Suppose a plant desires to increase production. How does one get increased flow from an existing pumping system? This issue discusses options and considerations to make when planning to increase throughput.

 Dale B. Andrews - Editor

In last month’s issue we discussed the fact that all centrifugal pumps operate at a flow rate that corresponds to the intersection of the pump curve and the system curve. To increase the flow rate from an existing pump either the pump operating characteristics or the system resistance characteristics need to change. If other modifications to the pumping system are being considered such as new piping, heat exchangers, etc., a new system curve needs to be developed as a baseline reference before considering any of the following modifications.

Options for increased flow from an existing pump

• Increase the impeller diameter
• Increase the pump speed
• Modify the impeller blades
• Install a different impeller
• Install a suction inlet splitter vane
• Decrease the system resistance

Impeller diameter/speed change

In systems where the primary resistance comes from a pressure vessel, any increase in flow will be primarily dependent on the shape of the pump characteristic curve. Flatter curves will experience a higher percentage of flow increase than steep curves. The pump operating point relative to the pump best efficiency point may change. The only way to predict the performance is to plot the intersection of the pump and system curves.

Impeller blade modifications

Installation of a different impeller

The available head from an impeller is largely a function of peripheral speed which is set by diameter and RPM. The available flow from an impeller operating at a constant peripheral speed is controlled by the blade angles at the impeller inlet, and the volume of the impeller vane passages. Many pump manufacturers offer multiple impeller designs to fit within a single pump body that, along with interchangeable diffusers or volute liners, offer a significant change in flow characteristic for a given TDH.

Installation of a suction splitter vane

Decrease system resistance

Upgrading valves, replacing heat exchangers, or modifying piping or other system components to lower the resistance head at any given flow will result on a pump that will operate at an increased flow. The pump will also have a reduced horsepower requirement as compared to the previous methods that increase pump TDH to achieve the desired increase in flow.

Other considerations

There are a number of associated mechanical and hydraulic issues that should also be evaluated when investigating modifications to increase a pump’s flow rate. It is a good idea to consult the OEM’s service department to assist in evaluating the pump requirements. Some of the items are as follows:

NPSHR - It is likely that the NPSHR will increase if either flow or pump RPM is increased.

Seal flush requirements – The pressure at the seal chamber will increase in proportion to the pump discharge pressure. Make sure that there is adequate seal flush pressure to give the required flow rate through the seal.

Power – Increasing flow rate will result in increased power draw unless it is accompanied by an adequate decrease in system head requirements. When changing speed or impeller diameter, power will vary approximately as the cube of the ratio of the change. Couplings, base plate dimensions, and electrical components will all require reevaluation.

Mechanical – Planned changes to the operating speed should be evaluated with respect to rotor critical speed, pressure containment, and mechanical seal limitations.

Operating Limits – Changes that move the pump further away from the best efficiency point may decrease pump reliability due to hydraulic instability.

Copyright 2007 Lawrence Pumps Inc. 371 Market Street
Lawrence, Massachusetts U.S.A. 01843, Tel:800-434-5248, Fax:978-975-4291