Knowing how to read a pump curve is essential to understanding your equipment and being able to troubleshoot issues as they arise. In a previous post, we discussed how to properly read a centrifugal pump curve. Here, we will discuss how to read and understand a positive displacement pump curve.
Before we learn how to read the curves, we have to first understand the how a positive displacement pump operates. A positive displacement pump has an expanding fluid cavity on the suction side of the pump, and a shrinking fluid cavity on the discharge side of the pump. The pumped fluid flows into the cavity as the suction side expands and flows out as the discharge cavity shrinks. Over a complete cycle of the pump, this volume remains constant. The image below shows how this operation happens.
Positive displacement pumps cover a wide range of pumps, including:
- Gear Pumps
- Rotary Lobe Pumps
- Progressive Cavity Pumps
- Piston Pumps
- Diaphragms Pumps
- Screw Pumps
- Vane Pumps
- Peristaltic Pumps
These pumps provide a constant flow of fluid at a given pump speed, regardless of the discharge pressure required. A pressure relief needs to be used with these pumps to protect both the pump and the process in which it is installed. If a relief is not provided, the pump will continue to develop pressure until the point that the pump, or the weakest point in the process, is damaged to relieve the pressure. This can be very costly and also very dangerous to anyone working around these pumps.
Enough about the pumps, on to the curves!
The majority of positive displacement pump curves will look like the curve shown below:
- PUMP INFORMATION
Provides information about which pump this curve refers to (ie. Pump model, size, etc.).
- PUMP SPEED
As discussed earlier, positive displacement pumps are dependent on the speed that the pump is operating at. Please note that this speed would be the pump speed, not the motor speed. Many positive displacement pumps use gear reducers, v-belt drives, hydraulic motors, or variable speed drives to control the operating speed of the pump. The pump speed information also states the maximum allowable speed for the pump, since several positive displacement pumps can’t run at full motor speeds.
The flow is dependent on the pump speed, so once that information is known, you can determine the flow the pump is providing. In this example, if you are operating at 300 RPM and working against a discharge pressure of 25 PSI, the flow from this pump would be approximately 450 GPM.
All pumps are affected by the viscosity (resistance to flow) of the fluid that they will be pumping, and positive displacement pumps are no different. However, a positive displacement pump has the ability to handle more viscous fluids than most centrifugal pumps. Viscosity can have a large impact on the size and speed that a positive displacement pump can operate. Higher viscosities can limit the allowable speed the pump can run at, and in some cases may push you to a larger pump to accommodate the flow at those speeds. It also has an effect on the next two items discussed, pump slip and pump required horsepower.
- PUMP SLIP
You will notice that this curve has two speed vs. flow lines at different discharge pressures. A positive displacement pump has a phenomenon known as “slip”, which is the recirculation of the pumped fluid from the discharge side of the pump back to the suction side of the pump. The amount of slip in a PD pump is affected by the system discharge pressure requirement and the fluid viscosity. As the discharge pressure increases, it will force more fluid from the discharge to the suction side of the pump. And as the fluid viscosity increases, the amount of slip will decrease due to the fact it is more difficult for a high viscosity fluid to slip back through the clearance in a pump than a thin fluid. Most pump manufacturers will provide multiple curves to correct for this slip.
- PUMP HORSEPOWER
The amount of power needed for the pump to operate is dependent on the discharge pressure required and the viscosity of the fluid pumped. The above curve only shows differences in horsepower because the viscosity is held constant. In the curve below for a rotary lobe pump, multiple viscosities are shown to determine the pump horsepower.
Positive displacement pumps can handle many difficult applications. Understanding how to read a positive displacement pump curve can help troubleshoot a currently installed system, or help to size equipment for the next project down the road.
Need further assistance with your pump curves? Ask us about it! We're happy to provide technical assistance to businesses in Wisconsin and Upper Michigan.