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1.6.B

Hydraulic Design Considerations (Cont.)

Mechanical Design Considerations

Torque for Hand Operation

Speed of Opening & Closing

Bearings

Needle Support

Allowable Stresses in Cast Iron

Reference
Drawings

HYDRAULIC VALVES (Continued)

VALVE DESIGN (Continued

passageway dimensions for any valve can be obtained from the dimension
ratio diagram, Figure 14, by multiplying the ratio figures by the inlet
diameter of the size of valve required. A layout drawing using these
figures should be prepared first as a guide in designing the valve.
The discharge for full valve opening is determined by the same formula
as for needle valves using values of C as follows:

C = 0.70 for normal full valve opening. (The coefficient
can be increased to 0.724 by increasing the
needle travel 5.5 percent; however the pressure
in the water passageway will be reduced to
atmospheric which is undesirable for continuous
operation.)

B. The valve is operated mechanically with the assistance of partial balancing
through fixed openings in the face of the needle. Pressure of water flowing
by the openings is transmitted through them to the inner chamber and,
since the pressure varies materially across the face of the needle, the
position of these openings should be carefully located. The required thrust,
in pounds, of the operating mechanism can be obtained by adding together
(1) the maximum unbalanced thrust of the needle which equals 0.112 awh,
where a equals the effective area in square feet of the balancing chamber,
h equals the effective head in feet as stated above, and w equals the unit
weight of water (62.4 pounds per cubic foot), (2) the friction resistance
from the seals, and (3) the friction resistance from the weight of the
moving parts. A sufficient factor of safety will be obtained if in designing
the screw a coefficient of friction of 0.2 at starting and 0.15 at running is
used.

(1) For hand-operated valves the normal running torque required on the
handwheel of the control stand should not exceed 30 pounds multiplied
by the radius of the handwheel, but the mechanism should be able to
withstand a torque of 75 pounds multiplied by the radius.

(2) For motor-operated valves the closing and opening speed of the needle
usually will not exceed 6 inches per minute, with reduced speed for
the last part of both the opening and closing travel.

(3) Self-alining, double-acting, cylindrical roller thrust bearings should
be used on the screws of the larger valves to eliminate excessive
friction.

(4) The neck of the needle support must have sufficient strength to resist the lateral thrust should some foreign object become accidently wedged between the needle and its seat.

(5) The valve mechanism should be able to withstand an overload of 250 percent above the normal running requirements at stresses not exceeding 75 percent of the yield of the various parts involved. The grades of gray iron ordinarily specified may be designed for 3,000 psi in tension, whereas 10,000 pounds may be used for cast steel. The various parts should have sufficient thickness for proper casting, regardless of design stresses.

C. Typical examples of the design of hollow-jet valves are presented in the following drawings:

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.7 Butterfly valves are frequently used in penstocks as a service or emergency gate and are usually placed immediately upstream from the turbine case. They are also used as both shutoff and regulating valves in outlets, although the Bureau has not heretofore used them extensively for regulation due to their tendency to become pitted and to cause considerable dispersion at part openings. Butterfly valves may be used in pipe lines on either or both sides of pumping units, with provision for automatic closing in the event of power failure or pump shutdown.

A butterfly valve consists essentially of a cylindrical or conical body with a circular leaf mounted on a transverse shaft which is carried in two bearings, diametrically opposite each other, in the valve body. An external operating mechanism rotates the leaf 90 degrees to move it from a fully closed to a fully opened position. The 168-inch butterfly valve used at Hoover Dam is shown in Figure 15, and the 76-inch butterfly valve for Granby Pumping Plant is shown in Figure 16.

A. To eliminate leakage past the closed leaf, especially at high heads, it is necessary to provide a form of adjustable sealing device.

B. The leaf and the valve body are shaped to avoid abrupt changes in velocity. Velocities may be held constant, although when the valve is used upstream and adjacent to a turbine it is advisable to shape the water passage to give a constant increase in velocity in the direction of flow.

BUTTERFLY
VALVES

Seals

Hydraulic Design Considerations

Mechanical
Design

C. For small-sized valves, operating at low or medium pressures, a gear-reducing unit, driven by an electric motor, may be used as an operating mechanism. For high heads and large valves it is usually necessary to Considerations operate the leaf by means of hydraulic cylinders or rotors.

D.

(1) The design head on the leaf will be the sum of the static head, any
vacuum head, and any water hammer that may occur.

(2) The theoretical torque required to operate the leaf will be the sum of
the hydraulic moment and the friction torque. The hydraulic moment
can be determined by the following formula:

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(3) The friction torque will be the sum of the trunnion, packing, and
seating friction. For design purposes, this theoretical value of the
torque should be raised about 25 percent.

