A pigging operation is 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 sulfide, can have fatal consequences.
Using valve interlocks in a pigging operation
There are a few simple steps to launching a pig safely using valve interlocks. Each step traps and releases a key, and can only be performed in a particular order.
Simple steps to securing a safe pigging operation
- To begin with, keys are used to unlock and open the vent and drain valve respectively. These actions release a key that can then be used to open the vessel door safely and load the pig.
- Once the pig is loaded and the vessel door is closed and locked, a key is released to close the drain and vent. This action releases the key that opens the kicker valve and launches the pig.
- Once the pig is launched, the trap is re-isolated by closing and locking the mainline valve. This releases a sequence of keys that depressurise the trap by closing the kicker and opening the vent and drain.
- The last step involves closing all valves and returning the final key to the control room key cabinet, where is it kept until the process starts again.
No steps can be bypassed in this sequence, nor can steps be taken out of order. This simple, fail-safe process ensures accidents cannot happen during a pigging operation.