While referring ASME B31.3 code we come across various types of fluid services. In day-to-day engineering design activities, we normally design and construct systems following the rules of Normal Fluid service. Category M fluid service is different from normal fluid service. As defined in clause 300.2 of ASME B31.3 Category M Fluid Service is a fluid service in which both of the following two conditions apply:
the fluid is so highly toxic (poisonous
or lethal) that a single exposure to a very small quantity of the fluid, caused
by leakage, can produce serious irreversible harm to persons on breathing or
bodily contact, even when prompt restorative measures are taken.
after consideration of piping
design, experience, service conditions, and location, the owner determines that
the requirements for Normal Fluid Service do not sufficiently provide the leak
tightness required to protect personnel from exposure.
Chapter VIII of ASME B31.3 code provides rules for piping designated by the owner as being in Category-M Fluid Service which means It is the owner’s responsibility to select the fluid service category. Selections of Category D or Category M cannot be made without the owner’s permission.
Also note that it is Category M Fluid Service and not Category M fluids, as it is not simply the fluid, but also the conditions of installation that are considered in making the designation. The owner is guided in the classification of the piping system by the definition of Category M Fluid Service in Chapter I of ASME B31.3. Additionally, a guide to the application of these rules is provided in ASME B31.3, Appendix M, which contains a flow chart to assist the owner in classifying fluid services. This Appendix is considered by the Code to provide guidance, not Code requirements.
It is not possible to create a list of Category M fluids, because the conditions of the installation must be considered in making the classification. For a fluid service to be Category M, the potential for personnel exposure must be judged to be significant. If piping is double-contained, for example, it could be judged that even highly toxic fluids (such as methyl isocyanate, phosgene, or nerve gas) do not make the system Category M, because the potential for personnel exposure is not significant. For Category M fluid service, the rules for Normal Fluid Service are not applicable. Instead, additional rules that lead to more costly construction, with provisions designed to enhance piping system tightness, are provided in Chapter VIII.
If higher
integrity piping is desired by the owner, even though the fluid service does
not meet the definition of Category M, the owner can still specify the
additional design, construction, examination, and testing requirements that are
provided in Chapter VIII. Hydrofluoric acid is one example of a fluid for which
many owners specify more stringent requirements than are provided in the Code
for Normal Fluid Service, although it actually would be considered Normal Fluid
Service.
Normally the
following should be applicable for Category M fluid services.
Use of any temperature other
than the fluid temperature as the design temperature shall be substantiated by
heat transfer calculations confirmed by tests or by experimental measurements.
Design, layout, and operation
of piping shall be conducted so as to minimize impact and shock loads.
Suitable dynamic analysis, such as computer simulation, shall be made where necessary to avoid or minimize conditions that lead to detrimental vibration, pulsation, or resonance effects in the piping.
Allowance for Pressure and
Temperature Variations, Metallic Piping is not permitted.
Valves having threaded bonnet joints (other than union joints) shall not be used.
Special consideration shall be given to valve design to prevent stem leakage into the environment.
Bonnet or cover plate closures and body joints shall be flanged and secured by at least four bolts with gasketing.
Single-welded slip-on flanges, expanded-joint flanges, slip-on flanges used as lapped flanges, and threaded metallic flanges, except those employing lens rings or similar gaskets and those used in the lined pipe where the liner extends over the gasket face, shall not be used.
Split backing rings shall not
be used.
Socket welded joints greater
than DN 50 (NPS 2) are not permitted.
Caulked joints shall not be
used.
Soldered, brazed, and braze welded joints shall not be used. adhesive joints and bell-type joints shall not be used.
Relief set pressure shall be in
accordance with ASME BPVC, Section VIII, Division 1.
The maximum relieving pressure
shall be in accordance with Section VIII, Division 1.
Materials of unknown
specification shall not be used.
A piping system designed for Category M service fluid preferably shall not go under severe cyclic conditions
Process Safety Management (PSM) is the management of hazards that can give rise to major accidents involving the release of potentially dangerous materials, the release of energy (such as fire or explosion), or both. It focuses on the prevention of leaks, spills, overpressure, equipment malfunction, excessive temperatures, metal fatigue, corrosion, and other similar conditions by the application of good engineering and design principles. In short, Process Safety Management focuses on controlling the release of highly hazardous substances. All plants handling highly hazardous materials must undergo a sound Process Safety Management program.
Functions of Process Safety Management
The foundation of a process safety management system is to manage risks and reduces them to a tolerable level, either by reducing the likelihood of the hazard release, reducing the consequences if does get released, or both. So, basically, process safety management is a blending of management and engineering skills to prevent catastrophic accidents. The main functions of process safety management are:
the proactive and systematic identification of unsafe releases
evaluation and mitigation or prevention of chemical releases
A sound Process Safety Management will ensure minimal risk from:
Fire
Explosion
Poisoning, and
Suffocation
Written procedures for managing those hazardous releases must be developed and implemented from the design stage itself to ensure proper process safety management.
The US Occupational Safety and Health Administration (OSHA) has issued a Process Safety Management standard “29 CFR 1910.119” for helping chemical companies to identify highly hazardous chemicals and take precautionary action for ensuring a safe and healthy workplace.
What is Process Safety?
