JBrendt
Mechanical
- Nov 18, 2006
- 1
Application is water at 85C in an open vessel (14.7 psia) with 1 ft of static suction head between pump centerline and liquid level. A centrifugal pump is currently in use. At the specified duty point, I've calculated that there is only 4 ft. NPSHA due to significant friction losses in suction pipe. NPSHR for the centrifugal is about 5 ft. In addition to the above, this pump is normally started "dry" with the vessel and suction pipe empty. After 1 minute or so, liquid enters the vessel and then travels down the suction pipe.
The problem is that sometimes the centrifugal pump does not transfer any liquid out of this particular vessel. My guess is that the centrifugal pump is becoming airbound upon startup and that there is not enough NPSH to push the air out.
The exact same pump is used successfully on other vessels with the only difference being that these vessels are elevated about 10 feet higher. I'm guessing for these vessels that the pump probably becomes airbound initially when started dry but eventually there is sufficient NPSH to push the air out?
To summarize, here are my questions:
1. Do you think the pump is becoming airbound because it is being started "dry"? Would the problem be solved just by waiting for suction to become fully primed before starting the pump? My guess is that even if the pump is fully primed it will still become airbound due to insufficient NPSHA.
2. It is being considered to replace this pump with a Liquid Ring design. I spoke to the Liquid Ring pump manufacturer and they said that their pump requires 9 ft. of NPSH - higher than the existing centrifugal. If a Liquid Ring pump is placed into service, and the software is changed to start the pump after the suction is fully primed, then how do you think the Liquid Ring pump would operate if the NPSHA is lower than NPSHR?
Thanks for reviewing this.
The problem is that sometimes the centrifugal pump does not transfer any liquid out of this particular vessel. My guess is that the centrifugal pump is becoming airbound upon startup and that there is not enough NPSH to push the air out.
The exact same pump is used successfully on other vessels with the only difference being that these vessels are elevated about 10 feet higher. I'm guessing for these vessels that the pump probably becomes airbound initially when started dry but eventually there is sufficient NPSH to push the air out?
To summarize, here are my questions:
1. Do you think the pump is becoming airbound because it is being started "dry"? Would the problem be solved just by waiting for suction to become fully primed before starting the pump? My guess is that even if the pump is fully primed it will still become airbound due to insufficient NPSHA.
2. It is being considered to replace this pump with a Liquid Ring design. I spoke to the Liquid Ring pump manufacturer and they said that their pump requires 9 ft. of NPSH - higher than the existing centrifugal. If a Liquid Ring pump is placed into service, and the software is changed to start the pump after the suction is fully primed, then how do you think the Liquid Ring pump would operate if the NPSHA is lower than NPSHR?
Thanks for reviewing this.
