Condition Monitoring, Troubleshooting and Reliability in Rotating Machinery

9Remember to Check the Rotational Speed When Encountering Process Machinery Flow Problems

By Robert X. Perez

Rotating machinery field problems are often associated with operating speed issues. Operating at the wrong speed can result in vibration problems, pressure and flow issues, and overloading. However, the most common speed related problems that I have encountered in the field are low flow issues due to operation below the design speed.

We know that the flows generated by most types of rotating machinery are a strong function of speed. For example, the performance of centrifugal, i.e., dynamic, pumps and compressors (Figure 9.1), tend to follow the affinity laws, which are a set of mathematical relationships that predict how capacity, head, and horsepower are affected by changes in impeller diameter (D) and shaft speed (N) (see a summary in Table 9.1). We can see from studying the table that, in centrifugal machines, flow (Q) is directly proportional to speed and that head (H) is proportional to speed squared. Therefore, if operating speed falls below the design speed, then the flow will also drop below the design flow accordingly, assuming there isn’t some other system issue.

In positive displacement compressors, such as reciprocating and screw compressors, gas flow is directly proportional to the rotational speed. This is because positive displacement compressor’s displacement rate, i.e., volume per unit time, is proportional to its rotational speed. Therefore, with all ...