2.4 Selection of pumps for waste water

Selection of pumps for waste water (sewage)

Selection of pumps for waste water (sewage) requires more to consideration then pumps that can handle a water suspension.

The properties of the pump must be formulated in terms of the waste water to be pumped. An up-to-date waste water pump must designed to pump, if possible without trouble, waste water of the types found today and likely to be found during the next 10 years. lt is possible to grade the water to some extent. The extreme cases range from very rural areas to big city centres. Whatever the scope of a definition of waste water, the following requirements apply:

• Minimum risk of blockage (non-c’ogging ).
• High degree of operational reliability (mechanical).
• Low running costs (energy consumption and service).
• Simple replaæment in the event of breakdown.

In recent years the definition self-cleaning (non-clogging) has been used more and more seldom to describe the desired pump. The expression was used to describe a pump equipped with a through flow impeller of channel type. Previously this type of pump could be considered, with regard to the waste water of that time, to have satisfied the special requirements for which it was designed. However, the waste water produced today and still more likely, that to be produced in the future, make it increasingly difficult for pumps with through-flow impellers (channel impellers) to meet the non-clogging requirement.

The dimensions and design of the channels in relation to the type of contaminant found in the liquid are of decisive importance to the non-clogging capability of the pump. The design of the cross section area of a channel impeller varies from rectangular to circular. lt is not known for certain, whether the design of the channel has any real effect on the non-clogging properties of the impeller. A circular through flow area may be assumed to offer superior characteristics for the pumping of spherical objects than an impeller with a rectangular cross section. In the case of textile contaminants a rectangular design may be preferable. Available statistics concerning breakdowns due to the blockage of channel impellers give Iittle guidance to the assessment of different designs of impeller, figure 2.41.

Impellers for waste water pumps are usually designed with one or two channels. Apart from reasons connected with the hydraulic design, which can motivate different numbers of channels, there is also a lack of reliable assessment of the respective through flow capacities of one and two channel impellers. In general it may be said that two outlets from the impeller are better than one, provided that the accessible area of each channel is of an acceptable size. The acceptable size of a channel area is of course dependent on the size and character of the contaminantsin the waste water. Certain guide values have established themselves in purchase specifications. Offen the area is indicated as corresponding to a spherical through flow of 75 mm for small pumps and of 80 or 100 mm for larger pumps. In some cases there is a requirement that the through flow area should be as large as that of the pressure (delivery) pipe. The small pump with a through flow of 75 mm can thus be accepted for the pumping of the same water where a large pump cannot be accepted with a through flow area of less than 100 mm. The fixation of these values probably has its basis in the fact that the magnitude of the Q-H curve in relation to the motor speeds in current use does not allow scope for larger impellers.

The speed of the impeller has a decisive effect on the rate of volume flow of the through flow impeller and thereby on the size of the through flow area. With the aid of the laws of affinity, it can be demonstrated that a given pump at a certain speed will, at double the speed, double its flow, the delivery head will increase four times while the power requirement is increased eight times. For pumps with small flow and pressure the speed must be determined on the basis of the minimum permissible through-flow area. The speed greatly influences dimensioning of the through flow area of a through flow impeller. On the Other hand the speed in itself is without significance with regard to ability of the pump to pump contaminants.

Thanks to the hydraulic design of the interior passages, the volume flow through a free flow pump, see Chapter 3, is not dependent on speed. The only function of the impeller is to supply energy to the Iiquid and it can be dimensioned without regard to the through flow area. The Iow symmetrical impeller imparts negligible deflection to the rotor shaft. This results in increased service life for shaft seals and bearings. For normal waste water the free flow pump may be regarded as non-clogging.

The selection of non-clogging pumps represents a compromise between non-clogging properties and energy consumption. On the grounds of experience it may be claimed that free-flow pumps are the best choice up to capacities of about 5 kW and up to about 10 kW at remotely located pumping stations. Travelling costs for cleaning and maintenance greatly affect the optimum selection. Waste water pumps of the future will probably take the form of submersible high speed and free flow pumps with capacities in excess of 30 kW. In the case of larger pumps, through flow types will probably continue to dominate the field.

For waste water pumps the manner of installation deserves attention. Submersible pumps are easy to install and replace for maintenance. Dry located pumps are usual in the case of high capacities and generally operate at a higher degree of efficiency. Their speed can be easily regulated and they can be more reliably supervised. They are, however, more expensive to install (more costly building design, foundations etc.). Due to maintenance problems and industrial safety requirements, the current trend is turning from submersible to dry mounted pumps.