Reducer In Relief valve inlet line- Pressure Loss 2

[reaxion]

I am sizing a relief valve inlet line size.
The following is the design data:

1. PSV Set Pressure 16 kg/cm2 g
2. Allowable overpressure 21%
3. Inlet line size estimated to meet 3% inlet loss requirement: 12"
4. Relief valve inlet size: 6"
Estimated pressure drop in 12" line is 0.35kg/cm2 which meets the 3% criteria.
However, if a 12"X6" reducer (Which is necessary) is incorporated incorporated in calculation, the pressure losses across the REDUCER alone are as follows:
A. Total (Recoverable+ non recoverable pressure loss): 2.0 kg/cm2 out of which,
B. Non Recoverable Pressure loss: 0.1 kg/cm2

If non- recoverable pressure loss is added to line loss, total pressure loss meets the 3% API criteria. However, if total pressure loss across reducer is considered, the loss is much higher than the 3% criteria.

In my openion, since the PSV will be located immediately downstream of reducer, we must consider Total pressure loss since pressure recovery is not possible downstream of reducer.

However, if this criteria is applied strictly to all designs, most of the inlet line sizes for most of safety valves will fail.

What is it correct to consider? Total pressure loss or only non recoverable pressure loss across reducer, in case of PSV Inlet line?

 

[sheiko]

This is a very relevant question to ask API as their requirement (cf API STD 520-Part II, and also ASME Section VIII, Div 1- Appendix M ) is to consider non-recoverable pressure drop (ie pressure loss).
In the meantime, i would follow the CODE and consider NON-RECOVERABLE pressure drop.
Or see if using required relieving flowrate instead of PSV rated capacity in your inlet loss calcs help...

"We don't believe things because they are true, things are true because we believe them."

 

[CJKruger]

I agree with sheiko, just use non-recoverable pressure drop.

Cilliers