Design of The Centrifugal Pumps...Design a closed impeller with one-side suction for a centrifugal pump, for a discharge capacity of 1.5 m3/min of water, at an operating head of 20

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Design of The Centrifugal Pumps

محمد السيد أبوعرب. د

أستاذ هندسة نظم الري والصرف المساعد

جامعة القاهرة–كلية الزراعة

http://eleg.oucu.edu.eg/moodle/file.php/56/1-1/1_1.htmhttp://eleg.oucu.edu.eg/moodle/mod/lesson/view.php?id=1814


Lecture Contents

1.Basic Hydraulic.

2.Bernoulli’s Equation for Stationary Conduit’s.

3.Velocity Triangle.

4.Leakage Losses.

5.Disc Friction Losses.

6.Mechanical Losses.

7.Overall Head Coefficient.

8.Specific Speed.

9.Design of Impeller.

10.Design of Impeller Vanes

11.Design of Volute.



Basic Hydraulic

The principles of fluid flow applicable to centrifugal pumps

include Bernoulli’s equation, velocity triangles, specific speed, total

head and computation of losses due to disc leakage, friction and

other mechanical losses.



Bernoulli’s Equation for Stationary Conduit’s



Velocity Triangle



A study of the components of flow through an impeller is best carried out

graphically by means of velocity vectors. The velocity vector diagram is

triangular and it is called a velocity triangle. It can be drawn for any point of

the flow path through the impeller. However, velocity triangles are usually

drawn on the entrance and discharge ends of the impeller vanes. Hence,

velocity triangles are called entrance and discharge triangles.

u = peripheral velocity of impeller, m/s

D = impeller diameter, cm

n = speed of impeller, rpm

ω = relative velocity of flow, m/s

C = absolute velocity of flow, m/s

Cm= radial component of absolute velocity of flow, m/s

Cu = tangential component of absolute velocity of flow (C cos α ), m/s

α = angle between C and u, degrees

β = angle between ω and u (extended) degrees

Velocity Triangle



Entrance and discharge velocity diagram of an impeller

with backward curved vanes



Virtual entrance and discharge velocity triangles

of the impeller



Correction Factor

Z 1 2 4 6 10 20 ∞

0.25 0.40 0.572 0.666 0.77 0.87 1.0



Leakage Losses



Disc Friction Losses



Mechanical Losses



Overall Head Coefficient



Specific Speed



Design of Impeller

With the help of the

calculated specific speed and

a given capacity, the

attainable efficiency of the

proposed impeller may be

predicated.

The impeller profile and

the layout of the vanes may

be done if the following

elements of an impeller are

known.

Radial velocities at the inlet and outlet

Outside diameter

of the impeller

Impeller vane inlet and outlet

angles



Relationship between specific speed, discharge and

efficiency of centrifugal pump (Wislicenus, 1947)



Definitions sketch for determining the dimensions of

different components of an impeller

oThe vane angle β2 is one of

the most important elements

in the design of the pump

impeller.

oFor a normal design, β2varies from 17.5° to 27.5°.



The design steps of the impeller

Shaft Diameter

(Ds)

Impeller Inlet Dimensions

and Vane Angle

Impeller Outlet

Dimensions and Vane

Angle

Design of Impeller Vanes



Shaft Diameter (Ds)



Impeller Inlet Dimensions and Vane Angle

Diameter of the

Suction Flanges

(Dm) Diameter of the Eye of Impeller

Inlet Vane Edge

Diameter

Passage Width at

Inlet

Inlet Vanes Angles



Diameter of the Suction Flanges (Dm)



Diameter of the Eye of Impeller



Inlet Vane Edge Diameter

oThe inlet vane edge diameter D1 is usually assumed to be the

same as the diameter of the eye of the impeller, in order to

ensure smooth flow without excessive turbulence.

o In case of a sloping inlet edge, the average value of the

diameter may be made equal to the eye diameter, D0.



