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Uniform velocity and pressure exist only at zero flow.
This uniformity is progressively destroyed as the capacity is increased, Therefore, at BEP the casing losses are generally greater than those of the conventional volute. For low specific speed pumps, however, there is some gain in efficiency due to a circular volute since the benefits of the Improved surface finish in the machined volute outweigh the problems created by the nonuniform pressure distribution.
A comparison of the efficiency of circular and conventional volutes is shown in Figure General Design Considerations It was pointed out previously that the casing itself represents only losses and does not add anything to the total energy developed by the pump. In designing pump casings it is therefore important to utilize all available means of minimizing casing losses. However, commercial considerations dictate some deviations from this approach, and experience Volute Design 57 Figure Efficiency comparison of circular and conventional volutes, has shown that these do not have a significant effect on casing losses.
The following design rules have shown themselves to be applicable to all casing designs: 1. Constant angles on the volute sidewalls should be used rather than different angles at each volute section. Experience has shown that these two approaches give as good results and the use of constant wall angles reduces pattern costs and saves manufacturing time. The volute space on both sides of the impeller shrouds should be symmetrical. All volute areas should be designed to provide a smooth change of areas.
Circular volutes should be considered for pumps below a specific speed of Circular volutes should not be considered for multistage pumps.
The total divergence angle of the diffusion chamber should be be tween 7 and 13 degrees. The final kinetic energy conversion is obtained in the discharge nozzle in a single-stage pump and in both the discharge nozzle and crossover in a multi-stage pump. In designing a volute, be liberal with the space surrounding the impeller.
In multi-stage pumps in particular, enough space should be provided between the volute walls and the impeller shroud to allow one-half inch each way for end float and casting variations, A volute that is tight in this area will create axial thrust and manufacturing problems. The Use of Universal Volute Sections for Standard Volute Designs It has been noted that when the volute sections of different pumps are factored to the same throat area; their contours are almost identical. Any differences that do exist can be traced to mechanical considerations or the designer's whim, rather than any important principle of hydraulic design.
Similarly, factoring the impeller width and the radial gap between the impeller and the cutwater reveals that the values of these parameters also lie in a very narrow random range. In other words, the entire discharge portion of the pump casing when viewed in cross section and factored to a common throat area has only minor variations throughout the entire specific speed spectrum.
This fact enables us to eliminate the usual trial-and-error method of designing volute sections while still consistently producing casings to a high standard of hydraulic design. To facilitate this process we have prepared a set of "universal" volute drawings on which the typical volute sections described above have been laid out for a 10 sq in.
Once the designer has chosen his throat area, he can quickly produce the required volute sections by factoring the sections shown for the "universal" volute.
Sections for a single-volute pump are shown in Figures and , and sections for a double volute pump are shown in Figure Volute Design 59 Figure Typical single-volute layout. The universal volute sections for such a design are shown in Figure A rectangular volute casing requires the same throat area as a standard volute casing and should be laid out according to the principle of constant velocity.
Rectangular volutes are widely used in small single-stage and multistage pumps. The benefits of the rectangular volute are strictly economical. The simple volute section yields a considerable cost savings due to reduced pattern costs and production time. Over the range of specific speeds where it is used the hydraulic losses are negligible.
The Design of Circular Volutes The details of a typical circular volute casing design are shown in Figure , The ratio between the impeller diameter, D2, and the volute di- 60 Centrifugal Pumps: Design and Application Figure Universal volute sections for single-volute pump. Universal volute sections and typical layout for trapezoidal doublevolute pump. Volute Design 61 Figure Universal volute sections and typical layout for rectangular double volute pump.
The volute width, t, should be chosen to accommodate the widest maximum flow impeller that will be used in the casing. The capacity at BEP can be controlled by the choice of the volute diameter, D4.
Generally, the best results are obtained by selecting the volute width and diameter for each flow requirement. To minimize liquid recirculation in the volute, a cutwater tongue should be added.
General Considerations in Casing Design There are several considerations in the casing design process that apply to all volute types. This area together with the impeller geometry at the periphery establishes the pump capacity at the best efficiency point.
