Underwater welding, as the name properly hints, is the welding inside the water. This is mainly used to repair offshore pipelines and structures, ships, submarines, and nuclear reactors. Joining two metal pieces underwater requires a lot of safety considerations. The purpose of this presentation is for general knowledge only on how “underwater welding” is carried out. Safety is also emphasized here as we are dealing with two types of activities, Diving, and Welding. One must remember that underwater welding is a different world, and so special precautions are adhered to for maximum safety of the welder/diver.
Introduction of Underwater Welding
Underwater welding began during World War 1 when the British Navy used it to make temporary repairs on ships. Those repairs consisted of welding around leaking rivets of ship hulls. Underwater welding was also restricted to salvage operations and emergency repair works only. In addition, it was limited to depths below the surface of not over 30 ft (10 meters).
At first, underwater welding was just applied to weld a patch until a more thorough repair could be performed. But as soon as more experience was gained, ambitious individuals and companies joined forces to improve results and to establish achievable specifications.
Use of Underwater Welding
While underwater welding has been used for new construction & installation of offshore structures, subsea pipelines, & harbor facilities, it is most often used for maintenance and repair applications. These include repair of damage caused by corrosion, fatigue, and accidents of offshore structures such as oil platforms, repair & replacement of damaged subsea pipeline sections, repairing holes in ship’s hulls, or collision damage to harbor facilities.
Judging from the photographs shown below (Fig. 1), it is obvious that these structures have to be repaired. And one of the tools of repair is underwater welding.
Fig. 1: Damaged Offshore Structure
The Welder
Welder – is a certified welder who is also a commercial diver, capable of performing tasks associated with commercial subsea work, weld setup and preparation, and who has the ability to weld in accordance with the AWS D3.6, Specification for Underwater Welding (wet or dry), and other related activities.
Welder qualifications required for a given assignment vary from project to project. Most diving contractors would like their welder/diver to be “a jack of all trades”. This means that the welder/diver must know how to do underwater cutting, fitting and rigging, inspection and nondestructive testing, and underwater photography.
Classification of Underwater Welding
Underwater welding is classified into two categories.
Welding in the wet environment = this was used primarily for emergency repairs and salvage operations in shallow waters due to poor quality welds.
Welding in-the-dry environment = this technique produces high-quality welds that meet X-ray and code requirements.
Welding in-the-wet environment
As the name implies, underwater wet welding is done in an environment where the base metal and the arc are surrounded entirely by water. The electrode types used conform to AWS E6013 classification. These electrodes are waterproofed by wrapping them with waterproof tape or by dipping it into special sodium silicate mixes and allowing them to dry. The power source is a direct current machine rated at 300 or 400 amperes.
The power of the arc generates a bubble of a mixture of gasses that lets metal melting and joining occur more or less normal as shown in Fig. 2.
Fig. 2: Welding in the wet and dry environment
Welding in-the-dry environment
Welding in-the-dry environment is again divided into two:
Hyperbaric welding, in which a chamber is sealed around the structure to be welded and is filled with breathable gas at the prevailing pressure.
Cofferdam welding, which is carried out in the dry, air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded and is open to the atmosphere.
Hyperbaric welding is done with the use of a welding chamber or habitat. This method provides high-quality weld joints that meet radiography and code requirements. The chamber is sealed into a structure or pipeline and filled with a breathable mixture of helium and oxygen (90-95% helium and 5-10% oxygen).
Cofferdam welding is also a type of dry welding where a rigid steel structure to house the welders is sealed against the side of the structure to be welded and is open to the atmosphere. It is normally used for harbor works or ship repair
Photographs in Fig. 3 are examples of Cofferdam Welding
Fig. 3: Cofferdam Welding
Principle of Operation
The figure (Fig. 4) below right shows the general arrangement for underwater welding. Underwater welding should always be a direct current machine grounded to the ship. The welding circuit includes a knife switch that is operated on the surface by an assistant upon the signal of the welder/diver. The knife switch cuts off the welding current and is designed this way for safety reasons. The electrode holder utilizes a twist-type head for gripping the electrode. The work lead is attached within 3 ft. from the point of welding and is perfectly insulated to avoid leaks. The welding circuit should be direct current electrode negative.
Fig. 4: General Arrangement for underwater welding
Underwater welding is covered under the following codes and standards:
AWS D3.6- Specification for Underwater Welding.
ASME N-516- Underwater Welding Section XI Div. 1
BS EN ISO 15618-1:2002- Qualification Testing of welders for underwater welding.
AWS D3.6M 1999 Specification for Underwater Welding
This specification covers the requirements for welding structures or components under the surface of the water. It includes welding in both dry and wet environments. Sections 1 through 6 constitute the general requirements for underwater welding while sections 7 through 10 contain the special requirements applicable to four individual classes of welding:
Class A – Comparable to above-water welding.
Class B – For less critical applications.
Class C – Where load-bearing is not a primary consideration.
Class O – To meet the requirements of another designated code or
Case N-516-3 Underwater Welding ASME Section XI
Scope and General Requirement –
Requirements for wet and dry underwater welding.
Additional variables for dry underwater Welding – Procedure and Performance Qualification.
Additional variables for wet underwater Welding – Procedure and Performance Qualification.
Filler metal qualification – Each filler metal is tested in accordance with applicable SFA specifications.
Alternative procedure qualification requirements – By Charpy V-notch.
Examination – By NDE.
Underwater Inspection
The underwater inspection also includes a visual and photographic examination of underwater structures and repairs, and a Non-Destructive Examination such as the Magnetic Test, Ultrasonic Test, and Radiographic Test.
Fig. 5: Various Inspection and Test methodologies
Non-destructive Testing like UT, RT, and MT can also be done underwater.
The photograph in figure 5 (Fig. 5) shows an underwater NDT technician using Magnetic Particle Testing on underwater structural supports.
Books are also available in the market for reference like;
Non-destructive Examination of Underwater Welded Steel Structures.
Underwater Wet Welding and Cutting.
Underwater Repair Technology.
Professional Diver’s Manual on wet Welding.
Visual, video, and photographic examination can also be carried out during maintenance inspection on any underwater structures as shown below
In order to carry out a proper visual and NDT check, blast cleaning has to be carried out to remove all seawater organisms that grow on the underwater structure as shown in the photograph in Fig. 5.
The latest development in underwater inspection is the use of ROV. These are (Fig. 6) machines operated by an ROV pilot.
Fig. 6: Latest development in the underwater inspection.
Risk Involved in underwater welding & inspection
Below are some risks involved in underwater welding:
Electric shock – there is a possibility of electric shock when the equipment is not properly insulated, or when the power supply is not shut off immediately when the welder terminates the arc during welding.
Explosion–arc welding produces hydrogen and oxygen. Pockets of gasses can build up and are potentially explosive.
Nitrogen Narcosis – a health hazard normally experienced by divers during diving activities when safety stops at a certain level is not adhered to. Curiously, the risk of drowning is not listed with the hazards of underwater welding.
For welded structures, an inspection of welds after welding may be more difficult than welds made above water. There is a risk of defects that may remain undetected and may cause failure in the long run.
Safety Standards for underwater welding & inspection
The following safety standard governs the activities for underwater welding and inspection
OSHA Standard 1915.6 – Commercial Diving
OSHA Standard 1910.424 – SCUBA Diving
Volume IV, Issue 3, 3rd Quarter 2002
Occupational Health Newsletter – Commercial
Safety during Welding
Necessary precautions should be carried out such as:
Follow employers’ safety practices.
Fumes and gasses can be hazardous to your health.
Arc rays can injure eyes and skin.
Use adequate ventilation while welding.
Wear suitable eye protection and protective clothing.
Do not touch live electrical parts.
Wear rubber gloves.
Only change the electrode when cold.
