GMSARN International Conference on Sustainable Development: Issues and Prospects for the GMS
12-14 Nov. 2008
Design and Construction of Single Stage Centrifugal Pump (Impeller) Khin Maung Htwe
Abstract— Abstract— This paper deals with the application and use of centrifugal pumps that are universal today. In the beginning of this paper, the introduction to centrifugal pump is briefly described. There are many components in this pump. So, the components and their functions are mentioned. Next, the review of centrifugal pump is described. According to theory, a centrifugal pump consists essentially of a runner or an impeller which carries number of backward curved vanes and rotates in a casing. Liquid enters the pump at the center and work is done on it as it passes centrifugally outwards so that it leaves the impeller with velocity and increased pressure. From this theory, theoretical head and slip are obtained. In this paper, the impeller is designed to transport water and this design is considered by using Kyushu Method based on flow rate of 3 0.25m /min, head of 16m and motor speed of 3000 rpm. The outlet blade angle is assumed 22.5°, too. Moreover, the impeller dimensions from the mentioned method are gotten by executing MathCAD software. From these dimensions, isometric drawing and 3D drawing of impeller are produced. Keywords— Keywords— Centrifugal pump, Impeller, Kyushu method, Pressure.
1.
INTRODUCTION
The centrifugal pump was developed in Europe in the late 1600’s and was seen in the United States in the early 1800’s. It has been occurred in widely used only in the last seventy-five years. Prior to that time, the vast majority of pumping applications involved positive displacements pumps [2]. The increased popularity of centrifugal pumps is due largely to the comparatively recent development of high speed electric motors, steam turbines, and internal combustion engines. The centrifugal pump is a relatively high speed machine and the development of high speed drivers has made possible the development of compact, efficient pumps. A pump is device which lifts water from a lower level to a higher level at the expense of mechanical energy. Thus, a pump is a p ower absorbing device used to increase the pressure energy of liquid. The pressure energy is increased by creating a region of low pressure (usually lower than the atmospheric pressure) near the inlet of the pump and a higher pressure at the outlet of the pump. Due to this low inlet pressure, the liquid rises to the pump from a low level reservoir (sump) and the high pressure created inside the pump forces the liquid out of the pump to be delivered to the reservoir [2] . Pumping may be defined as the addition of energy to a fluid to move it from one point to another. The energy given to the pump causes the fluid to do work flowing through the pipe rising to higher level. They can also be used for pumping liquid from a higher level to a lower one, as well as lower level to a higher level, of very high hydraulic resistance. They operate by creating a pressure
Khin Maung Htwe is with the Mandalay Technological University, Mandalay, Myanmar. E-Mail:
[email protected] [email protected]..
difference between the suction side and delivery side of the moving element of the pump, such as an impeller. A pump transfer mechanical energy from some external source to the liquid flowing through it. The pump thus increases the energy of the liquid which may then be used to lift the liquid and to overcome the hydraulic resistance of the delivery pipe. An arrangement consisting of a suction pipe, a pump and a delivery pipe is called a pumping system. The pump which rises water or a liquid from a lower level to a higher level by centrifugal force is known as a centrifugal pump as shown in Figure 1. Centrifugal force is defined as the action that causes something to move away from its center of rotation. A centrifugal pump consists of a shaft and impeller rotating within a casting. An impeller is a rotating disk with a set of vanes coupled to the engine or motor shaft that produces centrifugal force within the pump casing. A casing is the stationary housing that collect, discharge and recirculates water entering the pump. The impeller and casing from the heart of the pump and help the determination of its flow, pressure and solid handing capability [3].
Fig.1. Centrifugal Pump [3]
1
2.
FLOW CHART FOR PUMP DESIGN
Check D1m = D1 < D0 Start Input: β2 Input n, Q, H Calculate: β1 Calculate: ns, Qs Calculate: Z No: of blade.
Calculate: η0, ηv
Input: δ1, δ2 Blade widths
Calculate: Qs1
Calculate S1, S2
“Shaft diameter” Input: Ksh
Calculate b1, b2 Calculate: d c, dsh STOP
hub: dimension Calculate: dh, lh “Impeller eye diameter” Input: g, Kmo
Calculate: D0
Fig.2. Flow Chart
3.
