13 replies to this topic

[Julien123456789]

Dear All,

I have a very simple system as attached.

Which type of PSV I shall use ?

The vendor recommended using a PSV type conventional.

 

Using API 520, I found a criteria that when Ratio back pressure/ Set point > 10 % (1,013/2 = 50 %), I shall buy a PSV type pilot.

 

Am I misunderstanding this criteria ? 

Please correct me.

 

Thank you.

Attached Files

•  PSV.png   11.99KB   3 downloads

Edited by Julien123456789, 13 April 2016 - 07:40 AM.

[Mahdi1980]

Dear Julien

A conventional type is suitable for your case as vendor has said. This is a simple case and does not require Pilot operated type.

 

Mahdi

[Julien123456789]

Mahdi,

Thank you so much for you comment.

I am convinced that the vendor has reason.

However, how about the criteria of ratio back pressure to set point ?

I am worry to be misinterpreted this criteria.

 

Julien

Edited by Julien123456789, 13 April 2016 - 09:15 AM.

[fallah]

 

Which type of PSV I shall use ?

The vendor recommended using a PSV type conventional.

 

Using API 520, I found a criteria that when Ratio back pressure/ Set point > 10 % (1,013/2 = 50 %), I shall buy a PSV type pilot.

 

Am I misunderstanding this criteria ? 

Please correct me.

 

 

Julien,

 

In general, for liquid relief and high back pressure the pilot operated PSV is the best option...But if the only scenario is thermal expansion it might a small size of conventional one would be adequate...

 

But to make a proper decision for the case did present it's needed some additional info such as pump and downstream piping design pressure, piping design code, other possible scenarios...

Edited by fallah, 13 April 2016 - 09:55 AM.

[Julien123456789]

Mr. Fallah,

Thank you so much for your comment.

As showed on the attachement file, it is a very simple system.

There is only thermal expansion scenario for my system.

I want to respect API RP or Shell RP.

Seeing that is a small PSV, I dont need to consider so much for buying a PSV type Pilot if necessary.

However, I've never seen a PSV type Pilot installed in this application, mostly at lower conditions (1 to 3 bara, T = 25 oc, and small flow-rate).

The trouble is the criteria ratio back pressure/ set point that a lot of literature mentioned. If I respect this criteria, I shall buy a PSV type pilot. In this case, it looks like not realistic!

Is it not primary the above criteria in selecting PSV ?

 

Julien

Edited by Julien123456789, 13 April 2016 - 10:26 AM.

[chocobang]

Julien,

 

Conventional is usually the PSV for themal expansion application. 

 

BUT you yourself shall need to check or calculate the backpressure experience by the PSV. Pipe outlet routing and pipe outlet size are the main factors which determines built-up backpressure. 

 

Then you can calculate the built-up backpressure over set pressure (which in your case is 1 barg).

 

But with the flowrate you gave, I think conventional is enough unless the tailpipe outlet is higher than PSV (creating a liquid pocket). But its counter intuitive to route the tailpipe going up.

[cea]

As I understand, your set pressure is equal to or higher than pump shut off pressure.

 

As your shut of pressure is (guessed to be) less, set pressure is set accordingly & hence your problem.

 

However, piping & associated flanges are expected to be in line with 150# spec. Please confirm / correct my understanding.

 

If so, you can increase PSV set pressure to a value which will allows use of conventional PSV.

[Julien123456789]

Julien,

 

Conventional is usually the PSV for themal expansion application. 

 

BUT you yourself shall need to check or calculate the backpressure experience by the PSV. Pipe outlet routing and pipe outlet size are the main factors which determines built-up backpressure. 

 

Then you can calculate the built-up backpressure over set pressure (which in your case is 1 barg).

 

But with the flowrate you gave, I think conventional is enough unless the tailpipe outlet is higher than PSV (creating a liquid pocket). But its counter intuitive to route the tailpipe going up.

 

Christopherchoa,

Thank you so much for your comment.

I see what you mean. 

 

 

Julien

Edited by Julien123456789, 15 April 2016 - 01:57 AM.

[Julien123456789]

As I understand, your set pressure is equal to or higher than pump shut off pressure.

 

As your shut of pressure is (guessed to be) less, set pressure is set accordingly & hence your problem.

 

However, piping & associated flanges are expected to be in line with 150# spec. Please confirm / correct my understanding.

 

If so, you can increase PSV set pressure to a value which will allows use of conventional PSV.

 

Cea,

Thank you for your comment.

My piping & associated flanges of class 150 as you guessed.

I had a mistake in calculation before posting this thread. In fact, my back pressure is small. So, I can use a conventional one.

 

Julien

[quiet.life]

Julien,

 

The mistake in your calculation is probably using the absolute pressures in place of gauge pressures.

For ratio of back pressure to set pressure, pressures used should be in gauge.

 

Set pressure is around 1 barg.

 

Constant back pressure in your case is 0 barg.

Friction loss in the tail pipe (i.e superimposed back pressure) should be less than 0.1 bar, to use conventional type of RV. 

 

Thanks. 

[Julien123456789]

Quiet.life,

 

Exactly what you guessed.

 

Julien

Edited by Julien123456789, 15 April 2016 - 07:46 AM.

[quiet.life]

small correction. Please read superimposed back pressure as built up back pressure.

[Julien123456789]

Thank you Quiet.life,

I noted it.

 

Julien

[ChemEng01]

Typically you don't have to size for thermal expansion if your fluid doesn't flash? 

 

You will not get a steady state flow as the pressure will drop instantaneously. Therefore built-up backpressure is irrelevant for thermal expansion. That's why normally you would just select a DN 20 x DN 25 valve. (Unless you have large diameter pipelines above ground or large vessels). In your case I presume not.