The working stress in gray iron castings should be limited to about 10
percent of the ultimate tensile strength of the material, i.e., 3,000 psi for
the ordinarily specified grade. (See Subparagraph 1.6B(5).)

Design

Working

Stresses

1.7.E

Reference
Drawings

HYDRAULIC VALVES (Continued)

VALVE DESIGN (Continued)

E. Designs for typical butterfly valves are illustrated in the following drawings:

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FUNCTIONS

& TYPES

BASIS FOR SELECTION OF TYPE

GENERAL CONSIDERATIONS

.8 Hydraulic gates, with one exception, function in a nonregulatory capacity, and are constructed so that the closing member is completely removed from the water passageway when the gate is fully open. The hydraulic gates designed by the Bureau are of several types, peculiar in their closing members and operation, which fall into three general classifications:

A. The operating member moves on sliding surfaces to engage with the
sealing element.

B. The operating member moves on wheels or rollers to engage with the
sealing element.

C. The operating member rotates about a fixed or movable point to engage with the sealing element.

The discussion that follows will describe in detail current Bureau gate designs.

.9 The selection of a hydraulic gate for any given installation will depend, as with hydraulic valves, on service conditions to be encountered. The wheel- or roller-mounted type may be used for any operating head and present Bureau practice considers the sliding type satisfactory for heads up to approximately 400 feet when used as an emergency gate.

Hydraulic gates permit greater clearances between submerged moving parts than hydraulic valves and are therefore more suitable for use with sandy, silty, or carbonate waters.

PRELIM- .10 The requirements outlined in Paragraph 1.3 for hydraulic valves will in
INARY
general be applicable in determining the type and size of a hydraulic gate
installation.

INFORMATION
REQUIRED

INTAKE .11 BULKHEAD GATES

BULKHEAD GATES

These gates are of the sliding type falling in general category Subparagraph
1.8A above. They are usually located at the upstream end of river outlet
conduits or penstocks, where other equipment is used to shut off flow, and
are subjected to relatively high heads. They are operated under balanced
pressures only and are usually operated by gantry cranes, with or without
the help of gate-lifting devices. Their design is similar to that of the wheel-
or roller-mounted gates described in Paragraphs 1.15 and 1.16 except for
the omission of the wheels or rollers. When the gates are closed the cross-
beams of the gate carry the water load to single vertical girders at the sides,
which in turn transfer the load through metal bearing bars to the seal seats
mounted on structural beams embedded in the face of the dam. The skin plate
is usually located on the downstream side of the gate to eliminate the additional
stress which would occur in the top, bottom, and side beam webs if the skin
plate were located upstream. Rubber or rubber-brass seals are attached to
the downstream face of the gate skin plate and bear against the seal seats.

1.12

HYDRAULIC GATES (Continued)

BULKHEAD GATES (Continued)

Guides are provided in the face of the dam similar to those provided for coaster and fixed-wheel gates described in Paragraphs 1.15 and 1.16. Figure 17 shows a typical bulkhead gate installation.

.12 Draft-tube bulkhead gates are used to permit unwatering the draft tubes for inspection and repair of turbine parts and draft tubes. The nominal size of the bulkhead gate is the width by the height of the draft-tube portal. These gates are placed over the draft-tube portal under no-flow conditions, although there may be considerable tailrace turbulence at the time.

A. Bulkhead gates are preferable to stop logs for draft-tube closures because there are fewer longitudinal joints required, and because it is easier to obtain proper seating of one bulkhead than several logs.

B. On account of the tailrace turbulence, gates should be guided the entire distance through which they are lowered. Means for seating the gate securely against the seats of the draft-tube portal should also be provided, so as to counteract the forces due to turbulence during the start of the pumping out of the draft tube. The guide angles against which the gate springs seat should be fastened to the pier or to the gate frames by means of a relatively few bronze bolts, so as to allow the gate to be blown off its seat in case high-pressure penstock water is admitted to the turbine and draft tube while the gates are closed.

C. Bulkhead gates may be placed either by means of a gate-lifting beam or a
sling, direct-connected to a crane and permanently attached to each gate.
The sling arrangement should be utilized where sufficient clearance exists
between the deck and the crane hook, in the raised position. This method
simplifies the gate construction and eliminates the need of grappling for
the gate as well as the handling of a tag line for hook release. Where con-
ditions necessitate the use of a gate lifting beam, the connection between the
gate and lifting beam should consist of a centrally located connection.
Further discussion of lifting frames and lifting beams is presented in
Paragraph 1.34.

