I have an exchanger using steam as a heating medium. Steam pressure inside the exchanger is 600# psig. Steam trap outlet goes to a drum 300 feet away operating at 70 psig. Does it mean that the pressure drop is consumed by the trap or by the line and exit losses.

I am designing a PSV for the drum considering a case of fail open steam trap. The trap condensate line is 3". If I use the Cv value for a 3" open gate, is it reasonable that I get a steam load for the PSV?

Check with the steam trap supplier the Cv of the trap in the worst situation (typically 100% open).

Find the orifice size of the trap. Based on a failed trap, calculate new trap mass energy flow (based on the new trap upstream conditions)---Then do a mass energy balance of drum at your specified relieving conditions.

The pressure drop is consumed by the trap and the line losses, but I would not regard the velocity head at the exit from the line into the condensate vessel as a loss.  Because you are discharging this line into a pressurised vessel you will get some pressure recovery.

I believe you would be being unnecessarily conservative if you regard the trap as a 3" gate valve. As the others have said, get the actual orifice size or its Cv from the trap supplier and use that in your calculation. I wouldn't be surprised if taking the trap as a gate valve resulted in a PSV of double the capacity that would really be required.

A gate valve takes a relatively small pressure drop while a properly sized steam trap may very well consume a large pressure drop. Your system sounds as if the trap was so designed. Adding my opinion to what has already been said, you want to rate the trap for its hydraulic performance in a "failed open" case. You need to know the orifice size or some other key hydraulic performance parameter (e.g. Cv). Good luck.

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