Steam Relief Valve Discharge Pipework

[BenjyMac]

Hi,

I was wondering if someone could give their thoughts on the following.

I have calculated the back pressure on a steam relief valve on the discharge side of the valve. The relief valve is 4" x 6" relief valve with a set-point of 58 barg and rated capacity of 69,000 kg/h at 410 deg.C inlet temperature.

The discharge pipework consists of a 1m length of 6" Sched 40 pipe followed by a 6" 1.5D drip pan elbow. This discharges into (not connected for allowance of expansion) into an 8"-12" expander which is attached to 12" pipe up to a silencer.


When I perform the calculation for back pressure, which is basically the back-pressure caused by the 1m pipe and bend, I obviously get two results depending on whether I take into consideration the discharge from the elbow as an exit loss. If I include it my back pressure is greater than 18% of relief valve set-point. If I ignore it I am bang on 10% of relief valve set pressure. My thoughts are due to the fact I calculate the conditions for sonic velocity at the end of the drip pan elbow which obviously utilises the area of the 6" pipe and then work back over to work out the pressure loss and hence back pressure at the relief valve discharge then I do not need to include for the exit loss.

Am I correct in my thinking?

Many thanks in anticipation.

 

[EGT01]

BenjyMac,

If you are sonic at the end of the 6" pipe I believe you are correct that you don't have to account for an exit loss. You can find supporting information related to this in the AICHE, CCPS publication "Guidelines for Pressure Relief and Effluent Handling Systems".

I think you also need to be sure that the remaining 12" piping and silencer doesn't adversely affect your assumption of sonic conditions at the end of the 6". That could change the approach that is needed.

As additional reference, ASME B31.1 has a non-mandatory Appendix II that goes into some detail about calculating back pressures at various points in the type of vent system you are describing. Even if your system does not have to follow the requirements of the power piping code, you may want to review them as well to ensure the back pressure due to the 12" piping and silencer are acceptable and won't cause a blow back of steam where the 6" pipe enters the 8".