Process Safety is defined as “a discipline that focuses on the prevention of fires, explosions, and accidental chemical releases at chemical process facilities”. So it is a disciplined framework for managing the integrity of operating systems and processes that handle hazardous substances. It relies on good design principles, engineering, and operating and maintenance practices. It deals with the prevention and control of events that have the potential to release hazardous materials and energy. Such events don’t only happen at chemical facilities, they occur in refineries, offshore drilling facilities, petrochemical plants, solids handling facilities, water treatment plants, ammonia refrigeration plants, off-shore operations, etc.
Why is Process Safety important?
Process safety hazards can give rise to major accidents involving the release of potentially dangerous materials, the release of energy (such as fires and explosions), or both. Process safety incidents can have catastrophic effects and can result in multiple injuries and fatalities, as well as substantial economic, property, and environmental damage.
Process safety refinery incidents can affect workers inside the refinery and members of the public who reside nearby. Process safety in a refinery involves the prevention of leaks, spills, equipment malfunctions, over-pressures, excessive temperatures, corrosion, metal fatigue, and other similar conditions. Process safety programs focus on the design and engineering of facilities, hazard assessments, management of change, inspection, testing, and maintenance of equipment, effective alarms, effective process control, procedures, training of personnel, and human factors.
Several Major incidents in both the upstream and downstream industries have highlighted the importance of having robust processes and systems in place. A major incident is typically initiated by a hazardous release; it may also result from a structural failure or loss of stability that escalates to become a major incident. For the oil and gas industry, the emphasis of process safety and asset integrity is to prevent unplanned releases which could result in a major incident.
We cannot afford to rely solely on lessons from major process incidents, which happen relatively infrequently. To strengthen safety barriers and prevent these incidents from occurring at all, it is necessary to collect, collate and analyze data from less severe incidents and shortfalls in management system performance. Process safety is required as Failures of process safety management systems are deadly and costly. It impacts people, communities, assets, and the environment.
What is Process Safety Engineering?
Process safety
in Design Engineering means
Prevention and mitigation of incidents
Identifying Safety-critical equipment and linking them to performance standards
Compliance with applicable standards
Properly design, procure, build, install and test
Hand over safe facilities
Engineering process safety emphasizes existing good practices to reduce unsafe acts and conditions. It guides daily actions one can take to prevent these incidents from happening again.
The following image briefly explains the bare minimum steps to follow for process safety in engineering.
Elements of Process Safety Management
To manage chemical operations involving hazardous materials; Process Safety Management must be implemented. The main elements of Process Safety Management are
Technology,
Design and Engineering Facilities
Hazard Assessment
Management of Change
Regular Inspection and Testing
Equipment Maintenance
Effective Alarm Management
Implementation of process control,
Proper operating procedures
Maintaining all regulatory requirements
and Human factors.
Reasons for Process safety management
There are numerous reasons to implement process safety management in plants. The main reasons are:
Prevention of major accidents and potential impact of the same.
Managing the risks inside the plant.
increasing sustained value and boosting productivity.
produce a high-quality product at less cost.
Increasing profit.
Keeping the environment safe.
increase shareholder value.
So the main role of process safety management is to keep the hazard under control inside the equipment or pipes that are designed and maintained to handle them safely. It requires a commitment from design, engineering, operation, and maintenance personnel to follow all the roles without deviation.
Fundamental Questions for Safety Management Process
To reduce or manage the process safety risk inside a plant one must ask the following four fundamental questions:
What can go wrong?
Are proper controls in place if a major incident occurs?
What does each control deliver in terms of a “safety outcome”?
Is it confirmed that the controls will work as intended?
Process Safety Management Process
A sound process safety management can be achieved by ensuring the following:
Design Integrity
Technical Integrity, and
Operating Integrity
Design Integrity:
Design integrity ensures the integrity of all activities prior to commissioning. This leads to
compliance with the design and engineering standards
robust assurance process during the EPC phase.
design HSE cases
to identify the maintenance routine for Safety-Critical Equipment.
written Design and Operating philosophy
with clear agreement on maintenance and inspection strategies.
Technical Integrity:
It means the safeguarding of assets through effective maintenance, inspection, repair, and assurance that can be achieved through
In the piping and plumbing industry, a piping nipple is a pipe fitting, consisting of a short straight piece of pipe, usually provided with male pipe thread at both ends, for connecting two other female threaded fittings or pipes. It is one of the most popular categories of pipe fittings. Pipe Nipples are commonly used as adapters from one connection type to another.
Pipe nipples are mainly used in low-pressure piping systems. They are used to fit straight-end hoses or pipes. The working pressure of pipe nipples will vary with the size and construction of the pipe, temperature, and product being conveyed. Pipe nipples are available in the widest variety of wall thicknesses and materials in the industry.
Pipe nipples are fabricated by cutting a specified length of pipe and applying the desired end connections. The pipe nipple dimensions and material follow pipe specifications based on ASME code B16.11. Pipe nipples come in seamless, threaded, grooved, barbed, bent, or welded construction.
Applications of pipe nipples
Pipe nipples as pipe fittings in a wide variety of industrial applications like:
Oil & Gas
Chemical processing industries
Petrochemical
Pharmaceutical
Food & beverage
Pulp & paper
Shipbuilding/marine
Waste incineration
Machine building
Architectural
Semiconductor etc.
Pipe Nipple Ordering Information
When ordering pipe nipples, the following properties must be specified:
Technical Specifications like Pressures, Temperatures, Safety coefficients, Conformity to the standards, Certifications, Standard, Double duty, Extra duty, Extra double duty, etc.
The length of the pipe nipple is usually specified by the overall length including threads. Pipe Nipples can come in any specified length, but most commonly range between close to 12”. A close pipe nipple is the shortest piece of pipe necessary to allow for fully threaded end connections, where there is no smooth surface between threads. Threads used on nipples are BSP, BSPT, NPT, NPSM, and Metric.
Types of Pipe Nipples
There are
several different types of pipe nipples in common use. A short list includes:
Barrel Nipple
Close Nipple / Running Nipple
Hexagonal Nipple
Reducing Nipple / Unequal
Nipple
Hose Nipple
Welding Nipple
Swage Nipple
Fig. 1: Various types of Piping Nipples
Swage nipple
The basic purpose of a swage nipple is to bring the flow of fluids from one pipe size to another. These are available with plain, beveled, or threaded ends.
Close Nipple / Running Nipple
In its most basic form, a pipe
nipple is a short length of pipe with male pipe threads at both ends for
connecting other fittings. Generally, there is a short distance of unthreaded
pipe between the two threaded ends, depending on how far apart you need the
attached fittings to be. When there is no unthreaded pipe between the two
connecting ends, the pipe nipple is called a “close nipple” or a “running
nipple”. In that case, connected fittings come close to touching one another
and very little of the nipple can be seen.
Close nipples are difficult to
work with. A close nipple can only be unscrewed by gripping one threaded end
with a pipe wrench which will damage the threads and necessitate replacing the
nipple, or by using a specialty tool known as a nipple wrench which grips the
inside of the pipe, leaving the threads undamaged.
Hexagonal Nipple
In pipe nipples where there is a little space between both the threaded ends, there may have a hexagonal section in the center for a wrench to grasp the nipple. These nipples are called “hexagonal nipples”. This hexagonal section in the middle functions like a nut that can be gripped by a normal wrench, providing a greater mechanical advantage than a normally rounded pipe nipple. A hexagonal nipple with more distance between the threaded ends is called a “long hex nipple”.
Reducing Nipple / Unequal Nipple
For ping systems that require a change in pipe dimension, “reducing nipple” or “unequal nipple” is used. Reducing nipple takes a female fitting with a larger connection and attaches it to a smaller one. Care should be taken when using these parts since a reduction in pipe diameter can mean more pressure and a greater flow rate in the smaller pipe/fitting.
Hose Nipple
For piping systems that require pipe connection to tubing, a “hose nipple” is used. A hose nipple features a male threaded connection on one end and a hose barb on the other end. The hose barb may be the same size as the pipe connection or it may be of reduced size.
Welding Nipple
For piping systems that require to be connected to welded pipes or fittings, a “welding nipple” is used. The welding nipple has a threaded connection on one end and a normal cut pipe at the other end. The unthreaded end of the pipe provides more surface area for the use of welding materials to make a stronger connection. One main benefit of welding nipples is that once the unthreaded end is connected, connecting pipes or other fittings to the threaded end becomes much easier.
Pipe Nipple End Connections
The end connections need to be specified by the customer as well;
Plain Both Ends (PBE), Threaded Both Ends (TBE), Bevelled Both Ends (BBE), Threaded One End (TOE), Bevelled One End (BOE), Plain One End (POE), or a combination thereof, depending on the ends of the piping system into which the pipe nipple will be fitted.
A Plain Both Ends (PBE) pipe nipple, has both ends as plain ends, with no thread, typically used to fit a socket weld connection.
A Threaded Both Ends (TBE) pipe nipple has both ends as threaded ends and is used to fit female threaded connections.
A Bevelled Both Ends (BBE) pipe nipple has both ends as bevel ends and is used for welding purposes like a buttweld fitting.
A Heat exchanger is a device to transfer heat from one fluid (Liquid/Gas) to another. There are various types of heat exchangers used in process piping. Shell and Tube heat exchanger is the most common type of heat exchanger. This article will provide you with a guide to everything you need to know about shell and tube heat exchangers. The major topics covered in this article are:
Definition of Shell and Tube Heat Exchanger
Working Principle of Shell and Tube Heat Exchanger
Basic Components and Parts
Types of Shell and Tube Heat Exchanger
Codes and Standards for Shell and Tube Heat Exchangers
Design of Shell and Tube Heat Exchangers
And much more.
What is a Shell and Tube Heat Exchanger?
Shell and tube heat exchanger (STHE) is the most widely used heat exchanger and is among the most effective means of heat exchange. A shell and tube heat exchanger is a device where two working fluids exchange heat by thermal contact using tubes housed within a cylindrical shell. The fluid temperature inside the shell and tube are different and this temperature difference is the driving force for temperature exchange. Used for wide temperature and pressure ranges, Shell and tube heat exchangers are compact in design, easy to construct and maintain, and provide excellent heat exchange.
As the name specified, it consists of a shell and a number of tubes. The shell is the housing of the exchanger and tubes are mounted inside the cylindrical shell.
Working Principle of Shell and Tube Heat Exchanger
The working of a shell and tube heat exchanger is fairly simple. One fluid flows inside the tubes and the other through the shell. While flowing they exchange heat which means the cold fluid gains the heat from the hot fluid. So one cold fluid enters the shell (or tube side or channel side) inlet nozzle and comes out of the outlet nozzle as hot fluid. Obviously, the other fluid will become colder in the outlet than in the inlet. The heat transfer in a shell and tube heat exchanger is determined by the exposed surface area and is decided by the number of thermally conductive metal tubes. The fluid flow inside the shell and tube heat exchanger can be parallel flow or crossflow.
Fig 1 shows the typical working principle of a shell and tube heat exchanger.
Fig. 1: Working principle of a Typical Shell and Tube Heat Exchanger
The above figure shows both the inlet and outlet nozzle in the front header of the channel side. That means this exchanger consists of an even number of tube passes. However, there can be an odd number of tube passes. In that situation, the channel side outlet nozzle will be on the read header. Increasing the number of tube passes increases the heat transfer coefficient.
To increase the fluid turbulence in the tube and shell side flow, turbulators and baffles are installed inside tubes and shells respectively. This increases the heat transfer between the fluids.
Basic Components of Shell and Tube Heat Exchanger
Typically a Shell and Tube Heat Exchanger consists of two-compartment / sections; one is shell side and the other is channel/tube side
The shell side section consists of the following components: Shell, Cover, Body Flange, Nozzles, and Saddle support.
Channel / Tube side section consists of the following components: Channel, Cover, Body Flange, Nozzles, Tube Sheet, and Tubes (Tube Bundle)
Fig. 2: Components of a Shell and Tube Heat Exchanger
The heat exchanger is supported by saddles in the shell part.
Tube Bundle of Shell and Tube Heat Exchanger
Tube Bundle (Fig. 3) consists of the following components
Tube sheet
Tubes
Baffles
Tie rods and Spacers
Sliding strips
Tube bundles are removed during maintenance. Standard practice is to flow the corrosive fluid inside the tubes so that if corroded they can be easily replaced or repaired. Fig. 3 below shows a typical tube bundle.
Fig. 3: Typical tube bundle of a shell and tube heat exchanger
Tube Pattern inside Shell & Tube Heat exchanger
Normally tubes inside the exchanger are 0.5″ to 2″ in size and arranged in triangular or square patterns as shown in Fig. 4
Fig. 4: Typical tube patterns
Tube Pitch
The tube shall be placed with a minimum center-to-center distance of 1.25 times the tube outside diameter of the tube. When mechanical cleaning of the tube is specified then a minimum cleaning lane of 6.4 mm shall be provided.
Baffles
Baffles are installed in the shell of the shell and tube heat exchanger to create more turbulence and increase the flow time so that better heat exchange is possible. Baffles support the tubes so that damage and vibration of tubes are minimized.
Types of Shell and Tube Heat Exchangers
TEMA shell and tube heat exchanger types based on application
Class R Exchangers – Refinery and Petrochemical Application
Class C Exchangers – General Process Application
Class B Exchangers- Chemical Process Application
TEMA Shell and Tube Heat Exchanger Applicable Criteria
Inside diameter less than 2540 mm (100 inches)
Product of nominal diameter (mm) and design pressure (kPa) of 17.5 x 106
Design pressure of 3000 psig (20684 KPa)
The reason behind such limitation is to keep the maximum shell wall thickness below 3 in. (76 mm), and the maximum stud diameter below 4 in. (102 mm).
Shell and Tube heat exchanger types based on construction
Depending on various construction and configuration parameters following types of shell and tube heat exchangers are widely used in industries.
Fixed Tube Sheet Heat Exchanger
The tube sheet is fixed in the shell by welding and hence the term fixed tube sheet exchanger applies. This simple and economical construction allows the cleaning of the tube bores by mechanical or chemical means. An expansion bellow is installed in the shell when there is a large temperature difference between the shell and tube materials. Refer to Fig. 5 for an example of the fixed tube heat exchanger.
Fig. 5: Example of a typical fixed tube heat exchanger
Floating Head Heat Exchanger
In floating head construction, the rear header can float or move as it is not welded to the shell. The tube bundle can easily be removed during maintenance. Fig. 6 shows an example of a floating head heat exchanger.
Fig. 6: Example of a typical Floating Head Removable bundle heat exchanger
Stationary Tube sheet with removable tube bundle
Fig. 7 shows an example of a stationary tube sheet with a removable tube bundle.
Fig. 7: Example of a typical stationary tube-sheet type heat exchanger
U-tube Heat exchanger
U-tube exchangers are a type of shell and tube heat exchanger whose tube bundle is made of continuous tubes bent into a “U” shape. The bend side is free-floating and this helps in thermal expansion without requiring expansion joints. However, such bends are difficult to clean.
Fig. 8: Typical representation of U-tube Heat exchanger
Based on the number of times the tube-side/shell-side flows pass through the exchanger, the shell and tube heat exchanger is categorized as:
Single-Pass exchangers and
Multi-Pass exchangers
The full TEMA classification of shell and tube heat exchanger types is provided in Fig. 9 below:
Fig. 9: TEMA Shell and Tube Heat Exchanger types
Depending on the application of shell and tube heat exchangers, they are known as various types as listed below:
In this, two or three heat exchangers are placed one above the other. This is termed as 1 shell in parallel and 2 or 3 Shells in series. Refer to Fig. 10.
Fig. 10: Figure showing stack arrangement of heat exchangers
Codes and Standards for Shell and Tube Heat Exchangers
The following codes and standards govern the design of shell and tube heat exchangers.
API 660 ( Shell and Tube heat exchangers for general refinery service)
ISO 16812
ASME SECT.VIII Div.1 (UHX) or Div.2
PD 5500
EN 13445
AD 2000 Merkblatt.
