SOLU HYDROELECTRIC PROJECT

Construction Pvt. Ltd.Tripureshwor - 11, KathmanduP.O. Box: 1223Tel.: 977-1-4243120, 4252124E-mail: cecon@mail.com.npWebsite: www.ce-construction.com

(23.5 MW)

SOLU HYDROELECTRIC PROJECT (23.5 MW)DETAILED ENGINEERING DESIGN REPORT

Hydraulic Steel Structural Analysis and Design Report

Contents

1.0) Low Pressure Penstock Pipe (LPPP)....................................................................................... 1

1.1) General……………………………………………………………………………………………..1

1.2) Hydraulic Design and Low Pressure Penstock Pipe Optimization……………………………

1.3) Mechanical Design of Low Pressure Penstock Pipe……………………………………………

1.3.1) Design Consideration……………………………………………………………………….1.3.2) Shell Thickness Calculation………………………………………………………………..1.3.3) Maximum Span of Unsupported Length………………………………………………….

1.4) Hydro mechanical Design………………………………………………………………………..

1.4.1) Design Consideration……………………………………………………………………….

2.0) High Pressure Penstock Pipe………………………………………………………………………….

2.1) General………………………………………………………………………………………………

2.2) Hydraulic Design and High Pressure Penstock Pipe Optimization……………………………

2.3) Water Hammer Calculation………………………………………………………………………..

2.4) Mechanical Design of High Pressure Penstock Pipe…………………………………………..2.4.1) Design Consideration………………………………………………………………………..2.4.2) Shell Thickness Calculation………………………………………………………………….

List of Tables:

Table 1.3………………………………………………………………………………………………………..Table 1.4…………………………………………………………………………………………………………Table 1.5…………………………………………………………………………………………………………

List of Figures:

Figure 1…………………………………………………………………………………………………………Figure 2…………………………………………………………………………………………………………

Annex:

Design calculations for LPPP………………………………………………………………………………..Design calculations for HPPP…………………………………………………………………………………

1.0 Low Pressure Penstock Pipe (LPPP)

1.1 General

Low pressure penstock pipe conveys discharge from head pond to surge tank. The LPPP is

designed to convey a discharge of 12.26 m³/s and proposed buried in most of sections except for

river crossing and near buildings at chainage around 2+950 m to 3+180 m where pipe is shifted

to valley side to avoid the existing building along the pipe line. As the section is shifted to valley

side, the LPPP is proposed exposed in this section with saddle supports at intermediate section

and anchor block at deflection point. In other sections LPPP is buried except in Kholsi crossings.

The maximum cut along the LPPP is extended to 12.77 m while the maximum backfill is about

9.84 m from the bottom of LPPP. The minimum cover of 1 m thick backfill is proposed as

insulation layer which will prevent thermal expansion and contraction due to change in

temperature at surface. The low pressure penstock pipe consists of manholes, straight pipes, bend

pipes, seepage rings, thrust rings and other necessary accessories.

1.2 Hydraulic Design and Low Pressure Penstock Pipe Optimization

The circular steel low pressure penstock pipe is optimized considering the cost of pipe

construction and revenue loss resulting from head loss. When the pipe size is increased, the

corresponding construction cost will be high but the loss of energy will be less. Similarly, when

pipe size is small the cost of pipe construction will be low but corresponding head loss will be

high. So the optimization of pipe is to find the balance between increased cost of construction

and reduced cost of head loss. Excel sheet was developed to optimize the pipe diameter which is

presented in the Vol. III report prepared by the Consultants. The optimized diameter of pipe is

worked out as 2.5 m. The same diameter of pipe is used for detail layout and calculations.

The friction loss in pipe is estimated by using Darcy-Weissbach equation:

Where,

f = friction factor

L = length of pipe

D = diameter of pipe

v = velocity of water

g = acceleration due to gravity

The friction factor f is calculated using Colebrook-White equation Where,

e = Colebrook-White e (mm) = 0.06 mm for pipe

Re = Reynolds number

As there are many bends in pipe, the bend loss is also substantial. The bend loss is estimated using following equation:

Where, k = bend coefficient depends on angle and radius of bend, for 90 degree bend R/D= 5, k is taken 0.08 as per USBR 1987 , for other bends reduction factor suggested in Hydraulic Design by Lysne, 2003 was applied. The number and angle of bends are estimated from the layout map presented in Vol. II-A, Drawing 1.7.A. The detail calculation is presented in Vol. III, Appendix A of the report prepared by the Consultants.

1.3 Mechanical Design of Low Pressure Penstock Pipe

The principle characteristics of the low pressure penstock pipe:

Type: Buried

Structure type of low pressure conveyance system: Steel pipe

Length of low pressure penstock pipe: 3860 m

Internal diameter: 2.5 m

The design of the steel pipe is governed by the internal pressure, external pressure and

minimum thickness required for handling.

