PIG_TRAPS

Design and Application Manual

PIG TRAPS

CONTENTS - SECTION 6.0

6.1

Introduction

6.2

Design Parameters

6.3

Operating Method

6.4

Selection

© Copyright 1999 Pipeline Engineering & Supply Co Ltd

Page 1

Section 6


PIG TRAPS INTRODUCTION 6.1

INTRODUCTION

As part of a complete pigging system, pig traps allow pigs to be inserted into and removed from a pipeline which is to undergo a pigging program and which is likely to be under pressure.

Key a. b. c. d. e. f. g. h. i.

Fig 1.

Pressure Gauge Vent Pressure Relief Drain Pig Signaller Kicker Valve (Launcher) Bypass Valve (Receiver) Mainline Bypass Valve Mainline Trap Valve Quick Release Closure

Typical Trap Configuration (Horizontal Launcher/Receiver) Launcher/Receiv er)

Pig traps can be, and frequently are, referred to by different names some of which may be no more than alternatives and some which more accurately describe the trap’s function. Popular alternatives are: Launchers, Receivers, Scraper Traps, Scraper Barrels, Universal Traps, Bi-Directional Traps, Sphere Traps. Traps. The name may also describe the orientation of the trap, giving a clue to its intended purpose: Vertical, Horizontal, Inclined, Declined, Temporary. If a pipeline is to be pigged, launching and receiving facilities (pig traps) must be provided. For large diameter pipelines this results in pig traps up to 56” (nominal pipeline diameter) weighing many tonnes and represent a significant capital investment. Any company that manufactures this Fig 2. Trap Types & Orientation type of equipment requires the scope and experience to do so and must possess excellent engineering resources, both in its equipment and and its staff Entrusting the design design of these systems to non-specialists non-specialists armed with a few proprietary catalogues is a short-sighted and all too frequent shortlived economy. Pipeline Engineering’s Engineering’s technical engineers have many years of experience in pig trap design and are fully conversant with all the major design codes and make full use of state-of-the-art CAD facilities. facilities. All welders are are fully qualified qualified to both American and British welding codes while the Inspection and Quality Assurance systems ensure control at every stage of the manufacturing process from order to delivery.


Page 2

Section 6


PIG TRAPS 6.1.1

DESCRIPTION AND PURPOSE

a)

What is a Pig Trap? A simple definition is – a piece of pipeline equipment that allows easy loading or unloading of a pig into, or out of, the pipeline.

b)

What is its purpose? To provide in a safe manner and without flow interruption the means to either insert and launch a pig into the pipeline or receive and retrieve a pig from a pipeline.

Although the definitions are simple, in reality it is quite different because a Pig Trap is a vessel that:• •

•

•

•

•

•

•

Gives access to or from a pipeline. Provides a means by which this access can be closed between the surroundings and full pressure capability of the pipeline. Provides for the access to be opened or closed with speed, convenience and safety. Provides and internal holding or storage position in which pigs may rest until the desired travel movement is achieved or after travel is terminated. Provides a means of converting the pig from its free expanded state into its compressed travelling state. Includes, or is associated with, a means of controlling flow, pressure and/or mechanical movement to give the pigs a positive driving force into or out of the pipeline. Has properly engineered safe and practical provisions for connecting to the pipeline. Is properly supported in a way which will neither impose excessive strains on the pipeline nor will accept more force than it is safe, or desirable, from the pipeline and its associated systems.

What does a Pig Trap comprise of – simplistically: I. II. III. IV. V. VI.

A quick opening closure or blanked flanged end. A major diameter section, referred to as the Barrel. A reducer – normally eccentric on a launcher, concentric on receiver. A minor diameter section corresponding to the line pipe size and referred to as the Neck Pipe. Various nozzles such as vent, drain, pressure indicator, kicker, or bypass, release, equalising and pig signaller Lifting lugs, supports and earthing lugs


Page 3

Section 6


PIG TRAPS 6.1.2

FUNCTION AND CONSTRUCTION CONSTRUCTION

Figures 3a and 3b show typical components found in most types of trap and which include: 1. 2. 3. 4. 5.

A short minor diameter pipe section, often referred to as the NECK PIPE A REDUCER. REDUCER. This may be eccentric eccentric or concentric concentric A longer major diameter pipe section, often referred to as the BARREL NOZZLES, SUPPORTS and LIFTING LUGS An END CLOSURE or a blanked, removable flange

Fig 3a. Horizontal Pig Launcher

Fig 3b. Horizontal Pig Receiver


Page 4

Section 6


PIG TRAPS DESIGN PARAMETERS 6.2

DESIGN PARAMETERS

We have already said that pig traps are pressure vessels and, as such, pressure vessel design parameters parameters must apply. apply. However, unlike unlike pressure vessels vessels that contain only fluids, pig traps must be capable of retaining line pressure whilst allowing pigs to be launched and received. received. It is the type of pigs pigs to be used which which determine determine the overall trap dimensions. Metal bodied intelligent pigs are both heavy and long and must be considered along with the internal pressure requirements when designing supports, lifting lugs and assessing foundation loadings. Trap design must also satisfy applicable applicable codes. These may be be piping codes, in which case the trap is treated as part of the pipeline or there may be a specification ‘break’ between the pipeline and trap, resulting in a trap design which must meet pressure vessel codes such as ASME VIII or BS 5500. Whichever approach is used, the designer must ensure all appropriate loadings and conditions are addressed to produce a safe working design. Pig traps are designed and manufactured within the limits set by the Design Parameters. These are: 1. 2.

Basic design parameters Functional design parameters

There are instances where the requirements of the basic and functional design parameters are not compatible and a compromise compromise must be reached. reached. One frequent example is that of a trap having a design code of BS 5500 with a requirement for a full line size kicker connection. connection. As the code states that the nozzle should should not exceed one third of the ‘run’ size size some form of compromise is is required. In this case a full or reducing tee – designed to one of the major pipeline codes – is often acceptable. Basic Design Parameters cover the following aspects of pig trap design: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Design Code, Pressure and Temperature Materials and Certification requirements External loadings from pipework or external pressure (e.g. sub-sea) Cyclic requirements and nozzle reinforcements Support and lifting lug design Wind, Blast and Seismic loadings Ice and Snow loadings Inspection and Welding requirements Transportation loads

Design Codes can be any of the National or International standards, such as: BS 5500, BS 8010, BS 4515, ASME VIII, ANSI B31.3/4 or 8, Stoomwezen (Dutch), AD Merkblatte or DIN Standards (Germany). Pressures can be client specific or based upon the ASME/ANSI Pressure/Temperature Pressure/Temperature ratings. ratings. However, the the design pressure of of the trap should should never be less than that of the t he pipeline.


Page 5

Section 6


PIG TRAPS Temperature is also client specific but it should be remembered that the maximum design temperature should not be less than the maximum temperature which the pig trap system could attain, or to which it could be exposed during operation, start-up or shut-down. Materials are often specified by the client. However, it is better better to allow the manufacturers of the pig trap to select select the trap materials. This approach ensures ensures that the most suitable, cost effective and readily available materials are always selected. To aid the selection process, the line product must always be specified and whether it is ‘sour’, toxic or or corrosive. This will influence influence the selection selection of not only the metallic metallic elements, but of the elastomeric materials which, typically, constitute the sealing elements. All components in sour service should be resistant to HIC (Hydrogen Induced Cracking) and conform to NACE specifications. Finally, all materials should be compatible with its mating material, particularly with regard to its weldability, weldability, wall thickness and material grade. Certification can apply to just materials or extend as far as the manufacturing and design appraisal. •

•

•

For materials only – this is usually limited to a certificate showing the chemical and mechanical properties of the materials being used and issued by the suppliers. In the case of material traceability, certificates verified by an independent third party inspection authority may be required, in which case they are issued in accordance with BS EN 10204.3.1.C. Where certification is to cover manufacture and design appraisal, this is carried out by an independent third party inspection authority, usually appointed by the end client, with the scope of inspection being against an agreed quality control/inspection plan.

Welding should conform to procedures in accordance with the design code used for the trap whilst all welders, including operators of automatic welding equipment, should be qualified qualified in the procedures used. All completed welds should should be examined by a qualified weld inspector. NDE/NDT (Non-Destructive Examination, also known as Non-Destructive Testing) requires that all circumferential and longitudinal butt weld, where practical, should be examined by by radiographic radiographic methods. Welds that cannot easily be radiographed radiographed should be examined ultrasonically or by magnetic particle inspection. Pressure Testing: •

•

Hydrostatic – upon completion, each vessel should be subject to a hydrostatic test pressure at least equal to 1.25 times t imes the design pressure. Generally there is no upper limit for the hydrostatic test pressure, however, any pressure above 1.5 times the maximum working pressure should not be allowed to exceed wither intentionally or accidentally to the degree that the vessel is subjected to visible, permanent distortion.


