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.1 Hydraulic valves in general regulate or control flow in conduits, pipe lines, FUNCTIONS and penstocks. Their construction is such that the closing member operates & and remains within the water passageway. All valves designed by the Bureau TYPES 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 1 penstocks and pipe lines, positioned at some intermediate point in the passageway. 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 BASIS FOR. service conditions to be encountered. In general, valves here considered are SELECTION designed for operating heads of 75 feet or more. OF TYPE
A. Where sandy, silt-laden, or carbonate water is apt to be discharged through Influence the passageway, valves with concentric and fairly close-fitting parts that Of move upon each other should be avoided, as the clearances between parts Impurities are quite apt to become choked and the valve thus rendered inoperative. in Water For this type of service, butterfly valves may be found most Suitable.
B. Where spray from free-discharge valves is objectionable, as in the Dispersion vicinity of electrical installations, hollow-jet valves will be found to give Considerations the least disturbed discharge, with needle valves a close second. Butterfly valves, plug valves, and tube valves tend to cause considerable dispersion of discharged water at certain part-open positions. Because of these differences in discharge jets, some valves require larger or more expensive 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 Maintenance the ease and frequency of repairs will have an important bearing on the Considerations 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 Final installation, selection of the valve will depend on the initial cost, and cost Selection of maintenance. For Bureau-designed valves, the costs of free-discharge valves of all types are roughly proportional to the valve weights for any given 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.
HYDRAULIC VALVES (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.
In order that valve types and sizes may be intelligently selected for any given 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.
The term “needle valve” is usually applied to a valve having a circular
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 Types of first of which was originated in 1906. Although very few needle and tube Needle Valves valves are now being designed, the inclusion of several types in the following 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 Ensign face of the dam, and was the earliest Bureau type of needle valve. The Valve 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 cavitation 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 Motor-Operated Dam in 1919 to replace the original ensign valve installation. This Needle Valve 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 Balanced at Pathfinder Dam in 1922. This valve eliminated the bend in the con- Needle Valve 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.
(4) The internal differential needle valve (see Figure 5) is an improvement Internal in design over the balanced valve noted above. It was designed to Differential secure greater economy in weight and cost and applies the paradox Needle Valve
control to the internal hydraulic chambers to obtain movement of the
Interior Differential | codle Valve
HYDRAULIC VALVES (Continued)
(5) The interior differential needle valve (see Figure 6) is an improvement
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 V 29.h
b = effective head in feet at a point one pipe diameter
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
A. Tube valves used in the central portion of conduits have long bodies with a
1.5. B HYDRAULIC VALVES (Continued) VALVE DESIGN (Continued)
higher. The valves should not be operated for any appreciable length of
B. Tube valves used at the downstream ends of conduits have short bodies Free-Discharge with a 45-degree nozzle. This is the free-discharge type as contrasted Type 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 Hydraulic give a gradually increasing velocity without abrupt changes within the Design valve. No ratio diagram is available for either the long or short body type. Considerations 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 (309 nozzle) in-line type discharge
D. The required thrust of the operating mechanism can be obtained by adding Mechanical together (1) the end area of the tube and the central tip multiplied by 50 Design percent of the static head on the valve, (2) the friction resistance of the Considerations
seals, and (3) the friction resistance due to the weight of the moving parts.
.6 The hollow-jet valve is used primarily in outlet works and is located on the HOLLOW-JET downstream end of the outlet pipe. The water discharged may be closely VALVES 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 Hydraulic have been developed from model experiments to prevent Subatmospheric Design pressure and cavitation. A water passageway ratio layout diagram has Considerations
been prepared and is available for use in designing the valve. The water