pump volute 2009

8/19/2019 Pump Volute 2009

1/8

Experimental and FEM failure analysis and optimization

of a centrifugal-pump volute casing

Mona Golbabaei Asl a,*, Rouhollah Torabi b, S. Ahmad Nourbakhsh a

a Hydraulic Machinery Research Institute, Mechanical Engineering Department, Faculty of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran, Iranb EBARA Pumps Machinery Company (EPMC), P.O. Box: 15875-7653, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 21 September 2008

Received in revised form 17 January 2009

Accepted 2 February 2009

Available online 10 February 2009

Keywords:

Cast iron

Finite Element Analysis

Pump failures

Stress analysis

Structural optimizations

a b s t r a c t

Material and geometry are two key factors in ideal mechanical performance of centrifugal-

pump casings engaged in high pressures.

This paper presents the model generation, static structural analysis, and geometrical

modifications performed for a failed volute casing of a real centrifugal-pump. Failure would

be examined under hydrostatic test conditions. Finite Element Method is employed in

stage of theoretical problem investigation.

To control failure phenomenon, necessary geometrical modifications are applied to the

model. Geometrical modifications must have the least effect on hydraulic performance

and avoid excessive manufacturing costs.

Finally, some test volute casings with new geometry would be built to experimentally

validate the analytical results and inspect the hydraulic performance.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

The history of study and research about pump casing goes back to pump design. Pump and its components must have

reliable performance without any leakage. In particular Casing analysis is essential for special pumps in oil industry, aero-

space, power plants, etc. For instance, Rezvani et al. [1] executed structural analysis and evaluation of a mixer pump at the

Hanford Site. Rosu and Vasiliu [2] did research on the main components of a positive displacement pump by FEM.1 Shannon

[3] performed structural analysis for volute housing of high pressure oxidizer preburner pump in the space shuttle main en-

gine. Lienau and Welschinger [4] studied the deformation in HPDM pumps by FEM. They also optimized a large pump casing

with similar approaches [5].

Generally, optimum hydraulic design and proper material are two key factors for reliable pump performance. However,

material cost may burden their use for conventional pumps. The use of common cast iron has already been shown to reduce

fabrication cost. This is due to the fact that cast iron is low cost and offers a competitive manufacturing process in massproduction.

The casings of centrifugal-pumps are usually made of a volute shell with two nozzles for supplying and removing the

pumped fluid. The pump is divided along its peripheral volute path into areas with high and low flow capacity. The cross

area of the volute passage of pump casings at the periphery of impeller is determined from flow capacity and on the basis

of technological considerations, whereas, the wall has a constant thickness. It should be borne in mind that geometry of cas-

ings differs in details with pump type, number of stages, suction and discharge positions, and other parameters. The loading

conditions are determined from the internal pressure typical of the service conditions of hydraulic test. The strength of the

1350-6307/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2009.02.006

* Corresponding author. Tel.: +98 218 2084815; fax: +98 218 8338648.

E-mail addresses: [email protected], [email protected] (M. Golbabaei), [email protected] (R. Torabi), [email protected] (S.A. Nourbakhsh).1 Finite Element Method.

Engineering Failure Analysis 16 (2009) 1996–2003

Contents lists available at ScienceDirect

Engineering Failure Analysis

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a te / e n g f a i l a n a l

mailto:[email protected]:[email protected]:[email protected]:[email protected]://www.sciencedirect.com/science/journal/13506307http://www.elsevier.com/locate/engfailanalhttp://www.elsevier.com/locate/engfailanalhttp://www.sciencedirect.com/science/journal/13506307mailto:[email protected]:[email protected]:[email protected]:[email protected]

8/19/2019 Pump Volute 2009

2/8

casing depends not only on the casing load, geometrical parameters, and material, but also on the temperature of water

which characterizes its corrosion activity [6].

In this paper we focus on optimization of a real failed volute pump for its reliability. This is mainly due to the fact that

some extreme conditions of operation like corrosive fluid will lead to relatively high stress. This possibly makes the pump

unreliable to operate if the stress is beyond material strength due to brittle nature of cast iron. In order to ensure pump’s

high reliability, there is a need to predict the mechanical capability of pump casing and optimize its geometric dimensional

design. The results presented include comparisons of computational and experimental work.

