing such downtimes and improving equipment performance. Maintenance is basically a set of processes and practices that help in the continuous and efficient operation of assets. The major benefits of maintenance are

Increase in the life of the equipment and other assets.
Optimization of asset performance.
Reducing unwanted downtimes, thus increasing production
Minimizing cost

Types of Maintenance

Broadly, there are two main types of maintenance categories that can further be subdivided into various maintenance-type groups.

Proactive Maintenance or Preventive maintenance and
Responsive Maintenance or Corrective Maintenance

1. Types of Preventive Maintenance:

Preventive maintenance refers to the fixing of problems before they appear. This means such maintenance prevents the problem. Inspection of equipment at regular intervals to check the machine’s condition and take necessary action is the motto of preventive maintenance.

It involves scheduled inspections, adjustments, and minor repairs designed to prevent equipment failures before they occur. This strategy is proactive, focusing on keeping assets in good working condition. Fig. 1 below shows a typical workflow of the preventive maintenance philosophy.

**Fig. 1: Work-flow of Preventive Maintenance**

Preventive maintenance is categorized into the following five types

Time-Based Maintenance (TBM)
Predictive Maintenance (PDM)
Failure Finding Maintenance (FFM)
Condition-Based Maintenance (CBM)
Risk-Based Maintenance (RBM)

Time-Based Maintenance

Time-based maintenance or TBM calls for maintenance at a fixed time. Normally, taking guidance from the equipment manufacturer’s maintenance plan a fixed interval is scheduled and maintenance work is performed to restore equipment efficiency and performance. Time-based maintenance also requires the replacement of items based on their service life capability.

Predictive Maintenance (PDM)

As the name suggests, this type of maintenance refers to the prediction of the failure probability of equipment and scheduling maintenance to prevent failure. To correctly predict the equipment’s workability and perform predictive maintenance the organization should keep and analyze the following data:

Equipment history
All records of downtime, defects, performance, etc
Equipment condition with respect to working time.

After analysis of the above data and including the experience with similar equipment maintenance dates are fixed.

Failure Finding Maintenance

In failure finding maintenance, potential hidden failures are searched at regular intervals and if discovered are repaired to prevent major breakdowns. So basically this is not a specific type of maintenance but a functional check. Failure to find maintenance increases the system’s reliability.

Condition-Based Maintenance

In the Condition-based maintenance (CBM) strategy, the actual asset condition is monitored and further maintenance requirement is decided. In this type of maintenance, based on visual inspection, scheduled tests, performance data, etc the equipment condition is studied. When some sign of decreasing performance or failure is received, maintenance is scheduled.

Risk-Based Maintenance

Risk-based maintenance considers the philosophy of maintaining the assets carrying the most risk during failure. This philosophy determines the most economical use of the maintenance resources and optimizes the risk of failure. Risk-based maintenance strategy works on the following steps:

Data Collection
Risk Assessment and Evaluation of Consequence and Probability of failure
Ranking of Risks
Creating an Inspection Plan based on those risk ranking matrices.
Maintenance planning and Mitigation of risks.

Equipment carrying the greater risk and failure consequences is frequently monitored and maintained. This philosophy and method provide a systematic approach to determining the most appropriate asset maintenance plans in the most economic way.

Advantages and Disadvantages of Preventive Maintenance

Advantages

Reduced Downtime: Regular maintenance helps identify potential issues before they lead to breakdowns.
Increased Equipment Lifespan: Regular care can extend the life of machinery and equipment.
Cost Savings: Preventive measures can save money by reducing the need for major repairs.

Disadvantages

Time and Resource Intensive: Requires scheduling and can lead to temporary downtime during inspections.
Over-Maintenance Risk: Performing maintenance too frequently can waste resources if equipment is still in good condition.

Click on the following link to know more details about preventive maintenance: Preventive Maintenance: Definition, Types, Philosophy, Advantages, and Disadvantages

2. Types of Corrective Maintenance

Corrective maintenance is any maintenance task that resolves a problem with a piece of equipment and returns it to operational condition. This is also known as reactive maintenance. Corrective maintenance work can be both planned and unplanned. The following image shows the workflow of typical corrective maintenance philosophy.

**Fig. 2: Work-flow of Corrective Maintenance**

Normally there are three situations that call for corrective maintenance:

If a piece of equipment or part breaks down
If any issue is identified during condition monitoring
If routine inspection discovers any potential fault.

Reactive maintenance, often referred to as “breakdown” or “emergency” maintenance, occurs after equipment has failed or broken down. This type of maintenance is unplanned and is typically initiated in response to a malfunction.

There are two types of corrective maintenance

Planned or Scheduled corrective maintenance and
Unplanned or Unscheduled corrective maintenance

Planned or Scheduled Corrective Maintenance

Planned corrective maintenance is the corrective action that is not immediate but planned or scheduled depending on the urgency and nature of the deficiency identified. The risks involved and costs involved are major parameters to determine the planned corrective maintenance schedule. This is also known as deferred corrective maintenance. Example: An AC is not providing proper cooling due to refrigerant gas leakage. So, s work order is created to repair it during the next inspection.

Unplanned or Unscheduled corrective maintenance

Unplanned corrective maintenance needs immediate attention due to some kind of critical failure and must be repaired without delay as it directly relates to cost. This philosophy is also known as Immediate Corrective Maintenance. Example: A pump is inspected and repaired after every 200 hours but it breaks down after 150 hours of operation and it calls for an emergency repair. Similar cases are examples of unplanned corrective maintenance.

Advantages and Disadvantages of Corrective Maintenance

Major advantages of Corrective Maintenance are:

Minimal planning requirement
Lower short-term costs
Simplified maintenance process
For non-critical equipment, without much impact, this is the best maintenance philosophy and can be cheaper.

The main disadvantages of Corrective Maintenance are:

Increased Unpredictability
Paused operations and hence production loss
The increased cost of maintenance
Equipment life is not optimized.
Higher long-term costs
High safety concerns

Preventive Maintenance vs Corrective Maintenance

Let’s have a look at the major differences between Preventive and Corrective maintenance. For ease of understanding and comparing the differences between corrective and preventive maintenance are tabulated below:

Corrective Maintenance	Preventive Maintenance
Corrective maintenance refers to maintenance after the failure of an asset.	Preventive maintenance is aimed at preventing failure and is scheduled before equipment failure.
Corrective Maintenance is a less complex and simple process.	Preventive Maintenance involves proper planning to prevent equipment failure, Hence it is complex.
Corrective maintenance is normally more expensive as equipment has already failed and needs replacement or extensive repairs.	It prevents equipment failure and thus preventive maintenance is normally less expensive.
Loss of production and time due to asset failure	The chances of equipment failure are reduced to negligible production and time loss.
It is only performed when a breakdown occurs.	Preventive Maintenance is performed at regular intervals.
Corrective Maintenance increases the need for preventive actions.	Preventive Maintenance reduces the need for corrective actions.
Overall equipment lifecycle and efficiency reduces	Equipment lifespan and efficiency are increased by regular preventive maintenance
This is hazardous considering the safety	From the safety of employees and working environment considerations, preventive maintenance is better.
It requires a greater number of employees or technicians to perform corrective maintenance which increases the workload.	The smaller number of technicians perform preventive maintenance decreasing the workload.

Table-1: Corrective vs Preventive Maintenance

Online Courses on Maintenance Aspects

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Storage Tank Construction: Procedure and Method Statement

Written by Anup Kumar Dey in Mechanical,Piping Design Basics,Piping Interface

Storage tanks play a vital role in various industries, including oil and gas, chemicals, food and beverage, and water treatment. They are used for storing liquids, gases, and even solids. Understanding the construction of storage tanks is essential for professionals in these fields, as well as for those interested in engineering and infrastructure development.

The method statement for storage tank construction provides detailed information on the procedure and rules for conducting all fabrication, erection, and testing of the storage tanks and similar static equipment. All the tasks/activities should be completed with the utmost care, with good workmanship, and in accordance with the specifications to realize satisfactory completion of the entire activities. This document will provide a reference for a proper methodology to realize the activities during a proper sequence for fabrication, erection, and testing of the storage tanks.

What is Storage Tank Construction?

Storage tank construction refers to the process of fabricating and installing tanks used for storing liquids, gases, or solids. This involves various stages, including site preparation, foundation work, tank fabrication (either on-site or off-site), installation of piping and accessories, and adherence to safety and regulatory standards. The construction can involve different materials, such as steel, concrete, or fiberglass, depending on the intended use and environmental conditions. Proper design and construction ensure the tanks’ structural integrity, safety, and efficiency in storage operations.

Reference Codes and Specifications for storage tank construction

The required codes and specifications for the storage tank construction are

API 650
IS 803
ASME Sec IX
Approved Drawing / Specification
ASME Sec V

**Fig. 1: Storage Tanks in a Tank Farm**

Storage Tank Construction Methodology

The storage tank construction methodology can be performed in the sequence listed below:

Identification of Tank Materials
Tank Construction and Fabrication
- Annular and Bottom Plate Fabrication
- Shell Plate Fabrication
- Roof Plate Fabrication
- Appurtenances
- Spiral Stairway and handrailing
Storage Tank Erection
- Annular Plate laying
- Bottom Plate laying
- Erection of Cone Roof plates and Structures
- Shell Course Erection
- Appurtenances installation

1. Identification of Tank Materials

All items required for tank fabrication, like plates, fittings, and other components of the tankage system, must be clearly identified by marking heat numbers by hard-punching on each part. Each weld joint shall be marked with a joint number and a welder number.

2. Storage Tank Construction and Fabrication

All fabrication works like tank material identification, marking, cutting, rolling and welding must be performed as mentioned in the latest API 650. The workmanship shall be good in every respect. It must be meeting all safety, Quality Control & Non-Destructive Testing requirements, with the coordination of the Engineer-In-Charge.

Lay all the plates, Structural & pipes in such a fashion that facilitates proper checking of dimensions & heat no.
Mark the components as per the drawings.

