In its simplest form, a pigging system consists of a solid projectile or plug (the ‘pig’) with a diameter slightly larger than the pipeline or tubing that is transporting the liquid. Pipeline pigging works by introducing the pig into the pipeline and pushing it through the pipe. A launcher or receiver station is typically a vessel, connected in parallel to the existing pipeline. This allows launching or receiving the pig, using the pipeline’s normal operating pressure, and without interrupting the flow or production. The vessel is equipped with a closure door to load or offload a pig. Receiving or launching the pig involves several sequential valve operations, in combination with opening and closing the closure door. Pigging operations are dangerous, as opening a pig trap closure while there is pressure in the barrel can shoot the pig out of the launcher at high speeds. Attempting to pass a pig through a partially open outlet valve, or prematurely opening the pig trap in the presence of high levels of toxic hydrogen sulphide, can have fatal consequences.

Formal procedures can be set up to prevent opening the closure door under pressure. However, these are no guarantee that all safety criteria are taken into consideration. Many accidents have occurred all over the world during dangerous pigging maintenance operations, even with strict procedures in place. Operators can be trained to follow procedures and informed about potential dangers, but mistakes cannot be excluded.

Even though some larger valves are equipped with motor operated valves (MOV), the process cannot be fully automated, since the loading and offloading of the pig requires manual operations. Therefore, partially equipping MOV’s on valves does not sufficiently guarantee the safety of people and processes.

To avoid dangers and incidents, such as loss of production time, or environmental damage, most companies have standard written operating procedures in place. In addition. most vessels are equipped with pressure and temperature transmitters to raise alarms. Written procedures and operating guidelines alone cannot ensure safe operations, as the human factor is not excluded. Alarms on the other hand do inform about sudden pressure and temperature increases, but this information is presented too late to act and prevent damage.

Valve interlocking systems prevent operators to operate any process valve while the closure door is still open, or not in its fully locked closed position. This is because the key to start the operating sequence is still trapped in the closure door lock.

To ensure the safety of the operators while opening a closure door, the vessel involved needs to be vented, drained, purged, and isolated from the main process line. Vessel lay-outs vary, just as procedures, and from and HSE point of view it is therefore good practice to have valve interlocks mounted onto all critical valves. This way, these valves can only be operated according to procedure, and the vessel is clear of any pressure and toxic gasses.

H2S gasses are extremely dangerous. When H2S gases are trapped inside a vessel, it needs to be purged to remove these gases. The volume of the vessel and the H2S content determine how many times purging should take place. Often, vessels need to be purged at least 7 times before it’s safe to open the closure door. To guarantee that procedures are followed, an extra control unit is used in addition to standard valve interlocks. This unit guarantees correct purging procedures, without shortcuts. For example, when a vessel needs to be purged 7 times, the mechanical control unit is programmed to force operators to purge 7 times before opening the vessel. These steps cannot be skipped This will keep the integrity of the plant and the safety of the operator a priority.

Additional sequence control products address unusual conditions during a pigging procedure. These products combine digital technology with traditional interlocking. Sequence control products bring signals from other field devices, like pressure or H2S/CO2 sensors, into the interlocking sequence, and only release keys for mechanical interlocking if all conditions in the process have been met. process variables – for example vessel pressure or H2S-levels – can be incorporated within safeguarded operating procedures. Electro-mechanics enable mechanical valve interlocks to be fully integrated within the context of a common system for control and safety of pigging operations. Additional safeguards, such as electrical signals from valve positioners via a switchbox, can also be incorporated in the system.

Recent developments in mechanical valve interlock products also made it possible to integrate valve interlocks within sophisticated electro-mechanical and PLC controlled cabinets, combining mechanical and electronic safety measures into one single safety system. These solutions allow for communicating detailed instructions, monitoring the status of the procedure and operating part of the sequence from the control room.