Minimum Flow

MINIMUM FLOW M018

Sometimes called low flow it is a condition that can cause excessive heat inside the pump volute. A temperature rise of 10°C (18°F) across the operating pump is considered excessive. High heat is often caused by:

  • Excessive clearance between the open impeller and volute or back plate depending upon how the impeller is adjusted. This clearance will cause the fluid to recirculate inside the pump.
  • Excessive wear-ring clearance if you are using closed impellers.
  • Throttling a pump discharge valve because the pump was originally oversized as a result of safety factors that were introduced during the selection process.
  • Too much impeller to cut-water clearance

Throttling the pump and operating at a low capacity could lead to several problems:

  • An increase in the products temperature that could cause the fluid to:
    • Flash or vaporize
    • Crystallize
    • Build a film. Hot oil does this.
    • Increase the corrosion rate of the fluid.
    • Some fluids could become more viscous and in some cases solidify.
  • Create excessive radial thrust of the impeller.
  • Initiate suction recirculation.
  • Initiate discharge recirculation.
  • Increase the NPSH required.

A catastrophic failure of a centrifugal pump can occur if the liquid within the pump casing is allowed to vaporize. To prevent flashing due to overheating of the fluid, a flow must be maintained through the pump to keep the liquid below saturation temperature.

A commonly accepted practice limits temperature rise through a pump to 15° F. For most installations, this is adequate, and minimum flow may be calculated with this equation:

Where:

Q = Minimums flow rate gpm
Pp = input power at the minimum flow, HP
2.95 = constant
Cp = specific heat, BTU/lb °F
S = specific gravity

  • At minimum flow the power input is approximately the same as at shut-off.
  • Catastrophic failure of the pump and associated equipment may result if the liquid within the pump casing is allowed to vaporize.
  • To prevent vaporization or flashing, a flow must be maintained through the pump that will keep the liquid below its saturation temperature.
  • Minimum flow can be guaranteed by installing a bypass from the discharge line to some low-pressure point in the system.
  • The bypass should not lead directly back to the pump suction because you will heat the incoming fluid.
  • An orifice installed in the bypass line breaks down the differential pressure between the pump discharge and the low-pressure point in the system.
  • The bypass may be manually or automatically operated, but must be open during periods of light load, or when starting or stopping the pump

Operating at too high a flow can cause problems also. The problems would include:

  • Shaft deflection as you operate off the best efficiency point (BEP)
  • The net positive suction head required (NPSHR) increase with capacity. You could have a cavitation problem.
  • Erosion damage.
  • Vibration.
  • Too low an operating head or pressure.

See: Heat generated within the pump

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Posted

  • On February 16, 2018