Blocked In Exchanger Boil-Off Rate

[Casimo5]

In the case of an gas/liquid exchanger where the gas is the hot fluid and is on the tube side, and water is the cold fluid on the shell side, I am trying to calculate the relief rate when the water side is blocked in and the gas is still flowing. The gas is at a temperature where I would expect the water vapor pressure to exceed the set pressure of the relief valve and boil in the exchanger.

As far as thermal expansion, I know that a 3/4x1 will be adequate. However, if discovery and correction is not expected before liquid boiling, I need to size for the vapor generation rate (per API 521). I am assuming this is the case for my exchanger.

Speaking with the heat exchanger experts, I gather that using the UA specified on the data sheet is overly conservative, since the U of the exchanger that is blocked in on the water side is affected severly since heat exchange is now by conduction rather than convection. That is to say, the water is stagnant on the shell side and not moving around facilitating heat exchange. They go on to say that the actual U is extremely hard, if not impossible, to calculate. While this may be the case, I need some kind of basis for vapor generation.

Does anyone out there have or know of any type of correction factor used for this relieving scenario?

 

[Chance17]

Forget about the water being stagnant.
To calculate the safety valve, you need the steady state boiling rate at relief pressure.
This would be gas side "U" (low) to boiling liquid "U" (high.
The gas side very likely limits the heat transfer rate.
I would suspect the spec sheet UA to be a good estimate.