The usually overspeed limit is taken as about 10 to 12% of normal operating speed. Hence every turbine is privided with one or two overspeed trips which shut off the supply of steam to the turbine if the RPM exceeds a certain limit.
The overspeed tripping device consist of an unstable centrifugal governor (astatic regulator).
Figure 9.13 shows the main details of construction of such a ring-type astatic regulator. The eccentric ring 1 is directly mounted on the turbine shaft.
The eccentric ring is held in the position shown on the figure by spring 2. The eccentric e of the regulator is given by the distance between the axis of the turbine shaft and the centre of gravity of the regulator ring 1.
Distance a shows the regulator travel. The regulator ring 1 is displaced through a when the shaft speed exceeds the limiting speed.
Overspeed regulators are made of various different constructions as well.
One of the most widely used overspeed tripping device is shown in figure 9.14. Regulator pin 1, when overspeeding, trips lever 7 disconnecting the interlock 8. The tensile force of the helical spring rotates lever 2 and segment 3 in the clockwise direction.
Segment 3 now leaves its notch in sleeve 4 and allows the valve to close under the pressure of its own spring 5, thus shutting off steam supply to the turbine. If the stop valve is to be re-opened the following operation have to be carried out.
The hand-wheel of stop valve is rotated in the direction of closing. During this operation sleeve 4 is displaced upwards. Lever 7, 2 and segemens 3 are now brought back to their original position. The stop valve is now opened by means of the hand-wheel.
For the turbines of medium and large capacities closing the stop valve while overspeeding is brought about by means of hydraulic control as figure 9.15.
In the case of overspeeding beyond the limiting of speed pin 1 of the overspeed trip strikes against the left arm of the lever 3 and thus release lock 4.
Because of tension of spring 5 lever 6 pushes the servomotor piston7 downwards. With the displacement of the servomotor piston oil normally flowing from chamber 9 to chamber 10 is directed into the lubricating system of the turbine, and at the same time the oil from chamber 10 i,e regulating oil, find its way into the drain oil piping through chamber 8.
This drain of oil from the regulating system causes the regulating valves close and the shut off the supply of steam to the turbine.
No comments:
Post a Comment