polytropic compression of wet gas with Prode Properties

[mrbabu]

recently we changed the spec's of inlet line to our compressor (centrifugal, two stages on natural gas stream),
now we have about 1% fraction of condensate,
the manufacturer said it's acceptable but efficiency and power consumption have to be revealuated,
we have measured pressures, temperatures etc. on different sections and calculated the new efficiency with Prode Properties, the value is close to the original number for dry gas (that could be Ok since we have only 1% condensate),
however it would be interesting to get an opinion from other members of this group about the correct evaluation of polytropic efficiency and running a centrifugal compressors on wet gas,
does that mean more troubles ?

[cmarc]

mrbabu,
I post the answer given by PaoloPemi in Cheresources, it is very detailed and I think it could interest other readers

I am not aware of standards discussing the calculation procedure to estimate the polytropic efficiency of a compressor under wet gas (gas + liquid) conditions,
for gas + different amounts of liquid I use the same procedure (Prode) to evaluate the efficiency,
in your case considering the low amount of condensate you may select the Huntington method (which is limited to dry gas) and see how much the results differ (you may be required to alter some value in order to have dry gas at inlet).
About phase equilibria at inlet condition I would presume that Peng Robinson can give reliable results.
Finally, according my experience it is quite usual to get higher temperatures (compared with rigorous procedures) with ideal gas correlation,

consider the case of methane compressed from 300K 10Bar.a to 40Bar.a

with efficiency = 1 (isentropic compression) the ideal gas model gives

cp/cv (tin,pin) = 1.3327 (Soave Redlich Kwong)
(k-1)/(k*eff) = 0.249
tout = tin*(pout/pin)^(k-1)/(k*eff) = 424K

with the rigorous model (see above) we get as tout = 409K

relative error (424-409)/(409-300) = 13%

now if we set a efficiency = 0.5 we get

(k-1)/(k*eff) = 0.499
tout = tin*(pout/pin)^(k-1)/(k*eff) = 599K

with the rigorous model (see above) we get as tout = 525.7K

relative error (599-525.7)/(525.7-300) = 33%

now if we add the contribute of phase equilibria (polytropic solution with phase equilibria model) we can observe large differences in tout-tin mainly due to the contribute of enthalpy of vaporization,

consider the case of the mixture methane 0.8 n-Butane 0.2 (molar fractions) compressed from 10Bar.a to 40Bar.a
with a polytropic efficiency = 0.75 , model Peng-Robinson

this mixture has a dew point of 287.9 at 10Bar.a then at 288K is in vapor phase
with the polytropic solution with phase equilibria starting from 288K @ 10 Bar.a we get as tout = 393K

at 272.47K 10Bar.a the mixture shows a liquid fraction = 0.1
with the polytropic solution with phase equilibria starting from 272.46K @ 10 Bar.a we get as tout = 337.5K

if we compare the values of tout-tin we get 105K in the first case (all gas) and 65.04K in the second case (with 10% liquid molar fraction at inlet condition)
a quite large difference,
in your case the liquid fraction is very small but it has certainly some influence.