CENTRIFUGAL PUMP DESIGN1

The client will usually specify the desired head and pump capacity. The type and speed of the driver may also be specified. Speed is governed by considerations of cost and efficiency as well as drivers available to the client. Given these parameters, the task of the engineer is to minimize cost.

Which cost to minimize, first cost or life-cycle cost, however, is an important consideration. From a life cycle viewpoint, we must take into account power consumption and operation and maintenance costs. These considerations call for optimizing efficiency, reliability (the mean time between failure) and maintainability (the mean time to repair). In general, designing to optimize these categories results in increased costs. Often, these considerations are not very important and we can design for minimum first cost. In appropriate cases, the engineer should initiate a dialog with the client concerning available options. For example, designing a boiler feed pump that operates continuously would probably call for maximizing efficiency. Efficiency considerations would not be so important, however, for a drainage pump that is only required to operate occasionally.

PIPE CONNECTIONS AND VELOCITIES

The diameter of the suction pipe is usually made larger that the pump suction flange and both are made larger than the discharge flange and pipe. Church recommends keeping the velocity at the suction flange about 9 or 10 ft/s and that at the discharge flange between 18 and 25 ft/s.

LEAKAGE LOSSES

To design the impeller, account must be taken of leakage from the discharge side back to the suction side. To reduce the leakage, wearing rings are fitted to the impeller and casing. These rings are designed with specified clearances. The leakage across each ring can be calculated from the following formula:

where: C = flow coefficient2

A = leakage area = D = mean clearance diameter

s = diametrical clearance For small wearing rings with precise machining and ball bearings, the minimum clearance may be

reduced to 0.008 in.

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IMPELLER INLET DIMENSIONS AND VANE ANGLE

1 This section is based on Church, A.H., Centrifugal Pumps and Blowers,Ch. 6, John Wiley & Sons, 1950.2 Id. Fig. 6-1, p. 92.3 Attributed by Church to Stepanoff, A.J., Trans. A.S.M.E., HYD-54-5, 1932.

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The diameter of the impeller eye, Do, is dependent on the shaft diameter, Ds, which must initially be approximated. The hub diameter, DH

, is made 5/16 to ½ inch larger than Ds. After estimating Ds and DH , Do is based on the known flowrate. The inlet vane edge diameter, D1, is made about the same as Do to ensure smooth flow.

EXAMPLE OF IMPELLER DESIGN4

Specified conditions: Required head: hP = 150ft

Required flowrate: Q = 2500 gpmRequired speed N = 1760 rpm

1. Quantity flowrate:

2. Mass flowrate:

3. Specific speed: Assume a double suction impeller; then, Q = 2500/2 = 1250gpm, and:

For this specific speed, a radial flow pump is indicated.5

4. Water horsepower.

5. Shaft diameter. Calculate shaft diameter based on torque. Increase the calculated value somewhat to allow for bending moment which is unknown at this point and to ensure that the critical speed exceeds the operational speed by a reasonable margin. The bending moment will depend on the weight distribution of the shaft and any unbalanced radial thrust acting on the impeller. From the figure shown below, with the given flow of 2500 gpm and calculated value of specific speed of 1450, we select a tentative value of efficiency of 80%.

4 See, Church, p. 107-117.5 See,Munson, Fig. 12.18, p. 812.

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Thus:

The required shaft torque then is:

Assuming a shear stress of 4000 psi:

To account for the unknown bending moment and critical speed, increase the shaft diameter to 2 1/8 in. Church states that the hub diameter, DH , is made from 5/16 to ½ in. larger than Ds:

Let 2 ½ in.

6. Suction line velocity and diameter of suction flange.

Assume a velocity of 10 ft/s at the suction flange; thus:

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; or, ;thus,

Assume the velocity at the eye of the impeller is 11 ft/s.

For a double suction pump, assume that the leakage will not exceed 2%. Dividing the total flow by 2 gives:

7. Wheel inlet dimensions and angle.

Assume an inlet diameter, D1, of 7 5/16 in.

The radial velocity should be slightly higher than V0 because a converging shape is more efficient than a divergent one. Let Vr be 12 ft/s.

The inlet area will be decreased by the vane thickness. Assume a contraction factor, 16

, of 0.85; the entering width then is:

Inlet angle: Assume that water enters vanes radially.

1 is usually increased slightly to account for contraction of the stream as it passes the inlet edges as well as prerotation. The inlet angle is usually between 10 and 25 degrees7. Let 1 be 130.

8. Impeller diameter, D2.

The theoretical head can be found from integrating the force on a differential mass:

6 1 is generally between 0.8 and 0.9, Church, p. 95.7 Church, p. 95.

β1

Vr1

W1

U1

4

and ;

but and ;hence,

For a closed rotating cylinder containing a fluid, the pressure head developed at the outer rim is:

Substituting D2 /2() for U2 and solving for D2 :

(12)

Where: H2 is in feet; N is in rpm; D2 is in inches.Tests have shown that the required impeller diameter can be calculated from this expression by substituting the head corresponding to the best efficiency point for H2 and then multiplying the right side by an experimentally determined coefficient :

(13)

Church8 gives several charts for which have been based on a large number of tests. Most of the plotted points fall within a range of 0.9 to 1.1. Noting that if the head on test is found to be too high, the impeller diameter can be machined to an appropriate diameter, select 1.05 for ; then:

; say, 13 ½ in.

9. Outlet vane angle, 2, and impeller width.

The normal range for discharge angles is between 20 and 25 degrees9. Furthermore, 2 is usually made larger than the inlet angle. Assume 2 = 200.

