Problems with low-load operation of expansion compressors

There should be no problem at 50% load; the key issue is that the pressure at the compressor outlet might not meet the specified values. Specifically, it is necessary to check the performance curves for the expansion end and the compression end. As long as it is ensured that at this flow rate, the output power at the output side of the expander is greater than the input power of the compressor, there will be no problems. According to API 617, this should be included in the manufacturer’s documentation. Alternatively, you can also consult the manufacturer directly. If you have the drawings, you can send them over for a look. Furthermore, if you are a designer, it is recommended to work together with the process engineers; at low flow rates, the system friction decreases, which can compensate for the impact of reduced efficiency of the expander and compressor under such conditions on the product specifications.

I consulted the manufacturer, who stated that the flow rate should be no less than 600,000 cubic meters per day; operations were attempted at a flow rate of 550,000 cubic meters per day, and no abnormalities were observed in any of the operational parameters. Given the manufacturer’s advice, further attempts to reduce the gas flow rate were not made. At the outlet of the compression side, hot gas is transported outside, so there are no strict requirements regarding pressure.

In this case, consulting the manufacturer is indeed the best option. Since you have already started trial operations, in fact, by carefully examining and analyzing each start-up and shutdown process, some useful information should be obtainable. Because, every time the unit starts up or shuts down, there is definitely a gradual change in flow rate. One more thing that’s not entirely clear: is a throttle valve installed in the expansion compressor, and did the design institute carry out simulation calculations? According to the data sheets provided by the design institute, the unit we are designing is capable of operating at load levels ranging from 110% to 25%. I personally had some doubts about this, but when I discussed it with the manufacturer, they raised no objections.

There is an anti-surge valve on the compressor side, and the surge point lies within the range of 60~63% of the design flow rate. Where is the throttle valve you mentioned installed? In terms of the process, there is a J-T valve that is logically interlocked with the inlet valve at the expansion end; it’s from the skid manufacturer TDE. As for whether simulation calculations were carried out in the design phase, I’m not quite sure about that. In my opinion, it should be possible to operate the expansion compressor at low load; the cooling effect will be somewhat poor after expansion, but it will still be much better than that of a J-T valve. Based on actual observations (comparing 550,000 cubic meters per day to 700,000 cubic meters per day), the outlet pressure at the compression stage does not change much as the flow rate varies.