Miscellaneous Design Considerations

In addition to the matters discussed previously, there are certain other items that should be given consideration in the design of pumping installations.

Piping and Valves Suction and discharge piping should normally be sized so the maximum velocities do not exceed 5 and 8 ft/s (1.5 and 2.4 m/s), respectively. Higher velocities, however, may be justified by economic analysis for particular installations. Lines less than 4 in (102 mm) in diameter should not be used for raw sewage. Preferably, sludge lines should be at least 6 in (152 mm) in diameter; 4-in (102-mm) lines are sometimes used for dilute biological sludge.

Valves should be installed as required on the suction and discharge sides of each pump to allow removal and maintenance of individual pumping units without disturbing the function of the remainder of the installation. It is customary to use either ball or plug valves on raw sewage and concentrated sludge applications. Either plug or butterfly valves can be used for settled sewage or for dilute sludge.

Piping should be designed with sufficient flexibility to avoid stress on the pump flanges. Flange-coupling adapters are sometimes used for this purpose on both the suction and discharge sides of the pump.

Surge Control Careful attention should be given to surge control wherever a pump discharges into a force main of appreciable length. Generally this is a problem only in the design of raw sewage pumping stations located within the collection system. Changes in fluid motion caused by starting or stopping of pumps or by power failure can create surge conditions.

Surges caused by normal starting and stopping of pumps driven by electric motors may be controlled (1) by selecting individual pump capacities such that the change in velocity in the system when a single pump starts or stops will not result in excessive surges, (2) by using variable-speed drives to bring pumps gradually on or off line, or (3) by using power-operated valves that are controlled so the pumps are started and stopped against a closed valve.

Surges caused by power failure can be controlled by devices designed to open on an increase in pressure, by devices that will exhaust sewage from the system upon sudden pressure drop in anticipation of surge, or by a surge tank.

Pump Seals Most sewage and sludge pumps can be obtained with either mechanical seals or packed stuffing boxes. Conventional mechanical seals have the disadvantage of requiring a pump to be disassembled so the seal can be repaired. Present mechanical seal technology offers a solution to this in the form of a split mechanical seal. Such a seal can be removed and repaired or replaced without the necessity of disassembling the pumping unit.

Often it is easier to replace the seal rather than repair it, and it is desirable to keep a spare on hand for this purpose. Packed stuffing boxes provided with water-sealed lubrication are still the most common choice for non-submersible sludge and sewage pumps. Grease seals are sometimes used for some of the smaller sewage pumps that do not run continuously.

Water serves multiple purposes as a sealing medium: it seals, lubricates, and flushes. Flushing is particularly important where abrasive material is involved in that it helps prevent this material from entering the seal. Grit and ash are very abrasive, and either will cut the shaft sleeves in a relatively short time. Where pumps are controlled automatically, a solenoid valve interlock with the pump starting circuit should be provided in the seal water connection to each pump. A manual shutoff valve and strainer should be provided on each side of each solenoid valve, and a bypass line should be provided around it.

Mechanical seals are normally lubricated with a clean external water source supplied at a pressure and flow rate as recommended by the seal manufacturer. Seals can be lubricated by the product being pumped provided it is filtered and can provide a reliable pressure and flow rate to the seal cavity. In such cases, a connection is normally provided between the pump discharge and the seal with a 0.10 to 0.20 in (2.5 to 5 mm) in-line filter to prevent foreign material from entering the seal cavity.

Occasionally, a pump station is so remote that sealing water is not readily or economically available. Mechanical seals can be provided constructed of special materials to withstand such adverse conditions. Also, several types of "formed-in-place" packing are available that do not require a smooth surface on which to seal as the packing develops its own sealing surface. However, these packing materials are not normally recommended for temperatures over 130°F (54°C) or pressures over 60 lb/in2 (4 bar).

Pump Bearings Pump bearings must be adequate for the service and should be designed on the basis of not less than a minimum life of five years in accordance with the Anti-Friction Bearings Manufacturers Association life and thrust values. The larger sewage pumps are usually equipped with both case and impeller rings of bronze or chrome steel.

Cleanout Ports Pumps should be provided, where possible, with cleanout ports on both the suction and discharge sides of the impeller. These are desirable for inspection and maintenance purposes.

Wet-Well Design Raw sewage wet wells should not be so large that sewage is retained long enough to go septic. It is usually desirable to limit storage to a maximum of 30 minutes. Shorter retention time is desirable. With the variable-speed controls now available, many stations can be designed so that the pumping rate matches the inflow rate and the inherent difficulties of frequent pump cycling or long retention times in wet wells can be avoided.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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