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
- 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:
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.
- Too low an operating head or pressure.