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PSV discharge piping location for liquid and vapor relief

PSV discharge piping location for liquid and vapor relief

(OP)
Hi everyone,

I have a 3/4"x1" PSV protecting the shell-side of an exchanger containing cooling water from two relief scenarios: fire and thermal expansion (due to heat input from the hot side while blocked in). Fire case is the controlling scenario, as expected, but my question concerns how to route the tailpipe discharging to atmosphere. For the fire case, the relieved fluid is steam, while for the thermal expansion it is water. Typical industry practice is to discharge vapor/gas upwards to atmosphere, and discharge liquid to grade at a safe location. What are your recommendations for PSV tailpipes that have both liquid and vapor relief cases? As for this example, since steam would only relieve in the event of external pool fire, would it be ok to have steam relief to grade as far as personnel safety is concerned?

RE: PSV discharge piping location for liquid and vapor relief

For the case you have it is recommended and proper to discharge the (thermal) relief valve (often called a TSV) directly down to grade.
Reason- TSV Location - The TSV should be connected to one the "taps" located on the side of the Cooling Water Inlet Nozzle on the bottom of the of the Exchanger. This allows the TSV discharge to be Water (yes, hot water, but none the less water) instead of Steam.
Remember, even though the TSV is activated on high temperature it is still just releasing pressure.

Sometimes its possible to do all the right things and still get bad results

RE: PSV discharge piping location for liquid and vapor relief

(OP)
Thank you Pennpiper for an excellent point. If I were to design a new system, that is how I would do it. However, the mentioned system is existing, and the TSV is located on the outlet nozzle side of the exchanger which is located on the top of the exchanger. Assuming the TSV location cannot be changed, in the case of fire, the TSV is likely to relieve steam. Would it be acceptable to relieve steam directly down to grade? Thank you in advance for your advice.

RE: PSV discharge piping location for liquid and vapor relief

Your last question: "Would it be acceptable to relieve steam directly down to grade? "
In the case you have stated, I would say Yes. Why? because, in the case of fire a little hot water is the least of your troubles.

Sometimes its possible to do all the right things and still get bad results

RE: PSV discharge piping location for liquid and vapor relief

Pennpiper is correct.

You know you have two cases. The governing case is external fire. People get excited about this case and where to discharge the PSV. But you need to remember no one will be standing next to that heat exchanger while it is engulfed in a raging pool fire. So who cares where your little heat exchanger PSV discharges. But if you have a thermal relief, someone could be in the area because you'll have a process upset. So you want to make sure you protect personnel under this scenario.

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