What Is The Use Of Pressure Tunnel In Hydro Power Plant?

What Are Pressure Tunnels?

Pressure tunnels are closed conduits used to convey water from a reservoir to hydroelectric turbines in a hydropower plant, typically located below the dam’s tailwater level. They allow the water to flow completely under pressure due to the elevation difference between the reservoir and powerhouse (Source).

The purpose of pressure tunnels in hydroelectric plants is to deliver water to the turbines efficiently while minimizing energy losses from friction. They provide a closed channel for the water to flow in, protecting it from external environmental factors.

The main difference between pressure tunnels and open channel water conductor systems is that pressure tunnels are closed conduits made of concrete, steel, or rock. Water conductor systems like canals, flumes, and penstocks are open to the atmosphere. Pressure tunnels maintain the pressure energy of the water over long distances by preventing friction losses.

How Do Pressure Tunnels Work?

Pressure tunnels are a critical component of hydroelectric power plants that allow water to flow from the reservoir to the turbines under pressure. The path of water flow begins at the reservoir, where water builds up behind a dam. The water enters the tunnel inlet, which is equipped with trash racks and gates to control water flow.

Inside the pressure tunnel, steel or concrete lining maintains the integrity of the tunnel and prevents water from seeping into the surrounding rock. Surge tanks or relief valves are installed at intervals to absorb water hammer shocks and regulate pressure. At the end of the tunnel is the valve house, which controls the final flow of water into the penstock connected to the turbine (Source 1).

The downward slope of the tunnel coupled with the water already in the tunnel creates tremendous water pressure. This hydrostatic pressure builds up to levels as high as 10 MPa as it gets closer to the powerhouse. Proper pressure tunnel design must balance optimal pressure delivery to maximize turbine efficiency with safe operating limits (Source 2).

Advantages of Using Pressure Tunnels

Pressure tunnels provide several key advantages for hydroelectric power projects compared to using open canals:

Pressure tunnels allow for more efficient transport of water over long distances. As the NTNU article explains, tunnels minimize friction losses, allowing water to move quickly through penstocks to the turbines with minimal energy loss.

Tunnels also avoid the high rates of evaporation associated with transporting water long distances via open canals or flumes. Keeping the water enclosed preserves the maximum volume for power generation.

Finally, for long distance water transport, tunneling is often lower in construction costs compared to building open canals, aqueducts, or pipelines above ground. The Utilities One article notes tunneling leverages natural geologic formations, reducing the need for expensive supportive structures.

Challenges in Pressure Tunnel Construction

Building pressure tunnels presents several key challenges, particularly when dealing with high hydraulic pressures and difficult geotechnical conditions. Three major challenges include:

Dealing with leakage and seepage – Pressurized water can infiltrate even tiny cracks and fissures in the surrounding rock mass. Controlling water inflows requires careful grouting and waterproofing methods. According to the construction planning resource from Utilities One https://utilitiesone.com/overcoming-challenges-in-underground-construction-planning, effective grouting is essential to minimize leakage and seepage into tunnels under pressure.

Maintaining structural integrity under high pressure – The immense internal water pressure places massive loads on tunnel linings. Tunnel designers must account for water hammer pressures and hydraulic jacking forces. As noted in Zhu (2019) https://www.sciencedirect.com/science/article/pii/S2095809919304680, ultra-deep high pressure tunnels require very robust construction to withstand forces up to 100 times atmospheric pressure.

Surge control – Sudden flow changes when starting up or shutting down turbines can cause damaging pressure surges within tunnels. Careful hydraulic design of surge tanks, valves, and other protective measures is required to avoid surges. Pressure relief valves may also be installed to protect tunnels from excessive transient pressures.

Notable Pressure Tunnel Projects

Some of the most notable large-scale pressure tunnel projects around the world include:

• The Hálslón Headrace Tunnel in Iceland is 7.1 km long and has a diameter of 10 m. It was built for the Kárahnjúkar Hydropower Project at a cost of over $1 billion USD (source).
• The Ufsarlón Headrace Tunnel, also in Iceland, measures 16.4 km long with a diameter of 8.8 m. It provides water to the Fljótsdalur Hydropower Station (source).
• The Niagara Tunnel in Canada is 12.7 km long and has a diameter of 10.2 m. It cost over $1 billion USD to build and carries water to Sir Adam Beck Hydroelectric Generating Stations (source).
• The Tailrace Tunnel at Manapouri Power Station in New Zealand is 10 km long with a diameter of 8.5 m. It is one of the world’s longest underground hydroelectric tunnels (source).

Pressure Tunnel Design Considerations

Several factors go into the optimal design of a pressure tunnel. Two key considerations are determining the optimal diameter and thickness of the tunnel lining. Smaller diameter tunnels are generally less expensive to construct but result in higher water velocities and more frictional head loss. Larger diameters reduce velocity but cost more to excavate and line. Thickness of the lining must balance strength needs with material costs. According to EPRI’s Design Guidelines for Pressure Tunnels and Shafts (https://www.epri.com/research/products/AP-5273), linings generally range from 5 to 12 inches thick for concrete and 0.4 to 1 inch for steel.

Another major design decision is choosing an appropriate lining material like steel or concrete. Concrete is generally used for shorter tunnels with less than 300 psi working pressure. Steel allows for thinner linings and is often used for longer tunnels and higher pressures. However, steel is more susceptible to corrosion and needs protective coatings. Advanced 3D computer modeling techniques can help engineers optimize the diameter, thickness, lining material, and other design parameters.

