S H

Three typical arrangements for determining the cavitation characteristics of pumps are illustrated in Figure 5. In Figure 5A, the suction is taken from a sump with a constant-level surface. The liquid is drawn first through a valve (throttle) and then through a section of pipe containing screens and straightening devices, such as vanes and baffles. This setup will dissipate the turbulence created by the suction valve and will also straighten the flow so the pump suction flow will be relatively free from undue turbulence. In Figure 5B, the suction is taken from a relatively deep sump or well in which the water surface can be varied over a fairly large range to provide the designed variation in suction lift. In Figure 5C, the suction is taken from a closed vessel in a closed loop in which the pressure level

FIGURE 6 Functions of sigma at constant capacity and speed; suction pressure varied

FIGURE 7 Sigma capacities above and below normal; suction pressure varied

FIGURE 8 Typical cavitation curves at constant speed and suction pressure

FIGURE 7 Sigma capacities above and below normal; suction pressure varied

FIGURE 8 Typical cavitation curves at constant speed and suction pressure can be varied by a gas pressure over the liquid, by temperature of the liquid, or by a combination of these.

By using one of these cavitation test arrangements, the critical value of s (that is, the value at which cavitation will begin) can be found by one of the following two methods:

1. Constant speed and capacity vary the suction lift. Run the pump at constant speed and capacity with the suction lift varied to produce cavitation conditions. Plots of the head, efficiency, and power input against s as shown in Figure 6.

When the values of s are held high, the values of head, efficiency, and power should remain relatively constant. As s is reduced, a point is reached when the curves break from the normal, indicating an unstable condition. This breakaway condition may and usually does impair the operation of the pump. The extent of impairment depends on the size, specific speed, and service of the pump and on the characteristics of the pumped fluid. A variation of this method is to plot results using capacities both greater and less than normal, as shown in Figure 7.

2. Constant speed and suction lift vary the capacity. Run the pump at constant speed and suction lift and vary the capacity. For a given suction lift, the pumping head is plotted against capacity. A series of such tests will result in a family of curves, as shown in Figure 8.

Where the plotted curve for any suction condition breaks away from the normal, cavi-tation has occurred. The value of s may be calculated at the breakaway points by dividing NPSHA by total head H at the point under consideration.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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