Structural design of verticalliftgates. Control systems and operation. Oil hydraulic operation of gates in free surface flow. Information about new jydraulic of water-operated automatic gates, rolling weir gates, fuse gates and an extended part on barrier gates and their details The sections on seals, the trunnions of radial gates, ice formation, gate elwin and structural design have all been expanded New sections on hazard and reliability of gates, earthquake effects on gates and operating machinery, environmental impact and aesthetics, as well as maintenance An appendix on the calculation of hydrostatic loads on radial gates has been set out Hydraulic gates and valves in free surface flow and submerged outlets: Hydraulic considerations pertaining to gates.
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While similar types of gate are found in both groups, the design of gates in submerged outlets, especially at high-heads, is more demanding and requires special consideration.
Appendix 2. While there are many types of gate, a limited number predominate because of their advantages. In open channels, at spillways and barrages, radial gates are the first choice.
Except for very large span gates in navigable rivers, new vertical-lift gate installations are infrequent. This does not apply to rehabilitation of old barrages such as the Sukkur and Koti barrages on the river Indus in Pakistan where old gates are being replaced by similar vertical-lift gates of modem construction. Allo its ability to discharge debris over the gate may be important, A radial gate requires a flap section to carry out the same function.
In tidal barrages a tilting gate can completely prevent ingress of saline water to the upstream pond or reach. It is often chosen on this account. In undershoot gates under drowned flow conditions a lens of saline water can penetrate upstream against the flow of water. In conduits, vertical-lift gates are more frequently used than radial gates. This is in spite of the hydraulic problems due to gate slots at high velocity flow.
Detail aspects of gate design are dealt with in separate chapters because they apply to a number of different gates. The exception is radial automatic, gates where many design considerations are special to this type of gate.
So far they have not found more general application. Wherever this is the case consistent ST units can be used 4 Hydraulic gates and valves and scetor gates, Stoney-roller gates and others not dealt with in this chapter, have been largely superseded because of their complexity, cost and associated civil engineering construction cost.
Gates in free surface flow 2. Radial gates Radial gates are the most frequently used movable water control structures. The skin plate is stiffened by vertical and horizontal members which act compositely with the skin plate.
The skin plate assembly is supported by two or more radial struts which converge downstream to the pivot assemblies which are anchored to the piers and carry the entire thrust of the water load.
The hi load consists of the weight of the gate, the friction force between the side seals and the seal contact plates embedded in the piers and the moment of the frictional resistance at the pivots. Radial gates are under most conditions the simplest, most reliable and least expensive type of gate for the passage of large floods. This benefits pier structural design and hydraulic flow.
Picr slots can produce cavitation and at low flows collect silt. Integrity of the anchorages and distribution of the load into the piers require special consideration because of this factor. In gates of m? This can be overcome by adding a flap or overflow section to the top of the gate Figure 2. The overflow discharge section is curtailed on this type of gate for the nappe to clear the gate arms, Tilting crests may be required when ice floes have to be discharged in spring or where the river carries an abnormal amount of debris during part of the year.
If the crest seotion is curtailed the nappe will be vented and problems due to nappe collapse will not occur. Radial gates have been designed to be submergible to provide overflow Figure 2. Provided the skin plate has been formed in a true radius with the origin at the pivot point there are no unbalanced forces about the pivot due to hydrostatic forces.
When the gate is lifted the forces are the mass of the gate with the centre of gravity fairly close to the weir plate assembly, the frictional resistance of the side seals and the frictional resistance of pivot bearings.
Types of radial gate 6 Hydraulic gates and valves Figure 2. Hydrostatic forces acting on a radial gate Figure 24 shows the distribution of pressure head for a radial gate under free discharge conditions. If the pressure curves are avail analysis or actual measurement on model or prototype, the required fomponents of the forces can be obtained by graphical integration.
This will result ty an inate of the hydraulic forces acting on the gate. This leads t0 a closet approximation, Constructional features of gate arms Gate arms are usually offset Figure 2. Conventional arrangement of arms of a radial gate 8 Hydraulic gates and valves the pivot bearing to be recessed into the pier. In this case hydraulic considerations should override structural priorities so that members are disposed in the most efficient manner Figure 2,8 , The hydrodynamic effect on the gate is usually ignored in calculations of structural strength as well as the hydrodynamic downpull forces which become more significant when radial gates are used as culvert valves under high-head conditions Figure 2.
