Parameters affecting the selection of pumps

The main parameters affecting or determining the selection of a particular type of pump are as follows:
a)  Maximum operating pressure.
b)  Maximum delivery.
c)  Type of control.
d)  Pump drive speed.
e)  Type of fluid.
f)  Pump contamination tolerance.
g)  Pump noise.
h)  Size and weight of a pump
i)  Pump efficiency.
j)  Cost.
k)  Availability and interchangeability.
l)  Maintenance and spares.

(a) Maximum operating – This is determined by the power requirement of the circuit, the particular application, availability of components, type of fluid and to some extent the environment and level of labour both using and maintaining the equipment.

In general, the higher the operating pressure, the higher the component cost and the lower the choice of components. The main advantage of higher working pressures is the reduction in fluid flow rates for a given system power, resulting in smaller pumps, smaller bore pipes and smaller components. The disadvantage is that at higher working pressures, the compressibility of the fluid used can have considerable adverse effects where precision control is required over a wide range of loads.

The general tendency is toward increased operating pressures. The operating pressures of pumps depend to some extent on the fluid used. A fire-resistant fluid is generally not as good lubricant as a mineral oil. So to give a reasonable pump life expectancy when using a fire-resistant fluid, the maximum operating pressure must be reduced and it is advisable to consult the pump manufacturer.

b) Maximum Delivery – The pump system selected must be capable of delivering the maximum flow rate demanded by the circuit. If the circuit demand is reasonably constant, a fixed displacement pump is chosen. When the demand is at a series of fixed levels, a multi-pump system is used. For demands which vary within a relatively narrow band, a variable displacement pump is used. If there is a wide variance in system demand, an accumulator circuit may best satisfy the requirements.

Pump capacities are stated by manufactures for a particular viscosity fluid at given operating temperatures and pressures. Any increase in temperature and hence a reduction in viscosity or an increase in operating pressure causes more leakage across the pump and consequently reduces the pump delivery. As the pump wears the leakage will increase. It is usual to select a pump with a capacity about 10% higher than that required to make an allowance for the reduction in volumetric efficiency with wear.

c) Type of Control – Various types of pump controls are available such as manual servo control, pressure compensated control, constant power control and constant flow control. The choice of control is dependent upon the circuit requirement such as complexity, accuracy of control, cost, type of machining operation, etc. The designer has to choose carefully the type of control after a detailed study of system characteristics.

d) Pump Drive Speed – A majority of pumps are driven directly from the prime mover – electric motor or internal combustion engine–so the proposed drive speed is known. The fluid delivery rate is proportional to the speed of rotation. Each design has a minimum and maximum operating speed: the faster the pump runs, the shorter its life.

e) Types of Fluid – Pumps are designed to operate within a particular range of fluid viscosity. Mineral oils of the correct viscosity work satisfactory with most pumps provided the oil is clean. Operating with synthetic or water-based fluids reduces the working life of a pump that relies on the hydraulic fluid to lubricate the bearings and moving parts. When any fluid other than a mineral oil is to be used, it is advisable to seek the pump manufacturer’s advice.

f) Fluid Contamination – Any fluid contamination causes pump damage. Precision pumps with very fine clearances are more susceptible to damage. If a contaminated fluid has to be pumped, such as in a clean-up loop, particular attention must be paid to pump selection. Non-precision gear pumps, lobe pumps and gerotor pumps are the most dirt tolerant. Whichever type is used, a strainer must be fitted in the suction line. In the case of precision pumps, the manufacturer’s recommendation on filtration must be followed; otherwise the life of pump will be drastically reduced and the maker’s warranty voided.

g) Pump Noise – Noise has become increasingly important environmentally. Operating levels vary considerably between the pumps of the same type but of different makes. The manufacturers are working on those aspects which most affect the emission of noise– port plate design, bearings, flow passages, pressure controls, materials and methods of mounting. Generally, the sound generated increases with speed and pressure. Certain kinds do, however, propagate lower noise levels, in particular, those with internal gears.

h) Size and Weight of a Pump – Generally, not only the overall size and weight of a hydraulic system is important in mobile installations, but also the whole system is important, as the size and weight of a pump is only part of the whole system. In a mobile hydraulic field, the trend is to reduce the weight of the hydraulic system by increasing the operating pressure, reducing the size of the reservoir and using efficient oil coolers.

The actual size and weight of a pump depend upon the particular manufacture’s design. Very light compact units have been developed for use in the aerospace industry but these tend to be extremely expensive.

i) Pump Efficiency – Reciprocating pumps tend to have higher efficiencies than rotary pumps. The actual efficiency depends on design, operating pressure, speed and fluid viscosity.

j) Cost  – The initial cost of a pump is usually of secondary importance to running and maintenance costs. Gear pumps are cheaper, vane and piston pumps are expensive.

k) Availability and Interchangeability – A number of gear pump manufacturers produce units to ISO standards so far as the external dimensions are concerned. This gives direct interchangeability between gear pumps of different manufacturers. The shafts, mounting flanges and port connections of most of the other types also comply with various international standards allowing a degree of interchangeability.

l) Maintenance and Spares – In every type of pump, the components involved in pumping worn out after a time and need replacing. In gear pumps, it is usual to replace the entire pump. With some types of vane pumps, the wear parts are grouped together as a cartridge that can easily be replaced without dismantling the pump drive. In the case of piston pumps, it may be advisable to ensure that the manufacturer offers a fast overall service for critical applications to carry a spare pump in stock.