To Vacuum Producer

FIGURE 7 Vacuum-controlled central automatic priming

a vacuum switch and maintains a vacuum in the tank of 2 to 6 in Hg (50 to 150 mm Hg) above the amount needed to prime the pumps with the greatest suction lift. The priming connections on each pump served by the system are connected to the vacuum tank by automatic vent valves and piping (Figure 7). The vacuum tank is provided with a gage glass and a drain. If liquid is detected in the vacuum tank as the result of leakage in a vent valve, the tank can be drained. Automatic vent valves consist of a body containing a float that actuates a valve located in the upper part. The bottom of the body is connected to the space being vented. As air is vented out of the valve, water rises in the body until the float is lifted, and the valve is closed.

A typical vent valve designed basically for vacuum priming systems is illustrated in Figure 8. The valve is provided with auxiliary tapped openings on the lower part of the body for connection to any auxiliary vent points on the system—for instance, when air is to be exhausted simultaneously from the high point of the discharge volute and the high point of the suction passageways. When one or more of these vent points are points of higher pressure, such as the top of the volute of the pump, an orifice is used in the vent line to limit the flow of liquid. Otherwise, a relatively high constant flow of liquid from the discharge back to the suction would take place, causing a constant loss. Where a unit is used more or less constantly, a separate valve should be used for each venting point.

The system shown in Figure 7 is the most commonly used of central automatic priming systems. It can use either wet or dry motor-driven vacuum pumps. Most central priming systems are provided with two vacuum pumps. The usual practice is to have the control of one vacuum pump switched on at some predetermined vacuum and the control of the second switched on at a slightly lower vacuum.

Combination dry reciprocating-type vacuum pump and vacuum tank units are commercially available in various sizes. The operation of a typical unit can be explained as follows, referring to Figure 9.

The primer automatically stops and start itself to maintain a minimum vacuum in its tank at all times, regardless of whether the centrifugal pumps it serves are operating or not.

A vacuum header is run from the primer to the centrifugal pumps, and connection is made to the priming valve mounted on each pump. The vacuum in the primer tank

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Survival Treasure

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