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# Psv Outlet Nozzle Stream Temperature

5 replies to this topic
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### #1 shantanu.ghag

shantanu.ghag

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Posted 12 November 2019 - 10:14 PM

Dear All,

1. Could anyone explain how temperature at PSV outlet nozzle stream is calculated? Does it calculated based on back pressure?

I have calculated PSV relief pressure based on back pressure calculation as per API 520 in flare header.

2. I have seen in API 520 that temperature at relieving condition (PSV inlet) may be lower than operating temperature. how it can be?

Thanks,

Shantanu.

### #2 latexman

latexman

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Posted 13 November 2019 - 07:37 AM

5.10.5 The designer should also investigate the effect of any autorefrigeration that may arise from the flashing of liquid.

T - is the stagnation temperature at the pipe outlet, in °R [K]. Note that the stagnation temperature is often not available. If this is the case, a suitable approximation is the relieving temperature. The user is cautioned that while this approximation is conservative for determination of reaction forces, the relieving temperature shall not be used for other discharge piping design aspects as this does not consider auto-refrigeration or Joule-Thompson cooling effects.

### #3 Erwin APRIANDI

Erwin APRIANDI

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Posted 14 November 2019 - 02:28 AM

1.

Q: Could anyone explain how temperature at PSV outlet nozzle stream is calculated?

A: What scenario that you are asking?

Q: Does it calculated based on back pressure?

A: The Temperature is calculated by flashing the Relief Pressure Stream to The Calculated Back Pressure at the PSV Outlet

Info: I have calculated PSV relief pressure based on back pressure calculation as per API 520 in flare header.

Response : I'm curious how you calculate the PSV Relief Pressure by calculating Back Pressure, The PSV Set Pressure shall be same as the Protected Equipment/Piping Design Pressure, depend on the scenarios the maximum accumulated pressure is add up from the set pressure. For Fire Case it is 21% accumulated pressure and 10% for other cases. Please Refer to API STD 521

2.

Q: I have seen in API 520 that temperature at relieving condition (PSV inlet) may be lower than operating temperature. how it can be?

A : What is the scenario? It is Possible Especially for Blocked Discharge Case, since the pressure is higher the stream is not flashed to lower pressure (opt pressure) but it is only flashed to set pressure. Please follow API STD 521 not 520

PSV Blocked Discharge Case.xlsx   14.12KB   10 downloads

Edited by Erwin APRIANDI, 14 November 2019 - 02:34 AM.

### #4 shantanu.ghag

shantanu.ghag

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Posted 14 November 2019 - 11:35 AM

1.

Q: Could anyone explain how temperature at PSV outlet nozzle stream is calculated?

A: What scenario that you are asking? - In my case its Fire Case (within fire circle one vessel filled with HC gas & other vessel with liquid water). does this depends case by case?

Q: Does it calculated based on back pressure?

A: The Temperature is calculated by flashing the Relief Pressure Stream to The Calculated Back Pressure at the PSV Outlet.

I have observed PSV outlet temperature calculated at throat, vena contracta & then final exit at one of the discussion on this forum and thus looking for how this calculated. Please refer the URL " https://www.cheresou...ng-temperature/

Info: I have calculated PSV relief pressure based on back pressure calculation as per API 520 in flare header.

Response : I'm curious how you calculate the PSV Relief Pressure by calculating Back Pressure, The PSV Set Pressure shall be same as the Protected Equipment/Piping Design Pressure, depend on the scenarios the maximum accumulated pressure is add up from the set pressure. For Fire Case it is 21% accumulated pressure and 10% for other cases. Please Refer to API STD 521

excuse me for improper word, its pressure at PSV outlet nozzle. I have referred API 520 part 1 old edition for this. I will check API STD 521.

2.

Q: I have seen in API 520 that temperature at relieving condition (PSV inlet) may be lower than operating temperature. how it can be?

A : What is the scenario? It is Possible Especially for Blocked Discharge Case, since the pressure is higher the stream is not flashed to lower pressure (opt pressure) but it is only flashed to set pressure. Please follow API STD 521 not 520 ok I will check.

Dear Erwin sir, thank you. please find response in blue font.

### #5 shantanu.ghag

shantanu.ghag

Junior Member

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• 19 posts

Posted 14 November 2019 - 11:39 AM

5.10.5 The designer should also investigate the effect of any autorefrigeration that may arise from the flashing of liquid.

T - is the stagnation temperature at the pipe outlet, in °R [K]. Note that the stagnation temperature is often not available. If this is the case, a suitable approximation is the relieving temperature. The user is cautioned that while this approximation is conservative for determination of reaction forces, the relieving temperature shall not be used for other discharge piping design aspects as this does not consider auto-refrigeration or Joule-Thompson cooling effects.

Noted Latexman. Thanks.

### #6 Erwin APRIANDI

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Posted 14 November 2019 - 07:38 PM

1.

Q: Could anyone explain how temperature at PSV outlet nozzle stream is calculated?

A: What scenario that you are asking? -

In my case its Fire Case (within fire circle one vessel filled with HC gas & other vessel with liquid water). does this depends case by case?

A: Yes, relieving rates, accumulative pressure allowance and temperature of each scenarios is different, what you will have to do is to try to estimate using calculation all the possible case that may happen on your protected equipment (The PSV case stated on P&ID is only the governing case but not the only possible case) to understand this from API STD 521.

Q: Does it calculated based on back pressure?

A: The Temperature is calculated by flashing the Relief Pressure Stream to The Calculated Back Pressure at the PSV Outlet.

I have observed PSV outlet temperature calculated at throat, vena contracta & then final exit at one of the discussion on this forum and thus looking for how this calculated. Please refer the URL " https://www.cheresou...ng-temperature/

A: Yes it same, the results will be around 1-2% difference compare to simulation software as mentioned in the discussion, the calculation is to put the Inlet PSV conditions and calculate the flash effect by using the calculated back pressure to predict the outlet temperature.

Info: I have calculated PSV relief pressure based on back pressure calculation as per API 520 in flare header.

Response : I'm curious how you calculate the PSV Relief Pressure by calculating Back Pressure, The PSV Set Pressure shall be same as the Protected Equipment/Piping Design Pressure, depend on the scenarios the maximum accumulated pressure is add up from the set pressure. For Fire Case it is 21% accumulated pressure and 10% for other cases. Please Refer to API STD 521

excuse me for improper word, its pressure at PSV outlet nozzle. I have referred API 520 part 1 old edition for this. I will check API STD 521.

2.

Q: I have seen in API 520 that temperature at relieving condition (PSV inlet) may be lower than operating temperature. how it can be?

A : What is the scenario? It is Possible Especially for Blocked Discharge Case, since the pressure is higher the stream is not flashed to lower pressure (opt pressure) but it is only flashed to set pressure. Please follow API STD 521 not 520 ok I will check.

Dear Erwin sir, thank you. please find response in blue font.

Edited by Erwin APRIANDI, 14 November 2019 - 07:45 PM.