Discussion on ammonia compressor issues
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The compression section of the refrigeration system in my ammonia synthesis plant is equipped with ammonia synthesis compressors of the steam turbine type. There is a very tricky problem at present: the plant is designed to produce 300,000 tons of ammonia per year, yet the current daily production is less than 500 tons. Two ammonia coolers rely on the ammonia from the refrigeration system, and the low-temperature methanol washing unit also uses ammonia from this system. The turbine design of the ammonia synthesis compressors requires 70 tons of steam per day, but the actual steam consumption is 86 tons. Yet the load on the refrigeration system is far from reaching its full capacity. Our analysis shows that each stage of the compressor has a minimum required flow rate of process gas; this flow rate must be met to prevent surging. Therefore, it is necessary to return the hot ammonia from the output of the third stage back to the second and first stages in order to ensure that the minimum flow rates for each stage are satisfied. As a result, a large portion of the work done by the refrigeration machine is wasted. This creates a vicious cycle. We invite manufacturers facing similar problems or experts who understand this issue to join the discussion.
Find a compressor manufacturer to create a curve showing the mechanical properties of the compressor, adjust the anti-surge curve, close the return valve to improve the efficiency of the unit; however, careful planning and proper preventive measures are necessary to avoid damaging the unit.
1. The efficiency of centrifugal compressors does decrease significantly at low loads. 2. The steam consumption at low loads exceeds the value specified for the designed operating conditions; there is likely an issue with the operation. 3. Hot ammonia from the third stage returns to the second stage outlet, and hot ammonia from the second stage outlet returns to the first stage outlet..................... This results in lower energy consumption, as it is not possible for the ammonia to flow directly from the third stage back to the first stage. 4. To determine the anti-surge flow rate at different pressures, it may be useful to try reducing the inlet valve opening. If the anti-surge valve is already open, reducing the speed has little effect on energy savings.
1. First, reduce the compressor speed to the lowest controllable speed； 2. Generally, the minimum flow rate specified for anti-surge valves in design is much higher than the actual surge flow rate of the compressor; the minimum flow setting for the anti-surge valve can be adjusted by referring to the design curves and actual experimental data ； 3. In any case, the steam consumption at low load levels should not exceed the designed steam consumption; such issues usually indicate a problem with the turbine or that the actual load on the unit is higher than the designed load.
I analyze that the issue in your factory is related to modification, specifically changing the three-stage ammonia cooling in the synthesis circuit to two-stage ammonia cooling. The large amount of gaseous ammonia in the third stage causes an imbalance in the load across the various stages of the compressor. Take the following measures: 1. Close the connection valve between the ammonia tank and the refrigeration system; use an emergency chiller to ensure the safety of the ammonia tank and increase ammonia production. 2. Adjust the load on the ammonia cooler in the synthesis circuit to match it as closely as possible. 3. Only increase the load on the chiller balance system in the methanol washing unit. It seems that the original poster works for a large company; the load on the chiller is analyzed, so it can’t be a problem with the compressor, and its parameters certainly cannot be changed lightly.