The throat area should be sized to accommodate the capacity at which the utmost efficiency is required, using Figure The use of these figures will save the designer time and introduce consistency into the design process. Typical layout for circular volute pump.
Volute Design S3 Manufacturing Considerations Casings, particularly of the double-volute design, are very difficult to cast. In small- and medium-sized pumps the volute areas are small and the liquid passages are long, requiring long unsupported cores.
In volutetype multi-stage pumps the problem is more pronounced since there are several complicated cores in a single casing. Casing Surface Finish To minimize friction losses in the casing, the liquid passages should be as clean as possible.
Since cleaning pump casings is both difficult and time consuming, an extreme effort to produce smooth liquid passages should be made at the foundry. The use of special sand for cores, ceramic cores, or any other means of producing a smooth casting should be standard foundry practice for producing casings.
Particularly with multi-stage pumps, however, even the best foundry efforts should be supplemented by some hand polishing at points of high liquid velocities such as the volute surfaces surrounding the impeller and the area around the volute throat.
Both of these areas are generally accessible for hand polishing. In addition, both cutwater tongues should be sharpened and made equidistant from the horizontal centerline of each stage. The same distance should be maintained for each stage in a multistage pump. Casing Shrinkage Dimensional irregularities in pump casings due to shrinkage variations or core shifts are quite common.
Shrinkage variations can even occur in castings made of the same material and using the same pattern. The acceptance or rejection of these defects should be based upon engineering judgments. However, knowing that shrinkage and core shifts are quite common, the designer should allow sufficient space for rotating element end float.
The allowance for total end float should be a minimum of onehalf inch. Conclusion Although it is often claimed that casings are very efficient, this is misleading, since the hydraulic and friction losses that occur in the casing can only reduce the total pump output and never add to it.
It is the designer's responsibility to do his utmost to minimize these losses. Total impeller width including shrouds at D2 in. Experimental constant Specific gravity Reference Stepanoff, A. Many pumps in service are operating at 3, to 4, psi discharge pressure. Only when shaft diameter is too big or rotating speed too high, will Kingsbury-type bearings be required.
The impeller design for the multi-stage pump is the same as that for a one-stage unit, as described in Chapter 3. The double-volute design is also the same as that for a one-stage pump as described in Chapter 5, 65 Figure The term "crossover" refers to the channel leading from the volute throat of one stage to the suction of the next. Crossovers leading from one stage to the next are normally referred to as "short" crossovers and are similar to return channels in diffuser pumps.
These are normally designed in right hand or left hand configurations, depending upon the stage arrangement, Crossovers that lead from one end of the pump to the other or from the center of the pump to the end are normally referred to as "long" crossovers.
The stage arrangements used by various pump manufacturers are shown schematically in Figure Arrangement 1 minimizes the number of separate patterns required and results in a minimum capital investment and low manufacturing costs.
However, with this arrangement a balancing drum is required to reduce axial thrust. Arrangement 2 is used on barrel pumps with horizontally split inner volute casings.
Arrangement 3 is the most popular arrangement for horizontally split multi-stage pumps and is used by many manufacturers. Finally, with Arrangement 4 the series stages have double volutes while the two center stages have staggered volutes. This design achieves a balanced radial load and an efficient final discharge while requiring only one "long" crossover, thereby reducing pattern costs and casing weight.
Velocity cannot be efficiently converted into pressure if diffusion and turning are attempted simultaneously, since turning will produce higher velocities at the outer walls adversely affecting the diffusion process. Furthermore, a crossover channel that runs diagonally from the volute 68 Centrifugal Pumps: Design and Application Figure Multi-stage pump stage arrangements. For these reasons, the multi-stage pumps of 25 or more years ago were designed with high looping crossovers. To achieve radial balance these crossovers were in both the top and bottom casing halves.
This design, referred to as the "pretzel" casing, was very costly, difficult to cast, and limited to a maximum of eight stages. These problems prompted a study to evaluate the performance of various crossover shapes. A 4-in.
