Pump impellers diameter

Recognizing the flexibility of the affinity laws, it is better to select an original pump impeller diameter that is somewhat larger than required for the range of anticipated performance, and then cut this diameter dowm after in-service tests to a slightly smaller diameter. This new performance can be predicted in advance. Once the impeller diameter is too small, it cannot be enlarged. The only solution is to order the required large impeller from the manufacturer. 

The net positive suction head requirement has been investigated in an open tank-valve arrangement for a 0.5 by 0.45 m branch pump (impeller diameter 1.4 m) for sands with average particle sizes of 0.3 to 0.4 mm when pumped at slurry densities of 1230 and 1400 kg/m. It was found that the net positive suction head required in meters of slurry was about 50% higher than the corresponding water values. The effect of solids on the pump head and efficiency was less than 3%. 

The pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

According to HI convention, for double-suction pumps, Q is half of the total pump flow, ie, taken per impeller eye. The value of S is calculated at the best efficiency point (BEP) at maximum impeller diameter. 

For quick pump selec tion, manufacturers often give the most essential performance details for a whole range of pump sizes. Figure 10-30 shows typical performance data for a range of process pumps based on suction and discharge pipes and impeller diameters. The performance data consists of pump flow rate and head. Once a pump... 

Sample Specifications 13 in. (33 cm) outside diameter, gray cast iron pump impeller... 

Manipulating flow and controlling prcs.surc by varying the impeller diameter conserve.s kilowatts of energy, and this is the third affinity law in this group. A pump consuming 10 BHP with a 10 inch impeller, would only consume 7.3 horses with a 9 inch impeller. 

Impeller diameter - There will be an efficiency reduction with a reduction in the impeller diameter. For this reason, it s not recommended to reduce (trim) the impeller by more than 20%. For example, if a pump takes a fiill sized 10-ineh impeller, don t trim the impeller to less than 8-inches diameter. This would be a 20% reduction. 

Howe er, most conventional pump impellers receive the fluid into the impeller eye, at the center or inside diameter of the impeller. There are single suction impellers, and dual or double suction impellers with two eyes, one on each side. Dual suction impellers are mostly specified for low NPSH applications because the eye area is doubled (it ean reeeive twice as much fluid at a lower velocity head). Dual suction impellers arc mostly found on split case pumps where the shaft passes completely through the impeller. But they can afso be found mounted onto the end of the shaft in some special pump designs. 

Sometimes you hear people say that the pump is operating off its cw. t velocity, the impeller diameter and design are correct, if the pump installed and functioning correctly, including the mechanical seal and impossible to operate off the curve. The pump will be somewhere on between shut-off head and maximum flow a zero elevation. 

The family curve shows the range of different impeller diameters that ean run inside the pump volute. They re normally presented as various parallel H-Q eurves eorresponding to smaller diameter impellers. 

If the condition should be oecasional, the solution could be to install a variable speed motor. If the condition is permanent, the solution could be to reduce the impeller diameter, replace the pump, or increase the diameter of the pipe. If normal operations require living with the condition, then increase the diameter of the pump shaft to improve the L/D factor. 

Inadequate Pressure. Not enough velocity. Air or gases in pumped liquid. Impeller diameter too small Worn or damaged impeller Incorrect rotation... 

The larger the ratio of impeller diameter to tank diameter, the less mixer power required. Large, slow speed impellers require a low er horsepow er for a given pumping capacity, and solid suspension is governed by the circulation rate in the tank. 

The primary pumping capacity of an impeller is determined by the impeller diameter, the Pumping number, and the rotational speed. The Pumping number Nq is defined by [13]... 

Hicks et al. [8] developed a correlation involving the Pumping number and impeller Reynolds number for several ratios of impeller diameter to tank diameter (D /D ) for pitched-blade turbines. From this coiTclation, Qp can be determined, and thus the bulk fluid velocity from the cross-sectional area of the tank. The procedure for determining the parameters is iterative because the impeller diameter and rotational speed N appear in both dimensionless parameters (i.e., Npe and Nq). 

Figures 3-36A, 3-36B, and 3-36C represent typical and actual performance curves showing discharge total head (head pressure at pump outlet connection for any fluid), required minimum water horsepow er (for pumping water), and capacity or pumping volume of the pump (for any fluid) for several impeller diameters that would fit the same case (housing). In addition the important NPSHr (net positive suction head required by the pump) charac-...

Most standard pump curves illustrate the effect of changing impeller diameters on characteristic performance (Figure 3-36A). Note change as reflected in the different impeller diameters. However, the slight change in efficiency is not recorded over the allowable range of impeller change. 

Figures 32.12 and 32.13, and Figure 32.14 shows how the effect of changing speed or diameter of a pump impeller may be predicted, using the scaling laws ...

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