Cold Differential Test Pressure 1

[chisa3]

Hi,

I am doing a review of our PSV settings and have found the term "cold differential test pressure" on some of the data sheets. When reading definitions of this, I have seen that it is an adjustment to the set pressure of the valve when it is being set, as it is not at process conditions.

The definitions say the correction can be due to backpressure and/or temperature fluctuations. I understand the point about back pressure, as you have no backpressure on the testing bench, so you must correct for this.

I am, however, trying to figure out how the temperature will affect the lifting pressure. I thought it may be to do with the spring in the valve changing resistance at elevated temperatures, but this sits on the non-process side of the valve, so don't think this can be right?

I would really appreciate it if anyone can explain this to me....

Thanks for the help

Alex

 

[JoeTank]

If the valve in mounted on a hot vessel it can see elevated temperatures due to heat conduction thru the valve mount and body.

Joe Tank

 

[ApC2Kp]

chisa3,

The cold differential test pressure would relate to the flow rate with corresponding pressure drop across the relief valve when discharging. Particularly for gas vapor or steam flows, the hot temperatures would change the flow density, and the pressure drop / rated flow changes.
Most flow test lines only have water flow or compressed air flow lines, so data is given for cold test condition with factors applied to establish hot service operating values.
It would not seem to be any adjustment to setpoint. Hot temperatures cause metal parts and springs to expand, but these would expand together, unless a part was stainless steel expanding more than the carbon steel of corresponding part, for example, yoke versus stem.

 

[chisa3]

Thanks for that.
I agree about the use of different fluids/temperatures on the test rig meaning that the relieving rate will be different.
However, as I think you agree, the density changes etc should not really affect the lifting pressure of the valve. It will more affect the relieving rate once it is open though. As far as I know we only set the set point when we set them, there is no verification of relieving rates etc.
The thermal conductivity issue is the one that I think comes into play here. I guess it taked effect where there are different materials used and there may also be a temperature gradient from the process side to the top of the bonnet/spring assembly?