13 replies to this topic

[Hossein Deldari]

dear all
during determine relief condition of some of PSVs for a new plant, I found that based on psv inlet pressure and back pressure, critical flow will occur during releiving. for checking autorefrigeration effect on material type of PSV outlet pipe line, I have to calculate temeprature drop during pressure drop in PSV from inlet pressure to superimposed back pressure. the considered superimposed back pressure is fixed to equal 0.2 barg. I think during critical flow condition, i should split pressure drop to two segment. 1- from PSV inlet condition (set pressure + set pressure*overpressure) to critical pressure and 2- from critical pressure to back pressure.
I guess i should estimate tempearture drop in first segment based on isentropic method and for second segment isenthalpic method should be applied.
I am not assured that this procedure is ok or not
another problem is sbout flare header network sizing for PSVs are susceptible critical flow condition. based on API 520, pressure drop of these types of PSV can not be more than (releiving pressure - critical pressure). as you know flare header network sizing is a hydrauilc calculation based on backward pressure drop calculation. that is, considering a meaningfull pressure in flare stack and back calculte to PSV outlet points. so I don't know what is the permissible back pressure of PSVs which are susceptible criticalk flow condition. is it 10% of set pressure (for conventional type PSVs) or calculated critical pressure?
please quide me in this regards

[fallah]

dear all
during determine relief condition of some of PSVs for a new plant, I found that based on psv inlet pressure and back pressure, critical flow will occur during releiving. for checking autorefrigeration effect on material type of PSV outlet pipe line, I have to calculate temeprature drop during pressure drop in PSV from inlet pressure to superimposed back pressure. the considered superimposed back pressure is fixed to equal 0.2 barg. I think during critical flow condition, i should split pressure drop to two segment. 1- from PSV inlet condition (set pressure + set pressure*overpressure) to critical pressure and 2- from critical pressure to back pressure.
I guess i should estimate tempearture drop in first segment based on isentropic method and for second segment isenthalpic method should be applied.
I am not assured that this procedure is ok or not
another problem is sbout flare header network sizing for PSVs are susceptible critical flow condition. based on API 520, pressure drop of these types of PSV can not be more than (releiving pressure - critical pressure). as you know flare header network sizing is a hydrauilc calculation based on backward pressure drop calculation. that is, considering a meaningfull pressure in flare stack and back calculte to PSV outlet points. so I don't know what is the permissible back pressure of PSVs which are susceptible criticalk flow condition. is it 10% of set pressure (for conventional type PSVs) or calculated critical pressure?
please quide me in this regards

Your split regarding themodynamic behaviors between PSV inlet and tail pipe is correct.
For coventional type PSV the superimposed backpressure should be constant or less than 10% of set pressure (if variable).
Regards

[Hossein Deldari]

dear Fallah
thanks for your fast reply.
so, you believe that I should split pressure drop to two segment and with two different thermodynamic method calculate temperature drop for determination of autorefrifgerant effect. I will do it. I am using PROII software in this regards. in flash unit operation of this software only isentropic calculation has been supported. I don't know how can do isenthalpic calculation for second segment (from critical pressure to superimposed pressure) to determine temperature drop in PROII. do you have any experience in this regards
and about your answer to my second question, based on API 520, for PSVs are susceptible critical flow condition, pressure drop across PSV will be from releif pressure to critical pressure. so back pressure of these PSVs should be critical pressure not back pressure (superimposed+build up back pressure). ma I reight?

[fallah]

dear Fallah
thanks for your fast reply.
so, you believe that I should split pressure drop to two segment and with two different thermodynamic method calculate temperature drop for determination of autorefrifgerant effect. I will do it. I am using PROII software in this regards. in flash unit operation of this software only isentropic calculation has been supported. I don't know how can do isenthalpic calculation for second segment (from critical pressure to superimposed pressure) to determine temperature drop in PROII. do you have any experience in this regards
and about your answer to my second question, based on API 520, for PSVs are susceptible critical flow condition, pressure drop across PSV will be from releif pressure to critical pressure. so back pressure of these PSVs should be critical pressure not back pressure (superimposed+build up back pressure). ma I reight?

