In February's "Letters" section (p. 22), a reader questioned a quote provided by Bill Marscher in "Avoid the Fatal Five," which appeared in Chemical Processing's January 2003 issue. Mr. Marscher responds:
It is always best to operate pumps near their BEP ," that was a key point of my comments! It is impossible, for a variety of reasons (some of them nontechnical), for all pumps to operate at BEP. For example, purchasing will not spend the extra cost for a VFD; the nature of a process (e.g., de-coking) requires wide variations in flow; one pump in parallel operation is deteriorating faster than its sister, etc. Therefore, the question arises: How far off BEP can I run?
Until pump manufacturers began quoting minimum continuous flow, users had to answer that question themselves with a guess. Based on my own experiences in helping solve problems, I know that sometimes the user's guess was as low as 5 percent of BEP. This had dire reliability consequences, as the reader implies, since the truth in those cases [was] typically in the range of 25 percent to 50 percent of BEP. That being said, the great majority of pumps have a range of flow ," at flows above the onset of suction or discharge internal flow recirculation ," over which they will operate with little (not zero!) impact on reliability.
Through the concept of minimum continuous flow, pump manufacturers simply have tried to tell users what that range is. However, no one, particularly the pump manufacturers or me, would encourage a user to operate away from BEP if he has a choice.
Bill Marscher, P.E.
Mechanical Solutions Inc.
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