Issues related to the speed control and cooling capacity control of booster expanders
Thread Content
Experts, what is used to control the speed of a turboexpander? As far as I know for now, there are only the expansion-end nozzle and the pressure-boosting end circulation valve; reducing the rotation speed by closing the pressure-boosting end circulation valve or opening the expansion-end nozzle should both work. Analogously, in a steam turbine, when the speed decreases, it is necessary to open the turbine throttle valve or close the air compressor vent valve. I’m not sure if this understanding is correct? If that is correct, then if there is sufficient cooling capacity, reducing the degree of expansion should allow the liquid output to be decreased. To reduce the amount of expansion, the nozzle at the expansion end should be closed; this causes the speed of the booster expander to decrease, so it is necessary to close the circulation valve at the booster end as well. The limitation at this point is the surge distance at the pressurization side; if the surge distance is not a problem, then it’s impossible to reduce the valve opening any further to zero. So, under normal circumstances, what percentage of full load is the minimum load for a turboexpander?
I really hadn’t paid attention to the design load of the expander unit before, but most current air separation systems use a bottom-column sieve plate configuration, and they are also constrained by factors such as air compressors; generally, the adjustable range is between 75% and 105%~~
If I remember correctly, the expansion machine manual does not specify what the surge amount is; it only lists two critical speed values. Therefore, we can assume that once the bypass valve is fully closed and the nozzle speed continues to drop to the first critical speed, operation is of course not allowed due to resonance. But it is conceivable that if the system reaches the surge point, the amount of expansion at that time would be less than that during resonance. Therefore, I personally believe that it is not very meaningful to discuss the minimum load in terms of the performance of the expander equipment; the amount of gas expanded is mainly influenced by the flexibility of the air separation tower. The minimum flexible operating range for such towers is generally around 75%. Thus, to determine the minimum load for the expander, it is necessary to see whether the minimum liquid production level achieved when the air separation tower operates at 75% capacity is sufficient to support production
The speed can be controlled by regulating the differential pressure before and after the expander.