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Relationship of Flow Rate and Pressure for Positive Displacement and Centrifugal Pumps
Flow and pressure do not work the same way on a centrifugal pump as they do on a positive-displacement pump. This article uses Wanner Hydra-Cell Pro® sealless pumps and Stan-Cor centrifugal pumps as examples.
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Pumps Create Flow, Not Pressure
A pressure gauge reads resistance to flow. A centrifugal pump and a positive-displacement pump react to that resistance differently.
Centrifugal pumps operated at fixed shaft speed, flow and discharge pressure are tied together. Higher pressure reduces the flow rate due to the limit of the velocity the impeller can put on the liquid, plus the losses in the gap between impeller and volute.
A positive-displacement pump moves a fixed volume per shaft turn. Check valves in the pump head set flow direction, so pressure does not affect flow much. The effect of pressure on flow rate is about 1% for each 100% rise in pressure.
Comparing Performance Curves
On the semi-open impeller curve, about 34 feet of total head (almost 15 PSI) on a 6″ impeller is roughly 50 GPM. Add pipe and fittings and flow falls because that extra head has to come from somewhere. Raise head about 14% to 40 feet and flow drops about 40%, to roughly 30 GPM.
On the positive-displacement curve at about 740 RPM, flow is about 50 GPM at 200 PSI and about 48.6 GPM at 700 PSI — a 3% drop for a 350% rise in pressure.
So a centrifugal pumps flow follows the head it is pumping against. A positive-displacement pump’s flow follows shaft speed.
Troubleshooting Lack of Pressure
Gauge pressure is a function of flow. “No pressure” is usually “not enough flow.” Wiring and aerated liquid still matter. Common system causes for both pump types:
- Inlet: NPSHa too low, including a dirty filter
- Pump: worn parts or debris in the wet end
- Outlet: a worn bypass or regulating valve leaking flow off the process
Troubleshooting Wanner Hydra-Cell Pro® Pumps
A Hydra-Cell Pro® is a sealless positive-displacement pump. A hydraulically balanced diaphragm does the displacement. There is no packing or plunger seal, which is why the design holds up on diffult fluids. The same simple wet end makes troubleshooting simpler.
New Hydra-Cell Pro® Pumps
If flow or pressure is low on a new pump, the system is the first place to look. Every Hydra-Cell Pro is tested at full flow and pressure before it leaves the factory — not a sample lot.
Most low-flowing pumps are related to the inlet condition. There has to be enough liquid for the desired flow rate. These pumps will lift and they will self-prime, but if NPSHa is below NPSHr the pump starves and cavitates. You are trying to move more liquid than is there. Liquid flashes, the pump runs rough, and if you leave it that way the check valves take a beating.
Slow the shaft RPM. If the pump quiets down and pressure is steady, NPSHa is too low for the design flow rate.
A pump that starts smoothly and then becomes rough can be a dirty filter, air from a large bypass flow, or falling tank level cutting NPSHa.
A system that ran, then sat for months, can have check valves stuck from dried residue. City-water pressure on the inlet will often free them up.
On the discharge of a new system, set the regulating valve to its maximum level. If pressure is still low, is liquid going out the bypass? Pressure needs restriction. Open bypasses, leaks and oversized or worn nozzles all reduce that restriction.
Existing Hydra-Cell Pro Pumps
A pump that used to run well is easier to trouble-shoot. Rough running is usually worn check valves. At 1,750 RPM those valves seat about 29 times a second. The valve-to-seat face wears. Seal efficiency decrease and so does the flow rate. Valves are easy to change and offered in a variety of materials for chemistry and abrasion compatibility.
O-rings and diaphragms are not typical wear parts. You can estimate check-valve life but you cannot estimate elastomer wear the same way. Chemistry and temperature are usually responsible for elastomer failure. This is why many customers change elastomers when they change valves. The head is already open and the elastomers do not increase the cost of the kit by very much.
Troubleshooting Stan-Cor Centrifugal Pumps
Low NPSHa, air in the liquid, and clogs in the pipe or the pump reduce flow on a centrifugal pump the same way they do on a PD pump. A few extra points apply.
Stan-Cor pumps, like most centrifugal pumps, do not self-prime. Fill the casing and the suction line before you start, after that the vacuum at the eye will pull the liquid through the inlet. An inline check valve or foot valve on the inlet will maintain prime even with the pump isn't running.
There are no check valves setting direction. Clearance between impeller and volute is what makes discharge velocity. A worn or plugged impeller or volute will reduce flow drastically.
Selecting the Right Pump
Centrifugal pumps work best with water-like viscosities and higher flow rates at lower pressures. Positive-displacement pumps are what you use when pressure is high or the liquid is difficult to handle.