An automatic alternator
The centrifugal type of sewage pump has nonclogging impellers. It can be installed in either a horizontal configuration or in a vertical configuration with a suction lift. The centrifugal pump can be placed in either a wet or dry pit and is equipped with a float or diaphragm for automatic operation.
ADVANTAGES AND DISADVANTAGES OF CENTRIFUGAL PUMPS. - The advantages of centrifugal pumps include simplicity, compactness, weight saving, and adaptability to high-speed prime movers. One disadvantage of centrifugal pumps is their relatively poor suction power. When the pump end is dry, the rotation of the impeller, even at high speeds, is simply not sufficient to lift liquid into the pump; therefore, the pump must be primed before pumping can begin. For this reason, the suction lines and inlets of most centrifugal pumps are placed below the source level of the liquid pumped. The pump can then be primed by merely opening the suction stop valve and allowing the force of gravity to fill the pump with liquid. The static pressure of the liquid above the pump also adds to the suction pressure developed by the pump while it is in operation. Another dis- advantage of centrifugal pumps is that they develop CAVITATION. Cavitation occurs when the velocity of a liquid increases to the point where the consequent pressure drop reaches the pressure of vaporization of the liquid. When this happens, vapor pockets, or bubbles, form in the liquid and then later collapse when subjected to higher pressure at some other point in the flow. The collapse of the vapor bubbles can take place with considerable force. This effect, coupled with the rather corrosive action of the vapor bubbles moving at high speed, can severely pit and corrode impeller surfaces and sometimes even the pump casing. In extreme instances, cavitation has caused structural failure of the impeller blades. Whenever cavitation occurs, it is frequently signaled by a clearly audible noise and vibration (caused by the violent collapse of vapor bubbles in the pump).
Several conditions can cause cavitation, not the least of which is improper design of the pump or pumping system. For example, if the suction pressure is abnormally low (caused perhaps by high suction lift or friction losses in the suction piping), the subsequent pressure drop across the impellers may be sufficient to 6-19 reach the pressure of vaporization. A remedy might be to alter the pump design by installing larger piping to reduce friction loss or by installing a foot valve to reduce suction lift.
Cavitation can also be caused by improper operation of the pump. For instance, cavitation can occur when sudden and large demands for liquid are made upon the pump. As the liquid discharged from the pump is rapidly distributed and used downstream, a suction effect is created on the discharge side of the pump. Think of it as a pulling action on the discharge side that serves to increase the velocity of the liquid flowing through the pump. Thus, as the pressure head on the discharge decreases, the velocity of the liquid flowing across the impellers increases to the point where cavitation takes place. Perhaps the easiest way to avoid this condition is to regulate the liquid demand. If this is not possible, then increase the suction pressure by some means to maintain pressure in the pump under these conditions.
OPERATION AND MAINTENANCE OF CENTRIFUGAL PUMPS. - The operating procedures and maintenance schedules for centrifugal pumps are generally similar to those of the other pumps we have discussed previously. Centrifugal pumps are also fitted with stuffing boxes and various types of bearings that, of course, require periodic maintenance and inspection. Always refer to the manufacturer's instructions and locally prepared maintenance schedules for operating and maintenance procedures.
One operating practice is common to nearly all types of centrifugal pumps. Unlike positive displace- ment pumps, the discharge stop valve on centrifugal pumps must. be CLOSED before starting the pump. This action allows the pump to work against the sealed discharge and builds up an effective pressure head before attempting to move and distribute the liquid downstream. After the pump is up to speed and the discharge valve is opened, the pump continues to maintain that pressure head unless the operating conditions are altered. Note that there is no danger of hydraulicking while the pump is run with the discharge closed. If the centrifugal pump were to continue operation with the discharge sealed, it would simply build up toward its maximum discharge pressure. It would then begin to churn the liquid; that is, the discharge pressure would overcome the suction pressure and the liquid would continually slip back to the suction side of the pump. Nothing more would happen, except the pump would build up heat, since theContinue Reading