METROPOLITAN ENGINEERING & ENVIRONMENTAL SERVICES: WHAT IS A PIG LAUNCHER/RECEIVER FOR PIPELINE MAINTENANCE?

What is a Pig Launcher/Receiver for pipeline maintenance?

Pipelines are a fundamental part of the oil and gas industry and they are by far the primary transportation method for crude oil, natural gas and even the refined product. However, building a pipeline network is a massive project and though most pipelines ultimately “pay for themselves” with the product they transport, they are still an expensive investment. It is essential for this investment to be well maintained and monitored. One of the most effective ways of doing that is with PIGs and the PIG launchers and receivers that facilitate them.

Pigging in the maintenance of pipelines refers to the practice of using pipeline inspection gauges or 'pigs' to perform various operations on a pipeline without stopping the flow of the product in the pipeline. Pigs get their name from the squealing sound they make while traveling through a pipeline. These operations include but are not limited to cleaning and inspection of the pipeline. This is accomplished by inserting the pig into a Pig Launcher - a funnel shaped Y section in the pipeline. The launcher is then closed and the pressure of the product in the pipeline is used to push it along down the pipe until it reaches the receiving trap - the 'pig catcher'.

If the pipeline contains butterfly valves, the pipeline cannot be pigged. Ball valves cause no problems because the inside diameter of the ball can be specified to the same as that of the pipe.

Pigging has been used for many years to clean larger diameter pipelines in the oil industry. Today, however, the use of smaller diameter pigging systems is now increasing in many continuous and batch process plants as plant operators search for increased efficiencies.

Pigging can be used for almost any section of the transfer process between, for example, blending, storage or filling systems. Pigging systems are already installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, and foodstuffs.

Pigs are used in lube oil or painting blending: they are used to clean the pipes to avoid cross-contamination, and to empty the pipes into the product tanks (or sometimes to send a component back to its tank). Usually pigging is done at the beginning and at the end of each batch, but sometimes it is done in the midst of a batch, e.g. when producing a premix that will be used as an intermediate component.

Pigs are also used in oil and gas pipelines: they are used to clean the pipes but also there are "smart pigs" used to measure things like pipe thickness along the pipeline. They usually do not interrupt production, though some product can be lost when the pig is extracted. They can also be used to separate different products in a multi-product pipeline.

How Do PIG Launchers and Receivers Work?

The exact procedure for operating a PIG launcher or PIG receiver will vary somewhat depending on the particular pigging system being used. However, for the most part it will include the following steps:

Launcher:

Pipeline operator should make sure that the isolation valve and kicker valve are closed.

If the system is a liquid system then the drain valve and vent valve should then be opened to allow air to displace the liquid; if the system is a gas system then the vent should be opened so that the launcher reaches atmospheric pressure.

After the PIG launcher is completely drained to 0 psi, with the vent and drain valves still open, the trap door should then be opened.

The PIG should then be loaded with its nose in contact with the reducer.

Closure seals and other sealing surfaces should be cleaned and lubricated as needed and then the trap door should be closed and secured.

The drain valve is then closed and the trap is slowly filled by gradually opening the kicker valve.

Once filling is complete the vent valve is closed so that the pressure will equalize across the isolation valve.

The isolation valve is then opened and the PIG is ready for launching.

Next the main valve is gradually closed, increasing the flow through the kicker and behind the PIG until finally the PIG leaves trap altogether and enters the pipeline itself.

After the PIG leaves the launcher the mainline valve is fully opened and the isolation valve and kicker valve are closed.

Receiver:

The receiver should be pressurized.

The bypass valve should be fully opened.

The isolation valve should be fully opened and the mainline valve partially closed.

Once the PIG arrives the isolation and bypass valves should be closed.

The drain valve and vent valve are then opened.

Once the trap is fully depressurized to 0 psi the trap can be opened and the PIG removed.

The closure seal and other sealing surfaces should be cleaned and lubricated as needed and the trap door should then be re-shut and secured.

The receiver should then be repressurized and returned to its original condition.

These processes may differ somewhat on different systems and of course if the launcher will be launching multiple PIGs then they should all be loaded at the loading stage.

Why use a Pig Launcher/ Receiver?

A major advantage of piggable systems is the potential resulting product savings. At the end of each product transfer, it is possible to clear out the entire line contents with the pig, either forwards towards the receipt point, or backwards to the source tank. There is no requirement for extensive line flushing.

Without the need for line flushing, pigging offers the additional advantage of a much more rapid and reliable product changeover. Product sampling at the receipt point becomes faster because the interface between products is very clear, and the old method of checking at intervals, until the product is on-specification, is considerably shortened.

Pigging operations, involving the loading and unloading of cleaning or inspection pigs into the launcher or receiver have the potential cause serious injury, death and damage to equipment if not carried out under the strictest procedural guidelines.

Even the best training or written procedures cannot safeguard against human error. Fundamentally, the potential for incident occurs when opening the pig launcher or receiver door. At this time, the launcher/receiver vessel can be under pressure, charged with hazardous liquids and gases. Opening the vessel without correctly isolating, venting and draining can cause the trap door to fly open with the pig exiting the vessel like a missile.

There are anecdotal accounts of such accidents available on the internet. Two links are provided here.

http://www.ncbi.nlm.nih.gov/pubmed/15553280

http://www.bsee.gov/Inspection-and-Enforcement/Accidents-and-Incidents/incidents/96-0069-pdf

A key interlock system can negate any such risk. Interlock devices can be fitted to the pig trap vessel door, and interlocked with valves and other equipment on the vessel to ensure the correct isolation, vent and drain procedures are followed.

But unlike other valve interlock changeover procedures, pig trap interlocking is rarely a linear sequence. The stages of isolating, venting and draining are often complex and repetitive. In these cases, key sequence control units are often used. In absence of adequate training, operators may find following the complicated sequence tiresome and in some cases, set about deliberately trying to override the equipment or avoid some steps in the sequence. Keeping the sequence simple as practically possible is imperative to correct and safe operation.

In its simplest form, the interlock hardware need only comprise two elements; the trap door interlock and a secondary device to prove the absence of pressure in the vessel. This could be an interlock on the vent valve, or a solenoid key release device linked to a pressure switch. This type of interlock is used to release the key at a pre-determined setting on the pressure sensor.

Using two interlocks as described can significantly simplify the interlock sequence for operators and remove the need for complex sequence control units. Despite this, a risk assessment should be carried out to determine whether or not other elements of the isolation, vent and drain should be incorporated.