DESIGN DATA
In designing, flow rate, head, shaft speed and motor are main input data. Again, allowable factor f a, transmission efficiency, overall efficiency, velocity coefficients are as assignment data. Table 1 shows the specification for the impeller design. Table 1. Specification for Impeller Design
“Impeller Outlet diameter” PARAMETERS
Input: Ku, Km1, Km2, D1 /D2
Calculate: u2, cm1, cm2
Calculate: D2
Calculate: D1 “Impeller Inlet diameter” Calculate D1s, D1h, D1m
Rated power (Lr)
1.5 kW
Flow rate (Q)
0.25 m /min
Rotational speed (n)
3000 rpm
Density of water (ρ ( ρ)
10 kg/m
Pump head (H)
16 m
4.
3
3
3
IMPELLER DESIGN ANALYSIS
The impeller of the centrifugal pump is designed in this paper. For this design, the rated output power (L r) of electric motor is caclulated in Equation 1. This is expressed as:
Lr =
(1 + f a )L η tr × 1000
(1)
where, L is shaft power, ηtr is transmission efficiency, and
2
f a is allowable factor. The allowable factor is read in Table 2 and ηtr is 1.0 for direct coupling. The specific speed value (n s) is evaluated in Equation 2 [1].
ns
Q
n
=
1 2
(2)
H 3 / 4
where, Q is flow rate, n is rotational speed, and H is pump head. Then the shaft power (L) is determined as:
L=
ρQ s gH
(3)
η0
where, Qs is flow rate per second, g is gravitational acceleration, and η0 is overall efficiency. Table 2. Allowance Factor and Rated Output Pow er of Electric Motor [1]
Lr (kW)
0.4
f a
0.75
1.5
2.2
0.4
3.7
π Vd
=
(4)
D s2
π
D
(5)
2 d
(6)
n
where, dc is diameter of the end of main shaft, and k sh sh is permissible shear stress factor. The dimensions of hub at the impeller eye are usually decided from Equations 7 and 8.
Dh = (1.5 to 2.0) d sh
(7)
Lh= (1.0 to 2.0) dsh
(8)
where, Dh is hub diameter, d sh is shaft diameter at hub section, and Lh is hub length. Then the diameter of impeller eye (D0) is evaluated in Equation 9.
4Q s'
D0 =
π Vmo
+ D 2h
(9) ' Q s is total
Vmo = K mo 2gH
(10)
Kmo = (0.07 to 0.11) + 0.00023 n s
(11)
where, Kmo is velocity coefficient of impeller eye. And then the volumetric efficiency (η (ηv) is determined as:
1
ηv = 1+
1.124
4 Qs
Lr
flow rate.
A pump efficiency (η0) is assumed by using Figure 3. And also the diameter of suction pipe (D S or ds) can be estimated from this chart. The discharge pipe diameter (Dd or dd) is usually selected equal to or one size smaller than that of the suction pipe. Thus, velocities in these pipes are given by:
Vs =
3
where, Vmo is velocity of impeller eye, and
0.4 to 0.25
Qs
d c = k sh
4
Shaft material is chosen as S30c material that permissible shear stress factor is 0.125. Suction Pipe Diameter Ds
) % (
Q s' =
(12)
n s2 3
Qs
(13)
ηv
The stepanoff chart shown in Figure 4 is widely used to decide the impeller geometry. Peripheral velocity at outlet is presented as:
U 2 = K u 2gH
(14)
Vm1 = K m1 2gH
(15)
Vm2 = K m2 2gH
(16)
h
y c n e i c i f f E p m u P
where, Ku is velocity coefficient of impeller outlet, V m1 is meridional velocity at impeller profile entrance, V m2 is meridional velocity at impeller profile outlet, K m1 is design speed constant at impeller profile entrance, and Km2 is design speed constant at impeller profile outlet. The outlet diameter (D2) and the inlet diameter (D1) of the impeller are decided considering the following relationship.
