Centrifugal Pumps

Hello Norman Lieberman. We have a big flashed crude oil heater charge pump. It pumps flashed crude from the flash drum. When we start-up this pump (sometimes we bypass the flash drum and pump), the pump cavitates. Meaning, it’s hard to get the pump to take suction at first. I throttle way back on the discharge valve. I raise the level in the flash drum to maximum. Still the pump cavitates when we first start-up. Eventually we do get it running. Then it runs find, if we have not damaged the pump seal during all the start-up tries. Any ideas?

---Henry, Los Angeles

Henry – Your problem is lack of “Starting NPSH.” When a pump is started, the liquid in the suction line has to be accelerated from stagnant conditions to several feet per second. That energy required comes from the available NPSH needed to satisfy the pump’s steady state NPSH requirement. Hence the observed cavitation. To provide the starting NPSH, quickly raise the flash drum pressure. This will instantly increase the pressure at the pump’s suction. The vapor pressure of the liquid will also increase by the same amount. But it will take time for the more volatile liquid to flow down into the pump suction. During this short period, you will have a surge of available NPSH.

---Norm