How do determine amount of condensation in piping from relative humidity/pressure/flow rate etc... ? 1

[USAeng]

Background - total beginner on this topic - It has been over 10 years since I looked at enthalpy/entropy/saturation etc in school... honestly I cant remember almost any of it. Hopefully someone can guide me here

We have biogas at our plant that has a flow rate of 600scfm through an exposed steel pipe at about 100F , 100%relative humidity and .75inWC. We take it through a gas conditioning skid with a compressor and a chiller that boosts the pressure to 5psi and heats and chills then reheats the gas to remove moisture - their design says the expected relative humidity of the gas leaving their skid should be approx. 25% at 80F. After these units the piping is all exposed and we are getting considerable condensation in the piping before the engines that burn the gas. How can we determine how much water 25% relative humidity corresponds to if the temperature is 80F leaving the unit and then the temps later in the piping is 10-20F? It doesn't have to be super exact. Are there tables we can look at or some way of estimating?

Also, Is there anything we may want to check or test to see if their conditioning skid is functioning as required?

Thanks

 

[zdas04]

I've attached a version of the McKenna chart that I generated from ASTM data (it gets the same answers as the GPSA Figure 21-3, but I find the colors to be easier to read).

At slightly below atmospheric pressure and 100°F I get a value around 3600 lbm/MMSCF.

At 5 psig and 20 psia, 100% relative humidity is around 1100 lbm/MMSCF (so you should have dropped out 2500 lbm/MMSCF by this point). 25% RH is 275 lbm/MMSCF (a dew point of around 40°F at 21.75 psia). Call it 33 gallons/MMSCF. You are doing 0.8 MMSCF/day so if the system works perfectly you will still have about 28 gallons/day in the process after water removal.

When you drop the temp to 10°F at 5 psig, the 100% relative humidity point drops to around 70 lbm/MMSCF and you will get something like 19 gallons/day of condensation.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[USAeng]

NICE. Thanks David