D2 = 3
Discharge Q (m /min) Fig.3. Overall Efficiency Curve [1]
Hub dimension can be calculated based on shaft dimensions.
U 2 × 60 π×n
D 2 D 1
D1 = D 2
(17)
(18)
Then the peripheral velocity at inlet is designated by:
3
U1 =
πD1n 60
S1 =
(20)
Qs' πD 2 b2 = πD 2 Vm2 πD 2 − S2 Z
The blade angle at inlet (β ( β1) is:
K V β1 = tan b1 m1 U1 −1
The number of impeller blades is determined as the following:
D + D1 β1 + β 2 sin Z = 6.5 2 D D 2 − 1 2
δ1
(19)
(21)
where, Z is number of blades, β2 is impeller outlet blade angle, and K b1 is coefficient factor. When the impeller is made of bronze, the minimum blade thickness is 2.0 mm and shroud thickness is 2.5 mm for an i mpeller having the diameter less than 200 mm. mm. They are 2.5 and 3.0 mm respectively if D 2 is greater than 200 mm.
S2 =
(23)
sin β1
(24)
δ2
(25)
sin β 2
where, δ1 is blade thickness near the leading edge, δ2 is blade thickness near the trailing edge, S 1 is blade circumferentrial thickness at inlet, and S 2 is blade circumferential thickness at outlet. A method to draw the impeller blade by three circular arcs shown in Figure 5 is used for the present design. Each radius is given by the corresponding equations.
(R
2 A
− R 2B ) ρA = 2(R A cos β 2 − R Bcos β B )
(R
2 B
(R
2 C
(26)
− R C2 ) ρB = 2(R Bcos β B − R C cos β C )
(27)
− R D2 ) ρC = 2(R C cos β C − R D cos β1 )
u
K
5.
(28)
RESULTS FOR IMPELLER DESIGN
Dimensions calculated by Kyushu method are described in Table 3. Moreover, this design calculation procedure is executed by MathCAD software program in Figure 2. D2 = 60 Ku
Table 3. Dimensions of Centrifugal Pump
2gH /(π ⋅ n )
2
RESULTS
D / 1 D
Vm1 = K m1
2gH
Vm 2 = K m1
2gH
Impeller outlet diameter (mm)
119
Impeller inlet diameter (mm)
54
Impeller eye diameter (mm)
53
Outlet blade angle
1 m
K
22.5
Inlet blade angle
22
Impeller outlet width (mm)
7
Impeller inlet width (mm)
12
Impeller hub diameter (mm)
18
Impeller hub length (mm)
24
Impeller passage width at inlet (mm)
12
Impeller passage width at outlet (mm)
7
Number of blade
6
2 m
K 3
ns (rpm, m /min,m) = n Q / H 3 / 4 Fig.4. Stepanoff Chart [1]
The width at the inlet (b1) and that of outlet (b 2) are respectively decided based on Equations 22 where smooth variation in velocity is considered.
b1 =
Q s'
πD1
πD1Vm1
πD1 - S1Z
(22)
4
Fig.8. Sand Pattern for I mpeller Fig.5. Schematic of Impeller Blade [4]
6.
FABRICATION PROCESSES
In the fabrication of the impeller, 1. Firstly, the design parameters of the impeller are calculated by Kyushu method. 2. Secondly, the machining drawing is made for impeller pattern. 3. Thirdly, sand mold is made for the impeller pattern. 4. Fourthly, the raw impeller is cast and machining process and hardness test are becomed. 5. Finally, the installation process and running operation are made on pump casing. Fig.9. Preparation of the Sand Mold
Fig.10. Ready for Pouring Fig.6. Impeller Drawing
Fig.11. Pouring of Molten Metal Fig.7. Dimensional View of Impeller
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suction or double-suction. In this thesis, single-suction centrifugal pump impeller is designed by using Kyushu method and executing MathCAD software program. When this program is written, flow rate, head, shaft speed and motor are main main input data. Again, allowable allowable factor f a, transmission efficiency, overall efficiency, velocity coefficients are as assignment data. The output data are impeller’s outlet and input diameters, blade angles, widths and blade number. 8.
