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What is High Integrity Pressure Protection System (HIPPS)?

Written by Sanjeev Das in Instrumentation,Piping Interface

The term HIPS stands for ‘High Integrity Protection System’. In upstream Oil and Gas projects, HIPS is usually used for the protection of pipelines and associated equipment downstream of a production manifold, from an overpressure scenario, for example, due to a blocked outlet. Hence, HIPS is sometimes termed as HIPPS – ‘High Integrity Pressure Protection System’.

Simply told, HIPPS or high integrity pressure protection system is a highly reliable Safety Instrumented System (SIS) which, on detecting a high pressure, would cut off the pressure source to avoid damage to the downstream pipeline. In this article, we will use the term HIPPS in the above-specified context, though HIPPS may be used in other applications too.

As HIPPS, in effect, replaces the conventional pressure relief systems, the last safety barrier, it shall be extremely reliable, which means that the HIPPS shall be designed to achieve a high Safety Integrity Level (SIL). Implementation of HIPPS mandates stringent documentation, testing, and inspection plans.

HIPPS typically comprises of the following:

  1. Sensors- three Pressure Transmitters, configured in ‘two out of three (2oo3) voting logic
  2. Logic solver
  3. Final Elements – two high-integrity fast acting isolating valves

Typical Architecture of High Integrity Pressure Protection System

Typical Architecture of HIPPS
Fig. 1: Typical Architecture of HIPPS

HIPPS sensors/initiators – Three Pressure Transmitters PT1, PT2, and PT3, measure the pressure downstream of HIPPS.

On detection of high pressure (at least two out of three transmitters measuring high pressure), the Logic Solver will initiate the close commands for both the isolating valves XV1 and XV2 (final elements). This action will cut off the pressure source and protect the downstream pipeline and equipment, which are designed for low pressure. Such a low-pressure system design could give significant project cost savings, owing to large pipe diameter and long pipeline lengths. The above safety loop can be termed as a Safety Instrumented Function (SIF).

Sensors – Pressure Transmitters

The transmitters shall have independent process taps/root isolation valves provided on the piping; shared isolation valves are not permitted. The transmitters shall be certified for use in SIL 3 application.

The project specifications may require special manifolds for the transmitters. Such manifolds are provided with a mechanical valve interlock to ensure that not more than one transmitter can be removed out of service for the purposes of testing or maintenance.

Similarly, it may be required to provide transmitters of different makes or models to reduce the possibility of common mode/ cause failure.

Where heat tracing is required for the pressure transmitter impulse tubing and manifold, the same shall be provided with reliable components, suitable alarms, and diagnostic features.

Logic Solver

The HIPPS logic solver is usually an independent SIL 3 certified Programmable Logic Controller (PLC), where the 2oo3 high-pressure voting and isolation valve trip logic is executed. However, sometimes this logic can also be executed in the plant’s emergency shutdown system (ESD), subject to satisfying the requirements of the required SIL.

The HIPPS PLC shall be supplied complete with redundant Processors, IO modules, Power supplies, Communication modules, operator interface, local printer, system, and application software, cabinet, wiring, and interface with the plant control system as per the project specifications.

HIPPS Valves

The HIPPS valves shall be high-performance, quick-closing, tight shut-off, fail-closed Isolation Valves. SIL certificates/reliability data of the valves shall be provided by the valve vendors.

Two HIPPS valves shall be installed in series. It may be required that the valves be of different types, to avoid common modes of failure, like valves stuck at their open position.

The HIPPS valves shall utilize Instrument air as the medium for actuation. Usually, volume bottles with backup Instrument air are provided to ensure high availability. Where Instrument air is not available, like remote well pads, hydraulic oil can be used. Redundant hydraulic oil pumps and oil headers shall be used to avoid common mode failure of the hydraulic system.

If the valves are located within the fire zone, necessary fireproofing of the valve actuators, Instrument air volume bottle, or the hydraulic power unit (HPU) shall be considered.

Valve leakage, the material of construction, and other features shall be as per the Valve Datasheet. The valve closing time required shall be determined from the simulation study and the same shall be recorded in the datasheet.

HIPPS valves shall be provided with “smart” positioners to facilitate periodic and remote partial and full stroke testing.

HIPPS Interfaces

The HIPPS shall be provided with a redundant serial communication link with the Distributed Control System (DCS) and a hardwired connection with the ESD system for information exchange.

Following are the typical interfaces with the DCS / ESD:

  1. Interlocks with plant on HIPPS actuation
  2. ‘Reset’ / Opening of HIPPS valves once the trip initiators are healthy
  3. Graphic interface to the plant operator showing the status of HIPPS
  4. The sequence of Events (SOE), alarms, and HIPPS diagnostics,
  5. Time synchronization
  6. Reporting and printing

HIPPS can be supplied complete as a package, with all the components described above, by a single vendor/integrator. Alternatively, the EPC Contractor may source the various components from different suppliers. In both cases, the complete system shall be certified for the desired SIL.

Safety Integrity Level (SIL)

The criticality of a Safety Instrumented Function (SIF) is expressed in SIL classes 1 to 4 as per IEC 61508. SIL defines the target ‘Probability of Failure on Demand (PFD) or a target level of Risk Reduction. The Risk Reduction Factor (RRF) is defined as follows:

RRF = 1 / PFD

A SIL 1 safety function provides minimum Risk Reduction (RRF 10 to 100 times), has the highest Probability of Failure on Demand (PFD), and is considered the least reliable.

A SIL 4 safety function provides maximum Risk Reduction (RRF 10,000 to 100,000 times), has the lowest Probability of Failure on Demand (PFD), and is considered the most reliable.

The target SIL of the HIPPS is defined by the SIL assessment team in a SIL assessment workshop. HIPPS shall be designed to achieve or exceed the target SIL, typically SIL 3. This is achieved by using redundant design, with no common mode failure, having good diagnostics, and performing periodic testing of all the elements.

For example, to meet the SIL 3 requirements, the HIPPS shall have a PFD value between 10-3 and 10-4 which may be achieved by using the following configuration:

  • Initiators – three pressure transmitters in a 2oo3 voting configuration
  • Logic solver – an independent PLC
  • Final elements – two isolation valves installed in series

All the above components shall be certified for use in a SIL 3 safety loop.

Once the HIPPS transmitters, PLCs, and valves are finalized for ordering, the reliability data of these elements shall be obtained from the respective suppliers to perform the PFD calculations and to verify if the HIPPS is meeting the target SIL.

The Safety Requirement Specification (SRS) compiles all the data related to the HIPPS, such as the overpressure scenario details, the mitigation method, functional logic narratives, dynamic simulation recommendations, the target SIL, process safety time, proof test intervals, the HIPPS configuration and response time to achieve the target SIL, etc.

HIPPS response time shall be sufficient to prevent the overpressure scenario.

HIPPS response time = Response time of the initiators + (IO data processing time + scan time) of the logic solver + Valve opening time

Testing of HIPPS Elements

All the elements of HIPPS shall be proof tested at defined internals as defined in the SRS or as considered while calculating the PFD, to ensure maintain the validity of the loop SIL certification.

The typical test intervals may be as follows:

  • Logic Solver – 36 months
  • HIPPS valves – full stroke testing – 24 months, partial stroke testing – 3 months  
  • Pressure Transmitters – 24 months

What is Mach Number? Its Significance, Applications, and Formula

Written by Animesh Chakraborty in Process

The term Mach Number is very important in fluid mechanics and process engineering. Mach number is widely used for speed comparison purposes with respect to the speed of sound. It is a very important parameter in fluid dynamics study. Mach number is also used to determine the compressibility of a fluid. In this article, we will learn more about Mach Number, Its importance, and equations.

What is Mach Number?

The Mach number is defined as the velocity of an object in a medium divided by the sound velocity in that medium. It is a dimensionless number used to compare the velocity of an object to the speed of sound. Mach numbers are commonly represented by the symbol “M”.

Mach number (M) = [object velocity(u)] / [sound velocity(c)]

Mach numbers are dimensionless because they are defined as the ratio of two velocities. If the flow is quasi-steady and isothermal with M <0.2–0.3, the compressibility effect is small and the fluid can be treated as incompressible.

The Mach number is named after the Austrian philosopher and physicist Ernst Mach. Because this is a dimensionless quantity and not a measurable unit, the number is placed after the term Mach, such as Mach 4 instead of 4 Mach.

What is the Mach number used for?

Mach number is used to determine whether a flow is incompressible or compressible. The medium can be gaseous or liquid. There are many other applications of Mach number as listed below:

  • Speed or Velocity comparison
  • To find out the sonic condition of fluid while determining the probability of acoustic-induced vibration.
  • While studying the motion of rockets and planes, the Mach number is a very important parameter.
  • Mach number is an important parameter for studying choked flow and flow through nozzles.
  • The study of shock waves is also dependent on Mach Number.

Compressibility and Mach number

Compression and expansion are important properties of fluids. Fluids can be liquids or gases. A fluid is said to be compressible if compression and expansion have a large effect on the density (kg/m3) of the fluid. On the other hand, if compression and expansion do not significantly affect the density of the fluid, the fluid is said to be incompressible. The volume of an incompressible liquid does not change with changes in pressure or temperature, and the density is treated as constant. Liquids are always considered incompressible because their density changes less with pressure and temperature. Strictly speaking, no perfectly incompressible fluid exists. However, if the density change with pressure or temperature is small, approximating the fluid as incompressible simplifies the calculations considerably.

Fluids (air/gas) behave similarly under the influence of compression ratio at a particular Mach number, independent of other variables. The speed of sound is 340.3 m / s (1,116.5 ft / s; 761.23 mph), as modeled at the International Standard Atmosphere, dry air above sea level, and standard temperatures of 15 ° C (59 ° F).  The speed of sound is not constant. In gas, the temperature rises in proportion to the square root of the absolute temperature, and as the altitude rises to 11,000 meters (36,089 feet) above sea level, the temperature generally drops, so the speed of sound also drops. For example, the standard atmospheric model reduces the temperature to -56.5 ° C (-69.7 ° F) at an altitude of 11,000 meters (36,089 feet) and the corresponding speed of sound (Mach 1) is 295.0 meters / second (967.8 feet/second). 659.9 mph), 86.7% of sea level.

