buguias minihydro project structural design

SECTION 3: PROJECT DESIGN

Project Design

Introduction

Proposed Project Layout

The Buguias I Mini-hydro potential has a gross head of 152 meters. A low diversion weir will be constructed to divert the river flow into a 11 OOmm diameter low pressure headrace approximately seven hundred fifty meters long. A johnson type surge tank with 4 meters diameter and 10 meter high will be used at the end of the low pressure headrace, before the beginning of the penstock pipe.

A penstock pipe approximately 513 meters long will be laid on the mountain slope to convey pressurized water to the turbines. The penstock, with thickness ranging from 6 mm at the top to 16 mm thick at the bottom part, has a general diameter of 813mm. A portion of the penstock pipe will be buried where it traverses vegetable gardens and terraces. An asphaltic coating will be applied on the outer surface of the steel pipe to prevent COITOSIOn.

The power house, of concrete construction will house two (2) cross-flow type turbine units. Its inside dimensions are 8m wide by 16meters long. It is also equipped with a three-ton capacity overhead crane.

The two turbines will be installed driving synchronous alternators of750 kW capacity each. At the rated head of 145 meters, for the three turbine setting, each turbine has a rated flow of 700 liters per second (Ips) or a total required flow of 1,400 Ips.

The step-up transformers, 0.44/23kV, with a total rated capacity of 3 x 666kVa will be installed in the open air. The proposed transmission line tapping points for the interconnection of the proposed generating plant is the BENECO transmission line along the Halsema National Highway.

[Buguias I Mini Hydropower Project] Page 3-1

Project Design

Diversion Weir and Desilting Basin

Design Considerations

The Buguias I MHP dam or diversion weir is built primarily to raise the water level to the required elevation to allow entrance of water into the headrace entrance. .

The dam must be relatively impervious to water and capable of resisting the forces acting on it. It must be statically and dynamically stable, and the design must be such that stresses in the concrete do not exceed allowable limits.

In selecting the dam site, the following river characteristic were observed and considered;

The dam is located in a long uniform and straight reach of river. The foundation is sufficiently stable to sustain the weight of the dam

and should be nearly impervious. The river banks are firm and stable to provide good anchorage of the

dam abutments. The site is at the narrower part of the river but sufficiently wide

enough such that the ogee spillway will have adequate capacity to discharge the design flood at not too high level.

Suitable construction materials are available in the vicinity of the job site.

To address the expected severe erosion at the toe of the dam, an apron will be provided together with the formation of hydraulic jumps by providing chute blocks.

Structural Stability

Figure below is a free-body diagram of a section of a gravity dam for the purpose of analysis.


Project Design

Free-Body Diagram: Gravity Dam (Spillway Section)

Horizontal Loads

Headwater (Ht):

Hflood = m (assumed flood water level at elevation 122 masl)

Hsilt = 32m (assumed to be filled up to sluice gate level)

Tailwater (Hs) This effect will be disregarded

Seismic Force ~)-

HSF = Horizontal accelerations assumed 0.5g

Vertical Loads

Weight of Dam (V1)- The unit weight of material in the dam is determined as accurately as possible.

Unit wt of Concrete= 2,400 kg/cu.m

Unit wt of Rubble Masonry = 2,300 kg/cu.m


Project Design

Vertical Water Loading (V2)- Imposed on any sloping surface of the dam, usually the upstream face, but also on the downstream for overflow dams.

Uplift (V 3)- Hydrostatic forces acting within a dam and its foundations including interstitial or pore pressures. Drainages will be used to prevent occurence of uplift, assuming the drainage will be effective for the entire life on the dam, therefore some inclusion for uplift must be included in the design. See diagram for distribution of pressure. [k values vary between 0.25 to 0.50 depending on conditions.]

Seismic Force (V4)- Force acting on dam in vertical plane.

Other Loads to be Considered

Direction of Forces - The direction of resultant forces is important for gravity and buttress dams - especially on stratified rock.

Hydrostatic Loading within the Foundation or Abutment- Faults, cracks and joints are present in most damsites. Forces due to a dam may cause cracks to appear in the rock upstream from the dam, this may cause jacking loads that could cause failure. To avoid this, careful surveys should be made of the orientation and inclination of faults, joints and cracks.

s Fsf=-L:H

Fss= (L:H/L:V) -tan a!

1+ (L:H/L:V)tana!

cA s = ----- +L:Vtan(cf>+a!) COSO!(l-tancf>tanO!)

