1.1

HYDRAULIC VALVES

GENERAL CONSIDERATIONS

.1 Hydraulic valves in general regulate or control flow in conduits, pipe lines,
and penstocks. Their construction is such that the closing member operates
and remains within the water passageway. All valves designed by the Bureau
may be either opened or closed under unbalanced hydraulic conditions. Valves
may be classed as ensign valves, needle valves, tube valves, hollow-jet valves,
plug valves, and butterfly valves. Except for the ensign valves, which by their
construction are limited to installation at the entrance, and hollow-jet valves
which cannot economically have their air requirements satisfied except at the
outlet, hydraulic valves can be installed at the entrance, the outlet, or an
intermediate position in a water passageway, provided that provision is made
for the supply of an adequate amount of air at the proper point. In practice,
however, needle and tube valves are rarely used except in free-discharge
regulating service at the outlet end, whereas plug valves and butterfly valves
are most commonly used for emergency and maintenance shutoff service in ■
penstocks and pipe lines, positioned at some intermediate point in the passage-
way. When a valve is used for regulation, the potential energy at the valve
inlet is converted to kinetic energy at the outlet, where the velocity is equal
to the spouting velocity corresponding to the difference between the total
effective head at the inlet and the static head at the outlet. The amount of
power in large jets is enormous and in the design of regulating valves it is
essential that every precaution be observed to prevent damage to the valves
and to the surrounding structures.

.2 The type of valve selected for any given installation will depend primarily on service conditions to be encountered. In general, valves here considered are designed for operating heads of 75 feet or more.

A. Where sandy, silt-laden, or carbonate water is apt to be discharged through
the passageway, valves with concentric and fairly close-fitting parts that
move upon each other should be avoided, as the clearances between parts
are quite apt to become choked and the valve thus rendered inoperative.
For this type of service, butterfly valves may be found most suitable.

B. Where spray from free-discharge valves is objectionable, as in the
vicinity of electrical installations, hollow-jet valves will be found to give
the least disturbed discharge, with needle valves a close second. Butter-
fly valves, plug valves, and tube valves tend to cause considerable disper-
sion of discharged water at certain part-open positions. Because of these
differences in discharge jets, some valves require larger or more expen-
sive stilling basins than others. Needle valves, to some extent at least,
appear to require the least stilling-basin expenditures.

C. Maintenance should be given consideration in the selection of valves, as
the ease and frequency of repairs will have an important bearing on the
cost of operation and the reliability of the service affected. Cavitation is
an ever-present menace in any high-head valve, and has been a major cause
of the discontinuation of the procurement of the earlier types of needle
valves through its heavy maintenance requirements. Cavitation will be
more fully discussed in connection with the descriptions and data given
for each type of valve.

D. If two or more types of valves are equally suited functionally for a given installation, selection of the valve will depend on the initial cost, and cost of maintenance. For Bureau-designed valves, the costs of free-discharge valves of all types are roughly proportional to the valve weights for any giver capacity, and the weights of any type of valve will vary approximately as the cube of the inlet diameter. Comparative weights of some of the valves are shown on Figure 1.

FUNCTIONS &

TYPES

BASIS FOR SELECTION OF TYPE

Influence of

Impurities in Water

Dispersion Considerations

Maintenance Considerations

Final Selection

1.2.E

Emergency
Gates

REQUIRED

HYDRAULIC VALVES (Continued)

GENERAL CONSIDERATIONS (Continued)

In addition to the valves so far considered, certain gates can be and frequently are used for conduit and pipe-line regulation and control. High-pressure gates, slide gates, radial gates, jet-flow gates, and ring gates or cylinder gates are used for free-discharge regulation, while some of these as well as ring-follower and ring-seal gates are used for shutoff service.

E. Usually, for penstock or conduit service, provision of only one gate or valve is not sufficient. In addition to a regulating valve or gate, there must be selected also an emergency or shutoff gate or valve so located and so controlled that its closure under emergency unbalanced conditions is assured for any except extremely improbable combinations of circumstances.

PREDESIGN .3 In order that valve types and sizes may be intelligently selected for any given INFORMATION installation, certain fundamental data are required. The quantity of water to be discharged and the effective static and operating heads are most important. Usually both maximum and minimum effective heads are required. Frequency of valve operation and length of time a valve may be in service each year are important, as are also climatic conditions, control requirements, number of units desired or most favored, stilling-basin requirements for free-discharge valves, emergency cutoff requirements, clear or muddy water, amount and types of salts in solution in the water, valve positions in conduit or pipe line, and accessibility of valves and controls. The foregoing conditions usually determine the size and number of pipes and valves for the installation, but frequently special conditions are found that are decisive factors.

NEEDLE
VALVES

Types of
Discharge

Proportioning

Operation

VALVE DESIGN

.4 The term "needle valve" is usually applied to a valve having a circular
orifice which is closed by a conical plunger. The needle valve is used by the
Bureau primarily for regulating purposes, which necessitates operation of the
valve for long periods of time at any opening from fully closed to fully open.
Experience has proven that the following principles must be observed to secure
satisfactory operation of needle valves.

A. The valve should normally be installed at the downstream end of the outlet conduit, discharging directly into the atmosphere. In some cases it may be possible to discharge the needle valve submerged under water if adequate precautions are taken to prevent the formation of vacuum conditions and if provisions are made for dissipating the energy of the jet without damaging the surrounding structure.

B. The water passage through the valve must be carefully proportioned to prevent subatmospheric pressure and cavitation, and to provide smooth flow at any opening, with the important provision that the minimum opening or control orifice must be maintained at the extreme outlet end. The nozzle tip should be shaped to accelerate the water toward the exit, and should terminate at a sharp-edged flare below the seat to permit free access of air to the jet. In order to maintain the control orifice at the outlet end for any opening, it is also essential that the angle between the nozzle and needle be converging in the direction of flow.

C. In a needle valve, the hydraulic forces acting on the needle are approximately balanced so that the force required to move the needle through its entire travel can readily be provided by mechanical operation. However, to provide positive seating in the closed position to minimize leakage, a large force should be supplied. The heavy seating force required usually makes it more economical to move the needle by internal

1.4.D

HYDRAULIC VALVES (Continued)

VALVE DESIGN (Continued)

hydraulic chambers with the reservoir head providing the energy for motive power. To insure positive positioning of the needle, the water pressure is controlled by an automatic mechanical follow-up valve known as the paradox control. This control is arranged to cause the needle to follow the movement of the control stand handwheel and to automatically maintain the needle in the position set by the control stand handwheel. A position indicator is provided in the control stand to give the operator the exact needle position.

D. The needle valve has evolved through a number of successive designs, the
first of which was originated in 1906. Although very few needle and tube
valves are now being designed, the inclusion of several types in the follow-
ing discussion is for the purpose of acquainting the new engineer with
this equipment, and to facilitate necessary changes and repairs to existing
equipment now operating on the projects. The principal types of needle
valve designs are outlined below in the order of their development.

(1) The ensign valve (see Figure 2) is arranged to mount on the upstream
face of the dam, and was the earliest Bureau type of needle valve. The
first installation was made about 1909. The entire valve is submerged
in the reservoir with the needle moved by reservoir water pressure
by controlling the pressure in an interior chamber. Because of its
form and lateral admission of water, the ensign valve must be placed
at the intake end of the conduit and consequently is not protected by a
guard gate. It cannot be reached for servicing and repairs on the
pressure side of the seat except when the reservoir water level is
below the valve. The design of the valve is such that serious cavita-
tion has occurred in the throat ring, in the concrete downstream from
the throat ring, and on the face of the needle especially in the vicinity
of the top and bottom guides. Because of this difficulty and the
problems of maintaining the valve, it has not been favored in recent
designs.

(2) Figure 3 shows a motor-operated needle valve installed at Roosevelt
Dam in 1919 to replace the original ensign valve installation. This
valve was installed at the downstream end of the outlet conduits and
has given satisfactory service since the date of installation.

(3) Figure 4 shows a valve of the balanced needle type which was installed
at Pathfinder Dam in 1922. This valve eliminated the bend in the con-
duit and the operating rod in the stream flow, which were considered
to be objectionable features of the motor-operated needle valves at
Roosevelt Dam. Also, this valve was designed to take advantage of
the features of hydraulic operation. The needle is moved by water
pressure from the outlet conduit which acts on interior chambers in
the valve. The movement is controlled by a handwheel installed above
the valve, with the motion transmitted through shafting and gearing to
a positive positioning device located inside the valve.

Types of Needle Valves

Ensign

Valve

Motor-Operated
Needle Valve

(4) The internal differential needle valve (see Figure 5) is an improvement
in design over the balanced valve noted above. It was designed to
secure greater economy in weight and cost and applies the paradox
control to the internal hydraulic chambers to obtain movement of the
needle. However, because of the rounded nozzle, together with a
divergent angle between the nozzle and needle, the water passages in
this valve induce a cavitation condition when the valve is operated at
partial openings which results in serious pitting of the needle and nozzle.

Balanced Needle Valve

Internal Differential Needle Valve

1.5

Interior Differential

Needle Valve

HYDRAULIC VALVES (Continued)

VALVE DESIGN (Continued)

(5) The interior differential needle valve (see Figure 6) is an improvement
in mechanical design over the internal needle valve, with the needle
guided on the interior surfaces which eliminates the necessity for
guide ribs to support the needle. This valve was first designed with
a diverging water passage (36°22' nozzle and 42° needle), and with a
rounded discharge orifice on the nozzle and a coefficient of discharge
of 0.50. As previously stated, diverging-water-passage valves with
rounded discharge orifice are subject to serious cavitation and have
high maintenance costs. An interior-differential design with converg-
ing water passages (40° nozzle and 39° needle) and sharp-edged
orifice was therefore designed which superseded the previous types.
The redesign of the water passage in this valve to eliminate cavitation
resulted in the present improved design of needle valve. The needle
is moved by hydraulic water pressure from the reservoir with the
movement regulated by a paradox control.

