3.2.16 Sewage pump

Sewage pump

Sewage pump or Non-clogging pumps are mainly used for pumping untreated sewage from private dwellings, public buildings, and industry. This type of pump is also used for various applications for pumping to a sewage plant and pumping between the various processes within the sewage plant. Due to the dominance of the particular types of pump used for sewage handling, these are presented here divided in models for dry installation and with only the pump section immersed in the fluid and wet installation. Wet installations covers submersible models where both pump and drive motor can be operated below the surface of the fluid.

Sewage handling pumps are completely dominated by centrifugal pump designs, the following types are used:

• Centrifugal pumps with through-flow impellers (channelled impellers)
• Free-flow pumps (recessed impeller)
• Pumps having semi-axial impellers and axial pumps (propeller pumps) for larger volume flow rates and lower delivery heads.

The free area, the through-flow for non-clogging pumps is normally 60-100 mm for the smaller pump sizes and 125-150 mm for the larger pumps. The obviously desirable feature that particles as large as the connection diameter should be able to pass through the pump is not usually possible because of the hydraulic problems and pump efficiency. For small flow requirements in order to obtain sufficient through flow, oversize pumps must be used.

All sewage handling pumps comprise of single sided suction impellers. This is to avoid the necessity of locating the pump shaft in the intake. Pump materials chosen are usually grey cast iron for municipal sewage and grey cast iron or acid-resistant steel for industrial effluent.

Pumps with channel impellers have long since been the most well developed and constitute a special class of their own with regard to the number of different types and the number of pumps in use. In practice most pumps have impellers with single or double channels, although larger pumps sometimes have three channels. As implied by the description “through flow impeller” the pumped fluid, after leaving the pump intake, passes through the pump impeller and out through the delivery figure 3.216a.

Pumps with through flow impellers are characterized by their high degree of efficiency over a large part of the Q-H curve. The overall efficiency at the nominal duty point for medium size pumps is about 60%. The wear resistance of the impeller is moderately good when used in combined systems and good in separate systems.

The shape and design of free-flow (recessed impeller) pumps differs from through flow pumps in that the location of the impeller and the utilization of the pump casing is different, see figure 3.216b. The pump impeller is symmetrical with open blades. The profile is low and its pulled backed location, close to the gland packing, leaves the pump casing completely or partially free. The free-flow pump can be described as a centrifugal pump having very large sealing clearance. The fluid and contaminants flow freely under the pump impeller and out through the delivery connection. The absence of sealing clearance results in a reduction in efficiency compared to through flow pumps. The overall efficiency at the nominal duty point for small and medium size pumps is around 40 percent.

The submersible pump is today of predominant importance compared to the conventional dry located pump type. The submersible pumps are usually capable of functioning in dry locations and can thus be used in low locations where there is risk of flooding, as in the case of an electricity power cut for example.

Figure 3.216c Submersible sewage handling pump suitable for both dry and submersed installation.

Submersible pump construction is recognized by the fact that the impeller is located directly onto the combined rotor and pump shaft, figure 3.616c. The drive motor, a squirrel cage induction motor, is completely sealed by axial and radial O-rings and the pumped fluid is sealed from the drive motor by double mechanical seals which run in an oil bath. The type of bearings, size of shaft and method of sealing are deciding factors in reliability and operational safety of the whole system. Deflection of the shaft due to radial forces should not exceed 0.05 mm at the shaft seal.

The shaft seal components should be assembled in a seal cartridge, which enables assembly and pressure testing of the shaft seal prior to fitting a replacement, figure 3.216d, Seal cartridge with double mechanical seals for submersible sewage handling pump.

From the point of view of pump service and repair, it is advantageous to be able to fit separate motor units of varying sizes to a number of different pump casings. Such a system makes it possible to maintain a complete community with a small stock of motor parts. Some pump systems also require facilities for converting on site from through-flow to free-flow impeller.