Pigging

In pipeline transportation, pigging is the practice of using pipeline inspection gauges or gadgets, devices generally referred to as pigs or scrapers, to perform various maintenance operations. This is done without stopping the flow of the product in the pipeline.
These operations include but are not limited to cleaning and inspecting the pipeline. This is accomplished by inserting the pig into a "pig launcher" —an oversized section in the pipeline, reducing to the normal diameter. The launching station is then closed and the pressure-driven flow of the product in the pipeline is used to push the pig along the pipe until it reaches the receiving trap—the "pig catcher".

Applications

Pigging is used to clean large 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 and reduced costs.
Pigging can be used for almost any section of the transfer process between, for example, blending, storage or filling systems. Pigging systems are installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, cosmetics and foodstuffs.
Pigs are used in lube oil or paint blending to clean the pipes to avoid cross-contamination, and to empty the pipes into the product tanks. 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, such as when producing a premix that will be used as an intermediate component.
Pigs are also used in oil and gas industries to clean or clear pipelines. Intelligent or "Smart pigs" are used to inspect pipelines to assess their condition and to prevent leaks, which can be hazardous or harmful to the environment. They usually do not interrupt production, though some product can be lost when the pig is removed. They can also be used to separate different products in a multiproduct pipeline and to clear slugs of liquid from multiphase gas//liquid pipelines.
Pigging requires the pipeline to be designed to be pigged from the outset. If the pipeline contains topological variations including changes in diameter, butterfly valves, instrumentation, tight bends, pumps or reduced port ball valves, the pipeline cannot be traditionally pigged. In such instances, alternatives such as ice pigging may be employed. Full port ball valves cause no problems because the inside diameter of the ball opening is the same as that of the pipe.

Etymology

Some early cleaning "pigs" were made from straw bales wrapped in barbed wire while others used leather. Both made a squealing noise while traveling through the pipe, sounding to some like a pig squealing, which gave pigs their name.

In production environments

Product and time saving

A major advantage for multi-product pipelines of piggable systems is the potential of product savings. At the end of each product transfer, it is possible to segregate the next products using a pigging sphere. Alternatively it is possible to clear out the entire line contents with the pig, either forwards to 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 much more rapid and reliable product changeover. Product sampling at the receipt point is faster with pigs, because the interface between products is very clear; the old method of checking at intervals to determine where the product is on-specification takes considerably longer.
Pigging can also be operated totally by a programmable logic controller.

Environmental benefits

Pigging has a significant role to play in reducing the environmental impact of batch operations. Traditionally, the only way that an operator of a batch process could ensure a product was completely cleared from a line was to flush the line with a cleaning agent such as water or a solvent, or even with the next product. The cleaning agent then had to be subjected to effluent treatment or solvent recovery. If a product was used to clear the line, it was necessary to downgrade or dump the contaminated portion of the product. All of these problems can now be eliminated due to the very precise interface produced by modern pigging systems.

Safety considerations

Pigging systems are designed so that the pig is loaded into the launcher, which is pressured to launch the pig into the pipeline through a kicker line. In some cases, the pig is removed from the pipeline via the receiver at the end of each run. All systems must allow for the receipt of pigs at the launcher, as blockages in the pipeline may require the pigs to be pushed back to the launcher. Many systems are designed to pig the pipeline in either direction.
The pig is pushed either with a gas or a liquid; if pushed by gas, some systems can be adapted in the gas inlet in order to ensure pig's constant speed, whatever the flow pressure is. The pigs must be removed, as many pigs are rented, pigs wear and must be replaced, and cleaning pigs push contaminants from the pipeline such as wax, foreign objects, hydrates, etc., which must be removed from the pipeline. There are inherent risks in opening the barrel to atmospheric pressure so care must be taken to ensure that the barrel is depressurized prior to opening. If the barrel is not completely depressurized, the pig can be ejected from the barrel and operators have been severely injured when standing in front of an open pig door. A pig was once accidentally shot out of the end of a pipeline without a proper pig receiver and went through the side of a house away. When the product is sour, the barrel should be evacuated to a flare system where the sour gas is burnt. Operators should wear a self-contained breathing apparatus when working on sour systems.
A few pigging systems utilize a "captive pig", and the pipeline is only opened occasionally to check the condition of the pig. At all other times, the pig is shuttled up and down the pipeline at the end of each transfer, and the pipeline is never opened up during process operation. These systems are not common.

Pigging operations

See diagram on the right representing a pig launcher. In normal operation valves V2 and V4 will be open, all other valves and the door are closed and locked.
To launch a pig

Ensure pig launcher is drained and vented. Unlock and open, then close and relock the vent and drain valves.
Unlock and open door, load pig into launcher, close and lock door.
Unlock and open valves V1 and V3, to create a flow path through the launcher.
Partly close ‘kicker’ valve V2 to launch the pig.
The launch alarm XA will indicate when the pig has been launched.
Open valve V2 fully.
Close and lock valves V1 and V3.
Unlock and open vent and drain valves. Allow launcher to depressurise and drain.
Close and lock vent and drain valves.
Pig launcher is then ready for the next pig to launch.

See diagram on the right representing a pig receiver.
To receive a pig

Unlock and open valves V1 and V3, to create a flow path through the pig receiver.
Partly close ‘kicker’ valve V2 to induce flow through the receiver.
The receive alarm XA will indicate when the pig has been received.
Open valve V2 fully.
Close and lock valves V1 and V3.
Unlock and open vent and drain valves to empty the receiver.
Close and lock vent and drain valves.
Unlock and open door, remove pig from receiver, close and lock door.
Pig receiver is then ready to receive the next pig.

The pigging installation shown is known as an Above Ground Installation. It is part of the UK's National Transmission System for natural gas. It shows two pig launcher/ receivers at the end of two diameter pipelines that carry gas under the River Thames, between East Tilbury in Essex and Shorne in Kent. Should one pipeline be damaged, by a ship's anchor for example, that line can be isolated and the second pipeline allows gas to flow across the river.

Mechanical interlocks

There are many reports of incidents in which operators have been injured or even killed while performing a pigging operation. Common causes of such events are:

opening of the closure door while the vessel is still pressurized;
opening of the main process valve while the closure door is not fully closed;
opening of the closure door while a high concentration of H₂S or other toxins remains inside the vessel;
the vent valve remaining open while the vessel is being pressurized with its medium.

All these causes are directly related to improper operation of the process valves and the closure door. A common method of avoiding these kinds of incidents is to add valve interlocks, which has been adopted by all global oil and gas operating companies.
Safety during pigging relies on strict adherence to safety procedures, which give detailed description of a safe valve operating sequence. By physically blocking incorrect operations, valve interlocks enforce such sequences. Valve interlocks are permanently mounted to both manual and motor operated valves and the closure door. The interlocks block operation of a valve or door, unless the appropriate keys are inserted.
The principle of valve interlocking is the transfer of keys. Each lock is equipped with two keys: a key for the locked open position and one for the locked closed position. During an operating procedure, only one key at a time is free. This key only fits in the interlock belonging to the valve that is to be operated next in the operating procedure. All keys are uniquely coded to avoid the possibility that valves can be unlocked at an inappropriate time.
Nowadays intelligent interlocking solutions enable the integration of field devices like pressure or H₂S meters in a valve interlocking sequence. This increases safety by integrating operator procedures with DCS and SIS safety systems.