2.1 Pump selection

Pump selection

Pump selection for a given liquid transport requirement or circulation of liquid presupposes that the volume rate of flow, delivery head and properties of the Iiquid are known. The first in choosing a pump consists of analysing the conditions with respect to variations. Thus the following information must be collected:

• Desired volume rate of flow, minimum, mean and maximum. Annual period of operation. The variation of the rate of flow in time should also preferably be known, e.g. in the form of a consistancy (duration) diagram.
• Required delivery head, minimum, mean and maximum.
• Properties of the liquid during normal operation, starting and special circumstances, e.g. cleaning of the system or pressure testing. Varying Iiquid characteristics can cause drastic variations in delivery haed as a result of the different viscosities of the liquid when hot and cold.

Flow and head variations

Variations in volume rate of flow determine whether the flow should be shared by several pumps. In pump selection the system curve (variations in delivery head) determines whether the division should between units of equal or different sizes. Great variations in the viscosity of the Iiquid may mean that, for example, a centrifugal pump should be used altematively with a displacement pump in cases of Iow and high viscosity.

Variations in volume rate of flow and delivery head, determine regulation (control) of the pump has to be used. In connection to this issue, the working time of the pump is of considerable interest, as is the magnitude or degree of efficiency at the mean duty point of operation. In this respect, the maximum duty point ( = order data) are of minor interest as peak loads seldom occure. Also, the prefered type of pump control is often the one giving the most efficient operation from an energy point of view.

The energy costs for pumping can be as great as the purchase price of the pump, after only a few months of operation.

The site where the pump is installed also has a marked influence on the choice of pump type. The location determines the suction capacity requirements, NPSH and evacuation, the environment at the installation site may also raise special demands with regard to the driving device for the pump, etc.

The location will also give at hand if a dry mount, wet or submersible pump is to be used as well as requirements for space can lead to a choice between horizontal or vertical designs.

The viscosity of the Iiquid as well as the size and content of particles are of course, important factors; more “personal” properties of the Iiquid such as toxicity, tendency to explode, aggressiveness and gas content also influence choice. The rapid choice tables in this chapter provides a summary of the properties of the different types of pumps in these respects.

The pressure class (rating) of the pump, constitutes a safety requirement, but many others must be taken into consideration.

Operational reliability, time required for repairs and maintenance costs are factors, but difficult to assess unless experience is available with respect to similar pumps. Costs arising from loss of production in large-scale plants of today are so high, that only after only a few hours they more than outweigh the difference in price for more reliable pump.

When equipping plants, an effort should be made as far as possible to restrict the choice to as to a few types and sizes of pumps, even though a somewhat cheaper type of pump would now and again prove adequate. The additional costs pay for themselves by virtue of reduced maintenance and spare part stock costs.

For choice of pump the auxiliary systems necessary for the pump should also be taken into consideration. The pump itself must often be furnished with protection against dry running, against over-heating (when running against a closed valve) and in the case of displacement pumps against unintentional throttling in the system.

Shaft seals require in many cases sealing or cooling Iiquids at a correct pressure and with a guaranteed flow. Adequate space and lifting equipment for hoisting and dismantling must also be available.

Pump selection with respect to hydraulic performance

Pump selection for low viscosity liquids for suitable ranges of application and volume flow rates (Q) and head (H) are indicated based on experience for different types of pumps with low to medium specific speed, e.g. (turbine) centrifugal pumps in figure 2.11 and for displacement pumps in figure 2.12.

Figure 2.11 Diagram for choice of centrifugal pumps for liquids with low viscosity based on pump performance. The curve with broken lines indicates the range of application for liquid ring and peripheral pumps.

Figure 2.12 The approximate range of application for displacement pumps for low viscosity liquids. In many cases it is necessary for the liquid to have certain lubrication value, see futher Chapter 3 about displacement pumps.

Performance requirements, purchasing cost, running costs, estimated service life and reliability of operation must be weighed up when choosing pumps. Centrifugal pumps of various designs cover nearly 80% of all pump requirements. Exceptions are cases demanding very high increases in pressure, volume flows of less than approx. 2 m³/h and the transport of liquids having certain “personal” properties. The limits for turbine pumps pumping viscous liquids are shown in figure 2.13 in the next section and some views on the choice of pumps for contaminated liquids are expressed in the ensuing sections.

The choice between different types of pumps is comparatively simple. It is only in very special cases that entirely different types of pump need to be considered. The system for choosing pumps is more or less as follows:

• In many special applications there are special designs which almost without exception are the most suitable. Some examples of these are heating, water and sanitation pumps and foodstuff pumps.
• In certain borderline cases for centrifugal pumps it is possible to choose between single-stage and multi-stage pumps. An analysis of the degree of efficiency and running costs on one hand and of operational reliability on the other will provide sufficient grounds for a decision in such cases.
• The speed of the pump is a factor which is often discussed. In principle the choice should be in favour of a speed sufficiently high to ensure that the available NPSH in the plant is utilized. Possible fears of a reduction in reliability in the case of higher speeds should not be allowed to result in the choice of a lower speed, but rather in better specified quality requirements when purchasing. Requirements of this kind can refer to deflections of the shaft or the service life of ball bearings, for example. It should, however, be observed that a high speed electric motor often emits a higher noise level than a low speed motor.
• In the border zones between centrifugal and displacement pumps the form of the pump characteristics and method of regulation often influence the choice. Both types of pump have their advantages (see further Chapter 3 with regard to different pumps designs).
• A difficult problem for all pumps is if there is presence of air or gas in the liquid. Centrifugal pumps suffer from a reduction of efficiency when the gas content on the suction side is in excess of about 1 percent of the volume, but some special turbine pumps can deal with up to 5%-10%. Displacement pumps can cope with much larger contents. The only limit is if the displacement pump can run entirely dry or with sporadic filling.