Flange Bolt Torque Calculation and Pipe Flange Bolt Torque Chart

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

Proper bolt loading is essential for ensuring proper sealing of the flanged joint to avoid flange leakage problems. This bolt load for gasket sealing and flange seating is provided by Flange Bolt Torque. So Flange Bolt Torque calculation should be done with utmost care for the proper functioning of the flanged joints. However, there is no direct method of measuring this load on the gasket, but the applied torque on the flanged bolts can be measured and controlled. In this article, we will learn the basics of flange bolt torque calculation and some of the typical torque values in a chart format.

What is Flange Bolt Torque?

Torque is a measure of rotational force applied to a bolt or screw, and it plays a vital role in achieving the correct clamping force on flanged connections. Proper torque ensures that the flange bolts are neither too tight nor too loose, which is essential for preventing issues such as leaks, flange separation, and joint failure.

When a flange is bolted together, the torque applied to the bolts compresses the gasket or sealing material between the flanges, creating a tight seal. This clamping force must be carefully controlled to maintain the integrity of the seal and ensure the system functions correctly under operating conditions.

Flange Bolt Torque Calculation Formula

Bolt Torque is the twisting or turning force applied to tighten the nut on a bolt. Using a calibrated torque wrench (Manual or Hydraulic Torque Wrench), flange bolt torque can be measured during flange assembly. This torque creates an axial force in the bolt. More torque is applied the nut stretches the bolt more and the load on the gasket increases. Bolt torque is calculated for a flanged assembly using the following equation.

Applied Torque, T= (k∙f∙d)/12 in FPS Unit

Where:

T=Torque in ft-lb
k=Dimensionless nut factor or tightening factor
f=axial force in pounds
d=Nominal bolt diameter in inches

In the Metric System,

Torque Applied, T = (k.d.f)/1000

where

T = Torque in N-m
f = Bolt load in N
d = Bolt diameter in mm
k=Dimensionless nut factor or tightening factor

Nut Factor on Flange Bolt Torque Calculation

The nut factor or tightening factor (k) is a “modified” friction factor. It is an empirically derived correlation factor that includes the impact of friction. The nut factor depends on various factors including the following:

Geometric factor – shape or type of threads
The friction of the nut against the bearing surface of the flange
Friction between threads of nuts and bolts
Bolt diameter
Bolt material
Assembly temperature, etc.

Because of so many factors, the applied torque between the two fasteners always varies between 20-30%. A small change in the nut factor/tightening factor results in large changes in the gasket load. For same torque values with a 0.1 nut factor would produce twice the axial force as a 0.2 nut factor. That’s why well-lubricated bolts, nuts, and washers are always preferred.

Factors Affecting the Applied Torque

The required torque in a flanged joint is dependent on various factors:

Nut and bolt size, class, and material
Burr of the nuts
Lubrication
Dust, chips, and dirt on bolts and nuts
Notches
State of the flange surface on which to rotate
Gasket material and thickness

Pipe Flange Bolt Torque Chart

Even though flange blot toque calculation is possible, they are normally selected from the pipe flange bolt torque chart. The entire bolt pattern shall be tightened at least three times around the flange at 30%, 70%, and 100% of the torque value. For bolt diameters greater than 1.25”, Hydraulic Tensioning is recommended to achieve more uniform gasket stress. The following images provide some typical Pipe flange bolt torque charts.

**Fig. 2: Typical pipe flange bolt torque chart for Spiral Wound Gaskets**

**Fig. 3: Typical pipe flange bolt torque chart for Spiral Wound Gaskets with Inner and Outer Rings**

**Fig. 4: Typical Torque values for Line Seal gaskets**

Typical toque values for API 6A Wellhead Connection Flanges — **Fig. 5: Typical torque values for API 6A Wellhead Connection Flanges**

**Fig. 6: Typical Torque Values for Metallic Lined Pipes**

**Fig. 7: Typical Torque Values for HDPE Flange Adapters**

Flange Bolt Torque Sequence

The first step in flange bolt torquing is to examine the flange alignment and inspect the nut, gasket, stud, or bolts. Here’s a simplified version of the bolt-torquing sequence:

Finish All Pre-Checks: Before you start tightening, make sure everything is checked.
Tighten Bolts in a Criss-Cross Pattern: Follow the criss-cross pattern for tightening and make sure to use three passes with the final torque value.
Pass 1: Tighten bolts to 30% of the final torque. Check that the gasket is compressing evenly.
Pass 2: Tighten bolts to 60% of the final torque.
Pass 3: Tighten bolts to the full final torque (100%).
Final Check: After the three passes, go over the bolts again using the final torque in a criss-cross pattern until the nuts don’t turn anymore.

The following images (Fig. 8 to Fig. 10) show the torquing sequence with respect to the number of bolts:

**Fig. 8: Bolt Torquing Sequence for 4-Bolt and 8-Bolt Flanges**

**Fig. 9: Bolt Torquing Sequence for 12-Bolt and 16-Bolt Flanges**

Bolt Torquing Sequence for 20-Bolt and 24-Bolt Flanges — **Fig. 10: Bolt Torque Sequence for 20-Bolt and 24-Bolt Flanges**

The following table provides the required flange bolt torque sequence

Flange Bolt Torque Sequence Table (4-bolt to 32-bolt)
Number of Bolt / Stud (Refer to ASME B16.5 with Respective Pressure Class)	Required Bolt Tightening Sequence
4-bolt Flange	1,3,2,4
8-bolt Flange	1,5,3,7,2,6,4,8
12-bolt Flange	1,7,4,10,2,8,5,11,3,9,6,12
16-bolt Flange	1,9,5,13,3,11,7,15,2,10,6,14,4,12,8,16
20-bolt Flange	1,11,6,16,3,13,8,18,5,15,10,20,2,12,7,17,4,14,9,19
24-bolt Flange	1,13,7,19,4,16,10,22,2,14,8,20,5,17,11,23,6,18,12,24,3,15,9,21
28-bolt Flange	1,15,8,22,4,18,11,25,6,20,13,27,2,16,9,23,5,19,12,26,3,17,10,24,7,21,14,28
32-bolt Flange	1,17,9,25,5,21,13,29,3,19,11,27,7,23,15,31,2,18,10,26,6,22,14,30,8,24,16,32,4,20,12,28

Table 1: Flange Bolt Torque sequence for flanges with 4 to 32 bolts

Hot Torquing

When the tightening of all bolts of a flanged joint is performed at the operating temperature, the process is known as Hot Torquing. For flanges that are known to leak at elevated temperatures due to gasket relaxation, the Hot Torqueing method is applied. It is normally performed when the temperature of the flange or the bolts is between 150°C and 230°C, or within 24 hours of a unit start-up if the joint temperature remains below 150°C.

Flange Bolt Torquing Guidelines

The following guidelines should be followed

Flanges and Gaskets must be inspected prior to torque application.
All working surfaces must be cleaned and lubricated properly.
Gaskets must be new, Re-use is normally not permitted.
Flange Bolt torquing must be done following the appropriate flange bolt tightening sequence. To know more about the flange bolt tightening sequence, Read: Flange Bolt tightening Procedure/Bolt Tightening Steps

What is Pump Priming and Why it is Required? | Self-Priming Pumps

Written by Anup Kumar Dey in Mechanical,Piping Design Basics

Pump Priming is a manual or automatic process by which air present in a pump and its suction line is removed by filling liquid. In the pump-priming process, the pump is filled with the liquid to be pumped and that liquid forces to remove the air, gas, or vapor present. With the exception of a few self-primed pumps, mostly all pumps are primed. Before starting a pump, Pump Priming is the most important first step and it avoids the majority of the pump problems.

The majority of the pump problems are started by not priming a pump or not doing it properly. Problems associated with lack of priming usually cause financial impact due to pump maintenance and the downtime of the piping system due to a malfunctioning pump. So for proper reliable operation pump must be primed.

Reason for Pump Priming

During starting a pump, if air, gas, or vapor exists inside the pump casing, the pump will not be able to function properly. The pump will be subjected to the risk of damage. The air or gas present inside the pump will make it gas bound and the pump won’t be pumping the desired liquid. The pump will get overheated and it will damage the pump internals.

To reduce the risk of pump damage and reliable operation, the gas present in the pump must be removed. So, the pump must be fully primed.

When is Pump Priming Required?

The main objective of priming a pump is to remove the gas present. So, if the air or other gases are present inside the pump casing and suction line, it must be primed before starting. But if the pump suction line and the casing are already filled with liquid during start-up, priming is not required. The main reason behind locating most centrifugal pumps below the liquid source level is that the pump remains primed automatically. Refer to Fig. 1 below that clearly explains the requirement of priming a pump.

