9.9 Pump specification

Pump specification

Pump specification performance data is a necessity for accurately select a suitable pump to the pump system. All components in the pump installation have tolerances in respect of hydraulic performance. To ensure achievement of a certain level of flow through the pump installation it is necessary to consider the hydraulic tolerances when determining pump data.

Figure 9.9a Illustration of hydraulic tolerances for a centrifugal pump

The straight line in figure 9.9a representing calculated value and the dotted lines representing tolerance limits.

Pump hydraulic tolerances are carefully set out in the pump testing standards respectively, see Section 3.10 Pump standards. The maximum permissible deviations from the guarantee data are thus defined by specifying the standard according to which acceptance testing and approval is to be carried out. Special conditions must however be agreed upon in the case of high viscosity and non-Newtonian liquids.

It is however, considerably more difficult to determine the tolerance limits of the system curve (H_syst). Examples of the tolerances which accumulate in the complete system curve are; tolerances for pipe diameter, roughness, valves, bends, flowmeters, grills, filters etc together with uncertainties in respect of the characteristics of the process liquid and the methods of calculation.

Another type of uncertainty, when specifying the pump data, is the future flow requirement. This is however, a question which should be clarified before selecting a pump and which is not primarily a matter which can be solved by pump technology.

Because of the difficulty in determining the system curve tolerance limits a general allowance is often applied when calculating the necessary pump data. Such allowances are often jokingly referred to as the “adjustment factor” or “designer’s allowance”.

Centrifugal pump specification and tolerances

The magnitude of hydraulic tolerance allowances which are normally required are:

• 5% of the calculated delivery head (H) for larger pumps, P > 30 kW
• 10% of the calculated delivery head (H) for smaller pumps, P < 30 kW
• Allowances in respect of flow (Q) should not be made
• The percentage values given apply to Newtonian liquids. The uncertainty increases in the case of non-Newtonian liquids.

The consequences of the allowances are illustrated by means of an example. For a mass-produced centrifugal pump, grade/class 1, purchased according to a catalogue curve, the tolerances for delivery head are ± 6%. For a tolerance of ± 8% for H_syst at the desired flow, the allowance will be:

√(0,06² + 0,08²) = 0.10 i.e. given as 10%

The calculation means that there is a 95% likelihood that the actual flow will be equal to the desired flow. The formula also assumes that the production tolerances of the pump and system are independent of each other and equally distributed about their respective calculated values.

The smaller allowance (5%) for larger pumps is motivated by the stricter delivery tolerance requirements and more extensive system curve calculation.

Table 3.101 Technical specifications for centrifugal pumps according to class I, II and III

Displacement pumps

For displacement pumps, which have very steep Q-H curves, the system curve tolerances have little effect on the actual flow. The pump flow tolerances are completely decisive.

Figure 9.9b Illustration of hydraulic tolerances for displacement pumps.

The desired flow will most certainly be achieved if the following rules are applied:

• The pump flow tolerances are added to the desired flow.
• Allowances in respect of pressure (delivery head) should not be made.