There are so many parameters to consider. It is difficult to make general criteria without being too conservative in some cases and not conservative enough in others. We use the criteria that I posted above for existing installations. But, if we are building a new system, we use even tighter criteria (more in line with the Best Practice line in the figure you provided). For most new applications, we require that the pump system be designed to operate the pump between 85% and 105% of BEP. For any existing single pump running alone, we would not normally allow operation below minimum stable continuous flow for more than a few minutes. For pumps operating in parallel, we would strictly require that they meet the minimum requirement in my posting above. And for new applications, we hold them to the highest standard. This inconsistency may seem strange. But, when building a new system, we can insist on the highest standard without adding too much additional cost. Once the system is built, it is difficult to justify the cost to upgrade to this higher standard. But, our experience with pumps running in parallel tells us that we must increase the minimum flow requirement or we are very likely to have trouble.
And, on top of all of this, we have to consider the service. A failure of a pump in 500 °F Naphtha is much more of a concern than a failure of a cooling water pump. So, for non-hazardous services, we are more likely to compromise. And, some things are just impossible to predict. We have pumps in our products blending system that are regularly operated as low as 10% of BEP, but are among the most reliable pumps in the refinery. And we have others that will fail quickly if they are operated below 80% of BEP. So, we set criteria and try to minimize the exceptions we grant. And, then we use Root Cause Failure Analysis and vibration analysis to identify applications where the criteria are not stringent enough.
Johnny Pellin