Settle out pressure calculation

[joisy]

Guys,

I continue with calculations of my compressor unit. Can sommeone explain, please, if I do have a standard compresor unit system with inlet pressure, 1st stage compressor discharge pressure, and second stage compressore discharge pressure - which is also about the same as the discharge pressure of this system. There are also scrubbers and coolers in each stage. I know all the volumes (piping, coolers, scrubbers), and 1st inlet pressure.

How can I calculate settle out pressure - for this system, having the vendor data for the compressor (as Z factor, ratio...)? Or at least where I can refer to?

 

[TD2K]

When I've done this, I've estimated the volumes of the equipment at each pressure level, used the pressure and a typical temperature to estimate the mass (how many points you believe you need to do this calculation is a judgement call, do you want to do it before and after the intercooler or just use a estimate of the average temperature) to come up with a mass for that pressure level and at the end, a total system mass.

Since you know the volume of the system also, back calculate a pressure that gives you a density equaling the total mass/total volume from above.

Temperature's the tricky one since it will change at more locations than pressure. However, with a spreadsheet, it doesn't really take much longer to include those effects. You can also do an overall enthalphy balance to estimate the settle-out temperature for the last calculation of overall density.

 

[joisy]

OK,

I was going to do about the same, but - did you use the comperssibility factor? The difficulty is that it also depends on pressure and temperature.

thanks

 

[TD2K]

Yes, I used the compressibility factor based on the graphs in the GPSA Engineering data book for different MW gases.

I doubt you have likely more than a few pressure/temperature combinations you need to calculate the mass.

 

[PDEBoss]

Don't forget that there will be several different settle out pressure depending on how the compressor was operating just before it shut down.

Normal settle out pressure will be much lower than if the compressor discharge was blocked and the relief valve lifting. If the discharge was blocked, the suction pressure would rise too, giving you an even higher settle out pressure.

 

[joisy]

We have two PSD valves on discharge and on inlet sides of compressor unit. I've calculated it considering they've just closed simultaneously. Can drop off somewhere my calcs' sreadsheet if someone wants to look at.

cheers,
Joisy