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# CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

## CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

(OP)
Dears
We have one Lube oil PSV(Conventional design), which is located in one compressor. PSV set pressure is 11Kg/cm2g and there is 1 atm(stad)g Constant back pressure. While calculating CDTP, is it required to deduct the constant back pressure?

Or CDTP is same as Set pressure?

If the constant back pressure is atmospheric, is it required to consider while calculating the CDTP?

PSV data sheet is attached for your reference.

Pls advice us as soon as possible.

### RE: CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

Safety Valve activation = CDTP + Constant back pressure. If constant back pressue is atmospheric, activatio = CDTP

### RE: CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

Other reason to use a CDTP is if the bonnet & spring of the PRV are hot during operation - >100°C/200°F. The hot spring looses tension so the COLD setpoint is higher than the operating relieving pressure; you cannot calc this one. The PRV mfr will give it to you, from their testing history when their PRV's were originally certified. And remember, it is ONLY the temperature of the spring that matters, at the moments BEFORE the PRV starts relieving.

And yes, your PRV that you asked about will have a CDTP = Set Point. You have no 'extra' backpressure, above std. atmospheric pressure. Thus the testing conditions will match the "in the field, actually relieving" condition.

### RE: CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

(OP)
Dears
Thanks a lot for your feedback.
By Nasim

### RE: CDTP Calculation on Constant Atmospheric Pressure in Lube Oil PSVs

Just to clarify, from an ex manufacturing guy;-

Conventional spring operated safety relief valve
CDP (Cold Differential set Pressure)
= Set pressure - constant set pressure + manufacturers % temperature spring correction factor.
eg. Set 100, constant back 30, relief temp* 350°F (manufacturers corr factor 2%) CDP = (100-30)1.02 = 71.4
(or is the temp the operating since relief is at point of discharge ?)

Bellows (Supplementary balanced piston) spring safety relief valve CDP
= Set pressure + manufacturers % temperature correction factor.

Notes
1. Temperature correction not considered in external fire cases (nor is material selection).
2. Based on gauge pressure 0 PSIG
3. Applies to all fluids
4. Not generally applicable to open spring bonnet valves
5. % Correction varies by manufacturer.
6. Considered, or not, by in situ test providers.
7. Importantly, CDP is the shop test pressure only, since constant back pressure increases inlet opening pressure. Hence the spring correction.

Did not purposely discuss variable back pressure and its types and effects.

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