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3

Pressure Relief Valve Set Pressure

8 replies to this topic
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#1 salihbaykal

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Posted 17 August 2023 - 12:12 AM

1) Can PSV set pressure be lower than the design pressure of the protected equipment ? Generally in guide books, it is above the design and MAWP of the equipment. However, I was trying to do a more conservative design. For example for a fire case;

Design P of Protected Equipment : XX kg/cm2-g

90% of the Design P : XX*0.9 kg/cm2-g

Set Pressure of the PSV: (XX*0.9) / (1.21) kg/cm2-g (21% for fire scenario)

Relieving pressure of the PSV is (XX*0.9) kg/cm2-g for that case. Can it be done for conservative results ?

2) My second question is: If relieving temperature of the PSV is higher than the pipe spec, could it be a serious problem ? (The distance between the PSV and the equipment is not short.)

#2 fallah

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Posted 17 August 2023 - 02:04 AM

1) Can PSV set pressure be lower than the design pressure of the protected equipment ? Generally in guide books, it is above the design and MAWP of the equipment. However, I was trying to do a more conservative design. For example for a fire case;

Design P of Protected Equipment : XX kg/cm2-g

90% of the Design P : XX*0.9 kg/cm2-g

Set Pressure of the PSV: (XX*0.9) / (1.21) kg/cm2-g (21% for fire scenario)

Relieving pressure of the PSV is (XX*0.9) kg/cm2-g for that case. Can it be done for conservative results ?

2) My second question is: If relieving temperature of the PSV is higher than the pipe spec, could it be a serious problem ? (The distance between the PSV and the equipment is not short.)

Hi,

1) The PSV set pressure shall be equal to or lower than the MAWP of the relevant equipment; hence lower set pressure is more conservative but care should be taken not to be so close to the maximum operating pressure.

2) In general, the relieving temperature shouldn't be higher than the pipe spec; but the relieving temperature of the fire case is exceptional and in such case the relieving temperature could be higher than the pipe spec...

#3 latexman

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Posted 17 August 2023 - 06:27 AM

salihbaykai,

You are using numbers that make me believe you are using ASME Code.  Do you realize there is conservatism built into the ASME Code already?  Certified flow coefficients are downrated by 10%.  Vessels do not explode at their MAWP; it's more like 2X MAWP or higher.  If we, as Engineers, add conservatism on top of conservatism on top of conservatism, we do our companies a disservice and make them uncompetitive.  Think about it.  My \$0.02.

#4 Daniel2023

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Posted 14 September 2023 - 03:25 AM

Hi all

What might be the reason for activating tank PSV at pressure lower than it's set pressure?

#5 fallah

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Posted 14 September 2023 - 04:14 AM

Hi all

What might be the reason for activating tank PSV at pressure lower than it's set pressure?

Lower than its set pressure or design pressure?

#6 Daniel2023

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Posted 14 September 2023 - 05:58 AM

Hi

When a PSV is leaking/sweating through, under it's set pressure, is this due to seat or some soft parts that that is not tight?

#7 fallah

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Posted 14 September 2023 - 06:27 AM

Hi

When a PSV is leaking/sweating through, under it's set pressure, is this due to seat or some soft parts that that is not tight?

Hi,

Actually a PSV isn't a zero leakage valve at all; but when the set pressure of a PSV is so close to the operating pressure of the relevant vessel, the PSV getting start to leak or simmer at a pressure of 90% of the set pressure but simmer can be adjusted by adding or subtracting the blowdown.

#8 latexman

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Posted 14 September 2023 - 07:41 AM

When a PSV is leaking/sweating through, under it's set pressure, is this due to seat or some soft parts that that is not tight?

As a PSV approaches set pressure closely, leakage is normal due to the force balance within the PSV.

Some rules of thumb my company uses:

A PSV with metal-to-metal seats will start leaking appreciably (simmering) at about 90% of set pressure.

A PSV with soft seats (o-ring) will start leaking appreciably (simmering) at about 95% of set pressure.

A pilot operated PSV will start leaking appreciably (simmering) at about 98% of set pressure.

If a PSV is leaking appreciably below these rules of thumbs, it's time to pull the PSV, test, inspect, and correct!

#9 shvet1

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Posted 15 September 2023 - 12:21 AM

Not sure answers correspond question as topicstarter asked for tank overpressure. Note sure that limits are the same in range below 1 barg.