Pig Launcher/receiver interlocking logic

[chemiind]

I am currently working on logic for interlocking system for a pig launcher/receiver system in which we have a single barrel with two kickers one for launching and other for receiving with two block valves each and a balance line from barrel to launching kicker.

For launching of pig. I understand that we need to open the kicker valve on launching kicker followed by opening of balance line valve to pressurize both sides of pig and prevent the damage to valves on main line.

My question is do we need to close the balance line valve before opening the main line valves and throttling the bypass valve to effect launching of pig. Please see the attached layout for same.

 

[rneill]

First, completely isolate and depressure the trap so that it is safe to open to insert the pig. Once the pig is inserted, and the trap sealed up again, you would pressurize it.

When pressurizing the trap, you would open the balance line valve (V6) before opening the Launching Kicker Valve (V9) so that when you opened V9, you would simultaneously pressurize both sides of the pig.

What you do not want is a differential pressure acting across the pig. If you open V9 first, you will pressurize behind the pig before you have opened the balance line and will have a differential pressure acting on it wanting to move it before you have opened the mainline valves.

Once the trap is fully pressurized, you would leave valves V6 and V9 open and then you would open Valves V1 and V2 (with the main bypass valve still open). The pig should not move since you still have the flow bypassing the trap and the pressure is still equal both upstream and downstream of the pig.

Now that V1 and V2 are open, you would close the balance line V6. Following that, you would close the mainline bypass. Now the only source of pressure is behind the pig and it will start to move.

Typically, the balance line is small relative to the kicker line and so I'm not sure it makes a big difference if this line is still open when you close the mainline bypass valve but the proper thing to do is close it after you have opened V1 and V2 but before you open the mainline bypass.