Page 6

Section 6


PIG TRAPS •

Pneumatic Test – Some codes allow for the vessels to be pneumatically tested in lieu of hydrostatic testing. testing. However, it should should be noted that AIR or GAS is highly hazardous when when used as a testing medium. It is strongly strongly recommended recommended that special precautions be taken when air or gas is used for test purposes.

Data Dossiers: Copies of all certificates, examinations and inspection reports, together with weld procedures, other client specified documents and “as built” drawings, should form a data dossier for presentation to the client with the completed vessel. Functional Design Parameters can be explained most effectively by describing, in detail, each of the trap components. The barrel comprises the major diameter section of the trap, designed to be oversize so that the pigs can be easily loaded and unloaded. It is usually equipped with a quick opening end closure or, for temporary traps, a blind flange. For conventional pigs, the diameter of the barrel is generally 2” larger than the diameter of the line pipe whilst, for intelligent pigs, it is recommended that the diameter of the barrel is at least 4” larger than that of the line pipe. Barrel length is dependent on operating procedures, service, pig type, available space, etc. However, for launchers launchers deploying conventional conventional pigs, pigs, the recommended length of the barrel should be 1.5 x pig length, measured from the kicker connection to the reducer weld. weld. For receivers, the recommended recommended barrel barrel length is, again, again, 1.5 x pig length. However, this dimension dimension is measured measured from the kicker connection connection to the closure weld. When deploying intelligent pigs, barrel length should be decided only after consultation with the pig manufacturer. For multiple sphere launchers or receivers, inclined or declined trap barrels should be considered, along with a mechanism for releasing the spheres spheres (e.g. fingers, flaps or valves). Historically, trap barrels have been inclined at angles anywhere between10° and 45°. 45°. However, Pipeline Engineering recommend that barrels be inclined between 2° and 5°.

2°-5°

Fig 4. 4. Recommended angles for Inclined/declined traps

The barrel is also equipped with a reducer that is either concentric or eccentric, depending upon the client’s preference. However, an eccentric eccentric reducer allows pigs to be loaded more easily and is recommended for horizontal traps whilst a concentric reducer is preferred for vertical traps or when an internal tray, or basket, is fitted to horizontal traps. Fig 5. Types of Reducer


Page 7

Section 6


PIG TRAPS Neck Pipe is the minor diameter section of the pig trap and is usually only between 500 and 1000mm in length (with the exception of intelligent pig receivers where it may be as long as 4 meters) and is positioned between the reducer and the pig trap valve. On launchers, launchers, the neck pipe pipe provides head space for the pig and, and, during pressurisation, prevents contact between the steel nose of the pig and the trap valve, which may be damaged if struck by a pig under sufficient pressure. For traps up to and including 24” line size, the neck pipe is usually attached to the pipeline system system by a flanged flanged connection. connection. For traps above above 24”, connections connections to the pipeline system are usually welded. For receiving traps, the neck pipe usually incorporates a pig signaller. Nozzles is the collective term for the connections from the trap to its associated piping system. On simple traps there are normally only 3 nozzles: • • •

