Short Answer:

A surge tank is a protective device installed in a hydroelectric power plant between the dam (or reservoir) and the powerhouse. It is a vertical tank or chamber connected to the penstock that helps to control sudden changes in water pressure caused by quick load changes or valve operations.

The main purpose of a surge tank is to absorb excess pressure (called water hammer) and provide additional water when the flow suddenly increases. It ensures smooth and safe operation of the penstock and turbines, preventing damage due to pressure fluctuations in the system.

Detailed Explanation :

Surge Tank

A surge tank is an important safety component of a hydroelectric power plant. It is a vertical cylindrical tank or chamber installed near the beginning of a penstock to control pressure variations in the water flow. When the load on the turbine changes suddenly, the velocity of water inside the penstock also changes rapidly, creating pressure surges known as water hammer. These pressure fluctuations can damage pipes, valves, or turbines if not controlled properly.

The surge tank acts as a pressure relief system. It balances the water pressure by allowing the water level in the tank to rise or fall according to changes in flow demand. This helps maintain a steady flow of water to the turbine, ensuring safe and efficient operation of the power plant.

Need for a Surge Tank

In hydroelectric power plants, water flows from the reservoir through a long penstock to the turbine. When there is a sudden increase or decrease in turbine load (such as during power demand changes), the water velocity changes instantly. However, due to the large mass and inertia of water, it cannot adjust its speed immediately. This results in sudden pressure variations, leading to:

High pressure (water hammer): When the turbine gate closes suddenly, water pressure in the penstock increases sharply.
Low pressure (vacuum): When the turbine gate opens suddenly, the pressure may drop, causing a vacuum that can damage the penstock.

The surge tank helps prevent both conditions by acting as a buffer, allowing water to flow into or out of the tank to equalize the pressure changes.

Functions of a Surge Tank

Pressure Control:
It absorbs pressure surges and prevents the formation of dangerous water hammer waves in the penstock.
Flow Regulation:
It maintains a uniform flow of water to the turbine, even during sudden load changes.
Protection of Equipment:
It protects the penstock, valves, and turbines from excessive pressure and vibration.
Water Supply Stability:
During sudden increases in demand, water from the surge tank provides an immediate supply to the turbine until the main flow stabilizes.
Energy Efficiency:
By maintaining steady pressure, it ensures the turbine operates efficiently and with less mechanical stress.

Location of a Surge Tank

The surge tank is usually installed at the junction of the penstock and the headrace tunnel or pipe, near the powerhouse. The location is chosen to achieve quick pressure balancing and effective damping of water surges. It is placed as close as possible to the power station to reduce the risk of water hammer effects in the penstock.

Working of a Surge Tank

The working of a surge tank can be explained in three main stages:

Normal Operation:
When the turbine operates steadily, the water flow from the reservoir through the penstock remains constant, and the water level in the surge tank stays stable.
Sudden Decrease in Load:
If the turbine load decreases suddenly (for example, when power demand drops), the turbine gates close quickly. The flow of water in the penstock slows down, causing a rapid rise in pressure (water hammer). In this condition, the excess water enters the surge tank, causing the water level in the tank to rise. This prevents pressure buildup in the penstock.
Sudden Increase in Load:
When the load on the turbine increases suddenly (when more power is required), the turbine gates open wider, increasing water flow. The penstock pressure drops as more water is drawn toward the turbine. To balance this, water from the surge tank flows back into the penstock, preventing the formation of a vacuum and maintaining smooth flow.

Thus, the surge tank acts as a cushion between the reservoir and the turbine, reducing pressure oscillations and keeping the system stable.

Types of Surge Tanks

Simple Surge Tank:
The most basic type, consisting of a vertical open tank connected directly to the penstock. It allows water to rise or fall freely.
Restricted Orifice Surge Tank:
It has a narrow opening (or orifice) at the base to restrict water flow between the tank and penstock, providing better damping of pressure fluctuations.
Differential Surge Tank:
This type includes a central riser pipe and an outer chamber. It offers improved pressure control and faster response to load changes.
Air Cushion Surge Tank:
In this type, the tank is closed and partly filled with compressed air, which acts as an elastic cushion to absorb pressure surges.

Advantages of Surge Tank

Reduces harmful water hammer effects in the penstock.
Ensures stable and continuous water flow to the turbine.
Protects the pipeline and turbines from damage due to pressure changes.
Maintains efficient and safe operation of the hydroelectric system.
Reduces maintenance costs and extends the life of the plant.

Design Considerations

When designing a surge tank, the following factors are taken into account:

Head (height of water column): Determines the pressure level.
Length of penstock: Longer penstocks require larger surge tanks.
Rate of flow: Depends on turbine size and water discharge.
Type of turbine: Determines the reaction time needed for pressure control.
Location: Must be close enough to the powerhouse for quick response.

The shape, height, and diameter of the surge tank are designed to provide effective damping of pressure fluctuations and to ensure economic and safe operation.

Conclusion :

A surge tank is an essential safety device in a hydroelectric power plant that helps control pressure variations and ensures the safe and smooth operation of the system. It absorbs sudden changes in water pressure caused by load variations, preventing the dangerous effects of water hammer.

By balancing water flow and pressure, the surge tank protects the penstock and turbine from damage, increases the efficiency of power generation, and ensures reliable operation of the entire hydroelectric system. It is a crucial component for the stability and safety of modern hydropower plants.