CIRCULATING PUMP

through the tube. The flow restriction is the space between the float and the tube wall. This area increases as the float rises. The differential pressure is fixed depending upon the weight of the float and the buoyant forces resulting from the combination of float material and the distilled water’s specific gravity. The tapered tube of the rotameter is usually glass with calibration marks reading directly in gpm. The major advantage of a rotameter over the venturi orifice meter is the visibility of the coolant. This allows quick determination of excessive entrained air in the coolant. CIRCULATING PUMP Each cooling system has two secondary distilled water circulating pumps (fig. 1-24), which are identical in construction and capacity. One pump is designated for service, and the other is held in standby in case the designated pump fails. Should the pump designated for operation fail, then the standby pump would be used in its place. The pumps should be operated alternately (every other week) to prevent deterioration of their shaft seals, equalize wear, and permit PM actions to be performed regularly. The two circulating pumps used in the liquid cooling system are single-stage centrifugal pumps closely coupled (pump is built onto the motor) to a constant-speed electrical motor. You may run into an older system that uses a separate pump and motor joined by a flexible coupling. The centrifugal pump has two major elements—the impeller rotating on the extension of the electric motor shaft and a casing that is called the impeller chamber. The impeller imparts the initial velocity to the coolant. The impeller chamber collects the high-velocity coolant from the impeller and guides it to the pump outlet. A mechanical shaft seal is used to eliminate external leakage. This seal is lubricated and cooled by water ducted from a high-pressure zone of the pump. You will find a vent valve located on the top of the pump casing to remove air and ensure the pump is primed with coolant. Located at the outlet of each pump is a check valve to prevent coolant from the outlet side of the operating pump from circulating to the return side of the coolant system through the standby pump. Hand-operated valves at the pumps are used to isolate the pumps so they can be removed for maintenance. Each secondary circulating pump is rated in gallons per minute (gpm) output at a specified head pressure in pounds-force per square inch gauge (psig) pressure, or in feet of water. The rating is usually at the pump’s maximum efficiency point, and it will vary depending upon the pump design. On all pumps, as the output pressure increases, the output flow decreases, and vice versa. This relationship is almost linear but differs with different pump designs. However, this condition means that if a restriction is placed in the pump output lines, the pressure will increase and the flow will decrease. The restriction could be a partially closed hand valve, dirty filter, damaged or crimped piping or hose, and so forth. In figures 1-6, 1-7 and 1-8, the pump performance indicators are the suction and discharge pressure gauges and the system flowmeter. If you start a pump and pressure fails to buildup, you should exhaust air through the vent cock on the top of the pump casing. You should ensure that the suction valve is fully opened and there is Figure 1-24.—Distllled water circulating pump. 1-19