It is pretty clear that the trap isolation valve should be a full opening ball valve and the bypass/kicker valve should be a plug valve or other valve capable of throttling the flow. The question is what type of valve should be used for the main line valve or what some call the suction/discharge valve located between the bypass and the barred tee.

Should this valve also be a plug valve or other valve capable of throttling the flow, or do you use a ball valve since the differential pressure over the valve is manageable?

Plug valves have the same throttling characteristics as a gate valve (and they move much more jerkily).  There is really no place on a pig trap for plug valves.  

The valve sequence for launching a pig on the Giard web page is (I've used this sequence for thousands of pig runs):
1.  Blow down barrel and load pig
2.  Pressurize the barrel with the kicker line fully open
3.  Open the barrel-isolation valve
4.  Begin shutting the side valve until the pig leaves.
5.  Fully open the side valve, Shut the barrel isolation valve, and shut the kicker valve.

I use full-opening ball valves for all three.

MORE NEWS