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Design and Application Manual

PIPELINE PIGS & PIGGING

CONTENTS - SECTION 1.0

1.1

1.2

Tools and Techniques

1.1.1

Description (What are Pigs?)

1.1.2

Function (What do pigs do?)

1.1.3

Description of Pigging Functions

1.1.4

Method (How do pigs work?)

1.1.5

Selection (How do I choose a pig?)

Pipeline Design Factors

1.2.1

Internal Conditions (Conditions inside the pipe)

1.2.2

Pipeline Specifications (Dimensions & materials)

1.2.3

Fittings & Equipment (Components fitted to the pipe)

 Copyright 1999 Pipeline Engineering & Supply Co. Ltd

Page 1

Section 1


PIPELINE PIGS & PIGGING TOOLS AND TECHNIQUES 1.1.1

DESCRIPTION (“What are Pigs?”)

1.1.1.1

OVERVIEW

Pipeline Engineering’s formidable reputation is based on the high specification and quality of its pigs, traps and related equipment – which have been used to maintain pipelines world-wide for many years. But, what is a ‘pig’. The name refers to a mechanical device that can be inserted into a pipeline and used to carry out pre-defined tasks (depending upon its construction) at critical points, or along the full length of the pipeline. Pigs are driven by pressurising the pipeline behind them or, in most cases, inserting them into the existing product flow, using specialised laundering equipment. Whilst the potential applications for pigs are limited only by the imagination of the specifier and the manufacturer, most tasks have now been defined and accepted design characteristics are used by all the major suppliers. Pigs and spheres must be suitably equipped to carry out a function or ‘task’. These tasks are given names (cleaning, gauging, separation, drying, etc.) but the demands of any individual task may require very different qualities from the pig depending upon the stage in the development of the pipeline (pipeline development stage) at which the pig is to be used (construction, pre-commissioning, commissioning, etc.) Pipeline design factors influence the overall design of the pig and the types of seals and fittings to be used. 1.1.1.2

PIG TYPES

The chart in fig. 1 shows the way in which pigs are generally classified: Fig. 1 Pipeline Pigs Utility Pigs

In-Line Inspection (ILI) Tools

Metal-Bodied Pigs (Mandrel Pigs)

Solid Cast Pigs

Bi-Directional Uni-Directional (Disc) (Cup) Standard

Foam Pigs

Hard Medium Soft

Spheres

Special Pigs

Solid Inflatable

Conical


Page 2

Section 1


PIPELINE PIGS & PIGGING There are two types of pigs at the highest level: 1. 2.

In-line inspection tools (‘Intelligent Pigs’) Utility pigs

1.

In-Line Inspection Tools (I.L.I)

Because of the complexity of these pigs (which make extensive use of on-board electronics, they fall outside the scope of this manual. 2.

Utility Pigs

Utility pigs include all categories except intelligent pigs. They can be divided into 5 sub-groups based on their construction characteristics: 1. 2. 3. 4. 5.

Metal bodied pigs (often referred to as ‘mandrel’ pigs) – sealing components axially mounted on a tubular or solid metal body. Solid cast pigs – single piece polyurethane casting – usually ‘dumb-bell’ shaped Foam pigs – moulded polyurethane foam, usually in 3 ‘densities’: soft; medium; hard. Spheres – manufactured from cast polyurethane and various rubbers according to task Special pigs – construction is similar to metal bodied pigs.

Specialised categories are allocated according to function (the task, or type of work that the pigs will be required to perform). Although functions are referred to collectively (cleaning, gauging, etc.) operating conditions will vary according to the pipeline development stage. This will influence parameters such as component rates of wear which will, in turn, effect the quantity of spares or complete pigs required to complete the task. 1.1.1.3

CONSTRUCTION CHARACTERISTICS

Overview The length of metal-bodies, solid cast and foam pigs is approximately 1½ - 2 x nominal diameter of the pipeline in which they will be operating. Special pigs may not confirm to this formula due to other factors that may effect their design. Spheres are, obviously, spherical. There are 3 elements to be considered with regard to pig construction: • • •

The pig body The type of seal The fittings

Only metal-bodied and special pigs have independent bodies onto which seals and fittings can be attached. These bodies are usually made from carbon steel, although polyurethane can be specified in some types of pig for improved flexibility.


Page 3

Section 1


PIPELINE PIGS & PIGGING Solid cast pigs are constructed entirely from polyurethane whilst spheres can be mad from several types of elastometric material: • •

Polyurethanes Rubbers: Neoprene; Nitrile; Silicon; Viton Seals

There are 3 types of detachable seal: • • •

Discs Standard Cups Conical Cups

Seals made from polyurethane are specified for most pigging functions – it possesses excellent physical characteristics which can be formulated for specific applications. However, there are good reasons for specifying some rubbers, which are given later. Careful formulation of urethanes allows characteristics such as harness, tear strength, abrasion resistance, hydrocarbon and hydrolysis resistance, flex life and compression set to be modified. There are 2 types of polyurethane: • •

Ethers (Good hydrolysis resistance; high resilience; low compression set). Esters (Better physical properties than ethers).

As a general rule, it is not always possible to specify and formulate a particular urethane, unless all the operating conditions are taken into account. Although urethanes work in applications where rubbers could not, they must be carefully selected, especially for very critical tasks. Pipeline Engineering was one of the first companies to use polyurethane for pig seals in the late 1960’s. The company developed its own polyurethane, known as ‘Omnithane®’ that was very successful and is still being specified by clients. However, the drive for technical excellence led to the development of ‘Super Omnithane®’ an ester/MDI/diol system and ‘Hyper Omnithane®’ a TDI/amine system. These formulations are used for the seals of some of the most durable pigs in use today. Fittings Fittings are those items other than seals which can be attached and removed from the pig body; these include: brushes; scraper blades; plough blades; transmitter housings; gauging plates; magnets and any item not usually found on a standard pig. 1.1.1.4

PIG CHARACTERISTICS BY TYPE

Metal-Bodied Pigs This is the only pig type in which the 3 construction elements can be identified separately. This form of construction allows the specifier to develop a pig that accurately meets the demands of the task.


Page 4

Section 1


PIPELINE PIGS & PIGGING The pig body is manufactured from steel tube in pigs 8” and above and a steel bar in pigs below 8”. Other materials, including aluminium and polyurethane, are also used. Flanges are welded to the body at both ends of the pig for seals and spacers; studs and pads are provided for attachment of fittings. These pigs incorporate discs, standard cups and conical cups, manufactured primarily from polyurethane, although discs can also be supplied in several types of rubber, as previously described. The quantity and type of seal depends upon the pigging function and operating conditions. Metal-bodies pigs allow the greatest range of fittings to be attached. This range covers fittings of all types and includes brushes, scrapers, plough blades, transmitted housings, gauging plates, magnets and many other types necessary for specific tasks. Solid Cast Pigs These pigs are made from a single polyurethane casting. Therefore, the sealing elements are not interchangeable or replaceable. All seals are disc-type and comprise 4 sealing and 2 support discs. They are designed, primarily, for small-bore pipelines of approximately 2” to 12” nominal diameter. The one-piece construction method limits the choice of fittings. However, brushes, gauging plates and magnets (for tracking and location purposes) can be fitted to this type of pig. Foam Pigs Foam pigs are often specified in similar circumstances to metal-bodies pigs. However, they employ very different design characteristics. The pigs are normally bullet shaped and moulded from open cell polyurethane foam which is available in several densities (designated soft; medium; hard). External coats are applied to increase the pigs’ effectiveness in specified operating conditions. They have no independent sealing elements. The pig is compressed in the pipeline by the pressure and forced against the pipe wall to form a seal. The density of the pig is matched to the pigging function and operating conditions. ‘Hard’ pigs are tougher and last longer; ‘soft’ pigs are more commonly known as ‘swabs’ and are used for drying and other specialised tasks. A thin polyurethane coating is often applied to bare pigs to help increase the efficiency of the pig with respect to its function: ‘cross-cross’, spiral or fully coated. A coating of polyurethane is also applied to the base (the flat end) of most bar pigs. Although there are many options for foam pigs, there are relatively few fittings. Specialisation is controlled at the moulding stage when options are ‘built-in’: gritted bands, brushes, jetting holes, magnet inserts (for tracking and location), transmitted cavities, ropes, studs for gauging plates, stud inserts for scraping are some of the options available. Some fittings are removable and include magnets, gauging plates, transmitters and studs for scraping.


Page 5

Section 1


PIPELINE PIGS & PIGGING Spheres The spherical elastomeric body of a sphere forms a single-line seal in the pipe. Therefore, there are no independent sealing elements to consider. Spheres do not accept fittings (with the exception of some spheres which have magnets cast into them for tracking and location purposes). There are 2 types of sphere (with typical uses given): • •

Solid – automated pigging programmes; batching; condensate removal Inflatable – meter proving; batching; condensate removal

Once the type has been decided, the characteristics of the pig are dependent upon the formulation of the material from which the spheres are constructed. This will influence properties such as sealing efficiency, rate of wear, operating temperature limits and chemical resistance. Inflatable spheres have a removable valve and are usually filled with water or a water/glycol mixture. Meter proving spheres are produced in hardnesses of 65-70° Shore A while, for most other operational purposes, spheres tend to be specified with hardnesses of approximately 65 – 75° Shore A. Special Pigs ‘Special pigs define those pigs – usually metal-bodies – which have to be significantly modified (relative to the ‘ideal’ standard in terms of dimensions and seal configurations) in order to pass through the limitations imposed by the pipeline design factors which exist for that specific pigging application. Special pigs include: • • •

Dual diameter pigs Articulated pigs High differential pigs

By definition, special pigs must cope with increased problems imposed by the pipeline design factors. Therefore, their design is much more critical. Body, seals and fittings may use standard components, but it is much more likely that they will include characteristics which will require detailed information on all aspects of the pipeline and its design factors.


Page 6

Section 1


PIPELINE PIGS & PIGGING 1.1.2

FUNCTION – (What Do Pigs Do?)

1.1.2.1

OVERVIEW

Pigs must first be selected to carry out a specific function (task). This results in a pig which can generally be described by its function type. e.g cleaning pig, gauging pig, de-watering pig (section 1.4.0 explains this principle in more detail). For now, the scope of pigging functions is set out as follows: 1.1.2.2

PIGGING FUNCTIONS

As a result of the experience gained in pigging operations world-wide, most functions are now pre-defined and can be found somewhere within the following list: • • • • • • • • • • • • •

Debris removal Cleaning Gauging Filling De-watering Drying Separation (batching) Condensate removal Meter proving Product displacement Product conversion Gel pigging Coating application

These pre-defined functions all rely on one (or a combination of) the following aspects of the pig’s design characteristics (the ability(s) required to complete the task successfully): • • • • •

Ability to SEAL Ability to CLEAN Ability to GAUGE Ability to ABSORB Ability to RETAIN PRODUCTS

Some of the characteristics may need to be enhanced in order to provide the best pig for the task (e.g more – or improved – seals and fittings may be required). All will be influenced by the pipeline development stage.


Page 7

Section 1


PIPELINE PIGS & PIGGING 1.1.2.3

PIPELINE DEVELOPMENT STAGE

Pigging function depends upon the stage in the operational life of a pipeline at which the pig must carry out its task. Most stages in the development of a pipeline can be found in the following list: • • • • • • • •

Construction Pre-commissioning (including hydrostatic testing) Commissioning On-line operations Inspection Maintenance and repair Renovation and rehabilitation Decommissioning

Some pre-defined functions can take place at several stages in the development of the pipeline. However, pigs designed to carry out tasks at one stage of pipeline development may not possess features that are suitable to enable them to carry out the same task at other stages. This is due to a combination of the following factors: • • • •

Pigging distance Frictional resistance of the materials in contact Lubrication Pigging speeds

These factors are dealt with later when we consider pipeline design factors and the operating conditions that apply. However, for now we can consider what is involved in each of the pigging functions previously listed. 1.1.2.4

DESCRIPTIONS OF PIGGING FUNCTIONS

1.1.3.1

DEBRIS REMOVAL

Debris removal is generally the first operation to be considered. It occurs at the construction stage and involved the clearance of the constructor’s waste, rocks, sand, dead animals, etc. These items must be removed prior to cleaning and gauging operations at the pre-commissioning stage. This particular task is extremely demanding and requires a robust, well specified pig. Bi-directional pigs – either foam or metal-bodied – are required as it may be necessary to reverse them if they become stuck. The nature of the operation varies depending on whether it is carried our on land of offshore/sub-sea. Sub-sea operations are more complex and may involve the use of ‘lay-down heads’ with the pigs being pre-loaded for operations after the line is ‘tiedin’. Debris removal onshore usually involves pigging with compressed air between ‘test-ends’ (temporary pig traps).


Page 8

Section 1


PIPELINE PIGS & PIGGING Compressed air operations impose several conditions on the pig – no lubrication is available and pigging occurs in a series of high speed ‘excursions’ between localised restrictions. The pig stops and the pressure builds until there is sufficient energy to launch the pig past the obstacle. As the pig moves forward rapidly, the pressure is dissipated until the pig reaches the next restriction, causing the process to be repeated. 1.1.3.2

CLEANING

The specification of cleaning pigs depends upon the pipeline development stage at which cleaning is required – usually at one of the following: • • • • •

Pre-commissioning On-line operations Inspection Renovation and rehabilitation Decommissioning

Cleaning at each stage imposes a different set of demands on the pig. This could, but would not automatically, require variations in pig design. There are 2 main requirements for cleaning – irrespective of the type of pig. 1. 2.

Pigs must be fitted with cleaning devices – brushes (circular or spring mounted; scrapers; ploughs. ‘By-pass’ must be introduced across the pig and, in particular, the brushes.

Cleaning operations are usually carried out by either metal-bodied or foam pigs. Both have advantages, although metal-bodied pigs can be more readily adapted to suit specific conditions. Liquid flows improve cleaning efficiency by allowing the pig to maintain a constant velocity. Unfortunately, a liquid flow is not always available (at the construction stage, for example). Cleaning then has to be done with compressed air, where the efficiency is diminished as the pig speed cannot be easily controlled. Cleaning included the removal of ‘scale’ (mill scale, silica, coke, calcium) which often requires a much more aggressive pig. Conversely, pigs can be fitted with nylon brushes for cleaning pipes that are internally lined. Foam pigs are often used for this task – ‘hard’ foam pigs have inserts for studs integrally moulded into the pig body. These studs are made from steel and are available in several types to suit the operating conditions. Metal-bodied pigs are available with steel scraped blades and special pigs (such as the pin-wheel pig) have also been developed for this purpose. Old pipelines, or those which have never previously peen pigged, require a special approach. A progressive pigging programme must be carried out to avoid the risk of a more robust pig becoming stuck in a pipeline which cannot be accurately evaluated. This approach involves the step-by-step use of progressively harder, more aggressive foam pigs, run until the pipeline is considered acceptable for metal-bodies pigs to be used.


Page 9

Section 1



GAUGING

Gauging is commonly carried out at the following stages: • • •

Construction Maintenance and repair Decommissioning

It is necessary for 2 reasons: • •

To check that there are no unintended intrusions into the Pipeline Engineering To confirm that the ovality of the pipeline is within acceptable limits (tolerances)

Metal-bodied pigs (uni-directional or bi-directional) are fitted with a circular aluminium gauging plate (steel plates can also be used), which is usually machines to 95% of the smallest internal pipeline diameter. Foam pigs can also be adapted to accept gauging plates but they are not often used for this purpose. At the construction stage, gauging operations are usually conducted with compressed air. As with all other operations using compressed air as the pigging medium, it is extremely dangerous and less effective. 1.1.3.4

FILLING

Filling is carried out at the following stages: 1. 2.

Pre-commissioning Commissioning

1.

Pre-Commissioning

Filling is carried out in order to evacuate air and fill the line with a solid column of water prior to hydrostatic testing. The most important characteristic required is the ability of the pig to maintain an effective seal against the pipe wall in order to minimise leakage. For this reason metal-bodied bi-directional pigs using multiple sealing discs are specified. Pigs are run in front of the water column, evacuating the air from the pipeline that is vented at the receiver. It is extremely important that all pipeline design factors that apply are considered in order to produce a pig which is capable of travelling through the pipeline system successfully. 2.

Commissioning

Filling is carried our in order to bring the pipeline to the point of operation – when it has been filled with product. Several metal-bodied, disc-type pigs are required as filling involves ‘slugs’ of other liquids e.g methanol, glycol, held between pigs preceding the pipeline product. Commissioning is complex and required considerable expertise.


Page 10

Section 1



DE-WATERING

After hydrostatic testing the water must be removed from the pipeline prior to drying and subsequent commissioning. This is done by a series of pigs pushed through the pipeline using compressed air. The same qualities are required for de-watering as for filling – the most important characteristic again being the ability to maintain an effective seal in order to minimise leakage. All relevant pipeline design factors should be considered. Soft foam pigs (swabs) are often run after de-watering to swab away any water remaining on the pipe walls, or which has accumulated at low points in the pipeline. 1.1.3.6

DRYING

In on-shore pipelines, foam swabs are pigged with extremely dry compressed air (90°F atmospheric dew point temperature). Together with pigs and the air pick up the remaining water leaving behind a dry pipeline. However, it takes more than one swab to do this and is, of course, dependent on the length of the pipeline. Several hundred swabs may be required to fully dry a pipeline. After the pipeline is dry, brush pigs must be run to remove the rust, mill scale and any deposits that have settled out from the hydrostatic test. Again, several hundred swabs may be required to remove the material loosened by the brush pigs. These swabs will then be run until the air reaches a specified dewpoint. 1.1.3.7

SEPARATION (BATCHING)

Batching is a technique used during on-line operations, primarily on multi-product pipelines that transport different products in the same line at the same time. In order to minimise ‘interface mixing’ (caused when two products being transported in the same line come into contact with each other), the pig has to be inserted into the pipeline at exactly the right time. This task can be automated by using spheres preloaded into magazines. They can then be automatically inserted into the pipeline when required. 1.1.3.8

CONDENSATE REMOVAL

Condensate removal is a constant activity during on-line operations in untreated gas lines. It settles out in low spots in the pipeline, reducing efficiency. As it builds up, the condensate is pushed along the line by the flow until it reaches the receiver. The resulting slug can be so large that it frequently exceeds the capacity of the slug catcher set up to receive it. Sphering provides the ideal solution. Automatic launching from pre-loaded magazines allows un-manned pigging activity over extended periods. Spheres do not have to seal in the pipe – their purpose is to move the condensate into the slug catcher at the receiver.


Page 11

Section 1



METER PROVING

Meter proving describes the procedure for calibrating flow meters (e.g turbine and positive displacement meters) in which pigs are used to batch a known volume of product through the meter. This volume is then compared against the volume indicated by the meter. Early meter provers were simply a ‘measured mile’ of pipe in which batching pigs were used to push the product through the meter in one direction only. When the pigs had completed the task they were transported back to the launcher to be used again. To save on transportation and handling costs, bi-directional pigs began to be used as these pigs could be returned simply by reversing the flow of the pipeline. The next development saw spheres used in a continuous pipeline loop with an interchange valve closing the top from the bottom. These interchange valves subsequently proved to be inadequate and bi-directional meter provers became standard. In their simplest forms, meter provers consist of an internally coated pipe containing an inflatable sphere. Electrical pig signallers and a scraper trap (for launching and receiving spheres) are installed at each end of the pipe. The volume of product contained in the pipe between the detectors is accurately calibrated against certified columetric tanks and the meter to be proved s connected in series with the meter prover system. Outputs from both signallers and the flow meter are connected to a counter. When the pig trips signaller 1, pulses from the flow meter are counted. When the pig trips counter 2, counting stops and the count is recorded. The signal count is compared against the known volume to obtain a factor which can then be used to calibrate the meter. Only inflatable spheres with a hardness of between 65° - 70° Shore A are specified for use in meter prover loops. 1.1.3.10

PLUGGING

Plugging is carried out during the maintenance and repair phase of a pipeline’s life. The plug is most commonly a modified metal-bodied, bi-directional, disc-type pig in which the sealing elements not only seal but cause the pig to be an extremely tight fit in the pipeline. Depending upon the diameter of the pig, pressures of up to 8 bar may be required to move it in the pipeline – higher pressures are required to reverse the pig and ‘flip’ the discs. Plugging is often carried out in ‘trains’ where the pressure that a single pig can withstand s cumulative and which therefore allows a series of pigs to withstand greater differential pressures.


Page 12

Section 1



PRODUCT DISPLACEMENT

The same qualities are required for displacement as for filling and de-watering duties. Product displacement is carried out whenever the pipeline contents have to be evacuated, particularly at the decommissioning stage. 1.1.3.12

PRODUCT CONVERSION

This involves ‘sterilising’ the pipeline prior to running other products which may become contaminated. Pigs used for this purpose require similar quantities to pigs used for filling and de-watering duties. 1.1.3.13

GEL PIGGING

This technique does not involve utility pigs. A jelly-like substance is either injected directly into the pipeline or moulded into cylindrical ‘pigs’. The jelly picks up and holds within it deposits found in the pipeline. It is bio-degradable and can be broken up under pressure. 1.1.3.14

COATING APPLICATION

There are 2 pigging phases involved in the application of ‘in situ’ pipeline coatings: 1 2

Cleaning Coating application

1.

Cleaning

Cleaning prior to coating application requires as much debris to be removed from the pipe wall as possible. Therefore, it is usually done using methods other than cleaning pigs. e.g: grit blasting techniques). Pigs are then used to remove loosened deposits from the pipeline.


Page 13

Section 1



METHOD – (“How Do Pigs Work?”)

1.1.4.1

PROPULSION METHODS

Pigs travel through the pipeline under pressure from the pigging medium being pumped behind them. This medium takes on of two forms: 1 2

Gas Liquid

Whatever the medium is – gas or liquid – has a dramatic effect on operational pigging characteristics, especially in terms of the pig’s velocity profile. Irrespective of the type of pigging medium, pig velocity has a profound effect on pigging efficiency. Pigs are designed to work most efficiently within a limited velocity range which may be difficult to achieve, depending upon the pigging medium or other factors, such as the ability to regulate existing product flow rates. 1.1.4.2

PIGGING WITH A GAS

A stationary pig requires increased pressure to start it moving. Once this pressure is reached, the pig moves forward very quickly as the pressure begins to decrease behind it. Eventually the pig comes into contact with an obstruction, such as a weld bead, the pressure is insufficient to keep it moving forward and the pig stops. The cycle then begins again. This velocity profile is typical when pigging with gasses, due to their ability to be compressed. Once the pig begins to move this energy is released very rapidly. Pigs have been measured at speeds in excess of 100mph and have been know to burst out of thin walled pipes at bends in the pipeline. Aside from the safety aspects, pigging in this manner is inefficient, increases wear on the pig seals and brushes (if used) and is generally more destructive to the pig. 1.1.4.3

PIGGING WITH A LIQUID

This method is much more efficient. Velocities can be maintained, within limits. They are lower and controllable at the pump. Seal wear rates are also reduced as the product or pigging medium acts as a lubricant. The pressure required to move a pig depends upon a number of factors: • • • •

The pressure in front of the pig Condition of the pipe wall surface Pressure exerted by the seals on the pipe wall (the ‘fit’ of the pig in the pipe) The presence of a lubricant

Pigs move in the pipeline under the influence of the ‘differential ‘pressure across them. That is to say, the pressure behind the pig (P1) minus the pressure in front of the pig (P2). This differential pressure (‘delta P) gives the pig a velocity. (i.e. It gives the pig both speed and direction). Obviously if P1 is less than P2 the pig will move backwards – which is only of benefit when using bi-directional pigs (refer to illustrations on page 14).  Copyright 1999 Pipeline Engineering & Supply Co. Ltd

Page 14

Section 1


PIPELINE PIGS & PIGGING The differential pressure contributes to the efficiency of pigging operations in the following ways: • •

It improves sealing efficiency by forcing the seals against the pipe wall, making them act like non-return valves. It is used as ‘by-pass’ in cleaning operations

1.1.4.4

BENEFITS OF BY-PASS

‘By-Pass’ is the controlled flow of pigging medium, gas or liquid, through and/or past the pig. It has two effects: •

It reduces the pressure (P1) behind the pig and hence the differential pressure (delta P) across the pig – variations in the differential pressure effects the speed of the pig. The pig slows down as the differential pressure decreases.

•

It provides a very necessary jetting action which increases the efficiency of cleaning pigs by preventing brushes from becoming blocked with loosened deposits and holding these deposits in suspension in front of the pig rather than allowing them to form into a solid ‘slug’. These slugs are difficult to deal with at the receiver but, more importantly, may cause the pig to become stuck.

Studies have shown that it is better to drive the pig in the pipe using the front seals only, the rear seals act as support. Pluggable by-pass ports are built into the pig body for this purpose and allow the pressure to flow through the pig body and be directed into the space between the two sets of seals. More ports can be added to create flowpath right through the pig. This reduces the pressure P1 behind the pig and the differential pressure (delta P) across the pig. Consequently, the pig can be driven at slower speeds in pipelines with fixed flow rates. The relative importance of by-pass depends on the task which the pig has been designed to perform. These tasks can be divided into 2 groups: 1. 2.

Sealing Cleaning

1.

Sealing

If the pig is used for operations where its ability to seal in the pipe is most important (e.g filling, de-watering) then the principle benefit of by-pass is to allow the pig to be driven from the front set of seals. However, in addition, it is thought that by-pass may help prevent the formation of slugs of solids building up in the liquid which the pig would then tend to ride over and leave behind in the pipeline.


Page 15

Section 1


PIPELINE PIGS & PIGGING 2.

Cleaning

During cleaning operations, especially was removal, the deposits on the pipe wall very quickly build up in the brush reducing its effectiveness. To prevent this happening, limited by-pass can be used to create a flow path of pipeline product through and around the brushes. This tends to prevent the deposits removed from the pipe wall from building up in the brush by holding them in suspension within the product flow created by the by-pass. Pigs are often fitted with jetting nozzles (also known as ‘spider noses’) which are positioned in front of the leading set of seals. The nozzle jets are directed back towards the leading discs or brushes (depending on the pig type) and again are used to create a flow of product that is designed to hold in suspension any deposits removed from the pipe wall.


Page 16

Section 1


PIPELINE PIGS & PIGGING 1.1.5 SELECTION – (“How Do I Chose A Pig?”) 1.1.5.1

OVERVIEW

There are 3 fundamental questions to ask when selecting the appropriate type of pig: 1. What is the function, of task of, the pig? 2. At what state in pipeline development will the task be performed 3. Which pipeline design factors are relevant and to what extent will they influence pig design This approach will establish: 1. The most suitable type of pig and consequent arrangement of body, seals and fittings 2. The extent of any modifications and additions to the body, seals and fitting of standard pigs (resulting in the basis of a design or a special pig). All pre-defined tasks will be carried our by one of the following 5 types of pig: 1. 2. 3. 4. 5.

Metal-bodied pigs Solid cast pigs Foam pigs Spheres Special pigs

The construction characteristics of each type of pig influences its suitability for a particular task. Some types (metal-bodied and foam) include within their range pigs suitable for most tasks while others (solid cast and spheres) are themselves specialised and are limited in the number of options they can offer. Special pigs are those pigs which have had their body and seal configurations significantly modified as a result of pipeline design factors or which belong to no pre-defined category. In order to carry out the function for which they were designed, many pigs are equipped with fittings of some type. The range of fittings, and hence the range of tasks, are limited by the type of pig specified. Metal-bodied pigs, other than those specified solely for their sealing qualities, may incorporate any of the fittings listed below: • • • • • • •

Blades Brushes Scrapers Magnets Gauging plates Jetting nozzles Transmitters


Page 17

Section 1


PIPELINE PIGS & PIGGING The way in which some fittings are mounted will influence the pigging direction. The following table refers to those fittings which do influence the pigging direction: Uni-Directional

Bi-Directional

Brushes

‘U’-spring mounted Cantilever Mounted

Circular Arch-spring mounted

Blades


Arch-spring mounted

Scrapers


Arch-spring mounted

Each pig type incorporated very different design characteristics which are examined in more detail below: 1.1.5.2

METAL-BODIED PIGS

Metal-bodied pigs are divided into 2 groups according to their directional ability within the pipeline. This can be selected or, more likely, will be imposed by the operating conditions; • •

Uni-directional pigs travel through the pipeline in one direction only Bi-directional pigs travel through the pipeline in either direction

The directional ability of a metal-bodied pig depends upon 2 factors • •

Seal type Fittings

Seal Type There are 3 types of seal which are further classified by their suitability for use with regard to pigging direction: • • •

Discs – bi-directional Standard cups – uni-directional Conical cups – uni-directional

Discs Discs increase the scope of the pig to be used for a more diverse range of applications. They allow it to be reversed, they provide a scraping/swabbing action and are lighter and cheaper. However, they wear more quickly, are more prone to damage and are less able to pass obstructions and reductions in the pipeline than conical cups.


Page 18

Section 1


PIPELINE PIGS & PIGGING Cups • •

Standard – cost effective design Conical – greater flexibility of use

Selection depends on the pipeline design factors detailed in section 2.0.0. However, conical cups do offer one particular advantage over standard cups – an increased ability to pass restrictions in the pipeline (up to 20% of minimum pipeline internal diameter compared to 5% with standard cups). Other Factors After ensuring that the pig will travel through the pipeline, selection is based on the pigging function, particularly when the function relies on the qualities of the seal (e.g. de-watering, filling, pigging long distances) Unless operating conditions dictate otherwise, the pig body will be constructed relative to the diameter and length of the pig. This ratio is as follows and varies between pig types – it does not include spheres: Length of pig = nominal pipe diameter x ‘n’ (The value of n varies between approximately 1.4 – 2.0 according to pig type)

Fittings will then be sized and configured to match the pigging function. All of the fittings listed earlier in this section are available for selection for use with metalbodied pigs. 1.1.5.3

SOLID CAST PIGS

These pigs have limited application as they cannot be readily adapted to suit complex operating conditions. They are often used in the food process industry because they can be manufactured from a single piece of polyurethane that complies with food hygiene standards. They are typically supplied in diameters from 2” to 12” and the integrally moulded disc seals offer low leakage and high scraping efficiency compared with spheres. They are bi-directional and can have a limited range of brushes, gauging plates and magnets fitted (for tracking and location).


