For workshops with multiple sets of air separation devices, several centrifugal compressors were often used to work in parallel in the past. Some air separation devices also adopt the way of two compressors supplying air to one air separation tower in parallel.
When the two compressors are in parallel, regardless of whether the performance of the two compressors is the same or not, the pressure at the intersection of the outlet tubes of the two compressors should be equal. That is to say, if the performance curves of two (different) compressors are respectively shown in curve I and II in Figure 106, when working in parallel, the two machines should work under the condition of equal discharge pressure, and the total gas flow should be the sum of the two gas sets. Therefore, the total performance curve after parallel connection is the curve III obtained by the superposition of two machine flows under the same outlet pressure.
If the performance curve of the pipe network is constant, as shown in curve IV in the figure. When the compressor supplies air alone, the working points should be points B and C respectively, and the corresponding flow rates should be QB and Qc. When working in parallel, the total working point should be A, and the corresponding flow is QA. At this time, the working points of the two compressors are no longer B and C, but A1 and A2 respectively, and the corresponding flow rates are QA1 and QA2 respectively. In addition, QA = QAl QA2.
The characteristics of two centrifugal compressors running in parallel are:
1) the air volume provided by each compressor in parallel is smaller than that provided by them when they work alone, namely: QA12) the outlet pressure after parallel connection is higher than that when working alone.
The above features remind the operator that: if two different compressors work in parallel, if the characteristic curve of pipe network becomes steep, move to the position shown in dotted line V in Figure 106, at this time, although compressor I can still work normally, but compressor I has entered the surge boundary. That is to say, if the pipe network characteristic curve is shown as the dotted line in Figure 106, neither of the two compressors will surge when they work alone, but once they are connected in parallel, the compressor II will surge immediately. This kind of accident once happened in some sites, and it was not known by people for a long time, and even felt strange. This is the reason.
In addition, when two different compressors work in parallel, special attention should be paid to whether the outlet check valve of each parallel compressor is normal. If one of the compressor outlet check valve does not work, then when the compressor stops alone, a large amount of high pressure gas in the main gas pipeline will flow back to the compressor, making it reverse, which will cause serious accidents.