Dear Hossein,
For first part of your question,i think it could be considered isentropic totally for two segments with efficiency of around 60%-70%,but other members may submit better explanations in this regard.
For second part,considering nozzle model for PSV,critical pressure would exist in assumed nozzle throat and after this point there would be isenthalpic expansion from critical pressure to superimposed back pressure.
Hope above could help.
Regards

[Hossein Deldari]

dear Fallah
thanx for your reply specially to second part of my question.
for first segment I will wait to receive other replies
regards
hossein

[JoeWong]

For first part, the process is quick, almost frictionless, etc. Those, i would think it is isentropic process without any deficiency.

[Hossein Deldari]

have you seen any description in API 521 that releif temperature should be calculated based on only superimposed back pressure and without heat input (adiabatic calculation)?

[JoeWong]

Hossein,
This question has less /little relationship with earlier post. Suggest to open a new thread with right subject name for ease search.

[Hossein Deldari]

Wong
I know my question maybe is not in direct relation with my first question, but I am searching about calculation of PSV releif temperature yet. I had a discusion with an expert egineer in another site. I asked about is adiabatic or isentropic flash calculation should be applied during determination of minimum temperaure during releiving condition. he reply that "Read and study API RP521--all about design of relief systems. To determine the minimum temperature achieved in flashing a hydrocarbon, one should assume there is no built up back pressure (atm pressure flash,) and no heat input to the boiling fluid. This is an 'adiabatic' situation"
I can not find this description in API. so I plan it as a new question.
I think this new my question can be related to first segment of my first one.

[JoeWong]

Wong
I know my question maybe is not in direct relation with my first question, but I am searching about calculation of PSV releif temperature yet. I had a discusion with an expert egineer in another site. I asked about is adiabatic or isentropic flash calculation should be applied during determination of minimum temperaure during releiving condition. he reply that "Read and study API RP521--all about design of relief systems. To determine the minimum temperature achieved in flashing a hydrocarbon, one should assume there is no built up back pressure (atm pressure flash,) and no heat input to the boiling fluid. This is an 'adiabatic' situation"
I can not find this description in API. so I plan it as a new question.
I think this new my question can be related to first segment of my first one.

I guess no one document will 100% address everything. This is where engineer is for...

I don't think (at least i still don't find. I am glad to know) any clause stating the method to determine temperature. I think an engineering judgment is required to define the correct method.

I don't mean to challenge your trust to your expert engineer...

If you have superimposed back pressure and it is always there, then less or no risk to consider it instead of ATM. Nevertheless, considering back pressure of ATM is conservative from low temperature perspective.

Adiabatic flow is present in PSV but just a matter of whether you would like to into account of the frictionless nozzle flow which is highly isentropic...

[Hossein Deldari]

joewong
thanx for your complette reply. whatever I undestand from your reply is in real world releivng from PSV is isenthropic process not adiabatic. am i right?
regarding to which clause of API points to apply adaibatic process, I could not find it yet.

[JoeWong]

Hossein,
There are some misunderstanding here...

First idea...
Many engineers consider pressure drop across valve i.e control valve, pressure relief valve, etc is isenthalpic flow (adiabatic and irreversible). For PSV, they analogue it as "orifice". This is one idea...

Second idea...
As more and more studies on the PSV and enginerr found that the flow in the PSV is actually passing a nozzle instead of orifice. Due the the construction, it is almost close ideal frictionless nozzle. This make engineer consider isnetropic process (adiabatic and reversible) across the PSV nozzle and use to estimated the relief flow (more conservative).

Nevertheless,
In real world the process across PSV will not be isentropic, but it stay between isentropic and isenthalpic. This lead to the third idea. Isentropic via nozzle follow by isenthalpic after nozzle. Read more in "Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve "

[Hossein Deldari]

joewong
based on your complete explanation I undestand that during determination relief condition in PSV outlet (temperature drop due to presure drop), adiabtic calculation should never be used. to determine this condition either isentropic or isenthalpic calculation should be used. am i right?

[JoeWong]

joewong
based on your complete explanation I undestand that during determination relief condition in PSV outlet (temperature drop due to presure drop), adiabtic calculation should never be used. to determine this condition either isentropic or isenthalpic calculation should be used. am i right?

Some thermodynamic definitions...

Adiabatic...Check out here
Isentropic is adiabatic reversible...Check out here
Isenthalpic is adiabatic irreversible...Check out here


For first part of your question, both are adiabatic...
For second part of your question, please check my responses in post #7.

If your studies is for low temperature, then isentropic follow by isenthalpic would give you conservative value...
If your studies is for outlet Mach, then isenthalpic would be sufficient...