Parallel Pump Operation and Cavitation
Parallel Pump Operation and Cavitation
(OP)
We have three boiler feed pumps operating in parallel and suspect cavitation in the #1 & #3 Pumps. The pumps supply water to a boiler, and 90% of the time two pumps must operate to satisfied the steam demand. The original pumps were installed in 1970, and the #2 pump was replace last year due to failure. Since that time we've experience what we suspect as cavitation, #1 Pump failed from outboard bearing demise and the #3 Pump failed from the 1st stage impeller, #1 of six, being excessivley eroded to the point holes existed. We suspect cavitation has been present for quite some time because operational records indicate similar problems. Do you feel this is a correct assumption? The recirc line was removed years ago, and as of this writting, there is no ability to balance flow by recirculation, which is what we'd like to achieve as we feel this is the resolution to our problem. I am new at this, so please understand when I ask how is this accomplished. I would think installation of a recirc line would be somewhat simple, but have been insuccessful in finding any plans or what instrumention and equipment one should use to correct this type problem. I would appreciate any help one could provide, and will be looking forward to whatever responses I receive. Thank you.





RE: Parallel Pump Operation and Cavitation
RE: Parallel Pump Operation and Cavitation
RE: Parallel Pump Operation and Cavitation
Other than use of a recirculation line to avoid overheating at startup, there can be substantial energy usage penalties resulting from a recirculation flow. It is likely that the added energy costs can significantly exceed the pump damage and repair costs due to operation without a recirculation system. Your careful study of your system and possible options may lead to unexpected indications for actions to mitigate the problem.
You may want to consider improving the suction piping system to increase NPSHa. The suction and discharge conditions that the pumps actually experience are what really matter. Parallel pump operation does not necessarily imply greater risks of damaging conditions. Careful troubleshooting may disclose that damaging conditions are present in your system even when only one pump is operating.
RE: Parallel Pump Operation and Cavitation
for reading: http://www.engineeringpage.com/info/pump/pumpinfo.html
then use the links to the Pump Conditions and Centrifugal Pump calculator found at http://www.engineeringpage.com/nav/pump.html
RE: Parallel Pump Operation and Cavitation
RE: Parallel Pump Operation and Cavitation
RE: Parallel Pump Operation and Cavitation
For positive (flooded) suction:
NPSH = ha-hvpa+hst-hfs
where:
ha=ft of liquid absolute pressure on the surface of the liquid supply tank at suction (this will be barometric if from opent tank)
hvpa=head in ft of liquid corresponding to the vapor pressure of the liquid at the temperature being pumped
hst=static ht in ft the liquid supply level is above the pump impeller eye.
hfs=all suction line loss if ft of liquid, including entrance losses & friction losses through pipe, valves & fittings.