Compressor for Helium and Gas Leak Tests of Compressor Casing Based on API 617

Only recip compressors are suitable for small volume flows at up to 400barg.
The helium compressor can be kept running while the leak test is being done at some intermediate pressure - during this time, process controls can temporarily lower the discharge pressure of the helium compressor so that all gas compressed is recycled back to suction. Check if a suction gas cooler is required to cater for 100% recycle on a suitable process simulator - helium, like H2, has zero dipole moment, so it should be a well behaved gas, unlike CO2, even up to 400barg. However check that the simulator predicts inverse J-T effect correctly (for very high pressure letdown) for the JT letdown across the recycle valve.
The discharge pressure controller of the helium compressor can be set at local manually to suit any ramp up sequence.

Thanks George for your response…
But there are another questions:
-Do you mean the compressor should be operated in whole time of the test and just to be full recycled at intermediate constant pressures steps within ramp-up plan?
-Can this He compressor be also applied for Gas Leak Test by N2?
-Client wants having constant pressure increasing rate of 5 bar/min during the tests, can we achieve this constant rate while the discharge pressure are increasing at the casing under the test? and if so, how can we achieve?
-Do you know/have about a sample data sheet or spec for such compressor/test stand? if so, appreciated if you can share it…

N2 is much heavier than helium, so for a fixed sm3/hr rate, discharge temp will be much lower than with helium. Power demand may most likely remain about the same.
Achieving 5 bar / min ramp up rate in the test machine will require info on the volume of the largest of these test machines. The larger the test machine, the slower will be the ramp up rate, so ask the client for info on the largest of these machines to be tested.
These questions should be posed to a senior process or mechanical engineer within your org who should also know how to set up process controls. I dont have equipment DS for such a helium / N2 supply compressor kit. Else reach out to some local engineering company and get them to help you.

Largest volume of the casing to be tested is 400 Litres…
Should the recip compressor in such test stand be equipped with VFD system or having the constant pressure increasing rate (5 bar/min) during pressurizing steps and having no flow during no pressurization steps (in ramp-up diagram) can be handled by partial recycling and full recycling, respectively?

For a small He / N2 compressor, a fixed speed machine will do. And yes, ramp up profile can be achieved by varying recycle capacity, which is normally done through manual set point changes at discharge pressure controller.

Thanks a lot George…
I think with a fixed suction pressure (hence a fixed density), a fixed discharge volume (a compressor casing subject to test) and a fixed strokes per minute (fixed RPM) we would have a fixed mass flow rate hence a constant timely increase in discharge pressure (corresponding to the RPM based on which we can adjust the RPM with manual stroke adjustment for a required mass flow rate hence pressure increase rate); and we just need to have fully recycle the compressor output to have a constant pressure at intermediate stages of ramp-up diagram.
For another compressor casing (new discharge volume) we just need the RPM adjustment by manual stroke button for pressure increase rate based on new discharge volume, and yes by discharge pressure controller (set manually as you said) an automatic on/off valve at discharge can be closed at the set pressure to fully recycle the compressor output through a recirculating line along which an on/off valve will be opened at the same time of discharge on/off valve closure in order to have constant discharge pressure at intermediate stages of the ramp-up.
Please correct me if I’m wrong…

If you already have a compressor with manual stroke adjustment, then yes, you can use it to save some power when pressurisation rate allows. Otherwise, with fixed stroke, required pressurisation rate can be obtained merely by gradual set point adjustment.

"fully recycle the compressor output through a recirculating line along which an on/off valve will be opened at the same time of discharge on/off valve closure in order to have constant discharge pressure at intermediate stages of the ramp-up."

Obviously, the discharge on/off valve, during this part of the sequence, should be closed only after the recycle valve is fully open.