Issues related to flow rate regulation of centrifugal compressors
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Process gas centrifugal compressors have various control methods such as outlet throttling, inlet throttling, vane control, speed control, and bypass control. There is one issue I haven’t been able to understand: regarding the flow rate regulation range of 40%–100%, does this mean that the flow rate coming from the system is within 40%–100%, and the compressor adjusts itself to match this range, or rather, the flow rate entering the compressor is 100%, and the compressor then regulates it to 40%–100%.
The amount of flow that reaches here should be controlled by the conditions in the upstream section; if the flow rate in that upstream section is insufficient, then operating the load at a higher level will cause surge. At a normal flow rate, the flow can be controlled through a return valve.
I’m referring to the operational capacity; it should be at least 40%, as anything lower won’t work. As mentioned above, this can lead to surge phenomena. The amount of gas in the system depends on factors such as the composition of the raw materials and the degree of processing involved. It’s not correct to think that the system receives 100% of the required amount of gas, which is then adjusted to between 40% and 100% by the compressor
It should be that the system presents changing operating conditions, and the compressor adjusts itself through valve control or changes in speed in order to adapt to these changing conditions of the system – is that what it means? However, by reading various papers and books, it seems that through the adjustment of the inlet or outlet valve of the compressor, 100% of the operating conditions can be changed to other conditions via valve regulation.
Let’s ignore the issue of surge for now, and assume the range is 80–100% to avoid the surge point.
Is the design flow rate the flow rate of the process system?
It should be that: whether through export regulation, import regulation, vane regulation, speed regulation, bypass regulation, or changes in the actual system, the flow rate through the impeller (converted to inlet pressure) should remain within the range of 40% to 100% of the rated flow rate. What is called the adjustment range is actually the operating range. Of course, the scope of work is achieved through the aforementioned adjustment measures. It’s purely my personal understanding.
The value passing through the impeller is 40–100%; so should the value at the inlet throttle valve be 100% or 40–100%?
All the gas that passes through the inlet throttle valve enters the impeller; therefore, the flow rate through the inlet throttle valve is of course 40–100%.
Under normal operating conditions, it can be adjusted using a return valve.