4. Bottom Hinge Flap Gate

Article 56. Shape of the Bottom Hinge Flap Gate

The gate leaf of a bottom hinge flap gate shall be of such a shape and supporting structure that it does not generate detrimental vibrations when the gate is under overflow conditions at a given angle or when the gate is fully open.

Description:

The leaf bottom of a bottom hinge flap gate is fixed to the river bed, dam top or top of a fixed wheel gate by means of a hinge and thus the span can be long as compared with the gate height and the leaf can be thinner as compared with a fixed wheel gate of the same dimension, there­fore, if vibration takes place this affects the gate leaf to a great extent.

The overflow depth always fluctuates when the bottom hinge flap gate . is closed, fully opened or while the gate is moving back to a closed posi­tion and therefore the hydraulic pressure at the back of the gate leaf and the air pressure may become unstable because of back water turbulence and a water-screen type phenomenon of the overflowing water. It is there­fore required that the shape and structure of a gate leaf and its related structures be chosen to have good hydraulic properties and not produce any detrimental vibrations.

Article 57. Considerations for Sedimentary Sand and Drift

The gate leaf and its operating mechanism shall always have sure clo­sing and opening functions and its functions shall not be hampered by sedimentary sands or drifts.

Description:

In case of the gentle gradient of a river, sedimentary sands and drifts, are left at the back of the bottom hinge flap gate leaf in many cases and - thus complete opening may be impossible or the leaf or operating mechan­ism may be damaged.

Therefore, when installing the gate where sedimentary sands and drifts may occur, consideration should be given to an effective sand flushing device or controlling method so that the sedimentary sand does not ac- , cumulate.

When the operating mechanism is exposed to sedimentary sand and drifts, attention should be paid to impact and malfunctions due to sand and drifts stuck in the device.

Article 58. Structure of the Gate Leaf

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The gate leaf is subject to a twisting moment in addition to a bending moment and therefore the leaf shall be made safe against buckling due to these moments.

Description:

In the bottom hinge flap gate which is meant to support the hydraulic pressure on the gate leaf at one or both sides, a twisting moment in addi­tion to a bending moment occurs to a great extent on the leaf and thus each member should be checked for safety against shear buckling due to these moments.

There are various shapes of the leaf cross sections such as the fishbelly type, the torque shaft type, the horizontal main the girder type, etc.

Fig. 2.58-1 Types of Leaf Cross Sections

Horizontal main girder type

Article 59. Gate Leaf Support

The support of the bottom hinge flap gate shall be safe against uneven support reactions, uneven contact due to installation error or uneven sub­sidence of a river bed, and leaf expansion due to temperature change.

Description:

The load working on the support of the bottom hinge flap gate depends on the gate leaf structure and the gate operating mechanism. Particularly for construction where one or b$th ends of the leaf are driven and on which twisting load acts to a great extent in addition to bending load, sup­port reaction is not even and at the ends may become much larger than the mean value of the support reaction, thus this situation requires at­tention.

Since the bottom hinge flap gate has a long span, an error is likely to be generated in centering the supports during installation. An appropri­ate consideration should be taken into account for uneven contact due to uneven subsidence.

The gate leaf expands in the direction of the span due to temperature change and thus it should be noted that a large horizontal load acts if all the supports are fixed in the horizontal direction of the span.

Article 60. Gate Operating Force and Operating Mechanisms

The gate operating force varies with the overflow height, the gate posi­tion angle, the.upstream and downstream water levels, the leaf shape, etc. and thus the operating force shall be determined from its maximum mo­ment of rotation.

The gate hoist shall be so constructed.as to ensure the closing and open­ing of the gate as well as the retention of the gate in the required position.

Description:

The operating force for the bottom hinge flap gate is determined from a moment of rotation. The moment of rotation varies with the hydraulic load, the sediment pressure, the rotary shaft frictional force by the gate’s own weight, the frictional force of the rubber seals, the resistance of the operating mechanism, etc. When installing the gate in a river having a lot of sedimentary sand, and silt, the load due to the sand and silt should also be taken into consideration. The moment of rotation by the hydrau­lic load varies extensively with the water level upstream and downstream. It generally has the maximum value when the downstream water level is low and the gate position angle is about 30° from the horizontal.

The operating systems fdr the gate leaf are as follows:

1. Top Supporting Type

Both sides (or one side) of the gate leaf top are lifted up and sup- .• ported by wirerope, chain or rods, etc.

2. Pin Twisting Type

Both sides (or one side) of a hinge pin at the bottom of the gate leaf are supported by torque. The driving force is transmitted through a rocker arm installed in the pier by an oil hydraulic cylinder_K,etc.

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3. Downstream Supporting Type

Several places on the downstream side of the leaf are supported by an oil hydraulic cylinder installed at the bottom foundation down­stream of the gate leaf.

If no power is used when fully opening the bottom hinge flap gate, it should be open by its own weight. If it is oil hydraulically operated, it should be confirmed that the gate can be opened notwithstanding the above resistance, pressure loss of the oil hydraulic system or increased pressure of the operating unit.

The oil hydraulic system of the oil hydraulic bottom hinge flap gate is always pressurized by the water hydraulic load and thus a relatively low design pressureis preferred so ttiat no oil leak is produced.

A majority of the failures of the oil hydraulic gate hoist are due to for­eign matter contained in the hydraulic operating fluids. And so; the in­stallation of the hydraulic system shall be carefully performed by referring to Article 35. and its Description in this chapter.