In general, Centrifugal Pumps need priming. Submersible Pumps or vertical sump pumps do not require priming. Positive displacement pumps are considered self-primed pumps.

Pump Priming: Centrifugal vs Positive Displacement Pumps

Normally, Centrifugal pumps need priming and Positive Displacement Pumps (Rotary Pumps, Reciprocating Pumps) do not require priming. However, for the first-time operation, all pumps need priming to avoid overheating and failure in dry running conditions.

In a centrifugal pump, the liquid is pushed from suction to discharge. The pump works by the transfer of rotational energy from the impeller to the liquid. In between the suction and discharge sides of the pump, there are no seals. For this reason, when the liquid level is below that of the impeller, centrifugal pumps are ineffective with gases and incapable of evacuating air from a suction line. So centrifugal pumps must be primed for proper working.

On the other hand, all positive displacement pumps use close-tolerance parts to prevent fluid from returning from the discharge to the suction side. Hence, a positive displacement pump is capable of venting air from its suction line to some extent.

Pump Priming Methods

The pump can be primed by layout considerations or using external arrangements. A few of the external pump priming methods are:

Natural Priming
Manual Priming
Priming a pump with Vacuum Pump
Pump Priming with Jet Pump
Pump Priming using a Separator
Priming a pump by Installing a Foot Valve
Pump Priming with Ejector

Natural Pump Priming:

Natural pump priming can be achieved by maintaining the impeller eye below the surface of the water. So, naturally, water will flow into the suction pipe and casing removing all the air present by gravitational force as shown in Fig. 2

Manual Pump Priming Method:

In the manual method (Fig. 3) of pump-priming, pumping liquid is filled in the pump suction by manually pouring liquid directly into the suction using a funnel. The pump is manually primed due to gravity feed and the air present escapes through the air vent valve.

Priming with a Vacuum Pump:

To prime, the main centrifugal pump, An additional small-size vacuum pump or a self-priming pump, or a positive displacement pump is used. The discharge line of the main pump is connected to the suction line of the positive displacement priming pump that evacuates all the air in the primary pump and suction piping.

**Fig. 4: Pump-Priming using a Vacuum Pump**

Priming with Jet Pump:

Water available at the high head is allowed to flow through a nozzle in this method of pump priming. The nozzle is designed in such a way that at the jet outside the nozzle, the pressure is less than the atmospheric pressure which causes water to be sucked from the sump.

**Fig. 5: Pump Priming using a Jet Pump**

Priming with Separator:

On the discharge side of the pump, an air-water separation chamber or separator is provided for pump priming with a separator. A bent suction pipe portion is provided at the inlet of the pump that always maintains some liquid in the pump. Through pump discharge or an air vent, Air is separated and expelled and the liquid falls back into the separation chamber due to higher density.

**Fig. 6: Pump-Priming using Separator**

Priming by Installing Foot Valve:

A foot valve that acts like a non-return valve is installed in the suction piping in this pump-priming method. The foot valve does not allow the liquid to drain from the pump casing and suction line once the pump operation is stopped. So, while starting the pump for the next operation, the pump is already primed and can work.

**Fig. 7: Pump Priming using Ejector and Foot valve**

Priming with Ejector:

In this pump-priming method, an ejector (Fig. 7) is provided on the pump suction (or discharge) side. These Ejectors create a vacuum inside the pump suction line forcing the liquids to draw from the sump up to the pump elevation. However, Ejectors require energy input for their work.

Prevention of Pump Operation without Priming

Various methods are used to prevent a pump to operate without being primed. The basis behind such methods is to trigger some alarm or auto shutdown of the pump when the pump is not liquid-filled or primed. One example of such a scenario is provided below.

In some kinds of pumps, a float switch in a chamber connected to the suction line is usually used. When the chamber level is above the impeller’s eye of the pump, the float switch allows the pump to operate without a problem. However, If the liquid falls below a predetermined safe level, the float switch forces the pump to stop using its control mechanism. Thus it prevents the pump to start to sound an alarm, or lighting a warning lamp for necessary action.

What is a Self-Priming Pump?

