--- title: "Surge tank objectives and economic penstock diameter factors" subject: "Hydro Power" chapter: 5 key: "semester_6/hydro_power:surge-tank-objectives-economic-diameter" source: https://www.bctengineer.com/q/semester_6/hydro_power/surge-tank-objectives-economic-diameter site: BCT Engineering --- # Surge tank objectives and economic penstock diameter factors Hydro Power · Chapter 5: Components of Hydropower System - Asked in: 2075 Baisakh, 1+4 marks - Video: [SURGE TANK IN HYDROPOWER PROJECT (ENGLISH)](https://www.youtube.com/watch?v=0x6UH00o3YE) — CIVIL TALKS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/surge-tank-objectives-economic-diameter - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5 ## Question What are the objectives of the provision of surge tank for the hydropower project? Discuss the various factors which govern the determination of economical diameter of penstock. ## Short answer Surge tanks reduce water hammer, supply water quickly, store water, and improve speed regulation. Economic penstock diameter depends on discharge, head, length, material cost, energy value, load factor, interest rate, friction, water hammer, and transport limits; it minimizes total annual cost of pipe and energy loss. ## Answer #### Objectives of a surge tank A surge tank is an open shaft/chamber at the junction of the pressure tunnel and penstock. It is provided to: 1. **Reduce water hammer pressure** in the tunnel—the free surface reflects pressure waves so only the short penstock carries full water hammer. 2. **Supply water quickly** when turbine load increases (acts as a reservoir until tunnel flow accelerates). 3. **Store water** (rising level) when load is rejected, preventing excessive pressure rise. 4. **Improve speed regulation** of the turbine by reducing the effective length of the water column. #### Factors governing the economical diameter of penstock The economic diameter is the one giving minimum total annual cost (annual cost of the pipe + annual value of energy lost in friction). It depends on: 1. **Discharge $Q$:** larger flow needs larger diameter. 2. **Head $H$ and pressure:** higher heads need thicker plates, raising cost per unit diameter, so economic velocities are higher at high heads (smaller diameter relative to flow). 3. **Length of penstock:** head loss is proportional to length. 4. **Cost of material and fabrication:** steel price, plate thickness, welding, transport, anchor blocks and supports; higher cost favours smaller diameter. 5. **Value (tariff) of energy:** higher energy price favours a larger diameter to reduce losses. 6. **Plant load factor and hours of operation:** more running hours mean more energy lost through friction, favouring larger diameter. 7. **Interest rate and life of penstock:** affect the annual capital recovery cost. 8. **Friction coefficient:** pipe roughness (material, age, lining). 9. **Water hammer and allowable velocity:** larger velocity gives larger water hammer pressure and thicker pipe; erosion if silt-laden. 10. **Transport and handling limits:** maximum section size that can be carried on hill roads in Nepal. Empirical formulas (e.g. USBR, Sarkaria, Fahlbusch) give a first estimate, refined by the cost-curve (graphical) method. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a surge tank reduce water hammer in the tunnel? A. Its free surface reflects pressure waves so only the short penstock carries full water hammer B. It increases the tunnel velocity C. It removes silt from the flow D. It increases the pipe wall thickness Answer: A. The free water surface reflects waves, protecting the tunnel. (Key point: Surge tank purpose) 2. What does a surge tank do when turbine load suddenly increases? A. It stops the flow into the penstock B. It supplies water until the tunnel flow accelerates C. It stores extra water D. It closes the turbine gates Answer: B. It acts as a reservoir that supplies water quickly during load increase. (Key point: Load acceptance) 3. How does a higher value (tariff) of energy affect the economic diameter of a penstock? A. It favours a smaller diameter B. It has no effect C. It favours a larger diameter to reduce losses D. It makes the pipe thicker but same diameter Answer: C. Costly lost energy justifies spending more on a larger pipe with lower friction. (Key point: Energy price effect) 4. Higher cost of steel and fabrication tends to A. favour a larger diameter B. reduce the friction coefficient C. increase running hours D. favour a smaller diameter Answer: D. Costly pipe makes the construction component dominate, so a smaller pipe is economic. (Key point: Material cost effect) ## Related in Hydro Power - [Economic diameter of penstock](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-economic-diameter) — Hydro Power, chapter 5, asked 5× - [Penstock thickness and economic diameter](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-thickness-economic-diameter) — Hydro Power, chapter 5 - [Economic penstock diameter by empirical formulas](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apenstock-economic-diameter-formulas) — Hydro Power, chapter 5 - [Economic diameter of tunnel](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aeconomic-diameter-tunnel) — Hydro Power, chapter 5 - [Surge tank functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-functions-design) — Hydro Power, chapter 5, asked 2× - [Forebay function and surge tank types](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aforebay-function-surge-tank-types) — Hydro Power, chapter 5 ## Related in other subjects - [Economic diameter factors; penstock wall thickness (3 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aeconomic-diameter-and-wall-thickness-factors) — Hydropower Engineering, chapter 6 - [Economic diameter of penstock by graphical method](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aeconomic-diameter-penstock-graphical) — Hydropower Engineering, chapter 6 - [Economic diameter of penstock (analysis)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aeconomic-diameter-penstock-mathematical) — Hydropower Engineering, chapter 6, asked 2× - [Economic diameter; penstock thickness (1.25 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apenstock-economic-diameter-thickness-1-25m) — Hydropower Engineering, chapter 6