Pigging in the context of pipelines refers to the practice of using devices known as “pigs” 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” (or “launching station”) — 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 down the pipe until it reaches the receiving trap — the “pig catcher” (or “receiving station”).
Pigging has been used for many years 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 already installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, cosmetics and foodstuffs.
In production environments
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 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
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.
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 (and other) 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. By accident, one pig was shot out of the end of a pipeline without a proper pig receiver and went through the side of a house 500 feet 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.
There are many incidents recorded where operators get injured or even killed while performing a pigging operation. Common causes of such incidents are:
- closure door is opened, while the vessel is still pressurized;
- main process valve is opened, while the closure door is not yet fully closed;
- closure door is opened, while a high concentration of H2S or other toxins still remain inside the vessel;
- vent valve remains 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 to avoid these kinds of incidents is to add valve interlocks, which is nowadays adopted by all global oil and gas operating companies.
Safety during pigging relies on the 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, rather than interfering with them. Valve interlocks are permanently mounted to 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 original pigs were made from straw wrapped in wire and used for cleaning. They made a squealing noise while traveling through the pipe, sounding to some like a pig squealing, which gave pigs their name.”PIG” is sometimes claimed as an acronym or backronym derived from the initial letters of the term “Pipeline Inspection Gauge” or “Pipeline Intervention Gadget”.
A “pig” in the pipeline industry is a tool that is sent down a pipeline and propelled by the pressure of the product flow in the pipeline itself. There are four main uses for pigs:
- Physical separation between different fluids flowing through the pipeline
- Internal cleaning of pipelines
- Inspection of the condition of pipeline walls (also known as an Inline Inspection (ILI) tool)
- Capturing and recording geometric information relating to pipelines (e.g., size, position).
How Pigging work
Pigging works by propelling a device with wiping discs (the Pig) through the pipeline from the source to the destination. As the pig travels through the pipeline, it pushes out the trapped product and cleans the pipe walls leaving the pipeline ready for the next product transfer. The pig is most frequently propelled with air or nitrogen, but can be just as effectively be pushed with a liquid such as a cleaning or flush media or even another product. Typically any liquid, resin, or paste product that can be pumped can also be pigged.