Pressure relief valves and set pressure

[samabert]

Dear all,

We have a discussion about the following item.
If the pressure on the outlet side of the pressure relief valve (PRV) is higher than 1 Bara (atmosphere), what is the effect on the set pressure of the PRV? The PRV is used in kerosene.

For example, we have a PRV with 4 bar set pressure mounted between a fuel line, on both sides of the PRV we have 10 bar pressure. So when the pressure on the inlet side raise to, for example 14 bar, the pressure will depressurize in the outlet side with 10 bar.
Question: is the set pressure of 4 bar in this example OK or do we pay attention of the back pressure on the outlet side and set the PRV to 14 bar.

Thank you in advance.
Marc

[Cubeng]

First of all what type of pressure relief valve are you talking about?
- For standard PSV superimposed + accumulated back pressure can't be higher than 10% of the set pressure.
For example if the set pressure is 10 bar - pressure on the other side of the PSV cann't be higher than 1 bar.
- For ballanced bellows typethe back pressure ic 30-40% of the set pressure.
- For Pilot type it can almost reach 100%
So if your PSV is pilot type, and you need to relief pressure at 14 bar, you probably are ok but vendor shall be consulted to ensure that rated flow at this conditions is suffisient. Having the psv set at 4 bar as you mentioned will not achieve results simply because the outlet pressure is higher than set pressure.

Hope this helps.

[Nephilim]

First of all what type of pressure relief valve are you talking about?
- For standard PSV superimposed + accumulated back pressure can't be higher than 10% of the set pressure.

For a conventional PSV the built-up back pressure can't be higher than 10%, in case of 10% overpressure. If you have any superimposed back pressure you'll need to take this into account (CDTP) because you set pressure may never be higher than MAWP / design pressure.

[explosion]

Setting is a balance of forces: weight or spring load against pressure of the system. If there is backpressure it adds up to the setting, resp. the setting gives only the difference between pressures on inlet and outlet size. Of course the MAWP etc. must be maintained but if the setting is 4 bar an overpressure of 14 bar will be relieved into a 10-bar-pipe.
This is simple engineering !
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[explosion]

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