How to Read a Positive Displacement Pump Curve
In this 101 blog, we’ll show you how to read a positive displacement (PD) pump curve so you can base your pump selection on the specific needs of your application. A PD pump produces the same flow at a given speed (in revolutions per minute--RPM) no matter what the discharge pressure. Positive displacement pump curves give you the information you need to determine a pump's ability to produce flow under the conditions that affect pump performance.
PD pumps come in a variety of mechanical designs, to name a few:
- Circumferential Piston Pumps
- Rotary Lobe Pumps
- Twin-Screw Pumps
- Progressive Cavity Pumps
Alfa Laval Optilobe Series PD Pump
A pump curve answers several important questions during the pump selection process:
- What flow rate is the pump capable of?
- How much does slip affect the pump’s ability to perform?
- How much HP is required for the anticipated pressure?
Curves answer those questions by displaying intersections of several important variables, including capacity, work horsepower, viscous horsepower, and Net Positive Suction Head required (NPSHr).
Capacity
Capacity, as illustrated in Fig. 1, is the volume of fluid a pump can displace by RPM.
As RPM increases, the pump flow increases, from 0 gallons per minute or (GPM) at 0 RPM, to about 130 GPM at 500 RPM.
Fig. 1. A PD pump curve indicates pump capacity, on the horizontal lines, in units per minute. In this example, the curve indicates gallons per minute (GPM) and liters per minute (LPM) in the left margin and the vertical lines indicates pump speed in revolutions per minute (RPM).
The importance of viscosity in pump selection
Positive displacement pumps deliver a constant flow of fluid at a given pump speed. When viscosity increases, however, resistance to flow increases, so to maintain system flow at higher viscosities, pumps require more horsepower.
Low viscosity also affects pump performance in the form of slip. Slip is the internal recirculation of low viscosity 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 influenced by the fluid’s viscosity and the discharge pressure.
As discharge pressure increases, keeping viscosity constant, more fluid slips from the discharge side to the suction side of the pump, so the pump must spin at a higher RPM to maintain output.
In Fig. 2, a positive displacement pump curve shows the influence of viscosity on slip with a correction chart. With changes in viscosity and pressure, slip correction indicates that flow capacity drops from a high of about 7 GPM to a low of about 3.5 GPM. Once viscosity is over 1000 cPs, slip basically doesn’t occur in liquid sanitary pumps. If slip is not a factor, use the 0 PSI line to determine flow rate.
Because PD pumps generate flow to transport relatively high viscosity fluids, PD pump selection requires analysis of three key influences on fluid transfer:
The fluid’s dynamic viscosity, density, and response to shear.
Fig. 2. Slip correction accounts for variations in pump performance while factoring fluid viscosity (resistance to flow) and discharge pressure.
Dynamic viscosity
Dynamic viscosity is a measure of a fluid’s resistance to flow. By common sense alone, we can imagine that water is less viscous, or resistant to flow, than corn syrup, so corn syrup has a higher viscosity than water. We measure internal resistance to flow as absolute viscosity (also referred to as dynamic viscosity). It is critical for the viscosity used to be consistent with “in pump” shear conditions, or shear rates of 800 or more s-1 (inverse seconds).As the following comparison shows, differences in viscosity vary dramatically by fluid:
- At room temperature, the absolute viscosity of water is about 1 centipoise (cps)
- At room temperature, the absolute viscosity of corn syrup is about 5,000 centipoise (cps)
Density
Density is a measure of a fluid’s weight by volume. Water is less dense than corn syrup, for example, so if you put equal volumes of water and corn syrup side by side, the corn syrup would weigh more than the water. Also, due to the differences in density between water and corn syrup, water would float on top of the corn syrup if combined. The following comparison shows the difference in density between water and corn syrup in kilograms per cubic meter:
- Density of water: 1 g/cm³ or 997 kg/m³
- Density of corn syrup: 1.38 g/cm³ or 1380 kg/m³
Shear
Shear-sensitive liquids change viscosity when under stress, such as when they are hit by an impeller inside a pump. Some liquids become less viscous with increased force (called shear thinning), while others become more viscous with increased force (called shear thickening).
By comparison, Newtonian liquids, such as water, do not change their viscosity, regardless of shear.