Latest Developments
With the latest development in the construction of offshore oil platforms, there has been an increased demand for underwater welding. The use of hyperbaric chambers to produce code-quality welds is very expensive to operate.
Sea Grant Researcher Dr. Chon Tsai has developed a new welding electrode for wet welding nicknamed “Black Beauty” for the black appearance of its waterproof coating. The electrode exhibits excellent visual appearance and profile, micro-cracking of weld has been eliminated, operating characteristics are superior to other commercially available electrodes, and the electrode produces suitable results when used in any position.
Wet-Dry welding
Dry hyperbaric chambers or habitats are extremely expensive. This is because it must be built for special applications such as repairing or making tie-ins on horizontally laid pipes. Recent improvements allowed GMAW (Gas Metal Arc Welding) process to be used in underwater welding with the use of special nozzles, domes, or miniature chambers. In using this type of apparatus the welder/diver is in the water but the nozzle of the welding gun and material to be welded is in the dry atmosphere. These localized dry gas environment chambers are inexpensive, small, and lightweight. It is made of transparent material or has a sufficient number of windows so that the welder can see the inside to properly manipulate and direct the welding gun. This process can be utilized for welding up to 125 ft. (35m) below the water surface.
Fired Heaters, as the name specifies, are obviously heaters or furnaces. They are pieces of equipment used in processing facilities (refineries, power plants, petrochemical complexes, etc.) to heat fluids up to the desired temperature. So, the main purpose of fired heaters is to raise the temperature of the process fluid that flows through the tubes. The heat energy is supplied by combustion fuels. These fall in the static or stationary group of mechanical equipment and are designed based on the API 560 standard. Today we will study the details of Fired Heaters, their components, types, construction features, and maintenance requirements. Let’s dive into the article!
What is a Fired Heater?
A fired heater is a type of industrial equipment designed to generate heat through the combustion of fuel. The heat produced is transferred to a process fluid or other medium, which is then used in various industrial processes. Fired heaters are critical in applications where precise and controlled heating is required, such as in chemical processing, oil refining, and power generation.
Where are Fired Heaters used?
Fired heaters find wide applications throughout chemical industries like refineries, petrochemical and chemical industries, gas processing units, ammonia plants, olefin plants, fertilizer plants, etc. They are termed Feed Preheaters, Cracking Furnaces, Fractionators heaters, Steam reforming heaters, Crude Heaters, etc.
How does a Fired Heater work?
A fired heater works by direct heat transfer from the product of the combustion of fuels. The maximum flame temperature of hydrocarbons burned with stoichiometric air is about 3500 °F (1926 °C). This heat energy is released by combusting fuels into an open space and transferred to the fluids inside tubes, which are ranges along the walls and roofs of the combustion chamber.
What are the different modes of heat transfer in a Fired Heater or Furnace?
There are different modes of heat transfer that occur in fire heaters. The heat is transferred by direct radiation, convection, and also by reflection from refractory walls lining the chamber. These zones are identified in a typical heater such as that of Fig. 1. In the radiant zone, heat is transferred predominantly (about 90%) by radiation. The convection zone is “out of sight’’ of the burners; although some heat transfer occurs by radiation because the temperature is still high enough, most of the transfer here is by convection mode. The shield section is the name given to the first two rows leading into the convection section.
Fig. 1: Radiant, shield & convection section of a fired heater
Components of a Fired Heater
A fired heater consists of:
Casing
Tubes
Return bends
Tube supports
Burners
APH/SAPH
ID & FD fans
Pilot
Radiant, Shield, and Convection zone
Duct
Damper
Stack
Refractory
Louvers /Air registers
Casing
The casing of a fired heater acts as the external protective shell, encasing and shielding the internal mechanisms. Its main role is to offer structural support and insulation, thereby minimizing heat loss to the external environment. This outer layer is crucial for maintaining the heater’s durability and effectiveness by retaining the generated heat within the unit.
Tubes
In a fired heater, tubes serve as hollow channels through which hot gases flow, transferring heat to the process fluid. These metal pipes are vital for the thermal exchange, as they enable the combustion gases to deliver their heat to the material being processed, thus enhancing the overall system efficiency.
Return Bends
Return bends are U-shaped tubes that redirect hot gases back through the fired heater. This configuration improves heat transfer efficiency by making sure the combustion gases pass through the system multiple times. By optimizing the path of the gases, return bends help to maximize the heat utilization from the combustion process.
Tube Supports
Tube supports are structural components that stabilize and secure the tubes within the fired heater. They ensure correct spacing and alignment, preventing issues such as tube sagging or damage. Proper support of the tubes is essential for the heater’s longevity and consistent performance.
Burners
Burners are devices within the fired heater that mix fuel and air to create a flame for heat generation. They are crucial for starting and maintaining the combustion process, which directly affects the heater’s efficiency and performance.
APH/SAPH (Air Preheater/Steam Air Preheater)
Air Preheaters (APH) and Steam Air Preheaters (SAPH) are heat exchangers designed to preheat combustion air using residual heat from flue gases. This process boosts overall efficiency by recovering heat that would otherwise be wasted, improving the heater’s energy performance.
ID & FD Fans (Induced Draft & Forced Draft Fans)
Induced Draft (ID) and Forced Draft (FD) fans manage the movement of air and gases within the fired heater. These fans are essential for balancing air and fuel supply, ensuring effective combustion, and contributing to the heating system’s overall efficiency.
Pilot
The pilot flame is a small flame that ignites the main burners when the fired heater starts up. It plays a key role in initiating the combustion process in the main burners, ensuring a stable and controlled ignition.
Radiant, Shield, and Convection Zones
The fired heater is divided into different zones: radiant, shield, and convection. Radiant zones absorb and emit heat, shield zones protect sensitive parts, and convection zones enhance heat transfer efficiency, each contributing uniquely to the heat transfer process.
Duct
Ducts are channels that direct combustion gases from the burners to the heat exchanger. They are essential for controlling the flow of hot gases within the system, ensuring efficient and regulated movement throughout the heater.
Damper
Dampers are adjustable plates or valves that control the airflow within the fired heater. By regulating the airflow, dampers help manage the intensity of combustion, enabling operators to maintain optimal operating conditions for efficient and controlled combustion.
Stack
The stack is a vertical structure that allows the combustion gases to exit into the atmosphere. It is designed to safely release exhaust gases, minimizing environmental and safety risks associated with gas emissions.
Refractory
Refractory materials line the inside of the fired heater to provide heat resistance and protect the structural components. This lining guards against high temperatures and enhances insulation, which is vital for the heater’s durability and performance.
Louvers/Air Registers
Louvers or air registers are adjustable openings that control the intake of air into the fired heater. By managing the air supply for combustion, these components ensure efficient heat transfer and contribute to the overall effectiveness of the heating system.
What is a Pilot Burner?
A pilot burner is a small light that has a small flame of natural gas or LPG which acts as an ignition source of the main burner. So the pilot burner always keeps alight for uninterrupted heater operation. The pilot burner should have a minimum heat release of about 10,000 kcal/hr. The length of the flame of a pilot should be a minimum of 150 mm & stable.
What is a Burner?
A burner is a device that introduces fuel & air into the firebox at the desired ratio & velocity & concentrations to maintain proper combustion. It is classified by the type of fuel combusted. It is normally designed to provide 120% of its normal heat liberation at peak duty.
What is a Damper?
The damper is a device for introducing a variable resistance for controlling the flow of flue gas or air. The role of the stack damper is very significant in the operation of fired heaters for draft control, but unfortunately, little attention is paid to the design of the damper. Most of the dampers are left open in the fire heater; very few of them work properly. But proper designing of dampers can save energy. The damper needs to close to reduce oxygen in the fuel gas, increase firebox temperature, reduce stack temperature, & reduce draft at the radiant section.
