Factors to determine minimum allowable flow rate in centrifugal pumps

BIRMINGHAM — In Water Technology’s May article, “Centrifugal pumps and their piping for water applications: Part two,” Amin Almasi, contributing author, writes about centrifugal pump performance, site installation and operation.

This is part two of Water Technology’s April feature, “Centrifugal pumps and their piping for water applications: Part one.” In April, Almasi wrote about pump design and components, performance and rotordynamics. You can find Part one of this feature here.

Part two examines how a pump operating lower or higher than the best efficiency point (BEP) will have poor performance, lower efficiency, increased shaft deflection, seal problem and higher bearing loads.

“Particular attention is required for low flow rates,” notes Almasi. “Most centrifugal pumps should not be used at a flow rate less than a certain level (say as a very rough indication 50 percent of the flow rate at BEP) without a recirculation line.”

He continues by offering factors to determine the minimum allowable flow rate:

• Temperature rise of water: This is often established as 46.4° F or 50° F; if a pump operates at very low flow (say around shut-off), it could overheat.
• Radial hydraulic thrust on impellers: This is most serious with single volute pumps, and even at flow rates as high as 50 percent of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage.
• Flow recirculation in the pump impeller: This can also occur below the minimum flow (say 50 percent of BEP) causing noise, vibration, cavitation and mechanical damage. Recirculation can occur at both impeller inlet and outlet.
• Total head characteristic curve: Some pump curves droop toward shut-off and other curves show a dip in the curve. Operation in such regions should be avoided.

Read the entire May feature on centrifugal pump performance here.