Sonic Velocity

As the pressure at a point is increased the adjacent molecules undergo a small change in position. If the fluid is incompressible then the motion is infinitely quick. Most fluids are, to some degree compressible, therefore this time is finite. If there is a sudden change, or the fluid is moving quickly then this time is important.

For a thermally perfect gas the relationship is:

c=√(γRT)

We can express the velocity of gas (u) as a function of the sonic velocity

M= u/c

  • Where M is the Mach Number;
    • When M < 1    Subsonic
    •           M = 1    Sonic
    •           M > 1    Supersonic
  • c = Sonic velocity (m/s)
  • γ = Specific heat ratio
  • R = Specific gas constant ( J / mol . K)
  • T = Temperature of the fluid (K)

Formula for Mach Number

As already stated above, the equation for Mach number is given by

Mach Number, M=u/c

Where Mach number is M. Based on the limit values, the local flow velocity is u and the speed of sound in this medium is c.

It can be said that the speed of sound corresponds to the speed of Mach 1. Thus Mach 0.75 is 75% of the speed of sound, also known as subsonic, and Mach 1.65 is 65% faster than the speed of light, also known as supersonic.

The Mach number due to the local speed of sound depends on the surrounding medium at a given temperature and pressure. This flow can be determined as an incompressible flow using the Mach number. The medium can be either liquid or gas. The medium can flow while the boundary is stable, or the boundary can move in a stationary medium. Both the medium and the boundary can move at a certain speed, but the speed of each other is important. Media can be passed through multiple devices such as wind tunnels or submerged in media. The Mach number is called a dimensionless number because it represents the ratio of two velocities.

Classification and Importance of Mach Number

Using Mach numbers, compressible flows can be nominally categorized as:

 [1] Incompressible flow: M = 0. The liquid density does not change with the pressure of the flow field. The flowing fluid can be a compressible gas, but its density can be considered constant.

 [2] Subsonic flow: 0 <M<1. Mach numbers do not exceed 1 anywhere in the flow field. No shock wave is generated in the flow. In engineering practice, subsonic flows with M <0.3 are often treated as incompressible.

 [3] Transonic flow: The Mach number of the flow is in the range of 0.8 to 1.2. A shock wave may be generated. The analysis of transonic currents is difficult because the governing equations are non-linear and it is often impossible to separate the non-viscous and viscous aspects of the flow.

[4] Supersonic flow: M> 1. Shock waves are common. In many respects, analyzing a flow that is supersonic everywhere is easier than analyzing a subsonic or incompressible flow. This is because the information propagates along specific directions called properties, and determining these directions greatly simplifies the calculation of the flow field.

[5] Hypersonic flow: M> 3. Very high flow velocities combined with friction or shock waves can result in sufficiently large temperature rises in the liquid due to molecular dissociation and other chemical effects.

Mach Number and Flow Through Nozzles

For flow through nozzles and diffusers, the governing equation is:

(M2-1) * [∂( u)/u] =[∂(A)/A]

  • If we assume that the flow is subsonic (M < 1) then M2 – 1 < 0
    • Therefore ∂ (A) and ∂ (u) are opposite signs
    • In other words, as ∂ (A) increases then ∂ (u) decreases
  • If we assume that the flow is supersonic (M > 1) then M2 – 1 > 0
    • Therefore ∂ (A) and ∂ (u) are the same sign
    • In other words as ∂ (A) increases then ∂ (u) increases
  • For M = 1 then ∂ (A) must be zero. The second derivative of this is positive therefore A is minimum.
    • In other words, if the flow is sonic it must be sonic at the throat

Frequently Asked Questions related to Mach Numbers

1. What’s the highest Mach speed?

The Guinness World Records awarded NASA’s X-43A Scramjet a new world speed record for jet-engine aircraft, Mach 9.6, or about 7,000 miles per hour. The X-43A set new standards and set its own world record on its third and final flight on November 16, 2004.

2. Does pressure affect Mach Number?

As the Mach number increases, so does the intensity of the shock wave, and the Mach cone becomes narrower and narrower. As the fluid flow crosses the shock wave, it slows down and increases in temperature, pressure, and density.

3. Does temperature affect Mach Number?

The warmer the air, the lower the density, the slower the speed of sound, and the faster the speed on the ground to achieve the same Mach number.

4. Can anything go Mach 10?

On November 16, 2004, NASA made history by launching the first-ever air-breathing hypersonic aircraft, the X-43A, into the atmosphere and reaching Mach 10 speeds. Separated from the booster, the X-43A used scramjet propulsion to accelerate to about 110,000 feet at nearly 10 times the speed of sound (7000 MPH).

5. Is hypersonic faster than supersonic?

Supersonic is the speed of Mach 1, and hypersonic is the speed of Mach 5. Supersonic speeds are faster than the speed of sound, while hypersonic speeds are five times the speed of sound. The Concorde is the only supersonic airliner, but there is no such thing as a hypersonic airliner.

6. What Mach is supersonic?

Supersonic is the speed of an object that exceeds the speed of sound (Mach 1). For objects moving in dry air at a temperature of 20 ° C (68 ° F) above sea level, this velocity is approximately 343.2 m / s (1,126 ft / s; 768 mph; 1,236 km / H).

7. Which Mach number is considered the beginning of compressibility?

Gas is considered compressible only if its Mach number is greater than 0.3. Therefore, if the Mach number is less than 0.3, the change in liquid density is not significant and the gas is considered incompressible.

8. Which Mach number is less than 1 flow?

If the Mach number is less than 1 and M <1, a subsonic condition occurs. For the lowest subsonic conditions, the compression ratio is negligible. As the speed of the object approaches the speed of sound, the flight Mach number becomes approximately equal to 1 and M = 1, and the flow is said to be transonic.

9. Why is Mach number important in a compressible flow?

If the Mach number is very high, the fluid is considered compressible, and if the Mach number is low, the volume remains constant as the pressure changes and the flow becomes incompressible. If the Mach number is less than 0.3, the mechanical approach is incompressible flow.

10. At what speed does air become compressible?

At velocities below 100 m / s, the air is treated as an incompressible fluid, and at velocities above 100 m / s, the air is treated as a compressible fluid.

11. What are compressible and incompressible flows?

The difference between compressible and incompressible flows in fluid mechanics is conceptually simple. Compressible fluids can experience changes in density during flow, but incompressible fluids do not.

Compressible vs Incompressible fluid
Fig. 1: Compressible vs Incompressible fluid

12. What altitude do you switch from airspeed to Mach?

When flying with an autopilot over approximately 20,000 feet, the pilot switches from the airspeed indicator to the Mach (sound velocity) indicator because the airspeed readings are high altitude and inaccurate.

13. What happens when an Aeroplane climbs at a constant Mach number?

Atmospheric pressure, density, and temperature decrease with altitude, so when flying a constant Mach number, the aircraft actually constantly accelerates or decelerates to the air mass and constantly rises or falls.

14. Does Mach number increase or decrease with altitude?

Since the speed of sound increases with temperature and the temperature generally decreases with altitude, the true airspeed for a particular Mach number generally decreases with altitude.

Mach Number vs Altitude
Fig. 2: Mach Number vs Altitude

15. Why is the Mach number used at high altitudes?

Mach number is defined as the speed ratio associated with the speed of sound. In other words, as the temperature and density of air decrease with altitude, so does the speed of sound. Therefore, certain true velocities result in higher altitudes and higher Mach numbers.

What is a Septic Tank? Types, Working and Maintenance of Septic Tank Systems

Written by Anup Kumar Dey in Civil,Construction

Septic tanks are of vital importance for wastewater treatment as domestic sewage flows through them for basic sewage treatment. They are a type of onsite sewage facility and are widely found in areas not directly connected to a sewage system, mainly in rural areas. In this article, we will break down the working and fundamentals of a septic tank.

What is a Septic Tank?

A septic tank is an underground sedimentation tank in a wastewater treatment system. They are made of fiberglass, plastic, or concrete and can have one or more tanks. One end of the septic tank is connected to a wastewater inlet pipe and the other end to a septic drain field.

The term “septic” indicates to anaerobic bacterial environment developed in the tank to decompose the waste discharge in the tank. the design of the septic tank incorporates two chambers with an access opening and cover. A dividing wall having openings located about midway between the floor and roof of the tank separates them.

Natural proven technology and processes are used in a septic tank to treat wastewater from household plumbing produced by bathrooms, kitchen drains, and laundry. Septic tanks are usually rectangular or round in shape. The T-shaped outlet and compartments of the tank prevent the sludge and scum from leaving the tank and traveling into the drain field area. Septic tanks are always installed underground and usually 50 meters away from the household and regular maintenance is compulsory.

Working of a Septic Tank

All septic tanks are connected with two pipes; inlet and outlet. The water waste from the house is transported by the inlet piping and it is then collected in the septic tank. In the tank, the wastewater is stored for a long period of time so that the septic tank digests/decomposes the organic matter and separates floatable and solid matters like grease/oil from the wastewater.

The outlet pipe is also known as the drain field pipe which moves the processed wastewater from the septic tank to spread it in the soil and watercourses. The wastewater inside the septic tank is separated into 3 layers (Fig. 1).

  • The top layer consists of oils and grease. It floats above all the waste and is usually known as “scum”.
  • The middle layer contains the wastewater along with waste particles.
  • The third and bottom layer consists of heavy particles that form a layer of sludge.

The bacteria present in the septic tank break down the solid waste of the wastewater which is then separated and drained easily. the left-over inside the tank should be periodically removed during maintenance.

Underground Septic Tank
Fig. 1: Underground Septic Tank

The following is the basic step-by-step process by which a septic tank generally works:

  • Domestic water from the bathroom and kitchen runs through one main drainage pipe leading to the underground septic tank.
  • The wastewater is held in the septic tank for some time where the separation and decomposition process starts.
  • The liquid effluent exits the tank into the drain field.
  • This pretreated wastewater is then discharged through pipes onto porous surfaces that filter through the soil.
  • Finally, the wastewater naturally removes the harmful bacteria, viruses, and nutrients as it percolates into the soil.

EN12566 provides the general requirements for domestic water treatment facilities.

Septic Tank Maintenance

The maintenance of the septic system must be periodically done in order to achieve proper working of the system. In general, it is the responsibility of the property owner. Regular septic tank maintenance helps in

  • Clearing the inlet drain blocks generated due to excessive disposal of cooking oils and grease.
  • Cleaning the clogs with non-biodegradable waste items down the toilet such as cigarette butts, and cotton buds/swabs.
  • Remove the food wastes that rapidly overload the septic tank system.
  • Eliminating certain chemicals like pesticides, herbicides, bleach, caustic soda, etc which may damage the septic tank or kill the bacteria.
  • Cleaning the biofilms developed on the pipe can lead to blockage.

Some manufacturers promote the use of certain septic tank additives to improve the effluent quality from the septic tank. There could be some environmental problems due to septic tank operation like:

  • Odor and gas emissions.
  • Septic tank failures can create pathogens that can create dangerous diseases.
  • In highly dense areas groundwater and surface water pollution may occur

Types of Septic Tank Systems

Septic systems vary widely in their design and sizes. Various factors are responsible for these changes. Some of the important factors are:

  • household size,
  • soil type,
  • weather conditions,
  • site slope,
  • lot size,
  • proximity to sensitive water bodies,
  • even local regulations

Some of the most common types of septic tanks are:

  • Conventional septic system consisting of a septic tank and a trench or bed subsurface wastewater infiltration system.
  • Chamber septic system including open-bottom chambers, fabric-wrapped pipe, and synthetic materials.
  • Drip distribution septic system with a large dose tank after the septic tank.
  • Aerobic treatment unit.
  • Mound septic system
  • Recirculating sand filter septic system
  • Evapotranspiration bed septic system
  • Constructed wetland septic system
  • Cluster septic system.

What is Gas Piping? Materials, Sizing, Working, and Problems of Gas Piping System

Written by Anup Kumar Dey in Engineering Materials,Piping Design Basics

Gas piping is basically natural gas piping used widely in common households. The gas piping system carries natural gas from the supply to household ovens or heating systems. The complete gas piping system consists of branch lines that distribute natural gas to individual appliances throughout the home and drop lines. Proper working of gas piping is crucial for the safety of mankind and properties. In this article, we will learn about gas piping systems, their working, materials used, and types.

The piping or lines that supply natural gas inside the house is known as a gas supply line or building line. Supply lines transfer the gas to branch lines that run to individual appliances. The branch lines terminate in a vertical drop-down pipe known as a drop line which connects to the appliance. The gas piping system also has drip legs or sediment traps at the appliance connection point.

Working of Gas Piping Systems

The flow of natural gas through the gas piping systems relies on the internal pressure inside the piping system. It is well-known that gas flows from higher to lower pressure. After the extraction of the natural gas, it is transported through a long highway-like piping system that ends up in the distribution systems to bring the gas into a common household.

The distribution line is also known as the main line. Natural gas from the mainline runs into the service lines. Beyond these service lines, gas supply lines and equipment are connected. A pressure regulator controls the pressure of natural gas entering any individual household. The gas pressure is kept slightly higher than the atmospheric air pressure so that the gas flows out of the burner and into the heating to ignite it.

Materials for Gas Piping

For distributing natural gas, a range of materials are available. The most common gas piping materials are Galvanized Steel, PVC, black iron, HDPE, Copper, Aluminum Plastic Composite, Stainless Steel, etc. Local guidelines must be followed when deciding the ideal material for the specific gas piping system.

Flexible Corrugated Stainless Steel Tubing: This type of gas piping system is highly flexible and easy to install. In high-risk locations of natural disasters, flexible stainless steel works well. The flexibility of corrugated tubing minimizes damage and is widely used for indoor gas piping systems.

Galvanized Steel: Galvanized steel gas pipes are energy efficient, versatile, and durable. they find applications in both interior and exterior gas lines. However, as galvanized steel gas pipes are labor-intensive, they are not generally used in new constructions.

Black Iron Gas Pipes: This is the most common material for gas pipes. Used both in interiors and exteriors applications, this type of gas piping system is strong, heat resistant, and can form an airtight seal. However, black iron gas piping systems must be maintained regularly as they are prone to corrosion and their sealant can deteriorate.

PVC Gas Piping System: Due to their high corrosion resistance and durability, PVC gas pipes are widely used for underground exterior gas lines. PVC pipes are economic but need careful installation to avoid breakage.

HDPE Gas Pipes: HDPE pipes are also a good choice for underground exterior gas lines. HDPE gas pipes are flexible and relatively inexpensive.

Copper Gas Piping System: They find only limited use as some municipalities do not allow it. Copper gas pipes have strict code requirements and are usually designed for a 20 years life.

Aluminium-plastic Composite Pipe: They are a composite of polyethylene and aluminum. The inside is aluminum whereas the polymer is bonded outside using adhesives. Suitable for transporting both liquids and gases, this type of gas pipe is corrosion-resistant and handles relatively high temperatures.

Problems with Gas Piping

One of the main problems with gas piping is leakage which may cause poor air quality, fires, and even explosions. So, the gas piping system must be inspected and maintained regularly to check for potential problems.

