Bharatpur is a city in southern central Nepal with a population of 280,502.

It is the fourth largest city in

Nepal and the district headquarters of the Chitwan District, as well as a separate metropolitan authority.
Bharatpur is one of the fastest-growing cities in Nepa
Mount Everest, known in Nepali as Sagarmatha and in Tibetan as Chomolungma, is Earth's highest
mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The international
bord
pour should be limited to 70 feet, using 3-inch Waterstops across contraction joints are necessary to
aggregate temperature-controlled concrete. By prevent leakage and obtain satisfactory dry operating
using additional reinforcing to minimize crack widths and working conditions. They are required to exclude
for satisfying the watertight concrete requirements, water under head in the substructure and to ensure
pour widths can be increased. With the added weather- tightness of the joints in the superstructure.
reinforcing, and by carefully establishing the Material of rubber or polyvinylchloride (PVC) is
temperature control requirements, it is seldom suitable for this purpose. Extensive experience in
necessary to resort to segmented, waterstopped the use of molded rubber or extruded
roof placements used in the past for unlimited polyvinylchloride waterstops in joints of conduits and
concrete spiral cases. hydraulic structures have proved the practicability and
advantages of using either of these materials. Copper
waterstops were used in the past, however, they will
(9) When vertical construction joints are fail where yielding foundations or other conditions
required in the substructure, they should extend result in differential movement between monoliths.
upward through the massive part of the structure, They are also easily damaged during installa- tion.
but need not extend into less massive piers, slabs PVC or rubber waterstops with a center bulb can
or walls. Vertical construction joints should be keyed, withstand this type of movement and are
and adequate shear friction rein- forcing should be recommended for use in hydroelectric products. A
provided across the joint to develop the required wider width is indi- cated for waterstops in the
shear capacity. substructure where large

(10) A sloping construction joint should be

located at the top of the main intermediate piers in
the intake and draft tube. The roof of the water
passage is then placed across the top of the
prepared surface of the piers. Due to the slope of
the roof, several horizontally placed lifts are usually
required to complete the roof. Where the lift tends
to feather out to the roof form, the pour line should
be dubbed down 12 inches to eliminate the
feathered edge.

(11) Consideration should be given to the

location of horizontal construction joints on
exposed faces. A V-notch rustication can be
chamfering the joints in keep- ing with the
architectural treatment.

4-8.
Waterstops
aggregate is used and higher water pressures exist 4-10. Spiral
than for waterstops to be installed in low-pressure Cases
areas or for weather-tightness only. Waterstops
should be placed as near to the surface as practicable
without forming weak corners in the concrete that a. General. Spiral cases should be designed to
may spall as a result of weathering, or impact, and withstand the bursting pressure of maximum
should create a continuous barrier about the headwater plus water hammer.
protected area. All laps or joints in rubber
waterstops should be vulcanized or satisfactorily
cemented together, and joints in “PVC”
waterstops should be adequately heat sealed.
Waterstops in contact with headwater for structures
founded on rock should terminate in a recess
formed by drilling holes 6 inches deep into the
rocks and should be carefully grouted in place.
Occasionally, double waterstops are required in
pier joints, one on either side of a formed hole,
contain- ing bituminous material. In some
important locations, two waterstops and a drain
should be used to ensure water-tightness.

4-9. Draft
Tubes

The outline of the draft tube is usually determined by

the turbine manufacturer to suit the turbine
operating require- ments. However, in most cases,
the manufacturer will be limited by certain physical
requirements, such as the spacing and setting of the
units, depth of foundations, and elevation of
tailrace. The draft-tube portion of the substructure
should be designed to withstand all loads that may
be imposed on it, including superstructure loads,
foundation reactions on the piers, tailwater on the
roof, and the bursting effect of tailwater inside the
draft- tube. Uplift under the floor of the draft-tube
should also be considered in the design of the slab
even when relief drains are provided. The upstream
ends of intermediate piers should have heavy cast or
structural steel nosing (usually furnished by the
turbine manufacturer) to with- stand the
concentrated vertical load and to protect the piers
from erosion. Piers between adjacent draft-tubes
are usually bisected by the monolith contraction
joint from which water is excluded by seals near the
gate slot. Therefore, each half pier must be designed
for the pres- sure of tailwater on the inside of the
draft tube as a nor- mal condition. It is advisable
to consider also the possibility of unbalanced load
in the opposite direction in case of failure of a
contraction joint seal with the draft- tube
unwatered.
 b. Types of spiral cases. The type of spiral When a unit is unwatered, unbalanced hydrostatic
case depends on the power plant being considered. load will be shared by both piers. The contact strip is
constructed by injecting grout into a formed recess
about 3 inches wide and 12 inches high located in
(1) For low-head plants they may be of the contraction joint. A waterstop should be located
unlined concrete with engineered reinforcement to just above and below the recess to prevent grout- ing
withstand applied dead, hydraulic and equipment the entire joint.
loads.