TEMA -Tubular Exchanger Manufacturers Association
Shell DEP 31.22.20.31 and DEP 31.21.01.30
Design of Shell and Tube Heat Exchangers
Process Design of Shell and tube Heat exchanger
The design of the Shell and tube heat exchanger is a trial-and-error iterative process. In recent times, thermal design has been carried out by the process team using engineering software. However, the logic behind the calculations should be clearly understood. The shell and tube heat exchanger design calculations are based on the initial selection of a preliminary exchanger configuration and certain initial decisions like
the front and rear header type,
shell type,
the sides the fluids are allocated,
baffle type and baffle pitch
tube diameter, length, and tube layout
shell diameter, and
number of tube passes
Further steps for the shell and tube heat exchanger design consist of
Calculation of shell side flow distribution and heat transfer coefficient
Estimation of tube side heat transfer coefficient and pressure drop
Determination of wall resistance and overall heat transfer coefficient
Calculation of Mean temperature difference (log mean temperature difference) from the inlet and outlet temperatures of the two fluids.
Estimation of the required heat transfer area
Comparison of the calculated area with the assumed geometry
Comparison of shell and tube-side pressure drop with allowable pressure drop
If the pressure drop is within the allowable pressure drop design is acceptable. Otherwise, adjust the assumed geometry and repeat the above steps.
Once the requirements are met, a process datasheet is developed indicating all process design parameters of shell and tube heat exchanger design.
General design Considerations for shell and tube heat exchanger
Fluid Allocation: Shell Side vs. Tube Side
The following table (Table 1) provides general guidelines for shell and tube side fluid allocation in a shell and tube heat exchanger:
Fluid Parameters
Fluid Allocation-Shell Side
Fluid Allocation-Tube Side
High-Pressure Fluid Stream
X
Corrosive Fluid
X
High-fouling fluid stream
X
More Viscous fluid
X
Lower Flow Rate Fluid
X
Fluid with a low heat transfer coefficient
X
Toxic Fluid
X
Table 1: Shell and Tube-side Fluid Allocation
Fluid Velocity inside Shell and Tube
High fluid velocities increase heat transfer coefficients and reduce fouling but cause erosion and increase pressure drop. So velocity selected should be just enough to prevent the settling of suspended solids. Typical fluid velocities considered for the design of shell and tube heat exchangers are given in the following table (Table 2):
Fluid Types
Fluid Velocity-Shell Side
Fluid Velocity-Tube Side
Liquid
0.3 to 1 m/s
1 to 2 m/s
Gas /Vapor (Vacuum Pressure)
50 to 70 m/s
50 to 70 m/s
Gas /Vapor (Atmospheric Pressure)
10 to 30 m/s
10 to 30 m/s
Gas /Vapor (High Pressure)
5 to 10 m/s
5 to 10 m/s
Table 2: Typical fluid Velocities in Shell and Tube Heat Exchanger Design
Pressure Drop Consideration
Typical pressure drop values considered for shell and tube heat exchanger design are:
For liquids with Viscosity=1 to 10 mN- s/m2, ΔP= 50-70 kPa
Liquids without phase change= 70 kPa
Condensing streams= 14 kPa
For Vapor and gas services:
High vacuum Pressure: 0.4-0.8 kPa
Medium vacuum Pressure: 0.1 x absolute pressure
Pressure 1 to 2 bar: 0.5 x system gauge pressure
Pressure above 10 bar: 0.1 x system gauge pressure
Vapors without phase change= 14 kPa
Boiling Streams = 7 kPa
Software used for Thermal Design
The most popular software used for the thermal design of shell and tube heat exchanger are listed below
HTRI – Heat Transfer Research Institute
HTFS – Heat Transfer Research and fluid flow service
Mechanical design of Shell and Tube Heat exchanger
Mechanical design of shell and tube heat exchangers consists of calculation of shell thickness, flange thickness, etc. Various codes like ASME Sec VIII, PD 5500, TEMA, etc. provide guidelines for mechanical design. The following design guidelines can be followed:
Minimum Shell Thickness (Fig. 11) as per TEMA for Class – R
Fig. 11: Minimum Shell thickness of shell and tube heat exchanger
Baffle clearance, Baffle spacing, and thickness as per TEMA table RCB -4.3
Tie rod size and nos. as per TEMA table R- 4.71 for class – R
Peripheral Gasket: The minimum width of the peripheral ring gasket for external joints shall be 10 mm for shell sizes up to 584 mm and 12 mm for all larger shell sizes.
Pass Partition Gasket: The min. width of the gasket web for the pass partition of the channel shall not be less than 6.4 mm for shell sizes up to 584 mm and 9.5 mm for all larger shell sizes. The gasket joint shall be confined type
Shell and Head design is done as per selected Pressure Vessels Design Code such as ASME, EN, or AD
The most widely used design code across the world is ASME Sect. VIII Div.1 & 2
Body / Girth Flange Design as per Appendix -2 of ASME Sect. VIII Div.1
The tube sheet design is Mandatory as per UHX of ASME Sect. VIII Div.1
The tube sheet is designed for the following three cases.
Tube side pressure (Pt) acting and Shell side pressure (Ps) is Zero
Shell side pressure (Ps) acting and Tube side pressure (Pt) is Zero
Shell side pressure (Ps) acting and Tube side pressure (Pt) acting
Please consider the effect of Vacuum in the above load cases
Tube sheet Design formula based on the theory of Flat Plates
Shell and Tube Heat Exchanger Material of Construction (MOC)
The following materials are the most common as Shell and Tube Heat Exchanger MOC.