2. Description of the pump

The volute casing belongs to a centrifugal-pump with single axial suction and an impeller with six vanes. The schematic

view of the centrifugal-pump studied here is shown in Fig. 1. When the pump is run at 2900 rpm, the best efficiency point

corresponds to 85 l/min flow rate and 28 m head. The volute-shaped segment of the casing has circular section. As presented

by Fig. 2, the volute is built by distributing its cross-sectional area on a base circle. Beginning at the tongue, the cross-sec-

tional area of the volute passage is zero but it increases with angle (h) in the direction of rotation, ending up at area ( Amax)

near discharge nozzle.

The outside diameter of impeller is 160 mm and the flow inlet diameter is 32 mm.

The centrifugal-pump casing is shown in Fig. 3 in an exploded view. It is mainly composed of two parts: one cast iron (GG

25) casing body and one steel (AISI 304) casing cover. The cast iron body is fitted to the steel casing cover and it is bonded to

suction pipe on the opposite side, using threaded connection. The casing body is also fixed onto the base plate in two points.

A photo of cast iron pump casing is shown in Fig. 4. The detected cases of cracking in pump casing in service for a period of 2000 h represented crack initiation in stress raiser zone, i.e. near discharge nozzle at interface of volute-shaped segment and

body of casing on suction side. Initiated crack propagates into the external surface of the casing through the interface in

peripheral direction away from the discharge nozzle.

It should be mentioned that cast iron (GG 25) is an inexpensive material used to manufacture low cost pumps. It is advis-

able to use a different material rather than GG 25 and a further investigation of the casing body material is needed. Never-

theless, this issue is outside the scope of current paper.

3. Optimization of the centrifugal-pump volute casing

As a general rule, the more corrosive the pumping fluid, the lower the component strength. This notion leads to centrif-

ugal-pump casing reliability deficiency if the stress and strain is beyond material strength. In order to ensure the pump

Fig. 1. Cross-sectional view of centrifugal-pump.

M. Golbabaei Asl et al. / Engineering Failure Analysis 16 (2009) 1996–2003 1997

8/19/2019 Pump Volute 2009

3/8

works reliably, following paragraphs will study the pump and then based on the study, it will obtain an optimized dimen-

sional design for this volute casing.

Fig. 2. Volute cross-sectional area distribution in centrifugal-pump.

Fig. 3. Geometrical features of volute casing.

Fig. 4. Photo of breakage in real parts.

1998 M. Golbabaei Asl et al. / Engineering Failure Analysis 16 (2009) 1996–2003

8/19/2019 Pump Volute 2009

4/8

3.1. Finite Element Analysis

Finite Element Analysis (FEA) was utilized to calculate casing mechanical capability under hydrostatic test. Here, reliabil-

ity means that the maximum stress and strain generated in the steel casing cover and the cast iron casing should be below

material strength limits. So the maximum stress in casing (rmax) was chosen as the criterion to measure mechanical capa-bility of the casing. Hydrostatic test conditions were established in accordance with American Petroleum Institute standard

(API 610). Although API 610 usually address several key aspects of petroleum, petrochemical, and gas industry process ser-

vices, for comparative study reasons, it can be employed as completely reliable criteria [7].

The geometrical parameters of volute casing model are shown in Fig. 5. The volute casing has an overall dimension of

182 mm in diameter and an approximate overall weight of 2.8 kg. It is significant to note that the volute casing is a single-

piece casting with no welded joint. The wall-thickness (t c ) for volute circular section was fixed at 6 mm. The largest circular

cross area ( Amax) near the discharge nozzle was designed to be fixed at 104 mm2. The wall-thickness for steel casing cover (t s)

was fixed at 2 mm. As the gasket between cover and casing is relatively thin, its thickness is ignored in all simulations.

Finite elements were used considering 3D stress and strain of cast iron in elastoplastic region. GG 25 refers to a group of

grey cast iron with a microstructure containing flakes of graphite. Sharp edges of graphite flakes also tend to concentrate

stresses, allowing cracks to form much more easily. Cast iron property input for this problem employed the failure model

presented by Hjelm [8,9]. The material properties and geometrical dimensions used in the model are listed in Table 1. Non-

linear static analyses were performed to determine the maximum failure-equivalent stress in cast iron casing.