(2) The substructure may be “skeletonized” and

(2) For medium and high head plants, they should the downstream wall and portions of the side walls or
be made of steel plate with shop welded longitudinal piers completed prior to embedment of the spiral
joints. Circumferential joints may be either field case. In this case a minimum clearance of 2 to 3
welded or high strength bolted, depending on the feet must be left
turbine manufacturers design. Welded joints should
be double-vee butt joints made under strict quality
control and in accordance with the provisions of
ASME. It is preferred that the “c” sections of spiral
cases requiring field welding be butt- welded to skirt
plates which should be shop-welded to the stay
rings. All longitudinal welds should be radio-
graphed. Ordinarily, stress relieving will not be
required. When considering spiral cases under high
head, and when shipping, handling, and erection cost
would control, consideration should be given to the
use of high strength steels. Completed spiral cases
should be proof tested hydrostatically with a test
pressure equal to 1-1/2 times the maximum design
pressure.

c. Construction details.

(1) Consideration should be given to under-

drainage of the turbine floor to intercept seepage
upward through the spiral case roof. Under-drainage
should consist of a grid of shallow trenches in the
concrete subfloor covered by porous concrete
planks, and overlaid by a 3 inch gravel bed. The
vertical joint between the intake struc- ture and the
spiral case roof should contain a double waterstop
and drain. Where the spiral case piers are placed
in the first of a two-stage concreting operation
described in paragraph 4-7c, and extend above the
spiral case roof line, the joint between the piers and
roof should be double waterstopped. Consideration
should also be given to providing a grouted contact
strip in the contrac- tion joint between adjacent spiral
case piers at approxi- mately mid-height of the pier.
between the spiral case liner and the concrete attached at the inlet of the spiral case extension. The
walls of the recess. The transition section from spiral case is filled with water and pressurized to the
penstock to spiral case extension should not be test pressure or to a pressure equal to head water plus
encased. A penstock room or gallery should be water hammer while encasement concrete is placed.
provided to house the transition, penstock coupling, Grout and vent holes in the stay ring are fitted with
and when required, a butterfly or spherical closure plugs which remain in place until the spiral case is
valve. A Dresser-type coupling should be used to unwatered. After concrete has been placed
connect the penstock to the spiral case. The lifts of
concrete around the spiral case liner should be limited
to the depth specified in paragraph 4-7c. A mini- mum
of 72 hours per lift shall elapse between the placing
of each successive lift. Bent steel ’J’-pipes 6 inches
or more in diameter should be provided for placing
concrete under the stay ring, discharge ring, and
spiral case. The number of ’J’ pipes required depends
on the size of the spiral case. Concrete may be
pumped through the ’J’ pipes using a positive
displacement concrete pump. After concrete
placement is complete the ’J’ pipes should be filled
with concrete and left in place.

d. Embedment conditions. Two methods of

embed- ment of steel spiral cases in concrete are
commonly used:

(1) When steel spiral cases are to be embedded

in concrete in an unwatered condition, the top portion
of the spiral case should be covered with a
compressible mem- brane to ensure that the spiral
case liner resists internal pressure by ring tension with
only a small load being transmitted into the
surrounding concrete. The compress- ible membrane
should consist of sheets of closed cell foam
material with the property that a 1/4-inch-thick
piece deflects 0.10 inch under a 50-psi uniform
pressure applied normal to the surface.
Polyvinylchloride foam and polyurethane foam are
acceptable, and the sheets of this material should
be attached to the spiral case liner with an
adhesive. The thickness of the membrane depends
on the diameter and thickness of the spiral case liner,
and the internal pressure being resisted. The com-
pressible membrane should extend to the first
construc- tion joint below the horizontal centerline
of the spiral case. A drain should be provided along
the lower limits of the compressible membrane in
order to prevent trans- mitting stress to the concrete
through a water medium.

(2) When steel spiral cases are to be embedded

in concrete under a pressurized condition, a test
barrel is used to close off the opening between
the upper and lower stay rings, and a test head is
at least one lift above the top of the spiral case, parts, the heavy loads from the thrust bearing, and
and the top lift has set at least 72 hours, the spiral the short circuit torque of the genera- tor. It is
case should be unwatered and the test barrel and test usually designed to support the generator room floor
head removed. Workable concrete should then be also. Openings in the pedestal should be provided
placed through the bent steel ’J’ pipes to fill the void on all four sides when practicable for access to and
under the stay ring/discharge ring. Neither a ven- tilation of the turbine pit. Adequate head room
compressible membrane or drains are required for should be provided between the underside of the
embedding spiral cases under the pressurized generator and the generator platform, if one is used,
condition, except when low alloy steels or high and between this plat- form and the turbine walkway.
strength quenched and tempered alloy steels are
used for spiral case construction, consideration should
be given to providing drains.

e. Concrete placing. The use of mechanical

vibra- tors will not be permitted closer than 5 feet
to any part of the spiral case, stay ring/discharge ring,
or draft tube liner, except that small vibrators may be
lowered through the grout holes in the stay ring to
vibrate concrete placed through the steel ’J’ pipes.

