Apart from problems with real variable stiffness from one valve to another we have a major issue with the double seated expanding gate valves. These are used in a pigging service. They pass a pressure test and then whenever we have pigged for the first time they just leak. We have even pulled these valves out sent them off cleaned them up at the manufacturer and reinstalled them, they pass a pressure test clean, and then after first pig run, same thing happens they leak. The valves as found condition is always in good condition but they are extremely difficult to close , its a long shot but I was wondering if anyone could shed some light since I have never come across an expanding double seated gate valve before.
These are the old WKM syle gate valve.
They call these dual block and bleed valves - pressure from both sides and vent the center cavity. Do not bet on it. Most never reach that standard, at best they are downstream sealing. Sometimes you can pressure the center cavity and have both seats seal - whatever that proves.
In you application is sounds like the pig is focing debris between the seat and gate and messing up the seal and possible not allowing the gate to colapse when being operated - a possible source of the high torque. Sometimes cycling the valve will wipe the debris out and other times it will scratch the gate and a regrind is needed to solve the problem.
Another point, the gate can be machined wrong causing it to bind on the stem and stop the gate from sealing properly.
And the potential problems are endless for this type of valve.

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