Downstream and upstream sealing techniques for Intake Gate

[shineseb]

Before the 1960s, all intake gates had downstream skinplates. After 1960, roller wheeled gates with upstream skinplates were introduced when it was discovered that such gates eliminated one wall in the intake and had a lower downpull at part gate openings. But, increased risk and installation costs associated with upstream skinplated gates were overlooked

GATES with downstream skinplates are installed in a gate well upstream of the air vent well, and require a concrete wall to separate the two wells. The gates are generally equipped with rubber seals on the downstream face, which are forced against the sealing path by water pressure. As the gate deflects downstream under water pressure, the deflection increases the compression force on the seals, increasing the sealing pressure and reducing leakage. In the gate bedded parts, the roller path and the seal face are beside each other. This means that only one set of anchors on the heavy roller path need adjustment, which facilitates installation. With a separate air vent, the cold exterior air can be kept away from the gate well which can be heated in cold climates to prevent freezing. However, there is a major detraction in that downpull forces at near-closed openings are high, depending on the configuration of the bottom lip.

A downstream skinplated gate has several advantages. These include:

• Gate deflection increases sealing force.

• The gate well can be easily heated.

• Alignment of bedded parts is considered relatively simple.

However, it does have disadvantages, namely that a concrete wall is needed between gate well and air vent well, and downpull at part gate openings is high, requiring a stronger hoist.

Gates with upstream skinplates, however, are installed in the same well as the air vent, thus saving the expense of a wall between the gate well and the air vent well. With gate and vent in the same well, heating of the water surface is more difficult in cold climates. The seals are located on the upstream face and are pulled away from the sealing surface as the gate deflects downstream under water pressure. Hence gate deflection decreases sealing force and the seals need to have more flexibility to overcome this deflection. Due to gate deflection, there is an increased risk of top seal 'rollover' under pressure or on lowering the gate, resulting in damage to the seal. Alignment of the top seal is more difficult, and often the seal has to be shimmed in the centre to reduce leakage to acceptable levels. In the bedded parts within the gate check, the roller path is on the heavy downstream face and the seal path is on the lighter upstream face, separated by the diameter of the rollers. Alignment to within a fraction of a millimetre is more difficult, requiring the adjustment of two sets of anchors: those on the heavy downstream roller path and those on the lighter upstream seal path. On the other hand, downpull at part gate openings is negligible. As such, an upstream skinplate gate has the reverse of the advantages/disadvantages of a downstream skinplate gate.

Operating considerations

From a design standpoint, it is evident that an upstream skinplate gate is the preferred choice due to the lower cost of the installation. But what about operating aspects?

When the intake gate is located at the head of a long pipeline or tunnel, there are clear indications that a downstream skinplate gate is the preferred alternative.

Over the past few years both anecdotal and hard evidence has been accumulated on several incidents of inappropriate gate operation, resulting in damage to facilities but fortunately no loss of life. All incidents have resulted from accidental rapid opening of a gate when the pipe or tunnel was empty, or only partially full. All gates were equipped with controls to prevent such an operation but somehow the controls failed, or the operating instructions were overlooked.
How to open a Gate
Normal pipe filling is accomplished by lifting the gate about 10cm to a 'prime' position, and then waiting until the pipe is full before lifting the gate to the full open position. This operation takes from several minutes to several hours, depending on the volume of the empty pipe. This length of time allows for the steady release of air in the pipe being displaced by the water. Problems arise when the gate is lifted fully, without pausing at the 'prime' position. In such an event, the rapid flow of water down the pipe entrains a large volume of air, and the air/water mixture traps more air within the pipe to produce a large bubble of compressed air at the bottom of the pipe. As more water enters to fill the pipe, the downward water velocity reduces below the rate of air bubble rise, and the compressed air bubble/water mixture starts to rapidly ascend the pipe to emerge through the air vent with a highly explosive force.