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HIGH-PRESSURE RESERVOIR OUTLETS
BY P. A. KINZIE, SENIOR ENGINEER, BUREAU OF RECLAMATION
THE BUREAU OF RECLAMATION defines highpressure outlets as those which control and regulate the release of water from dams, reservoirs, and conduits where the effective head is more than 75 feet. For such heads the conventional slide gates and similar equipment commonly employed under lower heads are found to be inadequate, due to the more severe conditions of operation.
STANDARD HIGH-PRESSURE GATES A standard high-pressure-gate frame consists of two rectangular, heavily ribbed castings which form the water passage. Where the adjacent mating, vertical-flanged faces are brought into bolted and watertight engagement, they form a vertical slot or recess in the walls on each side of the water passage, within which the gate leaf is received and guided in its opening and closing movements.
The gate leaf is provided with bronze seal bars on the downstream face that rest upon bronze seat bars of a different composition attached to the downstream sides of the slot walls. When the gate is closed the full water load on the leaf is carried to its seal bars and transferred into the seat bars on the gate frames. Consequently, whenever the leaf is moved upward to open the gate, or downward to effect closure, the leaf seal bars act like runners on a sled, rubbing or sliding on the mating surfaces of their supporting seat bars.
Extensive tests were made by the Bureau to determine the bronzes best suited to carry heavy loads under sliding contact without causing abrasion. These tests led to the development of class C and class D bronzes. The two compositions, when used in rubbing or sliding contact against each other, are capable of sustaining 3,000 pounds per square inch load without seizure. However, for design purposes, the bearing pressures should not exceed 700 pounds per square inch.
The slot recesses in the gate frames are continued upward into the envelope or bonnet castings whose lower flanges are bolted to a flange formed on the top of the frame castings. The gate leaf, when raised to its open position, is received within this bonnet, leaving the water passage free. The gate frames and bonnets are strongly ribbed, both vertically and horizontally, to distribute the heavy reactions set up by the hydraulic hoists in the opening and closing cycles; but they are not designed to take the internal water pressures to which they are subjected. This is cared for by circling these parts with reinforcement bars and then encasing the gates in concrete up to the joint between the top bonnet flange, where the bonnet cover, which is
designed to resist the maximum internal water pressure, is firmly attached by bolts.
To the upper flange of the bonnet is bolted the bonnet cover (see fig. 1) which is heavily ribbed and converges into the circular flange that forms the lower head of the hydraulic hoist cylinder which is bolted to it. The hoist cylinder continues upward and terminates in a similar upper flange to which the cylinder head is bolted. Within the cylinder is a semisteel or bronze piston, attached to the upper end of the gate stem which extends downward through stuffing boxes in the bonnet cover. The lower end of the stem continues into the gate leaf and is fastened to it by a suitable nut. First-grade packings of graphited flax, or of the built-up “Chevron” type are used in the stuffing boxes.
The piston in the hydraulic cylinder is made oil tight by a series of from four to six conventional metal piston rings, closely fitted into mating slots machined in the periphery of the piston. The hydraulic hoists are designed for pressures ranging from 750 pounds per square inch up to 1,500 pounds per square inch, according to requirements. They are operated by either triplex oil pumps or highspeed nitrided steel gear pumps, with capacities ranging from 5 gallons per minute up to 30 gallons per minute, depending upon gate sizes and operating time cycle requirements. A light-grade, inexpensive, lubricating oil is used in the pressure systems, which consist of pressure delivery and exhaust return lines fitted with air chambers, pressure relief valves, and high-grade, straight-way, lever-operated plug valves for controlling gate operations.
The general arrangement of the gates insures easy assembly, facilitates maintenance, and provides a completely self-contained structure, capable of withstanding the reactions produced by the hydraulic cylinder which may in some instances exceed 1,000,000 pounds.
In the earlier installations the importance of avoiding abrupt changes in the water passage surfaces and the prime necessity for an ample air supply on the downstream side of the gate leaf, was unknown. Consequently, the earlier installations had been in service but a relatively short time when it was discovered that the metal on the downstream side of each recess, bolt, or other departure from continuity of the water passage surfaces, was being eaten away at an alarming rate. The gravity of this condition was accentuated by the fact that the cause was unknown; and, in consequence, preventative measures were of a very doubtful character.
Among the first of the gates of this general character to be put into service under relatively high heads, were two sets of three gates each, 5 by 10 feet in size, installed in the Roosevelt Dam, Salt River project, Arizona, in 1908. These gates were intended for service under a maximum head of 220 feet. They were operated by hoist cylinders in wells extending to the top of the dam from an elevation 33 feet above the sluiceway floor. The gates were put into service while the reservoir water surface was still much less than its maximum; but trouble soon developed, and when an inspection was made, it was revealed that both gates and the tunnel below had been seriously damaged.
Arizona spillway, Boulder Dam, looking downstream into entrance of inclined tunnel.
It was found that the concrete and metal linings had been loosened, eroded, or washed out; the bolts and fastenings of the gates had become loosened and in some cases were missing; the bronze seats had been damaged by blows from the loosened parts; and the bronze roller trains, used behind the gate leaves, had been either broken or carried away. The concrete in the roof, floor, and walls had been badly eroded by the high velocity. and holes had been torn in the tunnel floor from 4 to 6 feet deep. After the damage had been repaired, the gates were again put into service; but continued release of water damaged them still further, finally forcing their abandonment. The tunnels were accordingly plugged with concrete and two 38-inch needle valves, guarded by bronze slide gates were installed.
Four gates of the earlier type were installed at Pathfinder Dam, North Platte project, Wyoming, in 1909. These gates were 44 inches wide by 77 inches high, and were installed in the north tunnel which is cut through solid granite. When the discharge from the gates was sufficient nearly to fill the tunnel, reverberating and hammering sounds were heard at the tunnel outlet, the intensity of the sounds increasing as the flow increased until they finally attained such violence as to cause the dam and canyon walls to tremble.
When the water was shut off and an inspection made, a serious condition was disclosed. The solid granite walls, roof, and floor of the tunnel had large masses torn out, portions of the concrete below the gates were damaged or destroyed, the 24-inch plate steel linings were torn, anchor bolts were sheared off, and nuts stripped from the bolts. The gates had suffered but little injury, due primarily to their having been operated in wide-open positions. An inclined air shaft was cut through the roof of the tunnel immediately downstream from the gates, and the damage in the tunnel repaired. When water was again turned through the gates, it was found that the air shaft prevented further damage.
The experience derived from this and other early installations demonstrated that it was essential to admit large quantities of air near the downstream leaf faces, and to keep the walls of the water passages free from cavities or projections. Continued observations disclosed that when
these precautions were followed, vibrational phenomena and destructive erosive effects, later known as “cavitation”, were practically eliminated, even when operating under heads considerably in excess of those where troubles had been previously encountered. As a result of data derived from many later installations, air inlet pipe sizes required for different sizes of gates under varying heads were worked out as shown in figure 2. This chart can be used for square or rectangular gates by finding the equivalent crosssectional area of a round gate on the chart.
Experience at many installations under widely diversified conditions of operation shows conclusively that while gates of this type can be successfully operated in their fullyopened positions under heads as high as 300 feet, they are not satisfactory for continuous operation at partial openings for heads in excess of 75 feet. In other words, it is not recommended that they be used for regulation of water release under heads higher than 75 feet. When conditions of this character arise, the usual procedure is to place a needle valve below the gate and regulate the discharge by the needle valve, leaving the gate wide open as an emergency control.
Gates of this type have a wide field of use. They are used at storage reservoirs for outlet and control regulation where the head is less than 75 feet. An outstanding example is American Falls Dam, Minidoka Project, Idaho, where a battery of twenty 5- by 5-foot high-pressure gates are used to release and regulate the irrigation and power water under a maximum head of 70 feet.
Gates of this type are often installed in tandem in sluiceways through the bases of high dams. Under such conditions, no attempt is made to use them for regulating purposes, as they are either fully closed when not in service, or fully opened when in use. A notable example is the use of six 5-foot 8-inch by 10-foot sluiceways through the base of Madden Dam, Canal Zone, Panama, designs for which were prepared by the Bureau. Each of the six conduits is provided with a tandem pair of high-pressure gates, four gates for two conduits being included in each of the three operating chambers. The upstream gate of each tandem pair is held in reserve as an emergency gate, and is never opened or closed, except in emergencies, until after the downstream gate has been closed and the pressures balanced by means of bypasses. Normally, it is kept in the open position, suspended from a semi-automatic gate hanger hung from a bolt embedded in the concrete roof of the gate chamber. The downstream service gates are each provided with automatic hydraulic gate hangers, operated by oil pressure.
Standard designs for square and rectangular high-pressure gates, ranging in sizes from 2 feet 9 inches square to 5 feet square and from 3 by 4 feet to 4 feet 9 inches by 12 feet, have been developed. These gates are arranged for operation under heads up to 90 feet, when equipped with cast-iron leaves up to 140 feet, when supplied with semisteel leaves, and for heads up to 250 feet, when cast-steel leaves are used. The hydraulic hoist sizes are increased in similar increments to accommodate the three ranges of head, and the gate bonnet covers are made with three sizes of flanges to receive their respective hoist cylinders.
RING FOLLOWER GATES Ever mounting demands for larger gates and operation under higher heads led to the development of ring follower gates, as shown in figure 3. The water passageways through these gates, instead of being square or rectangular, as in the high-pressure gates, are circular. This has the advantage of eliminating transitions from round to square or rectangular on their upstream sides, and from square or rectangular back to round on their downstream sides, when installed in circular conduits.
A further advantage of these gates is that when fully opened they afford an unimpeded passageway which is
essentially the equivalent of the conduit in which they are installed. In other words, the open vertical slot in the wall on each side of the water passage, which is exposed to high velocity flow and common to all high-pressure gates, is eliminated in the ring-follower gates. This avoids the erosive action of the high-velocity flows that occur adjacent to the slots in high-pressure gates subjected to operating heads in excess of 250 feet.
This essential feature of smooth, continuous water passage surfaces is effected by attaching a metal ring, of the same internal diameter as the conduit, to the under edge of the gate leaf, placing it so that when the leaf is raised to the open position the interior surface of the ring is aligned with the circular wall surfaces.
The presence of the follower rings beneath the gate leaf necessitates a recess below the conduit to receive the ring when the gate leaf is lowered for closing. This recess is provided by a lower bonnet which is bolted to the under side of the gate frames in the same manner as the bonnet