by Hydraulic Institute

Q. What are the effects of viscosity on rotary pump and system performance?

A. The viscosity of a pumped fluid typically affects pump ratings in the following ways:

  • The net positive inlet pressure required (NPIPR) increases with increasing viscosity (see Figure 1).
  • The required pump input power (Pp) increases with increasing viscosity (see Figure 2).
  • The maximum allowable pump speed (n) decreases with increasing viscosity (see Figure 3).
NPIPR increases with viscosityFigure 1. NPIPR increases with viscosity. (Graphics courtesy of HI)
Required Pp increases with increasing viscosityFigure 2. The required Pp increases with increasing viscosity.
Pump slip decreases with increasing viscosityFigure 3. Pump slip decreases with increasing viscosity.

Care must be taken when applying these generalities to non-Newtonian fluids because shear may change the viscosity within the pump. When the apparent viscosity of a non-Newtonian fluid can be determined, these generalities can be applied.

Because the exact relationship between viscosity and pump ratings depends on the pump design and on application conditions, the pump manufacturer’s published data for each pump should be consulted. The manufacturer should also be consulted when considering viscous fluid pump applications.

Energy put into a fluid to overcome the resistance to shear causes a finite temperature rise of the fluid. Manufacturers should be consulted for recommendations on rotary pump applications involving fluids that are shear- or temperature-sensitive.

For more information on rotary pumps, see ANSI/HI 3.1-3.5, Rotary Pumps for Nomenclature, Definitions, Application and Operation.

Q. How can I reduce the level of extraneous noise when measuring sound levels throughout a pumping system?
A. Separation of the different sound sources may, at worst, not even be possible. At best, special measurement techniques may be required. Therefore, consideration must be given to reducing the level of extraneous noise. The following precautions during the test setup should help accomplish this:

  • Valves—Select low-noise type valves. Use two or more throttl­ing valves in series to reduce the differential pressure across the valve. Locate valves as far away from the pump as possible. Avoid putting them between the microphone and the pump. Cover noisy valves with an acoustical barrier.
  • Piping—Use pipe and fittings that are sized 1:1. Avoid situations that cause a change in velocity. Use straight runs of pipe from the pump to the supply tank. Cover noisy piping with an acoustical barrier material.
    Gears—Avoid high-ratio reducers and increasers. Cover test gears with an acoustical enclosure or barrier material.
  • Test foundations/bases—Use rigid foundations and bases to support the pump. Avoid using large, flat, thin material that vibrates and radiates noise.

For more information on similar topics, see ANSI/HI 9.1-9.5, Pumps – General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination.

Q. How can I monitor the power consumed by a rotodynamic pump?
A. The power consumed by a pump can be monitored in several ways. Some of these monitoring instrumentation/systems are outlined below: