Rotary pumps capacity

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

Rotary Pumps In rotary pumps the liquid is displaced by rotation of one or more members within a stationary housing. Because internal clearances, although minute, are a necessity in all but a few special types, capacity decreases somewhat with increasing pump differentia pressure. Therefore, these pumps are not truly positive-displacement pumps. However, for many other reasons they are considered as such. 

Other positive displacement pumps, such as rotary, gear, and diaphragm pumps, normally require PR valve protection for both the pump and downstream equipment. PR valves for all positive displacement pumps should have a capacity at least equal to the pump capacity. 

Suction and discharge heads are determined the same as for centrifugal pumps. Total head and capacity are used in selecting the proper rotary pump from a manufacturer s data or curves. Since viscosity is quite important in the... 

Rotary pumps with volume capacities up to 50 gal/min are considered to be small-volume capacity pumps. Pumps with volume capacities from 50 to 500 gal/min are moderate-volume capacity pumps. And pumps with volume capacities beyond 500 gal/min are large-volume capacity pumps. 

The capacity of a rotary pump operating with zero slip is called displacement capacity, q (gpm). Thus, the actual capacity of a rotary pump q (gpm) is... 

Performance data for rotary pumps is given in several forms. Figure 6.21 shows a typical plot of the head and capacity ranges of different types of rotary pumps. Reciprocating-pump capacity data are often tabulated, as in Table 6.26. 

FIGURE 6.21 Capacity ranges of some rotary pumps. (Worthington Corp.)... 

With this pump handling 8000-SSU liquid, a speed reduction of 40 percent is necessary, as shown in Table 6.28. Since the capacity of a rotary pump varies directly with its speed, the output of this pump when handling 8000-SSU liquid is (1000 gal/min) x (1.0 — 0.40) = 600 gal/min (37.9 L/s). 

Compute the effect of gas on the pump output. Entrained or dissolved gas reduces the output of a rotary pump, as shown in Table 6.29. The gas in the liquid expands when the inlet pressure of the pump is below atmospheric and the gas occupies part of the pump chamber, reducing the liquid capacity. 

The filling of filter presses and the maintenance of the proper pressure is done almost universally by pumps and montejus. Rotary pumps are most favored. Capacities for the square-leaved presses of one reputable maker are shown in the accompanying tables. 

Due to the close clearances in rotary pumps, it is necessary to operate these pumps at relatively low speed in order to secure reliable operation and maintain pump capacity over an extended period of time. Otherwise, the erosive action due to the high velocities of the liquid passing through the narrow clearance spaces would soon cause excessive wear and increased clearance, resulting in slippage. 

Figure 4.11 Characteristic curves (pump head, effective capacity and efficiency subject to the discharge rate) of a rotary pump from KSB.

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