The developed FE model of pump volute casing is shown in Fig. 6 with meshes. The model uses 10-node tetrahedral ele-

ments for volute casing and casing cover. The origin of the main cylindrical coordinate system is at center of the volute cas-

ing. Elements were sufficiently refined so that the analysis results became mesh independent. Simplifications were made in

simulation of non-critical sections such as bolt holes; whereas, elements in volute segment were set fine enough. Prior to

verification by experimental observations, a modal analysis was performed for the purpose of mesh validation.

3.2. FE analyses and results

3.2.1. Loading calculation

Gravity effect was neglected compared with high pressures engaged. According to API 610, hydrostatic pressure test for

the centrifugal-pump is given by Eq. (1) as follows:

Fig. 5. Geometrical parameters of volute casing model.


8/19/2019 Pump Volute 2009

5/8

P test ¼ P MAWP 1:5 ð1Þ

where P MAWP stands for the maximum allowable working pressure of the pump. The above equation means a hydrostatic test

pressure of 900 kPa for the volute casing. This load was applied at normal working temperature to the inside surface of the

volute casing.

3.2.2. Distribution of simulated stresses

With the dimensions listed in Table 1, maximum failure-equivalent stress of cast iron occurs at the interface of the volute-

shaped segment and casingbody and at an angular distance of 15 to dischargenozzle on suction sideof thecasing. Meanwhile,stress values remain far from the yield point in steel casing cover. Stress analysis results satisfactorily predict the location of

crack initiation as shown in Fig. 4. As presented by Table 2, a general comparison of various stress componentsfor volute casing

revealed that the axial stressr z and the radial stressrr are the major contributorsto the equivalent stress. Withthe knowledgegained from FEA results, it can be concluded that the optimized geometry of volute casing parameters must, in particular,

Fig. 6. Developed FE model of pump volute casing.

Table 2

FE analysis results for initial and modified volute casings.

Failure criteria Stress components Location of maximum equivalent stress

Maximum equivalent

stress re ðMPaÞMaximum radial

stress rr ðMPaÞMaximum tangential

stress rh ðMPaÞMaximum axial

stress r z ðMPaÞr ðmmÞ h (degrees) z ðmmÞ

Initial volute

casing

324 319 307 339 85 75 5

Optimized volute

casing

305 281 236 296 82 90 6

Table 1

Material properties and geometrical dimensions.

Steel casing cover dimensions Cast iron casing body dimensions

Outside diameter, ds (mm) 180 Outside diameter, ds (mm) 182

Thickness, t c (mm) 2 Volute thickness, t c (mm) 6

Steel casing cover Cast-iron casing body

Young’s modulus, E c (GPa) 193 Young’s modulus, E c (GPa) 118

Poisson’s ratio, ms 0.3 Poisson’s plastic ratio, m p,c 0.43Density, qs kg/m

3 8000 Density, qc kg/m3 7200

Tensile yield stress, rt (MPa) 450 Tensile yield stress, rt (MPa) 228Tensile ultimate strength, S t (MPa) 655 Tensile ultimate strength, S t (MPa) 350


8/19/2019 Pump Volute 2009

6/8

include structural improvements in wall-thickness of volute-shaped segment near discharge nozzle. In order to keep casting

and fabrication process as simple as before, it was determined to increase the inner wall-thickness of the volute-shaped seg-

ment. Hence, the wall-thickness increase starts from 2 mm at discharge nozzle and gradually ends to zero at the angular posi-

tionof 90. Since geometrical modifications led to thevolute cross areadecrease, a hydraulic testwas carriedout to examinethe

overall performance of the centrifugal-pump with new volute casing. Another FE analysis was performed for the optimized

geometry as well and the results are summarized in Table 2. It can be seen that the maximum equivalent stress occured at

the same position near discharge nozzle but the value has decreased up to 7% compared with the initial model.

4. Centrifugal-pump testing

After obtaining the optimized geometry parameters in the simulation, six new volute casings were cast and fabricated. In

order to verify mechanical capability of casings, hydrostatic pressures of crack initiation were measured by the use of a test

rig as shown in Fig. 7. Schematic view of a typical test rig is presented in Fig. 8. A primary pump was utilized to provide nec-

Fig. 7. Hydrostatic test rig.