D. The type of seating and bearing surfaces to be used on these gates depends
on the maximum tail-water head over the bottom of the draft tube and upon
tail-water turbulence. For small gates, where the maximum head will not
exceed 30 feet and the tail-water turbulence is not expected to be severe,
it is customary to use wood bearing and sealing surfaces, designed to bear
directly on the concrete powerhouse substructure. For gates where the
maximum head is greater than 30 feet, or where extreme tail-water tur-
bulence is expected due to adjacent spillways or high-velocity water, it is
customary to provide gates having metal bearing surfaces bearing on metal
seats. Gates of this type have bearing surfaces only on the two vertical
sides, but have a rubber sealing strip extending continuously around the
gate.

E. Where conditions are favorable it is customary to store the gates in the
upper portion of the gate guides, above the portal opening and below the
surface of the deck. The gates should be suspended from latches mounted
on the piers and recessed sufficiently to allow grating to be placed over
the gates being stored.

F. Maximum tail-water surface elevations are usually found on the power
plant structure drawings. However, if these elevations are not designated
as "maximum," the designer should obtain an authoritative determination
of the water surface to be used for gate designs. Maximum stresses are
calculated on the basis of a dry draft tube and maximum tail-water surface.
For gates fabricated from structural quality plates and shapes the maximum
tensile stress should not exceed 24,000 psi. Other stresses should conform

DRAFT-TUBE
BULKHEAD

GATES

Advantages
Over
Stop Logs

Guiding & Seating

Method of
Handling

Seating

& Bearing Surfaces

Storing

Design Considerations

1.12G

HYDRAULIC GATES (Continued)

BULKHEAD GATES (Continued)

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to the specifications of the American Institute of Steel Construction.
Normal working stresses are calculated on the basis of normal tail-water
surface elevation and for gates of structural plate and shapes.

(1) The bearing pressure between the bearing surfaces of the gate and its
seat under maximum load should not exceed 10,000 psi for metal-to-
metal bearings, and should not exceed 1,000 psi for wood gate facings
loaded perpendicular to grain and bearing against the concrete power-
house substructure. It is desirable to hold bearing pressures on wood
facing to not more than 600 psi while subjected to normal loading.
At 600 psi loading the wood will yield about 10 percent of its original
thickness; at 1,000 psi the yield will be approximately 25 percent.

(2) The pressure required to hold the gate securely against its seat prior
to the pumping out of the draft tube is dependent upon the size of the
gate and the degree of tail-water turbulence to be expected; thus the
total spring loading is considered as a constant loading per square
foot of gate area, the area being taken on the nominal size of the gate.
The degree of tail-water turbulence then becomes an index to the
constant loading per square foot of gate. It should not be less than
10 pounds, which should suffice for turbulence due to the operation of
an adjacent power unit. It should not be less than 25 pounds in cases
where the draft-tube portal is adjacent to high-velocity spillway or
outlet discharges. Intermediate values should be used for intermediate
degrees of tail-water turbulence. Usual spring deflections, stresses,
etc., are treated in Subparagraph 1.12G.

(3) The faceplate is usually on the pressure side of the gate, in contact
with and longitudinally connected to the compression flanges of outside
and intermediate supporting beams. When the faceplate is to be welded
to the edges of the intermediate beam flange, it is usual to select a
beam with a narrow flange width. This produces minimum flange
deflection, with the plate bearing on the outside edges of the flange.
Since negative moment in the faceplate over the top and bottom beams
is neglected, the plate is considered as a beam supported at the webs
of the outside beams and continuous over supports at each edge of the
intermediate beams. The plate is therefore subjected to compression
stresses due to flexure in the supporting beams and to flexure stresses
caused by the hydraulic load between the beam flanges. To calculate
the stress component caused by supporting beam flexure, a maximum
plate width of a beam flange plus 40 times the plate thickness should
be considered as combined with the flanges of intermediate beams,
and the plate lap on the beam plus 20 times the plate thickness should
be considered as combined with the flanges of the top and bottom beams.
The plate lap should be at least five times the plate thickness. The
intermediate beam flanges should also be examined for the combined
stresses caused by beam flexure and the loading of the outside edge of
the flange.

Four seal loading springs to a gate are usual, each spring supplying 25
percent of the total loading. The normal deflection of the spring is the
deflection when the gate is completely seated. This deflection should never
be less than one-half inch and preferably not less than five-eighths inch.
The maximum deflection of the spring should be the normal deflection plus
three-eighths inch for wood-faced gates, or the normal deflection plus one-
eighth inch for gates having metal-to-metal bearings. Spring stresses
under maximum deflection should not be greater than 0.7 of the yield point
for alloy steels. Stock sizes of spring material should be used, with proper
proportion of rise to effective length. A rise of one-ninth to one-seventh of
the length is usual. Springs with hot-formed eye ends are very satisfactory.
Drawings should specify hot forming, quenching in oil, and drawing to spring

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