A self-priming pump is a specially designed end suction centrifugal pump with an external casing that always “floods” the inner pump or volute. The self-priming pump has the ability to evacuate air from the suction side at startup and then it operates similarly to a normal pump. The external casing is filled with liquid and the pump is always ready to start. When the impeller of a self-priming pump rotates inside the casing, a low-pressure area (below atmospheric pressure) is formed at the eye of the impeller. As a result, the liquid is pushed up the suction pipe by atmospheric pressure along with the air present in the suction pipe.

This air is mixed with the recirculating fluid inside the casing. The air then separates from the liquid and is discharged from the casing. Once, all the air from the suction pipe is removed, the pump operates dynamically like any other centrifugal pump. So, A self-priming pump can lift fluid from a level below the pump and evacuates the air of the suction line without using other external auxiliary devices.

There are three types of self-priming pumps. they are:

Liquid Primed Self priming Pumps
Compressed Air Primed Self-priming Pumps, and
Vacuum Primed Self-priming Pumps

Liquid Primed Self Priming Pumps: They have their own in-built “Priming Chamber” that is filled with liquid in order to “self-prime” the pump. This initial liquid charging must be done for a liquid-primed self-priming pump to prime and work. External auxiliary devices are not required for liquid-primed self-priming pump operation. A Liquid Primed Self Priming Pump works in two phases of operation; “Priming Mode” and “Pumping Mode”.

Compressed Air Primed Self Priming Pumps: In Compressed Air Primed Self Priming Pumps, a jet blows the compressed air into a tapered tube. This creates a vacuum so that the air from the pump casing and suction line is drawn in with the compressed air and exhausted into the atmosphere. A check valve seals out the air from the discharge. Water or other pumping liquid then replaces the air and the pump starts pumping. The potential build-up of solids is also prevented by this type of self-priming pump.

Vacuum Primed Self Priming Pumps: Vacuum Primed Self Priming Pumps consist of a vacuum pump and positive sealing float box installed at the pump discharge. This forces you to pull a vacuum on the pump until it is full of water.

Self Priming Pump Precautions

Note that, a self-priming pump, too, needs priming for its first operation. A priming chamber or some portion of the volute must be filled prior to start-up.
The discharge line should not be blocked or pressurized.
The suction line must be air-tight. Otherwise, the pressure will not be reduced and fluid will not be drawn up the suction line.
To reduce the priming time, the volume of the suction piping shall be minimized.
If the liquid contains any solids, a strainer shall be required to keep solids from accumulating in the priming chamber and displacing the priming liquid.
The pump suction piping should be designed to avoid high points where air can be trapped/accumulated, thus preventing priming.

Advantages of self-priming Centrifugal Pumps

The main advantages that self-priming pumps provide are:

Can handle a variety of liquids
Work well with slurries, corrosive liquids, and suspended solids
Self-priming centrifugal pumps will continue to pump liquids even after the pump is not submerged in a liquid tank or vessel
As the steps involving pump priming on start-up are eliminated, they are ideal for frequent and intermittent pumping operations.

Disadvantages of self-priming Centrifugal Pumps

The main disadvantages of self-priming pumps are:

They can’t operate without the presence of the initial priming liquid in the prime chamber.
Due to the presence of a liquid reservoir, this type of centrifugal pump is usually larger than a standard model, which may cause issues in applications where space is limited.
To avoid depletion of the pump’s liquid reservoir during self-priming operations, They are required to be as close as possible to production lines.

Surge Relief Valve: Definition, Function, Types, Sizing, Selection

Written by Anup Kumar Dey in Health & Safety,Instrumentation,Pipeline,Piping Design Basics,Piping Interface,Process

What is a Surge Relief Valve?

A surge relief valve is a specially designed valve that protects piping and pipeline systems from pressure surge events caused by rapid valve closure, emergency shut-down, or pumps trip situations. As they are inexpensive and can be installed at any location of interest, Surge relief valves are reliably used in the Oil & Gas, Chemical, Petrochemical, Power, Marine, and Nuclear Industries. Surge relief valves can be used in all systems requiring some kind of pressure relief to avoid personnel, equipment, or asset loss due to failure from an overpressure situation.

Surge Relief Valve Function

The main function of a surge relief valve is to quickly open during a pressure surge incident and relieve the high-pressure fluid to any safe outlet. In the normal scenario, the valve remains in a closed position by the compression of a spring. When the pipeline pressure becomes more than the surge relief valve set pressure, the valve disc quickly opens to release the overpressure. Again, when the system pressure subsides and reduces below set pressure, the valve automatically closes.

