For a "large", "high pressure" pump, the risk of pump damage from running below minimum flow would tend to be greater than the cost of lost energy by spilling back some flow to achieve a better operating point.  I work in a large oil refinery. We almost never set up a pump such as this to trip on low flow or high discharge pressure because the upset to the process could create a dangerous situation. But, we would often program the minimum flow spill-back to open automatically to protect the pump.  We would also set an alarm on low flow to warn the operators if the pump strayed into a low flow condition.  

If you do want to have a shut-down system to protect the pump, I would use the flow rather than the pressure. Even if the flow meter has questionable accuracy, for a pump with a very flat curve, the flow would almost certainly be more accurate than the pressure in determining the onset of low flow.  

In any case, consider the economics.  If spilling back some flow costs you 100 HP you would waste less than US$40,000 per year (even if you ran that way 24/7).  But, running the pump to destruction at low flow could cost US$250,000 to repair.  These are numbers that might be typical in our plant.  You need to consider your own energy and repair costs.  

I was not aware of any API RP recommending high pressure trips. This seems very strange and I would take exception to it in most of my applications.  Self-priming pumps
  

2011-06-29

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