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FOAM PIGS

Foam pigs are initially selected according to pigging functions – the following types are commonly available for the task specified: • • • • • •

Bare – swabbing displacement Fully polyurethane coated – displacement Criss-cross polyurethane coated – displacement/light cleaning Wire brush – spiral pattern – cleaning Wire brush – fully covered – cleaning (long run) Silicon gritted – heavy cleaning/scale removal

The characteristics of foam pigs are more closely matched to the task by specifying the density of the open cell polyurethane foam: • • •

Soft – approximately 2lb/ft3 Medium – approximately 5lb/ft3 Hard – approximately 8lb/ft3

‘Hard’ foam pigs assist aggressive cleaning whilst ‘Soft’ foam pigs improve swabbing characteristics. Foam pigs are often specified as an alternative to metal-bodies pigs as they have several advantages. • • • • • •

They are relatively cheap They can pass through tight bends (ID depending on density and type) They are easier to handle than comparable metal-bodied pigs Softer, non-brush types do not require specialised launching facilities Swabs can be used for testing blockages in a pipeline (if they become stuck they will disintegrate under increased pressure) As a product group, they form the basis of a progressive pigging programme ( a vital approach for pipelines which have never been pigged previously).

Not all foam pigs are bi-directional but most can be suitably adapted at the design stage. Several types of fitting are available: • • • • • •

Jetting holes Magnet inserts Transmitter cavities (for tracking and location) Ropes for pulling (or being pulled) Studs for gauging plates Stud inserts for scraping


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PIPELINE PIGS & PIGGING 1.1.5.5.

SPHERES

Spheres play a vital role in pigging operation due, particularly, to their shape. It allows them to be pre-loaded into launcher magazines and used to carry out automated pigging operations (e.g. batching, condensate removal in untreated gas lines) where the launching facilities are un-manned for long periods. There are 2 types of sphere: • •

Solid Inflatable

Solid spheres can be made from various elastomer materials: • • •

Polyurethane Nitrile rubber Neoprene rubber

Solid spheres are used for batching and condensate removal from unmanned launching stations Inflatable spheres – made from polyurethane – are always specified for use in meter prover loops because the sphere can be precisely inflated to give a constant seal as the sphere wears with use. Meter prover spheres are softer than standard spheres (65-70° Shore A compared to 70 – 75° Shore A for standard spheres). Making the sphere softer improves the quality of the seal. Rubbers such at nitrile and neoprene allow spheres made of the materials to be used within a much greater temperature range than is available with spheres made from the standard formulations of polyurethane. However, polyurethane formulations do exist which greatly improve the sphere’s ability to operate in temperatures up to 150°C The best polyurethane spheres are of seamless construction which, although it is more technically difficult to achieve and requires significant investment by the manufacturer, results in a sphere of much higher quality. Due to their construction and sealing characteristics, spheres cannot accept fittings however, some spheres do have magnets incorporated during the moulding process for tracking and location purposes.


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SPECIAL PIGS

If standard pigs must be modified to such an extent that they no longer comply with an ‘ideal’ standard model in order to perform a function, then a ‘special’ pig must be designed. This usually involves changing the dimensions of a pig relative to the nominal pipeline diameter. The body dimensions may be changed or constructed in two modules (articulated), and the fittings and seals may also be suitably adapted. In some instances, the dimensions of the pig remain unchanged with modifications being confined to altering the specification of the materials used in the construction of the pig. Pigs designed for one-off tasks (tasks for which there is unlikely to be any further demand) can also be classified as special pigs.


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PIPELINE PIGS & PIGGING PIPELINE DESIGN FACTORS 1.2.1

INTERNAL CONDITIONS (“What Are Conditions Inside the Pipe?”)

This section considers the potential problems posed by factors such as the chemical compatibility of the pipeline product with the materials used in the construction of the pig. It also considers the contributory effects of temperature, pressure, flow and pipeline deposits on the pig and its components. 1.2.1.1.

PIPELINE PRODUCTS

Most pipelines carry one or more of the products listed below: • • • • • •

Hydrocarbons Petrochemicals Chemicals Water Food Products Gases

Each of these product groups has an effect on polyurethanes and rubbers to a greater or lesser extent. (Pig body materials are also effected by pipeline product). The typical effect of chemical incompatibility with polyurethanes and rubber leads to blistering, cracking and chemical breakdown (Material is described as having a ‘cheese-like’ consistency). Whilst it is extremely important to select a material which has good compatibility with the pipeline product, a compromise will almost certainly have to be made because the most compatible material with the product is unlikely to have the necessary physical characteristics. Most rubbers can be moulded into spheres or supplied in sheet form (hence can be supplied as discs) but they cannot be economically moulded into more complex cup shapes. Therefore, the choice of seal materials may have great effect on the overall pig design. 1.2.1.2

TEMPERATURE

Internal pipe temperatures primarily effect elastomeric components and, in particular, pig seals. Temperature increases the effect that product incompatibility has on elastomeric components. Elastomers, in this instance rubbers and polyurethanes, operate most effectively within specified limits. These limits are much wider in rubbers (Nitrile, Silicon, Neoprene, Viton) than they are in polyurethanes, which have relatively low upper operating limits. In almost all instances, rubbers withstand high temperatures better than polyurethanes but have significantly inferior physical characteristics, resulting in inferior wear rates, material strength, etc.


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PRESSURE

Pressure does not have a significant effect on the construction of utility pigs which contain no sealed units. In-Line (I.L.I) pigs do have sealed units and pipeline pressure has a much more significant effect on this type of equipment. The general effect of pressure is to move the pig in the pipeline. If this movement is smooth and constant then pigging will be much more efficient. In pipelines transporting liquids, this characteristic movement is not difficult to achieve. However, it is much more difficult in gas pipelines or where pigging is being carried out using compressed air. Minimum pressure levels need to be achieved in order to keep pigs moving past obstacles (typically weld beads) without stopping. IF a pig does stop, pressure will increase until it is sufficient to launch the pig past the obstacle. Initial acceleration will be rapid and overall velocity will be high with consequent safety implications, possible damage to pipelines and fittings and much lower pigging efficiency. These problems can be relieved by pressurising the pipeline ahead of the pig and venting at the receiver. Recommended minimum pigging pressures are often requested but are not easy to supply as they are a compound of many factors: function, pipeline, condition, deposit, lubricant, seal material and type, liquid or gas pigging. 1.2.1.4

PRODUCT FLOW

Maximum pigging efficiency id dependent upon a constant product flow that, by definition, is usually only available in on-stream applications. There are guidelines giving approximate pigging speeds for different types of pig however, the speed is dependent on the velocity of the product flow and may not be changeable. The main consideration is t move the pig smoothly at a constant speed through the pipeline. Pigging in gas lines is less efficient and moor likely to result in ‘speed excursions’. This is more destructive to the pigs as the higher speeds and more abrasive conditions cause greater wear on the pig components. Increased speed also causes a decreased pressure differential across the pig which, in turn, results in decreased sealing efficiency. 1.2.1.5

DEPOSITS

The main types of deposit are as follows: • • • • • •

Condensate (untreated gas lines) Water (most lines) Waxes (oil lines) Scale Mineral deposits Rust

The type of deposit to be removed influences the type of seal and cleaning elements required for carrying out the work most effectively.


