Governing of a Turbine: Oil Pressure Governor, Needle Valve and Surge Tank | Fluid Mechanics

Governing of a Turbine: Oil Pressure Governor, Needle Valve and Surge Tank!

Meaning of Governing of a Turbine:

Governing of a turbine means regulating the speed of the turbine. This is accomplished by the use of a turbine governor. A governor is the means to maintain the turbine at a constant design speed even though the load on the turbine may fluctuate. It is necessary to maintain a constant speed of the turbine so that the designed frequency and voltage may be maintained.

Under fluctuating load conditions also it is possible to maintain the turbine at a constant speed by regulating the discharge into the runner. The governor of a Pelton wheel turbine decreases or increases the outlet area of the nozzle by moving the needle valve. In the case of a Francis turbine or a Kaplan turbine, the governor decreases or increases the wicket gates.

In most of the installations, the turbine is directly coupled to the generator which has to run at a constant design speed. The speed of the generator must have a fixed value depending on the number of pairs of poles and the fixed frequency required. Thus the speed has to be maintained constant when the load may fluctuate. Thus, we realize the need for a speed regulating device.

The governor of a turbine has to be devised so as to act quickly in restoring the speed of the turbine to the normal design speed after any disturbance of the speed has taken place due to load variation. It is equally important the governor must bring about the change in the rate of flow in a short interval of time and at the same time, so as not to bring about severe hammer blow in the pipes.

Generally, to protect the pipe line from water hammer effects, it is a practice to provide also a safety device like a deflector or diffusor (in Pelton wheels) or relief valve (in reaction turbines) which are operated by the governor.

The Oil Pressure Governor:

Fig. 23.3. shows the components of an oil pressure governor.

The main components of the governor are the following:

(i) The Servomotor or Relay cylinder

(ii) The distribution valve or control valve

(iii) Actuator or pendulum,

(iv) Oil pump

(v) Gear pump which runs by tapping power from the power shaft by belt drive,

(vi) A pipe system communicating with the control valve servomotor and the pump.

When the turbine is subjected to its normal load, it runs at the normal speed N. When the load on the turbine increases or decreases, the speed of the turbine also will accordingly decrease or increase. The oil pressure governor will restore the speed to the normal value.

When the turbine is running at the normal speed under the normal load, the various components of the governor will be in the position shown in Fig. 23.4.

Suppose the load on the turbine increases. The speed will decrease. This reduces the speed of rotation of the central vertical bar which is run by the turbine shaft. The fly balls of the centrifugal governor are brought to a lower level due to the reduction in centrifugal force on them. This downward movement of the fly balls will bring about a downward displacement of the sleeve.

The sleeve and the piston rod of the distribution valve are connected by a rigid horizontal lever supported over a fulcrum. Thus, as the sleeve moves down, the lever turns about the fulcrum taking a slanting position and dragging the piston of the control valve upwards. This brings about the valve A to open while the valve B is kept in the closed condition.

Oil which is being pumped into the distribution valve cylinder will now flow into the relay cylinder and exert a pressure on the right side of its piston. Thus, the piston of the relay cylinder will move towards left. Accordingly, the piston rod of the relay cylinder will also move towards left. The other end of this piston rod opens or increases the inlet area of the discharge into the turbine.

Thus the discharge to the turbine and hence, the energy supplied to the turbine is increased and the turbine reaches its normal speed. When this normal speed is reached, the lever reaches the horizontal position and the various components reach their normal positions shown in the figure.

Similarly, when the load falls, the speed increases. The centrifugal fly balls reach a higher level moving the sleeve also upwards. The lever takes again an inclined position, this time closing the valve A and opening the valve B so that high pressure oil reaches the relay cylinder on the left side of its piston. A higher pressure is exerted on this piston and hence, this piston and the piston rod will move towards right.

The other end of the piston rod will bring about a decrease in the inlet area of the discharge into the turbine. Due to the reduction in the discharge to the turbine, the speed falls and ultimately reaches its normal value. When the normal speed is reached, the various components of the governor again reach their normal positions shown in the figure.

Needle Valve:

This is the device provided for a Pelton wheel turbine to regulate discharge to the turbine.

The needle valve Fig. 23.5 consists of a tapering needle or spear provided at the mouth of the nozzle. The needle is connected to the needle rod. The needle rod is nothing but the other end of the piston rod of the relay cylinder. By moving the needle towards the outlet of the nozzle, the opening area of the nozzle is decreased and thus, the discharge is decreased. If the needle is moved away from the nozzle outlet opening area of the nozzle is increased and thus, the discharge is also increased. The movement of the needle is brought about by the action of the governor.

Surge Tank:

When the inlet passages to the turbine are decreased by the action of the governor, the rate of flow in the pipe line is suddenly decreased. This sudden decrease in velocity will result in sudden rise of pressure. Thus, a surge tank is the necessary safeguard against pipe bursting.

The surge tank is an open topped large chamber provided so as to communicate freely with the pipe line bringing water from the reservoir. The upper lip of the surge tank is situated at a suitable height above the maximum water level in the reservoir. When the turbine is working under steady load and the flow through the pipe is uniform, there will be a normal pressure gradient oc. The water-level in the surge tank will be lower than that in the reservoir by ac which represents the loss of head in the pipe line due to friction.

If now the rate of flow in the pipe line is suddenly decreased by the action of the needle valve brought about by the governor, there will be a sudden pressure rise and this will result in a sudden rise in the water level in the surge tank so that the hydraulic gradient is now along ob. In this situation, the water level in the surge tank will be higher than that in the reservoir. This condition prevails only for a short duration.

The surge tank acts as an auxiliary storage reservoir to collect the flow down the pipe when the needle valve reduces the outlet area of the nozzle. The excess water is accumulated in the surge tank. This arrangement eliminates the instantaneous expansion of the pipe line and thus prevents pipe bursting.

Other Types of Surge Tanks:

Besides the simple cylindrical surge tank, other types are also adopted. See Fig. 23.7.

Fig. 23.7. (i) Shows a conical surge tank which is similar to the simple surge tank, except in this case, the tank has a conical shape.

Fig. 23.7. (ii) Shows a surge tank provided with internal bell-mouthed spill way. This arrangement allows the overflow to be conveniently disposed of.

Fig. 23.7. (iii) Shows a differential surge tank. This has the advantage that for the same stabilising effect its size can be very much less than that of the ordinary surge tank. Inside the surge tank there is a riser pipe provided with parts at its bottom.

When there is an increase in pressure in the pipe, some small quantity of water enters the surge tank through these ports but the major bulk of the incoming flow mounts to the top of the riser and then spills over into the tank. Thus this provides a substantial retarding head while in the ordinary surge tank the head only builds up gradually as the tank gets filled. It may further be realized that the water is not allowed to waste in the differential tank.