A dimensional ratio layout diagram has been prepared for use in designing needle valves (see Figure 7). The water passageway dimensions for any valve can be obtained from the ratio diagram 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 given by the following formula:

Q = CA √2gh

where

TUBE VALVES

In-Line
Туре

h

=

32.2 feet per second

effective head in feet at a point one pipe diameter
immediately upstream from the inlet flange of the
valve, and is the sum of the velocity head and the
pressure head when the valve is discharging.

An assembly drawing of this valve in the 86-inch size is shown in
Figure 6. The full range of these valves is covered in Figure 8, which
shows the capacity for varying heads, and Figure 9, which can be used
for estimating weights.

.5 Tube valves are used primarily in outlet works and may be located either in
the central portion of a conduit or at its downstream end. These valves were
developed from the internal-differential needle valve, and the water passages
are similar to those through this type of needle valve except that the downstream
end of the needle is omitted. A tube or hollow cylinder similar to that of the
cylinder gate, instead of a needle, comprises the moving part of the valve.
This is actuated by a hydraulic cylinder and piston and a pressure pump, or by
a screw with an electric motor, or by manual control. Refer to Figure 10.

A. Tube valves used in the central portion of conduits have long bodies with a
30-degree nozzle and are provided with air inlets to aerate the jet imme-
diately downstream from the valve seat. Four valves of this type are
installed at elevation 742 in the river outlets at Shasta Dam. The discharge
curves for the Shasta valves show that a cavitation range for partial dis-
charge begins with heads of about 100 feet, and increases as the head becomes

1.5.B

HYDRAULIC VALVES (Continued)

VALVE DESIGN (Continued)

higher. The valves should not be operated for any appreciable length of
time within the cavitation range.

B. Tube valves used at the downstream ends of conduits have short bodies
with a 45-degree nozzle. This is the free-discharge type as contrasted
to the in-line type described above. Two free-discharge 44-inch tube
valves are installed as outlets at Green Mountain Dam. The amount of
water discharged may be regulated between 30 and 100 percent of the
opening range. Between 0 and 30 percent, the jet is unstable and will not
stay within the area of the designed discharge pool. The valves should
not be allowed to discharge for any considerable length of time within the
0 to 30 percent gate opening range. For this reason the tube valve is not
entirely satisfactory as a regulating valve. An assembly drawing of the
44-inch tube valve is shown in Figure 10.

C. The shape of the water passages is important and should be designed to
give a gradually increasing velocity without abrupt changes within the
valve. No ratio diagram is available for either the long or short body type.
Dimensions for a layout drawing may be obtained by proportioning from
one of the valves designed previously. A layout drawing should be prepared
first as a guide in designing the valve. The discharge for full valve opening
is determined by the same formulas as in needle valves using values of
C as follows:

C = 0.52 for the short body (45° nozzle) free-discharge valve

C = 0.72 for the long body (30° nozzle) in-line type discharge
valve (Shasta Dam).

D. The required thrust of the operating mechanism can be obtained by adding together (1) the end area of the tube and the central tip multiplied by 50 percent of the static head on the valve, (2) the friction resistance of the seals, and (3) the friction resistance due to 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. The discussion of the torque and strength required in the operating parts and castings as given in the design of the hollow-jet valves, Paragraph 1.6, also applies to tube valves.

.6 The hollow-jet valve is used primarily in outlet works and is located on the downstream end of the outlet pipe. The water discharged may be closely regulated over the entire opening range of the valve. This valve is essentially a needle valve with the needle, or closure member, pointing upstream. The nozzle is eliminated allowing the water to discharge from the bell-shaped body in a tubular or hollow jet, the outside diameter of which does not change regardless of valve opening.

The jet leaves the valve with very little dispersion at any valve opening. The effect of the hollow form is to distribute the energy dissipation over a comparatively large area of the stilling basin, materially reducing the destructive effect but tending to increase the size of basin required. Directing the valve discharge downward at an angle of 20 to 30 degrees aids in reducing disturbances in the basin. An assembly drawing of the 72-inch hollow-jet valve is shown in Figure 11. The full range of these valves is covered in Figure 12, which shows the capacity for varying heads, and in Figure 13, which can be used for estimating weights.

A. The proportions of the water passage through the valve are important and have been developed from model experiments to prevent subatmospheric pressure and cavitation. A water passageway ratio layout diagram has been prepared and is available for use in designing the valve. The water

Free-Discharge
Туре

Hydraulic Design Considerations

Mechanical Design Considerations

HOLLOW-JET
VALVES

Hydraulic

Design Considerations