I was recently asked to investigate a loss of capacity problem with a drinking water pumping facility. I found two split case pumps with a shop drawing capacity of 800 GPM @ 214 feet each. Only one pump can run at a time (electrical interlock). Both pumps have a flooded suction with water service about 5 feet above the impeller, from a tank about 4 feet away. The curve indicates a required NPSH of 26 feet at 800 GPM. The compound suction gauge has always indicated a positive pressure. Tests indicated the pump (only one was operable) was only capable of reaching 85% of shut-off. I ordered a new rotating assembly. While I was there, I noticed the Operator had disabled the check valve (propped it open) in order to get some more flow from the pump. I cautioned him that a power failure would cerate havoc due to reverse flow.

Sure enough we had a failure last weekend. The pump now would not run and was torn down.

The water is from a Roaring Spring in Pennsylvania. Also using the spring is a bottled water company and a paper mill. The water is pumped from the spring to an 8,000-gallon concrete tank where chlorine is added. The pump(s) in question then transfer the water to a tank elevated about 210 feet. Almost all discharge head is static. The pump(s) ran at about 550-660 GPM about 5 hours a day. Lately, the system has gained some leaks and operating hours has gone to 10 hours per day. Typical Cent PA Municipal water system. The pumps are Aurora 413s, 4x5x10B with 8.1875-inch impellers. Chlorine is maintained at 0.3 to 0.4 mg.L. Records indicate the pumps produced about 660 GPM when new and are (were) capable of 550 when destroyed. New rotating assemblies were shipped yesterday. I have not seen the innards of the second pump. I probably can post pics of number two next week. There is and has been a flowmeter on the discharge since construction. I calculate NPSHR at the suction as being above 28 feet, however the Compound Gauge on the pump suction has never indicated a negative pressure.

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