Christmas Tree and Wellhead: Function, Components, Differences, Design Codes

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

In the world of oil and gas production, the terms “Christmas tree” and “wellhead” hold significant importance. These components are crucial for the safe and efficient extraction of hydrocarbons from beneath the Earth’s surface. While they might seem like simple structures, their design and functionality encompass a wide range of engineering principles, safety considerations, and operational strategies.

1. Christmas Tree in Oil & Gas

1.1 What is a Christmas Tree?

In petroleum and natural gas extraction, a Christmas tree, or “tree,” is an assembly of a multi-valved structure consisting of valves, pipe spools, and fittings used to control the ﬂow of oil from the well. The shape of the wellhead structure with valves looks like a pine tree, so the wellhead is called a “Christmas tree”. The Christmas Tree is connected to the top of the tubing and cemented to the top of the casing. These valves regulate pressure, control ﬂow, and allow access to the wellbore when further completion work is required. From the outlet valve of the Christmas Tree, the ﬂow can be connected to a distribution network of pipelines and tanks to distribute the product to reﬁneries, natural gas compressor stations, or oil export terminals.

The Christmas tree is the heart of the offshore hydrocarbon production system. It is the primary means of well control and plays a key role in the emergency shutdown system. The Christmas tree sits on the top of the wellhead casing system and represents the interface between the well and the production and process facility.

The Christmas tree consists of an assembly of a gate valve that controls the flow of hydrocarbons. It may consist of individual valves bolted together, or it may feature a cast or forged steel solid block into which the valve chests are machined. Occasionally it is a combination of the two. In all cases, the valve seats and gates are removable for replacement or repair.

**Fig. 1: Typical Wellhead and Christmas Tree Assembly**

1.2 The Function of a Christmas Tree

The primary function of a Christmas tree is to manage the flow of oil and gas from the well. It serves several key purposes:

Flow Control: Regulating the flow rate of oil and gas to optimize production.
Pressure Management: Maintaining appropriate pressure levels within the well to prevent blowouts.
Separation of Fluids: Allowing for the separation of oil, gas, and water as they are extracted.
Safety Measures: Providing emergency shut-off capabilities to prevent uncontrolled releases of hydrocarbons.

In producing wells, injection of chemicals, alcohols, or oil distillates to prevent or solve production problems (such as blockages) may be used.

A tree may also be used to control the injection of gas or water injection applications on a producing or non-producing well in order to sustain economic “production” volumes of gas from other wells (s) in the area (field).

The control system attached to the tree controls the downhole safety valve while the tree acts as an attachment and conduit means of the control system to the downhole safety valve.

1.3 Components of a Christmas Tree

A typical Christmas tree is composed of a master gate valve, a pressure gauge, a wing valve, a swab valve, and a choke as shown in Fig. 1. The Christmas tree may also have a number of check valves.

At the bottom, we find the casing head and casing hangers. The casing will be screwed, bolted, or welded to the hanger. Several valves and plugs will normally be fitted to give access to the casing. This will permit the casing to be opened, closed, bled down, and, in some cases, allow the flowing well to be produced through the casing as well as the tubing. The valve can be used to determine leaks in the casing, tubing, or packer, and will also be used for lift gas injection into the casing.

The tubing hanger (also called a donut) is used to position the tubing correctly in the well. Sealing also allows Christmas tree removal with pressure in the casing.

There are wells drilled into the reservoir, and the central conductor, along with the surrounding jackets/annulus, rises up to the production deck/cellar deck of the platform. On top of the wellhead, an assembly of valves is placed, which has the form of a cross. This assembly of valves together with the flanges is called a wellhead Christmas tree.

The Christmas tree has many Manual valves and a number of actuated valves. The actuated valves usually found on the Christmas tree are as follows:

1.3.1 Sub-Surface Safety Valve:

The Sub-Surface Safety valve is a hydraulic operated valve, the location of which is below sea level, above the sea bed. The actuator of this valve needs to be very small, as it gets enclosed within the Annulus of the conductor. The actuator is usually hydraulically operated. The control line for the hydraulic supply for the SSSV runs within the conductor and terminates at a connection on the Christmas tree.

1.3.2 Surface Safety Valve or Master Valve:

This isolates the tree from the production tubing. The Christmas tree has two master valves referred to as the upper and lower master valves. The lower master valves are opened first and closed last. This ensures a minimal flow of hydrocarbon over the valve seat, thus protecting it from abrasive particles and ensuring a good seal is maintained.

In most cases, the lower master valve is manually operated and the upper master valve is operated via a hydraulic or pneumatic actuator and is connected to the emergency shutdown system. The actuators are fail-safe in operation. The valve is held open by oil or pressure against a compressed coil spring.

The master gate valve is a high-quality valve. It will provide a full opening, which means that it opens to the same inside diameter as the tubing so that specialized tools may be run through it. It must be capable of holding the full pressure of the well safely for all anticipated purposes. This valve is usually left fully open and is not used to control flow.

A master valve is the first actuated valve on the Christmas tree, located above the Mezzanine deck of the platform. The Actuator is bigger and can be pneumatic or Hydraulic, based on the Christmas tree requirement

The pressure gauge:- The minimum instrumentation is a pressure gauge placed above the master gate valve before the wing valve. In addition, other instruments such as temperature will normally be fitted.

1.3.3 Wing Valve:

The wing valve comes on the arm of the Christmas tree, on the line where the flow line starts. The actuator is again hydraulic or pneumatic based on the requirement. The wing valve can be a gate valve or a ball valve. When shutting in the well, the wing gate or valve is normally used so that the tubing pressure can be easily read.

Christmas trees may be manufactured with one or two wing valves. One valve is permanently connected to the hydrocarbon process system and is fitted with a hydraulic or pneumatic actuator. The other valve is manual in operation and permits the injection of chemicals or gases into the well without disturbing production pipework.

Both valves are offset from the vertical lines so that a clear entry into the well is maintained through the swab valve for wireline work. The flow of gas from the well is regulated by wing valve operation or by choking fitted above the wing valve.

Hydraulic tree wing valves are usually built to be fail-safe closed, meaning they require active hydraulic pressure to stay open.

The right-hand valve is often called the flow wing valve or the production wing valve because it is in the flow path the hydrocarbons take to production facilities.

The left-hand valve is often called the kill wing valve. It is primarily used for the injection of fluids such as corrosion inhibitors or methanol to prevent hydrate formation.

1.3.4 Well Service Valve:

The Well Service Valve may be present on some Christmas trees, where Diesel pumping is required for initial start-up. It is on the other arm of the Christmas tree, and usually, the size is lesser than that of the wing valve.

The valve at the top is called the swab valve and lies in the path used for good interventions like wireline and coiled tubing. A ‘Choke’ is a device, either stationary or adjustable, used to control the gas flow, also known as volume, or create downstream pressure, also known as back pressure.