The viscosity of shear-sensitive substances through a process line does change, however. Common shear-sensitive substances include ketchup, shampoos, and polymers; as shear increases during ketchup processing, ketchup’s viscosity decreases.
Continuing with the ketchup processing example, the next section discusses additional important information on pump curves: work horsepower (WHP), viscous horsepower (VHP), and Net Positive Suction Head required (NPSHr).
Brake horsepower
When you size a PD pump it will be important to select the correct brake horsepower. Brake horsepower (BHP) is the power the pump requires to overcome the discharge pressure. BHP is determined by adding the work horsepower (WHP) and the viscous (VHP) horsepower.
BHP = WHP + VHP
To properly analyze brake horsepower, you must look at work horsepower versus viscous horsepower.
Work horsepower
Work horsepower (WHP) is the horsepower required for the selected PD pump to achieve the desired flow rate considering the anticipated pressure drop from system components. Components like valves, heat exchangers, and filter/strainers, to name a few. WHP is sometimes called external horsepower.
To determine WHP find the intersection of anticipated differential pressure (PSI) and RPM, as shown in Fig. 3. Recall the required RPM was a result of flow required coupled with slip correction, if any.
Fig. 3. Work horsepower (WHP), is the horsepower required to operate a Positive Displacement Pump. As pressure from the discharge side of the pump increases, the pump requires additional horsepower to operate. For example, at 300 RPM and with 150 PSI, the pump requires 6.7 working horsepower.
Viscous horsepower
Maintaining pump capacity at various viscosities requires meeting horsepower minimums, as shown in Fig. 4. There is a certain minimum horsepower requirement to force the rotating parts of the pump to turn, considering the viscosity of the fluid in the pump. VHP is sometimes called internal horsepower.
To arrive at required horsepower for an application, add WHP and VHP.
- WHP = 6.7
- VHP = 4
- Required HP is 6.7 + 4 = 10.7
Fig. 4. Viscous Horsepower (VHP) is the power needed to turn rotating parts of the pump against the fluid inside the pump. At 300 RPM and a viscosity of 500 CPS, a pump requires 4 VHP.
Net Positive Suction Head required
Finally, curves show NPSHr, which stands for Net Positive Suction Head Required. NPSHr is the minimum amount of pressure required on the suction side of the pump to avoid cavitation. Cavitation is the formation of vapor filled cavities, or bubble, which can rapidly expand and collapse causing damage to the pump and/or process piping. NPSHr is determined by the pump.
Net Positive Suction Head Available (NPSHa), is determined by the process piping. You always want NPSHa to be greater than NPSHr. AS Fig. 5. Illustrates, the lower the speed of a PD pump, the lower the NPSHr.
Fig. 5. Without enough net positive suction, the pump will cavitate, which affects performance and pump life. NPSHr varies as a function of speed (RPM).
What is Head?
Head is defined as the height to which a pump can raise water straight up. While raising water straight up may not always be useful, based on the layout of process piping, water creates resistance at predictable rates, so we can calculate head as the differential pressure that a pump has to overcome in order to displace the water. Common units are feet of head and pounds per square inch. As Figure 6 illustrates, every 2.31 feet of head equals 1 PSI of pressure.
The formula for PSI: Feet of head / 2.31 = PSI
What is total dynamic head?
While pump curves help you select the right pump for the job, you first have to know the total dynamic head for the application.
Total Dynamic Head (TDH) is the amount of head or PSI of pressure on the suction side of the pump (also called static lift), plus the total of 1) height that a fluid is to be pumped plus 2) friction loss caused by internal pipe roughness or corrosion.
TDH = Static Height + Static Lift + Friction Loss
- Static Lift is the height the water will rise before arriving at the suction side of the pump.
- Static Height is the maximum height reached by the pipe on the discharge side of the pump).
- Friction Loss (or Head Loss) are the losses due to friction in the pipe at a given flow rate.
NEXT STEPS
As a processor, you need a pump that transfers product safely and efficiently from point A to point B. But with such a large variety of pumps, motors, and applications, picking the right pump can be difficult.
That's where we come in!
CSI is known as the experts in the specification, sizing, and supplying of pumping technology for hygienic industry processes. Speak with our knowledgeable pump team today and be confident in your next pump purchase!
ABOUT CSI
Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.
A Guide to Choosing the Right Pump for Hygienic Applications
This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications.