Normal pipe valves are covered under Mechanical Auxiliary Equipment. The turbine inlet valve has special features, as described below:
Turbine Inlet Valve
The turbine inlet valve (TIV), also called the “penstock shut-off valve”, is installed on or near the turbine inlet pipe. It is sometimes included in the Generating Equipment supply contract.
The most common forms of TIV are butterfly valves and spherical valves. Butterfly valves have a multi-plate vane inside. When the vane is turned across the flow, it stops the flow. When the vane is aligned with the flow direction, it allows water to pass.
Click on the buttons to open and close the valve:
This exploded illustration of a butterfly TIV was modelled in Shapr3D. To examine the model in 3D, click on the following link:
Spherical valves have a spherical shutter inside a spherical valve body. When the valve is opened, a the shutter rotates so that a hole in the shutter aligns perfectly with the turbine inlet pipe upstream and downstream. For this reason, spherical valves add no head loss to the water conveyance. When a spherical valve is closed, two seals stop the flow of water, and a drain between the seals ensures that the second seal is not pressurized unless the upstream fails.
This “double block and bleed” between the high-pressure water of the penstock and the turbine satisfies worker safety requirements and permits workers to safely enter the spiral case or draft tube by closing only a single valve.
Spherical valves are more expensive than butterfly valves. However, they are necessary for high head turbines because they have higher capacity, and they are also sometimes used for low head stations where it is important to minimize head loss.
Both types of TIV are equipped with a counterweight that can close the valve even when there is no power (only a low-voltage control signal). The valves are opened, and maintained in the open position by a hydraulic cylinder acting on a lever arm attached to the valve stem.
Hydraulic gates are used to isolate parts of the water conveyance system and to control spillway discharges. The common types of gate are as follows:
The power intake is equipped with a control gate. In spite of its name, this gate does not control the flow through the turbine. (That is the role of the nozzles on a Pelton turbine and wicket gates on Francis and Kaplan turbines.) However, in the event of a turbine runaway or penstock rupture, these gates close (within two to five minutes) to shut off the flow from the reservoir. They are designed to close under a rupture flow, and are often designed to close without needing external power.
Intakes are usually equipped with fixed wheel control gates (gates with wheels running in the gate slot at each side of the gate to reduce friction as the gate is raised or lowered). They are opened by a hoist. The most common types of hoist in modern intakes are hydraulic hoists and wire rope hoists. Wire rope hoists are more common when the intake gate is located a long way below the intake deck.
Intake gates may be upstream-sealing or downstream-sealing. Although it might seem more logical to use the pressure of the reservoir to compress the gate onto its seals, upstream-sealing gates are usually preferred for several reasons:
- Because there is air above and behind the gate, they do not suffer from hydraulic down-pull as the gate approaches complete closure.
- It is possible to design an open shaft downstream of the gate to provide access to the penstock for inspection or maintenance.
- The shaft is also used to supply the air downstream of a gate as it is closed. This prevents the development of low pressure in the waterway that has caused penstocks to collapse during emergency intake gate closure.
Numerous types of gate are used to control spillway flows. One of the most efficient forms of gate is the radial gate, also called the Tainter Gate. Radial gates may be operated by wire rope or hydraulic hoists. Andritz Hydro manufactures radial gates with position indicators integrated within the hydraulic hoists. This allows both sides of the gate to be lifted at the same rate, and provides a very precise measure of the gate opening and hence the flow.
Radial gates tend to suffer from ice build-up, so vertical lift gates are more common in cold climates. Vertical lift gates tend to be roller gates or caterpillar gates (roller-mounted vertical lift gates supported along either side by stainless steel roller chains). They may be operative by screw, hydraulic, wire rope and chain hoists.
Flap gates are also used for spillways with heads less than about 5 m. The most common form of flap gate these days are those operated by an inflated rubber bladder.
Bulkhead Gates and Stoplogs
Bulkhead gates and stoplogs are used to provide a water barrier for maintenance, such as upstream of the intake control gate and at the downstream end of draft tubes. The difference between the two is that bulkhead gates are lowered in one piece, whereas stoplogs are inserted one by one in the gate slot.
Neither bulkhead gates nor stoplogs are equipped with wheels, so they cannot be installed when water is flowing through the opening, and they cannot be removed unless the water level upstream and downstream is the same.
To remove an intake bulkhead gate, for example, the control gate is first closed; then the space between the bulkhead gate and the control gate is flooded to balance the water pressure across the bulkhead gate, allowing the bulkhead gate to be removed.
Bulkhead gates and stoplogs are often equipped with valves to bypass water and balance the water pressure before they are removed.
Draft Tube Gates
Stoplogs or bulkhead gates and a gantry hoist are usually used to close the end of the draft tube when the turbine needs to be unwatered for inspection or maintenance. Since larger draft tubes have one or two intermediate piers in the draft tube, a set of closures (stoplogs or bulkheads) is required to close all the draft tube exits.
Only one set of closures is needed for a multi-unit powerhouse, since it is rarely necessary to unwater more than one unit at a time. The gantry travels the length of the tailrace deck, so it can transport and install the closures in any draft tube.
However, the construction schedule sometimes makes it necessary to flood the tailwater before all the turbines are sufficiently complete to accept water. In this case, an additional set of draft tube closures may be included as part of the original purchase.
When possible, a unit is be taken out of service for routine maintenance at a time of the year when water availability is low. When it is not possible to schedule such “planned outages” when units are not in use, bulkhead and stoplogs closures are evaluated by economic analysis:
- The cost of bulkhead and stoplog closures is similar.
- Stoplogs require a grappling beam for installation and removal; the bulkhead does not.
- Bulkhead gates require a gantry hoist with a lift height equal to the gate height plus handrail height plus a clearance margin; a far lower gantry is needed for stoplog handling;
- The capacity of the gantry hoist is far larger for a bulkhead gate than for stoplogs.
- The time for installation and removal of the draft tube closure is far larger with stoplogs than bulkhead gates. This means that for stoplog closures:
- the outage time is longer and therefore the loss of generation revenue is greater; and
- the labour cost for the outage is greater.
Trash racks are installed on the upstream face of the power intake to prevent large debris from entering the water conveyance and causing an obstruction in the turbines. The spacing of the trash rack bars is normally specified by the turbine manufacturer.
The primary bars of the rack are oriented vertically, and the whole rack is inclined at 8° to 15° to help the cleaning equipment.
Manual trash rack cleaning may be adequate for small intakes. In that case, a platforms may be provided for safe access to the face of the intake. For larger intakes, permanent mechanical rakes are installed if the debris load justifies their cost. If not, the intake gates may be closed while the racks are cleaned with a rake on a mobile crane.