8 Church, pp. 199-104. 9 Id., p. 35.

U

dφ

5

The radial outlet velocity, Vr2 , is made the same as, or slightly less than, the radial inlet velocity, Vr1. Assume Vr2 = 11 ft/s10.

Outlet area (based on required flow plus leakage).

Assume a contraction width, 2 , (based on experience) of 0.925:

10. Outlet velocity diagram.

The absolute outlet velocity, V2 , is used in the design of the volute. We proceed as follows:

Theoretical tangential outlet velocity, V2.

Actual tangential outlet velocity, V2’.11

The inertia of the rotating fluid causes a circulatory flow opposite to the direction of rotation of the impeller. This flow, superimposed on the outward flow, results in the fluid leaving the impeller at an angle less than that calculated from angular momentum theory. Thus 2 must be decreased and , therefore, the absolute angle, 2 , increased. The effect of circulatory flow is to reduce V2 and the theoretical head. Church defines a circulatory flow coefficient, , as:

Church assumes a value of of 0.7. This coefficient can be calculated from tests. Pump manufacturers will maintain records from which a reasonable value might be estimated for a given design.

The outlet vector diagram can now be drawn:

, say, 130

10 Id., p. 110.11 See, Church, p. 28 for a discussion of circulatory flow.

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11. Cross-section of impeller.

Wall and vane thicknesses are usually made a minimum consistent with good foundry practice. The stresses due to centrifugal force and fluid pressure are relatively low for average applications; otherwise, they need to be taken into account12.

Table of Calculated or Assumed Dimensions

b1 = 1.75 in per sideb2 = 1.90 inD2 = 13 ½ inD0 = 7 5/16 inDr = 8 ½ in (to outside of impeller wearing ring)Impeller shroud tip thickness - 3/16 in

Connect outlet to inlet by a straight line faired into entrance to provide a smooth transition. Make tip of hub core 3/16 in and fair into hub diameter. The drawing is shown in the figure on the following page.

12. Check leakage loss.

From the figure on page 8, the mean diameter of the clearance is 8 ½ in. Let s be the diametral clearance. Church states that the wearing ring clearance for good practice is 0.01 in for rings of 6 in diameter and less. For rings greater than 6 in, increase the clearance by 0.001 in for every inch of ring diameter greater than 6 in:

The clearance area is:

Head across the rings13:

12 Id,, p. 152. 13 Church attributes this equation to A.J. Stepanoff: “Leakage Loss and Axial Thrust in Centrifugal Pumps,” A.S.M.E. Trans., HYD-54-5, 1932.

V’θ2

Vθ2

U2

α'2

α2

Vr2 Vr2β2’

β2V2’ V2

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From Figure 6-1, p 92, Church, the flow coefficient for 1760 rpm and a 0.013 in clearance is 0.410. Thus, the leakage is:

The per cent leakage is ; or 1.35 %, which is, close enough to the assumed value of 2 %.

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DESIGN OF VANES

The entrance vane angle, , has been found to be 130 ; that at the exit, 200 . For smooth flow, we must design the vane such that this angle increases smoothly from 130 to 200 . We note also that the radial components of velocity to these two angles are 12 and 11 ft/s, respectively. We also see from the vector

diagram that . The relative velocities corresponding to the entrance and outlet stations are

then: and . To obtain intermediate values of radii corresponding to intermediate values of the position angle, , we proceed as follows (see Fig. 3):

1) Plot , Vr, and W against vane radius, r, for the entrance and outlet stations and connect by a straight line (or a smooth curve).

2) The corresponding values for vane angle, , are computed from . These values are also plotted against their radii.

Alternatively, write a computer program to perform the above functions. Referring to the figure below:

or

Note: Use MATLAB or other computer system to perform the integration. Use a sufficiently close spacing of r to obtain a smooth vane shape.

3) Plot the radii against to give the shape of the trailing edge

dθ r

rdθ

dr

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of the vane.

Draw the front edge of the vane with the same curvature as the back edge with a thickness of about 1/8 in14.

NUMBER OF VANES

The number of vanes is given by the Pfleiderer equation15. First, calculate the average vane angle:

; then,

The circumferential pitch of the vanes is:

Check the contraction factor:

The assumed values agree reasonably with those calculated.

SUMMARY

Diameter of suction flange, Dsu ------------------------------- 10inVelocity in suction flange, Vsu ---------------------------------10.22 ft/s

Shaft diameter, Ds -----------------------------------------------------------------------2 1/8 inImpeller hub diameter, DH --------------------------------------2 ½ inImpeller eye diameter, D0 --------------------------------------7 5/16 in

14 Church, p. 115.15 Id.

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Velocity through impeller eye,V0-------------------------------11 ft/sDiameter of inlet vane edge, D1--------------------------------7 5/16 in

Velocity at inlet vane edge, V1 = Vr1 ---------------------------12 ft/sPassage width at inlet, b1 -----------------------------------------1.75 in per sideTangential velocity of inlet vane edge, U1 -------------------56.2 ft/sVane angle at inlet, 1 --------------------------------------------130

Impeller outlet diameter, D2 -------------------------------------13 ½ inRadial component of outlet velocity, Vr2 ----------------------11 ft/sVane angle at outlet, 2 -------------------------------------------200

Total passage width at outlet, b2 ---------------------------------1.98 inTangential velocity of outlet vane edge, U2 -------------------103.7 ft/s

Absolute velocity leaving impeller, -------------------------52.5 ft/s

Tangential component of absolute leaving velocity, ---51.5 ft/s

Angle of water leaving impeller, ----------------------------130

Number of impeller vanes, z ------------------------------------6

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