Jeremy Salisbury of Brammer, a supplier of maintenance repair and overhaul (MRO) products and services, examines the key criteria in the selection of centrifugal pumps
Centrifugal (dynamic) pumps are used in a range of industrial applications to transfer liquids from one point to another. They do this by converting mechanical energy from a rotating impeller into pressure energy (head) that moves liquid through the pump casing and through the system pipework, valves and process equipment. The head difference between the inlet and the outlet, or total head, produced by the pump, is proportional to the impeller speed and diameter. To obtain a higher head, the rotational speed or the impeller diameter can be increased. In order to operate reliably, with optimal energy use and maximum life span, a pump’s design characteristics must suit the intended service. Therefore, the correct selection of the pump is the first step in guaranteeing efficient and appropriate performance, reliable operation and maximum service life.
Steps to consider
The primary factors in pump selection include: definition of the technological process outline and main process parameters, such as flow, pressure and temperature; determination of the required pumping services; complete description of the fluid to be handled in each pumping operation (type of fluid, temperature, density, viscosity, vapour pressure, solids in suspension, toxicity, volatility); general layout of the plant and determination of available space in three dimensions; general arrangement and dimension of the piping according to the recommended velocities for each fluid and type of pipe; determination of elevation for suction and discharge points of vessels, relative to the centre line of the pump; preliminary calculation of friction losses and plotting of system characteristic curves; definition of the working parameters of the pump, namely capacity, head, suction and discharge pressures – taking into account any possibility of variations in pressure or temperature at different pumping conditions; determination of any possible exceptional start, stop or running conditions; determination of available NPSH (Net Positive Suction Head); preliminary selection of the pump type, design, position, driver, type of sealing, and cooling of seal and bearings – if required; and establishing the type of drive unit (electric motor, steam turbine, etc) and its main operating parameters. In the case of an electric motor, only high-efficiency motors should be used, and variable speed drives should be considered to control the process.
Guidelines for selection
1. Select the pump based on rated conditions as per the data sheet.
2. Ideally the pump should have a suction specific speed of less than 11,000.
3. The BEP should be between the rated point and the normal operating point.
4. Remember that hydraulic efficiency is high at specific speed 2000 and 3000 and is low if the speed drops below 500. Higher efficiency means less vibration and noise.
5. Do not select a pump with maximum diameter impeller. The pump should be capable of a head increase at rated conditions by installing a larger impeller.
6. The head/capacity characteristic-curve should continuously rise to shut-off.
7. Minimum continuous flow should be based on hydraulic stability, not temperature rise. Furthermore, the pump should not be operated below minimum continuous flow rate.
8. Select a driver that allows operation to the end of the curve.
9. For safe operation, NPSHA should exceed NPSHR (net positive suction head required) by more than 1m at the rated condition. As the NPSHR varies, depending on the head and flow, it is safer to select the margin at the end of the curve.
10. Consider how the pump will be driven (with reference to energy efficiency). A high-efficiency motor should always be specified, while fitting a VSD may also be advisable.
Traditionally, centrifugal pumps have been limited to low-pressure-head applications, but modern pump designs have overcome this problem unless very high pressures are required.
In order for a centrifugal pump to run correctly, without wasting energy or sustaining internal damage, it must be specified to meet the intended service requirements.