4 replies to this topic

Dear All,

Would you please clarify to me the following isssues:

1-I read that by increasing the suction temperature of the natural gas compressor, the polytropic head and the power will go down. How?
2-Is it possible to start the natural gas centrifugal compressor with discharge valve open (anti-surge also open 100%) and the downstream pressure is atmospheric?

Please clarify and thanks in advance

[djack77494]

I think your queries are best resolved by the compressor's manufacturer. With increased suction temperature, the inlet density is decreased. The polytropic head should NOT be impacted, but the discharge pressure will drop due to lower gas density. Power will decrease because the density and the mass flowrate will decrease. Your second question is too machine-specific for me to attempt a response.

djack77494,

Thanks but would you please clarify more how the increasing in the suction temperature will reduce the mass flow rate since the volumetric flow will increase which may conpensate for the density decrease.

[ankur2061]

Dear All,

Would you please clarify to me the following isssues:

1-I read that by increasing the suction temperature of the natural gas compressor, the polytropic head and the power will go down. How?
2-Is it possible to start the natural gas centrifugal compressor with discharge valve open (anti-surge also open 100%) and the downstream pressure is atmospheric?

Please clarify and thanks in advance

babox75,

Here is the equation for Polytropic head

H_Poly = (8314/MW)*T₁*Z_avg*(n/n-1)*((P2/P1)^(n-1)/n - 1)

where:
H_Poly = Polytropic head N-m/kg
MW = molecular weigth of the gas, kg/kg-mol
T₁ = Absolute temp. at inlet conditions, K
Z_avg = avg. compressibility factor, dimensionless= (Z₁+Z₂/2), where 1 & 2 are inlet and discharge conditions respectively
P₁ = inlet presure in absolute units, kPa(a) or bar(a)
P₂ = discharge pressure in absolute units, kPa(a) or bar(a)
n =polytropic exponent

Now I think it would be obvious to you that what you have mentioned about polytropic head going down with increase in suction or inlet temperature is not correct. Obviously if the polytropic head is going to increase, so will the shaft power requirement.

Hope this clears the doubt in your mind.

Regards,
Ankur.

[tslim]

With regards to your 2nd question, I assume the compressor is running with fixed speed electrical motor drive. I would make sure the compressor motor is not overloaded and keep the operating point within the compressor curve. With minimum backpressure at discharge, compressor flow rate increased, so does the power.