So, to protect the system from dangerous surge conditions, surge relief valves must have the following characteristics:

Quick Opening Speed to rapidly relieve the surge pressure. Open fully in a very short time.
Non-Slam Capability so that it does not cause pressure rise during valve closure.
Sufficient capacity to relieve the entire high-pressure fluid.

Types of Surge Relief Valves

Two types of Surge relief valves are widely used in process industries:

Pilot-operated surge relief valve, and
Gas-loaded surge relief valve (Normally Nitrogen is used)

The main features of pilot-operated surge relief valves are:

Suitable for low-viscosity refined products
Completely self-contained/self-acting
Relatively slow response
Stable operation
Disc action is controlled by a mechanical pilot valve.
No external energy sources.

Pilot-Operated Surge Relief Valves are normally used for pump trip cases and installed at the inlet or outlet of the pump station.

On the other hand, gas-loaded relief valves are used in transportation pipeline systems. The main features of gas-loaded (nitrogen-loaded) pressure relief valves are:

Suitable for all types of petroleum products, including high-viscosity or dirty products
Exceptionally fast response
For more efficient operating conditions can maintain a minimum back pressure

**Fig. 2: Gas-Loaded Surge Relief Valve**

The system relief pressure and the amount of fluid the valve must pass when it relieves will govern the required pressure of the gas loading system. Accordingly, the surge relief valve size will be decided. Such systems offer flexibility in design according to what is desired, including extremely rapid opening, as illustrated in Fig. 3 below. It follows that the higher the differential pressure the faster the valve will open.

**Fig. 3: Gas-loaded Surge Relief Valve Instrumentation**

Surge Relief Valve Sizing

The sizing of the surge relief valve depends on various factors like

Potential for pressure rise
system capacity
location of the surge relief valve
other safety and control measures in the system
pipeline length, and thickness.
Maximum fluid velocity and maximum allowable pressure.

In normal practice, surge relief valves are normally selected following various charts and nomograms.

Selection of Surge Relief Valve

To safeguard the complete piping/pipeline system from surge pressure, the right surge pressure relief valve selection is highly important. Selection is dependent on various factors like:

Valve Response Time: Gas-loaded relief valve is faster as compared to pilot-operated ones.
Valve Co-efficient: It is recommended that the required Cv should not exceed 85% of the selected valve size actual Cv. Note that, the C_v of a Valve varies by size, type, and manufacturer.
Excess pressure above the set point to reach the required flow rate
Valve characteristic control curve

Skid-Based Surge Relief Valve System

In recent times, skid-based surge relief systems provide a complete solution for surge events. Such skids comprise of properly sized surge relief valve with the nitrogen system integrated into the skid and inlet & outlet manifolds with isolation valves upstream and downstream of the surge relief valves. Fig. 4 below shows such a typical surge relief valve skid.

Reference & Further Studies:

https://www.emerson.com/documents/automation/technical-guide-introduction-to-liquid-pipeline-surge-relief-en-43822.pdf

What is the meaning of Project Kick-Off Meeting? Its Purpose & Agenda

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

What is a Kick-Off Meeting?

A project kickoff meeting is the first meeting between a project team and the client or stakeholder/sponsor of a project to discuss the project’s basic elements and overall planning activities. The kick-off meeting for a project is normally conducted just before starting project execution. The project kick-off meeting is an important tool for the project management plan development process. So, the meaning of kick-off meeting is basically the first project meeting during the initiation phase of the project lifecycle. Kick-Off Meeting is sometimes referred to as KOM kickoff meeting to form an acronym.

Project Kick-off Meeting Purpose

The main purposes behind conducting a project kick-off meeting just after the contract have been signed are:

To lay the foundation for a successful project.
First Formal Meeting of Team Members with the Client: This meeting introduces the project and client team members and provides the opportunity to discuss the role of each team member.
Understanding the project background.
The overall Project Goal is set during the kick-off meeting.
Clarification of member roles/contributions in the project.
Mutual trust and understanding of each one’s responsibility.
Starting the project client relationship well with an agreement to work collaboratively.
Setting the tone of the project during the start itself.
Instigate confidence in the client.