1.3.1 Design Consideration

Highest upsurge level in the surge tank: El. 1997.49 m masl

Low pressure penstock pipe center line at the end: El. 1979.42 m masl

Full supply level at head pond: El. 1995.35 masl

Static head: 17.78 m

Maximum internal design pressure: 1.78 kg/cm²

Internal diameter of pipe: 2.5 m

External pressure: 2 kg/cm² (during dewatering)

Maximum height of backfilling: 7.5 meters

Material: Steel, JIS SM41B or equivalent

Allowable stress of steel: 1250 kg/cm²

Corrosion allowance: 2 mm

Design discharge: 12.26 m3/sec

Joint efficiency of welding: 0.9

1.3.2 Shell Thickness Calculation

The design of low pressure penstock pipe for internal pressure is computed by the following relationship:

t PD e st j = +2σ η

where, t is the thickness of the low pressure, ηj is the efficiency of welded joints and σst is the permissible stress in low pressure, P is design internal pressure and e is corrosion allowance.

The thickness of pipe is calculated assuming that 100% of the stress developed will be resisted by the pipe.

Table 1.3: Computation of Thickness of low pressure penstock pipe

Descriptions Low pressure penstock

pipe length(m)

Low pressure

pipe design

level (m)

High surge

level (m)

Static head (m)

Designinternalpressure(kg/cm2)

Thicknessof pipeforinternalpressure(mm)

Minimumthicknessforhandling(mm)

Thicknessadopted(mm)

LowPressure

Pipe

0 1991.25 1995.35 4.25 0.42 3 10.25 10

LowPressure

Pipe

3860 1979.42 1997.42 17.78 1.78 4 10.25 10

Minimum thickness has been calculated as 4 mm from the principles of hoop stress.

However, the thickness is also governed by other criteria such as buckling during erection.

The following formula which is also known as handling thickness has been applied to

estimate the minimum thickness: with D as diameter of pipe. Accordingly, minimum thickness as 10 mm has been found out.

The thickness required for withstanding external pressure of about 7.5 meters corresponding to height of backfilling above the pipe is less than the thickness required for handling.

The actual thickness of the low pressure penstock pipe is mainly governed by handling requirement, so taking this consideration into account results are depicted in Table 1.3.

1.3.3 Maximum Span of Unsupported Length

The maximum span of unsupported length between saddle supports is generally guided by the

deflection criteria. Even the deflection criteria permits the maximum unsupported length of 10.07

m, 8.50 m is adopted as unsupported length between two saddles which will ensure the structural

safety of low pressure penstock pipe if one saddle fails between two consecutive saddle supports.

The detail calculation is presented below.

Internal diameter of Pipe di 2.50 m

External diameters of Pipe di 2.52 m

Thickness of pipe t 0.010 m

Moment of inertia I 0.06 m4

Density of steel r 7850.00 kg/m³

Weight of steel Ws 621.47 kg/m

Weight of water in pipe Ww 4908.74 kg/m

Total weight W 5530.21 kg/m

Max. unsupported length l 17.26 m

Moment at mid span M 205853.03 kgf-m

Factored moment Mf 308779.55 kgf-m

Max. bending stress Sigma 621.54 kgf/cm²

Allowable stress 1250.00 kgf/cm²

Permissible deflection (L/65000 m)

Maximum unsupported length based on deflection criteria Ls 10.07 m

Adopted maximum unsupported length 8.50 m

1.4 Hydro mechanical Design

An inclined steel pipe portion of surge tank is designed for the dynamic load due the upsurge and

the down surge water level. The thickness of the steel surge tank is calculated for the internal

hoop stress and checked for the external overburden earth pressure (dry and submerged) at the

empty condition. An anchor block is designed at the base of steel pipe portion of surge tank

considering the force induced due to diversion of flow into the tank, weight of the tank, self-

weight and the overburden soil weight.

The design of the steel pipe is governed by the internal pressure, external pressure and minimum

thickness required for handling.

1.4.1Design Consideration

Design Parameters

Highest upsurge level in the surge tank: El. 1997.49 m masl

Full supply level at head pond: El. 1996 m masl

Maximum Internal Design pressure: 1.8 kg/cm2

Internal diameter of pipe: 4.8 m

External pressure: 2 kg/cm2 (During dewatering)

Material: Steel, JIS SM41B or equivalent

Allowable stress of steel: 1250 kg/cm2

Corrosion allowance: 2 mm

Joint Efficiency of Welding: 0.9

Table 1.4: Computation of Thickness of Steel Pipe Section of Surge Tank

Description Steel pipelength

(m)

Steel pipebottom

level (m)

Highsurge

level (m)

Statichead (m)

Max.design

internalpressure(kg/cm2)

Thicknessof pipe for

internalpressure

(mm)

Min.thickness

forhandling

(mm)

Thicknessadopted

(mm)

Steel Pipe 25 1990.57 1997.49 18.8 1.8 6 16 16

The same formulae, which are used for thickness calculation of LPPP, are used for the design of

surge-tank steel pipe. The thickness for hoop stress is calculated to be 6 mm but for handling

purpose the required thickness is calculated as 16 mm. The inputs and result for the calculation is

presented in Table 1.4.

