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Page 1 of 3Pipeline Pigging Operations - ChE Forums

12/18/2010http://www.cheresources.com/invision/topic/627-pipeline-pigging-operations/

Posted 23 April 2004 - 12:46 PM

Posted 23 November 2009 - 02:07 PM

Pipeline Pigging Operations Safety around pigging receivers

For those not familliar with pipeline operations, pigging is the practice of inserting a device (the "pig") into a flowing pipeline that will run through the pipeline. The pig may be used to scrape off deposits (wax, parafins) from the inside of the pipeline, or to push liquids through a multi-phase pipeline. The safety issues that come up are dealing with the operations involved in inserting a pig into the pipeline and removing it from the other end of the pipeline. Basically the pig launcher and receivers are connected to the pipeline with bypass and isolation valves that allow fluid to either flow through or around the pig launcher/ receiver. I'm going to focus on removing the pig from the receiver because this is the most dangerous operation. Once the pig has been caught in the receiver, the operator will open the bypass valve and close the isolation valves to the receiver. He then opens the drain valve to drain trapped liquids and opens the vent valve to depressure. There should normally be a pressure guage to check that the vessel has been de-pressured. Then the operator will open the hatch at the end of the receiver and remove the pig. There are a couple of potential problems that have caused accidents in the past. Usually it relates to the receiver not being properly de-pressured. With pressure behind the pig, it launch like a cannon out the end of the receiver the operator opens the hatch. Reasons that the receiver may not depressure properly include the following: - vent/ drain valves are prone to plugging. These are often small valves and can be plugged because of the dirty material that has been scraped from the pipeline by the pig. - stuck pig. The pig could be stuck, if there is a restriction, upstream of the vent valve there could be a trapped pocket of high pressure gas. - poor pressure indication. The pressure guages are often scaled to pipeline pressure which could be 1000 psig. Even relatively low pressure (20 psig) can launch the pig and cause serious injury. These low pressures can not be accurately read on most standard guages (scaled for high pressure). I have heard (although I don't have the references to back this up) that more people are killed by low pressure gas than high pressure gas. This is largely due to people not considering the real threat it poses. Some general rules: NEVER stand down stream of someone opening a pig receiver. Always stand to the side. Use of a mirror to check the inside of the pig receiver before removing the pig is recommended. A stuck pig can let go without warning when an operator is looking in the receiver. Locate the pressure gauge close to the inlet valve to detect trapped pressure. Use auxilliary pressure tap for portable electronic pressure indication (more accurate) with regular calibration. Size vent and drain valves at least 1" ball valves. Do not try to save money by using 1/2" or 3/4", they are easily plugged. Even 1" can be plugged, so consider the service to determine if larger valves may be required. Different operating companies have slightly different standards on the design and operation of pig receivers. The above reflects my opinions, anyone who has other ideas or additional information on this issue please feel free to comment. Shaun

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Other Replies To This Topic

Hi!! These detailes are very much relevant and useful in pipeline O & M, and only comes from experience like yours. We also use to do cleaning pigging in 2-3 years and intelligent pigging in 5- 10 years as per requirement.Once we had faced situation like this.PIG was stucked up in pipeline in between due to dent.Pipeline flow reduced to control pressure of pipeline immediately, we had had to shut down the

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pipeline as by increasing pressure and then lowering did not give any result.So another PIG is launched one after another this time all PIG are received with lot of muck and black powder in receiver.As receiver door is opennned PIGs came out with high speed and lot of muck. That was happenned because of liquid trapped in between PIG and that could not be vented properly to flare.

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