Pump Drivers

In the majority of cases, pumps are driven by electric motors. Sometimes, however, they are driven by gasoline, gas, or diesel units where firm power is not available or where pumping is required only at infrequent intervals. Variable-speed drivers are used extensively in sewage applications. These units generally consist of variable-speed motors or constant-speed motors with adjustable slip couplings of either the eddy-current or the fluid coupling type. Selection of the type of variable-speed driver to be used is usually based on space considerations, initial cost, operating cost over the expected life of the equipment, and customer preference. Emphasis is increasingly being placed on operating cost over the expected life due to government and environment requirements. See Section 6.2 for these and other types of speed-varying devices.

FIGURE 12 Typical head-capacity curves

Variable-speed drivers are particularly appropriate for raw sewage installations that discharge to a treatment plant. Use of this equipment allows the treatment facilities to operate continuously instead of intermittently surging the plant at incremental pumping rates. Variable-speed drivers are used to pump settled sewage and biological sludge where intermittent surging would adversely affect the process. Also, sludge pumps used to feed dewatering equipment are often equipped with variable-speed drives because it is necessary to vary the rate of discharge with the dewatering characteristics of the sludge.

For dry pit applications, the choice between horizontal- and vertical-drive motors depends considerably upon the station arrangement and available space. Horizontal motors are usually preferred, provided there is space and no potential flooding problem. Horizontal pumps are more easily maintained, and they are generally less expensive in first cost. Vertical drivers are generally used, however, for the pumping of raw sewage because of their smaller space requirements. Also, vertical units are advantageous in that the motor is located higher and is less susceptible to flooding.

In vertical dry-pit installations, intermediate shafting is normally preferred. This allows the drivers to be located above a potential flood level in the station. However, the use of intermediate shafting can become quite expensive in terms of initial installation costs and maintenance costs. Additionally, they may be prone to vibration problems if proper precautions are not taken in the design of the station. Vertical drive motor can also be mounted directly above a pump without the use of intermediate shafting. In doing so, it is connected to the pump by means of a suitable coupling. A separate support or bracing of the motor may also be required to provide for adequate installed stiffness.

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