fail safe design and analysis for the guide vane of a hydro turbine

87 1–8 Ó The Authors 2016 DOI: 10.1177/1687814016658178 aime.sagepub.com Fail-safe design and analysis for the guide vane of a hydro turbine Bentang Arief Budiman1, Djoko Suharto1, Indra

Advances in Mechanical Engineering

2016, Vol 8(7) 1–8

Ó The Author(s) 2016 DOI: 10.1177/1687814016658178 aime.sagepub.com

Fail-safe design and analysis for the

guide vane of a hydro turbine

Bentang Arief Budiman1, Djoko Suharto1, Indra Djodikusumo1,

Muhammad Aziz2and Firman Bagja Juangsa3

Abstract

A design for the fail-safe mechanism of a guide vane in a Francis-type hydro turbine is proposed and analyzed The mechanism that is based on a shear pin as a sacrificial component was designed to remain simple Unlike the require-ments of conventional designs, a shear pin must be able to withstand static and dynamic loads but must fail under a cer-tain overload that could damage a guide vane An accurate load determination and selection of the shear pin material were demonstrated The static load for various opening angles of the guide vane were calculated using the computational fluid dynamics Fluent and finite element method Ansys programs Furthermore, simulations for overload and dynamic load due to the waterhammer phenomenon were also conducted The results of load calculations were used to select

an appropriate shear pin material Quasi-static shear tests were performed for two shear pins of aluminum alloy Al2024 subjected to different aging treatments (i.e artificial and natural aging) The test results indicated that the Al2024 treated

by natural aging is an appropriate material for a shear pin designed to function as a fail-safe mechanism for the guide vanes of a Francis-type hydro turbine

Keywords

Fail-safe design, guide vane, shear pin, hydro turbine, stress analysis

Date received: 25 March 2016; accepted: 13 June 2016

Academic Editor: Shun-Peng Zhu

Introduction

Fail-safe mechanisms have been designed for various

mechanical systems to reduce losses in terms of cost,

time, and human life and to reduce environmental

damage.1–3 A fail-safe mechanism should be simply

implemented in a system and be assured to function

properly In mechanical systems, shear pins are widely

used for the fail-safe mechanism For example, shear

pins are installed on gear trains, aircraft mounting

engines,4 valves,5 couplings,6 and flocculators.7 The

most challenging problem in analyzing shear pins is the

requirement to simultaneously fulfill two constraints in

their design A shear pin must be able to withstand

cer-tain operational loads but fail when an overload

condi-tion occurs

The failure of a shear pin is intended to avoid heavy damage to the system caused by operational error The design requirement of shear pins is unique because it differs from the conventional design requirement to only avoid failure under specific loads.8Moreover, the

1 Department of Mechanical Engineering, Institut Teknologi Bandung, Bandung, Indonesia

2 Institute of Innovative Research, Tokyo Institute of Technology, Tokyo, Japan

3 Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, Tokyo, Japan

Corresponding author:

Bentang Arief Budiman, Department of Mechanical Engineering, Institut Teknologi Bandung, Ganesha street no 10, Bandung 40132, Indonesia Email: bentang@ftmd.itb.ac.id

Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License

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loads imposed on shear pins in a mechanical system

depend on the shear pin position An exact

determina-tion of both static and dynamic loadings and proper

material selection for shear pins are required The

improper analysis and design of shear pins for fail-safe

mechanisms might result in heavy losses

Failures of a fail-safe mechanism were reported

fre-quently by a hydropower company (GREAT Co., Ltd.)

in which the shear pin was not fail during the overload

condition In particular, a guide vane used in a

Francis-typed hydro turbine was broken prior to a shear pin

failure because of a foreign object entering the water

flow The material used for the shear pin, which is

alu-minum subjected to artificial aging, is supposed to be

unsuitable for protecting the guide vane

This article aims to design and analyze a fail-safe

mechanism using a shear pin for the guide vane The

robust design of a fail-safe mechanism for

sustainabil-ity of a hydro plant is demonstrated The guide vane

must be reliable for several reasons such as:

 The function of the guide vane is vital to

con-trolling water flow before it enters the hydro

tur-bine.9 Uncontrolled water flow due to

malfunction of the guide vane can potentially

result in severe damage to other components,

such as the turbine and penstock

 Guide vanes are expensive because of their

com-plex shape

 The installation of guide vanes is relatively

com-plicated and time-intensive.10

 Hydro plants usually operate to supply electricity

to remote areas, which means their maintenance

and repair will be relatively difficult

In this study, we focus on the load determination for

both the guide vane and the shear pin These analyses

can be accurately calculated using commercial engi-neering software such as computational fluid dynamics (CFD) software and finite element method (FEM) soft-ware.4,11,12We used the CFD Fluent and FEM Ansys programs to calculate the loads corresponding to the opening angles of the guide vane In addition, the dynamic load imposed on the shear pins by the water-hammer phenomenon was analyzed Moreover, single-notched shear pins made of aluminum and subjected to different aging treatments, that is, artificial aging and natural aging, were tested to determine the most appro-priate material to be used as the shear pin

Shear pin fail-safe mechanism

A fail-safe mechanism requires a simple and reliable design to ensure that it functions properly Figure 1(a) shows a schematic of the movement of guide vanes in a Francis-type hydro turbine Notably, the number of guide vanes depends on the hydro turbine size In this study, 16 guide vanes used for a mini-scale turbine were analyzed The guide vanes are numbered from 1 to 16

in counterclockwise order The guide vanes rotate under torque, which is controlled by hydraulic pressure

on the system

A shear pin was installed in the arm rod of each guide vane The shear pin was designed as a sacrificial component that would break under heavy load The broken shear pin will release load transfer, hence avoid-ing damage to the guide vane This mechanism can reduce the cost and time required for repair because a shear pin is relatively cheaper than a guide vane Moreover, the installation of a new shear pin to replace the broken one does not require overall disassembly of the hydro turbine Figure 1(b) shows the dimension

of the shear pin Note that necking diameter of the

Figure 1 (a) Schematic fail-safe mechanism for guide vanes using hydraulic as driving force and (b) shear pin dimension with unit

of mm.

shear pin has small tolerance, which indicates that it

should be carefully manufactured

The following three categories of loads influencing

both the guide vane and the shear pin were

investi-gated: the static operating load, static overload, and

dynamic load A static operating load occurs when the

turbine is generating electricity under specific

condi-tions The load originates from water pressure imposed

onto the guide vanes A static overload occurs when a

foreign object impedes the rotational movement of the

guide vane It might occur as a consequence of a

prob-lem or failure in the filtration system of the water inlet

A dynamic load occurs because of waterhammer

Generally, waterhammer occurs when the fluid flowing

through the penstock is forced to suddenly stop or

change direction.13 Specifically, it occurs when the

guide vanes close as soon as possible to avoid a sudden

increase in the rotation speed of the hydro turbine due

to disconnection of electric power.14 The guide vane

must be able to withstand all three load scenarios,

whereas a shear pin must withstand static operational

load but fail under static overload or dynamic load

The details of the load scenarios and requirements for

guide vanes and shear pins are shown in Table 1

Investigation of the various loads imposed onto guide

vanes is the main consideration in the design of their

fail-safe mechanism

Load analysis of guide vanes

Static load case

A guide vane and shear pin must endure their opening

angle when the turbine is in operation and producing

electric power We modeled the case of static operating

load using the Fluent-CFD software This modeling

was intended to determine the static torques that act on

the guide vanes at various opening angles With guide

vane no 1 used as reference for the opening angle,

guide vanes were analyzed at opening angles of 15°,

30°, 45°, 60°, and 75° with respect to the horizontal line

(x-axis) Notably, an angle of 15° means the guide

vanes are fully opened, whereas an angle of 75° means

the guide vanes are totally closed

The k-epsilon standard flow model of the

Fluent-CFD software was selected on the basis of its

advantages This model describes the full flow of fluid but with a relatively short computation time In addi-tion, the k-epsilon model is also suitable for field condi-tions of water flow.15,16 A two-dimensional model of chasing turbine, static vane, and guide vane was cre-ated for a steady-state condition Moreover, a segre-gated implicit solver was selected The segresegre-gated implicit solver calculates the Reynolds-averaged Navier–Stokes (RANS) equations in stages and solves the equations separately.17

