« PreviousContinue »
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
SIZES FOR RING-FOLLOWER AND PARADOX GATES. X.O.I88;
Figure 2.—Single air vent pipe sizes for ring follower and
above. The lower end of the lower bonnet is closed by a cover bolted to its lower flange face. In this lower cover, one or more drain outlets of ample area are provided to permit draining the interior of the entire gate, and to flush out any sediment which may collect in the bottom of the bonnet.
An air inlet manifold is cast integral with the downstream gate frame, around the upper exterior surface of the water passage wall; and a series of l^-inch diameter cored holes, equally spaced in staggered relationship in four circumferential rows, admit air to the conduit in close proximity to the downstream face of the gate leaf. Two symmetrically positioned inlet flanges admit air to this distributing manifold from an air inlet pipe carried to a screened inlet located at an elevation above high water level in the reservoir.
The gate leaf is usually made in two parts, bolted together, consisting of the upper bulkhead section and the lower ringfollower unit. On the downstream face of the leaf are facing rings and vertical runner strips of class D bronze. Similar rings and strips of class C bronze are fastened with flat head bronze screws to the upstream faces of the downstream gate frames and bonnets, to form the sliding ways
upon which the vertically moving leaf and ring follower are supported and seated.
The leaf castings are usually made of cast iron of about 50,000 pounds per square inch strength for the lesser heads, of steel castings for higher heads, and of alloy cast steel having a minimum ultimate strength of 80,000 pounds per square inch and a yield point of not less than 50,000 pounds per square inch, for still higher heads.
The method of attaching the gate stem to the leaf, the general construction of the upper bonnet cover, the hoist cylinder, and the piston is similar to the latest type of highpressure gate construction previously described. The upper cylinder head, however, is modified to receive a cylinder head gate hanger which is enclosed and contains a pair of swinging jaws operating in a horizontal plane that engage the under face of the stem extension cap. This cap is retained by a safety stud which is necked to break without injury to other parts of the equipment, should malfunctioning of the hanger occur. A small oil-operated cylinder wedges the jaws apart to release the hanger whenever oil pressure is introduced into the lines to operate the hoist.
As a companion to the ring-follower emergency gates, there has been developed by the engineers of the Bureau, a service gate for opening or closing off the flow through conduits of sizes up to 102 inches in diameter, under heads up to and in excess of 600 feet. Gates of this type, when used in conduits, are known as paradox gates.
Paradox gates have a number of features similar to ringfollower gates, inasmuch as they provide circular waterways with practically unbroken wall surfaces by similar ring-follower members beneath their leaves, as shown in figure 4. They likewise have bonnets extending beneath their conduits to receive the ring-follower members when the leaves are lowered for closure. They are also provided with air inlets in the upper portions of their waterways, immediately downstream from the gate leaf faces in substantially the same manner as in ring-follower gates.
Paradox gates descend or ascend in closing or opening, while supported by vertical endless trains or belts of rollers that extend along either side throughout practically the entire height of the gate leaf and its follower ring. The leaf is lowered in closing until the bronze seal ring affixed to the downstream face of its bulkhead portion is brought into horizontal alignment with the mating bronze seat ring enclosing the water passageway in the adjacent face of the downstream gate frame, whereupon further downward movement of the leaf is arrested by lugs on the under side of the follower ring coming into contact with stops arranged for that purpose in the bottom bonnet cover. Meanwhile the roller carriage continues its descent some 11 inches farther, withdrawing the inclined roller trains and permitting the gate leaf to move horizontally about one-eighth
inch downstream until its seal ring contacts the scat ring. This completes closure and allows the water load on the leaf to be transferred through its seal ring into the supporting seat ring of the gate frame.
In opening the gate, the roller carriages first start moving upward until they engage the inclined roller trains with the downstream inclined surfaces of the leaf. Continued upward movement of the carriages then forces the roller trains to wedge the leaf away from contact with its seat, causing a horizontal separating movement of about oneeighth inch. When this has been accomplished, the toggles connecting the leaf and the cross-head are extended; whereupon the gate leaf with its ring follower, the two inclined roller trains, and the two roller carriages all move upward in unison until the ring follower is brought into axial alignment with the water passage.