(1) Concrete placement through the steel ’J’ pipes

should be accomplished in a single lift operation which
brings the concrete to within about 1 inch of the
lower stay ring. After a minimum seven day cure
period, grout fill pipes shall be attached to grout holes
spaced evenly around the stay ring, and non-shrink
grout injected at a head not exceeding 6 feet. A
vent hole shall be located approximately midway
between grout holes to permit escape of entrapped
air and water.

(2) Concrete reinforcement above the spiral

case should be designed to distribute the generator
pedestal and turbine floor loads to the stay ring
and the spiral case piers and walls.

4-11. Generator
Pedestals

The generator, except in some low-head plants, is

usually supported on a heavy concrete pedestal.
The details of this pedestal will depend on the make
and type of gener- ator to be installed. It should be of
massive construction and should be designed to
resist vibrational forces from the moving mechanical
4-12. Bulb Turbine at sites with low maximum tailwater. The structural
Supports economy must, how- ever, be weighed against
increased cost of generator housing, greater crane
costs, and increased maintenance of equipment.
Bulb turbines are supported in a much different
manner than the typical vertical shaft Francis or
Kaplan unit. A typical cross section of a bulb unit
is shown in Fig- ure 4-1. The main element of
support for a bulb turbine is the stay column. It
must carry the weight of the rotat- ing parts, most of
the stationary parts, and the hydraulic loads due to
thrust, hydrostatic pressure, and transient loading. A
typical stay column consists of an upper and lower
support column fixed to the inner and outer stay
cones. The inner stay cone forms the inner water
pas- sage and houses the turbine parts. The outer
cone forms the outside water passage surface. This
cone and both support columns are embedded in
concrete, thereby trans- mitting all loads to the
structure.

4-13. Types of
Superstructures

The main superstructure may be one of three

types: indoor, semi-outdoor, or outdoor. The indoor
type com- pletely encloses the generators and the
erection bay and has an inside crane runway
supported by the walls of the structure. The semi-
outdoor type consists of a continu- ously reinforced
slab over generators and erection bay supported by
heavy transverse walls enclosing 2 or

3 units. The powerhouse crane is an outdoor gantry

with one runway rail on each side of the low
superstructure. Sliding hatches in the roof over each
generator and in the erection bay provide access for
handling equipment with the crane. In the outdoor
type, each generator is pro- tected by a light steel
housing which is removed by the outdoor gantry
crane when access to the machine is necessary for
other than routine maintenance. The erec- tion and
repair space is in the substructure and has a roof
hatch for equipment access. Choice of type should
be dictated by consideration of first cost of the
structure with all equipment in place, cost of
maintenance of building and equipment, and
protection from the ele- ments. The indoor type
affords greatest protection from the weather and
facilitates operation and maintenance of equipment.
The semi-outdoor type may sometimes have a
marginal economic advantage insofar as it pertains
to the cost of the structure, but this advantage
will not always offset the increased equipment cost.
The outdoor type is structurally the most economical
Figure 4-1. Typical cross section thru bulb unit

4-14. Superstructure - Indoor Powerhouse

b. Concrete walls. Concrete exterior walls may
be cast-in-place with uniform thickness, column and
a. Framing. The framing of the superstructure
spand- rel wall, precast panels, or prestressed
may be cast-in-place or precast reinforced
precast units. They must all be designed to withstand
concrete, pre- stressed concrete, structural steel, or
the stresses from possible loading combinations and
a combination. The choice is dependent on economy
at the same time pro- vide the necessary space
and architectural requirements.
requirements, both for embed- ded items and
interior clearances. Provisions must usually be Wall pours should not be made more than about
made for carrying loads imposed by bridge cranes and 10 feet in height. Horizontal rustications should be
the rails are usually supported by continuous corbels used at the pour joints on the exterior side where
or wall offsets at the elevation of the rails. Exterior practicable in order to prevent unsightly spalling;
concrete should not be rubbed nor should form liner hence, locations of joints and pour heights will
be used, but a rigid specification for forming should depend partly on the exterior archi- tectural
be set up to insure reasonably smooth, plane surfaces. treatment. In the interior of the powerhouse
In some cases, special finishes, such as bush where neat appearance is essential, it will be
hammered or striated concrete, may be considered necessary to provide a smooth, dustless surface. The
for special architec- tural effect. Matched tongue- requirements for obtaining such a surface are
and-groove lumber of contained in EM 1110-2-
2-inch nominal thickness is satisfactory for 2000. Sack rubbing of the concrete surfaces should
sheathing. be avoided insofar as practicable. Contraction joints
in the same vertical plane as those in the substructure
should be provided in the superstructure walls.
Criteria for open- ings in exterior walls are given in
paragraph 3-1.