The Wellhead valves are all controlled by a Well Head Control panel, which gives the hydraulic & pneumatic supply for opening/closing these valves. There is logically built in the WHCP for allowing the safe closure of all these wellhead valves, in case of an emergency, either due to process upset or due to emergency/fire. In addition to these valves, the other instrumentation which is associated with the Christmas tree is the Pressure gauges and Transmitters for monitoring the Annulus pressures, the Flowing Tube Head Pressure, etc.

1.4 Vertical Christmas Tree vs Horizontal Christmas Tree

1.4.1 Vertical Christmas Tree:

The master valves are located above the tubing hanger, and swab valves together with master valves are stacked vertically. The production and annulus bore lay vertically on the body of the tree. The well completion is finished before installing the vertical Christmas tree. Since the tubing hanger rests on the wellhead, the Christmas tree can be recovered without having to recover the downhole completion. This type is generally applied in subsea fields due to its flexibility in installation and operation.

1.4.2 Horizontal Christmas Tree:

In contrast to the vertical Christmas tree, the valves of the horizontal Christmas tree are located on the lateral sides of the horizontal Christmas tree, allowing for easy well intervention and tubing recovery, thus this type of tree is very feasible for the wells that need many interventions. The tubing hanger is installed in the tree body instead of the wellhead. Consequently, the tree is installed onto the wellhead before the completion of the well.

2. Wellhead in Oil and Gas

2.1 What is a Wellhead?

A wellhead is a structure that sits at the surface of an oil or gas well, providing a point of access for drilling and production operations. It is essentially the terminal point of the wellbore, serving as the interface between the well and the surface.

A wellhead skid controls the operation of the Christmas tree and mudline safety valves. The skid permits valves to be operated locally, remotely, or via an ESD system, and timing, mechanisms provide a means of controlling the speed and sequence of valve operation. This sequence would normally be close to the wing valve, master valve, and mudline safety valve.

During an ESD operation, complete closure of the Christmas tree valves should be effected within approximately 45 seconds according to API recommendations, the only organization to provide guidance on this particular aspect.

Wellheads can be Dry or Subsea completion. Dry completion means that the well is onshore on the topside structure on an offshore installation. Sub-sea wellheads are located underwater on a special sea bed template. The wellhead consists of the pieces of equipment mounted at the opening of the well to regulate and monitor the extraction of hydrocarbons from the underground formation. It also prevents the leaking of oil or natural gas out of the well and prevents blowouts due to high-pressure formations. Formations that are under high pressure typically require wellheads that can withstand a great deal of upward pressure from the escaping gases and liquids.

These wellheads must be able to withstand very high pressures of the order of 140 MPa (1400 Bar).

2.2 Components of a Wellhead

A typical wellhead assembly consists of several key components:

Casing Head: The part that connects the well casing to the wellhead assembly.
Casing Spool: A cylindrical section that provides a place for the casing to connect and helps manage pressure.
Christmas Tree: Often integrated with the wellhead, as discussed earlier.
Blowout Preventer (BOP): A safety device that prevents the uncontrolled release of oil or gas during drilling.
Valves: Various valves control the flow of fluids and provide safety shut-off options.

2.3 Importance of Wellheads

Wellheads are critical for the integrity and safety of drilling operations. They ensure that the well is sealed off to prevent contamination of groundwater and minimize environmental risks. They also facilitate maintenance and monitoring activities throughout the lifecycle of the well.

3. The Interplay Between Christmas Trees and Wellheads

Integration in Operations

While Christmas trees and wellheads serve distinct functions, they are often integrated into a single assembly for efficiency and safety. This integration allows for streamlined operations, from drilling to production.

Safety Considerations

Both Christmas trees and wellheads play crucial roles in the safety of oil and gas operations. They are designed to withstand extreme pressures and temperatures, and they include multiple redundancies to ensure reliable performance.

4. Materials Used

The materials used in constructing Christmas trees and wellheads must withstand harsh environmental conditions. Common materials include:

Alloy Steel: Utilized in high-pressure applications.
Carbon Steel: Used for structural components due to its strength and durability.
Stainless Steel: Often used for valves and fittings to resist corrosion.

5. Christmas Tree vs Wellhead?

In the complicated but vital industry of drawing oil and gas out of the earth and getting it to the surface, people often get confused about the difference between a Christmas tree assembly and a wellhead. Although the terms are used interchangeably, a wellhead and a Christmas tree are entirely separate pieces of equipment. Each has valves and related equipment that help it control and guide the flow of this precious resource. A wellhead must be present in order to utilize a Christmas tree and is used without a Christmas tree during drilling operations. Producing surface wells that require pumps (pump jacks, nodding donkeys, etc.) frequently do not utilize any tree due to no pressure containment requirement.

Basically, The Christmas tree and the wellhead work together to bring oil and gas to the surface.

A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means (such as annulus vent), well monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback, connection points for devices such as downhole pressure and temperature transducers.

6. Design Code for the Christmas tree and wellhead

Christmas Tree and Well Head Equipment is designed following API Specification 6A/ISO 10423. The design shall take into account the effects of pressure containment and other pressure and temperature-induced loads. The following Figure (Fig. 2) shows the typical nomenclature of parts used in describing the Christmas Tree and wellhead.

**Fig. 2: Typical Christmas Tree and Wellhead Assembly Parts**

The Christmas tree and wellhead are vital components of the oil and gas industry, playing critical roles in the extraction and management of hydrocarbons.

Caesar II Error “Could Not Open TEMPMAT”- Steps to Overcome it

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

Sometimes, while working with Caesar II software during opening a CAESAR II Piping Input file, many of you might have come across an error similar to the one shown in Fig. 1 (Could not open TEMPMAT). In this context, you click on the Ok button and proceed with your analysis. But every time you open the input spreadsheet the same error appears each time which is very annoying. So this small article will highlight the steps which you can take to avoid such errors.

TEMPMAT ERROR — **Fig.1: Caesar II TEMPMAT Error**

Now, what the TEMPMAT is, and how this error is generated? Basically, whenever the piping input file is opened a temporary material properties file, TEMPMAT is created for easier access to the material properties from the Caesar II material database. The file gets automatically closed and deleted when the Analysis begins or when the Piping Input file is closed by the user.

There could be various reasons for this error. The most probable cause for this error is an access violation or insufficient user access rights. The probable reasons and remedies for this error are listed below:

If sometimes while running the Caesar file, the run does not complete or while opening the input spreadsheet the input window does not open due to some reason, i.e, the Caesar II program does not complete its intended operation you will find the error to appear when you close the file and try to open again.
If the Piping Input file is on a shared network drive (Such as in some servers as people often keep files in a common server for easy access to all stress engineers) with several users having access to it at the same time the same error generates. In such a situation, it is recommended to create a UserID string in the C2 configuration file (Miscellaneous Options Category, “User ID” value) for each user PC. The UserID should be between 2 and 4 characters long and must not contain any special characters (stars, dots, etc.) or spaces. This is NOT a password of any kind; and is NOT a personal key. This UserID must be unique to the machine where the CAESAR II program is running. After adding the value, Save and Exit the Configuration Editor (use the blue diskette button on the upper left of the toolbar). All the CAESAR II intermediate workflow items will be appended with this unique UserID string to distinguish them from other users’ files.
If the job (Piping Input file) is located on a local machine, then verify the job directory “user permissions” to allow “full access” (create/modify/delete files and folders).
If the job (the Piping Input file) was recovered from a storage device (such as a CD/DVD, or a backup drive), then the file would be read-only by default. So, verify that the job itself is not write-protected or read-only.
If still the problem persists simply restart your PC and the problem will not appear.

Air Cooled Heat Exchangers (ACHE)

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

Air Cooled Heat Exchangers (ACHE) are one of the heat exchanger types frequently used in Process, Power, Steel, and several other Industries where a process system generates heat that must be removed, for which there is no local use. The main function of the Air-Cooled Heat exchanger is the direct cooling of various process mediums by atmospheric air. These heat exchangers are also known as Air Fin Fan Coolers, Air Fin Coolers, Air Coolers, or Fin-tube heat exchangers. Air-cooled heat exchangers (ACHEs) have emerged as a crucial technology in this domain. They offer a method of cooling process fluids using ambient air, avoiding the complexities and costs associated with water-cooled systems. This blog discusses the design, operation, and applications of air-cooled heat exchangers, providing a thorough understanding of their significance and functionality.

What is an Air-Cooled Heat Exchanger or Air Cooler?

An air-cooled heat exchanger is a heat rejection equipment that transfers excess heat from a process fluid to the ambient air. It works on the principle of convection and conduction to dissipate heat from process fluid to air. Unlike water-cooled heat exchangers, which use water as a cooling medium, ACHEs use air to dissipate heat. This process typically involves a heat exchange coil or a series of finned tubes where the hot fluid passes through, and the ambient air stream is passed over the tubes to carry away the heat. By effective selection of the tube material, ‘ACHE’ can effectively cool or condense process water, chemicals, petrochemicals, or any other heat transfer fluid.

Applications of Air-Cooled Heat Exchangers

Major applications of air-cooled heat exchanger are found in

1. Oil and Gas Industry

In the oil and gas industry, ACHEs are used for cooling fluids and gases in various applications, including compressors, pumps, and separators. Their ability to operate in remote locations where water is scarce makes them particularly valuable in this sector.

2. Chemical Processing

Chemical plants often use ACHEs to cool process fluids and reactors. The robust design of ACHEs makes them suitable for handling corrosive and hazardous substances.

3. Power Generation

In power plants, ACHEs are used to cool various components, including generators and turbines. They help maintain optimal operating temperatures and improve overall efficiency.

4. HVAC Systems

In large commercial and industrial buildings, ACHEs are used in HVAC systems to reject heat from refrigerants. They are a crucial component in maintaining comfortable indoor climates.

They are also sometimes found in the cement, pharmaceutical, sponge iron, and steel industries.

Advantages of Air Cooled Heat Exchanger

The main advantage of Air-Cooled Heat Exchangers (ACHE) is their very low maintenance and operating costs.

As compared to cooling towers and shell and tube heat exchangers, air coolers are a “green” solution. They do not require an auxiliary water supply because of the lost water due to drift and evaporation. Where there is no utility, such as water, available as a cooling medium. The main advantages of an air-cooled heat exchanger can be summarised as follows:

1. Reduced Water Usage

One of the most significant benefits of ACHEs is the elimination of water consumption. This is particularly advantageous in areas where water is scarce or where water discharge regulations are stringent.

2. Lower Maintenance Costs

Air-cooled systems generally have fewer components subject to corrosion and scaling compared to water-cooled systems. This results in lower maintenance and operational costs over time.

3. Environmental Benefits

By reducing the reliance on water, ACHEs help mitigate the environmental impact associated with water withdrawal and discharge. This makes them a more sustainable option in certain scenarios.

4. Simpler Operation

ACHEs are less complex to operate compared to water-cooled systems, which often require additional equipment such as cooling towers and water treatment systems.

How Air-Cooled Heat Exchangers Work

The operation of an air-cooled heat exchanger is relatively straightforward. Here’s a step-by-step explanation:

Hot Fluid Inlet: The hot process fluid enters the heat exchange coil from the inlet.
Heat Transfer: As the hot fluid flows through the coils, heat is transferred to the metal surface of the tubes and fins. The large surface area provided by the fins enhances this heat transfer.
Air Flow: Fans force ambient air over the coils. The air absorbs the heat from the fins, cooling the fluid inside the tubes.
Cool Fluid Outlet: The cooled fluid exits the coil and is directed to the next stage of the process or system.
Warm Air Discharge: The air that has absorbed heat is expelled from the system.

ACHEs are usually used when the outlet temperature is more than about 20 °F above the maximum expected ambient air temperature. They can be used with closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger. Fig. 1 shows the operating principle of a typical air-cooled heat exchanger.

**Fig. 1: Operating principle of Air Cooled Heat Exchanger**

Air Cooled Heat Exchanger Construction

The construction of an Air-Cooled Heat Exchanger is fairly simple. Fig. 2 and Fig. 3 provide the component parts of an air fin fan cooler. Refer to the animated video at the end of the article for proper understanding.

Typically air-cooled exchangers consist of a finned tube bundle with rectangular box Headers on both ends of the tubes.
Cooling air is provided by one or more fans.
Usually, air blows upwards through a horizontal tube bundle.
The fans can be either forced or induced draft depending on whether the air is pushed or pulled through the tube bundle.
The space between the fans and the tube bundle is enclosed by a plenum chamber that directs the air. The whole assembly is usually mounted on legs or a pipe rack.

Diagram of Air Cooled Heat Exchangers — **Fig. 2: Diagram of Air-Cooled Heat Exchangers**

**Fig. 3: Construction of Air Fin Fan coolers**

Components of an Air-Cooled Heat Exchanger

An Air Cooled Heat Exchanger consists of the following primary components (Refer to Fig. 11):

One or more bundles of the heat transfer surface.
A fan or blower that moves the air.
A mechanical driver and power transmission for running the fan or blower.
A plenum
A supporting structure
Header and Fan maintenance platforms.
Optional louvers and recirculation ducts
Optional variable-pitch fan hub or variable-frequency drive for temperature control and power savings.