Stability Safeguards of a Dam


Project Design

A gravity dam or weir must be designed to safeguard against overturning and sliding. For the former it is usual to design the dam so that the resultant of all forces intersects the base within its middle third. This will provide a factor of safety in excess of 2.

The ratio of the sum of the horizontal forces to the sum of the vertical forces is referred to as the sliding factor (Fss). This is usually about 0.75 but must not exceed 0.90 under extreme loading. These figures represent the range of the coefficient of static friction normally encountered at the site of a gravity dam.

At or in the foundations, the horizontal loading will be resisted by cohesion and friction. The ratio of the total resistance by cohesion and friction to the horizontal load is termed the shear friction factor (Fsf). Most countries accept 4 as a minimum value. In practice the foundation is usually prepared in steps or is sloped upward in a downstream direction to provide resistance to failure far in excess of the above figure.

Range of shearing resistance parameters.

Location of plane of shearing/sliding Cohesion (c) Friction tan o mass concrete intact 1.5-3.5 1.0-1.5 mass concrete horizontal construction joint conrete/rock interface rock mass sound rock mass inferior

0.8-2.5 1.0-3.0 1.0-3.0

Project Design

Design of Spillway

The discharge over a spillway crest is given by the formula:

Q=C.L.H312

where Q=discharge, C=coefficient, L=length of the crest, H=effective head of water.

Crest Profile the crest of an overfall "' ~ spillway is usually dimensioned to conform to the underside of the nappe of the free-falling jet. Greater efficiency is obtained by operating a spillway at greater than design head, as can be seen in the figure showing the effect of nappe profile on coefficient.

I , f.-----------,

o 0 .2 o.~ o.a o.a 1.0 1.-2 t4 l{e:sd on trl!'t1 r:r~lic- HeQd lllot Iii' r'IOI)~e

It is common practice to choose the design head for the nappe as 75o/o-80% of the maximum expected head. When the spillway so designed does pass the greater flows, pressures lower than atmosphere will occur over the crest, causing problems associated with cavitation.

The flow over a spillway gives rise to self-excited vibration, in which three coupled elements are involved; the jet, the overflow crest and the air cushion between dam and jet. This can be avoided by using splitters on the crest. The passage of flood waters from upper level to lower level will involve the dissipation of vast energy. The velocities and pressures involved are huge and destructive.

Flip Bucket Spillways - the purpose of this type is to throw the water well clear of the structure. The jet of a ski jump spillway leaves horizontally whereas the jet of a flip bucket is deflected upwards to induce disintegration in the air. The spray produced can cause damage to the countryside and may adversely affect nearby electrical installations.


Project Design

SWZ>=< B. Deflector Bucket ~:;:;;; ............. ~

~

Dissipation of energy depends upon the formation of rollers, turbulence and/or standing waves. Dentations are provided to assist in dissipation of energy. The blocks are subjected to highly fluctuating pressures of such low minimum values that serious cavitation and destruction can occur.

Sluice Gates

The Buguais I MHP diversion or weir will be installed with sluice gate to rid the upstream of the dam from accumulated silt and bed load periodically. The gates will be manually operated.

General Information on the Diversion Weir:

Length of Spillway Height of Weir Profile type Length of Apron Sluice Gate

Estimation of design flood

30 meters 2 meters Ogee Type Profile 20 meters Mechanically Raised 1.5m x 1.5m

There are two methods now commonly used;

The statistical analysis of past floods with extrapolation to estimate the magnitude and probability of occurence of future floods, and;

The estimation of probable maximum precipitation on to the particular catchment under the worst meteorological conditions likely to occur over the catchment, followed by an estimation of the run-off that would result from such a storm.


Project Design

The determination of probable maximum precipitation for a particular drainage basin requires comprehensive study of major storms on record and is a job for experts. One is limited by the lack of data, records usually do not go back more than 50 years, which makes prediction of more than the 1 00 year flood impossible. As it is, 50 years of data will predict a 100 year flood to within 25%, and 115 years will predict it to 1 0%.

The Engineer is fa~ed with conflicting requirements in terms of safety and economy, he is therefore obliged to use to the best advantage the data and procedures that are available;

Statistical analysis of past flow records at the site - and extrapolation; As above, but with extension of the flow records by correlation with

flows from adjacent catchments; Statistical analysis of rainfall records and extrapolation; As preceeding, but with extension of data by correlation with other

stations; Correlation studies including both rainfall and flow records;


Project Design

Low Pressure Headrace


The conveyance system shall be designed with a capacity of 1.5 cu.m per second flow. A design slope of 111000 shall be adopted to minimize headloss.