c. Steel framing and walls. While self-supporting

concrete walls are usually preferred in the generator
and erection bays, their justification is dependent
upon econ- omy and tailwater limitations. Where
economically feasible, steel framing with curtain
walls or insulated panels should be used where the
generator floor is above the maximum tailwater.
The use of steel framing may, in some cases, be
desirable to permit the early installa- tion and use
of the crane runways and crane. This advantage
should, however, be evaluated only in terms of
overall cost. The steel framing for each monolith through all floor slabs. Shrinkage and temperature
should be a separate unit, with no steel except the steel should be provided in the tops of all slabs. At
crane rails crossing the contraction joints. Crane rail hatchways through floors, flush sockets should be
splices should be staggered a few inches with the provided for the installation of temporary railings for
joints. One bay in each monolith should be protection of person- nel at times when the covers
diagonally braced in both roof and walls. Bents may are removed.
be composed of columns support- ing simple beams
or girders or may be rigid frames. Trusses should
be used only if dictated by unusually heavy loads e. Roofs. Roof framing will usually consist of
and long spans. In case it will usually be pre- cast, prestressed units such as tee’s, double
advantageous to weld shop connections and bolt field tee’s or hol- low core plank. If structural steel framing
connections. Exceptions should be made for field is used, it will usually be fabricated steel girders
splices in long rigid frames, which should be welded supporting steel
for struc- tural continuity.

(1) If steel framing is used in other parts of

the structure, it is usually of the conventional beam
and column type used in office buildings.

(2) Suspended ceilings should be avoided unless

economically justifiable. Inner tile walls to conceal
the steel columns or rigid frames should be
avoided. Not only is concealment of the steel
considered unjustified but high thin walls are a
hazard structurally.

d. Floors. Floors systems should be

reinforced concrete flat slabs or one or two-way slabs
with a 6-inch minimum thickness. The types of floor
finishes and cove details designated for the various
parts of the super- structure are given in paragraph 3-
4a. Placement of concrete in floor slabs should be
stopped sufficiently below finish grade to allow for
appropriate finishes. The thickness of the structural
slab will in many cases be determined by the member
and size of electrical conduits which it must encase.
Separate concrete fill placed on top of the
structural slabs to encase conduits is uneco-
nomical but can be used where large numbers of
conduits must be accommodated, or where
reinforcement in the structural slab is closely spaced.
If used, it is recom- mended that the generator room
floor be designed as a slab of uniform thickness,
supported on the upstream and downstream walls
and at the generator pedestal and car- ried on
double columns at the contraction joints. Build- ing
contraction joints with water stops should be carried
purlins, which in turn, support the roof deck. Slope 4-15.
for drainage should be provided by the slope of Intakes
the roof deck or the use of sloped insulation. The
use of light- weight or sawdust concrete should be
avoided. Insula- tion, embedded in hot bitumen or a. Type of intakes. Intakes may be classified as
mechanically fastened should be applied over a low pressure, or high pressure, according to the
vapor seal course to the roof slab or deck. Foam head on the
insulation should be avoided due to the unevenness
of application. Thickness of insulation should be
determined by an analysis of heating and cool- ing
requirements. Roofing criteria are given in
paragraph 3-2.

f. Future extensions. A temporary end wall must

be provided for a superstructure which will at some
future time, be extended to house additional units.
The con- struction of the temporary wall should be
such that it may be easily removed, and with a
minimum of interfer- ence in the operation of the
station. The temporary wall below the maximum
tailwater elevation should be made of precast
concrete slabs, supported on a steel frame- work,
designed to resist the tailwater pressure, and sealed
with rubber or polyvinylchloride water seals at all
joints. The remainder of the temporary wall could be
made of prefabricated metal panels which can be
removed and possibly utilized in the future
permanent end wall.

g. Vibration. The superstructure in the generator

monoliths will be subject to vibration caused by the
generating units. In order to minimize the effect of
vibration on the main structural framing, the
superstruc- ture should be made as rigid as
practicable. Concrete columns and walls should be
integral with floor slabs and girders. Steel beams
should be framed into columns and girders with full
depth connections and not with seat angles alone.
In some cases it may be desirable to use top flange
clip angles also. Special attention should be paid
to framing connections in light floors, balconies,
stairways, and roofs and to fastenings of gratings,
prefab- ricated metal panels, precast slabs and
handrails. Threaded or welded handrail connections
are preferred to pin connections. The effect of
vibration in a generator monolith is, of course,
greater on members close to the unit than on those
at a distance. Also, members in parts of the
structure separated from the generator monoliths
by contraction joints will be less affected. Therefore,
the designer must use careful judgment in
determining the extent to which vibration will
influence the design of such members.
inlet, but there is no definite line of demarcation
separat- ing the two types. For low-head plants and
(1) The racks are usually designed for an
for develop- ments where the pool drawdown is
unbalanced head of 10 to 20 feet of water and are
small, low-pressure intakes are used. If the pool
fabricated by welding in sections of a size
drawdown is to be large, such as on many
convenient for handling. For low-head intakes,
multipurpose projects, the intake will be of the high-
stresses due to complete stoppage and full head
pressure type. It is advantageous to locate the
should be investigated and should not exceed 150
intake high as practicable in order to minimize
percent of normal stresses. If the racks are to be
weights and travel distance of gates, size of hoist, etc.,
sheathed for the purpose of unwatering the intake,
as well to keep the sill above possible silt deposits.
case II working stresses should not be exceeded for
Low- pressure intakes are usually incorporated in the
that loading condition. The clear distance between
dam and, for low-head plants, are also part of the
rack bars
powerhouse structure. High-pressure intakes may be
in the dam itself or may be in a separate structure or
structures in the forebay. The essential requirement
if the two types are the same, but the details and
equipment may be radically different. Features
common to practically all intakes are: trash racks,
gates, steel bulkheads, concrete stoplogs, or all
three, and converging water passage or passages.