Fig. 8. Schematic view of hydrostatic test rig.

Table 3

Hydrostatic test results for six sample volute casings with optimized geometry.

Number of failed casings 1 1 1 1 2

Hydrostatic test pressure (kPa) 1600 1850 2000 2250 above 2500


8/19/2019 Pump Volute 2009

7/8

essary hydrostatic test pressures. Suction nozzle of casing was connected to the discharge nozzle of the primary pump using

suitable pipe and fittings. A pressure gauge and a butterfly valve were installed on the discharge of the casing to adjust the

applied pressure. It was seen that crack initiated in volute casings in pressures much greater than previous pressure and the

crack trend satisfactorily agrees with simulation results in Section 3. Test results are summarized in Table 3 and it can be

observed that failure did not occur in two volute casings even at hydrostatic pressures of 2500 kPa. To supply such high pres-

sures, the centrifugal primary pump was replaced with a reciprocating pump. Table 4 gives the working life of initial and

optimized volute casings.

It should be mentioned that there are some differences between the simulated and tested results in mechanical strength

comparisons. The possible reasons are that:

1. Measurement of FE cast iron parameters may vary from actual pump parameters.

2. FE analysis genuinely includes some approximations.

3. A nonlinear FE analysis was performed in elastoplastic region of cast iron.

Once acceptable mechanical capability was obtained, optimized casings were examined under hydraulic performance

test. Fig. 9 represents the influence of wall-thickness increase on pump performance. From this figure, it can be seen that

at pump rated point, pressure/head (H ) drop is less than 2%. Hence, overall pump efficiency decrease can be neglected. In

conclusion, test results revealed a good hydraulic performance as well as enhanced mechanical capability.

5. Conclusions

An optimized design of the pump volute casing with respect to critical structural regions has been obtained. To be sure

about the simulation results some volute casings with new dimensions were fabricated and tested. The hydrostatic test re-

sults show that casings with such optimized dimensions have high mechanical capability. The hydraulic test results proved

that the overall performance and efficiency decrease due to changes in hydraulic design of the volute-shaped segment are

negligible. Furthermore, modifications in casing can be easily done in manufacturing line which do not impose further costs.

References

[1] Rezvani MA, Strehlow JP, Baliga R. Structural analysis and evolution of a mixer pump in a double-shell tank at the hanford site. In: 4th Department of

energy natural phenomena hazards mitigation conference; 1993.

[2] Rosu C, Vasiliu N. Researches on the main components of a positive displacement pump by FEM. In: The 2nd FPNI – PhD symposium; 2002.

[3] Shannon R. Space shuttle main engine structural analysis and data reduction evaluation. High pressure oxidizer turbo-pump preburner pump housing

stress analysis report; 2005.

Table 4

Typical working life in corrosive water services for centrifugal-pumps with initial and optimized volute casings.

Hydrostatic test pressure ðkPaÞ Working life before failure of volute casings

(h)

Centrifugal-pump before casing

optimization

600 (equal to maximum allowable working pressure) 2000

Centrifugal-pump after casing

optimization

900 (equal to 1.5 times maximum allowable working

pressure)

10,000

Fig. 9. Pump performance curve in initial and optimized casings.


http://-/?-http://-/?-

8/19/2019 Pump Volute 2009

8/8

[4] Lienau W, Welschinger Th. Early optimization of large water transport pump casing. Sulzer Tech Rev 2005.

[5] Lienau W, Welschinger Th. Finite element testing – a solution to deformation in HPDM pumps. World Pump; 2006.

[6] Nourbakhsh A, Jaumotte A, Hirsch Ch, Parizi H. Turbopumps and pumping systems. Springer; 2007.

[7] ANSI/API Standard 610. Centrifugal pumps for petroleum, petrochemical and natural gas industries. American Petroleum Institute; 2004.

[8] Hjelm HE. Yield surface for gray cast iron under biaxial stress. J Eng Mater Technol 1994;116:148–54.

[9] Chen WF, Han DJ. Plasticity for structural engineers. New York: Springer-Verlag; 1988.


pump volute 2009

Documents