The full length of the low pressure conveyance system will involve a steel p1pe conveyance.

Headrace Total Length 750m Thickness:

Diameter 1.2m 6mm thkMS Slope 1:1000m

Due to the utilization of a surge tank the hydraulic conveyance shall fall under the Full Flowing pipe.

Outcropped Steel Conveyance

The whole portion of the Buguias I MHP conveyance line is an outcropped conveyance steel closed conduit type The moderately sloped geologic feature of the mountain side from that point to the proposed desilting basin to the surgetank site allows construction of benches on which the circular type steel conduit will be laid.

Pipe Sleepers and Coatings

Where the steel conduit pipes are exposed and unprotected from excessive or corrosive condition, pipe sleepers or block carrying the pipe off the ground shall be employed to prolong life of steel pipe materials. Where the steel pipes will be subject to harsh conditions or will be subject to be buried, a bituminous coating will be applied on the external surface of the steel pipe.


Project Design

Other Water Conductor Structures

Where topography is unfavourable other types of water conductor structures may be required. In such cases the engineer will have to develop more detailed layouts in accordance with the relevant standards and guidelines.

Hydraulic Designs .

Trash rack Looses:

(~ .16)

I /(

I, /f

v .l ---- //

-'I / il

/I t( . I I_____ I 1~. ~--------------~!

t f- = heacloss :m11) = ba 1ickn~ss lmm) l!. l 0 J ~ tl b :: V'tidt betwee ba"S ( ,, V =aJoroac JelcCJty 11.s) ~ g = gravi aliCtlal ctt~s'a I

'== 2.4 . 8 .s 1.7 1.0 0.8 cp = a

Project Design

(-, 11" ~ . ')

For a ratio up to d D = 0.76. Kc approximately follmvs. the furmtlla:-

(2.18)

Loses due to friction:

Empirical formulae

O.~r the ye~ many empirical formulae. ba!>M on accumulated expenence-. have been de.-e'loped. They are. generally. nor based on sound phy!>ics principles and eeu. occasionally. lack dime-mional coherence. but are innutiYely ba->ed on rhe lx-lief that the friction on a dosed full pipe is:

l. Independent of the water pres!>llr

Project Design

-t Proportional m a cerrnu1 exponent of rh~ wat~r wlocity

In turbulent t1ows it ~~ influenced by the wall roughn~s

One of these furmuL1e. ,,idely u-;ed m estun.1te the tlo\Y in open channels. but al that de.-eloped by ~1all1llllg (n.~sp . Stnckler):

Q=..:... ! I

~ ( l.-1 : -~ .)

p: ~

u is the fanrung roughness coeffictenr (s nu ~- Ks-:rd.L!r=l n) P is the wetted perimeter (m) A 1s cross-ectional area of the pipe (tn: l. and S if> the hydraulic gradient or head loss br linear meter h L).

(2 .13)

Applying the abow fommlae to a full do!>Cd cu-cular cross section pipe :

' 10.29 1i 2 Q~ s = .,.. . D"

In Table ~.2 the ~L-uwing coeffiClent n for se-..eral commeraal p1pes 1s sho\\n:

Tab)(> 2-2 :\Ianning C'O(>fficiPnt n for '>f'n>ral comm(>rcial pipf"> Kmdofpi~u welded '>ted Polyethylene (PE) PYC Asbestos cement Ductile iron Ca'it iron V/ood-staw (new) Concrete (steel forms smooth fuush)

[Buguias I Mini Hydropower Project]

0.012 0.009 0.009 0.011 0.015 0 .. 01-t 0.012 0.01-t

(2.1-1-)

(2 .1-1-a)

Page 3-12

Project Design

Surge Tank and Penstock


Surge tanks

Surge tanks are required to protect long penstocks from excessive water hammer pressure rise, to control excessive generator runaway speeds and to contribute to system speed regulation. Alternatives to surge tanks providing some of the benefits of surge tanks, include:

-addition of extra machine inertia (typically by adding a flywheel to a horizontal axis unit or extra mass to a vertical axis generator).

- installing turbine bypass valves. - pressure relief devices.

A preliminary design methodology for surge tanks is outlined below. It is conservative.