Bay Arrangement in Air Cooler

One or more tube bundles, serviced by two or more fans, including the structure, plenum, and other attendant equipment is called a bay in an air-cooled heat exchanger. Refer to Fig. 4 for typical bay arrangements.

**Fig. 4: Bay arrangements in Air Coolers**

Air Cooled Heat Exchanger Tubes

Wall thickness for tubes with an OD of 1 inch (25.4 mm) to 11/2 (38.1 mm) shall not be less than specified in the table below: (Fig. 5)

**Fig. 5: Minimum wall thickness requirements for Tube Material**

What kind of finned tubes are used for Air Coolers?

Fin types
- Embedded
- Extruded
- Footed
- Externally bonded
- Elliptical
Fin density (FPI) – 7 to 16
Fin height – 3/8 inch to 5/8 inch
Fin thickness – 0.012 to 0.02 inch

Fin Types (Fig. 6):

**Fig. 6: Figure showing various types of fins**

Fin Selection (Fig. 7):

Refer to Fig. 7 for limiting temperatures of various types of fins.

**Fig. 7: Limiting temperatures for various types of Fins.**

Fin Material

Typical materials used for air-cooled heat exchanger Fin construction are

Aluminum
Carbon steel
Carbon steel finned tubes can be hot-dip galvanized to prevent corrosion & to provide a metallic bond between the fin and the bare tube.
Copper

Air-cooled heat exchanger tube bundle orientation:

Three types of ACHE tube bundle orientation are possible; horizontal, vertical, and sloped orientation.

Horizontal orientation is the most preferred and common type and is widely used in process industries.

Orienting the tube bundles in a vertical direction (Fig. 8) can save a considerable area but the air cooler performance is greatly dependent on prevailing wind speed and direction. So if the prevailing wind is in the opposite direction, it will greatly reduce the performance of the air-cooled heat exchanger.

Tube Bundle Orientation of air cooled heat exchanger — **Fig. 8: Tube Bundle Orientation of air-cooled heat exchanger**

A-frame or V-frame air-cooled heat exchanger units are a compromise between the ground area requirement and exchanger performance. In this type of design, two bundles are sloped at 45°-60° and joined by their headers at the top or bottom. Thus it takes the shape of A or V. In steam condensing applications, the A-frame type (Fig. 8) with forced draft fans is used.

What are Air Cooler Headers?

Headers of air coolers are the boxes at the ends of the tubes that distribute the fluid from the piping to the tubes. The cover plate header design shall permit the removal of the cover without disturbing header piping connections. Fig. 9 shows a typical construction of tube bundles with removable cover plate headers.

a typical header for air fin fan cooler — **Fig. 9: Figure showing a typical header for an air fin fan cooler**

The bonnet header design shall permit the removal of the bonnet with the minimum dismantling of header piping connections. Threaded plug holes shall be provided opposite the ends of each tube for access. Holes shall be threaded to the full depth of the plug sheet or 50 mm (2 inches), whichever is less.

Common header types (Fig. 10) used in air-cooled heat exchanger construction are

Plug Box Header: Widely used
Bolted Cover Header
Bonnet Header
Manifold Header
Pipe Header: Cylindrical header for very high-pressure services (>200 bar)

**Fig. 10: common air cooled heat exchanger header types**

Why are some coolers forced drafts & some induced drafts (Fig. 11), and which one is better?

Mostly air-cooled heat exchangers are of forced draft construction. Forced draft units are easier to manufacture and maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed & replaced. The fan shaft is short since it does not have to extend from the drive unit through the tube bundle and plenum to the fan as in an induced draft design. Forced draft units require slightly less horsepower.
Since the fan is moving a lower volume of air at the inlet than it would at the outlet. If the process fluid is very hot, the cooling air is hot at the outlet, this could cause problems with some fans or fan actuators. If the fan is exposed to very hot exhaust air since forced draft coolers do not have the fans exposed to hot exhaust air.

In addition to the above, The major differences between an Induced Draft and a Forced Draft Air Cooled Heat exchanger are listed below with respect to some design parameters:

Sr. No	Parameter	Induced Draft Air Cooler	Forced Draft Air Cooler
1	Air Distribution	Better distribution of air across the bundle.	Less uniform distribution of air over the bundle.
2	Air Re-circulation	Less possibility of hot effluent air recirculating into the intake	Increased possibility of hot air recirculation
3	Process Control and Stability	Better process control and stability. The plenum covers 60% of the bundle face area which reduces the effects of sun, rain, and hail.	Less Process Control
4	Fan Failure	Increased capacity in the fan failure	The low natural draft capability of fan failure
5	Horsepower Requirement	Possibly higher horsepower requirements if the effluent air is very hot.	Possibly lower horsepower requirements if the effluent air is very hot
6	Temperature Limitation	Effluent air temperature should be limited to 220°F to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air stream	Accommodates higher process inlet temperatures
7	Fan Maintenance Access	Fans are less accessible for maintenance	Better accessibility of fans and upper bearings for maintenance.
8	Bundle Maintenance Access	Plenums must be removed to replace bundles	Better accessibility of bundles for replacement
9	Structural Cost	Increased structural cost and lower mechanical life.	Reduced Structural costs and improved mechanical life.

Table: Difference between Induced Draft and Forced draft Air Coolers

**Fig. 11: Types of Air Fin Fan Coolers**

What kinds of Air Cooler controls are used?

As one might expect the best kind of control scheme depends on the application. Does the process require very tight control of the process outlet temperature or is it better to allow the process temperature to go down with the ambient air temperature? Following is a list of some of the commonly used control devices for air coolers, but in no particular order.

Manually operated louvers
Pneumatically activated automatic variable pitch fans.
Variable frequency fan drive (VFD).

VFD if used shall suit the motor nameplate rating and similarly for the Belt Pulley transmission, it shall suit the motor nameplate rating.

What types of Air Cooled Heat Exchangers are used for Cold Climates?

For extremely cold regions like Canada, Siberia, etc an air-cooled heat exchanger with an internal recirculation system is used. Such systems can control the cooling air temperature regardless of ambient temperature.

Positive or Negative Step Auto variable fans are used for such systems. By using one fixed-pitch fan blowing upward and one Auto-variable pitch fan, which is capable of negative pitch and thus of blowing air downward, it is possible to temper the air to the coldest portion of the tubes and thus prevent freezing. Normally, forced draft units have a negative pitch fan at the outlet end, while induced draft units have a positive pitch fan at the outlet end.

**Fig. 12: Air Cooled Heat Exchanger diagram with internal recirculation**

Codes and Standards for air-cooled exchanger Design

The following codes and standards govern the design of air-fin fan coolers.

