Types of Surge Tank
A surge tank can be used to control the surge or sudden pressure changes in a pipe line which may be due to following :
- Sudden closure and opening of valves.
- At the time of starting or stopping the turbine.
- Closure of turbine gates during the governing operation of turbines.
Main types of surge tanks as shown in Figures are :
(a) Simple or open surge tank.
(b) Restricted orifice surge tank.
(c) Differential surge tank.
(a) Simple or open type surge tank :
These are directly connected to the pipe line and they are open at the top. Shape of the tank may be cylindrical or conical as shown in Figure A.
In these tanks, the accelerating and retarding heads induced by the change of water surface is set up slowly, thus their action is somewhat sluggish.
In small tanks, the oscillations set up are large. These oscillations can be reduced by use of large tanks but their cost becomes prohibitive.
These type of surge tanks are not used except for small pumping stations.
(b) Restricted orifice type of surge tank :
As shown in Figure B, it has a restricted opening at the bottom of the surge tank connecting to the pipe line. It is also known as throttled surge tank.
Flow through the restricted opening between the tank and pipe causes high friction loss therefore, it produces large accelerating and retarding heads in a very short time. Hence, the heads available at turbine becomes variable. It makes the design of governing mechanism complicated and costly. For this reason, these type of surge tanks are not used, though the size of surge tank required is much smaller compared to open type of surge tank due to development of higher accelerating and retarding heads.
(c) Differential surge tank :
This type of surge tank is shown in Figure C. It combines the advantages of open type and throttled type of surge tanks with an additional feature of an internal riser. The diameter of internal riser connecting to the penstock is slightly less than the total height of surge tank. The riser is provided with large number of ports at its bottom. When the load on the turbine decreases, water rises fast in the internal riser and it spills over into the tank.
Thus, it provides a large retarding effect in the pipe line. The spilling of water from internal riser into the tank establishes a differential head on the port. The differential head is responsible for forcing the water by the turbine, through the port into tank.
Similarly, when the load increases, the water flows out from the riser rapidly into the turbine. It provides an accelerating head in pipe line. The additional water than flows from the tank through the ports to pipe line.
Thus the differential surge tank acts rapidly like a throttled surge tank but the pressure rise is not so rapid as in case of simple surge tank. Also, the size of differential surge tank required is less than that of a simple surge tank.