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Centrifugal pumps are designed to move a volume of water while creating differential pressure from the pump suction to the discharge. The pump has a flow range designated on paper from no flow to the maximum flow.

 

However, the curve does not indicate that the pump can or should be used at any condition on the curve.

 

Centrifugal pumps all have a “Best Efficiency Point” (BEP). Recommended operating ranges for the pumps vary from size to size, but most recommend pumping no less than fifty percent of the flow at BEP. So, here is what to look for under low flow or no flow conditions.

 

Low Flow

Conditions below the recommended margin of BEP become an area of low flow. These conditions can cause issues with the pump and pre-mature failure. One of the issues with low flow is recirculation within the volute or case of the pump. This results in recirculation of the fluid due to higher pressure, causing water to move at high velocity and wearing on the pump components.

 

In addition to that, because the pressure differential at the discharge is much higher than near the inlet side of the volute, the shaft will be unbalanced and try to deflect. This can stress the shaft as well as cause seal and bearing damage due to the deflection. Cavitation can occur within the pump causing a turbulent discharge.  

 

No Flow

Even more damaging than a continuous low flow condition is a no flow or dead headed condition. The flow, from the lack of fluid to the suction, causes the pump to run dry. It then has no lubrication, and heat builds within thermoplastic components, bearings and seals causing eventual damage to all components. Pumps should have a no flow protective device installed.

 

Another no flow condition is dead heading or pumping against a closed system. This can happen when a valve is accidentally closed or when a pump is pumping against a head equal to or exceeding the shutoff pressure of the pump. This condition also causes dramatic temperature rises as the entire fluid is recirculating in contact with all components.

 

Additionally, all the conditions of low flow are present. So, temperatures can easily reach boiling points for melting plastic components or elastomers in seals, resulting in bearing failures. Water well pumps that try to pump against a frozen pipe, for instance, will sometimes see so much heat the PVC pipe on the discharge will melt even while submerged in well water. The heat is quite significant.

 

Pump Selection

Ultimately, good design and pump selection based on the pumping system will aid in avoiding the low flow and no flow conditions. When selecting the pump, work with your sales rep or the manufacturer to confirm the pump you select will fit your needs.

 

Centrifugal pumps are designed for a long-life span when properly chosen for the job they are intended for and regularly maintained.

 

Webtrol’ s customer service team can help you find the BEP and determine the right Webtrol pump for your specific application.