CIRCULATING PUMPS

Figure 2-18.—Rotameter. CIRCULATING PUMPS Each cooling system has two secondary distilled- water circulating pumps. These pumps 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. If the pump designated for operation fails, then the standby pump is used in its place. The pumps should be operated alternately (every other week) to prevent deterioration of the shaft seals, to equalize wear, and to permit Planned Maintenance System (PMS) actions to be performed regularly. The two circulating pumps used in liquid-cooling systems are single-stage centrifugal pumps, closely coupled to a constant-speed electrical motor (the pump is built onto the motor). (Some older systems use 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 2-19 motor shaft, and the casing (the impeller chamber). The impeller imparts the initial velocity to the coolant and collects the high-velocity coolant from the impel- ler 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. A vent valve is on the top of the pump casing to remove air and to ensure that 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 that they can be removed for maintenance. Each secondary circulating pump is rated in a gpm output at a specified head pressure in pounds per- square-inch-gage (psig) pressure, or in feet of water. The rating is usually at the pump’s maximum effi- ciency point and varies depending on the pump de- sign. On all pumps, as the output pressure increases, the output flow decreases, and vice versa. This rela- tionship is almost linear, but varies 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, a dirty filter, a damaged or crimped piping or hose, etc. The pump performance indicators are the suction and discharge pressure gages and the system flow- meter. If you start a pump and the pressure fails to build up, 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 that there is pres- sure on the pump suction pressure gage. If the flow doesn’t develop, you should check for clogging and wear. Never operate a pump without coolant flow. Some pumps have a small recirculating line that enables the pump to recirculate coolant from the dis- charge side of the pump to the suction side to provide for a flow of coolant through the pump if an inlet or outlet valve to the pump is closed with the pump run- ning. Whatever the case, keep in mind that the opera- tion of a pump without the normal flow of coolant