Common signs of gas piping problems are:

  • Improper connections
  • Leaks
  • Rusting
  • Inadequate support
  • Missing shut-off valve
  • Hissing sounds
  • Plastic pipe exposed above grade
  • Issues with gas-powered appliances
  • Rotten egg smells
  • High energy bills
  • No drip leg
  • Inappropriate materials
  • Copper tubing is not properly labeled
  • Piping in chimneys or duct systems

So it is always suggested to be cautious and follow the following safety guides:

  • Turn off all gas mains before starting work.
  • Use the right kind of safety harness (gloves, a hard hat, boots, and reflective clothing) while working. If you detect a rotten egg smell (natural gas leaking), immediately get out of that place.
  • Barricade excavations while working underground gas pipe installation.
  • Use correct tools for gas pipe handling.
  • Ensure proper sign and warning conventions to inform others about the ongoing work.

Gas Piping Legislation

All gas piping must follow the local fuel gas legislation and the design of the gas piping system must be 100% compliant. All the guidelines must be followed which are usually developed based on various international codes like the building code, the plumbing code, the fire code, the property maintenance code, the residential code, the mechanical code, the fuel gas code, etc.

Gas Piping Size/ Natural Gas Pipe Sizing

Gas piping size or natural gas pipe sizing may vary depending on the fuel gas code of that locality. In general, gas pipe sizing depends on various parameters as listed below:

  • Total connected gas load
  • The longest length of the gas piping in the building.
  • Horizontal and vertical distances between the meter and the longest fuel-burning appliance.
  • The material of the gas pipe being proposed
  • The pressure inside the building.
  • The applicable Fuel Gas Code. There will be different tables in the fuel gas code depending on gas pipe materials.

Once these parameters are known, the Sizing of natural gas pipes is done following specific tables mentioned in the relevant fuel gas code. A typical natural gas pipe size chart can be accessed by clicking here.

What is a Refractory Lining? Materials, Selection, and Types of Refractory Lining

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics

A refractory lining is a safety protective layer in industrial furnaces, pipes, or equipment that provides high-temperature resistance and protects the structure from thermal shock, wear and erosion. The refractory lining protects the pipe or equipment material from direct exposure to heat from fire or fluids. They are very important components for boilers, furnaces, and certain pipes. The correct installation of refractory is essential to ensure the safe and efficient operation of furnaces and boilers. In this article, we will learn about the materials, selection, and procedure for the refractory lining. Let’s dive into the subject starting from its definition.

What is a Refractory Lining?

Refractory lining provides a protective layer inside the equipment as a form of insulation for elevated temperature services. The refractory lining withstands high temperatures, thermal shock, wear, and chemical corrosion. In general, the refractory lining for furnaces is made of refractory brick or amorphous refractory. Typical examples of refractory lining are fired heaters, blast furnaces, electric furnaces, boilers, drying equipment, high-temperature differential heat meter, muffle furnaces, etc.

Purpose of Refractory Lining

Refractory lining is widely used in very high-temperature services in order to

  • Serve as a thermal barrier between the pipe/equipment wall and hot medium.
  • Withstand physical stresses.
  • Protect against corrosion and erosion.
  • Provide thermal insulation

As a reason refractories are found in various useful applications. They are extensively used in furnaces, kilns, reactors, fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, utility boilers, catalytic cracking units, air heaters, sulfur furnaces, and various other vessels handling hot mediums such as metal and slag.

Refractory Lining Materials

The common materials that are used as refractory lining materials are:

  • Alumina or Aluminum oxide (High Alumina bricks)
  • Silicon oxide
  • Magnesium oxide
  • Calcium oxide
  • Fire clays (Clay bricks)
  • Zirconia
  • Silicon carbide
  • Tungsten carbide
  • Boron nitride
  • Hafnium carbide
  • Molybdenum disilicide
  • Tantalum hafnium carbide
  • Corundum bricks
  • Plastic refractory

Refractory lining material consists of refractory aggregate, admixture, powder, binder, water, or other liquid, made of amorphous refractory products or fixed refractory products.

Types of Refractory Lining Materials

Refractories can be classified based on various different parameters in multiple ways as listed below:

  • Based on Chemical composition:
    • Acidic refractories (Silica refractories, Zirconia refractories, Aluminosilicate refractories);
    • Basic refractories (Magnesite refractories, Dolomite refractories, Magnesia-chrome refractories);
    • Neutral refractories (Carbon graphite refractories, Alumina refractories, Chromite refractories)
  • Method of manufacture: Dry press process, Hand molded, Fused cast, Formed, Unformed.
  • Fusion temperature: Normal refractories, High refractories, Super refractories.
  • Refractoriness: Super duty, High duty, Intermediate duty, Low duty.
  • Thermal conductivity:
    • Heat-resistant (temperatures≤ 1100 °C),
    • Refractory (temperatures≤ 1400 °C),
    • High refractory (temperatures≤ 1700 °C),
    • Ultra-high refractory (temperatures ≤ 2000 °C).

Refractory Lining Thickness

Depending on the type of the furnace and the substance of the smelting, the thickness of the refractory lining is usually between 80mm-300mm.

In certain applications, there may be two layers of refractory lining of different materials. For example, the refractory lining of the converter has two layers. The outer layer consists of magnesium refractory of 50-100mm thickness and the inner layer is composed of magnesium brick with 300-500mm thickness.