Fig.12. After Pouring
Fig.13. Cast Impeller
CONCLUSION
In this thesis, the centrifugal pump is end-suction single stage pump. Impeller designed should be in such a way that, losses must be as low as possible. Major losses are disk friction losses and leakage losses. Leakage loss decreases rapidly with increasing specific speed. Leakage loss is low for closed impeller. So, the Impeller is closed impeller. The blade outlet angles for centrifugal pumps are backwards (b2<90°). (b2< 90°) which gives the decrease in head with increasing discharge is commonly adopted at the design of a centrifugal pump. Centrifugal 3 pump has the capacity of 0.25 m / min and 14m of head. The power of 1.5kW and efficiency is 0.58 (58%). The inlet diameter of impeller is 54 mm and the outlet diameter is 119 mm. The number of blade is 6 blades. The width at the inlet b 1 and that of outlet b 2 are 12mm and 7mm. The impeller design of lab-scale centrifugal pump in M.T.U Hydraulic laboratory is calculated by Kyushu method. By using this method, the existing dimensions of this impeller are the same as the calculated dimensions by adjusting the coefficients factors within their limits. Although the impeller can be fabricated, the characteristic testing cannot be made. Therefore, the author wishes to new generation of mechanical engineering students who are interested in the fluid field to make an experiment for that impeller design. This design can be employed not only for honoree but also agricultural and industrial sectors. The design calculation is based on Kyushu design. Therefore it is believed that this design can keep abreast with the international standards. The require material and thing are early available in the local areas. It should be continued by engineers who are expert in design by using methods. ACKNOWLEDGEMENTS
Fig.14. Impeller after Machining
7.
The author greatfully acknowledges the financial support by the Ministry of Science and Technology, Myanmar to carry out this research. The author would like to express his special thanks to Dr. Khin Maung Aye of West Yangon Technological University, Yangon, Myanmar. NOMENCLATURE
DISCUSSIONS
The application and use of centrifugal pumps today are universal. Modern public utilities, chemical plants, municipal gas, water and swage works, and other fields too numerous to mention would be seriously handicapped if these machines did not exist. In today’s competitive world, pump designers are tempted to just quickly copy existing pumps, or fall back on empirical design formulas established by statistical surveys of existing pumps. Statistically average designs may be adequate. A centrifugal pump consists essentially of one or more impellers equipped with vanes, mounted on a rotating shaft and enclosed by casing. Impeller may be single-
b1 b2 D0 D1 D2 Dh Dd Ds Dsh Ksh Ku dsh
Inlet width, mm Outlet width, mm Diameter of impeller eye, mm Inlet diameter, mm Outlet diameter, mm Diameter of hub, mm Diameter of discharge, mm Diameter of suction, mm Diameter of shaft, mm Factor Speed constant Shaft diameter, mm
6
H L Lh Lr Ns N Q U1 U2 V Vmo Z
β1 β2 ρ δ1 δ2
Pump Head, m Input power, kW Length of Head, m Rate output of an electric motor, kW Specific speed, rps Speed, rpm 3 Pump Discharge, m /min Peripheral velocity at the inlet, m/s Peripheral velocity at the outlet, m/s Velocity of flow, m/s Velocity at the eye section, m/s Blade number Blade inlet angle, degree Blade outlet angle, degree 3 Density of water, kg/m Blade thickness near the leading edge, mm Blade thickness near the trailing edge, mm REFERENCES
[1]
[2] [3] [4]
Kyushu Institute Technology, 1996, Training Course, “Fluid Mechanics of Turbomachinery”, Japan: Kyushu Institute of Technology. Austin .H .Church, 1972, “Centrifugal Pump and Blowers”, John Wiley and Sons, Inc, New York. Igor. J. K, William. C. K, Warren. H. F and Joseph. P. M, M, 1976, “Pump Handbook”. Touzson. J, 2000, “Centrifugal Pump Design”, John Wiley and Sons, Inc.
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