Kicker Drain Vent

On more complex traps, additional nozzles are incorporated for: • • • •

Blowdown Balance lines Pressurising lines Thermal relief valves

The kicker nozzle is usually the largest nozzle on a trap and is often referred to as the ‘bypass’ or ‘bridle’. ‘bridle’. Pipeline Engineering Engineering defines the kicker kicker nozzle as the off-take on the trap barrel which which connects the barrel barrel to the bypass line pipe. pipe. The bypass is the off-take after the trap valve on the main pipeline. For launchers, kicker connections are attached to the barrel near the closure end whilst for receivers, the connection is made near the reducer end. Universal and Bi-directional traps incorporate a single connection located midway along the barrel, or twin connections with one connection in the launch position and the other in the receive receive position. Ideally, the diameter diameter of the kicker nozzle should should not exceed 25% of the main line pipe diameter. Kicker connections should not be positioned at the 6 o’clock position, historically this position causes most damage to pigs. Drain connections should be provided near the end closure for horizontal launchers and near the pig trap valve valve for vertical launchers. launchers. For receivers on liquid liquid or gas lines where liquids could be present, a drain point should be provided near the trap valve. For receivers that are sloped for the use of spheres, two drain points may be located together near the end closure but should be separated by half a sphere diameter. This prevents the drains being blocked by the spheres. For traps up to and including 14”ns (nominal diameter diameter of the pipeline), the diameter of the drain nozzle should should be 2”. For traps above 14” ns the diameter should should be 4”. © Copyright 1999 Pipeline Engineering & Supply Co Ltd

Page 8

Section 6


PIG TRAPS Vent connections should should be provided near the closure closure end or highest point. A further connection may be considered near the trap neck end flange to ensure depressurisation depressurisation behind the pig in the event of it becoming stuck in the neck pipe. Size of vent connections should not be less that ½” ns. Blowdown. Blowdown. On high pressure pressure gas systems, systems, consideration consideration should be given to the provision of a blowdown line incorporating a globe valve or restriction orifice for controlled depressurisation. depressurisation. Size should not exceed 2” ns. Balance Lines can be provided on launchers to enable the barrel to be filled and pressurised on both both sides of the pig at the same time. time. This prevents a pig moving moving forward from the launch position hitting, and possibly damaging, the trap valve or moving backwards and losing the seal in the reducer. For receivers, balance lines will prevent any possible pressure differential across the pig and should always be considered for inclusion. inclusion. Balance line connections connections are approximately approximately 2” in diameter. Pressurising Lines may be required around kicker valves for several reasons: • • •

Speed of operation Control of barrel pressurisation To avoid damage to the kicker valve seats or other internals

Pressurising lines around bypass valves should also be considered to equalise possible high pressure pressure differentials. Pressurising line connections connections are usually smaller than balancing line diameters. Thermal Relief Valve connections can be provided at locations where the anticipated shut-in pressure of trapped fluid could exceed the design pressure. Pressure Indicator (Pressure Gauge) should be fitted towards the closure end and visible to the operator. May be incorporated with the vent connection. connection. Size should should be in the region of ½” to 1”. Gauge dial to be 4” or 6” size. Other items that may be fitted to the Pig Traps: Supports, Supports, as the name suggests, should permanently support and restrain the Pig Trap. They should be designed to carry carry the weight of the pig trap system filled filled with water (or other fluid if their density is greater), together with the weight of the associated heaviest pig. Supports under the barrel should normally be of the sliding type to compensate for expansion of the unrestrained unrestrained part of the pipeline. Other supports may be fixed if the design calculations indicate that sufficient flexibility is incorporated in the pipework to compensate for any axial and and transverse movements. movements. Where cathodic cathodic protection isolation joints are used, the supports should allow sufficient movement to avoid stressing of the joint joint above its design design limits. Where isolation isolation joints are not used used the supports may need to be electrically isolated. Lifting Lugs are designed to facilitate the lifting of the complete trap during installation stage. stage. Unless specifically specifically requested, they are not proof tested.


Page 9

Section 6


PIG TRAPS Earthing Lugs are designed to help help prevent the build-up build-up of static electricity. Static is a seriously under-estimated under-estimated yet ever present present hazard. Being invisible invisible to the naked naked eye, it tends to be ignored. ignored. Yet an undischarged undischarged build-up of electrostatic electrostatic can take hours, even days, to relax back into equilibrium, resulting in a potentially lethal workplace. If the accumulated static is suddenly discharged within a hazardous atmosphere, the resulting spark may easily act as the ignition source for an explosion. End Closures are fittings, including removable parts and assemblies, which provide quick and easy access to the barrel when open and which seal the barrel when closed. All closures should should be fitted with a vent/safety vent/safety bleed device that that forms part of the door locking mechanism and which, along with a system of interlocks sequencing the operation of the various valves and end closures, is designed to ensure the safety of personnel operating the trap. Pig Signallers (also known as Pig/Scraper Detectors) are devices set on or into the pipeline which indicate the momentary presence of a pig at a precise location. Signallers should should be installed installed on both both sides of the trap valve. For launchers, the signaller should be sited on the main pipeline and separated from the pig trap valve by a distance that is at least the length of the longest longest pig. For receivers, the signaller signaller should be positioned on the neck pipe and separated from the pig trap valve by a distance that is equal to the length of the longest pig. Sphere Release Mechanisms may involve any of the following: • • •

