TEMPERATURE AT RELIEF PRESSURE 2

[Buchi]

Hi guys,

i am sizing a PSV for blocked outlet for pressure vessel. the challenge i am having now is knowing the temperature to use at relieving pressure(set pressure +overpressure +atm). I feel particularly uneasy to use the operating temperature because i believe that when PSV is relieving, the temperature should change.

i have searched this forum to see if a similar question has been posted before and the closest i saw has to do with the relieve temperature for "fire scenario". i dont know if this applies to "blocked outlet" as well.

thanks for your response as usual.

Buchi

Buchi

 

[tickle]

I think that more information is required to answer your question.

Is it a gas or liquid?

A blocked discharge is only applicable if a blocked discharge would cause the pressure to increase above the MAOP of the vessel, for example if there ia a compressor or pump upstream of the vessel. In this case, why would the temperature not be the operating temperature or even the design temperature?

Is the design for a "shut in case" where the contents could heat up and expand? Then the relieving temperature should be calculated. This will be a lower temperature for incompressible fluids than for gases, similar to the fire relief scenario, but the relief rate would be significantly lower.

 

[Montemayor]

Buchi:

Please give us the basic data - all the basic data required to answer your question with reasonable logic.

How can we address your question if you don't even tell us the identity of the fluid and the scenario involved? I can understand that you may never have designed a relief device before, but you are a chemical engineer by your handle description and we ChemEs are trained and drilled in discussing and exchanging basic data and identifying hazardous scenarios.

You say you have a "blocked scenario"; fine, but this is a general description. We need the details. Is it a reciprocating compressor's discharge that is blocked? or is it a heat exchanger? Is it a reactor? It makes a LOT of difference!

We'll await your response, hoping to be of some help.

Art Montemayor
Spring, TX

 

[Buchi]

Ok guys,

sorry if i was too economical with information. It is a blocked dischage of the first stage separator overhead. so while it does not have a compressor upstream to create over pressure situation, the high well head pressure can.

I am assuming a two-phase flow. and tickle, concerning your suggestion on using "operating temperature", i was originally inclined to this until i stumbled into a paragraph on API 520 part 1 page 4 (seventh edition) which says "The temperature ogf the flowing fluid at relieving conditions may be lower or higher than the operating temperature.

And for Montemayor, this is not the first time i am sizing a PSV. I have sized quite a few in the past. But sometimes, we professionals need to rub minds on issues and that is why documents like API standards are constantly undergoing evolution. I have heard very sharp engineers differ on this seemingly trivial issue of "relief temperature" some think it should be design temperature, some think it should be operating temperature and some think it is normail boiling point. I am not sure if API 520 was specific on this one. which is why i have resorted to tap from the repertoir of knowledge in this FORUM.

i appreciate your contributions though.

Buchi

Buchi

 

[Homayun]

Buchi,
I assume that you have a simulation model of your plant. What I normally do is run the model with the fludis being at relief pressure. The corresponding properties then I would use for PSV calculation. Usually you'd see that some of the higher hydrocarbon vapors would condense at relief pressure and hence resultin a smaller PSV size.

regards

 

[MortenA]

If its blocked outlet then you should consider what could cause the temperature increase?

The only thing that i could think of rigt here is the effect of compression. While the upstream gas that causes the increase in pressure may maintain the same temperature then the gas in the seperator is actually not moving (and perhaps not beeing mixed with the upstream gas?) Anyway I personally find this a bit academic and would size for the operating temperature.

Best regards

Morten

 

[sailoday28]

If for an event you know the inlet flows and enthalpies, then based upon your relieving pressure and all outlet flows at the relieving pressure,
---with a simple model assuming a common outlet temperature a mass energy balance will justify your relieving temp. For this simple model, keep assuming outlet temp until the energy is balanced.
For two phase, multi component, I'm sure the process will be more complicated.