CODE OF GOOD PENSTOCK CONSTRUCTION AND ERECTION WORKFLOW

Introduction

Hydroelectric power remains a cornerstone of low-carbon electricity generation, offering a reliable and flexible energy source that complements variable renewable energies (EPRI, n.d.). At the heart of every conventional hydropower scheme lies the penstock, a vital conduit responsible for channeling water to the turbines under immense pressure. The integrity and performance of penstocks are critical, as failures can lead to significant financial losses, environmental damage, and, in severe cases, loss of life. Therefore, adhering to a comprehensive code of good construction and erection practices is not merely a recommendation but a fundamental requirement for the sustainable operation of hydroelectric assets.

Importance of Penstocks

Penstocks serve as the main arteries of hydroelectric power plants, conveying pressurized water from intake to turbine. Their structural integrity and operational reliability directly impact energy production, safety, and longevity of the facility (IEA, 2021).

 Their design and construction directly influence plant efficiency, safety, and longevity.

This article presents a structured methodology for good an robust practical recommendations for penstock construction and erection practices, drawing from the Comité Européen de la Chaudronnerie et de la Tôlerie (CECT, 1979/1984), modern engineering standards, and real-world case studies. It aims to guide engineers, project managers, and stakeholders in making informed decisions that balance technical, environmental, and economic factors.

This article aims to provide construction and erection workflow, emphasizing critical considerations from design to commissioning.

Penstock Classification

2.1 By Installation Category

Aboveground (Exposed): Visible for easy inspection but vulnerable to external loads and temperature variations.

These penstocks are visible and run along the surface, supported by piers or saddles. They are susceptible to external environmental factors such as temperature fluctuations, wind, and seismic activity, requiring robust external protection and structural design (CECT, 1979).

Buried: Protected from environmental conditions but demands intensive geotechnical assessment.

Installed underground, buried penstocks are less exposed to atmospheric conditions but must withstand soil pressures, corrosion from ground contact, and potential ground movements.

Semi-buried: Combines benefits of above and underground placements.

 A hybrid approach, where parts of the penstock are buried, and others are exposed. This requires careful consideration of transitions between different sections and varying external loads.

In Tunnels (Lined Shafts or Tunnels): High protection from environmental and seismic loads; ideal for mountainous terrain.

Penstocks installed within tunnels are protected from direct external weather and seismic impacts but are subjected to rock and grouting pressures, as well as internal water pressure and potential interstitial water (CECT, 1979).

The Nachtigal Hydropower project utilizes lined steel sections within its power waterway

By Material Type

Welded Steel: The most common material due to its high strength, ductility, and adaptability to varying pressures and diameters. Steel penstocks are typically fabricated in sections and welded on-site. The European Committee for Boilermaking and Kindred Steel Structures (CECT) provides extensive recommendations for the design, manufacture, and erection of welded steel penstocks (CECT, 1979).

Prestressed Concrete: Used for lower-pressure applications, offering good corrosion resistance and reduced maintenance. However, they are heavier and less flexible than steel.

Used in large-diameter applications (e.g., dams).

Ductile Iron: Often used for smaller diameter penstocks or where corrosion resistance is a primary concern. They are robust but have limitations in high-pressure scenarios.

Suitable for moderate pressures but less common in large hydro projects.

Composites (GRP/FRP - Glass Reinforced Plastic/Fiber Reinforced Polymer): Lightweight and highly corrosion-resistant, composites are increasingly used for their ease of installation and excellent hydraulic properties, particularly in aggressive environments.

Strapped Timber: Historically used, but largely superseded by more durable materials. Still found in some older, smaller-scale projects.

By Technical Class

Penstocks are also characterized by various technical specifications that dictate their design and manufacturing complexity:

Nominal Diameter (DN): Ranging from small diameters for low-head plants to very large diameters (several meters) for high-capacity projects.

Determines flow capacity, typically ranging from 0.5 m to over 10 m in large projects like GERD.

Wall Thickness: Directly proportional to the internal pressure and diameter, calculated to withstand operational and transient pressures (CECT, 1979, Annex II).

 Varies based on pressure and material, critical for withstanding hydraulic transients (EPRI, 2023).

Pressure Classes: Defined by the maximum internal and external pressures the penstock is designed to endure, including static pressure, water hammer effects, and vacuum conditions (CECT, 1979, Annex II).

Low (<10 bar), medium (10–50 bar), high (>50 bar), aligned with turbine requirements.

Interior/Exterior Cladding: Protective coatings and linings applied to prevent corrosion and reduce friction, enhancing durability and hydraulic efficiency (CECT, 1979, Annex VI), (e.g., epoxy coatings) or abrasion from sediment-laden water.

Assembly Methods: Dictated by material, size, and site conditions, including welding (manual or automatic), bolting, or specialized jointing techniques.

Shop-welded, field-welded, or bolted flanges, with welding predominant in steel penstocks (CECT, 2020).

