Surge tank :- A surge tank is the small reservoir or tank. It is open at the top. It is installed near Vale house of turbine. It avoids water hammer effect when load on turbine reduces. (It stores immediately the rejected water. It avoids cavity effect when load on turbine increases. (It supply immediately the water)
Spill way :- It acts as a safety value for a dam. It discharges excess water when head of water increases above predetermine maximum level, at the time of heavy rainfall & during floods etc. In this way spill way avoids damage of dam due to excess pressure of water.