b. Shape of intake. The lines of the intake should

be carefully laid out to obtain water velocities
increasing gradually from the racks to the penstock, or
to the spiral entrance. Abrupt changes in area of the
water passage should be avoided in order to minimize
turbulence and consequent power loss. The sections
between the rectan- gular gate and the round
penstock entrance is particularly important. The
transition is ordinarily made in a distance about equal
to the diameter of the penstock. Model tests are of
great value in determining a satisfactory shape of
intake, especially if Juvenile Fish Bypass is a design
consideration.

c. Trash racks. Trash racks are usually vertical

in order to economize on length if intake structure.
For low-head intakes, however, where the increase
in length of structure would be small or where
considerable trash accumulation may be expected,
they are often sloped to facilitate raking. Water
velocities at the racks should be kept as low as
economically practicable with a maxi- mum, for
low-pressure intakes, of about 4 feet per sec- ond.
For high-pressure intakes, greater velocities are
permissible but should not exceed about 10 feet
per second.
varies from two to six inches or more, depending slots, since a higher velocity may be tolerated for a
on the size and type of turbine and the minimum short time. In any case, however, it is advisable to
operating clearances. Bar thickness should be keep the maximum velocity V in feet per second
consistent with structural design requirements, below that given by the expression:
with the vibrational effects resulting from flowing
water being considered. A thick bar should be used
V 0.12 2gh
with the depth of the bar con- trolled by the allowable
working stress.

(2) The design of the guides and centering

devices for the rack sections should receive careful
attention. Clearances should be small enough to
prevent offsets interfering with removal of the
racks, or operation of a rake if one is provided.
Embedded members on the guide slots should have
corrosion-resisting exposed sur- faces. Corrosion-
resisting clad steel is satisfactory for the purpose.

(3) For high-pressure intakes in concrete dams,

the trash rack supporting structure is sometimes
built out from the face of the dam in the form of
a semicircle in order to gain rack area to maintain
low velocities.

d. Gates. Provisions for emergency closure of the

intake downstream from the racks is necessary to
protect the generator unit. A vertical lift gate in
each water passage is usually provided and is
normally suspended just above the roof of the intake
from a fixed hoist. On very low-head multiunit plants
a single set of intake gates operated by a gantry crane
is adequate and will be less expensive than individual
gates operated by fixed hoists. In either type of
installation, self-closing tractor gates capable of
operating under full flow are provided. Fixed wheel
gates may be the most economical type for intakes
where a gantry crane will be used for operation.
Bronze- brushed wheel bearings should be used if
the wheels will be submerged when the gate is in the
stored position, otherwise, antifriction bearings can
be used. For all but the lowest head intakes,
“caterpillar” type gates with corrosion-resisting steel
rollers and tracks have been found to be the most
economical.

(1) In selecting the position of the gate slots,

limit- ing velocities of flow as well as economical
gate size should be considered. If the slots are
located too far downstream, where the opening is
small, power reduction from eddy losses may be
more costly than a larger gate. The duration of peak
demand on the plant will also affect the location of the
in which h is the head on the center line of the low- pressure intakes, and the design of plants in
gate at normal power pool. northern latitudes should take this into account.

(2) Slots for stop logs or bulkhead gates are (1) Frazil and anchor ice may cause loss of head
usually provided just upstream from the gates so by forming on, or clogging rack openings, or may
the gate slots may be unwatered for maintenance immobi- lize racks and gates by massing in the slots.
operations. In case where headwater is never far Continuous surface ice tends to prevent the formation
above the top of the intake, the racks are of frazil and anchor ice, and for this reason an ice
sometimes designed to support sheathing for sheet in the forebay is more beneficial than
unwatering. otherwise, except as it interferes with raking, or at
breakup when it must be chuted to the tailrace or
passed over the dam.
(3) Essential fixed metal in the slop-log slots
should have corrosion-resisting exposed surfaces,
since these slots cannot be easily unwatered for
repairs.

e. Air vents. Since emergency closure must be

made under full flow conditions, negative pressures
will tend to buildup at the top of the intake just
downstream from a downstream-sealing gate as the
gate is lowered. To prevent excessive negative
pressures from occurring in the penstock during
emergency gate closure and to exhaust air during
penstock filling operations, one or more air vents
should be provided just downstream from the gate.
The air vents should be of sufficient size to maintain a
pressure of not less than 1/2 atmosphere in the
penstock at the maximum rate of depletion of water
from the penstock under emergency closure
conditions. The opening in the intake roof should be
as close as practicable to the gate. The upper end of
the air passage should be open to the atmosphere
well above maximum headwater and in a location not
readily accessible to personnel. Gates sealing on the
upstream side are some- times used. Air vents in the
penstock may then be elimi- nated, as enough air will
be introduced into the water passage through the
opening between the downstream side of the gate
and the concrete structure. Gate slots for both the
upstream and downstream seal gates should be
adequately vented by the use of open grating
covers or by other means.

f. Prevention of ice troubles. Periods of freezing

weather are likely to cause trouble with ice at
 (2) Formation of ice may be prevented in the
slots by means of electric heaters in casings
(2) This includes conditions during filling and
embedded in the piers next to the guides and on the
drain- age of the penstock or surge tank and
racks by a bubbler system with outlet nozzles just
seismic loads during normal operation. The
below the bottom of the racks and far enough
allowable stress for this condition is equal to 1/3 of
upstream for the released air to carry the ice
the specified tensile stress or
particles to the surface without coming in contact
with the racks. At the surface the ice is sluiced to 2/3 of the specified yield point, whichever is
the tailrace. less.

4-16. Penstocks and Surge

Tanks

a. Details and design. The determination of the

diameter of penstock and the selection of size, type,
and location of surge tank, if one is used, involve
rather complex economic considerations.
Therefore, only details of design will be discussed.

b. Free standing penstocks and surge tanks. The

penstock should be designed for full pressure due
to static head caused by maximum elevation of the
operat- ing range for the intake pool plus
waterhammer. Waterhammer studies should be
conducted to determine transient pressures at any
point along its length. The following design
conditions and their corresponding allowable stresses
for carbon steels (see ASME, Boiler and Vessel
Pressure Code, Sections 8 and 9 when other steels are
used) are to be considered for these structures.

(1) This condition includes maximum,

minimum, and rated turbine static heads plus
waterhammer due to normal operation, load
rejection and load acceptance. It also includes
stresses due to gravity loads and longitudi- nal
stresses due to penstock movement. The allowable
stress for this basic condition is equal to the smaller
of

1/4 of the specified tensile strength or 1/2 of the

speci- fied yield strength. The load acceptance
condition includes minimum static head and loading
the turbine from speed no load to full gate opening
at the maximum rate of gate opening. This condition
will indicate a mini- mum pressure grade line for the
determination of sub atmospheric conditions.
Amstutz buckling criteria should be used for
embedded conduits. Stewart buckling criteria should
be used for non-embedded conduits. If a valve is used
as an emergency closure device, the conditions at
maximum flow and maximum head with maximum
valve closure rate must be analyzed.
 (3) This condition includes the governor the shop after they have been radiographed. The
cushioning stroke being inoperative and partial gate pressure should be applied three times, being
closure in 2 L/a seconds at a maximum rate, where L increased and decreased slowly at the uni- form
equals the conduit length in feet and a equals the rate. The test pressure should be held for a length
pressure wave velocity in feet per second. The of time sufficient for the inspection of all plates,
allowable stress for this condition is equal to 1/2 of joints, and connections for leaks or signs of distress. It
the specified tensile strength, but in no case shall this is desir- able that the test be performed when the pipe
stress exceed the specified minimum yield stress. and water have a temperature of not less than 60 °F.
The penstock should be vented at high points during
filling to prevent formation of air pockets.
(a) After combining longitudinal and
circumferential stresses in accordance with the
Hencky-Mises Theory, where Se² = Sx²+SxSy+Sy², the
allowable stresses are not to be exceeded by the
resulting equivalent stress at any point on the
penstock or surge tank.

(b) Minimum shell thicknesses are recommended

for all steel penstocks to provide the rigidity needed
during fabrication and handling. This minimum may
be com- puted from the formula T = D + 20/400,
where D equals the diameter in inches and T equals
the minimum shell thickness in inches. A thinner
shell may, in some cases, be used if proper stiffeners
are provided during fabrica- tion, handling, and
installation.

(c) Welded joints should be butt-welds made

under strict procedure control by qualified operators,
and in accordance with the provisions of CW-05550,
welded power penstock and surge tanks. All
longitudinal seams should be examined
radiographically in accordance with CW-05550.