Conventional calculations of the strength of the weir plate assembly ignore the additional strength and rigidity due to its curvature. Effect of reverse roller on the lowest arm of a radial gate under drowned discharge conditions Figure 2.
Steps in the analysis of a weir plate assembly of a radial gate 10 Hydraulic gates and valves Radial gates can be operated by electric motor driven hoists or by hydraulic rams.
Some arrangements of hoists and of hydraulic rame are described and illustrated in Chapter 6, 2. They require no outside source of power, are simple and reliable.
Provided that they are correctly designed hydraulically they will operate consistently with minimum attention and require very little maintenance. The former is usual it rivers and the latter in irrigation channels.
Figures 2. Operation is through displacers in each side pier. The displacers have to overcome side seal and pivot friction which act in both directions as shown in Figure 2. The intake is controlled by a sluice valve and dis. The discharge over the weir flows into the dlsplacer chambers which are interconnected by a pipe passing under.
An outlet from the displacer chamber, also controlled by a sluice valve, returns the flow downstream of the gate. This is shown in Figure 2. An increase in the upstream level causes flow through the intake pipe into the weir chamber and discharge over the weir. This in turn causes a rise tn evel in the displacer chambers and flow to the river downstream of the gate. Due to increased buoyancy of the displacers, the gate rises and discharge Gccurs under the gate, Rise of the gate may cause a slight drop in upstream water level, which will result in reduced flow over the weit and hence a lowering of the level in the displacer chambers.
The gate may close slightly 3s 2 Consequence until balanced conditions are achieved. It should be eer pacing. The head loss which should be 50 to 6 a is provided bythe setting ofthe inlet valve. Figure 2. A stub wall st the end of the weir permits aeration of the nappe. Displacers and displacer chambers The flow from the weir is baffled to Provide quiescent conditions in the splacer chambers.
Similarly the frictional resistance of the side seals referred to the displacer buoyancy is amplified by the same ratio. Since the direction of the friction force changes on opening and on closing of the gate, the change in water level within the displacer chamber required to reverse the movement of the gate will be double. Upstream level retention within close limits is possible which can be as low as 12 mm, although under high flow conditions 50 to 80 mm rise will be required for the gate to discharge the increased flow.
The gate will therefore lift more rapidly than the rise in flood water and will cut out of the water to permit unobstructed flow under these conditions. This can also be of importance during a falling flood when the downstream level falls more slowly than the upstream level.
This can result in a condition when the downstream water level keeps the gate open, causing loss of retention. The construction takes the form of a stiffened box with watertight access covers, for loading weights. The total assembly forward of the pivot is out of balance to the extent that the displacers are half immersed when the gate is in a steady condition. Equal forces are then available for opening and closing the gate.
A gate with displacers sized on this basis would have no margin and would provide very coarse level control. With displacers designed three times this size they will give good service. If self-aligning roller bearings are used for the pivots the effort at the displacers to overcome friction is negligible. For bronze bushed bearings it will amount to It is the usual practice to use displacers.
There are some gates where floats have been used. In a 6 m wide gate, as shown in Figure 2. Departure from this is possible, ings. These have a cocificient of friction of 0. The gates in Figures 2. Their coefficient of friction is 0. Self lubricated bearings, mentioned in Chapter 7, and illustrated in Figure 7. These are suitable only for small gates, because the discharge under the gate causes turbulent conditions. Larger gates have to be severely damped, or displacer chambers in the piers have to be constructed asing an intake positioned downstream of any turbulence.
It is usual to baffie the intake, Gates of this type are not subject to size limitations. Causes of gate instability and malfunction Gate instability can be due to: Insufficient head loss in the inlet system. Reflux of flow around piers dividing two sluiceways. Downstream evel control gate Tipes of gate W Computer programme for radial gates Because unstable operating conditions can arise in radial automatic gates, and because design and testing for instability can be carried out only on a trial and error basis, varying parameters in turn, a computer programme forms a useful design tool so that changes in variables ex, be rapidly examined, The programme output has to determine the relationship between eter an, ttl downstream water levels, gate Opening and the discharge under the gate, The relationships have to be computed for the rising and falling river or reservoir stages and also when the gate is clear of the y Such a programme was run for the gate shown in Figure 2.
Hydraulic Gates and Valves in Free Surface Flow and Submerged Outlets
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Hydraulic Gates and Valves - Jack Lewin