Carbon steel and Cladding Plates
Stainless Steel
Duplex Stainless steel
Tubes – Carbon steel, Stainless steel, Duplex stainless steel, Exotic material such as copper, Inconel, Titanium
Maintenance of Shell and Tube Heat exchangers
Depending on user experience and manufacturer guidelines, shell and tube heat exchangers should be inspected at regular intervals. A shell and tube heat exchanger can fail by one or more of the following factors:
Improper design.
Excessive fouling.
Air or gas binding resulting from improper piping installation or lack of suitable vents.
Excessive clearances between the baffles and shell and/or tubes, due to corrosion.
Operating conditions differ from design conditions.
Maldistribution of flow in the unit, etc
Following preventive maintenance steps at regular intervals can reduce the risk of equipment failure. Following maintenance steps can be followed to enhance shell and tube heat exchanger performance:
Cleaning periodically to avoid fouling
Inspection of tubes
Gasket replacement
Repairing leaks if detected during inspection.
Application of Shell and Tube Heat Exchangers
Shell & Tube Heat Exchangers find their application in the following Industries-
Refinery and Petrochemical
Fertilizer
Oil and Gas
Chemical
Power Plants
Advantages of Shell and Tube Heat Exchanger
Shell and tube heat exchangers are widely used in various industrial processes due to their numerous advantages. These advantages make them a preferred choice for many applications where efficient heat transfer is essential. Here are some of the key advantages of shell and tube heat exchangers:
High Heat Transfer Efficiency: Shell and tube heat exchangers are known for their excellent heat transfer capabilities. The design, which features a large surface area for heat exchange, allows for the efficient transfer of thermal energy between the fluids. This results in rapid heating or cooling, making them highly efficient in heat transfer applications.
Versatility: Shell and tube heat exchangers are versatile and can handle a wide range of fluids, including gases and liquids. They can be used for both heating and cooling processes and are compatible with various industries, including chemical processing, power generation, food and beverage, and HVAC systems. They are available in a range of configurations to suit various operations.
Robust and Durable Construction: These heat exchangers are typically built with materials like stainless steel, copper, or titanium, which offer excellent resistance to corrosion and wear. This durability ensures a long operational lifespan, reducing maintenance and replacement costs.
Ease of Maintenance: Shell and tube heat exchangers are relatively easy to maintain. The tube bundle can be accessed and cleaned or replaced without the need to disassemble the entire unit. This feature minimizes downtime and reduces maintenance costs.
Compact Design: Despite their high efficiency, shell and tube heat exchangers have a compact design, which is particularly beneficial in applications with limited space. Their compactness allows for efficient heat transfer within a relatively small footprint.
High Pressure and Temperature Capabilities: Shell and tube heat exchangers are capable of handling high-pressure and high-temperature applications. This makes them suitable for use in industries such as oil refining and chemical processing, where extreme conditions are common.
Customizable Configurations: These heat exchangers can be customized to meet specific requirements. Engineers can adjust the number of tubes, tube diameter, tube arrangement, and other design parameters to optimize performance for a particular application.
Resistance to Fouling: The design of shell and tube heat exchangers often includes baffles and turbulence-inducing features, which help reduce fouling by preventing the buildup of deposits on the tube surfaces. This resistance to fouling ensures consistent heat transfer efficiency over time.
Long Service Life: When properly maintained, shell and tube heat exchangers can have a significantly long service life, making them a cost-effective investment for industries that rely on continuous heat exchange operations.
Energy Efficiency: Their high heat transfer efficiency translates into energy savings. Shell and tube heat exchangers can help reduce energy consumption in processes that require heating or cooling, contributing to overall operational cost reductions.
Disadvantages of Shell and Tube Heat Exchanger
While shell and tube heat exchangers offer numerous advantages, they also have some disadvantages and limitations. It’s essential to consider these drawbacks when selecting a heat exchanger for a specific application. Here are some of the disadvantages of shell and tube heat exchangers:
High Initial Cost
Large Footprint
Limited Heat Transfer for Low Temperature Differences
Control valves are control devices that are used to manage and control fluid flow, pressure, temperature, or liquid level by varying the flow passage size. In process systems of production wells, oil and gas plants, Chemical and Petrochemical industries, refineries, and power plants, Control valves are frequently used to control or manage any of the process parameters.
The piping systems of industrial, commercial, residential, and other civic facilities carry the lifeblood of modern civilization, like arteries and veins. And the valves in those piping systems serve the functions of allowing, stopping, regulating, and controlling the flow, to fulfill the intended objectives of the system.
Valves are an essential part of any piping system that conveys liquids, gases, vapors, slurries, and mixtures of liquid and gaseous phases of various flow media.
Some valves are self-actuated while others are manually operated or have actuators that are powered with electric motors, pneumatic or hydraulic, or a combination to operate the valve. Valves are manufactured with metals and non-metals.
What is a Control Valve?
The control valve is an automated valve that can make precise adjustments to regulate and monitor any commodity flowing through a piping system. The function of a control valve is to provide throttling control in response to signals from a control system, using an actuator and a positioner. They are considered the ‘‘final control element’’ in an automated and usually very sophisticated ‘‘control loop.’’
A control valve receives the information from various sensors and transmitters in a control loop and processes that data to manage the control of the fluid parameter. The following figure shows a typical control loop for a control valve
Typical Control Loop of a Control Valve
Components of Control Valve
Control valves have basically three interactive components:
a valve body subassembly (either with a reciprocating or rotating stem),
an actuating device (usually a spring diaphragm type),
a valve positioner (an instrument that converts an electronic control signal from a controller, or computer, into an air signal to control the position of the control valve stem), and
an air set or regulator to supply air pressure to the positioner.
Parts of a Typical Control Valve
Control Valve Features
The most common valve body style used as a control valve is the globe valve. Although many other body styles such as angle valves, Three-way valves, Eccentric rotary plug valves, semispherical ball valves, Ball Valves, Butterfly valves, etc are used, the globe valve provides the most effective means to regulate and control flow.
Control valves use signals received from instruments positioned throughout the piping system to automatically make adjustments that regulate the commodity within the pipe. Though control valves can perform many functions, they are typically used to control the flow of a commodity within a pipe or to limit its pressure.
Control valves must be arranged within a run of the pipe so that they can be easily operated. To achieve this, control valve manifolds are configured. Control valve manifolds make control valves readily accessible to plant workers.
Control Valve Actuators
The actuator of a control valve is the assembly that provides power for moving the control valve mechanisms. In a control valve loop, actuators move the plug, ball, or vane upon receipt of a signal from the control system to allow or disallow full or partial flow. There are three types of actuators in a Control Valve. They are
Pneumatic Actuators
Electric Actuators and
Hydraulic Actuators
Pneumatic Actuators:
Pneumatic Actuators are the most basic and widely used control valve actuators that use an air or gas signal from an external source to produce a modulating control action. The top port sends the pneumatic signal to the actuator that exerts pressure on the diaphragm plate to move the valve stem. On loss of driver power, pneumatic actuators provide a fail-safe response.
Electric Actuators:
Electric Actuators of a control valve are motor-driven devices. A motor rotates when an electrical signal is received. A gear reduction drive converts this rotating motion into a linear motion to drive the control valve stem for flow modulation. They are used for On-OFF applications in isolation services and for continuous positioning control.
Hydraulic Actuators:
Control Valve Hydraulic actuators use hydraulic oil as the signal fluid. When the force required to move the valve stem is high, hydraulic actuators are used. Due to the non-compressibility of the liquid they exhibit stable positioning.
Specifying Control Valve
The first step in specifying a control valve is to define its function in the given application. In some, it will operate as an on-off valve that opens or closes following the commands of a programmable controller on, say, a batch process. In others, it will be used to remotely set a flow rate in a process—that is, it will be used as a manually controlled variable orifice in a pipe (an open-loop application).
Finally, in more sophisticated applications, the control valve will serve as the final control element in a process control loop and respond to the sometimes infinitely small variations of a signal coming from a controller (typically a computer). The signal will be generated in response to a deviation in the desired temperature, pressure, or level of a process fluid as measured by a transmitter.
More than 90 percent of all control valves use pneumatic actuating devices—either spring-opposed diaphragm types or piston-actuated.
Materials for Control Valves
For noncorrosive use, the material of choice is carbon steel (ASTM A216 Grade WCB, if cast; and A105 when forged). For mild, corrosive applications, valve housings are made from type CF8M (316 stainless steel). However, Teflon-lined housings and exotic alloys, such as Hastelloy, Monel, or Titanium are available for highly corrosive fluids.
Characteristics of different valves to serve as Control Valve
Control Valves play a major role in the everyday effort to increase process plant profitability and conserve energy. Proper selection of these valves can have a significant financial impact on the overall cost of a project and how well the process can be controlled. To narrow down the choices, the engineer must understand how the general characteristics of each type of valve match up with the design requirements. Refer to table 1 for the general characteristics of a few valve types.
General Features of Each type of Valve
Control Valve Characteristics
Each control valve has a flow characteristic, which describes the relationship between the flow rate and the control valve travel. As a valve opens, the flow characteristic, which is inherent to the design of the selected valve, allows a certain amount of flow through the valve at a particular percentage of the stroke. This enables flow regulation through the control valve in a predictable manner. The three most common types of control valve flow characteristics are:
Linear
Equal percentage
Quick opening
Control Valve Selection
There are two major parameters on which the control valve selection is dependent-
Service condition and
Load characteristics
Other parameters that determine the control valve selection are
Ability to control the flow rate;
Lack of turbulence or resistance to flow when fully open – turbulence reduces head pressure;
Quick opening and closing mechanism – rapid response is many times needed in an emergency or for safety;
Tight shut-off – prevents leaks against high pressure;
Ability to allow flow in one direction only – prevents return;
Opening at a pre-set pressure – procedure control to prevent equipment damage; and
Ability to handle abrasive fluids – hardened material prevents rapid wear.
Control Valve Types
Depending on various parameters control valves are classified as follows:
Types of control Valve
Pressure Control Valve
Pressure Control valves help in controlling system pressure within a limit. They are installed in all systems where pressure fluctuations can occur and can cause a hazard if not controlled. Pressure control valves are basically closed valves with a restriction for pressure control. There are two mechanisms on which pressure control valves operate:
The symbols used for different types of control valves in P&ID are as given below:
Control Valve Symbols in P&ID
How to install a Control Valve?