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PIPELINE PIGS & PIGGING Condensates and water are swabbed our using pigs with seal discs, spheres or foam pigs. Polyurethane ploughs and scrapers are recommended for removing soft and hard waxes respectively. Scale and mineral deposits often have to be removed using foam stud pigs to break up the hard scale initially then followed using cleaning pigs equipped with brushes. Rust (corrosion) can be removed using brush-equipped cleaning pigs, possibly with magnets fitted to pick up the ferrous debris. Foam pigs gritted with silicon carbide can be used to burnish the pipeline, which will improve frictional losses and which, in turn, lead to increased pipeline output.


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PIPELINE SPECIFICATIONS (“What Are the Pipeline Dimensions and Materials?”)

1.2.2.1

PIPELINE LENGTH

It is not the overall pipeline length that is of interest but the maximum pigging distance – the distance between launching and receiving stations. This factor influences the wear rate of the seals along with several other factors: • • • • • • •

Pipe wall condition Pipe wall lining (if any) Product compatibility Seal material Pigging medium Presence of lubricant Pigging speed

Throughout the world, pigging distances range from a few metres to hundreds of kilometres. Whilst the pig design for the two extremes may possibly involve similar characteristics, it is essential that, along with the factors mentioned above, the maximum pigging distance is specified. 1.2.2.2

PIPELINE DIAMETER

Most pipelines have a constant external diameter while their wall thickness changes to accommodate design pressure, etc. This variation in wall thickness can greatly effect the function of the pig, whether it is batching, cleaning or gauging. At the extremes, the variation in wall thickness is equivalent to specifying a dual diameter pig and many ‘standard’ pigs would possibly become stuck in the narrower bores. The other aspect relating to pipeline diameter is the tolerance on ovality, particularly important to consider when gauging. 1.2.2.3

MULTIPLE PIPELINE DIAMETERS

Pipeline systems can include pipes of two or more nominal diameters (say 8” and 10” for example). This can result in a very significant narrowing of the internal diameter of the pipe. In order to overcome this problem, dual-diameter pigs are available which are usually designed to cope with two changes in pipe diameter (e.g. 10” to 8”; 12” to 10”). Ideally, pigging should be carried out from the large diameter into the small diameter pipe. To assist the pig, the transition from diameter to diameter should be smooth and gradual. Reducers should be concentric with a taper no steeper than 1:5.


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PIPELINE MATERIALS

Pipelines are made from many different materials: • • • • • •

Steel, including stainless steel. Cast iron Wrought iron Cement Reinforced concrete Plastic

Whilst there are many reasons for selection of the line pipe, compatibility with the proposed product is extremely important. If the product is particularly aggressive, the pipe can be lined (this also improves the flow characteristics of the product in the pipe. 1.2.2.5

INTERNAL LININGS

Linings minimise corrosion of the pipe walls and improve the flow characteristics of the product. They can be applied when the pipe is constructed or after it has been laid. There are several types of lining: • • •

Epoxy linings Concrete linings Plastic lining (sleeve)

Pigs must be carefully chosen for use in lined pipe – all points of contact with the pipe (brushes, seals) should be constructed from non-metallic materials (polyurethane, nylon, etc.). 1.2.2.6

EXTERNAL COATINGS

External coatings do not directly influence pig design but can influence the reception of signals from pig mounted transmitters fitted for tracking and location purposes. Any external coating and the depth and nature of ground cover should be made known to the pig manufacturer who can then offer advice on suitably powerful transmitters, should they be required. 1.2.2.7 1. 2.

FLEXIBLES

Smooth bore – plastic lined for chemical/water injection systems Rough bore – for flowlines, jumpers, risers.

Smooth bore pipelines are not suitable for pigging and any metal-to-metal contact with pigs should be avoided in rough bore pipelines.


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BUNDLES

Bundles allow several pipes to be laid together. The operational pipelines are contained within a large conduit that is laid as a single pipeline in the normal manner. Bundled pipelines do not present any problems to utility pigging. 1.2.2.9

RELATIVE POSITION FEATURES

This refers to the minimum distance between any two significant pipeline features that might influence the efficiency of a pig or cause it to become stuck. These features are as previously listed and are particularly relevant to the positioning of: • • •

Valves Offtakes (tees and laterals) Bends

The general rule is that a minimum distance of 3 x pipeline diameters should be allowed between any two features.


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FITTINGS AND EQUIPMENT (“What Components Are Fitted to the Pipe?”)

1.2.3.1

VALVES

Valves present major obstacles to successful pipeline pigging. There are several types: • • • • •

Ball valves Gate vales Check valves Butterfly valves Plug type valves

Some types cannot be pigged but others can. Certain features can be included in the valve design to assist pigging. Only those valves which can be pigged are illustrated, with relevant features noted. There are two types of valve that cannot be pigged: •

Butterfly/Plug type

There are three types of valve that can be pigged: 1. Ball valves 2. Gate vales 3. Check valves Valve design should include the following features: 1. Ball valve: • Solid ball • Full bore • Concentric bore • Smooth, shallow transitions between valve bore and pipe 2. Gate valve (Through-conduit only) • Full bore • No gaps or intrusions • Concentric bores • Smooth, shallow transitions between valve bore and pipe 3. Check Valve • Full bore • Concentric bores • Smooth, shallow transitions between valve bore and pipe • Contoured (shaped) valve clapper • Side hinged valve clapper improves piggability


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PIPELINE PIGS & PIGGING Possibly the single most important factor applies to all valves, irrespective of type. They must all be OPEN. Appropriate mechanisms or checking procedures must be in place to ensure that this happens as the consequences of not doing so involve great expense. Valves should always be fully specified as there will usually be some compromise with regard to the most effective pig design for the function. Specialised components may need to be added to the pig to improve its ability to pass safely through any valves. 1.2.3.2

OFFTAKES

All offtakes present a potential hazard to pigging. It is easy for pigs, especially spheres, to be diverted into an offtake and become stuck or continue their journey down the offtake with obvious and expensive implications. Alternatively, if the size and relative positioning of offtakes are not offered for consideration at the pig design stage, it is possible that the pig may stall at the offtake as a flowpath opens up for product/pressure to bypass the pig. Spheres are particularly vulnerable as they incorporate a ‘single line seal’ (the circumference of the sphere I contact with the pipe). Offtakes are sub-divided into 2 groups: 1. 2.

