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# Psv Required Flow

10 replies to this topic
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### #1 CS10

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Posted 07 February 2019 - 10:20 PM

Dear All,

I got to do a calculation to determine what is the required flow of a PSV.

Has anybody had experience to share about the PSV required flow calculation?

or share any standard or reference that I can use to do the calculation?

I have attached a sketch to show the diagram of what I am trying to work on?

Best regards,

Cozta

### #2 breizh

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Posted 07 February 2019 - 11:02 PM

Hi ,

Consider these resources to support your work .

https://www.leser.co...ls/engineering/

Good luck.

Breizh

#### Attached Files

Edited by breizh, 07 February 2019 - 11:04 PM.

### #3 Bobby Strain

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Posted 07 February 2019 - 11:49 PM

Something is wrong with your picture.

Bobby

### #4 CS10

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Posted 08 February 2019 - 01:12 AM

Thanks @breizh for sharing those references

@Bobby, do you mean the pressure regulator drawing?

I have updated the drawing and re-attached.

### #5 fallah

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Posted 08 February 2019 - 01:27 AM

I have updated the drawing and re-attached.

Hi,

Pressure regulator "wide open" and "blocked outlet"...double jeopardy?

### #6 CS10

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Posted 08 February 2019 - 01:56 AM

Thanks @fallah for the correction.

What actually I was trying to point out is the scenario during the control valve is in fully open position.

Form the PSV datasheet from vendor, it says for blocked outlet scenario the required flow is 2100 kg/h.

For this calculation, I want to check if the control valve 100% open how much flow can pass through the valve (Pressure Regulator) and go to pressure relief valve. So then this can confirm that the required flow from vendor in the DS is correct.

### #7 fallah

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Posted 08 February 2019 - 05:00 AM

What actually I was trying to point out is the scenario during the control valve is in fully open position.

Form the PSV datasheet from vendor, it says for blocked outlet scenario the required flow is 2100 kg/h.

For this calculation, I want to check if the control valve 100% open how much flow can pass through the valve (Pressure Regulator) and go to pressure relief valve. So then this can confirm that the required flow from vendor in the DS is correct.

Hi,

The required flow, or relief load, for a PSV should be specified by the customer not vendor; hence the vendor should specify just the PSV rated capacity.

### #8 latexman

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Posted 08 February 2019 - 07:59 AM

The PSV set pressure should be higher than the regulator set pressure or it may be open all the time.  I suspect this is what Bobby Strain saw.

Edited by latexman, 08 February 2019 - 08:00 AM.

### #9 CS10

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Posted 11 February 2019 - 06:47 PM

Hi,

This regulator actually has the set point of 490 kPa (Operating Pressure). I have stated before the wrong set point of the regulator of 1500 kPa, which is the set point of the upstream PSV of the regulator.

Thanks a lot for your feedback and correction.

### #10 Fletch

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Posted 18 February 2019 - 12:40 AM

Focus on the control valve/regulator, you are wanting to determine the flow across the fully open regulator with a specified pressure drop across the regulator (upstream pressure - downstream pressure).

What is the 100% open Cv value for the regulator? (check datasheet or use model number and search online)

The upstream pressure is the maximum possible upstream pressure (e.g. 2210 kPag or 1500 kPag if you want to credit the upstream regulator is still operating)

The downstream pressure is the relieving pressure of the PSV (700 kPag + accumulation)

Get out your control valve formulas and work out the flow rate based on the above information.

Compare this to the rated capacity of the relief device?

The calculation above will give you a conservative result which could be within the capacity of the relief device.

Though, as this is a control valve failure, then you could also credit the normal flow to the downstream users, so you could subtract this normal flow from the relief rate as we are saying there is a second open/available path for the gas to go.

Edited by Fletch, 18 February 2019 - 12:48 AM.

### #11 CS10

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Posted 25 February 2019 - 11:04 PM

Thanks all for your valuable inputs.

@Fletch: I have done a search online based on the model number of the control valve and I have got already the Cv when 100% open (Cv = 4).

One more question, with that Cv I have calculated the flow during control valve fully open with HYSYS model. But, I am not really sure with the result from my model.

If I specify valve sizing methods in HYSYS valve rating tab with ANSI/ISA it gives me higher flow rate compare to Manufacturer specific method. Both methods using the same Cv of 4 for the modelling.

The results flow rates from ANSI/ISA and Manufacturer methods are less than the flow capacity of the PSV which means the PSV is OK.

My question is which methods is considered accurate (ANSI/ISA or Manufacturer) to use?

Thank you,