--- title: "Governing of hydraulic turbines" subject: "Fluid Machines" chapter: 3 key: "semester_5/fluid_machines:turbine-governing" source: https://www.bctengineer.com/q/semester_5/fluid_machines/turbine-governing site: BCT Engineering --- # Governing of hydraulic turbines Fluid Machines · Chapter 3: Water Turbine - Asked in: year not given, 8 marks - Video: [Governing of Pelton Wheel Turbine With Animation](https://www.youtube.com/watch?v=KRm5b10n1aI) — BEST MECHANICAL ENGINEERING - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/fluid_machines/turbine-governing - Whole chapter: https://www.bctengineer.com/md/semester_5/fluid_machines/chapter-3 ## Question What is governing of a turbine? Explain the principle and working of an oil-pressure (servo-operated) governor for a Pelton turbine and for a reaction turbine. State the qualities of a good governor. ## Short answer Governing automatically regulates turbine speed by adjusting water flow to match load changes. An oil-pressure governor uses a fly-ball speed sensor to move a pilot valve, which directs high-pressure oil to a servomotor piston that adjusts the spear (Pelton) or guide vanes (reaction). Feedback restores the pilot valve to neutral. A good governor is stable, sensitive, quick, has small speed droop, and is reliable. ## Answer **Governing** is the automatic regulation of the turbine speed to keep it constant (so that the generator frequency stays constant) when the electrical load varies. When load falls, the speed rises, and the governor reduces the water flow; when load rises, it increases the flow. #### Principle Output power $=\rho g Q H\eta$. With the head fixed, power is changed by changing the discharge $Q$: - *Pelton turbine:* by moving the spear (needle) in the nozzle, and by the jet deflector for sudden load drops. - *Francis and Kaplan turbines:* by changing the opening of the guide vanes (wicket gates); in Kaplan, runner blade angle is also adjusted. #### Oil-pressure relay governor ```text Speed Pilot valve Servomotor (relay cylinder) sensor -> +--+ +--------------------+ (fly-ball -| |<-oil in | piston -> rod ---+--> spear / or electric) +--+ +--------------------+ gate ring ^ | | | | v v | +---- feedback linkage (restoring) <-------------------+ Oil sump <- gear pump -> oil under pressure ``` 1. A **fly-ball (or electronic) speed sensor** driven from the turbine shaft senses the speed. 2. If the load decreases, the speed increases; the balls move outward and raise the sleeve, which moves the **pilot (control) valve**. 3. The pilot valve admits high-pressure oil (from a pump driven by the turbine) to one side of the **servomotor piston**; oil on the other side goes to the sump. 4. The piston moves and, through the linkage, **moves the spear** forward (Pelton) or **closes the guide vanes** (Francis, Kaplan), reducing discharge and bringing the speed back. 5. A **feedback (restoring) mechanism** returns the pilot valve to the neutral position as soon as the correct opening is reached, which prevents overshoot and hunting. 6. For a sudden large load rejection on a Pelton, the **jet deflector** acts first to divert the jet, and the spear closes slowly, so water hammer in the penstock is avoided. For reaction turbines, a surge tank and slow closing of gates do the same. Because oil pressure supplies the force, the governor can handle a much larger power than a direct mechanical linkage could. #### Qualities of a good governor - **Stability:** returns the speed to normal without hunting (continuous oscillations). - **Sensitivity:** responds to small speed changes. - **Quick response (promptness):** small time lag in changing the gate opening, but not so fast that water hammer is caused. - **Small speed regulation (droop):** the change of speed between no load and full load should be small, typically 3 to 5 %. - **Isochronism:** ability to give exactly constant speed at all loads (zero droop). This is an ideal case. A permanent restoring (feedback) linkage gives some droop, so true isochronism needs temporary (dashpot) feedback or integral action, as in electronic governors. An isochronous governor cannot be used for parallel operation of generators. - Reliability, simple maintenance and the ability to be used for parallel operation of generators (which needs a small droop). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does governing of a turbine achieve when the electrical load changes? A. It keeps the head constant B. It keeps the turbine speed (and generator frequency) constant C. It keeps the efficiency at maximum D. It keeps the discharge constant Answer: B. Governing automatically regulates the discharge so the speed, and hence frequency, stays constant as load varies. (Key point: Purpose of governing) 2. Since power = rho g Q H eta and head is fixed, how does a Pelton turbine governor change the output? A. By changing the head B. By moving the spear in the nozzle to change the discharge C. By changing the runner diameter D. By changing the number of buckets Answer: B. With head fixed, power is changed through Q, and in a Pelton turbine the spear (needle) varies the jet flow. (Key point: How power is changed) 3. What is the purpose of the feedback (restoring) mechanism in the oil-pressure governor? A. To increase the oil pressure B. To return the pilot valve to neutral once the correct opening is reached, preventing overshoot and hunting C. To open the deflector D. To drive the fly-balls Answer: B. Restoring linkage neutralises the pilot valve at the right opening so the system does not oscillate. (Key point: Role of feedback mechanism) 4. For a sudden large load rejection on a Pelton turbine, what acts first to avoid water hammer? A. The spear closes instantly B. The jet deflector diverts the jet while the spear closes slowly C. The draft tube is opened D. The surge tank is emptied Answer: B. The deflector removes the jet from the buckets quickly, allowing the spear to close slowly so the penstock pressure surge is limited. (Key point: Sudden load rejection on Pelton) ## Related in Fluid Machines - [Impulse and reaction turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-vs-reaction-turbine) — Fluid Machines, chapter 3 - [Impulse and reaction steam turbines compared](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aimpulse-reaction-steam-turbine-types) — Fluid Machines, chapter 5 - [Pelton wheel: components and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apelton-wheel-working-components) — Fluid Machines, chapter 3 - [Francis turbine: construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Afrancis-turbine-working-components) — Fluid Machines, chapter 3 - [Kaplan turbine construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Akaplan-turbine-working) — Fluid Machines, chapter 3 - [Specific speed of a turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aspecific-speed-turbine-derivation) — Fluid Machines, chapter 3 ## Related in other subjects - [Purpose of governing hydraulic turbines](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aturbine-governing-purpose) — Hydro Power, chapter 7, asked 3× - [Purpose of governor and governing of Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agoverning-francis-turbine) — Hydro Power, chapter 7 - [Leadership and qualities of a good leader](https://www.bctengineer.com/question.md?key=semester_7%2Forganization_and_management%3Aleadership-good-leader-qualities) — Organization and Management, chapter 8, asked 2× - [Low specific speed reaction turbines](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Alow-specific-speed-reaction) — Power Plant Design, chapter 3