2.1 Tank Annular and Bottom Plate Fabrication

Mark the plates as per the drawing
Perform the cutting operation after proper inspection.
All the cutting operations shall be administered by Gas Cutting.
Grind the cut edges smoothly to get rid of burrs and slag.
Stack the plates at designated places within the fabrication yard in proper Sequence.
The backing strip shall be fitted and tacked with an annular plate as per the drawing.
After completion of the fabrication, the underside of the annular and bottom plates shall be blast cleaned and painted as per the specification to the satisfaction of the engineer-in-charge.
Check the joint position and root gap for welding after welding inspection by weld visual & NDT as per API 650.

**Fig. 2: Tank Shell and Bottom Plate Fabrication**

2.2 Tank Shell Plate Fabrication

Lay all the plates within the open area such as how easy it is for marking and cutting.
Mark the plates as per the drawing and to the satisfaction of the engineer in charge.
The cutting operation including beveling shall be administered by the gas-cutting method on all four sides.
Grind the cut edges smoothly to get rid of slag and burr.
The stings-prepared plates shall be shifted by crane to the rolling area.
The plates shall be fed to the rolling machine. by means of a crane & proper lifting tools and tackles.
The plate shall be rolled as per drawing details and therefore the radius shall be checked using a Template.
Proper care shall be taken for the graceful curvature. 36’’ Long (5 to 6 mm thick). The template shall be wont to check rolling curvature accurately.
The Template shall be checked and cleared by the inspection authority.
The rolled plates shall be inspected and shall be within 3 mm of the tolerance limit.
The rolled plates shall be shifted to the shot blasting yard to hold out the shot blast as per specification.
Final profile and dimensions check shall be administered before sending for erection.

2.3 Tank Roof Plate Fabrication

Mark the roof plate as per the drawing.
All the cutting operations as per requirement shall be administered after getting clearance from the inspection authority.
All the cutting operations shall be administered by the gas-cutting method on all four sides. Grind the cut edges smoothly to get rid of burrs & slag.
Structural items of the roof shall be fabricated as per the drawing.
These roof plates shall be shifted to the blasting yard to hold out the blasting and painting

2.4 Tank Appurtenances

Mark the specified items of the Appurtenances as per the drawing. All the cutting operations as per requirement shall be administered after getting clearance from the inspection authority.
All the cutting operations shall be administered by the gas-cutting method.
Grind the cut edges smoothly to get rid of burrs & slag.
The Flange face is covered with an appropriate cover to guard against damage during handling, fabrication, and transportation.
The whole shell nozzle, roof appurtenance, i.e. flanges, flanges to pipe joint, and other requirements, etc. Shall be fabricated as per approved drawings.
All the fabrication and welding activities shall be administered after the stage-wise inspection wherever required.
The NDT requirements are as per code specification.

2.5 Spiral Stairway, Hand Railing of Storage Tanks

All the structural items shall be straightened before marking and cutting.
Mark the components as per the drawing.
All the cutting operations, as per requirement shall be administered after getting the clearance from the inspection authority.
All the cutting operations shall be administered by the gas-cutting method.
Grind the cut edges smoothly to get rid of burrs & slag.
Fabrication like fitting, welding, drilling, etc. shall be administered as per drawing and after clearance from the inspection authority.
After fabrication, these structural items shall be blasted and painted as per requirement.

3. Storage Tank Erection

There are two methods for storage tank erection:

the Jacking method &
the conventional method.

The conventional method is tough & unsafe as compared to the jacking method that’s why the jacking method for Tank erection is used everywhere.

In the jacking method, we calculate the overall weight of tank ages except for the bottom & deck, and accordingly, jacks are used. The number of jacks to be used is directly subjected to the weight of the tank to be lifted.

**Fig. 4: Jacking method of tank erection**

3.1 Laying of Storage Tank Annular Plate

Check the extent of the foundation as per specification Latest API 650 clause 8.4.2. After getting clearance for annular plate laying, mark the 0 degrees, 90 degrees, 180 degrees, and 270 degrees coordinates on the inspiration from the point of reference.
Lay the annular plate as per the approved drawing. Out radius of the annular plate shall be on the positive side (5 to 10 mm.) so as to realize the ultimately required radius after weld shrinkage.
The orientation of the annular plate joint shall be as per the approved drawing.
Fit from the annular plate’s joints shall be administered using proper jigs and fixtures as shown in the drawing.
Care shall be taken while fit-up, such that there shouldn’t be any gap between the annular plate and backing strip.
Annular plate joint welding shall be administered by welding alternative joints in four quadrants.
Qualified welders shall be engaged for the welding work consistent with WPS. If any defect is found, the defect weld shall be removed by grinding and re-welding and conducting the LPT check test.
Repeat the sequence until the defect is cleared.
Complete the welding, and clean the ultimate weld surface by wire brushing and grinding.
Remove the jigs and fixtures that were used for the fit-up of the annular joint and grind the tack.
Radiography shall be taken as per API-650 Sec-8.

3.2 Laying of Storage Tank Bottom Plate

Lay the middle plate on the inspiration top as per the drawing.
With the coordination of the Centre, plates lay rock bottom plates consecutively as per the drawing.
Laps shall be maintained while the fit-up of a short seam and long seam as per drawing.
Temporary tack welding is to be performed on the long seam to avoid uneven movements, while the fit-up and welding of the short seam are.
Short seam welding is going to be administered alternatively to avoid distortion.
After the completion of short-seam welding, remove the temporary tacks on the Long- seam by grinding to facilitate the long-seam fit-up. Short seam welding is going to be administered alternatively to avoid distortion.
After the completion of short-seam welding, remove the temporary tacks on the Long- seam by grinding to facilitate the long-seam fit-up.
Minimum laps shall be maintained while the fit-up of the long seam is as per the approved drawing.
Joggling shall be administered by hammering wherever necessary. (Three plates Joining junction.)
Before starting the welding, channels shall be tacked along the long seam to avoid distortion. After completion of shot seam welding long seam welding is going to be administered alternatively to avoid distortion. a professional welder shall be engaged and welding shall be performed as per the approved WPS.
After completion of welding, thoroughly clean the weld joint by wire brushing and grinding.
Sketch to annular plate joint shall be welded only after shell-to-bottom joint welding.
All rock bottom plate joint vacuum box tests shall be administered as per the approved process and code specifications.
If any defect is found, the defect weld shall be removed by grinding and re-welding and conducting the vacuum box test. Repeat the sequence until the defect is cleared.

3.3 Erection of Cone Roof Plates and Structures

After completion of the highest two shell courses erection, fit-up, welding, and curb ring fit & welding shall be done.
Erect the fabricated Centre Drum, Roof Truss, and cross girders as per drawing.
Complete the welding of the Roof structure by approved welders and as per approved WPS. Erect and Lay the Roof plates on the structure as per the Drawing. While fit from the short seams and long seams Lap to be maintained as per the Drawing.
Weld the short seams by welding the alternative joint or sequence mentioned within the drawing to stop the distortion.
Provide proper support lengthwise of the long seam and weld the joints as per the drawing sequence.
Roof Nozzles and top shell nozzles fit-up and welding shall be carried out as per the approved drawing and subsequently, it’s to be correlated with the priority piping drawing.

**Fig. 5: Spiral Stairway of Storage Tanks**

3.4 Erection of Storage Tank Shell

After the completion of the welding of the Annular plates and bottom plates, mark the tank’s inner radius on the annular plates.
Fix 25 Nos. of erection tools at equal intervals on the annular Plates & transfer the within tank diameter on the stools.
The last 2 shell courses shall be erected by the conventional method.
Balance shell courses shall be erected by the Jacking method.
The rolled shell plates shall be shifted to the tank foundation area stacked around the periphery by using a crane.
Proper care shall be taken while handling the rolled plates.
Care shall be taken that the shell plate is erected to the diameter marked on the annular plate. Jigs and fixtures shall be used to align the shell plates.
Complete the fit-up except for the ultimate joint which shall be fitted and welded after completion of the welding of the opposite joints. (To avoid shrinkage).
Peaking shall be checked at the top, middle, and bottom of the vertical joints employing a Sweep board of 36” long. Plumpness shall be checked for the verticality of the shell course at every 60° and shall be within the tolerance (tolerance 1/200 of the entire shell height).
For perfect verticality, channels shall be provided at regular intervals inside the shell course (3 to five meters), providing the channels shall facilitate the alignment of the of the shell course.
Tack welds of the fitted vertical joints shall be ground smooth. Offer for inspection and obtain clearance from Engineer-in-charge for the vertical fit-up Complete the primary side welding by using qualified welders.
Care shall be taken while welding to avoid peaking and therefore the roundness distortion.
After completion of the first side welding, the back-chip shall be administered for sound metal from the other side of the weld by grinding.
Back-chipped grooves shall be offered for inspection before starting the welding.
Complete the 2nd side welding using qualified welders.
Joint Nos. & welder No. shall be marked on both sides of the weld joint. Care shall be taken to avoid peaking and roundness distortion. Clean weld joints from each side by wire brushing and grinding. After completion of welding from each side remove all the temporary jigs and fixtures and flush grind the tacks. (Do the weld refill wherever required) and therefore the portion checked by M.P.T.
Check the plumpness, circumference, and therefore the radius, and offer for inspection to the satisfaction of the Engineer-in-Charge. While erecting subsequent coarse 3 mm-thick spacers shall be kept between the shell courses.
Erection channels shall be fixed between the jacked shell and erected shell course plates at regular intervals to align and hold the last shell in a vertical position.
Check to peak at vertical joints of shell employing a sweep board of 36″wide, acceptable tolerance shall the as per API-650.
Complete welding of last course vertical seam inside after getting clearance from the Engineer-in-charge. Back chip & welding shall be administered following as same as for other shell courses to the satisfaction of Engineer – in – Charge.
After completion of welding, weld visuals, verticality, & circumference shall be verified and recorded to the satisfaction of the Engineer-in-Charge. Fit up the horizontal seam between the 2nd and 1st shell courses.
Check the verticality of the last shell course, Verticality (plumpness) tolerance shall be as per API – 650 (maximum out of plumpness at the highest of the shell relative to the rock bottom of the shell to not exceed 1/200 of total shell height from top of the last shell to bottom).
While welding, care shall be taken for banding and therefore the roundness. Back-chip shall be administered by grinding for the sound metal and to the satisfaction of the Engineer-in-Charge. Complete the 2nd side welding and clean the joint thoroughly from each side by grinding and wire brushing. Check banding, and plumpness, and record within the approved format to the satisfaction of Engineer-in-Charge.
Offer welds visually to the satisfaction of the Engineer-in-Charge. Mark the RT spots as per the instruction of the Engineer-in-Charge, and complete the RT as per API-650 requirements. Offer the RT film for review to the Engineer-in-Charge, and if any repair occurs, the repair spot shall be repaired by grinding for the sound metal, Re-weld the repair spot as per code API -650 sec-8 requirements.
Take the repair spot RT and re-offer for the inspection to the satisfaction of the Engineer-in-Charge. Spiral staircase erection shall be administered as per drawing, including brackets & avoiding fouling with welds.
Remove all the temporary cleats and tacks by grinding. If any defect is found the defect shall be repaired by grinding the defect area for the sound metal plus 150mm from both ends of the defect.
Conduct the DP test on the repair spot. perform the entire
Process until the repair is cleared to the satisfaction of the Engineer-in-Charge.
After completion of the shell to the bottom outside welding visual inspection is going to be administered to the satisfaction of the Engineer-In Charge.
On completion of the shell-to-bottom welding/NDT and having completed all erection and welding work on the tank inside associated with the roof and roof structure, all unwanted materials, and scrap shall be far away from inside the tank.
RT of the vertical and horizontal joints shall be completed.
Sketch to annular plate joint fit-up shall be administered after completing the shell-to-bottom welds.
A vacuum box test shall be administered for the rock bottom plate short seam, long seam, and sketch to the annular plate joint. If any repair occurs, an equivalent shall be repaired and re-tested as per the approved procedure and API-650.

