Short Answer:

A surge tank is a small reservoir or tank installed in a hydraulic power plant between the penstock and the main pipeline to control water pressure variations. It helps in protecting the system from water hammer effects caused by sudden changes in flow or load on the turbine. The surge tank allows the water level to rise or fall depending on the flow conditions, maintaining safe and stable operation of the system.

It acts as a safety device that absorbs pressure fluctuations in the water column. When the turbine load decreases suddenly, excess water flows into the surge tank; and when the load increases, the stored water from the surge tank flows back into the penstock. This helps in reducing pressure surges and prevents damage to the pipeline and turbine.

Detailed Explanation :

Surge Tank

A surge tank is an essential component of a hydraulic power plant used to maintain steady water flow and prevent pressure fluctuations within the penstock system. It is a vertical or open tank connected to the pipeline carrying water from the dam or reservoir to the turbine. When there is a sudden change in water demand or turbine load, the surge tank helps balance the pressure difference by temporarily storing or releasing water.

In a hydroelectric power plant, water flows from a reservoir through a penstock to the turbine. If the turbine load suddenly decreases, the water flow is abruptly reduced. However, due to the inertia of moving water, it cannot stop instantly, leading to a sudden rise in pressure called water hammer. This pressure surge can damage the penstock and other equipment. The surge tank absorbs this extra pressure by allowing the excess water to rise within it. Similarly, when the turbine load suddenly increases, water flows down from the surge tank to maintain adequate supply and prevent pressure drop.

Thus, the surge tank ensures smooth operation, reduces the effects of water hammer, and maintains the longevity of the hydraulic system.

Functions of Surge Tank

1. Pressure Regulation:
   The surge tank maintains a balance between water pressure and flow rate, preventing sudden pressure changes in the penstock.
2. Protection Against Water Hammer:
   It reduces or eliminates the harmful effect of water hammer by allowing excess water to move into the tank during pressure rise.
3. Ensuring Continuous Water Supply:
   During a sudden increase in turbine demand, the surge tank provides additional water, ensuring consistent flow.
4. Energy Conservation:
   The controlled water flow reduces energy losses due to turbulence and sudden changes in momentum.
5. System Stability:
   It helps the entire hydraulic system operate smoothly and prevents mechanical strain on the penstock and turbine.

Working of Surge Tank

The working principle of a surge tank depends on pressure variations in the penstock:

• When the turbine load decreases, less water is required by the turbine. The inertia of the water column causes the water to continue moving towards the turbine, resulting in a rise in pressure. This excess water flows upward into the surge tank, where it is temporarily stored, thus preventing high-pressure damage.
• When the turbine load increases, more water is needed. The level of water in the surge tank falls as it releases stored water into the penstock to maintain the required flow rate.
• The water level in the surge tank fluctuates up and down until it reaches equilibrium between the reservoir and turbine flow rates.

This self-regulating action of the surge tank maintains steady operation and prevents the destructive effects of water hammer or pressure surges.

Types of Surge Tanks

1. Simple Surge Tank:
   This is an open vertical cylindrical tank directly connected to the penstock. It is simple in design and used in small plants where pressure variations are minimal.
2. Restricted Orifice Surge Tank:
   In this type, a small opening or orifice connects the tank and penstock. It reduces the rate of water exchange, making it more effective in damping water hammer effects.
3. Differential Surge Tank:
   It consists of an inner and outer chamber separated by an orifice. It is used in large hydraulic systems where precise control of water flow and pressure is required.
4. Elevated Surge Tank:
   This type is installed at a higher level to provide additional head to the penstock and to help control water flow more effectively in steep terrains.

Advantages of Surge Tank

• Prevents damage to pipelines due to water hammer.
• Provides continuous and stable water flow to the turbine.
• Maintains steady operation during load variations.
• Reduces maintenance cost of the hydraulic system.
• Increases overall safety and efficiency of the power plant.

Design Considerations

• The size and height of the surge tank must be adequate to handle the expected pressure variations.
• The location should be near the beginning of the penstock for quick response to pressure changes.
• The shape and type depend on the hydraulic characteristics of the system and terrain conditions.

Conclusion

A surge tank plays a crucial role in hydraulic power plants by balancing water pressure and flow rate during sudden load changes. It protects the penstock and turbine from pressure surges or water hammer and ensures continuous, smooth operation of the system. The type and design of the surge tank are selected based on the plant’s capacity, pressure conditions, and site layout. In short, it serves as both a safety device and an energy stabilizer for efficient plant operation.