API 661 / ISO 13076 ( Air-cooled heat exchangers for general refinery service )
ASME SECT.VIII Div.1 or Div.2
Shell DEP 31.21.70.31

SHELL DEP special requirement for air-cooled exchangers:

The removable Bonnet type Header Shall not be used.
Plug-type headers shall be used for all air coolers in hydrocarbon gas or liquid service.
Removable cover plate-type headers shall be utilized in auxiliary utility services such as lube oil, hot oil, and cooling circuits.
Bolted joints (stud construction or flanged construction) shall be designed with confined gaskets
Tubes shall be provided with aluminum fins. The fins for all the air cooler tubes shall be of the extruded type with 394 fins per meter

Maintenance and Inspection of Air-Cooled Heat exchangers

The performance and reliability of ACHE depend on proper inspection and maintenance of components. Regular maintenance and inspection of Air Cooler systems include

Lubrication of motor and fan bearings (once a month if coolers work continuously)
Checking belts for wear and ensuring their tension.
Inspect fans, blades, bolting, etc at regular intervals.
Maintaining proper oil levels at gearboxes.
Checking the condition of tube bundles.

Disadvantages of Air-Cooled Heat Exchangers

Some of the disadvantages of air-cooled heat exchangers are

1. Space Requirements

ACHEs typically require more space compared to their water-cooled counterparts. The need for large fan arrays and ample clearance for airflow can be a limitation in space-constrained environments.

2. Limited Cooling Capacity

The cooling capacity of ACHEs is directly affected by ambient air temperature. In very hot climates, the efficiency of heat removal can be reduced compared to water-cooled systems.

3. Noise Levels

Fans used in ACHEs can produce significant noise, which might be a concern in noise-sensitive areas or residential environments. Proper design and placement can help mitigate this issue.

Some more useful resources for you..

Basic Considerations for Equipment and Piping Layout of Air Cooled Heat Exchanger Piping
Considerations for development of Plant Layout: A brief presentation
Some articles related to heat exchangers

Animated video tutorial of the Assembly Sequence of an Air Cooled Heat Exchanger

Refer to the below attached animated video for proper understanding and visualization of each part of a forced draft type air fin fan cooler. The assembling sequence of one single bay is shown in the video.

Air Cooler Assembly Sequence

Air-cooled Heat Exchanger Manufacturers

When it comes to air-cooled heat exchangers (ACHEs), several manufacturers are recognized globally for their expertise, innovation, and high-quality products. Here’s a rundown of some of the world-renowned ACHE manufacturers:

SPX Cooling Technologies, USA
API Heat Transfer, USA
GEA Group, Germany
Thermax, Germany
KTI-Plersch Kältetechnik GmbH, Germany
Fives, France
Hamon & Cie (International) SA, Belgium
Heatex, Denmark
Parker Hannifin, USA
Sustainable Technologies (S&T), India
Bharat Heavy Electricals Limited (BHEL), India

In conclusion, Air-cooled heat exchangers play a vital role in a wide range of industrial applications by providing an efficient and sustainable method of heat removal. Their design and operation involve careful consideration of various factors, including ambient conditions, fluid characteristics, and space requirements. Despite some disadvantages, their benefits—such as reduced water usage and lower maintenance costs—make them a valuable component in many systems.

References and Further Reading for Air Cooled Heat Exchanger (pdf)

https://files.chartindustries.com/hudson/BasicsofACHEBrochure-Web.pdf

What is Post Weld Heat Treatment (PWHT)? Its Procedure, Advantages, and Requirements

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

Welding is a critical process in manufacturing and construction, but it can also introduce stresses and imperfections in the welded joints. To mitigate these issues, post-weld heat treatment (PWHT) is employed to enhance the integrity and performance of welded components. This article provides a comprehensive overview of PWHT, including its principles, methods, applications, and benefits.

What is PWHT or Post Weld Heat Treatment?

The full form of PWHT is post-weld heat treatment. PWHT is a controlled process that involves reheating the metal below its lower critical transformation temperature, following a welding process. The material is then held at the elevated temperature for a predetermined period of time to alleviate residual stresses, increase the strength, increase or decrease the hardness, and reduce the risk of cracking by microstructural changes. An array of heating processes can be used to carry out post-weld heat treatment. The primary objectives of PWHT include:

Reducing Brittle Phase Formation: In some materials, PWHT helps to minimize the formation of brittle phases that can lead to premature failure.
Relieving Residual Stresses: Welding generates thermal gradients that can create internal stresses. PWHT helps to redistribute these stresses.
Enhancing Mechanical Properties: Heat treatment can improve ductility, toughness, and hardness in the heat-affected zone (HAZ) and base material.

The Need for PWHT

The necessity for Post Weld Heat Treatment (PWHT) primarily arises from the residual stresses and microstructural changes that occur after welding. During the welding process, a significant temperature gradient develops between the weld metal and the parent material. As the weld cools, it generates residual stresses, which can become unacceptable, particularly in thicker materials, potentially exceeding design limits. To mitigate these stresses, the part is heated to a specified temperature for a designated period, allowing the stresses to be reduced to acceptable levels.

In addition to residual stresses, the high temperatures associated with welding can alter the material’s microstructure, often increasing hardness while diminishing toughness and ductility. PWHT effectively addresses these changes, helping to lower hardness levels and restore toughness and ductility to meet design specifications.

The necessity of PWHT largely depends on the materials being welded, the service conditions, and the specific application. For instance, high-strength steels, low-alloy steels, and certain alloys are more susceptible to issues that PWHT can address. Industries like oil and gas, nuclear, and power generation often mandate PWHT to ensure structural integrity.

When is PWHT required for Carbon Steel?

Post Weld Heat Treatment or PWHT, of Carbon Steel must be performed after every welding in order to ensure the material strength of the part is retained. The exact criteria for PWHT of carbon steel are mentioned in the ASME BPVC code. PWHT ensures the reduction of residual stresses, controlling material hardness, and enhancement of mechanical strength.

Advantages of Post Weld Heat Treatment

If PWHT is neglected or performed incorrectly, the residual stresses can combine with service load stresses. The value may exceed a material’s design limitations, leading to weld failures, higher cracking potential, and increased susceptibility to brittle fracture. PWHT significantly enhances the quality of welds, leading to fewer failures and increased longevity of structures.

Other benefits of PWHT are listed below:

Improved metallurgical structure
Improved ductility of the material
Reduced risk of brittle fracture as ductility increases
Relaxed thermal stress due to the redistribution of residual stresses.
Tempered metal
Reduced chance of cracking
Reduced hardness
Removal of diffusible hydrogen, which helps in preventing Hydrogen-Induced Cracking (HIC)
Increased stability of the weld in further mechanical works
Improved corrosion resistance.
Many industries have strict codes and standards that require PWHT, making it essential for compliance.

