Tuesday, February 3, 2009
Posted by Ankit Chugh on 8:43 AM
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
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).
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 constant.
Pigs are used in all types of pipelines to increase efficiency and avoid problems at pump or compressor stations that could result from the presence of unwanted materials. Brushes and scrapers on a cleaning pig remove dirt and scale from the pipeline walls. Brush and scraper pigs feature longitudinal boles, which pass through the body of the pig. The holes allow a flow of fluid through the pig to prevent the build-up of wax or debris in front of the pig.
A pig can remove very large amounts of debris if it is run over a long distance.
For example, assume a pig is run in a 24 in. pipeline, 100 miles long, and removes 0.016 in. of wax material from the wall of the pipeline. After 100 miles, a plug about 1,450 ft long would form. For this reason, pipelines are operated to very definite pigging programmes.
Pipelines are often pigged first during testing following construction. Most pipelines are tested with water (hydrostatic testing) either in sections or over the entire length. A foam pig or pigs is normally sent ahead of the water when filling the test section to prevent mixing the test water with air in the line. Internallycoated pipelines are often flushed with water ahead of a pig to prevent debris from being dragged along the inside surface, damaging the coating.
After testing, the water is usually displaced with the fluid to be transported in the pipeline. A pig is run between the two fluids to separate them. In gas pipelines, the pig is used to “dewater” the pipeline by running it behind the test water. Additional pigs may also be run to ensure that as much moisture as possible is removed from the line.
Equipment is required to introduce the pig into the pipeline and to retrieve the pig at the end of the segment being pigged. A launcher is required at the upstream of the section and a receiver at the down-stream end.
The distance between these pig “traps” depends on service, location of pump or compressor stations, operating procedures and the material used in the pig.
The design of pig launchers, pig traps and related equipment is done in accordance with standards developed by several organizations. Traps for brush pigs, squeegees and foam pigs include a barrel, short pup joint, a trap valve, a side valve and a bypass line. The barrel holds the pig for loading and unloading and is equipped with a quick-opening closure or blind flange. A barrel diameter larger than the diameter of the pipeline served is required in order to allow the Pig to be successfully launched or retrieved. Barrel length depends on operating procedures, service and available space.
Sphere launchers are often designed for multiple sphere launching and recovery duties and the barrels for sphere launchers are typically longer than those for other types of pigs. The operator can load these magazines with several spheres that can be launched automatically. This approach is often used in two-phase pipelines where the barrels may be designed to accommodate over lo spheres. The sphere launcher consists of the barrel, a launching mechanism, an isolation valve, an equalizer valve and a reducing tee. A drain can serve as an equalizing line.
Figure 4.32 illustrates a sphere launcher/receiver system
Combination pig and sphere launchers can also be designed if both cleaning pigs and spheres for liquid control are needed.
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.
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.
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 over-sized and it is jammed in the exit to the launcher.
3. The pig is too far back in the launcher.
The pig is launched successfully but fails to arrive on time with no major changes in pipeline pressures or flows. The possible causes are:
1. The pig is too small (wrong size) and cannot climb the riser into the receiver.
2. The pig has disintegrated into its component parts.
3. The pig is hung on a bend and the cups have “flipped” forwards to allow full flow.
The pig is launched successfully but fails to arrive on time and there is an increase in pipeline pressure drop in pipeline flow. The possible causes are:
1. The pig has hung up on a bend or ’T ’piece (pig is too long for bend radius).
2. The wrong size of pig was launched (too large in diameter).
3. The pipeline has been dented and the pig is stuck at the damaged section.
The pigs “leap-frog” each other in the pipeline; (usually foam pigs). The possible causes are:
1. The operator launched them 1, 3, 2 but did not realize (most common);
2. The front pig hangs up on an obstruction and is only cleared by the second pig rolling over it.
Spheres arrive with huge chunks missing. The most likely cause is that the launcher valve has taken a bite out of the sphere as it was launched.
Launcher valves are often half-cup ball valves, which rotate through 180º to launch the sphere. Over-sized spheres hang over the side of the cup and are sliced as the cup rotates.
Pigs and spheres go into by-pass lines, junction ‘T pieces or other pipelines. Operator error or process upsets may often create situations where the sphere or pig can deviate from its normal path. In one known instance a 28” diameter neoprene sphere traveled into a 12” diameter pipe for some considerable distance before flow was stopped.
Whatever the “Causes” of pigging problems, the “effects” can be severe and in some instances the pipeline has had to be cut out to remove the offending pig.