(d) Completed penstocks with an operating

head greater than 100 feet, should be
hydrostatically tested with an internal pressure that
will produce a hoop stress of 1.5 times the allowable
stress. Penstocks with operat- ing heads less than
100 feet should be pressure tested if they are
unusually long, as may be the case of power tunnels
or some conduits. Care should be taken when
specifying test pressures to indicate where on the
pen- stock the pressure is to be measured. This
will ensure that the penstock is not overstressed
during the test. If the entire penstock cannot be
tested as a unit, individual sections are to be tested in
 (e) Upon completion, and prior to insulation and These changes in flow rate can be caused by the
painting, surge tanks should be tested by filling the turbine wicket gate motions due to power changes
tank with water to a point 1 foot from the top of the or load rejections, unit runaway, and closure of the
shell and maintaining this water level for not less than emergency valve or gate. The magnitude of the
24 hours, or such additional time as may be pressure variation is depen- dent upon the length
required to inspect all plates, joints, and connections of penstock, the velocity of the
for leaks or signs of dis- tress. Preferred water and
shell temperatures for the test should be not less
than 60 °F.

(f) In long penstocks, a surge tank may be

necessary to prevent the fluctuation of water-
hammer flow from seriously interfering with turbine
regulation (see 4-16(f)). Free-standing penstocks
should be constructed so as to permit any leakage to
drain to tailwater without pressur- izing the
surrounding regulating outlet conduit. Careful
attention should be given to anchorage of the
penstock against longitudinal thrust.

c. Power conduit linings. The function and many

of the details of construction and erection for an
integrally embedded steel liner are similar to a free-
standing pen- stock; however, the loading conditions
are different. The steel lining, concrete encasement,
and if present, the surrounding rock act together to
resist the pressures. EM 1110-2-2901 outlines in
detail the loading conditions and allowable stresses
for a conduit under embankments or rock. In both
instances, external pressures must be accounted for
as well as the internal pressures.

d. Water velocities and water hammer. The

velocity of flow in penstocks depends largely upon
turbine regula- tions but is seldom lower than 6 feet
per second. In very high-head plants velocities as high
as 30 feet per second have been used. For medium-
head plants at maximum discharge, velocities of
about 12 to 18 feet per second are typical. It
should be noted that the allowable stresses for the
components of the turbine spiral case, spiral case
extension, valve, and valve extensions are different
than those for the penstock. Refer to the guide
specifications (CE-2201.01, CE-2201.02, and CE-
2201.03, etc.) for these allowable stresses. The
point of division between the penstock and spiral
case/valve extension is custom- arily defined as the
limit of supply for the turbine and/or valve
manufacturer.

(1) Changes in the rate of flow in penstocks

cause variations in pressure known as water hammer.
water, and the rate of change of the flow. When sometimes with embedded steel girders and tie rods,
the turbine gates close due to a decrease in load, the to prevent deforma- tion and concentration of
pressure increases above the steady full load gradient. stresses in the shell. An exam- ple is the tee at a
As the gate movement ceases, the gradient drops surge tank riser. Stress analysis of wyes and tees
below that for steady full load, then fluctuates with can be done as outlined in “Design of Wye
diminishing ampli- tude between the maximum and Branches for Steel Pipe” published in the June 1955
minimum positions until the movement is damped edition of Journal of The American Water Works
out by friction. When an increase in load causes Associ- ation. For large structures or unusual
the turbine gates to open, the gradient first drops configuration, a finite element analysis may be
below that for steady full load, then fluctuates in a necessary.
manner similar to that described for gate closure. The
penstock must be designed at every point to
withstand both the maximum and minimum
pressure at that point as determined by the highest
and lowest posi- tion of the water-hammer pressure
gradient.

(2) The subject of water hammer is covered in

Hydroelectric Handbook by Creager and Justin,
Hand- book of Applied Hydraulics by Davis,
Engineering Fluid Mechanics by Jaeger, and
Waterhammer Analysis by John Parmakian. Prior to
development of plans and specifications, the
hydraulic system should be modeled using a digital
computer to simulate the various design conditions
and configurations of the hydroelectric facility. The
Corps has had a computer program (WHAMO)
especially developed to simulate water hammer and
mass oscillation in hydro-power and pumping
facilities. This program or one equal to it should
be used for this purpose. The Hydroelectric Design
Center should be consulted prior to usage of
WHAMO.

