Our recycle gas compressor is giving higher flow than it is designed for at the same loading. What can be the reasons?
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05/08/2020
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Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Your question doesn’t tell us what type of process unit this is, or what type of compressor it is. Most recycle gas compressors are reciprocating or centrifugal.
If it is a reciprocating compressor, a reasonable engineering approximation is to assume that the actual volume moved per revolution is constant. If it is driven by a fixed-speed motor, the actual flow-rate can be reduced with valve unloaders, but there is no way the actual flow can be higher than design. The measured flow usually varies, but this is because the measurement is sensitive to molecular weight, pressure and temperature. If your question relates to a fixed-speed reciprocating compressor, correct the measured flow for temperature, pressure and molecular weight and you will see a steady flow. If it is a variable speed reciprocating compressor, correcting for shaft speed is straightforward.
If the machine is a centrifugal machine, the situation is more complex. Sometimes, a flow vs pressure curve is available, but there are many assumptions hidden behind that curve that make it unhelpful for anything other than a fixed-speed, fixed-composition, fixed-pressure system. The manufacturer should have provided a polytropic head curve. Plotting the machine performance on this curve will take a bit more work, but should be within the skill-set of a competent process or mechanical engineer. It will require accurate data for the gas composition, suction & discharge temperature, suction & discharge pressure and shaft speed.
In both cases, molecular weight is a big variable. As I said at the start, I don’t know what process unit you are referring to, but recycle compressors are used in hydrotreaters, hydrocrackers and reformers - all these units involve hydrogen. Because of the large MW difference between hydrogen and hydrocarbons, small changes in composition give large changes in indicated flow. Simply looking at indicated flow gives very little information about the performance of the machine.
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22/07/2020
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Carlos Mendiondo, CHM Consult, Carloshugomendiondo@gamil.com
Check the flow control system and the site where a valve is placed altering the resistance of recycle circuit . This valve can be located in suction or in discharge. If it is located in discharge, the volumetric flow drops. But if is placed in suction, the pressure in the compressor will decrease so that with the same mass flow you will have more volumetric flow and less pressure will be dissipated in the valve. The recommendation is always to place the valve in suction.
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20/07/2020
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keith bowers, B and B Consulting, kebowers47@gmail.com
The compressor is delivering 'better than design' value which probably includes a 'contingency value'.
The actual operating conditions are 'better' than the 'design' values.
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