So, basically, the project kick-off meeting generates enthusiasm for the customer to provide a full summary of the project. By describing the goal and steps on how to reach it, the customer gains confidence in the team’s ability to deliver the work.

Kick-off Meeting Agenda

The agenda for every kick-off meeting must be well thought off and complete considering essential project priorities. Even though the kick-off meeting agenda varies from project to project, still there are some basic elements that are followed in every project kick-off meeting as agenda. These are:

Welcome Notes & Introduction of Team Members.
Project Background and Project Objectives.
Project Plan/Approach presentation.
Statement of work, Scopes, and Deliverables.
Roles and Responsibilities.
Collaboration and Communication Plan.
Establish rules for future communication and meetings.
Schedules, Timelines (Key dates), and Progress Tracking.
Question and Answer session.

Advantages of a Project Kick-Off Meeting

The major advantages of any project kick-off meeting are:

Team members get acquainted with each other.
Team members’ understanding of the project objectives is clarified.
It allows stakeholders to understand the milestones, risks, assumptions, and constraints of the project.
It gets stakeholders on the same page.
A project kick-off meeting assists the project manager to gain support from stakeholders.
Attendees get an opportunity to clarify their doubts.

Who Should Attend a Project Kick-off Meeting

Ideally, all of the people who will work on the project should attend the kick-off meeting. But for large projects with big teams, it may be unfeasible to include everyone involved. In such a case, Project Managers, all discipline lead members, and the key working person should be invited to attend the kick-off meeting. However, it’s always a good idea to share the meeting with all team members through video or web conferencing.

Duration of a Project Kick-Off Meeting

Ideally, all project kick-off meetings should end within an hour. But, for larger projects, it may extend up to two hours. For very big projects including separate phases, a project kick-off meeting for each phase can be conducted.

Kick-off Meeting Template

As the project kick-off meeting is held in the initial phase of the project cycle, there will be a lot of questions in mind. The one responsible for the kickoff meeting should address those questions as part of the session. But, addressing all doubts will increase the duration of the meeting. Hence, a project kickoff meeting template can help the responsible person in managing the time. A typical kickoff meeting template usually includes the important activities mentioning the time to devote for discussing that activity during the kick-off meeting. A project kickoff meeting template should address the following points:

Project background
Purpose
Scope
Project Timeline
Roles and Responsibilities of key contributing members
Address any misunderstanding if any among group members

Project Kick-Off Meeting Checklist

To make the project kick-off meeting organized the following key points in the form of a kick-off meeting checklist can be followed:

Defining Project Scope and Objectives.
Risk Identification.
Establishing time-bound deliverable lists.
Proper Team Building with defined roles and responsibilities.
Appropriate Communication Plan.
Tracking Progress and Milestones.
Tools for team members.
Agenda, handouts, and Presentation Slides.

Once the above-listed items are addressed, schedule the kick-off meeting without hesitation.

Pre-Kickoff Meeting

It is always better to have a pre-kickoff meeting to plan everything beforehand for the success of the kick-off meeting. The pre-kickoff meeting serves various purposes like:

Understanding the full project scope clearly.
Creating an inventory of required stuff.
Looking into existing data from other projects which may be useful for the new project, etc.

Summary:

A kick-off meeting is significant for successful project completion with minimal obstruction. Note that, Kick-Off meetings are not problem-solving meetings and hence should not discuss problems. Kick-off meetings should focus solely on information sharing and setting the tone for the rest of the project.

What is a Slug Catcher? Its Types, Working, Selection, & Design Steps

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

Pipelines handling multiphase flows of gas and liquid, slug flow can pose significant operational challenges. To mitigate these challenges, slug catchers serve as critical components in pipeline systems. In this article, we will explore what slug catchers are, how they work, their types, and their benefits.

What is a Slug Catcher?

A slug catcher is a piece of static equipment in the form of a vessel or piping network. It contains sufficient buffer volume to handle the largest expected slug from the oil & gas pipeline systems or flowlines. Flowlines carrying multiphase fluids (Crude Oil, Gas, Water, mixtures) often form damaging slugs. Slug catchers protect the equipment and support from abrupt failure due to forces generated because of slug flow. They are normally located (Fig. 1) before transferring the fluid into the processing equipment.

