## Significance Of Ondesign Operating Conditions

With reference to Figure 3, assume that the pump is operating on-design at Mop = ^Mmsp; that is, on the left branch of the efficiency "parabola." Primary flow rate Q1 and suction pressure Ps are constant. If the back pressure Pd is then reduced, the secondary flow rate Q2 (and hence Mop) will rise and pressure ratio Nop will fall. The new operating points will move to the right, along the characteristic curves for N and h. This increased-flow response to lowered Pd will end when cavitation-limited flow ratio ML is reached, assuming the NPSH is low enough to allow cavitation. Any further reduction of Pd will cause the operating points to leave the N and h theoretical curves on paths vertically downward as shown by the dashed line in Fig. 3. Consider next the same initial condition (on design: Mop = 2Mmep) followed by an increase in jet-pump discharge pressure Pd. In response, the secondary flow Q2 (and hence M) will decrease, the Nop and hop operating points moving left along the pump's N and h characteristic curves. Ultimately, increasing the back-pressure will result in reaching the M = 0 point, where N = No. (See Table 1, showing N = No = .680 at M = 0.)

Mechanical efficiency is important in most jet pump installations. Two low-efficiency situations that should obviously be avoided are

• Throttling the pump discharge pressure (to raise the Pd to the design point) wastes energy. Design Pd should match the pressure of the system into which the pump is to discharge.

• Operating the LJL pump with a discharge pressure Pd less than the design Pd (same Q1 and Ps) will yield more Q2 flow (M > Mop) and slightly higher efficiency. But such an increase in M may reach the cavitation point, that is, where M = ML, causing a marked departure of actual/measured operating points downward from the N and h characteristic curves.

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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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