A solid grasp of pump sizing allows engineers to make effective economic and practical decisions about process pumps. This one-page reference provides information about two key parameters and other considerations for pump sizing.
Pump sizing steps
Sizing a pump requires engineers to estimate the temperature, density, viscosity and vapor pressure of the fluid being pumped. Pump sizing can be accomplished in six general steps:
- Find the total dynamic head (TDH), which is a function of the four key parameters of a pumping system, shown in Figure 1.
- Correct for the viscosity of the fluid, since pump charts and data are given for water with a viscosity of 1 cP. Viscosity of other process fluids can differ dramatically.
- Calculate the net positive suction head (NPSH) to select a pump that will not undergo cavitation.
- Check the value of suction-specific speed to see if a commercial pump is readily available.
- Check for potentially suitable pumps using a composite performance curve and an individual pump performance curve.
- Compare the energy consumption and lifecycle cost of operating the selected pumps.
Total dynamic head
A key parameter in characterizing a pump is the total dynamic head (TDH), which is the difference between the dynamic pressure of the discharge side (after the pump) and the suction side (before the pump). Dynamic pressure represents the energy required to do the following: (1) to raise the liquid level from the suction tank to the discharge tank; (2) to provide liquid velocity inside both suction and discharge piping; (3) to overcome frictional losses in both suction and discharge piping; and (4) to pump the liquid against the pressure difference between the suction and discharge tanks.
To find TDH, the difference between the discharge velocity head (hD) and the suction velocity head (hs) needs to be calculated.
TDH depends on the elevation difference between the discharge and suction tanks. In Equations (2) and (3), P is the pressure of the suction or discharge side, converted to units of length using the specific gravity of the fluid as in Equation (4). The TDH represents the difference between Equations (2) and (3), in which users actually add together the velocity head and the frictional head loss for both the suction and discharge sides of the pump.
Pumps must overcome the frictional losses of the fluid in order for the fluid to flow in the suction and discharge lines. Frictional losses depend on pipe roughness, as well as valves, fittings, pipe contractions, enlargements, pipe length, flowrate and liquid viscosity. To calculate the frictional head losses, in feet of liquid being pumped, on the suction (hs,f) and discharge (hd,f) side of the pump, Equation (5) can be used. The same equation can be applied to calculate the frictional losses of the discharge side, but with the appropriate values correlating to the discharge side of the pump.
Net positive suction head
NPSH is the net pressure available for pump suction after all deductions, such as line losses and vapor pressure, are taken into account. It is the pressure available in excess over the vapor pressure to prevent the pumping fluid from boiling. The aim with NPSH is to provide an adequate amount of head that exceeds the fluid’s vapor pressure to prevent the fluid from boiling at the pump inlet. This excess head is defined as NPSH.
The NPSH value is used in the determination of whether the liquid on the suction side of the selected pump will vaporize at the pumping temperature, thus causing cavitation and rendering the pump inoperable. NPSH varies with impeller speed and flowrate. The following data are required for NPSH calculations.
Site atmospheric pressure. NPSH calculations are impacted by the site’s local atmospheric pressure. This value is used in NPSH calculations, and the higher the atmospheric pressure, the better, with regard to NPSH.
Suction piping layout. The physical layout of the suction piping is important in determining NPSH. This must include the exact number of pipe fittings in order to properly determine the suction-piping pressure drop.
Vapor pressure of the pumping fluid. Vapor pressure depends on operating temperature. Vapor pressure for pure substances can be found in literature, such as “Perry’s Chemical Engineers’ Handbook.” To determine the vapor pressure, the operating temperature must be provided.
Suction vessel elevation and operating pressure. The elevation of the suction vessel itself is also important. Additionally, the operating pressure of the suction vessel must be known. n
Editor’s note: This column is adapted from the following articles: Sarver, J., Finkenauer, B., and Liu, Y.A., Pump Sizing and Selection Made Easy, Chem. Eng., January 2018, pp. 34–43; and Raza, A., Calculate NPSH with Confidence, Chem. Eng., September 2015, pp. 46–51.
Additional information on nomenclature can be found at: https://www.chemengonline.com/pump-sizing-selection-made-easy/ and https://www.chemengonline.com/addendum-to-pump-sizing-and-selection-made-easy/