2.0 High Pressure Penstock Pipe

2.1 General

A buried type high pressure penstock pipe is proposed to convey water under pressure to the

turbine. The foundation of HPPP is proposed mostly to the rock except for the portion near to

powerhouse. Hence the trench cutting depth is extended up to 17 m while the backfill varies

from minimum of 2 m to maximum of 14 m near powerhouse. The water management and slope

stability measures (drains grouted pitching and anchor breast walls) are proposed to stabilize the

backfill slope which is more elaborated in Chapter 4: Geological/Geotechnical Studies in the

report prepared by the Consultants.

The high pressure penstock pipe consists of straight pipes, bend pipes, bifurcation, reducing

pipes, seepage rings, thrust rings, drain pipes and all other necessary accessories. The inside

diameter of high pressure penstock pipe is 1.9 m up to the bifurcation. The thickness of the pipe

shell is designed to resist both the internal and external pressure and other loads. The thickness is

calculated taking into account of water hammer effects. The total length of 1.9 m dia. (which

starts from reducer) high pressure penstock pipe including bifurcation pipe is about 401 m.

Anchor blocks are provided at each change of direction in high pressure penstock pipe to provide

necessary weight to counteract the resultant to all forces and to transmit them safety of the

ground.

An ellipse type manhole of 500 mm by 600 mm in size will be provided on the high pressure

penstock pipe at downstream of emergency closing valve for inspection and maintenance of

pipes.

2.2 Hydraulic Design and High Pressure Penstock Pipe Optimization

The similar methods and procedures, which are used in the hydraulic design and optimization of

low pressure penstock pipe, are used for the design and optimization of high pressure penstock

pipe. The optimization study reveals the optimum diameter of high pressure penstock as 1.9 m.

The detail calculation is presented in Vol. III, Appendix A of the report prepared by the

Consultants.

2.3 Water Hammer Calculation

Allieve graphical solution has been used to estimate water hammer pressure for gradual closer.

The detail calculations is presented below:

Total length of penstock (Δx) L = 388.00 m

Design discharge Q = 12.26 m³/s

Diameter for penstock D = 1.90 m

Mean area A = 2.84 m2

Average flow velocity in penstock V = 4.32 m/sec

Maximum gross head H = 228.65 m

Bulk modulus of water k = 2.10E+09 N/m2

Modulus of elasticity of pipe material

E = 2.10E+11 N/m2

Assumed pipe thickness 25.80 mm

Pressure wave speed c = 1099.72 m/sec

Critical closure time Tc = 0.71 sec

Valve closure time Tv = 6.00 sec

Check Tv>Tc OK

θ 8.50

ρ 1.06

Z2 1.14

Water hammer head 32.01 m (14%)

Total design head 261.66 m

Therefore maximum internal water pressure for penstock design is estimated to be 26.17 kg/cm2.

2.4 Mechanical Design of High Pressure Penstock Pipe

The principle characteristics of the high pressure penstock pipe:

Type: buried type

Quantity: one lane

Length of high pressure penstock pipe: 388 m

Internal diameter: 1.9 m

Shell thickness: 9 mm to 28 mm

Bifurcation pipe

Diameter: 1.9 to 1.2 m, two lanes

End pipe diameter: 1.2 m (two lanes)

Shell thickness: 22 mm

Bifurcating angle: 90 degree

2.4.1 Design Consideration

The design of the steel pipe is governed by the internal pressure, external pressure and minimum

thickness required for handling.

Design Parameters:

Internal diameter: 1.9 m

Design discharge: 12.26 m3/sec

FSL at head pond: El.1995.35 masl

Turbine center level: El. 1764.594 masl

Highest upsurge level in the surge tank: El. 1997.49 masl

Net head: 218.66m

Static head: 228.65 m

Pressure rise head: 32.01 m (14% pressure rise)

Static head: 261.66 m (including water hammer)

Maximum internal water pressure: 26.17 kg/cm2

External pressure: 2 kg/cm2 (during dewatering)

Maximum height of backfilling: 14 meters

Material: steel, JIS SM41B or equivalent

Allowable stress: 1250 kg/cm2

Corrosion allowance: 2 mm

Welding efficiency: 90%

2.4.2 Shell Thickness Calculation

The same formulae and assumptions, which are used for LPPP, are used to calculate the shell

thickness of HPPP. The minimum thickness for handling requirement is computed as 9 mm. The

thickness required to withstanding external pressure of 10.5 meters corresponding to height of

backfilling above the pipe is less than the thickness required for handling at beginning of high

pressure pipe and internal pressure at remaining portions.

For inclined portion, the hoop stress is calculated considering linear distribution of pressure with

respect to the elevation. Hence the calculated thickness of HPPP is presented in Table 2.4.

Table 2.4: Computation of Thickness of HPPP at different Sections

Descriptions Accumulatedhigh pressurepipe length(m)

Pressurerise (m)

Statichead(m)

Totalinternalpressure(kg/cm2)

Thicknessof pipe forinternalpressure(mm)

Minimumthicknessforhandling(mm)

Thicknessadoptaded(mm)

Pipe 38 7.876 16.3 2.59 3.37 8.75 9PipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipePipe