Range of pump operation 

When sizing and selecting centrifugal pumps for a given application, the pump efficiency 

at design should be taken into consideration. The pump characteristics indicate the point 

of BEP and its variation with respect to the flow rate Q. 

This is an important factor; however, it is almost improbable that the pump operation in 

actual service would be at the BEP. Thus, if the above is not possible in most of the cases, 

the selection of the pump has to be made in a manner so that the range of operation is 

near the BEP. 

This range of operation is important to define to avoid excessive hydraulic thrust, 

temperature rise, and erosion and separation cavitation. 

These phenomena occur when the operation of a centrifugal pump is to the furthest 

left or right of the Q–H curves. Performance in these areas induces premature bearing 

and mechanical seal failures due to shaft deflection, and an increase in temperature of 

the process fluid in the pump casing causing seizure of close tolerance parts and 

cavitation. 

5.13.1 Pump operation to the left of BEP 

In a centrifugal pump when operating toward shut-off or the far left on the Q–H curve, a 

percentage of the process fluid recirculates in the eye of the impeller and between the 

impeller shroud and back-plate. Evidence of minimum flow problems is more dramatic 

on applications where NPSH-a exceeds the NPSH-r by a given pump two-, three- and 

fourfold. 

This liquid that churns in the casing keeps absorbing the ‘inefficiency’ of the pump. 

Efficiency is a factor of useful conversion of mechanical energy into liquid head and 

flow. However, the inefficient part of the power goes into heating of the liquid. 

Heating up of the liquid can lead to potential problems such as vapor formation, 

expansion of internals leading to seizure, crossing the operating temperature limits of the 

material of construction, or any other. 

To avoid thermal problems during low flow operation and prevent a potentially 

hazardous temperature rise within the pump, the temperature rise at shut-off and theminimum flow required for thermal protection must be calculated and the required 

volume of the fluid to be bypassed to dissipate this heat established. 

Prior to calculating the minimum flow required for a given application, the maximum 

allowable temperature rise must be established. This defines the temperature that exceeds 

the corresponding saturation temperature of the impeller eye. 

While most maximum allowable temperature increases are based on the temperature 

where flashing; vaporizing, of the process fluid occurs, it is important to realize other 

pump components may dictate a lower temperature to insure long trouble-free service. 

For example, while the maximum allowable temperature to avoid cavitation maybe 

210 o

F, the upper temperature limitations for a polypropylene pump maybe 180 o

F. 

Other pump components that may require consideration will include mechanical seals, 

packing and bearings, and wear ring tolerances. 

When handling non-Newtonian, viscous, and shear sensitive fluids, where temperature may 

affect product integrity, the maximum allowable temperature introduced by the pump at the 

design capacity should be considered. The temperature rise at various parts on a centrifugal 

pump head capacity curve should be identified and the required bypass volume established. 

During the centrifugal pump selection process, consideration must also be given to 

minimum flow for mechanical protection. With cool clean liquids, the required minimum 

flow for thermal protection maybe minimal. 

However, excessive shaft deflection due to unbalanced radial loads, vibration and 

rotating element instability will result, should the mechanical minimum flow 

requirements not be met. These scenarios become more evident as suction pressures 

increase further beyond the NPSH-r by the pump. 

The rate of temperature rise when a pump operate with fully closed discharge valve can 

be calculated from: 

This equation neglects the heat loss through the pump case so its result is conservative. 

The volume of the casing can be geometrically estimated. 

Based on the above rate of temperature rise, the duration of safe operation at shut-off 

can then be calculated by dividing the allowable temperature rise with the rate of 

temperature rise. 

For safe operation the allowable temperature rise should be limited to 50 °F maximum. A 

lower value of allowable temperature rise should be considered if the 50° limit would result 

in exceeding the case design temperature, or to an increase in vapor pressure that could 

result in critical NPSH or to potentially damaging vaporization at mechanical seal face. As 

in the case of boiler feed water applications, this rise in temperature is limited to 15 °F. 

At any other point of operation other than the shut-off conditions, the following 

equation can be used to estimate the temperature rise of a liquid being