Pumping Tips Low Flow

Pump Selection and Low Flow Operation

1. Introduction

The pump is not an all purpose piece of equipment. In order to provide reliable efficient operation it must be operated correctly and preferably as close as possible to the best efficiency point.

2. The Ideal Operating Point

Ideally we'd like to operate our pumps at the Best Efficiency Point (the point where efficiency is at its highest). At this point the flow through the impeller is smoothest and losses are at their minimum. Away from this point the efficiency reduces. It reduces because the flow no longer travels through the pump in a smooth fashion. The flow becomes turbulent and liquid recirculates within the pump.

Operation at the best efficiency point will ensure the most economical and most reliable operation.

3. Why our Pumps Rarely Operate at BEP

In practice we rarely get a pump that operates exactly at Best Efficiency Point.

Pump selection is usually a compromise between what is commercially available and the constraints of the system.

The reasons that your pumps are rarely operating at Best Efficiency Point (BEP) can be summarised as follows:

Pumps are selected from a manufacturer's standard product range. They are not designed for your application. A perfect fit is therefore not always available
Your required flowrate may be very low. Impeller design limitations may dictate that all available pumps would be operating close to minimum flow (specific speed limitations)
Low NPSHA may limit the available range of pumps
There may be a variety of operating conditions that the pump has to meet
There may be cost constraints which limit the available pumps that can be considered.

4. Operation Away From BEP

When operating away from BEP the fluid flow is not as smooth. There may be liquid recirculating in and out of the impeller (recirculation). This can lead to increased vibration levels, unacceptable fluid temperature rise and damage to the impeller. Pumps are also designed so that the radial and axial loads are at a minimum when operating at BEP. Operation at low flowrates can cause an unacceptable increase in radial loads.

Note these phenomenon are more severe with high power per stage pumps and pumps with lower shaft stiffness. Generally they are not a problem on small, low energy pumps.

The problems encountered when operating off design are summarised as follows:

discharge recirculation
suction recirculation
increased radial and axial loads
fluid temperature rise

These problems will cause the following:

increased vibration
poor mechanical seal life
poor bearing life
reduced impeller life
early pump failure

5. Practical Pump Selection

In practice we try to select pumps operating between approximately 50% of BEP and 110% of BEP. In the majority of cases this will mean we are operating in a position on the curve where the pump will not experience these problems. (Note this is a “Rule of Thumb” and will not be applicable in 100% of cases).

Pump selection is discussed in more detail in our training courses. Return to Top of Page