Selecting Refractory Lining

Selecting a refractory lining is not easy and straightforward. The selection of refractory lining depends on various factors like:

Thermal Requirements: The refractory lining material chosen must meet the maximum operating and design temperatures that they will be subjected to. The material must have high thermal shock resistance, and be strong against thermal fatigue, excessive expansion, etc.

Life Span: During operation, the refractory material is bound to experience mechanical and thermal loads which may cause wear and failure. So, the selected material must be able to absorb dynamic loads, mechanical impacts, severe erosion and corrosion, tensile loads, large hydraulic loads, pinch spalling, etc to increase its lifespan.

Chemical Attack: There could be the possibility of a chemical reaction with the content at high temperatures. So, the refractory material selected should be inert against them.

Installation: The refractory lining material must be easily available and installed. The chosen refractory material must be quickly delivered to the required location during repair or maintenance times.

Cost: Finally, cost or economy governs all decisions. The chosen material must be economic for the range of services.

Refractory Lined Pipes

In petrochemical plants and refineries, while handling very high-temperature fluids the use of internally insulated pipe is desired due to practical and economic reasons. These internally lined/insulated pipes use the refractory material to resist high fluid temperatures and leave the pipe wall at a near-ambient temperature.

The internally refractory lined pipes are used in many instances. For example, some process fluids can simply be too hot for any type of available pipe material without internal protection. Even if the fluid temperature is within the applicable temperature range of the available material, it is very often too expensive to use when the temperature is at the higher end of the applicable range.

Again, some corrosive fluids also call for the use of internal insulation to protect the pipe. Besides the material concern, the other major reason to use internally refractory lines pipe is to reduce the amount of thermal expansion by reducing the pipe wall temperature. Smaller expansion eliminates the requirement of an expansion joint or expansion loop, which may not be easily accommodated due to either process or space limitations.

The use of refractory-lined pipes for very high temperatures can substantially reduce the metal temperature. The lining also resists erosion while at the same time permitting the use of normal carbon steel with a fluid that would otherwise be prohibitively hot for its use.

Stress Analysis of Refractory Lined Pipes

The main concern for using a refractory lined pipe is the simulation of the combined pipe and refractory stiffness. In contrast to external thermal insulation, whose stiffness is usually ignored, the stiffness of internal refractory can not be ignored during stress analysis of refractory-lined pipes.

The normal practice followed among organization for stress analysis of refractory lined pipes are to use an equivalent modulus of elasticity with the actual pipe wall thickness. This method avoids underestimating the stress due to the increase in section modulus by the increase in wall thickness.

The weight of the system is calculated based on the actual pipe material weight, fluid weight, and refractory weight. The stress analysis is performed based on the actual pipe cross-section and geometry, which is applied with the total weight and an equivalent modulus of elasticity.

The equivalent modulus of elasticity calculation assumes that the refractory material, just like the pipe material, has similar tensile and compressive properties. Even though this assumption is not correct because refractory material is stronger in compression than in tension. However, the analysis of piping generally deals with three-dimensional systems. It is impossible to foresee which part of the piping system will have tensile stress and which part will have compressive stress. The equal tensile and compressive characteristics assumption appears to be the only one that is practical.

References and Further Studies: Pipe Stress Analysis by L C Peng

What Are Refractory Metals? Properties & Applications of Refractory Metals

Written by Anup Kumar Dey in Engineering Materials

Refractory metals are a unique category of metallic materials known for their exceptional resistance to heat and wear, making them invaluable in various high-temperature applications. “Refractory Metals” in engineering refers to a group of materials having an extremely high melting point. They can retain their shape and usefulness at high temperatures and have a very high melting point. In general, refractory metals meet the following two basic criteria:

  • Their melting point is above 4000° F (2200° C) and
  • Their creep resistance is above 2700° F (1500° C)

In this article, we will explore the properties, types, and common applications of refractory metals.

What Are Refractory Metals?

Refractory metals are defined as metals that have exceptionally high melting points and retain their strength and stability at elevated temperatures. These metals are crucial in applications that require materials to withstand extreme conditions without deforming or losing integrity.

The study of refractory metals began in the early 20th century, primarily focused on tungsten and molybdenum, which were used in electrical contacts and filaments. Over time, the unique properties of these metals led to their adoption in a wide range of industries, particularly as technology advanced and the demand for materials that could endure harsh environments increased.

Refractory metals are a broad class of metals having excellent heat resistance, extreme wear resistance, and very high hardness at room temperature. There are 5 metals that are undisputedly considered refractory metals. They are:

  • Tungsten (W): Melting Point 3420°C, BCC
  • Rhenium (Re): Melting Point 3185°C, HCP
  • Tantalum (Ta): Melting Point 3017°C, BCC
  • Molybdenum (Mo): Melting Point 2623°C, BCC
  • Niobium (Nb): Melting Point 2477°C, BCC

There are some other metals that display similar properties and are broadly referred to as refractory metals. they are:

  • Osmium (Os): Melting Point 3027°C, HCP
  • Iridium (Ir): Melting Point 2447°C, FCC
  • Ruthenium (Ru): Melting Point 2250°C, HCP
  • Hafnium (Hf): Melting Point 2227°C, HCP
  • Technetium (Tc): Melting Point 2200°C, HCP (Radioactive)
  • Rhodium (Rh): Melting Point 1963°C, FCC
  • Vanadium (V): Melting Point 1902°C, BCC
  • Chromium (Cr): Melting Point 1857°C, BCC
  • Zirconium (Zr): Melting Point 1852°C, HCP
  • Titanium (Ti): Melting Point 1670°C, HCP

Properties of Refractory Metals

All refractory metals have some unique and desirable properties as listed below:

Physical Properties of Refractory Metals

Refractory metals are distinct for the following key physical properties:

  • Very high boiling point.
  • High creep resistance.
  • High density
  • Heat and electrical conductivity.
  • All refractory metals have a close-packed or nearly close-packed crystal structure.