What is a stack?
Stack is the vertical pipe through which combusted gas or flue gas is vented out into the atmosphere. It is often called a chimney. It helps ventilation as well as air ingression to the fire heater based on buoyancy which is generated due to density difference. We all know air density depends on air temperature. The velocity of flue gas through the stack is maintained between 25 to 40 ft/sec. Stacks are mostly made of steel plates of minimum 6 mm thickness and lined with 50 mm insulating castable. At the top of the stack, absolute pressure should be 2.5 mm WC below the atmosphere to keep the heater at the negative draft.
As the high temperature is generated inside the heater, it is necessary to prevent the environment from exposure to high temperatures. For this purpose, refractories are used, which is a material resistant to decomposition by high heat. Radiant section linings are exposed to firebox temperatures of more than 1000°C & therefore require high-quality insulating refractory materials to tolerate high temperatures. Convection sections are lined with a castable blanket. Heat losses are kept between 1.5%-3%.
Types of Fired Heaters with Different Coil Arrangements
Depending on the arrangement of tube banks and combustion chambers there are several types of fired heaters that are used in industries. Some of the common types of fired heaters are
Type A-Box heater with arbor coil
Type D-Box heater with vertical tube coil
Type E-Cylindrical heater with vertical coil
Type F-Box heater with horizontal tube coil
Fig. 2: Types of Fired Heaters
The disadvantage of vertical types of radiant tubes is their difficulty in replacement due to the smaller gap between the wall and the tube.
Horizontal-type radiant tubes are weldable outside the heater firebox due to more space available in return header bends and plugs.
Constructional Features of Fired Heaters
Fired Heater Casing:
The metal plate is used to enclose the fired heater. Normally, CS plates 6 mm thick are used as casing material. Casing design temperature Outside 82 deg. C, Radiant floor – 91 deg. C. Max. temperature, CS can withstand 440 degrees C, however, oxidation starts at 270 deg. C.
Failure of internal refractory lining causes overheating of underlying steel casing. This will be revealed by local hot spots.
To prevent further damage to the casing plate
Apply air
Apply steam
In extreme cases put water
Put an additional refractory lined casing plate over the hot spot area.
Fig. 3: Fired Heaters in Refineries
Radiant section of a fired heater:
The portion of the heater in which the heat is transferred to the tubes primarily by radiation is known as the radiant section.
Convection section:
The portion of the heater in which the heat is transferred to tubes primarily by convection.
Bridge wall:
The section separates the radiant & convection sections. The temperature of flue gas leaving the radiation section is called the bridge wall
Arch:
A flat or sloped portion of the heater radiant section opposite the floor.
Radiant Coils:
The radiant coils are located in the radiant section of the furnace where the heat picks from flame & high-temperature flue gas & hot refractory.
The radiant tubes may be either vertical or horizontal, depending on the construction of the furnace
Main Sections of a Fired Heater
Convection section:
Bank of coils which receive the heat from hot flue gases mainly by convection.
Finned/studded tubes are often used in convection coils due to lower flue gas temperatures. Finned tubes ( 1.3 mm thick strip 200turn/meter) are difficult to clean when compared to studded (12.7 mm dia)
The rate of heat absorption tends to be high at the entrance to the convection section in heaters, where the convection section is right above the radiant section. Tubes in this section are called shock/shield tubes. Normally, the first two rows absorb half of their heat in this section.
Consists of a large tube support plate located in the convection section and supports. The material of end supports & intermediate supports is usually low-alloy steel.
Replacing /Repairing of Convection tube support sheet is difficult & calls for the removal of all convection coils or it is necessary to lower the entire module.
Tube support sheets are 25 cr-20 Ni or 50 cr -50 ni MOC.
Plug header:
A bend, provided with one or more openings for the purpose of inspection, initial measurement of coke before cleaning.
Ensure proper depressurization before opening the plug.
Ensure the Arrow mark is maintained on the plug to ensure the plug nut is guiding
After the repair /replacement of the plug hydro test of the coil pass is recommended.
Fired Heater Internal Tube supports:
Tube supports are metal devices that support the weight of the
The tube guide is used to direct the movement of tubes in one particular direction.
These are metallic members able to withstand high temperatures used to prevent the sagging/bowing/buckling/ swaying of tubes
Tube supports are more prone to high-temperature oxidation and fuel ash corrosion.
Horizontal roof tubes of box-type heaters are supported by means of hangers
Tube supports must be aligned perfectly in one straight line.
The use of fillers of any kind is prohibited.
Ensure perfect contact between supports and tubes.
Coils shall rest uniformly all over the supports.
Failure of tube supports may take place due to mechanical overloading caused by the bowing of tubes, loss of strength of supports, and tube vibration.
The tube support/hangers/guides shall be examined for cracks, oxidation, missing sections, and missing/broken or oxidized bolts.
Fig. 4: Figure showing typical tube supports
Common problems associated with Fired Heater tubes
These are some common problems associated with fired heater tubes:
Tubes are designed for approx 1 year life-1,00,000 hrs.
Tube distortion – Hot spots, Sagging, Bowing, Touching of tubes.
Tube surface – Pitting, Scale, Evidence of overheating.
Observe & monitor the skin temperature, and compare the residual life of the tube.
Fig. 5: Heater Tubes in an operating Fired Heater
Fired Heater Tube cleaning
Generally, tubes are cleaned manually making scaffolding inside the heater.
Ensure All burner tips are covered while cleaning.
Ensure Fire bricks are covered to avoid ingress of foreign particles between the bricks to provide expansion of refractory during operation.
Ensure no damage to refractory while making scaffolding.
Hydro-testing of the fired heater coils
The hydro test is performed when the new coil is installed/repaired in the coil is
Coils shall be hydrostatically tested, thoroughly drained after the test is completed and to be drained by blowing compressed air to avoid hammering &
During the hydro test due to return bends & elevation differences adequate care is to be taken to vent air.
Stack
Cylindrical steel is an insulated shell that carries flue gases to the atmosphere & provides the necessary draft. The stacks shall be externally inspected for hot spots and external corrosion. Check, if any unusual vibration of the stack exists.
Burners of a fired heater
Burner: Introduces fuel & air into the heater at the desired velocities, turbulence, and concentration to establish and maintain proper ignition and combustion.
Pilot: A smaller burner that provides ignition energy to light the main burner.
Plenum or wind box: A chamber surrounding the burners that are used to distribute air to the burners or reduce the combustion noise.
What is a fired heater Draft?
A draft is the pressure differential that persists between air/fuel gas in the combustion chamber and atmospheric air. The draft is caused due to density difference between hot fuel gas and ambient air.
A negative draft must be maintained in every part of the fire heater so that hot fuel gas cannot be leaked out. Draft reading in the middle of the furnace is used to control the draft & excess air. A heater draft is required to pull out fuel gases from the heater.
How draft is generated?
The draft can be created by the following means,
Full open the damper and close the louvers.
Open purging/snuffing of steam
Cut off the steam flow.
A close damper as per draft requirement.
Draft Profile across the fired heater furnace
Fig. 6: Typical draft profile across the fired heater
In the above image,
(SE)r is the Stack effect in the radiant section
(SE)c is the stack effect in the convection section &
(SE)s is the stack effect in the stack
Draft & Excess air Control Scheme:
Draft and air are closely linked together & they should act together. The main objective should be achieving the optimum air level for the complete combustion of fuel.
Fig. 7: Draft and Air Control Scheme
Natural Draft fired heater:
In this type of heater fuel gas or air is injected into the heater by using atmospheric pressure & the combusted gas is vented out through the stack. No external means are used. This is happened due to density differences as hot gases are having a lower density than the normal atmospheric air.
