The basic guidance for wetted surface area, used world-wide, is found in API 521. Relief designers should be familiar with that. It discusses wetted surface area for the different types/orientations of vessels. Read and understand that. Then, when you have a task like this, use that knowledge appropriately for the task at hand.
I'm guessing this pig trap is fabricated according to pipe code rather than vessel code. API 521 is written for ASME Sec VIII pressure vessels. The owner can choose to protect this "segment of pipe" as though it's a vessel, but personally I wouldn't in this case. My resoning is based partially on the fact that this is pipe rather than a vessel, but it's also based on the fact that pig traps are normally empty of all liquid contents, except during those infrequent pigging operations. The decision of wheteher to treat this like a vessel should be based on a risk assessment.
The bottom line is that I think you're spending time and money on something that (a) is not required, (b) is a very low risk, and (c) can't be adequately protected by a relief valve anyway (relief valves can't prevent failure of a gas/vapor filled container exposed to fire).