Strength and Hardness

Refractory metals exhibit excellent mechanical properties, including high strength and hardness, even at elevated temperatures. This makes them suitable for use in high-stress applications, such as turbine components and cutting tools.

Oxidation Resistance

Many refractory metals possess good oxidation resistance, particularly at high temperatures. This characteristic is crucial for applications in oxidizing environments, such as aerospace and nuclear applications, where prolonged exposure to oxygen can lead to material degradation.

Thermal Conductivity

Refractory metals typically have high thermal conductivity, which allows for efficient heat dissipation. This property is particularly valuable in applications involving heat sinks and thermal barriers.

Chemical and Mechanical Properties of Refractory Metals

Some of the chemical and mechanical properties of refractory metals are:

  • They oxidize easily and create a stable oxide outer layer which gives them high corrosion resistance.
  • Thermal shock resistance.
  • Refractory metals are usually stable against acids.
  • Refractory metals have high strength and extreme hardness.
  • Outstanding Abrasion and Wear Resistance
  • Low diffusion rates

Processing of Refractory Metals

The processing of refractory metals presents unique challenges due to their hardness and melting points. Below are some common techniques employed to fabricate these metals.

Powder Metallurgy

Powder metallurgy involves the use of metal powders to create components through compaction and sintering. This method is particularly effective for refractory metals, as it allows for precise control over the microstructure and properties of the final product.

Arc Melting

Arc melting is a technique used to melt and refine refractory metals by creating an electric arc between two electrodes. This method is useful for producing high-purity alloys and is often employed in research and small-scale production.

Chemical Vapor Deposition

Chemical vapor deposition (CVD) is a process used to produce thin films of refractory metals on substrates. CVD is valuable for applications requiring precise coatings, such as semiconductor manufacturing.

Additive Manufacturing

Additive manufacturing, or 3D printing, is becoming increasingly important for refractory metals. This technique allows for the production of complex geometries and reduces material waste, making it an attractive option for many industries.

Applications of Refractory Metals

The most commonly used refractory metals are tantalum, niobium, molybdenum, and tungsten. They are combined with other materials to create an extensive range of refractory compounds. the major applications of refractory metals are listed below:

Electronics and Semiconductors Industry: Due to strong electrical conductivity, refractory metals are used to produce durable electrical components by alloying with copper, gold, or silver. They are widely used for producing cases, anodes, and cathodes.

Industrial Parts: Refractory metals are suitable for applications involving extreme thermal and mechanical stresses. Hence, they are widely used in glass melting electrodes, furnace boats, sintering trays, rods, sheets, crucibles, shields, tubes, and nozzles.

Medical Industry: Various medical devices are produced using refractory metals. For example, Tantalum is used in dental devices, surgical clips, bone grafts, Plates for cranioplasties, etc. Molybdenum is used in medical scanning tools, Tungsten is used for radiation shields, and tantalum and niobium are used in MRI scanners.

Nuclear Applications: Niobium, molybdenum, and tungsten are widely used in the nuclear industry. Tungsten is utilized in radiation shielding, whereas niobium zirconium alloy is used for nuclear reactor structural components.

Aerospace and Defence Applications: Due to very high thermal and mechanical durability, refractory metals find wide applications in the defense and aerospace industry. Molybdenum, tungsten, niobium, and tantalum have got a range of aerospace applications in forging dies, thrusters, shields, and balance weights.

Chemical Industry: Tantalum is widely used in heat exchangers, reaction vessels, and spargers to reject the corrosive effect of various acids and chemicals.

Superconductors: Tantalum and niobium are widely used in low-temperature superconductor applications, mass spectrometry, NRM, particle accelerators, MRI medical imaging, and various analytical and experimental equipment.

Powder Metallurgy: Refractory metals are highly popular in powder metallurgy. They are processed in specific powder sizes and forms and then blended to get the required mixture of properties.

Superalloys: Rhenium is an additive for producing superalloys. The creep strength of superalloys increases by a great extent with the addition of rhenium in conjunction with iron, cobalt, nickel, tungsten, and molybdenum which makes these alloys suitable for gas turbine and jet engine part manufacturing.

Tungsten is widely used in lighting applications, industrial ovens, and various types of probes.

In conclusion, Refractory metals are a vital component of modern technology, offering unparalleled properties that make them suitable for a wide range of high-temperature applications. From aerospace to electronics, their unique characteristics enable innovations that drive progress across various industries. Despite the challenges associated with their use, ongoing research and development promise to unlock new potential for these remarkable materials, ensuring their relevance in the future. As we move towards a more advanced and sustainable world, the importance of refractory metals will only continue to grow.