Mechanical fingers Flaps Valves

Mechanical Fingers are the most popular but are not really practical for use with traps in which the the spheres spheres exceed 20”ns. Above this diameter it is more practical to use flaps which are designed to absorb the high loads that a large sphere, weighing up to 500 kilos, can impose. Fig 6. Sphere Release Fingers

It is normal to fit two fingers or flaps to launchers so that multiple spheres can be loaded into the barrel after which single spheres can then be launched, at a predetermined rate, by sequencing the operation of the t he fingers or flaps. For declined receivers, it is common to fit a single finger to prevent the sphere from rolling onto the operator as the closure door is opened. Fingers and flaps can also be used in traps intended for conventional and intelligent pigs. Sphere Valves are basically ball valves where the hole in the ball does not go all the way through. A sphere enters enters the valve and on rotation through through 180 degrees degrees the sphere drops out to roll and engage with an inclined tee for pick up by the product flow. On rotation back through through 180 degrees degrees the valve is reloaded. reloaded.


Page 10

Section 6


PIG TRAPS If pigging facilities are required then consideration had to be given to providing vertical launcher for the pigs with the sphere valve and its storage magazine forming an angled branch branch into this. Naturally this method method is both costly and and bulky. Other factors also influence influence pig trap design. design. These are related to: • •

Layout Ancillary Facilities

Layout involves the siting of pig trap systems and the possible adverse environmental effects that could result during construction and operation. Pig traps should also be sited so that the end closures are pointing away from personnel areas areas and critical critical items of equipment. equipment. This will minimise minimise any damage damage resulting from the unlikely event of a pig being ejected from the trap whilst under pressure. Ancillary Facilities generally refer to pig pig handling equipment equipment and systems. systems. Nothing does more to improve efficiency, safety and cleanliness of pigging operations than a purpose designed designed system. With the correct equipment, equipment, heavy pigs and spheres spheres can be moved efficiently between traps, benches and vehicles, free of the danger and dirty conditions usually associated with manual operations and the largest pigs and spheres can be correctly and safely handled into, and out of, their traps. Handling equipment should always be designed as an integral part of trap installations. Typical arrangements arrangements are as as follows: • • •

Davit and Bench Cassettes Cradles

Davit and Bench refers to a manually operated system consisting of a cradle bench with a winch and and a free-standing swing jib crane. The cradle bench bench can be either trolley mounted or suitable for fixing to the floor next to the end closure door. For positive launching, the pig is inserted into the reducer by a winch-operated pusher mechanism mechanism on the cradle bench. bench. At the receiving trap the pig is attached by cable to the winch and withdrawn withdrawn onto the cradle bench. bench. The free-standing jib crane is used for hoisting and positioning of pigs. On multiple trap installations, all traps can be served by a single mobile handling system. Cassettes, also known as magazines, offer a solution to the problem of limited space on offshore platforms. platforms. They enable pig or spheres spheres to be pre-loaded pre-loaded in multiples multiples at the onshore terminal and then transported to the platform as a single single unit. The cassette is loaded into the launching trap from where the pig or spheres can then be launched at at a pre-determined pre-determined rate. At the receiving receiving trap the loaded cassette cassette is removed and the complete unit can again be transferred, after inspection, to the launcher for re-loading.


Page 11

Section 6


PIG TRAPS Cradles, also known as half baskets or trays, can handle pigs or spheres of any type. When used with launching traps, the cradle is loaded externally and then moves the pig or sphere into its launch position position inside the trap. For receivers, the incoming incoming pig comes to rest on the cradle cradle that can then be withdrawn from from the trap. Any debris collected in the trap as a result of pigging will also be removed. A free-standing jib crane can be sited next to the trap t rap to handle and position the pigs. The systems described are designed to provide a cost-effective means of handling pigs and spheres throughout throughout the operational lifetime lifetime of the pipeline. It is important that all pig handling systems are manufactured as an integrated part of the trap in order to preserve its integrity as a pressure vessel. There are several types of trap which do not confirm to the conventional horizontal horizontal layout and which are known as Special traps. traps. The following are examples: • • • •

Vertical traps Temporary traps Bi-directional traps Sphere Launcher and Receiver

Vertical Traps are usually used where space is at a premium (e.g. offshore platforms). Their design and configuration shows very little difference to that that of horizontal traps. In fact, fact, the only major differences occur in the design of the supports and of the end closure hinging. End closures swing to the side on horizontal traps but, for vertical traps they have to be either sprung, fitted with a counterweight or jacked out on a davit and screwed rod).