The foregoing description shows that no sliding contact between seating and sealing surfaces occurs while the gates are passing through their opening or closing cycles. Consequently, the surfaces are not subjected to the rapid wear and deterioration that results when sliding contact occurs under high intensity loadings. Inasmuch as all movements of the gate leaf occur while it is supported on roller train systems, the force or power input required for operation is relatively small.
Paradox gates are not suitable for operation at partial openings. They are used as service gates at Grand Coulee Dam because the quantities of water handled arc so large that ample regulation of reservoir release can be obtained by opening or closing one or more of the battery of twenty 102-inch gates in the lower tier, or similar gates in the two upper tiers.
When the rectangular gates at Roosevelt Dam demonstrated the impracticability of reservoir outlet regulation with that type of equipment in 1908, O. H. Ensign, then chief electrical engineer of the Bureau, seeking a means to overcome the trouble, invented the Ensign valve. Ensign valves were installed at Roosevelt, Arrowrock, and Belle Fourche Dams, and in the South Tunnel Outlet of Pathfinder Dam. They were placed on the water faces to control discharges through conduits piercing the dams. They proved successful where the reservoirs are drawn down at frequent intervals, so that the valves are exposed for servicing and maintenance work. At large hold-over reservoirs, where the outlets may be submerged for several years, the valves may become inoperative due to corrosion or scale encrusting and may require the draining of the reservoirs to a level where repairs can be made.
This valve is generally known as a balanced valve of the Ensign type. It consists of a central, horizontally mounted, cylindrical piston, with the downstream end curved and pointed to guide the discharge into the conduit, the opposite end being enlarged to form the bull ring.
The bull ring is closely fitted into a bronze-lined cylinder which is bolted to a heavy radially ribbed cage anchored to the face of the dam. A circular flange is provided, surrounding the discharge passage and concentric therewith. The inner edge of this flange forms the seat against which the forward end of the needle rests when the valve is closed.
A slight clearance is provided around the circumference of the bull ring to permit passage of water to equalize the pressures on both sides of the piston when the control pipe is closed. When the pressures are thus equalized the valve is closed and the piston or needle is held against its seat, the pointed end in the water passage being subjected to atmospheric pressure and the back of the piston and bull rins being under full reservoir pressure. By opening the control valve, whose discharge area is larger than the clearancearea around the bull ring, the pressure on the upstream side of the bull ring and piston is reduced, whereupon the reservoir pressure, acting against the annular downstream face of the bull ring, causes the needle to move upstream, thus opening the valve. Conversely, by closing the conirol valve, the pressure on the upstream side of the bull rine and piston is increased; and, as this area is much greater than the annular area on the downstream face of the bull ring, the excess force causes the needle to move toward its closed position.
Discharge conduits from balanced valves have been made of cast iron, plate steel, or, in some instances, have been formed in concrete and lined with a carefully placed and smoothed rich cement mortar. Cavitation, resulting from a vacuum immediately beyond the inlet entrance to the conduits, has seriously damaged cast iron and steel linings. The conduits in the South Tunnel Outlet at Pathfinder Dam were repaired by installing bronze bellmouth inlet castings, then lining the balance of the conduits with rich concrete. A number of 1 },-inch pipes, connected with the bronze bellmouths, were embedded to supply air to the annular space around the exterior of the bellmouths, and from which air gained entrance to the conduits through a series of 1-inch holes in the bellmouth walls. The new linings have been in use 14 years and show but little signs of deterioration.
Experience with Ensign balanced valves showed that such equipment should be placed where it is readily accessible at all times. This led to the development of what are known as balanced needle valves.
BALANCED NEEDLE VALVES
Balanced needle valves, as first developed by the Bureau, were placed in the downstream ends of the conduits instead of the upstream ends. The hydraulic conditions of operation and control were practically the same as for the Ensign valves. One great advantage which the new type possessed, besides that of accessibility, was that the enormous energy stored in the water passing through the valve was released