Draft tube objectives and governors

Objectives of a draft tube for a reaction turbine

A draft tube is a diverging pipe from the runner exit of a reaction turbine to the tailrace, with its outlet submerged. It is provided:

1. To recover kinetic energy at the runner exit by reducing velocity, converting it into pressure energy.
2. To create a suction head (pressure below atmospheric) at the runner exit, so that the head between the runner and tailwater is not lost.
3. To allow the turbine to be set above tailwater for easy access, inspection and repair without loss of head.
4. To increase the net working head, efficiency and output of the turbine.
5. To discharge water to the tailrace with low velocity, preventing scour.

Types: conical (straight divergent), elbow with varying section, simple elbow, Moody spreading tube. Its depth is limited by cavitation (Thoma's criterion).

Working principle of governors

A governor automatically keeps the turbine speed constant by matching the water supply to the load. A conventional oil-pressure (hydraulic) governor has:

• Speed sensor: centrifugal (fly-ball) governor or electronic speed transducer driven from the turbine shaft.
• Control (relay/distributor) valve: directs pressure oil to either side of a servomotor piston.
• Servomotor: a cylinder and piston that moves the guide vanes (Francis/Kaplan) or the spear and deflector (Pelton).
• Oil sump, pump and pressure accumulator: supply oil under pressure.
• Feedback (restoring) mechanism / dashpot: prevents over-correction and hunting.

 turbine shaft -> fly-ball -> lever -> control valve
                                        |   oil
                                        v
  guide vanes / spear <---- servomotor piston
          ^                         |
          |____ feedback linkage ___|

Sequence (load decreases):

1. The turbine speed rises; fly-balls move outward and the sleeve rises.
2. The lever moves the control valve so that pressure oil enters one side of the servomotor.
3. The servomotor piston moves and closes the guide vanes (or pushes the spear forward), reducing discharge.
4. The speed returns to normal; the feedback linkage brings the control valve back to the neutral position.

When the load increases, the reverse happens and the gates open. Modern plants use electro-hydraulic or digital (PID) governors, which sense frequency electronically but still use oil servomotors to move the gates. For Pelton turbines a jet deflector acts first on sudden load rejection to avoid water hammer, while the spear closes slowly.