Minimum Continuous Flow
There are three factors which must be looked at with respect to minimum flow. These are:
- Mechanical ability - this is the ability of the pump to withstand all of the forces (side thrust, axial thrust, vibration, etc. ) associated with operating the pump at low flows.
- Hydraulic stability - this is the stability of the pump to operate at flow rates other than the Best Efficiency Point (or nearby), particularly low flow rates as a percentage of the BEP, without detrimental internal recirculation, hydraulic instabilities, etc. that can cause pump damage.
- Thermal stability - has to with the properties of the particular liquid being pumped and the effects of heat (temperature) rise to liquid. See HEAT INPUT above for a discussion of temperature rise. A commonly accepted conservative temperature rise through a pump is 15 deg. F. For a 15 deg. F rise, the minimum flow can be calculated as follows:
|Where Qmm||=||minimum flow rate for 15 deg. F temp rise. (GPM)|
|BHP||=||the power input at the minimum flow. (BHP)|
|2.95||=||constant, for 15 deg. F rise (HP-Lb-Min-deg. F/BTU-Gal.)|
|Note: for 20 deg. F rise use 3.93 instead of 2.95.|
|Note: for 10 deg. F rise use 1.96 instead of 2.95.|
|Note: for 5 deg. F rise use 1.00 instead of 2.95.|
|Cp||=||specific heat, (BTU/lb-deg. F).|
|SpGr||=||Specific Gravity of the pumped fluid.|
It is important to understand that a catastrophic failure of any pump can occur if the liquid within the pump casing is allowed to vaporize. To prevent flashing, a flow must be maintained through the pump which will keep the liquid below its saturation temperature.