« PreviousContinue »
into the open instead of being largely expended in the destruction of the conduit.
The two 58-inch balanced needle valves in the North Tunnel Outlet at Pathfinder Dam, were the first valves of this type installed by the Bureau. These valves have been in successful operation since 1921, the only trouble experienced being some damage to the inner control mechanism. This damage was caused by failure of the operator to vent the air from the interior of the valve as it was being filled with water by opening the emergency gate behind it.
Profiting from experience derived at Pathfinder Dam, the 60-inch balanced needle valves for Tieton and Lahontan Dams were designed with the control apparatus placed outside the valves, in cases bolted to flanges provided on the upper sides of the valve bodies. This type of control has proved satisfactory. Operation of the Tieton valves revealed that the cylinder walls of the control had not been made as heavy as required and consequently went "out of round" where the ports pierced the walls. This condition was relieved by admitting conduit pressure to the closed side of the outer sleeve by means of a ^-inch pipe, and by providing high-pressure lubrication with a heavy oil.
In designing the Tieton valves, the water passages were proportioned so that the velocity is constant until the water reaches the forward portion of the valve and starts to converge inward to pass along the conically pointed needle and out through the nozzle. When this point of travel is reached, the passages are shaped and proportioned so that the velocity is gradually accelerated on a sine curve until maximum velocity is reached as the water issues from the nozzle. This change was based on a series of tests conducted on a 4-inch experimental model. It resulted in a considerable improvement in performance and efficiency over that obtained with the Pathfinder valves. The two 60inch balanced needle valves at Tieton Dam were tested by the salt brine injection method and the coefficient of discharge, based on the total effective head at the entrance flanges and the gross nozzle areas, was found to be 72% percent as compared with 62 percent for the Pathfinder valves.
When the cylinder controls were being designed for the 48-inch balanced needle valves for McKay Dam, advantage was taken of the experience obtained at Tieton Dam. The control cylinders were made heavier, and when later put into operation proved very satisfactory.
INTERNAL DIFFERENTIAL NEEDLE VALVES
In 1928, a new principle of operation was developed, which resulted in materially reducing the physical dimensions, weight, and cost of needle valves. The new valves are arranged and constructed so that the annular external ring around the needle or piston, commonly known as the "bull ring," is eliminated, making possible a marked reduction in the external diameters of the valves, accompanied by an even more marked reduction in the over-all
lengths. These values are known as "internal differential needle valves."
In valves of this design, the interior is divided into three tandem pressure chambers, designated A, B, and C. These chambers are formed by a fixed diaphragm, inside the needle, supported by a heavy diaphragm tube concentric with the axis of the valve, with the rear end terminating in a flange bolted to the valve body. The rear end of the needle is closed by a hemispherical head. The head is provided with a bushed hub which rides on the diaphragm tube as the needle and its attached head move back and forth in opening or closing the valve. The exterior cylindrical surface of the needle is telescopically mounted in an enclosing cylinder, supported by radial ribs extending through the water passage from the walls of the exterior shell.
Chambers A and C are interconnected by passages formed in the diaphragm tube; so that water can readily pass from one to the other in either direction. Consequently, the pressures in these two chambers are always equalized. Pressure introduced into the chambers causes the needle to move in the closing direction. When the pressure is released in chambers A and C, the pressure in chamber B produces a force on the inner face of the hemispherical needle head which causes the needle to move in the opening direction. Chamber B is connected to the conduit at all times and consequently has reservoir pressure therein.
The sliding clearance between the bore of the bushing in the hemispherical needle head and the cylindrical surface of the diaphragm tube, permits a slow transfer of water from chamber B to chamber A, then into chamber C through the interconnecting passageways in the diaphragm tube. From this it will be seen that if water is prevented from escaping from chambers A and C, the infiltration from chamber B will soon establish substantially equal pressure in all three chambers. When this occurs, the preponderance of closing forces so engendered in chambers A and C are greatly in excess of the opening force produced in chamber B, and the needle will start moving toward the closed position. This will diminish the cubical contents of chamber B, causing part of the water to be forced back into the conduit.
Water from chambers A and C is released through the needle tip by a manually controlled spear operating in conjunction with a tube carried in the tip of the needle. The forces acting upon the needle and piston of this valve are different from those on the valve previously described and are as follows:
Opening forces.—(a) Conduit pressure within chamber B acting against the concave face of the movable needle head.
(b) Conduit pressure acting against that part of the downstream conical end face of the needle that is of greater diameter than that of its line contact with the valve seat when the needle is in the closed position.
(b-1) Jet reaction and pressure upon the downstream
conical and curved surfaces of the needle produced by the high-velocity flow whenever the valve is either partially or wholly opened.
(b-2) Were this valve to be used in a pipe line or penstock, then (a) would be augmented by conduit pressure over the full area of the downstream conical and curved face of the needle.
Closing forces, (c) Conduit pressure in chamber A against the convex upstream face of the movable needle head.
(d) Conduit pressure in chamber C against the interior conical end curving surfaces of the discharge end of the needle.
With conduit pressure in chambers A and C, closure can be effected with conduit pressure present in chamber B or with the pressure in this chamber reduced.
With the forces produced by the above pressure combinations, the piston or needle may be moved to the opened or closed positions or to any intermediate position desired and maintained there indefinitely by regulating the exhaust from chambers A and C and from chamber B. This valve provides a larger effective opening force and a larger differential closing force over the opening force than any of the previous needle valves, due to the fixed diaphragm inside the needle and the three tandem pressure chambers. This increase in available operating force permits operation under lower heads at the close of the irrigation season when
the reservoir levels are drawn down. When the valves have been idle for a long time, the friction of the needles may be increased by corrosion or scale depositions and additional force may be required to operate them.
The needles are prevented from "slamming" when they reach the limits of their travel in either an opening or closing direction, by arranging the port apertures so that the speed is gradually reduced. Control and air-vent stands for the valves may be mounted either upon a floor above or directly on the upper side of the valve body.
Two 60-inch valves of this type were installed at Gibson Dam, another pair of the same size at Echo Dam, and still another pair at Coolidge Dam. As a result of the experience derived from the operation of these and subsequent installations, additional improvements and revisions have been made in design.
Some difficulty was experienced in preventing too much leakage from passing between the bore of the needle body and the exterior diameter of the diaphragm, from chamber B into chamber C, and thence into A. This was corrected by putting a heavy snap piston ring around the outer diameter of the diaphragm.
When the valves were wide open, the high velocity of flow passing over the conical face of the needle was found to be producing a very pronounced suction or aspirator effect upon chamber A, by drawing water out of the chamber and its interconnected chamber C, through the clearance between the exterior cylindrical surface of the needle and the bore of the bronze liner in the front end of the cylinder. This made it hard to get the needle started moving toward the closed position under certain conditions of operation. It was also discovered that when the valves were closed, the conduit pressure in the water passageway surrounding the needle forced water inward through the same clearance into chamber A and from there into C, causing a force tending to hold the needle against its seat. This condition was only partially relieved, under certain operating conditions, when the control spear was withdrawn from the tube in the end of the needle.
No practicable means were found for preventing the outward or inward flow of water through this clearance. This condition was finally corrected by changing chamber
A into a plain cavity with a liberal drain to carry the inflow away before it could build up any appreciable pressure, as shown in figure 5, and by discontinuing the interconnecting passage between chambers A and C. By this arrangement chamber B was used as before to open the valve, and chamber C was used only to close the valve. Chamber B is exhausted to the atmosphere during the closing cycle by means of a new control known as the paradox control, see figure 6, located beneath the valve and placed in communication with chambers B and C by means of cored passageways formed in the bottom radial rib. This control utilizes two moving elements, consisting of a cylindrical male member with a coarse, rapid pitch multiple thread upon its external diameter, meshing with the female member having a similar thread tapped into the hub of a three-spool valve mounted to travel vertically in the multiported control body bolted to the bottom of the needle valve.
Valves equipped with paradox control have a number of advantages over their predecessors. The control system makes it possible to hydraulically move the needles throughout the extreme ranges of travel without requiring the presence of water in the conduits. This feature has proved to be most advantageous, as it makes the routine inspection and scale removal operations much easier to perform.
Twelve 84-inch valves of this type are installed in the canyon-wall outlets at Boulder Dam and twelve 72-inch valves of similar construction arc installed in the tunnel-plug outlets. Valves of this type have proved to be economical to build and capable of giving satisfactory and dependable service. The marked reduction in the physical dimensions of the valves makes possible substantial reductions in the size of the control structures and a corresponding reduction in cost.
Two 84-inch valves of this type were installed at Madden
Dam. These valves were tested by piezometers in the supply conduits to determine flow and discharge capacities. Results of the tests, which have not been completed, indicate that the coefficient of discharge will exceed 75 percent, based on the total net effective head at the inlet flanges and the gross nozzle areas. Pressure-rise tests were likewise made by closing the valves as rapidly as possible. It was found that the maximum pressure rise attainable was less than 2 pounds per square inch.
Preliminary tests conducted on the 84-inch internal differential needle valves at Boulder Dam indicated coefficients ranging from 75% to more than 80 percent. Further tests on the same valves indicated a coefficient of discharge of 78 percent. The valves were tested by a calibrated Pitot tube and piezometers at the outlet conduits.
Figure 8 shows capacity curves for balanced needle valves, based on tests of the Tieton valves. The curves are conservative for valves of later design.
The marked scale depositing characteristics of waters