Fig 7. Vertical Trap Trap with Stepped Stepped Basket (and pig)

Vertical receivers are usually fitted with an internal stepped basket into which the pig arrives. As the pig cups expand, expand, the step prevents the pig dropping back back into the reducer. The basket also allows allows the pigs to be removed easily. easily. Temporary Pig Traps should never be used as an excuse to avoid the design parameters previously discussed as most of them still apply with equal force. Because of the circumstances which prevail on construction sites (where there is often an absence of trained routine) greater attention must be given to safety. Remember – pressure can kill. However, where there is less incentive towards the time and labour saving properties of modern quick opening closures, temporary traps can be designed and made Fig 8. Temporary Launcher to less stringent codes than those required for long (With Pig) term capital equipment – although quality assurance must never be neglected.


Page 12

Section 6


PIG TRAPS Bi-Directional Traps, Traps, with sleeves, were originally designed for shuttling spheres back and forth but are equally suitable for use with bi-directional bi-directional pigs. These units comprise a sliding sleeve inside the major barrel that can be positioned so that a single pig or sphere can be pre-loaded and held in the trap until it is ready for launch. Initially, the flow inside the trap bypasses the pig until the sleeve is moved into the launch position at which point the flow is directed behind the pig launching it into the pipeline. At the receiving end, the sleeve is positioned so that flow passes through it (the sleeve) until the pig arrives. The incoming pig is then captured by the sleeve and moves along the trap until the flow can bypass the pig. Fig 9. Bi-Directional Bi-Directional Trap Trap

Sphere Launchers and Receivers (Automated Pig Traps) Automatic pig launching and, to a lesser extent, receiving may be considered for unmanned installations where there is a requirement for frequent pigging of lines. The types of pigs used are either either sphere or batching batching pigs. The general principle principle for the handling of each each is the same. However, advantage advantage is taken of the spheres ability ability to roll. Pig launching is usually achieved from a vertical launcher whilst for sphere launching vertical or inclined can be utilised. The rolling feature of the spheres makes them readily adaptable to unmanned faciliti4es in that the operator can load a trap with several spheres and the launching can be activated either either manually or or automatically. Various launching launching mechanisms mechanisms are available, as already described, and the selection is largely a matter of client preference, bearing in mind the design constraints of each. In the case of sphere receivers, the barrel is declined and it is best to have a horizontal pup pipe near the closure with a sphere stop fitted so that incoming spheres do not impact against the closure door and also to prevent spillage onto the operator when he opens the closure door for unloading.


Page 13

Section 6


PIG TRAPS Safety and Interlock Systems Safety issues are always always at the heart of of pig trap design. Whilst pigging is a common procedure, typically carried out when a pipeline needs purging, cleaning or surveying, it can involve a high risk of human error, high enough to warrant pig traps being described as ‘primary grade sources of hazard’. Probably the greatest associated danger is when a trap is opened accidentally whilst still under pressure pressure – usually to insert insert or remove a pig. To overcome this danger danger it is vital that a system is incorporated which ensures that the trap is fully vented before the door can be opened. Venting depressurises depressurises the trap and removes removes the force necessary to propel the pig. Pig trap systems systems also involve involve other more complex, procedures. procedures. Even in a basic basic system, safe operation of the closure require it to be correctly sequenced with certain valves which are incorporated into the trap – these being the pig trap valves, drain and kicker valves. The pig trap line valve governs governs the piping connection connection between the pig trap and the main pipeline whilst the kicker valve is on a secondary piping connection used used to move the pig into and out of the trap. trap. Both valves must must be closed, isolating the trap from the main pipeline, before the trap is drained.