Passage Width at Inlet



Inlet Vanes Angles



Impeller Outlet Dimensions and Vane Angle

Outlet Diamete

r (D2)

Outlet Vane Angle

Outlet Passage Width (b2)

Outlet Velocity Diagram



Outlet Diameter



Outlet Vane Angle

The outlet vane angle β2 may be selected within a fairly wide limit.

Usually, its value varies from 15° to 40°.

With a view to attaining a smooth and continuous passage, β2 is

assumed to be larger than the inlet angle β1.



Outlet Passage Width



Design of Impeller Vanes

Number of Vanes

Vane Curvature

Vane Thickness

Passage Width



Number of Vanes



Vane Curvature

Church and Lal (1973) suggested two methods for the

construction of the vane shape, using the vane angle curve plotted

between the inlet and outlet radii of the impeller.

The methods are (i) tangent arc method, and (ii) polar coordinate

method. The first method, which is more common, is discussed in this

lecture.



Tangent Arc Method



Vane Thickness

o The vane thickness, t, in case of a closed impeller having shrouds on both

sides, can be determined by assuming the vane to be a beam fixed at the

ends and loaded uniformly.

o However, the thickness so calculated is usually less than the minimum

thickness recommended.

o The minimum thickness of the vane, at the end, usually recommended is 3

mm.

o The vanes may be of uniform thickness throughout or the thickness may be

progressively increased from inlet to outlet.



Passage Width



Passage Width

o Most single-stage centrifugal pumps,

except vertical pumps (vertical

turbine and submersible pumps), are

usually of the volute type.

o Though a casing with diffuser vanes

is more efficient, the volute-type

casing is adopted because of its

simplicity.

o The volute consists of a casing

surrounding the impeller.

o The cross-sectional area of the

volute increases gradually from the

tongue to the throat.



Design of Volute

Volute Area

Tongue Angle

Discharge Flange



Volute Area



Tongue Angle



Tongue Angle

The diameter of the discharge flange is generally based

upon the average liquid velocity of 5.5 to 7.5 m/s at the design

point.



Example

Determine the inlet and outlet dimensions and angles of a

double-suction radial impeller, for an operating head of 15 m

and discharge of 0.04 m3/s. The pump is to be directly

connected with a motor operating at 1450 rpm.



Solution



Example

Design a closed impeller with one-side suction for a

centrifugal pump, for a discharge capacity of 1.5 m3/min of

water, at an operating head of 20 m. The pump is to be directly

coupled to an electric motor operating at 1450 rpm.



Solution



Design of Vanes

o The radial components of absolute velocity at the inlet and outlet

ends, Cm1 and Cm2 are 3.5 m/s and 3.25 m/s, respectively.

o The relative velocities w1 and w2 are calculated from the relationship

sin β = Cm/w. The relative velocities w1 and w2, corresponding to

vane angles β1 (22º 46 57) and β2 (30º) and Cm1 and Cm2 are 8.8 and

7.0 m/s at the inlet and outlet ends, respectively.

o For the known values of the radial component of absolute velocities,

relative velocities, and corresponding radii (5.5 cm and 13.7 cm at

the inlet and outlet, as D1 = 11cm and D2 = 27.4cm), the

intermediate values of radial components of absolute velocities, and

relative velocities, corresponding to various radii, are calculated

proportionately.

o The values of corresponding to the established intermediate values

of Cm and w are calculated.



Tangent Arcs



Design of Vanes using

Tangent Arcs



Computed data for determining the passage width

for impeller



The dimensions of the designed impeller



Example

Design a volute to fit the impeller designed in previous example.



Solution



Tongue Radius

The volute starts at

the tongue with a radius rt,

which is 5 to 10% greater

than the impeller radius r2.

Therefore, tongue radius =

1.05 r2 to 1.10 r2, r2 =

13.7 cm. Hence, a value

of 14.5 cm may be used.



Design Sketch of

Volute

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Design of The Centrifugal Pumps...Design a closed impeller with one-side suction for a centrifugal pump, for a discharge capacity of 1.5 m3/min of water, at an operating head of 20