Cross-section area of surge tank (As) =1.6AU2gcHo (m2)

Where:

Highest up-surge:

A = cross section area of upstream pipe (m2) L =length of pipe surge tank to reservoir (m) c =head loss factor as hi= cV2 (m-l.s2) Ho= steady state head on turbine

In order to dimension the surge tanks it is also necessary to know the maximum and minimum water levels that can be expected. An approximate method is shown below that is based on Parmakian' s method for balanced design (Parrnakian - 1960). This method provides equations relating the following parameters from which the maximum and minimum surge levels can be calculated:


-Project Design

QO =initial steady state flow (m3/s) As =cross-section area of surge tank (m2) g =acceleration due to gravity(= 9.8 ms-2) L =length of pipeline between forebay reservoir and surge tank (m) A= cross section area of pipeline (m2) SA= upswing (m) SB =downswing (m) HO =steady state w~ter level in surge tank (m) Hs = static water level in surge tank (m) Hf and bo as defmed below:

For maximum upsurge calculate:

Hf = pipe friction loss + minor losses + Vo2/2g

Bo =Hf/Qe * (Asg/(L/A))"0.5

Sb = l.OSb"-0.89 * Hf

Maximum W .L. in surge tank= Ho- Hf + Sa

Lowest down surge:

For lowest downswing calculate

Hf =pipe friction losses+ minor losses+ Ve"212g

(where Qe = flow demanded by turbine)

Bo = Hf I Qe * (Asg/(L/A))"0.5

Sb = 0. 88bo"-O. 91 * Sf

Minimum W.L. in surge tank= Hs - Hf- SB


Project Design

Penstocks

Penstock Intake

The concrete volume of a typical penstock intake is approximately 15 .QP rm and net cost can be estimated as:

Where:

CI = 15.Qp.fi

Qp= plant flow (m3/s) fl =unit price of reinforced concrete (Php/m3) C 1 = cost of intake (P).

The penstock intake should be protected with trash racks but gates can be omitted for mini-hydro plants.

Penstock

Check head /length (H/L) ratio of the proposed penstock layout, if HIL > 5 a surge tank or turbine bypass valve may be required. Exceptions to these requirements are: -Mini hydro plants with load controller. - High head plants with Pelton turbines

If BIL > 5, then calculate maximum length of penstock:

Lmax = 3.14 Hn* TeN (m)

Where: Hn= net head on turbine (m) Te = effective governor closure time, max = 6.0 sees V =flow velocity in penstock (m/s)

[for penstocks with varying diameters Aequiv = VEAi!Li and V = Q/ equiv].

H L< Lmax, no surge tank is required.


Project Design

The economic diameter of a penstock can be estimated as below:

D = 3.55 (Q"2 I 2gH) AQ.25 (Sarkaria's Equation)

or D = ( 4/pi)AQ.5 * Q/3.0 based on V = 3.0 m/s

Use the lesser of the two values.


Project Design

Powerhouse


The power house shall be of the conventional type concrete and steel construction to house and protect the generating units and other control and auxiliaries are located on the ground floor with the turbines. The draft tubes are cast integrally with the substructure when it is poured in, with steel liners serving as forms.

A working bay is provided at one end of the power house where equipment maybe unloaded and repaired.

An overhead crane, with a capacity large enough to lift the heaviest piece of equipment over other machinery, is installed to lift and move the heavy equipment to and from the repair bay for replacement into their positions.

Tailrace

The tailrace is designed taking into consideration the maximum gross head that can be utilized. Due to the proximity of the power house to the discharging stream, a wide tailrace is selected to minimize tailwater elevation rise resulting in decrease of gross head for the plant, particularly during flood flow conditions.

To unwater draft tubes for repairs, stoplogs or sectional gates will be installed between the tailrace piers to shut off water from the draft tube during unwatering. A hoisting mechanism shall be provided for the gates or stop logs.


Project Design

Electro-Mechanical Equipment

Selection

The criteria for the selection of the electro-mechanical equipment will be based mainly on the optimisation of the resource and economy. The cost of each alternative is computed including the associated civil works particularly on the power house aspect as required by each equipment configuration and are compared against each other on the basis of the lowest specific cost per kilo-watt-hour produced. Another aspect to be considered will be the ease of transportation of the bulk components particularly at the site. Large machinery components which cannot be disassembled into smaller parcel will be hard to transport without constructing an extensive transportation system.

Figure: Turbine Selection Chart

S'n '~ 'h. S'a ~_,o -o _ D 't> Po

... ~ ~.... ~~ ~?

Project Design

Based on available selection charts, available choices of turbine includes pelton type turbines, turgo impulse turbines or francis type turbines.

Turbine type will be selected primarily on the economic and financial basis.


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buguias minihydro project structural design

Documents

dam site

project design diversion

mhp dam

dam abutments

meters diameter

river flow

project layout

design flood