Control valves are normally installed in a horizontal orientation in a straight pipe run preferably away from elbows. The actuator is kept in a vertical position. A bypass valve is installed for continuous operation during control valve inspection and maintenance. Upstream and downstream isolation valves and drains are added to the piping system to isolate the control valve for maintenance. The following figure shows a typical control valve installed in a piping system.
The sealing quality of a metal seated control valve is very important. Normally, The higher the pressure class of the valve, the lower the allowable leakage rate in a closed position.
Control Valve Assembly
The control valve assembly is used for controlling process parameters in piping systems. The following figures show typical different arrangements of control valve assemblies
Control Valve Assembly Arrangement
Various types of control Valve Assembly Arrangements
Example of Control Valve Assembly
The following videos will explain more details about control valves and their working methodology:
Control Valve Basics
The following video briefly describes the basics of Industrial Control Valves.
Flow control valves
Flow control valves are devices that manage the rate of fluid flow in part of a hydraulic circuit. The following video tutorial explains the basics of flow control valves
A Material Take Off (MTO) is a process to study a drawing and find out all the materials required to construct the drawing in physical form. In each individual drawing, specific items are listed along with approximate quantities. Those materials are then tabulated in excel or in other company-specific standard forms as per size and quantity involved to make a comprehensive material listing which is sent to the procurement department for purchasing or to the construction team for fabrication. Material Take Off or MTO is a very important step in materials management for procurement and construction.
Depending on the project size and complexity, the materials list or material take-off can be long or short. Without proper management of these materials, unnecessary delay and expense in project execution can happen.
Major Steps in Piping Material Management
The major steps involved in Piping Material Management are:
Material Take Off (MTO) of piping items or piping material take-off is a detailed listing of piping components required for a given project. It includes:
Commodity Code,
Size,
Quantity and
Purchase description for all the items.
Purpose of Material Take Off
The Material Take Off or MTO is used for the purpose of
Making Proposal,
Material Estimation &
Preparation of Purchase Requisitions.
Material Take Off helps in a rough estimation of the project cost
Initial MTO (also called First MTO) is generated manually at the beginning of the project. Very often, the 3-D model is not available or complete at that stage. Hence, Initial Material Take Off is generated on the basis of P&ID and Project Specifications.
This is required as the complete modeling takes time and we need the MTO at the earliest to initiate procurement activities.
As the 3D modeling progresses, detailed MTO is generated electronically by the 3D modeling software (PDS/PDMS, etc). The Piping Material Specification is generated in electronic format and is linked to the 3-D modeling software for the purpose.
Accuracy and correctness are of prime importance and digital material take-off provides more accurate estimation in less time with respect to manual MTO estimation.
Following input, documents are required for working out the initial (First) MTO.
Piping & Instrumentation Diagram (P&ID)
Line List
Standard/Project PMS.
Project specification & standard drawings.
Equipment Drawings/Data Sheet.
Instrument Hook-up Sketches.
Manual Material Take-Off Preparation
Fig. 1 below shows a flow diagram of how manual material take-off is generated. It is a time taking process and must be thoroughly checked as manual estimation is prone to errors.
Fig. 1: Flow Diagram for Manual MTO Generation.
Steps for Manual MTO Preparation
Manual Material Take Off is generated in the following steps:
Make a folder of all P&IDs as a master copy for 1st Bulk MTO.
All pick-able items are entered in an MTO format excel File, refer next sheet.
All entries must be taken against a specific line number. In case a line number is not marked on P&ID, it has to be assumed by adding suffixes A, B, etc. with the main header line.
Queries shall be raised for components that are not available in PMS. All such queries shall be documented & MTO shall be updated as & when queries are resolved.
All assumptions made are documented & such items shall be marked as Hold. All assumed data is maintained in a separate file & tracked. MTO shall be updated whenever confirmed data is received.
The basis of Material Take Off is a properly issued version of P & ID or documented communication.
Prepare a datasheet having all Piping / Material data of special items.
Send datasheet to process & related department for review & incorporating missing data & comments. All the missing data/information required to be filled by other disciplines shall be highlighted by the material team.
Prepare IFP datasheet, when commented & updated datasheet is received.
Non Pickable Items
Pipe Length & Elbows quantity is generated by 3-D modeling.
Generation of Total MTO
Combine pick-able bulk items and non-pickable items (except piping special items)
Combined data is processed and validated against the piping material specification (PMS).
Check & Rectify errors.
The final BOM (Bill Of Material) is received from the successful run file.
Piping specials are processed separately
Preparation of Material Requisition
Material requisition for purchase includes:
Bill of materials having commodity codes, sizes, material specifications, and quantity
Additional technical requirements, inspection & test guidelines, documentation requirements, and guidelines for bid submission
Receipt of quotations from prospective bidders and Technical Bid Evaluation
Vendor offers are invited for the material requisition
Received offers are evaluated to ensure technical compliance to material and project specifications
TBE report is sent to the procurement department
The purchase order is placed on a technically acceptable vendor after commercial negotiations
Vendor document review/approval
After the purchase order is placed on a particular vendor, the vendor submits all the documents required per material requisition.
These documents are reviewed and commented on to ensure compliance with all the specifications
After the approval of the documents, the actual manufacturing of the material begins.
Inspection and testing of materials are carried out at the vendor shop according to approved ITP/QAP
Material is dispatched to the site after successful inspection and quality checks.
Normally every organization has its own standard format for each of the below-mentioned Material Take Off categories:
