« PreviousContinue »
HYDRAULIC GATES (Continued)
ST gate-seat reaction or gate arresting force
W weight of gate section acting at the center of
gravity of the proposed design layout
a,b,c,d = moment arms of the respective forces.
Similarly, the hinge-pin reactions can be computed and checked independently by graphic static methods. The reactions can then be diagrammatically illustrated for various gate positions and the maximum head condition. The friction losses through bearings and seals may be neglected since they are not of sufficient magnitude to alter the theoretical values adversely.
The hinge-pin location and the gate contour must be proportioned in such a manner that the gate sustains the reservoir pressure with safety. The buoyancy ratio represents the quotient of all rotational moments tending to raise the gate and the moments tending to lower the gate. Practical values for this moment ratio, Ra, vary from 1.4 to 1.6, depending on
trash and ice loads or overtopping water conditions.
The upstream and downstream girders of the transverse section of the gate represent a curved beam with a wide, thin flange. The full width of the skin-plate section is only partially effective in the evaluation of moment of inertia.
L = 1.574/TT
On the straight bottom girder, the effective skin plate width of the section is assumed to be 100 percent when the girder spacing is below 15 percent of the bottom girder span. On various projects, access to the gate interior is not possible for years at a time; therefore, the girder sections are computed under an assumed corrosion allowance of one-sixteenth inch on the skin plates, the girder angles, and the girder web plates.
The principal bending stress in the girders combined, according to Westergaard’s “Criteria of Failure,” with the lateral skin-plate bending stress, should not exceed 18,000 to 20,000 psi for an arbitrary 50 percent overload condition assumed for possible misalinement of hinge bearings. In this connection, the direct compression skin plate stress resulting from the water pressure on the side bulkheads should be considered and the slenderness ratio of the gate shell investigated.
The hinge clearance for lateral movement should be liberal to allow for field erection inaccuracy and for expansion and contraction of the gate resulting from temperature variations. The side clearance should be governed by the prevailing temperature at the spillway during erection plus a maximum summer and winter variation.
Longitudinal bending on the center part of the gate through flexure on the part of the Spillway is avoided through the introduction of special contraction joints at the junction of the piers and the spillway nose. Since no
1.23J HYDRAULIC GATES (Continued) HINGED-TYPE GATES (Continued)
restraint exists at these junction points, the spillway nose deflects uniformly for its entire length when under water load.
To obtain the most economical proportions for the gate members, the stresses should be balanced in the interior and exterior flanges. This will require a succession of design approximations.
J. For the final design analysis and the evaluation of true moments, thrusts, Final Design and shears, the following methods are recommended: Analysis
Analytical mathematical method
The column analogy (as applied by Professors Cross and Morgan) lends itself particularly well to the solution of this type of rigid-frame structure. For the resolution of forces and simple static moment diagrams, the method by graphic statics is advocated; consequently, the amount of work for the evaluation of the analogous column loads is greatly simplified. For the solution of the indeterminate moments, it is important to make corrections to dissymmetry and centroids since the elastic center of the frame structure is arbitrarily selected.
Circumstances under which special analyses are requested are covered in another part of this Volume.
K. The gate hinges are commonly provided with a high-grade bronze bushing Hinge-Pins containing “Lubrite” inserts which are designed to carry a slow-moving & Bushings load and operate under water. Hinge-pins are made of high-strength alloy steel to carry the combined bending and torsion through pin friction, for the assumed condition that every alternate hinge-pin is out of bearing contact.
L. A series of stationary cast-steel hinge anchors, evenly spaced between Hinge contraction joints of the Spillway, are held in place on a continuous Anchorage structural-steel base by anchor bolts. The anchor bolts are generally provided with upset ends and should be threaded for ASA Standard coarse threads.
Stresses at the root area of the threads should not be excessive for the assumed condition that every alternate hinge is out of bearing contact. The embedded bolt length should be calculated from the bond strength of the concrete.
M. The rising gate will be arrested by the contact of the gate stop with the Gate-Seat protruding nose of the gate-seat casting. The embedded castings run the Anchorage full length of the downstream lip of the recess chamber and are made of either high-strength cast iron or cast steel, depending on the magnitude of the seat reactions. The flanged connections of the castings are provided with sliding dowels to allow separation at the contraction joints of the dam, otherwise the castings are held firmly in place by anchor bolts of similar material and design as the ones for the hinge-anchor castings. Stresses should be calculated for an assumed 50 percent overload condition. The rubber cushion between seat and gate assures better load equalization and diminishes impact loads on the casting as well as on the gate.
HYDRAULIC GATES (Continued)
N. Prior to the grouting of the cast-iron faceplate sections in the piers, the erection trusses are embedded in the concrete and the protruding turnbuckles are utilized to properly place and aline the individual plate castings.
O. The gate-control mechanism should be designed to prevent the dropping of the gate in case of a control failure or faulty valve action.
Where there is no reduction of the inflow into the recess chamber or an increase in the outflow, a continually rising reservoir would force the gate to the upper position and eventually overflow the gate, but when chamber inflow is reduced by the action of the inlet valve, or the outflow is increased by the control valve, the gate can be lowered during high runoff.
The inlet is governed by an electrically driven butterfly valve which is ordinarily set to a constant partial opening. The outlet valve controls the discharge from the recess chamber and is a modified type of needle valve. It is indirectly connected with and made responsive to the rotation of the gate.
Where maintenance of constant water Surface is important, automatic float, manual, or remote electric control is used.
P. Typical designs of drum gates are illustrated in the following drawings:
100- x 18-foot Drum Gate General
100- x 18-foot Drum Gate, Typical
100- x 18-foot Drum Gate, Hinge
100- x 18-foot Drum Gate, Control
Flap valves or gates are used at the outlet end of pump-discharge pipes in order to prevent a return flow from the discharge line or discharge bay when the pump is stopped. In Some instances these gates will close with an objectionable impact, indicating the need for a method of reducing slam. When the pump is stopped the velocity of the water in the discharge pipe will quickly diminish to zero and the flow will reverse. During this interval the gate will start to close due to its own weight and the pressure of water in the afterbay. At the instant of closure the inertia of the downflowing column of water below the gate will tend to form a vacuum, with a resulting force much greater than the water pressure above the gate. The three forces-—the inertia of the gate, water pressure above the gate, and a partial vacuum below the gate--when combined may cause a severe shock as the gate suddenly strikes the gate seat.
A large air vent located just below the gate is generally provided to partly eliminate the inertia effect of the column of water. However, if the pump runner is located above the water surface and it is necessary to maintain the prime, a vent cannot be used. Gates 24 inches in diameter and smaller which are located at a distance from the inlet of less than seven times the total pumping head, may be rubber-cushioned to absorb shock as shown on Figure 37. Larger gates having an unfavorable “length to pumping head ratio” should be equipped with an oil-operated check or dashpot installed above the gate and above the water surface similar to that shown on Figure 39. The cylinder is provided with two regulating rods with tapered slots to permit the flow of oil through the piston on the down stroke. As the piston nears the bottom of its
1.24A HYDRAULIC GATES (Continued) HINGED-TYPE GATES (Continued)
travel the area of the tapered slots will decrease and cause increasing resistance to the final closing of the gate. The regulating rods are threaded into the top cylinder head so that the position of the tapered slots can be adjusted as required after installation. The piston is provided with a relief valve, which will permit the piston to raise and the gate to open with very little resistance.
A. The following are some reference drawings on flap gates: Reference Drawings 21-inch Flap Gate (Figure 37) 12-D-836
60-inch Flap Gate (Figure 38) 3-D-1787
Ring gates are installed with relative infrequency and are used as spillway RING GATES gates only. Before a gate of this type is selected, many factors should be considered, including hydraulic, topographic, and geologic conditions at the dam Site, especially where a nonoverflow dam is to be built. The dimensions of the dam compared with spillway requirements may not facilitate the installation of other types of gates. The amount of concrete required for the spillways may be a deciding factor in favor of the ring gate. The first floating ring gate for a “glory-hole” type spillway was installed by the Bureau at Owyhee Dam in Oregon. Figure 40 shows the general outline and construction features of this gate, while Figure 41 shows the control assembly. The gate has a 60-foot crest diameter with a 12-foot overflow controlling height, and is installed at the circular overflow weir. The steel ring gate is a hollow, annular drum, seated within a hydraulic chamber. The upper surface of the chamber in conjunction with the upper surface of the lowered ring gate forms the spillway crest. The gate is raised or lowered as one complete unit by its own buoyancy in water introduced under control from the reservoir into the chamber. Seals are provided in the spillway crest for the purpose of acting against the inner and outer gate faces to prevent water not under control from entering or leaving the gate chamber. Gates must necessarily be of welded construction or provided with countersunk-head rivets in the sealing faces. Water leaking into the annular gate space is removed through hinged drain pipes. A leveling device keeps the gate in a horizontal plane at all times. Within a certain range of reservoir levels, the normal operation of the gate can be made automatic, but hand controls are provided so that the gate may be held in a predetermined position should this be desirable.
Cylinder gates are used primarily in intake towers upstream from dams for CYLINDER closing off the water to penstock and control valves, or in outlet works for GATES regulating the volume of water where they may have a free discharge. (See Drawings Nos. 26-D-971 and 26-D-972.) A drawing of the 32-foot cylinder gates at Hoover Dam is shown in Figure 42.
A. The water passage in the entrance liners should be streamlined to produce Design the most favorable condition for the flow of the water. Sharp corners and Considerations abrupt curves should be avoided. The gates may be designed for either external or internal water pressure. When designed for external water pressure, the circular ring, or cylinder, should be carefully analyzed for flexural rigidity both longitudinally and axially. Formulas for determining the gate dimensions under critical pressure conditions can be obtained from “Strength of Materials,” by S. Timoshenko, Part 2, second edition, August 1941, pages 216 to 219, inclusive. Where the gate is made in two or more segments, the bolted joints should have the same rigidity as the
HYDRAULIC GATES (Continued)
gate segment. For regulating gates under heads where vibration is likely
B. The cylinder gate leaf is generally lifted by three stems mechanically connected in such a manner that each stem carries its full share of the load. Secondary stresses at the stem connection should also be considered in the design of the leaf.
C. The hoists lifting these gates are of the screw-stem type and have as many heads as there are stems. These heads are operated by a central drive which synchronizes the lifting speed of the stems. (See Drawing No. 26-D-971.)
The required lifting and lowering capacity of the hoist should be approximately 25 percent more than the estimated force required, which is the sum of the weight of the gate and stems, guide friction, and uplift or downpull.
Gate-stem guides should be spaced as required by the compression in the
D. Typical designs of cylinder gates are illustrated in the following drawings: 38-foot Diameter Cylinder Gate--Boulder
Dam (Figure 42) 45-D-2260
LOW-HEAD AND TURNOUT GATES
.27 Cast-iron gates are used along canals for controlling the flow of water into
laterals, and in outlet works in low-head dams. In the latter case they are
A. The gate seats are cast iron for 0- to 15-foot and 15- to 30-foot heads, but for 30- to 50-foot heads the gate seats are bronze strips. Cast-iron stem guides are provided at intervals depending upon the length and diameter of the stem. The hoists are of the pedestal type, with a gear ratio dependent upon the load to be lifted, and are operated by a hand crank. The larger gates may be arranged for motor operation when desired.