**Fig. 6: Storage Tank Construction at Site**

3.5 Installation of Tank Appurtenances

Flanges to pipe joint shall be prefabricated and required NDT shall be completed before erection.
Mark the nozzle location as per the drawings. Cut the openings by gas cutting after proper Inspection-by-inspection authority.
Erect the nozzles as per the orientation & the elevation shown within the drawing. Install RF. pads wherever required before nozzle erection.
Suitable jigs & fixtures shall be provided to stop the distortion during the welding.
Orientation, elevation, & projection shall be maintained as per the drawing and offer for the inspection clearance. Proper care shall be taken for welding by providing jigs & fixtures to stop distortion.
Welding shall be as per WPS and to be welded by the qualified welder.
The man-hole neck shall be fabricated, and therefore the longitudinal joint shall be radiographed. All the RF Pad welds shall be pneumatically tested at a pressure of 1.05 Kg/Cm². The pneumatic test shall be administered to the satisfaction of the engineer in charge.
All the shell nozzles finally weld from each side and RF Pads welds shall be inspected visually and by LPT, to the satisfaction of the engineer-in-charge.

Storage tank construction is a complex and multifaceted process that requires careful planning, design, and execution. Whether for oil and gas, chemicals, or other industries, the principles of storage tank construction remain critical for protecting resources and the environment while ensuring the safety of personnel and the surrounding community.

Differences Between Jacking and Conventional Methods of Tank Erection

The major differences between the storage tank erection methods by conventional and jacking methods are provided here: Storage Tank Erection: Conventional vs Jacking Method

Click here to know more details about storage tanks

Pressure Relief Valve (PRV): Definition, Types, Working, Location, Sizing, Codes and Standards

Written by Anup Kumar Dey in Instrumentation,Mechanical,Pipeline,Piping Design Basics,Piping Interface,Process

A pressure relief valve is used to release excess pressure from a system during overpressure situations thus avoiding catastrophic failure. So, a Pressure relief valve is an important process safety device and is widely used in the chemical, petrochemical, power, and oil and gas industries. The pressure relief valve (PRV) is designed to open at a predefined set pressure. So whenever the system pressure exceeds the set pressure, the PRV pops and releases the overpressure and when the excess pressure is removed the PRV closes again. The main advantages of installing a pressure relief valve in a system are:

They vent the fluid to safeguard the system from overpressure.
They reclose and prevent loss of fluid when system pressure returns back to acceptable.
Installation of the PRV system minimizes damage to system components.
They are reliable and versatile

What are Relief Events?

Relief events are obligatory events that prevent efficiency or performance and increase cost but must be met considering the safety of the operating plant. Examples of potential relief events are

External fire
Flow from a high-pressure source
Heat input from associated equipment/ external source
Pumps and compressors or other equipment failures.
Failure of Cooling Medium
Ambient heat transfer
Failure of the Control system
Liquid expansion in pipes and surge
Blocked discharge, Gas blowby
Failure of the Condenser system
Chemical reactions
Operating error
Closed Outlets
The entrance of Volatile Fluid

Potential Lines of Defense against Relief Events

To act against the above-mentioned potential relief events the following defense methods are followed.

Inherently Safe Design
Low-pressure processes
Passive Control
Overdesign of process equipment
Active Control
Install Relief Systems

What is a Relief System?

A relief system is an emergency system used to safeguard plants during relief events by reducing pressure or discharging gas during abnormal situations. The relief system consists of

A relief device, and
Associated lines and process equipment to safely handle the material ejected

Why Use a Relief System?

Installing relief systems in operating plants is a must from the process and technical safety points as

Inherently Safe Design simply can’t eliminate every pressure hazard
Passive designs can be exceedingly expensive and cumbersome
Relief systems work!

Code Requirements for relief system design

General Code requirements include:

ASME Boiler & Pressure Vessel Codes
ASME B31.3 / Petroleum Refinery Piping
ASME B16.5 / Flanges & Flanged Fittings

Relieving pressure shall not exceed MAWP (accumulation) by more than:

3% for fired and unfired steam boilers
10% for vessels equipped with a single pressure relief device
16% for vessels equipped with multiple pressure relief devices
21% for fire contingency

Locating Pressure Relief Valves

The location of Pressure Relief valves is decided by Process Engineers. They mention the PRV requirements in the P&ID. Below are the General guidelines on where relief devices are required, although there are likely to be other special cases in any process.

All vessels
Blocked in sections of cool liquid lines that are exposed to heat
Discharge sides of positive displacement pumps, compressors, and turbines
Vessel steam jackets
Low-pressure storage tanks require both vacuum and pressure relief devices since tanks are typically not designed for full vacuum.
Wherever formal hazard identification procedures such as Hazard and Operability (HAZOP), Process Hazard Analysis (PHA) indicates
Piping systems where overpressure can arise due to process control failure.

Types of Pressure Relief Valves

Conventionally Pressure relief valves are categorized into the following three groups:

Relief Valve
- Adjustable
- Electronic
Safety Valve
- Low Lift
- High Lift
- Full Lift
Safety Relief Valve
- Conventional spring-loaded safety relief valve pilot-operated
- relief valve
- Balanced-bellows type relief valve
- Power actuated
- Temperature and Pressure actuated relief valve

The above-mentioned pressure relief valve types are produced in graphical form in Fig. 1 below

**Fig. 1: Types of Pressure Relief Valves**

Relief valves are spring-loaded and characterized by gradual opening and closing. They are actuated by the upstream pressure and are suitable for incompressible fluids. Adjustable relief valves allow the pressure setting adjustment through the outlet port. Electronic relief valves offer zero leakage with electric controls to monitor and regulate the system pressure.

On the other hand, safety valves are used for compressible fluids (gas and vapors) and are characterized by the rapid action of opening and closing. Safety valves are widely used in steam plants for boiler overpressure protection. They are classified into three groups based on the amount of travel or lift during the pop-up. Low-lift safety valves have a small capacity and the valve lifts 1/24^th of the bore diameter. High-lift safety valves travel 1/12^th of the bore diameter. Whereas Full-lift safety valves travel at least 1/4^th of the bore diameter and are best suited for steam services.

The safety relief valve can be used for gas or liquid service depending on the application. They have the characteristic of both rapid and gradual opening.

Conventional Pressure Relief Valve

Spring-loaded conventional pressure relief valves are best suited for applications where excessive back pressure is absent. The back-pressure directly affects the operational characteristics of these PRVs. Refer to Fig. 2 which represents a conventional safety relief valve with its basic elements.

**Fig. 2: Conventional type Pressure Relief Valve**

There are three basic components of a conventional pressure relief valve

An inlet nozzle to be connected to the system requiring protection.
A movable disk for fluid flow control, and
A spring for controlling the disk position.

While designing a conventional pressure relief valve, consideration of seat leakage to be checked as leakage means continuous loss of system fluid and the valve seating surface can be damaged. Depending on the seating material, conventional pressure relief valves are classified into the following two types:

Metal-seated valves, and
Soft-seated valve

Pros & Cons of Conventional Pressure Relief Valves

Advantages

Most reliable type if properly sized and operated
Versatile — can be used in many services

Disadvantages

Relieving pressure affected by back pressure
Susceptible to chatter if built-up back pressure is too high

Balanced Bellows Type Pressure Relief valve

To reduce the effects of backpressure, spring-loaded balanced bellows pressure relief valves (Fig. 3) are developed. The PRV design incorporates a bellow that offsets the effect of back pressure. The bellow isolates the spring, bonnet, and guiding surfaces from direct contact with the process fluid.

Typically when back pressure is variable and exceeds 10% of the set pressure, a balanced-bellows type pressure relief valve is used.

Balanced Bellow type Pressure relief valve — **Fig. 3: Bonnet Bellow type PRV**

Pros & Cons of Balanced Bellows type Pressure Relief Valve

Advantages

Relieving pressure not affected by back pressure
Can handle higher built-up backpressure
Protects spring from corrosion
Possess good temperature and chemical properties

Disadvantages

Bellows are susceptible to fatigue/rupture
May release flammables/toxics into the atmosphere
Requires a separate venting system

There are two types of balanced bellows safety relief valves:

Balanced bellows
Balanced bellows with auxiliary balancing piston

Pilot-operated Pressure Relief Valves

A pilot-operated safety relief valve is a pressure relief valve where a self-actuated auxiliary pressure relief controls the pressure-relieving. The opening or closing of the relief valve is governed by the pressure of the flowing medium. A pilot is used to sense the process pressure and to pressurize or vent the dome pressure chamber, which controls the valve opening or closing. Three main components consist of a pilot-operated pressure relief valve (Fig. 4)

the main valve,
a floating, unbalanced piston assembly, and
an external pilot.

The pressure on the top side of the main valve’s unbalanced moving chamber is controlled by the pilot. Generally, a resilient seat is attached to the lower end.

Advantages of Pilot-Operated Pressure Relief Valve

The main advantages of pilot-operated safety relief valves are:

The set pressure is unaffected by the valve backpressure.
As the system operating pressure decides the opening of the relief valve, the system can be operated at maximum working pressure.
Economical as compared to other types.
Less susceptibility to chatter.

Pilot-operated Pressure relief valves can be classified based on various parameters as mentioned shown below:

Depending on the type of moving members
- A piston-type.
- A diaphragm-type.
Based on the type of pilots
- A pop-action pilot
- A modulating-action pilot
Based on the flow of pilots
- A flowing-type pilot.
- A non-flowing-type pilot

Power Actuated Pressure Relief Valve

Power-actuated pressure relief valves (Fig. 5) are controlled by a device requiring an external power source. Energy sources like water, electricity, or steam control the opening and closing of the pressure relief valve. They are mostly used for forced-flow steam generators with no fixed steam or waterline and in nuclear power plants.

Temperature Pressure actuated Pressure Relief Valves

A temperature and pressure-actuated pressure relief valve (also known as T&P safety relief valve-Fig. 5) is actuated by the temperature or pressure of the inlet side of the relief valve. The valve consists of two primary controlling elements, a spring, and a thermal probe. They serve dual purposes.

Prevention of temperature rise above specified and
Prevention of over-pressure from rising above a specified value.

They are mostly used for vessels, tanks, and heaters carrying hot fluids.

**Fig. 5: Power and Temperature Actuated Safety Relief Valve**

Vacuum Relief Valve

A Vacuum Relief Valve is designed to prevent an excessive internal vacuum by admitting fluid. Once the normal condition is restored, they reclose and prevent further fluid flow.

When to Use a Spring-Operated Pressure Relief Valve

Losing entire contents is unacceptable
- Fluids above the normal boiling point
- Toxic fluids
Need to avoid failing low
Return to normal operations quickly
Withstand process pressure changes, including vacuum

Pressure Relief Valve Accessories

A number of pressure relief valve accessories help the valve in its operations to achieve the intended use. They are:

Test gags hold the safety valve closed during the hydrostatic test.
Lifting mechanisms to lift the valve disk. Available in three types
- plain lever,
- packaged lever, and
- air-operated lifting devices.
Bolted caps are available for standard pressure relief valves in addition to the screwed caps.
Valve position indicators for remote indication of the PRV opening

Working of a Pressure Relief Valve

For a spring-loaded pressure relief valve, the spring force holds the disk in position keeping the valve in a closed position. When the pressure of the line exceeds the set pressure, the disk starts to lift allowing the fluid to flow through the outlet and release pressure. With a further increase in inlet pressure, the disk lifts further. When the disk has traveled to its designed value, the valve is fully open and the system pressure is released.
Once the overpressure inside the system falls below the spring force the spring pushes back the disk in positive to close the valve preventing further release of fluid.

For pilot-operated pressure relief valves, the inlet pressure is directed to a small safety valve that acts on top of the piston. As the top area of the piston is designed greater than the bottom area under fluid contact, the pressure on top is higher which pushes the piston to close the relief valve. When the inlet pressure rises above the set pressure, a net upward force acts on the piston forcing the piston to pop up and release the pressure.

Codes and Standards for Pressure Relief Valve

Pressure relief valves are governed by codes and standards. The most widely used pressure relief valve codes and standards are:

ASME BPVC (Sec I, Sec III, Sec IV and Sec VIII)
ISO 4126
API 520
API 521
API 526
API 527
PED 97/23/EC
EN4126
JIS B 8210 (Japan)
KS B 6216 (Korea)
SAA AS 1271 (Australia)

Relief Valves – Pressure Terminologies

Let us understand some additional basic terms which are widely used in relation to relief valves.

The set pressure is the pressure at which the relief valve starts to open. It is normally the same as design pressure and is measured at the valve inlet. For spring-operated relief valves small amount of leakage (simmer) starts at 92-95% of the set pressure.

Overpressure is the pressure increase over the set pressure of a pressure relief device, during discharge and is usually expressed as a percentage of the set pressure (normally overpressure is set to pressure +10%) Relief valve achieves its full discharge capacity at overpressure

Accumulation is the pressure increase over the DP/ MAWP of equipment during discharge through the protecting pressure relief valve and is usually expressed as a percentage of the DP/MAWP.

Generally there is confusion between the terms Accumulation and Overpressure. When we say ‘accumulation’, it means we are talking about the vessel, and when we say ‘overpressure’, we are talking about the pressure relief valve

Blowdown is the pressure difference between the set pressure and the pressure at which the valve reseats. It is usually expressed as % of the set pressure and refers to how much the pressure needs to drop before the valve reseats.

Reseat Pressure is the pressure at which the valve is fully closed.

The cold differential test pressure is the pressure at which the valve is adjusted to open on the test stand, and incorporates the effects of superimposed back pressure and operating temperatures

Pressure Relief Valve Sizing

Correct sizing of Relief Valves is crucial. If the relief valve is undersized it may not relieve sufficient quantity of fluid to prevent pressure build-up. This consequently may result in high pressure. If the relief device is oversized, the relief valve may become unstable during operation.

The sizing of a pressure relief valve is done by the Process team based on the governing codes and standards. The most widely used reference for pressure relief valve sizing is API 520. The parameters that affect the PRV sizing and selection are

Set the Pressure of the relief valve
Process Design temperature and pressure
Size of inlet and outlet piping
Backpressure on the pressure relief valve outlet
Fluid Service
The required capacity of the relief valve
Flow condition (liquid flow, gas flow (critical and sub-critical), steam flow, and two-phase flow)

The sizing of the pressure relief valve is a complex method requiring a multi-step process as listed below:

Defining the Protected System
Locating the relief valve
Defining the over-pressure condition
Selecting the relief device
Obtaining Data for Relief valve Sizing
Determining the flow condition types

The above steps can be easily shown in the form of a flowchart as shown in Fig. 6.

**Fig. 6: Pressure Relief valve Sizing Flowchart**

Most major pressure relief valve manufacturers provide sizing software having the unlimited capability to accept wide variability of fluid properties and decide the right pressure relief valve. Some typical software for pressure relief valve sizing is developed by:

Anderson Greenwood Crosby
PRV2SIZE software Emerson Automation Solutions
PRV2SIZE software Pentair Software
VALVESTAR^® by LESER Safety Valves
SIZEMASTER – Relief System Sizing Software by Farris
VALVIO by HEROSE
Fluid-Flow

In absence of pressure relief valve sizing software or manufacturer’s standard tables, the effective orifice area can be manually calculated using the following equations:

**Fig. 7: Pressure Relief Valve Sizing Equations**

After getting the effective area, the Standard pressure relief valve orifice designation (size) is selected from the following table:

**Fig. 8: Standard Orifice Designation for Pressure Relief Valve**

Pressure Relief Valve Symbols

Pressure relief valves are designated by special symbols as shown below:

Fig. 9 also provides the P&ID representation of a typical Pressure Safety Valve. Set pressure and Orifice Designation is clearly mentioned in the P&ID, along with the identifier and symbol of the pressure relief valve.

Relief Valve Chattering

Chattering is the rapid opening and closing of a pressure relief valve at low flow rates. Under normal process conditions the vessel pressure is below the set pressure of the relief valve. As the pressure increases and exceeds the relief valve set pressure the valve opens. As soon as the valve opens there is flow resulting in a pressure drop between the vessel and the valve. If this pressure drop is large enough, the pressure at the relief valve can be low enough so that the relief valve closes. The flow stops, the pressure at the relief valve increases back to the vessel pressure because there is no flow to cause a pressure drop and the relief valve opens again.

Spring relief devices require 25-30% of maximum flow capacity to maintain the valve seat in the open position
Lower flows result in chattering, caused by rapid opening and closing of the valve disc
This can lead to the destruction of the device and a dangerous situation

Chatter – Principal Causes

Valve Issues

Oversized valve
Valve handling widely differing rates
Relief System Issues
- Excessive inlet pressure drop
- Excessive built-up backpressure

Oversized valves will partially lift at set pressure & then re-seat resulting in “chattering” of the disc which could damage the seat/disc surfaces and cause the relief valve to fail. It is good practice to install multiple relief valves for varying loads to minimize chattering on small discharges.

The general rule to prevent relief valve chattering: line pressure drop between equipment and inlet of RV during relief case must be <3% of RV set value.

Difference between a PSV and PRV

A Pressure Relief Valve (PRV) opens gradually in relation to the pressure, on the other hand when the pressure reaches a certain value a Pressure Safety Valve or PSV opens suddenly to release the overpressure.
PRV is normally used for liquid systems while PSV is for gaseous systems.
The set point of PRV is usually 10% above the working pressure while the set pressure in PSV is generally 3% above the working limit.

Online Courses on Pressure Relief Valve

If you still have doubts, undertake the specially designed below-mentioned courses to improve your understanding of the subject:

Few more useful Resources for you…

Pre-Commissioning and Commissioning Checklist for Flare Package
Flare systems: Major thrust points for stress analysis
Stress Analysis of PSV connected Piping Systems Using Caesar II
Articles related to Process Design
Piping Layout and Design Basics
Piping Stress Analysis Basics

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