PUMP CHARACTERISTICS

For these parameters, the equations predict a much higher vacuum (24.5 in Hg or 230 percent of the shortcut method) than the gravity-discharge case. Of course, different tank dimensions and pump characteristics coiild give different comparisons between cases. If conditions are such that the pump can completely empty the tank before backflow occurs, the vacuum is Rest calculated from Eq. (26-57). 

Parameters q and W are variables when filtration conditions are changed. Coefficient (rj, is a function of pressure (rj, = f(P). The exact relationship can be derived from experiments in a device called a compression-permeability cell. Once this relationship is defined, the integral of the right hand side of the above equation may be evaluated analytically. Or, if the relationship is in the form of a curve, the evaluation may be made graphically. The interrelation between W and P, is established by the pump characteristics, which define q = f(W) in the integral. Filtration time may then be determined from dq/dt = W, from which we may state ... 

If the calculated blending time is longer than desired, nozzle discharge flow rate can be increased or, nozzle diameter increased. Consideration of nozzle recirculation line pressure drops and pump characteristics is required to select the parameter to change. 

The removal of all gases in the water by means of traps or chambers will improve the pumping characteristics and reduce corro,sion and noise. 

Figure 3-50. Typical centrifugal pump characteristic curve vrith auxiliary specific speed curve. Double-suction, single-stage, 6-in. pump, operating at 1760 rpm constant speed. (By permission, Karassik, I. and Carter, B., Centrifugal Pumps, McGraw-Hill Book Co., inc., 1360, p. 197.)...

Figure 3-55. Relation of speed change to pump characteristics.

Values of Q and h are plotted in Figure 8.49 and the discharge at the point of intersection between the pump characteristic equation and the line of the above equation is 0.0054 m3/s. 

When selecting a centrifugal pump for a given duty, it is important to match the pump characteristic with system curve. The operating point should be as close as is practical to the point of maximum pump efficiency, allowing for the range of flow-rate over which the pump may be required to operate. 

Most centrifugal pumps are controlled by throttling the flow with a valve on the pump discharge, see Section 5.8.3. This varies the dynamic pressure loss, and so the position of the operating point on the pump characteristic curve. 

Plot the system curve on the pump characteristic given in Figure A and determine the operating point and pump efficiency. 

Plotting these values on the pump characteristic gives the operating point as 18.5 m at 40.0 m3/h and the pump efficiency as 79 per cent. 

Figure 8-2 Typical pump characteristic curves. (From TRW Mission Pump...

Most pump manufacturers provide composite curves, such as those shown in Fig. 8-3, that show the operating range of various pumps. For each pump that provides the required flow rate and head, the individual pump characteristics (such as those shown in Fig. 8-2 and Appendix H) are then consulted. The intersection of the system curve with the pump characteristic curve for a given impeller determines the pump operating point. The impeller diameter is selected that will produce the required head (or greater at the specified flow rate). This is repeated for all possible pump, impeller, and speed combinations to determine the combination that results in the highest efficiency (i.e., least power requirement). Note that if the operating point (Hp, Q) does not fall exactly on one of the (impeller) curves, then the... 

The flow rate, head, and impeller speed at the maximum or best efficiency point (BEP) of the pump characteristic can be used to define a dimensionless group called the specific speed ... 

You must select a centrifugal pump that will develop a pressure of 40 psi when pumping a liquid with an SG of 0.88 at a rate of 300 gpm. From all the pump characteristic curves in Appendix H, select the best pump for this job. Specify pump head, impeller diameter, motor speed, efficiency, and motor horsepower. 

You need a pump that will develop at least 40psi at a flow rate of 300 gpm of water. What combination of pump size, motor speed, and impeller diameter from the pump characteristics in Appendix H would be the best for this application State your reasons for the choice you make. What are the pump efficiency, motor horsepower and torque requirement, and NPSH for the pump you choose at these operating conditions ... 

You must chose a centrifugal pump to pump a coal slurry. You have determined that the pump must deliver 200 gpm at a pressure of at least 35 psi. Given the pump characteristic curves in Appendix H, tell which pump you would specify (give pump size, speed, and impeller diameter) and why What is the efficiency of this pump at its operating point, what horsepower motor would be required to drive the pump, and what is the required NPSH of the pump The specific gravity of the slurry is 1.35. 

Likewise, the pump characteristic can usually be described by a quadratic equation of the form... 

Pump characteristic Simple single- head Single- head smooth pulse Simple dual-head Dual-head, compressibility-corrected, smooth pulse Dual-head, closed loop flow control Triple head low-volume Syringe- type Hydraulic amplifier Simple Amplifier Amplifier with flow control... 

Table 1.2. The effect of the size of a duct on the pumping characteristics...

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