Offtakes which connect to the main pipeline at 90° - ‘Tees’ Offtakes which connect to the main pipeline at other anges – ‘Laterals’

1.

Tees

There are three types of tee: • Unbarred • Barred • Sphere (or flow) Unbarred Tees Most pigs can pass tees where the inside diameter (ID) is less then 70% of the main pipeline ID. However, spheres would undoubtedly be lost at this point. It is always a good practice, therefore, to install guide bars when designing pipelines for pigging. Always make the presence of unbarred offtakes – tees or laterals – known to the pig designer. Barred Tees This is the preferred condition of all tees in a pigging system (except sphere tees). Sphere (Flow) Tees Sphere tees have been designed for use in pipelines that are regularly pigged with spheres. If spheres do not disappear into an offtake, they are highly likely to stall opposite the offtake. This is because their spherical shape results in a single line seal in the pipe. When the pig reaches an offtake, the seal is broken and flow bypasses the sphere.


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PIPELINE PIGS & PIGGING Sphere Tees allow product flow into and out of the offtake through slots around the inner sleeve preventing the sphere from pushing into the offtake. 2.

Laterals

Laterals are offtakes that connect to the main pipeline at angles other than 90°. They are either barred or unbarred although, when designing pipelines for pigging, they should always be barred. Laterals present an increased opening when compared to Tees. Therefore, pig design must take account of this. Pigs have to be made long enough to span the opening to prevent stalling. 1.2.3.3

WYES

For the purpose of pigging, wyes join two lines together, both of which must be pigged. The pig emerges from a branch pipeline into the main pipeline (‘main’ and ‘branch’ do not imply any difference in pipe diameter) where it changes direction by bouncing off the opposite wall of the pipeline into which it has emerged. The convergence angle between the two pipelines is between 22° - 30°. Wyes can be pigged on one direction only and there is therefore no requirement for the pigs to be bi-directional although disc-type pigs offer the most popular solutions. There are two types of wye, each of which requires a pig possessing different design characteristics: 1. 2.

Wyes with parallel bores Wyes with over-size bores

1.

Wyes with Parallel Bores

The pig must be long enough to emerge into the main pipeline whilst still being driven in the old pipeline. As with laterals, the convergence angle extends the opening at the junction of the two pipes which allows product/pressure to bypass the pig before it seals in the new pipe. Articulated pigs are usually specified to overcome this problem. 2.

Wyes with Over-size Bores

Standard size pigs are used and are intended to ’float’ into the main pipeline. The pig is driven out of the branch pipeline under pressure and into the over-size bore where product/pressure bypasses the seals. The pig then drifts on the flow until it hits the wall of the main pipeline where it seals again and continues its journey. In both examples, pigs are equipped with special polyurethane bumper noses to assist with shock absorption and to help locate the pig correctly in the main pipeline.


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DIVERTERS

Diverters are designed to guide pigs from a main pipeline into a branch (lateral) – the opposite of wye pigging. The pipeline product is not diverted – only the pig. Most diverters use mechanical methods but some divert pigs by means of magnets which attract the metal bodied pigs. 1.2.3.5

BENDS

There are three types of bend: 1. 2. 3.

Forged (or factory) bends Field bends Mitred bends

1.

Forged (Factory) Bends

Forged bends are classified according to their ‘bend radius’ which is expressed in terms of ‘diameters or ‘D’. Standard sizes are produced as follows: • • • • • 2.

ID or ‘short radius bend’ should not be included in a system designed for pigging. However, if they are present, it may be possible to manufacture special pigs to cope. 1½D or ‘long radius bend – suitable for: spheres, foam pigs, specified metal bodied pigs 3D – most utility pigs are suitable 5D – all pigs are suitable 10D – all pigs are suitable Field Bends

These are ‘cold’ bends made by machine before laying. Bend radii are extremely large and will not cause problems for pigs – localised pipeline deformation as a result of bending/laying is a much greater problem. 3.

Mitred Bends

Mitred bends are sections of pipe cut and welded at an angle in order to change the direction of a pipeline. They are not recommended as part of a piggable pipeline system. However, it may be possible to design a pig for some applications but detailed specifications of the bend would need to be supplied to the pig manufacturer. Bends have other characteristics that influence pig design: • • • •

Bend angle – 22°; 45°; 90°. Increased wall thickness in forged bends Ovality in forged bends Distance between bends in a pipeline system (Minimum 3 x pipeline diameter).


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PIPELINE PIGS & PIGGING The minimum bend radius is almost always the first piece of information requested by the pig manufacturer after establishing the pipeline diameter and pigging function. It is important that as much relevant information as possible is made available. 1.2.3.6

TRAPS AND PIG HANDLING EQUIPMENT

To some extend, pig design is dependent upon the type of launching and receiving equipment installed. Some systems will only accept their own specially designed pigs and some systems will allow pig design within the limits of the trap dimensions. These systems may be specified in applications where complex industrial pigging strategies are necessary or where trap size must be restricted because of space limitations. Where there are no such restrictions, the following points should be considered: • • • • •

Are handling facilities adequate for pig types and weights? Are the traps sufficiently oversize to allow the pigs to be properly launched/received? Are the traps large enough to meet the requirements of the pigging programme? Do the traps have the necessary characteristics for launching and receiving? Are the traps suitably equipped for automated pigging programmes?


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Pipeline Pigging (Visit http://Piping-Info.Blogspot.com)

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