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Centrifugal Compression Train with Lower MW Gas

Centrifugal Compression Train with Lower MW Gas

(OP)
We are running a centrifugal gas compressor train (x 3 compressors in series).

The compressors were designed for gas MW of 23. Due to a change in gas composition, we may need to periodically operate the compression train on gas with MW 17.5.

We have confirmed that the compressors can provide the required head based on the compressor curve (with flows in excess of surge line).

I’m not sure if the low MW can affect any other aspects of the compressor operation e.g. dry gas seals. Are there any other checks that we should be doing based on the lower gas MW before operating with the new gas?

Thanks.

RE: Centrifugal Compression Train with Lower MW Gas

Discharge temp on each stage will be higher with lower mol wt - check TSHH setpoint / margin to design temp for affected kit.

If you running at constant mmscfd for both cases, then heat rejection rate at interstage and final coolers may not be affected, since increase in feed temp to each cooler would be offset by lower mass rate - to be checked.

The operating point on each stage may be much closer to the surge line at design speed - check with antisurge controller supplier if they are comfortable working with the closer approach.

RE: Centrifugal Compression Train with Lower MW Gas

The compressor will also be running at higher speed to develop the higher head - check if you are approaching the high speed alarm limit at the required mmmscfd.

RE: Centrifugal Compression Train with Lower MW Gas

- Check new seal gas phase envelop vs. risk of condensation in dry gas seal
- Check gas composition versus current material selection (presence of CO2, H2S or chlorides etc.)
- Rotating speed shall be within 100% speed
- Thrust bearings might be sized considering limit points on map which you may be now operating on continuously, this to be verified.
- Check the seal gas system vs. new operating pressures - head being equal (flow to the seals, orifice sizing, utility consumptions, external seal gas etc.)
Other questions:
- How have you ensured the head-flow point is still in the operating envelop considering new MW with map shape having changed ? have you received revised maps including pressure vs. flow maps from OEM or have you approximated this yourself ? in the old map you may be fine (not surging) but with the map fitted for new MW you may be running at different distance from surge - I don't remember from top of my head if MW going down would play in your favor or not.
- What happens to the alternative cases - if this is applicable, is everything within the performance envelop or are you running one set of process conditions only ?
Best advice as usual is: contact the OEM and have the machine qualified for this new gas.

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