Benefits of Slug Catchers

Slug catchers are widely used in multiphase flow lines to serve the following functions:

To protect the downstream system and equipment from the damaging effects of slug flow.
To efficiently manage high volumes of liquids (slugs) generated.
To separate part of the liquid from gases in multiphase gas processing plants (Product Separation). The preliminary separation of liquid and gas phases occurs due to the density difference inside the slug catcher.
To reduce the probability of further slug formation in the downstream processes.
To allow the liquid to follow into downstream facilities and equipment at a lower rate which can be properly handled.
Slug catchers can also work as temporary storage devices.
To stabilize the flow by capturing the generated slug.
To increase efficiency by preventing slugs from disrupting flows.

Depending on the type and frequency of slug generation, slug catchers can be used permanently or intermittently. When slugging behavior is difficult to predict, they are used continuously. However, for purposely generated slugs like during pipeline pigging operations, slug catchers are used during requirement only.

**Fig. 1: Typical Location of a Slug Catcher**

Types of Slug Catchers

Based on their design construction, four types of slug catchers are found in industrial use. They are:

Vessel Type Slug Catcher
Finger Type Slug Catcher
Stored loop type/ Parking Loop type Slug catcher and
Hybrid Slug Catcher

1. Vessel Type Slug Catcher:

As the name suggests, vessel-type slug catchers are basically two-phase separation vessels. The main design consideration is the vessel volume to contain the largest expected slug from the pipelines. However, the vessel-type slug catchers must be strong enough to sustain the high pipeline pressures. The vessel can be horizontal or vertical. Vertical vessels have more efficiency as compared to horizontal vessel slug catchers. They are simple for design and maintenance but with an increase in volume requirement and pressure, this equipment becomes too costly.

2. Finger Slug Catcher or Harp Slug Catcher:

A number of large-diameter pipes are used to construct a Finger type or Harp slug catcher. As piping design to withstand high pressure is easier, finger-type slug catchers are widely used due to their economic reasons. Fig. 2 below shows a typical example of a Harp Slug catcher.

Finger-type slug catchers consist of

Fingers have three distinct sections
- Gas/Liquid Separation Section
- Intermediate Section, and
- Storage Section
Gas Risers that are connected to each finger at the transition zone between separation and intermediate sections.
Gas equalization lines are located on each finger within the slug storage section.
Liquid header configured perpendicular to the fingers.

3. Stored loop type / Parking Loop type Slug catcher:

The features of both the vessel and finger types are combined into a Parking Loop slug catcher. The Separation of the gas and liquid phases occurs in the Vessel, while the parking loop-shaped fingers provide the buffer volume for storing the liquid.

4. Hybrid Slug Catcher:

The high efficiency of a vessel separator and the large storage volume of a harp slug catcher combines in a hybrid slug catcher.

Slug Catcher Type Selection

The basic selection between the “Vessel Type” and “Finger Type” is done considering

Economical aspects (Slug Catcher Design and Installation costs)
Equipment characteristics
Volume Requirement (Thumb rule is to use Vessel type for volumes less than 100 m³. For high steady-state gas flow: vertical type and for low steady-state gas flow: horizontal type)
Site Conditions like Space Requirements. Finger-type slug catchers occupy huge plot space.
Design Pressure (Low Design Pressure-Vessel Type; High Design Pressure-Finger Type)
Transportation feasibility, etc.

Slug Catcher Design Steps

As Slug catchers are used in both oil/gas multiphase production systems and in gas/condensate systems to handle the dangerous effects of slugs, they must be properly sized to dampen the slugs to a level that the downstream processing equipment can easily process. Information on liquid handling rate, pigging frequency, slugging possibility, slug duration, ramp-up rates, etc is required to estimate the required size of a slug catcher. To determine the slug arrival duration, volume, and other relevant information, a transient simulation of the arrival scenarios like pigging, flow ramp-up, etc. is normally performed. The main slug catcher design steps consist of the following:

Determination of Slug Catcher function and location.
Based on Design criteria and data, estimating slug catcher volume (Dimensions)
Selecting Slug catcher types or configurations considering the design and economic consideration
Feasibility study

Slug Catcher Working Principle

The basic workings of a slug catcher is explained nicely in the following animated video:

Slug Catcher Operation

To conclude, slug catchers are indispensable devices for managing slug flow in pipelines, contributing to flow stability, equipment protection, and overall operational efficiency.

