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Specifying centrifugal pumps for industrial applications

Specifying centrifugal pumps for industrial applications Jeremy Salisbury of Brammer examines key criteria in centrifugal pump selection and options available for creating an energy-efficient, low maintenance, pump solution.

Centrifugal pumps move liquids by converting mechanical energy from a rotating impeller into pressure energy (head) that moves liquid through the pump casing, system pipework, valves and process equipment. The head difference between inlet and outlet - or total head - is proportional to impeller speed and diameter. To increase head, rotational speed or impeller diameter can be increased. To operate reliably, with optimal energy use and maximum life, pump design characteristics must suit the intended service. Therefore, correct pump selection is key to efficient, appropriate performance, reliable operation and maximum service life.

A centrifugal pump directs liquid into the pump's suction port and then into the impeller inlet. The rotating impeller moves the liquid along the spinning vanes, increasing its velocity energy. On leaving the impeller vanes, the liquid moves into the pump volute or diffuser casing, where its velocity is converted into pressure. The fluid is then guided into the pump's discharge port and out into the system, or onto the next stage in the case of a multi-stage centrifugal pump. Pumping applications include constant or variable flow rate requirements, serving single or networked loads, and comprising open loops (non-return of liquid delivery) or closed loops (return systems).

There are four general types of centrifugal pumps: rotary, overhung, submersible and horizontal self-priming. Rotary pumps function with close clearances, displacing a fixed volume of liquid with each revolution. Overhung pumps have cantilevered impellers from their bearing assemblies. In submersible pumps, the pump and inside driver components are completely surrounded by the pumped fluid. Horizontal self-priming pumps create a vacuum at the pump inlet, enabling the pump to 'suck' fluid into its casing. The suction nozzle can therefore be located above the liquid.

Primary considerations before selection include: defining technological process outline and main process parameters, such as flow, pressure and temperature; determining required pumping services; complete description of the fluid to be handled (type, temperature, density, viscosity, vapour pressure, solids in suspension, toxicity, volatility); general plant layout and available space; general arrangement and dimension of piping according to the recommended velocities for each fluid and pipe type; determining elevation for suction and discharge points of vessels, relative to the pump centre line; preliminary calculation of friction losses and plotting system characteristic curves; defining pump working parameters, namely capacity, head, suction and discharge pressures - considering any possibility of variations in pressure or temperature at different pumping conditions; determining any possible exceptional start, stop or running conditions; determining of available NPSH (Net Positive Suction Head); preliminary selection of 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. With an electric motor, special attention should be paid to efficiency (only high-efficiency motors should be specified) and the advisability of using a variable speed drive (VSD) to control the process.

Key selection criteria
Most pumping applications use centrifugal pumps operating within ranges of head and velocity. For high-head or high-flow applications, pumps may be used in series or in parallel. When running in series, heads are added with total capacity equal to the smallest capacity pump. In parallel, pump capacities are added and the head of all pumps will be equal at the point where discharged liquids recombine.

Pump and motor selection should ensure high-efficiency values, aiming for lowest possible energy consumption per volume of pumped fluid. The best option is to specify high-efficiency motors, which may cost more initially, but will reduce total ownership cost through lower energy consumption.

Engineers should first calculate pumping conditions and then decide which type of pump to use. Engineers should consider process requirements consider the conditions and physical properties of the liquid (flow rate, pressure, density and viscosity). The head depends on fluid density and viscosity while flow rate determines pump capacity. Required flow rate is usually determined by material and energy balances with a design margin typically up to 25%. This margin accounts for unexpected variations in properties and conditions, or to ensure the plant meets performance criteria.

Minimum flow protection is often added as continuous circulation. Before selecting a pump, examine its performance curve, indicated by its head-flow rate or operating curve. The curve shows the pump's capacity, plotted against total developed head. It also shows efficiency (percentage), required power input (in brake-horsepower) and suction head requirements over a range of flow rates. Furthermore, pump curves indicate pump size and type, operating speed and impeller size, as well as showing they show the pump's BEP (Best Efficiency Point).

Selecting efficient pumps and motors alone will not achieve cost-effective and reliable operation. Pump characteristics must properly fit system requirements throughout any process variations, ensuring operation as close as possible to the BEP for the majority of operating time. Operating a pump away from its BEP can cause adverse effects, such as cavitation. This can occur where the liquid vaporises at the eye of the pump impeller, notably where pressurised liquids are being handled. Such liquids are subjected to rapid pressure changes, causing cavity formation in the fluid's lower pressure regions. When entering high-pressure areas, these bubbles collapse on a metal surface continuously, causing cyclic stressing of the surface, resulting in surface fatigue.

Centrifugal pumps are used in more industrial applications than any other kind of pump. Primarily, this is because they offer low initial and upkeep costs. Traditionally, these 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, its design characteristics must be suitable for the intended service. Therefore, the correct selection of the pump is integral to optimising performance, operation and product life.
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