Principles of Post-Weld Heat Treatment

Basic Concepts

PWHT involves heating the welded assembly to a specified temperature for a defined duration, followed by controlled cooling. The key principles include:

Temperature Control: The temperature is critical; it should be high enough to achieve the desired material properties but low enough to avoid adverse effects such as excessive grain growth.
Soaking Time: The duration at which the material is held at the target temperature is equally important. This ensures uniform temperature distribution and effective stress relief.
Cooling Rate: Controlled cooling is essential to prevent the formation of new stresses. Rapid cooling can lead to hardening and cracking.

Metallurgical Considerations

The metallurgical characteristics of materials are significantly affected by the heat treatment process:

Phase Transformations: Heating can induce phase changes in materials, which can enhance their mechanical properties.
Grain Refinement: Proper heat treatment can lead to finer grain structures, resulting in improved strength and toughness.
Stress Relief: PWHT promotes dislocation movement within the crystal structure, helping to relieve residual stresses.

Post Weld Heat Treatment Method and Equipment

The local post-weld heat treatment of the welded joints on the pipes shall be carried out by the electric-resistance method after the completion of all welding or repair operations.
The resistance heater is electrically and thermally self-insulated and is built to size for each individual pipe.
The applied voltages across the coils are either 220 or 380 volts AC, depending on the power requirements.

The power-control panel of post-weld heat treatment is composed of:

A temperature controller indicator and recorder of digital type.
A potentiometer device that controls the percentage of power input to the coils.
A switch On and Off indicator lights, input, and output terminals for power and thermocouple connection.
Electrical power contactors of the proper rating.

Each panel will supply a single heating station and therefore for each heating operation, one panel will be needed. Heating and cooling rates are adjusted by manual selection of the percentage of power input by means of potentiometers. Fig. 1 below shows an example of PWHT in the piping applications.

Requirements for Post Weld Heat Treatment or PWHT

Before applying for the detailed PWHT requirements and exemption in these paragraphs, satisfactory weld procedure qualifications of the welding procedures specification to be used shall be performed in accordance with all the essential variables of ASME SECTION IX including conditions of post-weld heat treatment and other restrictions listed below.

While carrying out local post-weld heat treatment, the technique of application of heat must ensure uniform temperature attainment at all points of the portion being heat treated. Care shall be taken to ensure that the width of the heated band on either side of the weld edge shall not be less than four (4) times pipe thickness or 2″, whichever is greater.

Throughout the cycle of post-weld heat treatment, the portion outside the heated band shall be suitably wrapped under insulation so as to avoid any harmful temperature gradient at the exposed surface of the pipe. For this purpose, the temperature at the exposed surface of the pipe should not be allowed to exceed 400 °c.

The valves, instruments, and other special items with welding ends shall be protected because of the risk of damage during post-weld heat treatment.

No welding shall be performed after PWHT.

Automatic temperature recorders that have been suitably calibrated shall be employed. The calibration chart of each recorder shall be submitted to the owner prior to starting the heat treatment operations and his approval shall be obtained. Recording equipment shall be calibrated at least once every 12 months. Also, the instrument equipment (potentiometer) which is used for the calibration of recorders should be supported by a related certificate.

Preparation and Attachment of Thermocouple for PWHT

After performing a visual inspection and removing surface defects and temporary tack welds (if any) an adequate number of thermocouples (based on the diameter of pipes) shall be attached to the pipe directly and in an equally spaced location along the periphery of the pipe joint. The minimum number of thermocouples attached per joint shall be

1 for up to 3″ diameter,
2 for up to 6″ diameter,
3 for up to 10″, and
4 for up to 12″ diameter and above.

However, the required minimum number of thermocouples to be attached can be increased if it is found necessary.

The thermocouples shall be placed on the joint and in firm contact with the pipe as near as possible to the weld area. Thermocouples should be directly tack welded to the joint or heating band jointly provided that they have a tail of the same material and approved filler wire or electrode not larger than 2.5 mm in diameter is used for tack welding.

In order to avoid incorrect temperature readings due to direct radiation to thermocouples, it shall be protected by ceramic fiber blanked or any other suitable insulation material.

Heating resistance elements shall be laid over the attached thermocouples throughout the heating band and shall be insulated as shown in Fig. 2 below.

Insulating materials shall be mineral wool/glass wool, which can overcome the temperature employed. The minimum insulation thickness shall be 50 mm. To hold the insulation material in position, wire mesh shall be wrapped around and tied or tied by other suitable means.

PWHT Temperature, Time Record

The post-weld heat treatment temperature and time and its heating and cooling rates shall be recorded automatically and present the actual temperature of the weld area. Each thermocouple shall be connected to the controlling and recording instrument for each treated joint.

Heating, Holding, and Cooling in PWHT

The heating temperature above 300°c shall be recorded and the heating and cooling rate shall not be more than that specified in related WPS and standards but in no case, more than 200°c/hr and the difference between the temperatures measured by various thermocouples shall be within the range specified.

The heat treatment soaking temperature and holding time shall be as specified in related welding procedure specifications. For easy reference, the values for different types of steel are given in the following table.

The cooling down to 300°c shall be controlled cooling. Below that, the cooling down to ambient temperature shall take place under insulation coating without control.

**Fig. 2: PWHT-TABLE FOR A106-B MATERIAL**

The post-weld heat treatment temperature for ASTM A672, ASTM A234, ASTM A671, and ASTM A420 is generally 635 (+/-15) Degrees Centigrade.

Post-Weld Heat Treatment Temperature as per ASME B31.3

ASME B31.3 specifies the PWHT temperatures with respect to the P Number of the material. Table 331.1.1 provides the PWHT holding temperature range. The same is reproduced below (Fig. 3) for your reference.

**Fig. 3: PWHT Temperature range as per ASME B31.3**

Additionally, table 331.1.3 provides certain exemptions from mandatory heat treatment. So, both Tables must be referred to concurrently.

Exemption Rules for Mandatory Post Weld Heat Treatment — **Fig. 4: Exemption Rules for Mandatory Post-Weld Heat Treatment as per ASME B31.3**

Types of Post-Weld Heat Treatment

The PWHT process can be classified into two groups:

Full PWHT, where the entire sample is subjected to heat treatment in a furnace, and
Local PWHT, where localized heating is performed near the welding area only

Post Weld Heat Treatment Techniques

Industrial post-weld heat treatment is carried out using different techniques, as listed below:

Conductive (electrical resistance) Method of Post Weld Heat Treatment:

The conductive method of PWWHT uses ceramic fiber, band, snake, and other resistors based on the object dimension, together with precision control, programming, and automatic recording equipment. This method of heat treatment using electrical resistance is a common method for local PWHT in the field.

Inductive Post Weld Heat Treatment Method:

In this method, Currents induced by high-frequency coils generate the required heat. The generated heat increases the metal temperature as per the PWHT requirement. The number of coils and length of cable needed to achieve the objective is pre-determined.

Convective PWHT Method:

This method is also known as internal combustion treatment. In the convective post-weld heat treatment technique, the heat generated with the help of gas/diesel burners is transferred to the weld by the convection process. This method is widely used for large objects like pieces of equipment requiring very high power levels. This type of post-weld heat treatment is also recommended for pipe prefabrication works where the processing volume is very high.

Some Important Notes related to PWHT

PWHT operation shall be performed only by trained personnel having a similar experience and approved by the owner.
During PWHT joints shall be protected from rain and wind by adequate rain cover and windshield.
Hardness tests after PWHT shall be performed to determine if heat treatment has been performed effectively. Normally for Carbon Steel, the maximum Brinell Hardness is 200 HB.

Safety Precautions during PWHT

The following safety precautions shall be provided during PWHT:

Equipment and panels shall be properly earthed.
Electrical technicians shall work with proper safety wear such as rubber gloves, shoes, etc.
Only certified electricians will work.
Joints under PWHT shall be well cordoned with red tape/red light and danger display to avoid unknown persons coming in contact with high voltage electrical connections.
An adequate platform shall be made for in situ joints to avoid the fall of a person.

Stainless Steel PWHT

PWHT for stainless steel is usually not required. However, to increase the corrosion resistance or reduce stress corrosion cracking susceptibility, stainless steel PWHT may be used depending on the service conditions encountered.

PWHT for Piping

The requirements for PWHT for piping are dependent on the P-numbers and Group numbers of the pipe material. Clause 331.1.1, along with Table 331.1.1 and Table 331.1.2 of ASME B31.3, provides the requirements for PWHT depending on pipe materials. However, if the proper pre-heat temperature is applied during the welding of pipes of specified thicknesses, the mandatory post-weld heat treatment can be exempted. Such rules are provided in Table 331.1.3 of ASME B31.3.

Applications of Post-Weld Heat Treatment

PWHT is used across various industries, each with specific requirements and standards. Here are some prominent applications:

Oil and Gas Industry

In the oil and gas sector, pipelines and pressure vessels are subjected to extreme conditions. PWHT ensures that welded joints can withstand high pressures and corrosive environments. Specific standards, such as ASME Section VIII, often mandate PWHT for certain materials.

Nuclear Industry

Nuclear power plants require the highest standards of safety and reliability. PWHT is critical for ensuring the integrity of welded joints in reactor vessels and piping systems. Compliance with stringent regulatory standards is essential.

Power Generation

For power plants, particularly those using fossil fuels or nuclear energy, PWHT is vital for components such as boiler tubes and heat exchangers. It helps to improve resistance to thermal fatigue and enhances overall reliability.

Structural Fabrication

In structural engineering, PWHT is used in the fabrication of steel structures, especially in high-rise buildings and bridges. It ensures that the material properties meet design specifications and safety standards.

Disadvantages of PWHT

Post Weld Heat Treatment (PWHT) offers many benefits, but it also comes with several disadvantages. Here are some common drawbacks:

Cost: PWHT can be expensive due to the need for specialized equipment, facilities, and skilled labor, which can increase project costs significantly.
Time-Consuming: The heat treatment process can be lengthy, potentially delaying project timelines. Both heating and cooling cycles require careful control and monitoring.
Equipment Limitations: The need for large furnaces or other heating equipment can pose logistical challenges, especially for large components that may not fit easily.
Potential for Distortion: Improper heat treatment can lead to warping or distortion of the welded component, necessitating additional corrective measures.
Material Limitations: Not all materials respond positively to PWHT. Some alloys may become brittle or lose desirable properties if subjected to heat treatment.
Quality Control Challenges: Ensuring uniform temperature distribution and proper monitoring throughout the process can be challenging. Inadequate control may lead to suboptimal results.
Skill Requirement: PWHT requires skilled personnel to manage the process effectively, and finding qualified workers can be difficult.
Post-Treatment Inspection: Additional inspections are often necessary after PWHT to ensure that the desired mechanical properties have been achieved, further adding to project timelines and costs.
Thermal Fatigue: Repeated heating and cooling cycles can lead to thermal fatigue in some materials, potentially reducing their overall lifespan.
Compliance and Documentation: Meeting industry standards and maintaining thorough documentation can be cumbersome, especially in regulated industries.

Frequently asked questions related to PWHT

In the following section, I will address some of the most frequently asked questions related to PWHT.

What is the full form of PWHT?

PWHT is an acronym for the full form of Post Weld Heat Treatment.

When is PWHT required?

Most of the conventional welding processes generate residual stresses in the welding regions. These stresses in certain conditions can approach the yield strength of the material. Such joints are prone to failure and can not be used directly. PWHT is applied to such welded steel assemblies to reduce the likelihood of brittle failure. PWHT significantly reduces the residual stresses in the weld joints and thereby reduces the failure potential. So, wherever there is a risk of environmentally assisted cracking, PWHT is required.

What are the ASME B31.3 PWHT requirements?

The ASME B31.3 PWHT requirements for piping materials are established in clause 331.1.1 along with Table 331.1.1, 331.1.2, and 331.1.3.

How to calculate PWHT temperature?

PWHT temperature is not calculated. The range of PWHT temperatures is mentioned in tabular formats in the governing codes. So, the PWHT temperature is selected from the tables of the governing code. For example, table 331.1.1 of ASME B31.3 for process piping systems.

What is the difference between Post Weld Heat Treatment and Stress Relieving?

Stress-relieving is a process to reduce residual stresses in an object. Heat treatments like normalizing, annealing, quenching, tempering, etc are some of the processes to relieve the residual stresses generated by welding. PWHT is also a process in which stress relief is achieved. However, the main difference between Stress relieving and PWHT is that the stress-relieving can be made at temperatures below the minimum transformation temperature of the steel where no micro-structural change of the material occurs.

Some more Useful Resources for you.

General requirements for Field Welding
Underwater Welding & Inspection: A short Presentation
13 major differences between Seamless and Welded Pipe
Procedure for Post Weld Heat Treatment (PWHT) in Carbon Steel and Low Alloy Steel Materials

Piping Elbow or Bend SIF (Stress Intensification Factor)

Written by Anup Kumar Dey in Piping Design Basics,Piping Stress Analysis,Piping Stress Basics

What is the Stress Intensification Factor or SIF?

The term SIF (Stress Intensification Factor) is too confusing for many piping engineers. That’s why the following article is taken to explain the bend SIFs in simple language. Every piping Engineer who possesses a little basic of the Piping Stress Analysis theory must be aware of the term SIF or Stress Intensification Factor.

