Safety Relief Valve - recip compressor closed outlet
Safety Relief Valve - recip compressor closed outlet
(OP)
Dears,
I would like to consult you in solving an issue regarding a closed outlet scenario at the outlet of a reciprocating compressor.
Data:
I have a 2 stage reciprocating compressor, compressing a mixture of isobutane, hexane and nitrogen.
The 2'nd stage suction parameters are P0=4.14 bar and T0=35 C and 342 m3/h (@760 mmHG, 0C)which is almost equivalent to 563 kg/h. The normal discharge parameters of the 2'nd stage are P1=16.3 bar and T1=110 C (predicted; and 35 C adiabatic, don't know exactly what it means). The kW for 2'nd stage is 20.
Requirements:
I have to check the relief valve downstream of this compressor (relief flow, relief conditions and valve size) for the CLOSED OUTLET scenario. The relief pressure is 110%xSPrv = 1.1 x 17.2 barg = 18.9 barg. The relief flow I suppose it's the flow that compressor is able to provide, 563 kh/h, but I don't know exactly HOW TO CALCULATE THE RELIEF TEMPERATURE.
Cmpressibility of gas(@inlet pressure) is 0.985. I assume ideal gas behaviour
Cp/Cv = 1.22.
In case of closed outlet, is the compression process still polytropic?
Can I use the equation: T2 = T1(P2/P1)^(1-(1/n))?
Does the compressor efficiency matters in calculating the temperature? If I had an efficiency curve, could I have been able to use Hysys for calculation?
Thank you in advance for your support,
I would like to consult you in solving an issue regarding a closed outlet scenario at the outlet of a reciprocating compressor.
Data:
I have a 2 stage reciprocating compressor, compressing a mixture of isobutane, hexane and nitrogen.
The 2'nd stage suction parameters are P0=4.14 bar and T0=35 C and 342 m3/h (@760 mmHG, 0C)which is almost equivalent to 563 kg/h. The normal discharge parameters of the 2'nd stage are P1=16.3 bar and T1=110 C (predicted; and 35 C adiabatic, don't know exactly what it means). The kW for 2'nd stage is 20.
Requirements:
I have to check the relief valve downstream of this compressor (relief flow, relief conditions and valve size) for the CLOSED OUTLET scenario. The relief pressure is 110%xSPrv = 1.1 x 17.2 barg = 18.9 barg. The relief flow I suppose it's the flow that compressor is able to provide, 563 kh/h, but I don't know exactly HOW TO CALCULATE THE RELIEF TEMPERATURE.
Cmpressibility of gas(@inlet pressure) is 0.985. I assume ideal gas behaviour
Cp/Cv = 1.22.
In case of closed outlet, is the compression process still polytropic?
Can I use the equation: T2 = T1(P2/P1)^(1-(1/n))?
Does the compressor efficiency matters in calculating the temperature? If I had an efficiency curve, could I have been able to use Hysys for calculation?
Thank you in advance for your support,





RE: Safety Relief Valve - recip compressor closed outlet
T2 = T1(P2/P1)^(1-(1/k)) where k = Cp/Cv
Good luck,
Latexman
RE: Safety Relief Valve - recip compressor closed outlet
The PSV is between 2'nd stage discharge vessel and aftercooler; the valve that might be closed is downstream of the aftercooler. I suppose it will have a bit of effect on gas cooling.
Kind regards,
RE: Safety Relief Valve - recip compressor closed outlet
The P1 and P2 that you use are really important. The equation that Latexman gave you works after the suction check valves and before the discharge check valves. Both those valves have a non-zero cracking pressure. I always use P1=P(suction header)+P(atm)-5 psi and P2=P(discharge header)+P(atm)+10 psig. This minor tweak will make a significant difference in calculated temperature. If you know actual cracking pressure, use them, otherwise the 5 and 10 psi gets very close to the assumptions in the manufactures programs.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Safety Relief Valve - recip compressor closed outlet
Good luck,
Latexman
RE: Safety Relief Valve - recip compressor closed outlet
I apologize, in my first post it's an error. The question should have been if i could calculate the relieving temperature with the following relation: T2 = T0(P2/P0)^(1-(1/k)) (and not using T1 and P1 which are the normal discharge parameters).
T0 = suction pressure of the 2'nd stage, K
P0 = suction pressure of the 2'nd stage, atm
P2 = relief pressure, atm
k = Cp/Cv
RE: Safety Relief Valve - recip compressor closed outlet
The equation you put in is not the equation for cooling through a PSV. It is the equation for adiabatic heating. Like in a PD compressor
130648,
That is exactly the equation I would use for the case you were asking about. Needs to be in absolute units and I would account for the cracking pressure of the valves, but that is the right equation.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Safety Relief Valve - recip compressor closed outlet
Good luck,
Latexman
RE: Safety Relief Valve - recip compressor closed outlet
zadas, I have had plenty of discharge reliefs go off and the engine keeps on going In this case, the poster needs to clarify his set pressure, is the maop of the system 17.2 barg and therefore the relief must me set at 17.2 and be fully opened at 18.9? In any case a 1 to 2 barg increase in pressure isn't that much work that the engine has take up.
RE: Safety Relief Valve - recip compressor closed outlet
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Safety Relief Valve - recip compressor closed outlet