e. Bends, wyes, and tees. The distance from

an elbow or bend in a penstock to the turbine inlet
should be as great as the layout will permit in order
that distur- bances in the flow at the bend will not
affect turbine performance. If a butterfly valve is
used, its center line should be at least 3 penstock
diameters upstream from the center line of the
unit. The penstock must be anchored at bends
to withstand the centrifugal forces of the water as
it changes direction. Anchorages are usually blocks of
mass concrete encasing the pipe. Wyes and tees
involve internal pressures on noncircular sections
and require special design. Often the entire wye
branch or tee is encased in reinforced concrete,
 f. Surge tanks and stability. For isochronous equipment into position without de-energizing
(iso- lated from the power grid) operation, a minimum existing buses and equipment. A chain link woven
ratio of water-starting time to mechanical starting wire fence approximately 7 feet with lockable gates
time is required for stability. Usage of a surge tank should be provided to enclose the entire switchyard.
(which decreases water starting time) or a flywheel
(increases mechanical starting time) may be
employed. Surge tanks also mod- erate water-
hammer pressures. In long penstocks the fluctuation
of water-hammer pressure may seriously interfere
with turbine regulation unless relief is provided. For
this purpose, a surge tank is generally used at the
lower end of a penstock longer than about 400 feet.
For simple surge tanks, the minimum area is usually 50
per- cent larger than the Thoma area. Isochronous
operation capability should be provided for all but
the smallest units. Surge tanks are of three basic
types: simple, restricted-orifices, and differential.
Also for underground stations where the rock is
suitable, a surge chamber (accumulator) can be
employed. A discussion of the advantages and
disadvantages of each type as well as an outline of
design procedures,is contained in Chapter 35 of
Hydroelectric Handbook by Creager and Justin.
Waterhammer Analysis by Parmakian also covers
solu- tions of simple surge tanks. The WHAMO
program is also capable of medeling all types of
surge tanks as well as predicting hydraulic
instability. It is recommended that the
Hydroelectric Design Center or an engineer who has
a successful record in surge tank design be retained
to analyze the flow regulation problem and design
the tank at any power project where long penstocks
are to be used and isochronous unit operation is a
requirement.

4-17. Switchyard
Structures

The most suitable and economical general

arrangement and design of outdoor high-voltage
switchyards should be based on consideration of the
scheme of high-voltage switching employed, the
voltage and capacity of the main buses and
transmission lines, the number of generator or
transformer and transmission line bays required, the
location of the main power transformers, the
direction of transmission lines leaving the yard, and
size and topo- graphy of the space available.

a. The switchyard should be arranged to provide

adequate space for the safe movement of
maintenance equipment and for the future
movement of circuit breakers and other major
 b. An arrangement using high truss-type structures and either strain or rigid-type buses required
a minimum of ground area and is generally used for yards rated

161 kv and below. An arrangement using low flat-type structures with rigid buses is generally used
for yards
230 kv and above and may also be used for lower- voltage yards where adequate space is available.
This design utilizes separate A-frame structures for dead- ending the transmission lines and individual
lightweight structures for supporting the buses, disconnects and other equipment. This arrangement is
considered the most reliable and all equipment is easily accessible for inspec- tion and maintenance.

c. The switchyard structures and transmission line take-off towers have special requirements in
regard to loading, rigidity, resistance to shock, installation, and maintenance. Standards for the design
of switchyard structures to meet these special requirements have been developed on the basis of long
experience of the power industry and are summarized in NEMA Publication SG 67, “Power
Switching Equipment.”

d. The switchyard structures should be designed for the initial power installation but with provision
for expansion as additional generating units and transmission lines are installed in the future.

4-18. Reinforcing Steel

Reinforcement should be designed using the requirements set forth in the latest edition of ACI 318
“Building Code Requirements for Reinforced Concrete,” and as amended by EM 1110-2-2104
“Strength Design for Reinforced Concrete Hydraulic Structures.” Guide specification CW-03210
“Steel Bars, Welded Wire Fabric and Acces- sories for Concrete Reinforcement” provides the neces-
sary details for tests, cutting, bending, and splicing of reinforcement.

4-19. Encasement of Structural Steel

When the framing of the powerhouse is structural steel, the members shall not be encased, except
for certain locations where appearance is a factor, such as office space and lobby, or where a fire
hazard exists.
4-20. Retaining Walls

Walls subject to earth pressure, such as tailrace walls and foundation walls at the shore end of the
powerhouse, may be of the gravity, semi-gravity or cantilever type, depending upon economy, and
should be founded on solid rock wherever possible. Where sound rocks rises above the bottom of the
tailrace at the shore side, exca- vation may be saved by anchoring a concrete facing to the rock and
building the gravity wall above.

4-21. Area Drainage

Roof drains should be provided with basket-type strainers and should be connected to interior leaders
discharging into the headwater or the tailrace. All floors should have flush drains to carry wash water,
seepage, and possible leakage from tanks to the station sump or, if the area drained is well above
maximum tailwater, to the tailrace.

a. Angles and abrupt bends in drain lines should be avoided insofar as possible, and cleanouts should
be provided where necessary to facilitate clearing the pipes.

b. Floors should be sloped so that drains are well removed from electrical equipment, and particular
atten- tion should be paid to all details to avoid damage to such equipment caused by leaks or clogged
drains.

c. In cold climates drain piping must be located where temperatures will not drop below freezing
or must be properly insulated. Outlets should be well below tailwater to prevent formation of ice at
the discharge.

4-22. Chamfers, Grooves, and Rustications

Exterior square corners are undesirable in concrete con- struction because of their tendency to break
removal of forms or as a result of weathering or impact. Chamfers are usually formed on all exposed
corners, but particular attention should be paid to this detail on the exterior walls. Chamfers of
ample size should be provided at the ends of monoliths, forming V-grooves at the contraction joints.
Horizontal V-grooves, or rustications are some- times used for architectural reasons and lift heights
should then be planned so that the rustication will occur at the horizontal joints.
er between Nepal and China runs across its summit point.