The term SIF indicates a multiplier of Bending and Torsional stresses. This Intensifier acts locally on a piping Component ( tees, elbows, bends, Olets, etc.) and Its value depends on component geometry. The minimum value of the SIF is 1.0. It is widely used by piping stress engineers in places where the actual stress calculation is quite difficult due to its difficult geometry (Varying thickness, cross-section, curvature, etc.) as, unlike straight Pipes, the simple Beam theory is not applicable. So in this situation, it is required to assume additional stresses by suitably incorporating an SIF. The following article will provide an example of SIF calculation of piping elbow or piping bends following process piping code ASME B31.3.

SIF — **Fig.1: In-Plane, Out-Plane Concept as per Code**

Stress Intensification Factor for a Piping Bend/Elbow

In layman’s language, the SIF of a bend or elbow can be defined as the ratio of bending stress of an elbow to that of a straight pipe of the same diameter and thickness when subjected to the same bending moment. Whenever the same bending moment is applied to a bend because of ovalization the bending stress of the elbow will be much higher than that of a straight pipe. That is why the SIF value will always be greater than or equal to 1.0 (for straight pipe).

The process piping code ASME B31J (Appendix D of ASME B31.3 till edition 2018 of ASME B31.3) provides a simple formula to calculate the SIF of a bend or elbow. As per that code

SIF_in-plane = 0.9 / h^(2/3)
SIF_out-plane = 0.75 / h^(2/3)

Here h=T R₁ / r₂²

h =Flexibility characteristics, dimensionless

T =Nominal wall thickness of bend, in
R₁ =Bend radius, in
r₂ =Mean radius of the matching pipe, in

In-plane and Out-plane SIF

The in-plane and out-plane SIF concept for a bend can be obtained from the attached figure from code or in layman’s language the same can be explained as follows:

The in-plane bending moment is the bending moment that causes the elbow to close or open in the plane formed by two limbs of the elbow. In a similar way, the out-plane bending moment can be defined as the bending moment that causes one limb of the elbow to move out of the plane keeping another limb steady.

From the above-mentioned equations, the following can be interpreted: For the same pipe size and same pipe thickness

A short-radius elbow has more SIF as compared to a long-radius elbow.
With an increase in the bend radius, the SIF decreases and finally reaches 1.0 for the straight pipe.
The SIF for a 45-degree elbow and a 90-degree elbow is the same and the bend radius is the same.
With an increase in nominal pipe thickness or schedule, the SIF of a bend (90-degree) keeps on decreasing till its value is equal to 1.0.

Few more Resources for you..

How to use ASME B31J-2017 and FEM for SIF and k-factors for Stress Analysis
Importance & Impact of Stress Intensification Factor (SIF): A Presentation
Piping Stress Analysis Basics
Piping Design and Layout Basics
Piping Materials Basics

Vacuum Box Testing of Butt, Fillet, and Lap Weld

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Interface

Vacuum box testing is a non-destructive examination (NDE/NDT) used for locating welding leaks. A vacuum box and a compressor create a high or low-pressure vacuum and a detergent solution is applied to the test area. The detergent bubbles help to identify the leaks within the created pressure envelope.

The main objective of the Vacuum box testing technique is to locate leaks in welds due to through-thickness discontinuities. This is accomplished by applying a solution to a weld and creating a differential pressure across the weld causing the formation of bubbles as leakage gas passes through the solution. This testing is to be performed prior to any main vessel or tank testing following the completion of all welding. This article will briefly explain the leak testing procedure using the vacuum box method that can be used for all metals.

Required Equipment for Vacuum Box Testing

The vacuum box test method employs the following equipment for welding checks

a compressor: approximate flow 6 m³ per minute under 7 bar
An air pipe between the compressor and vacuum box.
A vacuum box, which enables it to obtain a depression of 500 m-bar, can be controlled by means of a vacuum manometer located on the box. The manometer shall have a range of 0 psi (0 m-bar) to 15 psi (1020 m-bar).
A liquid detergent mixed with water makes soapy water.
Bubble solution which produces a film. That does not break away from the area to be tested and the bubbles formed shall not break rapidly due to air drying or low face tension.
Soups or detergents for cleaning shall not be used.
If the ambient temperature is below freezing, add ethylene glycol to the solution (antifreeze).
General lighting or individual lighting if necessary. The minimum light level shall be 100 foot-candles (1000 lux)
A brush (diameter not less than 50 mm)
A metallic brush and/or a grinding machine.
Fig. 1 shows a typical vacuum test box arrangement.

**Fig. 1: Typical Vacuum Box Testing Arrangement**

Vacuum Testing Procedure

Remove weld slag. Mud dirt and other debris from the weld joint may prevent bubble formation. This is achieved, if necessary, by strong brushing.
If freezing weather conditions exist at the time of the test, heat the weld joint to be vacuum box tested until the metal is slightly warm to the hand in order to avoid any ice, that could possibly be plugging existing leaks. Limit the areas heated to those which can be tested before the metal cools to freezing temperatures. The temperature of the part to be examined shall not be below 40 f (5 ˚c) nor above 125 f (52 ˚c ) during the examination.
Provide adequate lighting for this examination.
Apply leak detector soap solution to the weld in a continuous film relatively free of bubbles about 20 inches (500 mm) and on about 4 inches (100 mm) wide. This must be achieved at least 1 minute before applying the vacuum box test.
Put the vacuum box on the area to be examined:
Open the valve of the air ejector.
Press on the vacuum box in order to seat it on the plate.
Check the pressure on the vacuum box manometer: between 204 m-bar (3 psi) and 340 m-bar (5 psi) Note: the API 620 requirement is a partial vacuum of at least 3 psi.
Observe the detector solution for leakage for at least 10 seconds. If there is any leakage, bubbles will appear through the Plexiglas of the vacuum box.
Repeat again the operation after having displaced the vacuum box on the welds to be examined. Overlap successive settings of the vacuum box should be at least 50 to 100 mm ( 2 inches to 4 inches), to assure complete coverage of the weld inspected (thickness of the sponge rubber gasket of the vacuum box is about 30 to 35 mm)
When testing large areas, such as roof or bottom joints, mark inspected areas to prevent missing an area of the weld.

Acceptance Criteria for Vacuum Test Welding

The presence of continuous bubble formation or growth on the surface being examined indicates leakage through an orifice passage(s) in the area under examination. Any indicated leakage shall be considered unacceptable. Some large leaks may not be detected by bubble formation because the strong stream of air may break the bubble film as soon as it forms. To avoid missing this type of leak, the pressure shall be monitored for a variation (decrease).

Repairs after the vacuum test

All indicated leaks, regardless of size, shall be marked and repaired by relieving the pressure, removing the defective weld, and re-welding using qualified welding procedures, welders, and welding operators, and the repaired area shall be retested by the same steps stated above.

Report: For all leak testing by means of the vacuum box testing method, one report is established by the examiner. This report is reviewed and signed by the constructor quality control supervisor.

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