By establishing safety guidelines, a certain level of control over pig trap operations can be imposed but these usually rely on the voluntary compliance of personnel operating the the plant, pig traps and handling handling equipment. equipment. IF these guidelines guidelines are contravened due to human error, or malicious malpractice, serious accidents will occur. Accordingly, the need for a safety system that positively controls the entire process, without without dependence on human judgement, judgement, is clear. The many separate separate operations involved when launching and receiving pigs must be made to follow a safe, predetermined path and the most widely accepted and reliable method for achieving this is by Key Transfer Interlocking. Mechanical key transfer interlocking has developed from the principle that actions performed in the correct sequence are safe but potentially lethal if performed out of sequence. Therefore, the use of trapped trapped key interlocks in pig trap trap operations will will limit the sequence of valve valve and end closure closure operations operations to a single, unchanging unchanging path. By fitting interlocks to all relevant valves, as well as to the end closures, it becomes impossible to load or retrieve a pig without first depressurising the pig trap. In summary, key transfer interlocking provides a logical method or controlling pig launching and receiving receiving procedures, procedures, no matter how complex. complex. It ensures that procedures can only be performed in the correct sequence and eliminates the possibility of human error.


Page 14

Section 6


PIG TRAPS OPERATING METHOD 6.3

OPERATING METHOD

The following section describes typical operating sequences for launching and receiving pigs in liquid service pipelines. The sequences are general in nature and are intended solely as a means of explaining the working principles of some of the many types of trap in existence. existence. They (the sequences) are are not definitive and should should not be used for training pig trap operators, nor should they form any part of the operating procedures for specific launching and receiving installations. 1.

Launching

Key a. b. c. d. e. f. g. h.

i.

Pressure Gauge Vent Pressure Relief Drain Pig Signaller Kicker Valve Mainline Bypass Valve Mainline Trap Valve Quick Release Closure

Assumptions prior to launching: • • •

Trap is full (of pipeline product) and is under pressure Valves (f), (g), (h) are open Valves (d) and (b) are closed

Launching Procedure: 1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

Close valves (f) and (h) Open valve (d) followed by valve (b). Air will displace the liquid and the trap will drain. When the trap has been fully drained (0 psig), open the closure door and push the pig into the trap until the first cup (or seal) forms a tight fit in the reducer Close and secure the closure door, following the manufacturer’s operating procedure, and close valve (d) Open valve (f) slowly. The trap will begin to fill and any residual air will be forced out through valve (b). Slowly continue to fill the trap. When the trap is full, close valve (b) and allow the pressure to equalise. Close valve (f) Open valve (h) and then open valve (f). The pig is now ready for launching Partially close valve (g). The liquid flow through valve (f) behind the pig will increase. Continue to close valve (g) (g) until the pig signaller signaller (e) indicates that the pig has moved out of the trap into the mainline stream. When the pig signaller signaller (e) (e) has indicated that the pig has left the trap, fully open valve (g)


Page 15

Section 6


PIG TRAPS 3.

Receiving Key a. b. c. d. e. f. g. h. i.

Pressure Gauge Vent Pressure Relief Drain

Pig Signaller Bypass Valve Mainline Bypass Valve Mainline Trap Valve Quick Release Closure

Assumptions Prior to Receiving: • • • •

Trap is empty of all product and is at atmospheric pressure Valves (b), (d) and (g) are open Valve (f) and (h) are closed Closure door is closed in accordance with the manufacturer’s operating procedures

Receiving Procedure: 1.

6.

Close valve (d) and slowly open valve (f). Th trap will begin to fill and any residual air will be vented through valve (b) Close valve (b) and allow trap pressure to equalise through valve (f) Open valve (h). Trap is now ready to receive pig On its arrival, if the pig stops at the point marked ‘X’, partially close valve (g). Increased flow through valve (f) will force the pig into the trap When the pig signaller (e) indicated that the pig has entered the trap, fully open valve (g) and close valve (f) and (h) Open valves (d) and (b). The trap will drain

6.4

SELECTION

2. 3. 4. 5.

To provide the user with the most suitable equipment for their application, Pipeline Engineering would appreciate the following information: • • • •

Pipeline diameter and wall thickness Pipeline pressure, temperature and product Design requirements: code, pressure and temperature Inspection/certification Inspection/certification requirements


Page 16

Section 6

PIG_TRAPS

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