Surge tank
A surge tank is used to reduce the pressure surge in the pipeline of a hydropower plant, arising from the closing of the valves in the line.
General Description
The moated castle is a waterproof, mostly in the mountain befindlicher shaft, a large-volume, indoor pool or a kettle -shaped water tank. It is located at the end of a pressure tunnel or penstock, the water supply will feed on each lowest point. Inside the surge tank, the water level in the pressure equalization can now hang freely. When Pumpspeicherwerk Niederwartha in Dresden three round water containers are for example provided (see photo), because there are three pipes should be run separately.
A surge tank is required to compensate for pressure fluctuations in the pressure pipe lines of a hydroelectric power plant (storage facility or pumped storage power plant). These occur when the valves are closed, thereby changing the flow rates in the pressure tubes. Occur especially at high acceleration or braking forces when this happens quickly. The flow through the turbines must be regulated, because the demand for electrical energy changes in the power grid. On settlement of the turbines strong pressure surges may occur, which propagate at high speed in the water in the pipe. Due to this pressure surges the pipes may burst.
The moated castle dampens the effect of these shocks by collects the fast-flowing water and redirects. The water that moves through the pressure tube, dodges when closing in the moated castle. The water level fluctuates up and down then, until he gradually comes to rest. The kinetic energy of flowing water in the headrace tunnel is converted into the potential energy of elevated water level in the surge tank.
The moated castle is as close as possible to the hydroelectric power plant at the upper end of the chute. It must be so high that the water can not spill out, that is, the top must be higher than the highest water level in the upper reservoir and higher than the power line in the penstock.
Burden on the hydroelectric plant ( Joukowsky Bump)
As described at the outset, incurred as a result of rapid closure of pipelines, turbine control, opening and closing of Regulierverschlüssen in pressure pipes flow changes and thus pressure surges. These result in a vacuum forms when opening or pressure when closing.
Since pressure surges can reach a multiple of the operating pressure, this load must be taken into account in the planning of the system. On the one hand this can be done in an increasing the cross -section (reduction of flow rate ), on the other hand, may take appropriate steps, pressure shocks for the pipeline to be rendered harmless. The cost-effectiveness due to higher takes place at high pressure hydro power plants, the arrangement of a moated castle, with the aid of the affected area of pressure surges can be limited. For systems with long headrace tunnel, this building will be built between tunnels and penstock. Thus must only part of the supply line, the relatively short pressure shaft, are measured on the high loads. This is usually done by an armored version of the line in the bay area.
The pressure propagation speed in water is 1000 m / s, as a function of modulus of elasticity of the pipe, the pipe diameter, the wall thickness, as well as the bulk modulus of water.
The pressure surge size in turn depends on the rate of change per unit time. For quick closing ( speed difference between flow velocity and zero), the maximum surge pressure, the so-called surge (also Joukowsky shock after Nikolai Zhukovsky Jegorowitsch ) can form.
The maximum occurring pressure pulse only occurs when the closing timing of the closing element is shorter than the time required for the pressure to the inlet of the engine water line and back to the lock back () to get - the so-called reflection time of the pressure wave.
Moves the closing time on, the surge will be lower, since within the reflection time, the flow is partly restricted. Rather than to throttle the speed of a quick closing of the shutoff rapidly, the water can escape into the chamber of the water castle. Kinetic energy in the pressure tunnel is thereby converted into potential energy in the water lock chamber. Within this creates a back pressure which lowers the speed of the pressure tunnel and then slowly goes into an oscillating movement of the water surface in the chamber, the gradually ausdämpft. This so-called surge oscillation is compared to the pressure surge an inert mass vibration dar. The occurring in the pressure tunnel maximum pressure now corresponds to the maximum water level in the surge tank. The affected by the surge portion of the lead then extends only to the area of the pressure shaft. Its size is due to the small size of the shaft and the relatively long closing times are usually less than the maximum Joukowsky shock.
The surge tank
A similar device is the water tower, the surge chamber. This is arranged in the underwater pressure compensation structure is required if the tailrace discharges into a river with strongly fluctuating water levels. Another application arises in long free- tailrace tunnel, which at times act due to heavy load fluctuations as penstocks.