Technical Selection Criteria

The selection of penstock type and design is a complex process influenced by several critical technical factors.

Site Topography: The geological and topographical conditions of the site significantly impact the choice of penstock installation. Steep inclines may favour aboveground or tunnel installations, while flatter terrains might allow for buried sections. Geological stability, rock formations, and potential for landslides or rockfalls are paramount (CECT, 1979, Annex I).

Influences route selection, slope, and anchoring requirements.

Operating Pressures: The maximum static head and transient pressures (water hammer) are primary drivers for determining wall thickness, material strength, and the need for stiffeners or anchor blocks (EPRI, n.d.; CECT, 1979, Annex II).

Designed for low, medium, or high-pressure systems, aligned with turbine requirements.

Surge chambers or pressure relief valves mitigate transient pressures.

Seismic Risks: In seismically active regions, penstocks must be designed to withstand earthquake forces, requiring specific considerations for material ductility, joint flexibility, and anchoring systems (CECT, 1979, Annex I) - (Eurocode 3).

Favor flexible joints and ductile materials reinforced supports, and dynamic analysis (ASCE MOP 79, 2012) in high-risk zones.

Cost/Durability: A balance must be struck between initial investment costs and long-term durability, maintenance requirements, and operational efficiency. Steel offers a good balance for many high-pressure applications, while concrete or composites may be more economical for lower pressures or specific environmental conditions.

Balance between capital expense and life-cycle performance.

 Good Construction and Assembly Practices

A rigorous workflow encompassing preliminary studies, manufacturing, on-site assembly, and testing is essential for successful penstock projects. The Nachtigal Hydropower project's installation procedure provides a detailed example of such practices

4.1 Preliminary Studies

Before any physical work begins, extensive preliminary studies are indispensable.

Site Investigations: Comprehensive geotechnical surveys, hydrological assessments, and topographical mapping are crucial for understanding ground conditions, water characteristics, and potential environmental challenges (CECT, 1979, Annex I).

Design Review and Optimization: Detailed design reviews, often involving collaboration between the client and manufacturer, are necessary to optimize penstock layout, material selection, and structural details based on site-specific data. This includes determining optimal anchor block locations and support spacing.

Logistics and Access Planning: Planning for the transportation of large penstock sections and heavy equipment to the site is vital. This includes assessing road conditions, bridge capacities, and the need for specialized transport vehicles like lowbed trailers (IH_NACHT_DEV_X_440_CHY_150002-B, 2022). Access to work areas via scaffolding and platforms must also be meticulously planned for safe and efficient operations (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

Safety Planning: A comprehensive safety plan, adhering to applicable regulations and addressing site-specific hazards, is paramount. This includes providing appropriate personal protective equipment (PPE) and ensuring qualified personnel for all tasks (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

Studies Practices

Conduct thorough geotechnical and hydrological investigations.

Integrate digital twins for predictive performance analysis (EPRI, 2023).

Verify access routes and site logistics early.

Manufacturing & Quality Control

Manufacturing quality directly impacts penstock integrity.

Material Selection and Traceability: Strict adherence to material specifications (e.g., steel plate grades as per CECT Annex III) and maintaining full traceability of all components from raw material to finished product are essential (CECT, 1979, Annex III).

 Fabrication Procedures: Fabrication in controlled workshop environments minimizes defects. This includes precise cutting, rolling, and welding of penstock sections, following approved welding procedures and qualified personnel (CECT, 1979, Annex IV).

Non-Destructive Testing (NDT): Extensive NDT methods, such as ultrasonic examination and radiographic examinations, are critical at various stages of fabrication to detect internal flaws in welds and materials (CECT, 1979, Annex III & IV). Surface defects must be rectified according to strict standards (CECT, 1979, Annex IIIA).

Surface Preparation and Coating: Proper surface preparation (e.g., cleaning to white metal) and application of anti-corrosion coatings are vital for long-term durability. This includes both internal and external protection, with careful attention to environmental conditions during application (CECT, 1979, Annex VI).

Implement ISO 9001-compliant quality systems.

Ensure certified steel grades, weld filler materials, and traceability.

Use shop-fabricated segments to reduce on-site variability (CECT, 1979).

Follow CECT Annex III for steel plate and welding specifications.

Perform NDTs (radiography, ultrasonic) as per ISO 9712 and ASME Sec. V. - (CECT, 1984).

On-Site Assembly

On-site assembly demands precision and coordination.

Foundation and Anchor Block Preparation: Accurate construction of foundations, pedestal supports, and anchor blocks is crucial. The manufacturer provides the forces acting on these structures, which the civil contractor uses for their design and construction (CECT, 1979).

Section Alignment and Positioning: Penstock sections must be accurately aligned and positioned using topographical equipment, plumb lines, and measuring tapes. Temporary supports and stoppers are critical to prevent displacement during assembly (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

Welding and Jointing: On-site welding requires highly qualified staff and strict adherence to welding procedures (CECT, 1979, Annex IV). Environmental factors like temperature must be considered, especially for closure welds (IH_NACHT_DEV_X_440_CHY_150002-B, 2022). Non-destructive testing must be performed on site welds (CECT, 1979).

Concreting and Grouting: For buried or tunnel-lined penstocks, concreting and grouting operations must be carefully controlled to avoid excessive pressure and deformation on the pipes. Maximum pouring heights and holding times are critical (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

 Scaffolding and Erection Equipment: Appropriate erection equipment, such as derrick cranes, internal erection trolleys, and external platforms, are necessary for safe and efficient handling and positioning of penstock sections (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

On-Site Assembly Practices

Safety Protocols: Equip workers with PPE (helmets, safety belts) and implement HSE checks on scaffolding, as per Nachtigal safety guidelines.

Equipment Selection: Use derrick cranes (32-ton capacity) and internal/external erection platforms for precise positioning, as outlined in the Nachtigal workflow (Document IH-NACHT-DEV-X-440-CHY-150002-B).

Alignment & support spacing: Prevents excessive deflection (AWWA M11).

Sequential Erection: Follow a phased approach (e.g., lower elbows, inclined sections, upper elbows) to ensure alignment and stability, with temporary supports to prevent pipe movement.

Follow a structured sequence: elbows → inclined → upper sections (IH Nachtigal, 2023).

Welding and Alignment: Use topographic equipment and hydraulic jacks for alignment, with closure welds performed post-concrete hardening to accommodate thermal expansion (EPRI, 2023).

Use internal trolleys and external platforms for alignment and welding.

Field welding: Preheating required thick plates to avoid cracking.

Apply real-time monitoring to detect stress accumulation.

Testing & Commissioning

Thorough testing and commissioning ensure operational readiness.

Dimensional Checks: Final dimensional checks, including ovality and out-of-roundness, must be conducted to ensure the penstock meets specified tolerances after erection and concreting (CECT, 1979, Annex IIE).

Hydraulic Pressure Testing: A critical step, where the penstock is subjected to hydraulic pressure tests to verify its structural integrity and identify any leaks (CECT, 1979, Annex V).

Non-Destructive Testing (Post-Erection): Additional NDT on critical site welds and areas subject to high stress should be performed after erection and concreting (CECT, 1979, Annex IV).

Acceptance and Handover: Formal acceptance by the client, based on satisfactory test results and adherence to specifications, marks the completion of the construction and erection phase (CECT, 1979, Annex V).

Performance Monitoring: For ongoing operational safety, real-time performance monitoring, potentially utilizing digital twin technology, can enhance understanding of stress-time history and identify fatigue-critical points (EPRI, n.d.).

Verify alignment, anchorage, and valve operation.

Document all inspections and obtain client sign-off before energizing.

Hydrostatic test: 1.5x design pressure for 30+ minutes (ASME B31).

Leak checks & strain measurements ensure structural integrity.

Digital Twin Monitoring: Implement real-time monitoring systems to track pressure and stress, as tested in the EPRI project (EPRI, 2023).

Case Studies (Projects)

Examining real-world projects illustrates the application of these principles. While detailed construction workflows for all listed projects are extensive, these examples represent diverse penstock applications and challenges:

Nachtigal Hydropower Project (Cameroon): The provided document outlines detailed procedures for the installation of steel lining, including preliminary activities, sequence of assembly, use of specialized erection equipment like derrick cranes and internal trolleys, and concreting considerations (IH_NACHT_DEV_X_440_CHY_150002-B, 2022).

 This project exemplifies the meticulous planning and execution required for large-scale penstock installations.

Challenge: High-pressure steel penstocks in tropical conditions.

Solution: Zinc-aluminum coating + cathodic protection for corrosion resistance.

Toktogul Dam (Kyrgyzstan): A large hydroelectric power plant with significant penstock infrastructure, representing challenges in mountainous terrain and high-pressure conditions.

Rehabilitation integrated with digital twin stress monitoring (EPRI, 2023).

Inga Dams (Democratic Republic of Congo): A massive complex of hydroelectric dams, featuring very large diameter penstocks and extensive construction challenges.

Grand Ethiopian Renaissance Dam (GERD - Ethiopia): A monumental project with significant penstock components, showcasing the engineering complexities of large-scale hydro development in challenging environments.

Challenge: Massive penstock diameter (7.5 m).

Solution: High-strength steel with internal epoxy lining.

Kariba Dam (Zambia/Zimbabwe): An older, but still operational, large-scale project that provides insights into long-term performance and maintenance considerations for penstocks over decades.

Challenge: Seismic activity risk.

Solution: Flexible expansion joints and reinforced anchor blocks.

 Author: Hervé YIMGNA MENGOUO