Sophisticated software like Phase2, RS2, and Ansys enable advanced finite element analysis for pressure tunnel designs. These programs can simulate the structural stresses and fluid dynamics involved under various operating conditions. Computer modeling provides data to help engineers select appropriate factors of safety and specifications to withstand water pressures over decades of operation.

Construction Methods for Pressure Tunnels

There are several key construction methods used for excavating pressure tunnels:

Drilling Methods

– Tunnel Boring Machines (TBMs) – TBMs are large mechanized excavators that continuously tunnel by cutting through rock or soil. They are efficient at excavating long straight tunnels like pressure tunnels. There are two main types – gripper TBMs brace against the tunnel walls for thrust, while shield TBMs have a protective shield. TBMs minimize disturbance to the rock mass around the tunnel.^[1]

– Drill and Blast – This traditional method uses explosives and drilling to progressively break out rock and excavate the tunnel. It can be challenging to maintain smooth tunnel walls suitable for lining using this method.

Support Systems

– Rock Bolts – Rock bolts anchored into the bedrock are commonly used as temporary or permanent rock support in pressure tunnels. Fiber reinforced shotcrete can also be sprayed for added support and stabilization.

– Steel Ribs and Lagging – Used for short term ground support. Steel ribs are installed against the rock surface and lagging (wood or concrete boards) is placed between the ribs.

Tunnel Boring Machine Types

– Main Beam TBM – For hard rock tunneling. Uses a single shielded cutterhead attached to a sturdy main beam.

– Double Shield TBM – Has two circular shields for protection from broken rock and water ingress, best for mixed ground conditions.^[2]

– Gripper TBM – Braces against tunnel walls with gripper pads for thrust during excavation. No circular shield.

– Single Shield TBM – One forward shield for protection. Can handle soft ground and high water pressure conditions.

Testing and Commissioning of Pressure Tunnels

Hydrostatic testing is a critical procedure conducted during the commissioning of pressure tunnels to ensure structural integrity and ability to withstand design pressures. The tunnel is filled with water, pressurized up to 1.1 to 1.25 times the maximum design pressure, and held at that pressure for a specified duration per quality standards and codes.

Per standards like ASME B31.10, the test pressure should be held for a minimum of 24 hours. The allowable leakage rate is calculated and measurements taken to ensure the tunnel passes. Strain gauges, pressure transducers, and flow meters are installed to monitor stress distribution, pressure retention, and leakage.

Maintaining quality is crucial as any undetected defects can have catastrophic effects when the tunnel becomes operational. Factors like water quality, dissolved gas levels, and temperature are closely controlled. Test equipment is precisely calibrated and inspected. Stringent pass/fail criteria are set. The test results are thoroughly documented and reviewed before final commissioning sign-off.

With large diameter and long distance tunnels, it may take weeks to fill, pressurize, hold test pressure, depressurize and dewater after testing. Overall, rigorous testing and quality control ensures these critical pressure tunnels safely deliver water for hydropower generation.

Operation and Maintenance

Proper operation and maintenance of pressure tunnels is critical to ensure efficient and safe functioning of hydroelectric power plants. Some key aspects of pressure tunnel maintenance include:

Monitoring for leaks/cracks – Regular inspection and monitoring of the tunnel lining using instruments like extensometers and instrumentation is important to detect any cracks or leaks early on. Acoustic emission testing and ground penetrating radar can also be used. Early remedial action can prevent major failures.

Desilting tanks – Silt and debris deposited in desilting tanks and forebays needs to be cleared periodically to maintain storage capacity and prevent flow disturbances. Desilting is done using excavators, underwater suction pumps etc.

Valve maintenance – The valves along the tunnel and penstock need to be inspected and maintained regularly. This includes checking for leaks, servicing, replacing worn out parts etc. Valves need to be tested for proper operation.

Tunnels also need regular cleaning and drainage maintenance. Ventilation systems, lighting, access tunnels and other ancillary structures associated with the pressure tunnel also require periodic upkeep and maintenance. Personnel need to be trained in tunnel safety procedures.

Future of Pressure Tunnel Technology

The future looks bright for innovations in pressure tunnel technology and construction techniques. Some key areas of development include:

Improving construction techniques: New tunnel boring machines, drilling methods, and materials are enabling faster and more efficient tunnel construction. Techniques like sequential excavation and precast segmental lining are speeding up the process while maintaining safety and stability. Using fiber reinforced polymers as tunnel supports is an emerging innovation.

Smart monitoring systems: Integrating sensor networks and data analytics into tunnels allows for real-time monitoring of conditions like water pressure, vibrations, strains, and temperatures. This helps detect issues early on and enables predictive maintenance. Fiber optic monitoring is a promising new approach for comprehensive tunnel diagnostics.

Innovations in design: Advances in design tools like computational fluid dynamics and finite element modeling facilitate optimized tunnel designs. Concepts like developing modular, standardized tunnels or using variable diameters can improve constructability and operations. Designers are also finding ways to build tunnels to better withstand seismic events.

As technology progresses, pressure tunnels are poised to become even more efficient, safe, and cost-effective. Their essential role in hydroelectric power generation will continue advancing along with underground construction techniques.