Force Draft fire heater:
In this type of heater fuel gas or air is pushed into the heater by means of an external means like a fan. It is often called an FD fan, it provides air or fuel gas. The FD fan is installed before the furnace.
Induced Draft fire heater:
In this type, the fired-heater fan is installed above the heater so that it can induce air through the combustion chamber into the burner. This fan causes a negative draft which pushes the burnt air out through the ventilation system.
Advantages of using Force draft:
The forced draft system requires a lower level of excess oxygen. The flame becomes stable & small size of the burner is required. FD fan maintains an optimum ratio of air to fuel gas.
What is Bridge Wall Temperature?
It is the temperature of the flue gas which is generated due to the combustion of fuel gas at the radiant section and entered into the convection section. The rate of heat transfer at the convection section is governed by the bridge wall temperature. It should be in the range of 760-900ᵒC.
Why snuffing steam is used in fire heaters?
The main purpose of using snuffing steam is to snuff unwanted fire (that can cause due to tube leakage) by excluding air ingress or prevent potential fuel from air exposure as well as it carries away heat to some extent. The amount of snuffing can be based on the requirement of 8 lb/hr per cubic foot of furnace volume. Normally LP steam is used for this purpose. During the start-up of the heater operation snuffing steam is also used to remove combustible gas & excess air as well as create a negative draft.
What is Puffing?
It actually indicates a huge vibration of furnaces. If a burner is seriously out of fire, opening air control without reducing the firing rate can cause a hazardous situation called puffing. To prevent such a scenario first slow down the firing & then adjust the air louvers.
Start-up of fired heaters
Make sure all the utilities are supplied as per requirement.
Ensure every instrument & safety device are in operation.
Ensure the fuel for the burner with sufficient operating pressure.
Purge combustible gas inside the furnace by snuffing steam to cause a negative draft of -5 to -15 mm H2O in the radiant section. This is done by fully closing the louvers & opening the stack damper completely.
Ignite the pilot burner and then the main burner.
Check the concentration of O2 in flue gas and heater draft.
The ramp of raising process fluid temperature at 30-50 C/hr to prevent overfiring.
Once the furnace has been brought up to a steady state, then switch the control mode from Manual to Auto mode.
The control scheme of the fired heater
The following image shows a typical control scheme for fired heaters.
Fig. 8: Typical Fired Heater Control Scheme
Heater Dry-Out
It is a very important operation of a fired heater prior start-up of a fired heater from a long shutdown or the start-up of a new fire heater. Heater dry-out is usually done to remove moisture contained in the refractories as refractories contain a large amount of moisture absorbed from the atmosphere. Ramp up of temperature is very crucial as a fast temperature increase may damage the refractory lining & surface shrinkage. Refer to the following figure that provides a Heating curve for heater dry-out.
Fig. 9: Heating curve for heater dry out
Annual Maintenance
Tubes visual inspection prior to cleaning
Inspection after cleaning
Dimensional check-up (OD of a tube), thickness.
Visual inspection of header plug leaks
Inspection – tubes supports, hangers, etc.
Inspection burner assemblies
Inspection of refractory
Inspection of explosion doors
Dampers external, internal, operating linkages, etc.
Due to exposure to high temperatures inside the fire heater coke is deposited in the tube which may lead to a reduction of heat transfer & the tube can be choked. So decking is a necessary operation that is performed by using the variable size of pigs, chemical & combustion methods. Mostly pig decoking is preferred over another. A pig has a uniformly studded pin around its surface which helps removals of carbon depositions inside the tube walls.
A new method of decoking the tubes is to steam, and then use water pressure to push Styrofoam pigs with studs and grit on the exterior through the tubes and around u-bends (even u-bends with clean-out plugs). The pigs scrape out the coke without scratching the tube walls.
The improper size of the pig may leave scratches on the tube walls, hence a selection of the correct size of the pig is
Pigging is faster than steam-air decking, and refiners generally have longer campaigns on the heater compared to steam air decoking.
Pigging will not provide temperature shocks & hence pigging has been found effective.
PIGGING – Double Pumping Unit
Set up:
The connection is made to a pair of passes (coils) with flow/return piping. There are four separate piping links with the furnace & pumping unit.
Launchers/receiver units complete with full port ball valves to be connected to Coils horizontally.
Ensure safe access to pig launchers/receivers.
Launchers/receivers are provided with hammerlock couplings to connect flexible piping.
Fig. 10: A typical figure showing the Pigging method
Cleaning of Pigging:
These are some procedures for the cleaning of Pigging:
Water fill-up.
Water circulation for removing hydrocarbons and loose debris.
Special density foam pig launch
Decoke pig selection to clean
Increase pig size incrementally.
Polishing by using oversize abrasive-coated foam pig.
Air-to-Fuel Ratio
It is an important factor to maintain in fire heater operation. Basically, it is the mass ratio of air to fuel present in the combustion process. For controlling air pollution to meet the regulatory norms it is an important parameter to measure & maintain. Under ideal conditions, fuel mixes with air to perform complete combustion. At the end of the combustion no excess oxygen & unburned fuels are left in the combustion chamber, it is called stoichiometric combustion. But in the real scenario, some amount of excess air should be present to ensure complete combustion of the fuel. Otherwise, significant amounts of CO are produced, reducing efficiency & increasing pollution levels.
Effects of excess fuels result in loss of fuel, CO production & caused heavy smoke while effects of excess air result in a reduction of temperature & excessive heat losses.
Troubleshooting of Fired Heaters
Problem
Reason
Recommendation
High flue gas temperature
Fouling in convection section Burnt off fire Over-firing
Piping Strainers (or filters) arrest debris such as scale, rust, jointing compound, and weld metal in pipelines, protecting equipment and processes. A strainer is a device that provides a means of mechanically removing solids from a flowing fluid or gas in a pipeline by utilizing a perforated or mesh straining element. Pipe Strainers are very important components in piping systems to protect costly equipment from potential damage caused by foreign particles carried by the process fluid. Piping Strainers are also known as Strainer Filters.
The following figure shows various types of pipe strainers normally used in Pump or Compressor Suction lines in the process piping industry.
Various types of Pump Strainers
Fig.1: Example of a typical Strainer
Application of Piping Strainers
To ensure against the untimely shutdown of equipment, strainers should be installed ahead of pumps, loading valves, control valves, meters, steam traps, turbines, compressors, solenoid valves, nozzles, pressure regulators, burners, unit heaters, and other sensitive equipment. The most common range of strainer particle retention is 1 inch to 40 microns (0.00156 inches ).
Strainers in Sensitive Static Equipment
Even though static equipment is normally not considered as that sensitive, still sometimes strainers are installed near the following equipment.
For the following sensitive and vibration-prone equipment use of a strainer is a must.
Pumps
Compressors
Turbines
Types of Piping Strainers/Strainer Types
Depending on the use, two types of strainers are found in industries.
Permanent Strainers and
Temporary Strainers
Permanent Pipe Strainers
These strainers will be installed permanently in the piping system. Examples of permanent strainers are
Y type strainers(Fig. 2)
Basket Type strainers( Simplex & Duplex construction) (Fig. 3) and
Y-Type Strainers
This type of strainer got its name from the shape as it resembles the alphabet “Y”. They are low-cost strainers and are used in pressurized lines with low debris or foreign particle concentration. They can be installed in horizontal or vertical lines keeping the filtering element towards the ground. As the retaining capacity of Y-Strainers (Fig. 2) are normally small, they must be cleaned frequently.
Fig. 2: Figure showing an example of a typical Y-type Strainer
Basket Strainer / Bucket Strainers
Basket Strainers or Bucket Strainers are closed vessels with a filter screen inside them. They have a high capacity to retain foreign particles and are hence widely used. Basket strainers (Fig. 3) are used only in horizontal lines; mostly for liquid services with high flow capacity. Bucket strainers can be independently supported like equipment in case their weight is more, or they can be supported inline from pipe supports. As they resemble the alphabet “T” of the English language, they are often termed as T-Strainers.
Basket Strainers are of three types
Simplex Style Basket Strainer (Fig. 3) and
Duplex Style Basket Strainer (Consists of two parallel basket filters with by-pass Valves as shown in Fig. 3)
Automatic Strainers
Fig. 3: Typical Basket type filters
Basket filters can be easily cleaned by opening the top cover. Duplex Basket strainers are cleaned online when the pipeline is in operation simply by diverting the flow to the other filter.
Automatic strainers have self-cleaning baskets that are controlled by using pressure drop settings or times; Hence, the cleaning operation is never interrupted.
Temporary Pipe Strainers
Temporary strainers are used for a small period of time. Examples of temporary strainers are
Cone-type strainer and
Truncated Cone type strainer
Refer to Fig. 4 which shows typical cone and truncated cone-type temporary strainers.
Fig. 4: Typical Temporary Strainers
Design standards for Piping Strainers
Strainers or filters are normally designed following the below-mentioned International Standards:
Perforated screens or strainers are formed by punching a large number of holes in a flat sheet of the required material using multiple punches. These are relatively coarse screens and hole sizes typically range from 0.8 mm to 3.2 mm
Mesh screens
Fine wire is formed into a grid or mesh arrangement. This is then commonly layered over a perforated screen, which acts as a support cage for the mesh.
Mesh Screen terminology: e.g. 3 mesh screen
We shall always ask the process to give Maximum allowable pressure drop at % clogged condition.
Mesh screens are usually specified in terms of ‘mesh’; which represents the number of openings per linear inch of screen, measured from the center line of the wire.
Fig. 7: Example of Mesh Size
Mesh is not the only thing to be asked for but hole size is also important.
The corresponding hole size in the mesh screen is determined from knowledge of the wire diameter and the mesh size
Selecting Mesh Size for Piping Strainers:
While selecting the proper mesh size the following factors must be considered.
the maximum particle size that the downstream equipment can handle safely.
the working temperature and pressure ranges.
the maximum allowed pressure drop.
the fluid service or nature of the conveyed fluid.
Strainer options
Nowadays various strainer options are available to the user like
Magnetic inserts
Self-cleaning strainers
Mechanical-type self-cleaning strainers
Backwashing type strainers
Temporary strainers
Y type Pipe strainer on various fluid
Fig. 8: Y-type strainer on various fluid
Selection of Pipe Strainers
The success of a specific type of pipe strainer solely depends on the proper selection of the piping strainer. The main parameters that affect the piping strainer selection process are:
Flow Rate: A flow rate in excess of 150 GPM requires a basket strainer.
The dirtiness of the flowing fluid: The basket strainer has more dirt-holding capacity as compared to the Y-strainers.
Application Requirement:
A y-type pipe strainer is suitable for applications requiring frequent cleaning.
For continuous operation, the ideal choice will be a duplex basket-type pipe strainer.
Strainer Orientation: For vertical orientation, Y-type is the only option.
Pressure Loss: Basket strainers exhibit less pressure loss as compared to Y-strainers. That’s why when there is doubt a basket strainer can easily be installed. It will cost more but serve all the purposes.
Note that, purchasing a spare pipe strainer is always a good engineering practice to avoid unnecessary delays in cleaning and installation for line operation.
Pipe Strainer Dimensions
Dimensions of pipe strainers vary with respect to flange rating, end connection, and pipe strainer types. For flanged piping strainers, with an increase in flange rating the dimension and weight increase. Normally, the pipe strainer dimensions are vendor specific. This is the reason during the initial stages of piping design the length of piping strainers is kept as hold in piping isometrics. Later, when the specific vendor data is available the same is updated in piping isometrics and adjustments are made in piping length. The following table provides some typical pipe strainer dimensions and weights for Y-type and Basket-type strainers as samples. However, Final vendor details need to be verified.
Pipe Size (Inch)
Pressure rating
Y Type Strainer Length- Flange face to Flange Face (mm)
Y Type Pipe Strainer Weight (Kg)
Basket Strainer Length- Flange face to Flange Face (mm)
Basket Pipe Strainer Weight (Kg)
1/2″
150
170
3
3/4″
150
170
3.5
224
7
1″
150
185
5.5
225
13.5
1-1/2″
150
270
10
240
19
2″
150
275
16
315
27.5
3″
150
360
30
365
46.5
4″
150
435
50
445
70
6″
150
560
92
600
167
8″
150
655
180
715
310
10″
150
880
340
855
550
12″
150
970
385
1015
600
1/2″
300
175
3.5
–
–
3/4″
300
175
4.5
224
7.5
1″
300
195
6.5
232
14.5
1-1/2″
300
275
11.5
245
21.5
2″
300
315
18
321
29.5
3″
300
405
35.5
375
49
4″
300
490
65.5
455
92
6″
300
582
125
620
225
8″
300
730
233
740
405
10″
300
910
420
890
655
12″
300
1070
445
1050
700
Table 1: Pipe Strainer dimensions
Comparison between Filter and Strainer
Nowadays, the terms filter and strainer are used interchangeably in industries. However, there are differences between the two. The main differences between a Filter and a Strainer are tabulated below
Filter
Strainer
A device that eliminates unwanted particles from the fluid is known as a filter.
Strainer also serves the same function.
The filtering medium in a filter is normally disposable.
Strainer uses a Reusable filter that is used again after cleaning.
The Filters normally filter out smaller particles (smaller than 40 microns).
Strainers remove comparatively larger particles ( larger than 40 microns)
Normally filters remove elements invisible to the naked eye.
Dirt removed by strainers is visible.
Filters remove particles by obstruction as well as chemical action.
Strainers remove particles by construction only.
Filters use soft media on hard surfaces to remove contaminants.
Strainers use hard mesh or rigid materials to remove debris.
Providing the required flexibility for absorbing the thermal movements of the piping system is one of the important activities in piping design. A rigid piping system experiences stress during operation at high temperatures. Providing sufficient flexibility in routing will ensure stresses in the piping system is within acceptable limits. So it is always suggested to be focused on providing flexible routing for large bore and temperature-critical piping. Flexibility in piping connected to strain-sensitive equipment like pumps, compressors, columns, turbines, plate heat exchangers, etc. is a must.
What it means!!!
The piping designer should be aware of the stress and support concepts in piping layout which are the guiding principles behind flexible pipe routing.
The piping designer should be aware of what pipe expansion means, how rigid piping induces stress and how to provide minimum expansion length using guided cantilever tables.
The stress engineer is always there to review flexibility but the piping designer uses his best judgment and coordinates with the stress engineer as needed to design a flexible and stable pipe route upfront.
How does it help?
A basic stress concept will help the piping designer in many ways, like
Understanding stress and flexibility concepts will help designers in reducing cycle time for the preparation of an effective layout that is acceptable to the stress engineer.
Increases the technical expertise of the designer in piping layout and design and also gives a good understanding of pipe supporting norms.
This will lead to better design, more effective and accurate material take-off & shorter schedule.
Provide sufficient pipe leg perpendicular to the pump suction axis so as to absorb suction line growth. Refer to Figure 1.
Case 1 piping is more flexible as the column nozzle is perpendicular to the pump axis and pumps set equidistant from the column centerline help minimize differential thermal growth across the pump axis.
Fig. 1: Typical pump routings
Case 2 has a column nozzle parallel to the pump axis and this layout is less flexible because thermal growth along the pump axis has to be absorbed by the offset loop.
Case 3 are preferred arrangement for higher temps and higher suction/discharge pipe sizes.
Provide min 5D straight run from the first elbow to pump suction.
First base support shall be adjustable.
Consider a low-friction slide plate where required.
Furnace/Reactor layout and flexibility in piping design when interspaced with a pipe rack. Refer to Fig. 2.
(Case 1) A common practice is to locate both Furnace & Reactor on the same side of the pipe rack so that the connected line on the rack moves away from the equipment during thermal expansion
(Case 2) In an alternate layout, the midsection of the line on the rack acts as a pivot allowing the pipe on either side to move away from equipment due to thermal expansion (more loads on nozzles).
Fig. 2: Typical Furnace reactor Layout
Tall Column Piping (Fig. 3)
Considering column expansion is very critical to effective and efficient piping design.
Column skirt expansion is also critical.
Use the tables for standard pipe guide spacing for supporting of vertical lines.
Avoid loop on vertical downcomer alongside the column.
First support is almost always a “Rest Support”.
Fig. 3: Typical column piping
Conclusions
Reduce stress by providing flexibility in the piping system and properly balanced support to ensure uniform distribution of piping loads.
Remember that pipe moves when hot and movement shall not be restrained by the adjacent pipe.
Understand that different materials have a different thermal coefficients of linear expansion (i.e. SS expands more than CS).
Most sustained-case loads (dead loads) in a piping system can be addressed by the designer by use of support span table and guide tables.
Avoid fitting to fitting routing of lines especially for higher sizes and temps
Remember that expansion leg in the other direction/plane is good but in third direction/plane is better (3-D loop better than 2-D)
Piping design owns the ultimate responsibility for effective, economical and efficient design
The full form of HVAC is Heating, Ventilation, and Air Conditioning. Appropriate HVAC provides comfort for people & maintains an effective Environment for surroundings. Proper HVAC Allows humans to exist under adverse conditions. The requirement of HVAC Systems is a must for critical areas like Control rooms, Electronic Equipment Rooms, switchgear halls, Battery rooms, workshop areas, etc. in a petrochemical plant. HVAC systems are designed following basic principles of thermodynamics, heat transfer, and fluid mechanics. The main purpose of the HVAC system is to adjust and change the outdoor air conditions to the desired conditions of occupied buildings for achieving the thermal comfort of occupants. Outside air is drawn inside the buildings and either heated or cooled depending on the outdoor condition. The air is then distributed into the occupied spaces.
Objectives of HVAC Systems
The Primary Intent of HVAC systems is the Comfort of occupants. Other major objectives are
Maintaining environmental conditions (temperature and humidity) appropriate to the operating requirements.
Increase in Productivity.
Building & Equipment Durability.
Maintaining pressurization between hazardous and non-hazardous areas.
Increase Life & Health.
Dilution and removal of potentially hazardous concentrations of flammable/toxic gaseous mixtures in hazardous areas.
Filtration of dust, chemical contaminants, and odors through chemical and carbon-activated filters.
Inside condition of 21–24°C & 50–60% Relative Humidity is most comfortable & purity of air.
The isolation of individual areas and control of ventilation in emergency conditions, through interface with the shutdown logic of the fire and gas detection and alarm safety systems.
However, note that an HVAC system is not intended to prevent catastrophic events such as the release of toxic and/or hazardous gases. Also, HVAC can not compensate on its own, for the intrinsic safety design features such as structural stability, coatings, area segregation, fire protection systems, etc. It only aids the safety process.
HVAC system components
The HVAC system components that condition the air to maintain the thermal comfort of space and occupants and achieve the required indoor air quality are:
There are four major types of HVAC systems available in different sizes. They are:
Split HVAC systems
Hybrid Heat Split HVAC systems,
Duct-free or Ductless HVAC systems, and
Packaged heating and air HVAC systems.
Split HVAC Systems:
A split HVAC system is the most common and classic type of HVAC system providing energy efficiency at lower costs. In this model, components are kept both outsides and inside the building and the system is split between two main units, one for heating, and one for cooling. The installation of split HVAC systems is quite complex; So must be performed under expert supervision. Typically, a split HVAC system consists of
An outdoor component, or a condenser, such as an air conditioner or a heat pump.
An indoor component that consists of an evaporator coil or fan, along with furnaces that convert refrigerant and help circulate air.
A system of ducts that circulate air from the HVAC unit throughout the building.
A programmable or non-programmable thermostat to manage the system.
Accessories like purifiers, air cleaners, UV lamps, or humidifiers improve indoor air quality and comfort.
Hybrid Heat Split HVAC systems:
Hybrid Heat Split HVAC systems are basically an advanced form of split system with improved energy efficiency and lower utility bills. This type of HVAC system typically consists of
An evaporator coil and furnaces that work to convert refrigerant and circulate air.
A heat pump to cool or heat refrigerant.
An oil or gas furnace.
Ducts to take the warm or cool air throughout the building.
Accessories to improve indoor air quality.
Duct-free or Ductless HVAC systems:
A duct-free or mini-split HVAC system is a unique ultra-efficient HVAC system with large upfront costs. It provides big benefits for certain needs and applications where conventional systems of ducts cannot be used. Duct-free or ductless HVAC systems consist of the following components:
An air conditioner or heat pump to cool or heat refrigerant.
Wires and tubes to connect refrigerant from the outdoor unit to the fan coil.
A compact fan coil.
A thermostat to manage the system.
Indoor air quality accessories.
Packaged heating and air HVAC systems:
Packaged heating and air HVAC systems are best suited for buildings without enough indoor spaces for split system components. In such HVAC systems, all components come together in one single package. So, easier to install. But the packed system is outside and exposed to extreme weather conditions, so can be damaged easily. A packaged heating and air HVAC system generally consist of:
A heat pump, gas furnace, air conditioner, and the fan coil and evaporator reside in one unit
An interface/thermostat on the front of the unit controls the system.
Optional indoor air quality accessories.
Depending on the primary equipment location, HVAC systems can be classified as
Centralized HVAC Systems and
Decentralized or Local HVAC Systems.
The major differences between Centralized and Decentralized HVAC Systems are provided in the following table:
Centralized HVAC system
Decentralized or Local HVAC system
Visually appealing as mechanical equipment is hidden in the mechanical room.
Visually not appealing.
Less Flexible.
The flexibility of operating different building parts at selected times. Also, flexible to maintain the different temperatures at different building parts.
Less Energy Efficient
Efficiency is more.
Higher installation and operating cost
Lower operating and installation costs.
Easy maintenance and Lower maintenance cost
Difficult to maintain and the cost is higher.
Standby equipment is accommodated for troubleshooting and maintenance
No backup or standby equipment
Long service life
Reliable equipment, but the estimated service life is normally less.
Centralized vs Decentralized HVAC System
HVAC System Selection
The selection of HVAC systems for a given building depends on various factors as mentioned below:
The climate.
The shape, size, function, architectural design, and age of the building.
The individual preferences of the owner/HVAC designer.
An HVAC system is primarily based on the following four requirements:
Primary equipment
Space requirement,
Air distribution, and
Piping
Primary equipment includes heating equipment (steam boilers/hot water boilers), air delivery equipment (centrifugal fans, axial fans, and plug or plenum fans), and refrigeration equipment (water chillers or refrigerants).
To specify an HVAC system as per design requirements, Space is required for the following facilities:
Equipment rooms
HVAC facilities
Fan rooms
Vertical shaft.
Equipment access space for installation, replacement, and maintenance.
Air distribution represents the ductwork for delivering the conditioned air to the desired area in a direct, quiet, and economical way. It consists of air terminal units, fan-powered terminal units, variable air volume terminal units, all-air induction terminal units, and air-water induction terminal units. To prevent heat loss and save energy, all the ductwork and piping should be insulated. To host ductwork, buildings should have enough ceiling space.
The piping system is essential to deliver refrigerant, cooled water, steam, hot water, gas, and condensate to and from HVAC equipment in a direct, quiet, and affordable way. Piping systems in HVAC design are normally divided into two parts: the piping in the central plant equipment room and the delivery piping. Depending on the governing code requirement, HVAC piping may or may not be insulated.
How does an HVAC System Work?
Fig. 1 below shows a schematic diagram explaining the basic function of an HVAC system:
Fig. 1: Schematic of basic HVAC system
Basic Refrigeration Cycle:
Fig. 2: Vapor Compression Refrigeration Cycle
Air Conditioning Capacity – The “Ton”:
One Ton of Refrigeration (TR) is defined as the quantity of heat removed to freeze 1 Ton (1 American short ton is 2000 pounds) of water into ice from and at 0°C in 24 hours.
1 Ton of Refrigeration = 12000 Btu/hr or 3024 Kcal/hr
TR = Q x Cp x (Ti – To) / 3024
Q = mass flow rate of coolant in kg/hr
Cp = coolant specific heat in KCal /kg deg C
Ti = inlet, the temperature of the coolant to the evaporator (chiller) in °C
To = outlet temperature of coolant from the evaporator (chiller) in °C
Major Work in HVAC (Fig. 3):
The design of an HVAC system for plant buildings, requires specific temp., humidity, internal pressure, and cleanliness under various conditions.
Preparation of Heat load Calculations, System Diagram, and Duct/Piping Plan.
Equipment Sizing & Selection.
HVAC Work Flow:
Fig. 3: HVAC System Design
Tools and Products:
E20-II, Elite
System Capacity Report (Fan, Cooling, Heating, Humidification)
Year Simulation (Power, Operation, Cost)
Load Analysis by Each Room
Fig. 4: Simulation of HVAC using tools
Scope of work:
Preliminary Design
Vendor Print Checking
Engineering
Field Engineering
Basic HVAC Equipment:
Fans / Blowers
Air Handling Units (AHU)
Filters
Compressor
Condensing units
Evaporator (cooling coil)
Control System
Air Distribution System
Common HVAC System Types:
Window / Split AC
Duct-able Splits
Packaged Units
Fan Coil Units
VRV Units
Direct Expansion System
Chilled Water System
Split System:
Fig. 5: An outline diagram of a typical split system
Air Handling Unit (AHU):
Fig. 6: Diagram showing an AHU
HVAC Equipment (Fig. 7):
Chillers
Heat Exchangers
Cooling Towers
Humidifiers
Fig. 7: Diagram Showing Major equipment
Air Distribution:
Ductwork
Metal
Flexible
Duct board
Grilles, Louvers, & Registers
Dampers
Shut off
Fire
Smoke
Sealants
Supports
HVAC Codes and Standards
Various well-established codes and standards that are followed at different stages of HVAC system design are listed below:
ISO 15138, Heating, ventilation, and air-conditioning.
NORSOK Standard H-001, Heating Ventilation, and Air-conditioning.
ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy.
ANSI/API RP 500: Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities of Class 1 Division 1 or Division 2.
ASHRAE Standard 62.1 – Ventilation for Acceptable Indoor Air Quality.
BS 5925: Ventilation principles and designing for natural ventilation.
ASHRAE Standard 90.1 – Energy Standard for Buildings Except for Low-Rise Residential Buildings.
BS EN 60079-10: Electrical apparatus for explosive gas atmospheres, Part 10. Classification of hazardous areas.
IP, Model Code of Safe Practice for the Petroleum Industry: Part 15
IS – 12332 For Ventilation in Petrochemical Plants & Refineries.
IS – 3103 For Industrial Ventilation.
OISD – 163 Safety of Control room for the Hydrocarbon industry.
Online HVAC Courses
The below-mentioned online courses will help to upgrade your knowledge to the next level. So, click on the subject you are interested in learning, review it first, and then enroll if you feel suitable for you:
Compressed air systems are an essential component in many industrial and commercial operations. They provide a versatile and reliable source of energy that powers various tools and processes.
What is a Compressed Air System?
A compressed air system is a network of equipment designed to convert atmospheric air into a high-pressure source of energy. This system consists of several key components, including air compressors, storage tanks, air treatment equipment, and distribution piping. Compressed air can be used for a multitude of applications, ranging from powering pneumatic tools to actuating machinery and performing process operations.
Why do we need a Compressed Air System?
Compressed Air (CA) is a major prime-mover for the modern industry. Compressed Air is referred to as the fourth utility after electricity, gas, and water. A properly managed Compressed Air system can:
save energy
reduce maintenance & decrease downtime
increase throughput
improve product quality depending on its end-use
Compressed Air Quality
Compressed Air quality ranges from Plant air to Breathing air depending upon the end-user. Quality is determined by the dryness & the contaminant level. Higher the quality, the higher the cost. The following figure (Fig. 1) gives the applications of compressed air with respect to quality.
Fig. 1: Application of Compressed Air.
Components of a Compressed Air system
Compressed air systems consist of:
a supply side which includes compressors and air treatment
a demand side, which includes distribution and storage systems and end-use equipment.
A Compressed Air system broadly consists of:
Air Compressor
The heart of the compressed air system, the air compressor is responsible for drawing in ambient air and compressing it to a higher pressure. There are several types of compressors, including:
Reciprocating Compressors: Utilize a piston mechanism to compress air.
Rotary Screw Compressors: Use two interlocking screws to compress air, providing a continuous flow.
Centrifugal Compressors: Use a rotating impeller to compress air, suitable for high-capacity applications.
Air Storage Tanks
These tanks store compressed air, helping to balance the supply and demand. They allow for peak demand management and reduce the frequency of compressor operation, leading to increased efficiency and extended equipment life.
Air Treatment Equipment
To ensure the quality of the compressed air, treatment equipment is necessary. This includes:
Filters: Remove dirt, moisture, and other contaminants.
Dryers: Reduce humidity in the air to prevent corrosion and other issues in pneumatic tools.
Regulators: Control the pressure of the air delivered to various tools and machinery.
Distribution System
A network of pipes and hoses transports compressed air from the compressor and storage tanks to the point of use. Proper design and maintenance of this system are critical to minimize pressure drops and energy losses.
Control Systems
Advanced compressed air systems may incorporate control systems that monitor and adjust the operation of the compressor, storage, and treatment equipment to optimize performance and energy efficiency.
Air Compressor Types (Fig. 2)
The Air compressor is the heart of any CA system.
There are two basic compressor types: positive-displacement and dynamic.
Fig. 2: Basic Compressor Types
Reciprocating Compressor (Fig. 3)
This is a very versatile type of compressor and can be used for nearly all industrial applications. These are constant-capacity machines & deliver the air in pulses.
Characteristics are:
High discharge pressures & relatively low to moderate volumetric flows (600-3000 cfm with a pressure range of 2000-5000 PSIG)
Generally more maintenance intensive due to the many wearing parts
It can be single-acting or double-acting, single-stage or multi-stage, air-cooled or water-cooled and lubricated or non-lubricated.
High efficiencies
Occupy larger footprint
Higher capital cost
Control is usually by Load-unload with 3 or 5 step capacity control
Fig. 3: A typical reciprocating compressor
Multi-staging
Multistage machines are used in place of single-stage ones for the following reasons:
Single-stage compression would generate excessive heat from compression
MOC would have to be of high grade and hence expensive
The power consumption of a single machine would be higher
Better efficiency
Rotary Compressor (Fig. 4)
The most common type of rotary compressor is the helical-twin, screw type. Less common types include sliding vane, liquid-ring, and lobe.
Characteristics are:
Not usually suited for high discharge pressures & are most efficient for moderate air flows & low pressures (3000-6000 cfm with a pressure range of 300-400 PSIG)
Low initial cost, compact size, low weight, and easy to maintain.
Centrifugal compressor develops pressure by increasing the velocity of the air going through the impeller & then recovering the velocity in a controlled manner to achieve the desired flow and pressure.
Best suited for continuous air flows in large quantities.
Characteristics are:
The heat generated & power consumption is lower.
The space requirement & maintenance is minimum.
Inherently non-lubricated.
Available for flows ranging from 300 to more than 100,000 cfm, but the common ones are 1200-5000 cfm with a pressure range of up to 125 PSIG.
Capacity control by inlet valve/guide vane throttling
Surge/choke phenomena
Fig. 5: A typical Centrifugal Compressor
Characteristic curves
The characteristic curve (Fig. 6) of a compressor plots its discharge pressure as a function of flow.
Fig. 6: A typical characteristic curve
Selection criteria
Flow rate
Discharge pressure
End-use of the air
Energy efficiency
Reliability
Compressor sizing
Estimation of Compressed air consumption:
Instrument air requirement
5 nm3/hr per CV
0 nm3/hr per ROV
The plant (Utility) air requirement
Compressed air requirement for pneumatic equipment, etc. operating in full load condition
Compressor Discharge pressure
end-use pressure (for instrument air, minimum 7 bar g)
plus all the pressure drops in the system.
Compressor Controls
Compressed air system controls serve to match compressor supply with system demand. Proper control is essential to efficient operation & high performance.
System controls include:
Start/Stop
Load/Unload
Dual control
Modulating
Speed variation
Pressure/Flow controls
Minimum Instrumentation required
Indications/alarms/trips consist of 3 major systems: Compressor, Lube oil & Cooling Water
Lube Oil system
Minimum alarms
Low oil pressure
Low oil level in the reservoir
High oil filter differential pressure
High oil temperature
The high thrust bearing metal temperature
Temperature gauges
Oil piping to & from coolers
The outlet of each radial & thrust bearing
Pressure gauges
Discharge of the oil pump
On bearing header
On control oil line & seal oil line
Compressor
Pressure indicator at inlet, inter-stage & discharge
Pressure switch at discharge
Temperature indicator/alarm at inlet, inter-stage, and outlet
The temperature gauge on the bearings
Vibration switches
A safety valve on each stage (for reciprocating type only)
Flowmeter (if required)
CW System
Pressure & temperature gauge on CW inlet
The temperature gauge on CW outlet
The thermal relief valve on CW outlet
Contaminants in Compressed air
The 3 major contaminants in Compressed Air are:
Water
Oil
Dirt
Compressed Air System Components/Accessories
The standard components/accessories include:
Prime Mover
The prime mover is the main power source providing energy to drive the compressor. This power can be provided by any of the following sources:
Electric motors: Economic, reliable, efficient
Diesel/natural gas engines: Fuel availability, higher maintenance, high cost/uncertainty of power
Types – Impingement baffle type, Centrifugal type, with Demister pads.
Pulsation Dampeners
Reduce/eliminate pulsations of reciprocating machines
Installed at the outlet of each stage
Receivers
Storage for utilization at peak load
Draining of condensed water
Reduce pulsations from reciprocating machines
Receiver sizing is based on hold-up for a drop in pressure level, say 10 minutes for a pressure drop of 3 bar.
Provided with standard accessories.
Air filters
Suction filters or Post-compression filters.
Felt cloth filters are used for suction. Compressed air filters can be:
Coarse particle filters (filter media can be a ceramic candle, felt cloth, etc)
Coalescing & activated carbon filters
Microfilter (high efficiency for special uses such as breathing air, etc) Minimum pressure drop
Drain traps
Manual
Mechanical float type
Electronic timer operated
Auto drain traps – condensate sensing
Regular maintenance is required to avoid CA wastage
Lube oil coolers
To remove heat from the lube oil
Usually Shell & tube type with CW
Air distribution piping
Least pressure drop in the system to reduce operating costs. The maximum pressure drop between the compressor and the farthest end of compressed air consumption should be around 0.3 bar
Velocities between 6-10m/s in air mains; this will limit the DP & thus energy consumption and also allow moisture to precipitate
Minimum bends & joints (long radius bends to be used)
Arrangement for draining of moisture at regular intervals, slope provision
Minimum expanders/reducers
Leakproof joints, proper piping supports
Gauges are to be provided at different locations to monitor the system pressure & temperature.
Compressor Cooling system
Cooling plays an important role in energy efficiency, two types are:
Air-cooled – In-efficient, preferred only for low-capacity compressors
Water-cooled – Efficient, used for high-capacity compressors
Water is circulated in a jacket around the cylinders to remove the heat resulting from compression & friction due to the sliding of pistons. Water is also required for the inter/aftercoolers and lube-oil coolers.
CW consumption for inter/aftercoolers can be estimated based on the compression ratio per stage assuming an adiabatic efficiency.
Generally, per the thumb rule,
Compressed air plant layout and distribution
Plant layout:
Centralized layout
All compressors installed in a single house, cost-effectiveness as maximum plant space utilization
More pressure drop expected
Decentralized layout
Suitable for large industries, different levels/pressures of air
Compressor situated at the maximum user location, less pressure drop, max energy utilization
This can lead to noise and heat inside the plant
Compressor location
The location should be such that compressor can induct clean, dry, and cool air
Interesting fact:- every 4°C reduction in air intake temp reduces power consumption by 1%
Points to be remembered while selecting the location of the air compressor:
Low humidity to reduce water entrainment
Adequate ventilation especially for air-cooled units
Minimum suction piping
Minimum bends
Compressed air distribution (Fig. 7)
Fig. 7: Different compressed air layouts
Codes
API 617 Centrifugal compressors
API 618 Reciprocating compressors
API 619 Rotary-type positive displacement compressors
API 672 Packaged integrally geared centrifugal air compressors
Applications of Compressed Air Systems
Compressed air systems have a wide range of applications across various industries. Some common uses include:
1. Manufacturing and Assembly
Pneumatic Tools: Tools such as drills, wrenches, and hammers are powered by compressed air, allowing for greater efficiency compared to electric counterparts.
Material Handling: Compressed air is used in conveyor systems and automated guided vehicles (AGVs) for transporting materials.
2. Food and Beverage Industry
Packaging: Compressed air is used for sealing and packaging products, ensuring hygiene and efficiency.
Process Control: Many processes in food and beverage production require precise control of air pressure and flow.
3. Automotive Industry
Paint Spraying: Compressed air systems are crucial for paint application in vehicle manufacturing, providing a smooth and even coat.
Assembly Line Operations: Pneumatic tools are widely used in assembly lines for tasks like fastening and lifting.
4. Healthcare
Medical Equipment: Compressed air is utilized in various medical devices, such as ventilators and dental chairs, ensuring reliability and safety.
5. Construction
Heavy Machinery: Tools like jackhammers and nail guns rely on compressed air for their operation, improving efficiency on job sites.
Benefits of Compressed Air Systems
Compressed air systems offer numerous advantages that make them a preferred choice for many industries:
1. Versatility
Compressed air can power a wide array of tools and equipment, making it suitable for various applications across different sectors.
2. Safety
Unlike electricity, compressed air poses minimal fire and explosion risks, making it a safer alternative in hazardous environments.
3. Clean Energy Source
When properly treated, compressed air is clean and free from contaminants, making it ideal for applications in sensitive environments, such as food production and pharmaceuticals.
4. Reliability
Compressed air systems can provide a consistent and uninterrupted power source, crucial for maintaining productivity in manufacturing and other operations.
5. Energy Efficiency
Advanced control systems and energy-saving technologies can help optimize compressed air systems, leading to significant reductions in energy consumption and operational costs.
In conclusion, the compressed air systems are a vital component of many industrial processes, offering versatility, reliability, and safety. Understanding their components, applications, and challenges is essential for optimizing performance and efficiency.
