# An Automatic Fault Diagnosis Method for the Reciprocating Compressor Based on HMT and ANN
## 4 Results and Discussion  
### The Test Results  
Figure 7 compares the differences of \(p\)-\(V\) diagrams of the seven faulted cases and that of the faultless case, respectively.  
From Figure 7a,b, for fault 1 and fault 2, it can be seen that, the gradient of compression curve increased, and the gradient of expansion curve declined compared to those in the faultless case. This is because the gas of high pressure in the discharge chamber leaked into the cylinder through the cracked discharge valve during the compression process, which leads to the result that the pressure increased more quickly in the compression process. Moreover, the gas of high pressure also leaked into the cylinder during the expansion process, which resulted in that more high pressure gas expanded and the pressure declined more slowly. Additionally, since the gas leaking into cylinder occupied the working volume, the flow rate of the compressor dropped. With the fixed regulating valve opening, the discharge pressure went down. Besides, the discharge curve showed fluctuation at the end of discharge process. It is due to the fact that the piston speed slowed down when the piston was close to the TDC and the discharge pressure was lower than that under the faultless condition, both of which leads to a lower gas thrust. The lower gas thrust could not overcome the spring force, so the discharge valve closed in advance, whereas the compression went on and the compressed gas pushed the discharge valve to open again soon. Therefore, the quick closing and opening of the discharge valve cause the fluctuation at the end of the discharge process. The shape change of diagram of Fault 2 is more obvious  
Figure 6: Faulted compressor components.  
compared to that of Fault 1. This is due to the fact that Fault 2 is much severer than Fault 1 and Fault 1 can develop to Fault 2 in on-site application.  
In Figure 7c, the faulty and faultless diagrams almost overlapped with each other. This is because in Fault 3 the designed degradation of the spring is slight, and the fault almost cannot be reflected in \(p\)-\(V\) diagrams. This fault is corresponding to the case that incipient defects of the spring arise. If the defects develop as much severer faults, the discharge valve will open earlier and the discharge pressure will also drop.  
For Fault 4, 5, and 6, Figure 7d-f illustrated that the gradient of compression curve declined and the gradient of expansion curve increased. The reason is that, during the compression and expansion processes, the gas of high pressure leaked out of the cylinder through the wearing piston ring or the cracked suction valve, respectively, both of which lead to a faster descent of pressure during the expansion process and a slower climb of pressure during the compression process. Therefore, when the compressor is single-acting, the impact of Fault 4 on the \(p\)-\(V\) diagram is similar with that of Fault 5 and 6.  
Figure 7g revealed that the suction pressure in the fault 7 is higher than that under faultless condition. This is because the degradation of the spring led to the earlier opening of the suction valve. Further, the gradient of the compression curve increased, which is because the higher initial pressure made the compression proceed more quickly.  
Figure 7: _Cont._