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Emergency Shutdown System or ESD System

Written by Anup Kumar Dey in Health & Safety,Instrumentation,Piping Interface,Process

An emergency shutdown system or ESD system is a highly reliable control system for providing a safety layer during emergency situations. It helps to prevent situations from having catastrophic impacts economically, environmentally, or operationally. Emergency Shutdown Systems in any plant minimize injury to working personnel & the environment or damage to equipment, by protecting against leaks, hydrocarbon escape, fire outbreaks, explosions, etc. The application of emergency shutdown systems has been substantiated in the oilfields (oil wellheads), Nuclear plants, oil and gas processing plants, steam and gas turbine power plants, chemical & petrochemical plants, boilers, geothermal industries, etc. During an emergency situation, the process operations are stopped by the ESD system, therefore, isolating the hazard to escalate.

Functions of an Emergency Shutdown System

All emergency shutdown systems should always work at the back end throughout the plant operation as it is one of the main security systems. The major functions of an emergency shutdown system are:

Shut down of the system or equipment during a critical situation
Isolate electrical equipment
Proper control of ventilation during an emergency
Stop or isolate hydrocarbon sources from potential hazard situations.
Blowdown and depressurization.
Prevent dangerous event escalation like prevention of ignition and explosion.
To protect personnel, asset, and the environment.

Note that critical situations may be triggered in any plant by various factors but emergency shutdown systems should be able to handle those in an effective manner.

Emergency Shutdown system design considerations

The design of the Emergency Shutdown or ESD system shall take into account the needs resulting from normal operation and shall also fulfill the requirements that may arise during other possible (and likely to occur) abnormal or down-graded configurations. Depending on the type of operating plant and functions, ESD system design will vary.

However, the below-listed issues shall be adequately addressed when relevant:

Tripping or stopping a unit or equipment does not necessarily eliminate all sources of hazards.
Due to the loss of essential utilities like air, essential power, hydraulics, etc., new hazards can appear anytime. The emergency shutdown system should be designed to identify and mitigate or alarm regarding the risk of such hazards.
All operating configurations that the ESD system generates shall be stable, safe, and reversible.
The ESD system shall be compatible with the re-start philosophy. The inevitable inhibitions of the control and safety systems during the re-start sequence shall be identified and shall be limited in number, time, and duration.
ESD system design shall provide specific attention to non-routine operating conditions, simultaneous operations, and down-graded situations.
Particular operating conditions may require a different shutdown logic than that, or the combination of those, applicable under normal circumstances. For example, An installation normally operates under different conditions, e.g. high, medium, or low pressure. Each condition may require a different ESD logic, but the differences shall be limited to process shutdowns. Emergency shutdowns shall result in the same actions independent of the condition. Before switching over between different ESD logics, the proper line-up of equipment and the status of valves need to be verified.
The Emergency Shutdown system shall be used to continuously monitor the safety parameters of the plant and shall take actions to maintain the safety of the plants on demand.
The ESD system diagnostics shall show the following minimum fault / healthy state status but not limited to:
- Circuit breakers tripped
- Power feeders healthy
- Fuse Failure
- Power supply removed
- CPU fault
- Battery failure
- Power supply failure
- Communication Failure
- Input/ Output Module failure
- The input/ Output Module removed
- Each channel failure
- Panel internal temperature high
- Others as supplied by the manufacturer.

Working of Emergency Shutdown System

An emergency shutdown system works by monitoring the plant condition using field-mounted sensors, valves, trip relays, and inputs to a control system as alarms. The control system performs a cause-and-effect analysis of the above parameters to determine plant health. The system will minimize the effects in case of abnormal behavior by reducing the number of plant items available or shutting down part of the system. For example, In case of a fire hazard, a Fire Damper control system may override the existing controls to open or close vents as needed, and close fire doors.

Normally, for plants, a shutdown matrix is defined. Three to four shutdown levels based on decreasing criticality are decided and the complete plant is categorized. In the process control system, various safety loops and devices are organized as complementary barriers. For each installation, an ESD/SD logic shall be defined covering all the installations and represented in an ESD/SD logic diagram.

Components of an Emergency Shutdown System

The following components shall be part of an emergency shutdown system:

Dedicated Process Transmitters
Shut Down Valves, Normally Fail to Close Type
Logic Solver
Blowdown valves

Fig. 1 below shows a Typical Emergency Shutdown System in its basic form.

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