Figure 2 shows a typical result of the CFD Fluent model describing the pressure contour acting in a chas-ing turbine for a guide vane in the case of an openchas-ing angle of 30° A water discharging rate of 2.3 m3

s21was applied in the water inlet with a pipe of 0.8-m diameter The largest torque due to pressure and the friction force

of the water was received by guide vane no 13 of the turbine at an opening angle of 30° A maximum torque

of 649.9 N m in a clockwise direction was observed These results were obtained because guide vane no 13

is positioned in front of the water inlet and is therefore directly facing high water pressure The details of the torque in each guide vane are shown in Figure 3 A neg-ative sign of torque indicates torque applied in the clockwise direction All guide vanes and shear pins must be able to withstand these torques

Table 1 Load scenarios and requirements for guide vane and shear pin.

Static overload Withstand Fail When foreign object flows to water inlet of the turbine

Dynamic Withstand Fail Waterhammer phenomenon in load rejection case;

shear pin fails if closing time of the guide vane is less than the designed time

Figure 2 Pressure contour acting in the chasing turbine An opening angle of guide vanes is 30°.

Static overload case

For static overload analysis, a foreign object, such as a

stone or a wooden rod, is considered to flow into the

water inlet and inhibit the movement of the guide vane

The presence of this object prevents the guide vane

from completely closing However, the guide vane is

nonetheless continuously forced to close The resulting

static overload could result in failure and possibly

per-manent deformation of the guide vane The shear pin is

designed to fail to release the load before the guide

vane fails

The static overload model was conducted using the

FEM Ansys program integrated into the Autodesk

Inventor 2010 CAD software The base material of the

guide vane is martensitic stainless steel with a yield

strength (syield) of 551 MPa Fixed and pinned

con-straints were applied for certain guide vane surfaces A

pressure of 0.46 MPa, which is equivalent to the water

pressure under operating conditions, was also applied

to the guide vane A foreign object was represented by

an incident load imposing the guide vane edge This

edge is in farthest position to rotation axis of the guide

vane The edge position was selected in order to

gener-ate a highest torque with minimum incident load, which

is a worst case of static overload analysis

Figure 4 shows the FEM modeling result for static

overload case The guide vane was fail indicated by

safety factor of 1 when the incident load of 12 kN was

applied The von Mises failure criterion was employed

in the analysis The guide vane failure was predicted to

occur in the top of the guide of the vane shaft The

maximum torque imposed by the guide vane of

1135.37 N m in a clockwise direction was recorded on

the FEM modeling result

Dynamic load case

The waterhammer phenomenon causes the water flow

pressure to abruptly change.18 The resulting high

pressure can initiate the failure of both the guide vane and the penstock In the electrical load rejection case, the waterhammer effect can be reduced by extending the closing time of the guide vane which will reduce water-flow-speed alteration in the penstock The rela-tionship between the pressure alteration and water-flow-speed alteration can be expressed as a differential equation proposed by Joukowsky (see Ghidaoui et al.19,

Eq 1) as follows

dp

dt = ra

dC

where p is the water pressure, C is the water flow speed during opening or closing of the guide vane, and a is the speed of pressure waves that move along the pen-stock The speed of the pressure waves is calculated using equation (2)

a = ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

r 1

k + d Ee

where k is the bulk modulus of water, E is the modulus

of elasticity of the pipe, d is the pipe inner diameter, e

is the pipe thickness, and r is the specific mass of the water Note that the longer closing time can cause an uncontrollable increase in the speed and unbalance the rotor turbine The appropriate closing time should be selected with consideration of these constraints

The maximum head of the turbine during a certain closing time can also be obtained from the Allievi graphs with appropriate nondimensional parameters.19 The parameters are g, Z, and u, which are expressed as shown in equations (3)–(5), respectively

g= aC 2gH0

ð3Þ

u=aT 2L =

T

Figure 3 Torque acting in guide vanes for several opening

angles of guide vanes.

Figure 4 Guide vane modeling for overload case.

Z2=H0+ hmax

where g is gravitational acceleration, H0 is the initial

head, L is the pipe length, T is the closure time, m is the

critical time, and Hmaxis the maximum head

Table 2 shows the parameters required for

calcula-tion of the waterhammer phenomenon, as obtained

from GREAT Co., Ltd Figure 5 shows the curves of

head rise corresponding to closing time Notably, a

short closing time of the guide vane leads to a high

head rise, which means high pressure occurs The

clos-ing time must be carefully selected to avoid operational

error that could damage both the penstock and the

guide vane

Penstock pressure increases during the closing

pro-cess of the guide vane A rapid alteration of the pressure

and a high maximum pressure can cause a dynamic

loading, which might result in failures in the penstock

and the guide vane The type of the load can be

deter-mined through analysis of its strain rate.20 The strain

rate can be calculated using equation (6)

de

dt =

rgdHdtðd=2Þ

where dH/dt is the rate of head alteration Equation (7)

is used to determine the safety factor based on von Mises theory, where st, sa, and sr are the principle stresses for the penstock The results from the calcula-tions of the waterhammer effect on the penstock are shown in Table 3

s t s a

ð Þ 2 + s ð a s r Þ 2 + s ð t s r Þ 2

2

Notably, a low strain rate occurred in the penstock for each designed closing time, even though the dynamic load from the waterhammer effect was imposed The low strain rate indicates that a quasi-static analysis is sufficient Moreover, the safety factor

of the penstock is relatively high We used the CFD Fluent software to calculate the torque of guide vane during the closing time Dynamic head values of five opening angles, that is, 15°, 30°, 45°, 60°, and 75° were used as input data to determine the torque of the guide vane The closing process of the guide vane was assumed to occur at a constant speed Figure 6 shows the results of the modeling for guide vane no 13 which imposes highest torque

Material selection of shear pin The concept of a fail-safe mechanism is to sacrifice the shear pin to save the guide vane in the event of unex-pected conditions The static load scenarios for shear pin failure are described in Table 1 To satisfy the requirements, the boundaries of an allowable ultimate shear strength of the shear pin (tu) should be properly investigated The boundaries consist of a lower limit and an upper limit of tu The lower limit of tu was obtained by calculating the static operating load case The shear pin must be able to withstand the shear stress due to load transfer from the guide vane under a static operating load By contrast, the shear pin must fail before the shear stress exceeds the upper limit of t

Table 2 Input data for calculations of the waterhammer

phenomenon.

Gravitational acceleration (g) 9.8 m s22

Figure 5 Head alteration versus closing time of the guide vane.

Figure 6 Torque in a guide vane no 13 for several closing times.

The upper limit of tuwas obtained from calculations of

the static overload case when the guide vane begins to

fail or plastically deform

Upper and lower limits of tufor a static load

A free body diagram of the shear pin and guide vane is

shown in Figure 7 Shear stress in the shear pin was

transferred from the guide-vane torque The shear stress

was simply calculated using equation (8)

t=Fs

A =

MRcosa

where Fsis shear force, A is the cross-sectional area of the shear pin, R is the length between the center of the guide vane and the center of the shear pin (90 mm), a is the force angle, and r is the designed shear-pin radius The force angle a changes according to the opening or closing position angle of the guide vane A force angle

of 0° was used to obtain maximum shear stress in the shear pin in the shear stress calculations

On the basis of the guide vane torque results from the static operating load case, we calculated the shear stress in the shear pins using equation (6) The calcula-tion results are plotted in Figure 8 Guide vane no 13, which is subjected to a maximum torque of 647.9 N m

in the clockwise direction (Figure 3), exhibits the high-est shear stress in its shear pin (143.22 MPa) Therefore, this shear stress value becomes the lower limit of tu It

is reasonable because the guide vane and its shear pin were directly exposed by water inlet which causes high-est water pressure The upper limit of tu was deter-mined in the same manner as the lower limit of tuusing the maximum torque from the static overload case of 1135.37 N m (in the clockwise direction) Hence, the upper limit tu is 250.97 MPa On the basis of these results, the allowable tu due to the static loading is defined as the dark area in Figure 8

Table 3 The results of calculations of the strain rate and safety factor based on von Mises criteria.

Figure 7 Free body diagram of a guide vane and shear pin.

Figure 8 Allowable t due to static load.

Designed closing time of guide vane

On the basis of the guide vane torque results from the

dynamic load case, we again calculated the shear

stres-ses in the shear pins using equation (8) We then

deter-mined a new allowable tu by combining the analysis

results for static operating load, overload, and dynamic

loads due to the waterhammer effect The dynamic

load analysis only focuses on the shear pin no 13

because it imposes the highest shear stress during

nor-mal operation Figure 9 shows the

waterhammer-induced changes in shear stress on the shear pin no 13

for various closing times Notably, shear stress in the

shear pin increases dramatically when the closing time

of the guide vane is 2 s By contrast, for closing times

of 4, 7, and 10 s, the shear stress changes only slightly

The increase in speed of the turbine rotor and the

pressure rise in the penstock are the two constraints

that should be considered in determining the closing

time of a guide vane in the electric power rejection case

To avoid an increase in speed, the closing time of the

guide vane must be short In this case, 2 s of closing

time is considered as the suitable option

Unfortunately, the short closing time will cause small

band of shear stress, which means the shear pin should

be selected very strictly A slower closing time is also

required to prevent the pressure rise In this case, 10 s

of closing time appears to be more appropriate

After careful consideration of the aspects of both

speed and pressure increases, 4 s of closing time was

determined to be the best closing time that balances the

increases in both turbine speed and penstock pressure

The lower limit of tuis changed to 165.07 MPa and is

indicated by a dashed line in Figure 9 Thus, to save

the guide vane, the shear pin will fail if the closing time

is less than 4 s, as specified in Table 1 Note that on the

short closing time case, only shear pin no 13 fails and

causes guide vane no 13 to lose its control and leave

water flow to the turbine It reduces the pressure rise

but the turbine speed will not be out of control because

the water flow is considerably small, which is 1/16 of total water flow

Experimental validation

We conducted an experimental study to determine the suitable shear pin material for use as the sacrificial component The shear pin must have a tuin the range

of the allowable tu values A simple direct shear test was designed to conduct the validation test using a ten-sile testing machine equipped with a special shear-test fixture

Two types of specimens were subjected to different aging treatments and subsequently tested: aluminum alloy Al2024 with artificial aging (heated at 210°C for

5 h) and aluminum alloy Al2024 with natural aging (left

at room temperature) Three samples were tested for each type of specimen to observe the statistical varia-tion and consistency of the test procedure The results

of shear strength tests are shown in Figure 10 On the basis of the experimental tests, Al2024 with natural aging treatment is suitable for use as shear pins because

it exhibits an average shear strength of 186 MPa, which

is slightly higher than the lower tuof 165 MPa The tu

of the artificially aged Al2024 approaches the tuupper limit, which possibly cause the breakage of guide vane during overload condition

Conclusion

A design of the fail-safe mechanism for a guide vane of

a hydro turbine was studied and analyzed under vari-ous loading scenarios The loading calculations for the guide vane and shear pin were conducted using finite element software (CFD Fluent and FEM Ansys) to assure accuracy of the shear pin material selection According to calculations for static load and overload conditions, the allowable tuvalues are between 143.22 and 250.97 MPa In the case of a dynamic load from the waterhammer phenomenon, the lower limit of the

Figure 9 The allowable t u after combining three different load

cases.

Figure 10 Shear strengths of Al2024 shear pins subjected to artificial aging and natural aging treatments.

allowable tuincreases slightly to 165.07 MPa This

con-dition occurs when the closing time of the guide vane is

4 s Validation experiments indicated that Al2024

sub-jected to a natural aging treatment is a suitable shear

pin material, with an average tuof 186 MPa This study

focused on a mini-scale Francis-type hydro turbine with

16 guide vanes However, an identical procedure for the

design and analysis of the fail-safe mechanism could be

applied to a Francis-type hydro turbine of any scale In

addition, a grading design method for shear pin as a

sacrificed material of fail-safe mechanism might be

interesting as a future work

Acknowledgements

The authors would like to thank GREAT Co., Ltd., a hydro

turbine company, for providing the shear pin material,

techni-cal design, and valuable discussions.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with

respect to the research, authorship, and/or publication of this

article.

Funding

The author(s) received no financial support for the research,

authorship, and/or publication of this article.

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