Pipeline Pigging

[Freeman]

4.5.8 Pipeline Pigging

4.5.8.1 General

Pipeline pigs and spheres are used for a variety of purposes in both liquids and natural gas pipelines.

Pigs and spheres are forced through the pipeline by the pressure of the flowing fluid. A pig usually consists of a steel body with rubber or plastic cups attached to seal against the inside of the pipeline and to allow pressure to move the pig along the pipeline. Different types of brushes and scrapers can be attached to the body of the pig for cleaning or to perform other functions.

Figure 4.28 illustrates a variety of pipeline pigs

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Pipeline pigging is done for the following reasons:

• To clean up pipelines before use (foam pigs);

• To fill lines for hydrostatic testing, dewatering following hydrostatic testing, and drying and purging operations (spheres and foam pigs);

• To periodically remove wax, dirt and water from the pipeline (scraper pigs and brush pigs);

• To sweep liquids from gas pipelines (spheres)

• To separate products to reduce the amount of mixing between different types of crude oil or refined products (squeegee pigs and “Go-Devil” pigs);

• To control liquids in a pipeline, including two-phase pipelines (spheres and foam pigs);

• To inspect pipelines for defects such as dents, buckles or corrosion (“intelligent-pigs or caliper pigs).

(Figure 4.29 illustrates a kaliper pig.)

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Differential pressure is required to move a pig or sphere through the pipeline. The force required depends on elevation changes in the pipeline, friction between the pig and the pipe wall and the amount of lubrication available in the line. (A dry gas pipeline provides less lubrication tan a crude oil pipeline, for example).

Cups are designed to seal against the wall by making them larger than the inside diameter of the pipe. As the cups become worn, the amount of blow-by fluid by-passing the pigs increases because the seal is not as effective.

In the case of spheres, a certain amount of over-inflation is required to provide a seal. (In two-phase pipelines, spheres are sometimes under-inflated to allow some blow-by to lower the density of the fluid ahead of the sphere).

Pigs and spheres travel at about the same velocity as the fluid in the pipeline and travel speed is relatively cons

[Freeman]

4.5.8.4 Pig Launching And Receiving Procedures

Pig launching and receiving procedures are often supervised by senior operations staff and fully monitored by all pipeline users but the actual procedures laid down for each pig launching/pig receiving facility will vary.

4.5.8.5 Pigging Problems

The pig launcher-receiver is probably the only high-pressure vessel on the facility, in hydrocarbon service, which is regularly opened to the atmosphere and then pressured as a normal operating procedure.

If the launcher/receiver is incorrectly purged and pressured, an explosion becomes a major possibility. To reduce the chances of such an incident, the relative procedures are commonly backed up by an “interlock-system”, which prevents the movement of valves and door closure devices until certain criteria have been met within the system.

Figure 4.33 illustrates the logic of a simple interlock system.

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In the last decade at least two launchers have been involved in major explosions in Britain.

When pigs are launched into a pipeline there is always the possibility that the pig will stop or reduce the flow of fluid through the pipeline. The most common incidents and their causes are:

The pig fails to launch (this only becomes apparent alter the launch procedure is at its final stages. The possible causes are:

1. The pig is too small (wrong pig or under- sized) and the flow cannot pick up the pig in the launcher barrel.

2. The pig is too large, wrong pig or oversized and it is jammed in the exit to the launcher.

3. The pig is too far back in the launcher.

The pig indicator, Figure 4.35 should show that the pig has launched. They are, however, not always reliable.

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[nwingwon]

Dear Brothers

Pipeline Pigging and Inspection Technology (Second Edition)
by John Tiratsoo

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Enjoy

[gepachir]

Thank you friend

[omli]

thank you master

[fatahjalbani]

thaks

[settimana]

Thank you very much for sharing

[guptah]

thanks

[rafik b]

merci

[eagle_gold]

Thank you friend

[lulliri]

Thanks.................

[spk]

HI all!!
If anybody have any book on Slug Catcher, pl. share it.
Thank you in anticipation.