For these tests the pump hydraulic passages were highly polished micro-inches , ring clearances were minimized and component crossover parts were carefully matched using a template. Configuration 1 was designed with a total divergence angle of Design of Multi-Stage Casing 69 Figure Configurations evaluated during crossover performance study. From this point the area was held constant to the impeller eye. To prevent prerotation a splitter was added to the suction channel.
Crossovers 2 and 3 were designed maintaining the same areas at sections A, B, and C with the same divergence angle but progressively reducing the radial extent of the crossover. The "U" bend on Crossovers 1 and 2 were cast separately from the casing and highly polished before welding. Crossover 3 was cast as a single piece, and the "U" bend polished only in the accessible areas. The results of testing all three crossover configurations are shown on Figure The tests indicated that Crossover 1 yielded a peak efficiency four points higher than Crossover 3.
Subsequent testing of commercial units, however, indicated the difference to be only two points. The difference in improvement was attributed to the poor quality of the commercial castings and the use of normal ring clearances. The two-point efficiency loss associated with Crossover 3 was deemed commercially acceptable and was incorporated in multi-stage pumps of up to fourteen stages by all the West Coast manufacturers.
These pumps were suitable for higher pressures, easily adaptable to any number of stages, odd or even, and readily castable even in double-volute configurations. These three items become the basic design requirements during the layout of horizontally split multi-stage pumps.
Hydraulically, the pump design should achieve the best possible efficiency, as well as the highest head per stage, thereby minimizing the number of stages required.
The best available technology should therefore be utilized to produce the most efficient volutes and impellers. Although crossover design has only a secondary effect on pump efficiency, it too should use every available "trick" to achieve the best possible results.
Figure shows short and long configurations of the two basic types of crossovers normally used on multi-stage pumps. Both have been tested by the West Coast pump companies. Results of these tests indicate that the radial diffusion type is approximately one point more efficient than the diagonal diffusion type.
Results of crossover performance study. Design of Multi-Stage Casing 71 Figure Radial and diagonal diffusion crossovers. This point should be reached before the "U" bend to the suction channel. The suction channel should be sized to accommodate the largest capacity impeller that will be used in the pump.
The area of the suction return channel should be consistent immediately after the "U" bend. Some designers prefer to decelerate slightly at the impeller eye; however, recent tests indicate that better efficiency is obtained if the liquid is accelerated as it approaches the impeller eye. However, the cost of adding this splitter is generally prohibitive, and it is not generally used. From a theoretical viewpoint, crossover channels should have a circular cross section to minimize friction losses.
However, all the designs on the market today have rectangular shapes for practical reasons. The best overall results are obtained by placing two splitters at the casing parting split as shown In Figure The long crossover is identical to the short or series configuration yp to the area "B," where the long channel that traverses the pump begins, This long channel should be designed with a "window" at the top for cleaning and a properly shaped plate matched to the crossover opening before welding.
The configuration of the long crossover is also shown in Figure Crossovers with Diagonal Diffusion Sections The diagonal diffusion type crossover shown in Figure leads the liquid from the volute throat to the suction of the next impeller while traveling diagonally around the periphery of the volute. This design has one long radius turn as compared to the "U" bend used in the radial diffusion type. Other than these differences, both types of crossover have the same diffusion, area progression etc.
Figure also shows the configuration in which the long crossover channel climbs over the short crossover. Even though this crossover has only a single long-radius turn, it is not as efficient as the radial type.
This can be attributed to the diagonally located channel, which imparts a spiral motion to the fluid leaving the volute throat, resulting in hydraulic losses larger than those in the "U" bend. Mechanical Suggestions In previous chapters, we have described design procedures for centrifugal pump impellers and volutes applicable to one-stage or multi-stage units and in this chapter crossovers for multi-stage pumps only. However, hydraulic considerations alone for multi-stage pumps are not sufficient to complete a final unit.
Mechanical details must be considered. This refers to patterns, foundry methods, mechanical bolting, and quality controls, Patterns Multi-stage casings have quite complex shapes of liquid passages, including crossovers, crossunders, double volutes at each stage, etc. For this reason, pattern equipment must be of high quality sectional design to allow for variations in number of stages. Other Items In This Category.
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