The function of a governor is to control the fluctuation of speed of a prime mover within the prescribed limits with the variation of loads on it. In case of steam turbines when it is connected to drive an alternator for converting its mechanical energy into electrical energy, device is used to vary the turbine output according to the load on the alternator with very small fluctuations in speed, called governor.
The methods used for governing the steam turbines are: 1. Throttle Governing 2. Nozzle-Control Governing 3. By-Pass Governing’s 4. Extraction Pressure Regulating System 5. Over-Speed Tripping System/Emergency Governor.
Method # 1. Throttle Governing:
In this type of governing the steam is throttled down to lower pressure according to the load on the turbine before it is supplied to the turbine. It reduces the enthalpy drop i.e. the available energy of the steam. Such a method is useful for small capacity power plants since the mechanism is simple with initial low cost.
The schematic arrangement employing the throttle governing. The throttling of steam is achieved with the help of balanced throttle valve which is controlled by a centrifugal governor. In case of small turbines the throttle valve can be actuated directly with the help of governor through linkages since the steam flow rates would be small and the valves needed are high.
However, in case of medium and large power plants the effort of the governor may not be sufficient to actuate the throttle valve due to this an oil operated relay is incorporated in the circuit as shown in figure. This rely magnifies the small force produced by the governor for a small change of speed to produce a large force to move the throttle valve.
The working of the mechanism is as follows. Consider the case when the load on turbine shaft is equal to the power developed by the turbine, the speed is constant and the system is in equilibrium. Let us assume that the load on the turbine is reduced suddenly. At this state since the power developed is more than the load, the turbine and governor speeds will increase due to the excess energy developed by the turbine.
The governor balls will fly out and it will raise the leave of the governor, consequently, the different lever will cause the pilot piston to be raised. The oil which is supplied under a pressure of 3 – 4 bar will flow through the pipe A to the cylinder of relay piston and it would force the relay piston to move downwards, while the oil from relay piston cylinder is drained out through the pipe B.
The downward movement of the relay piston operates the throttle valve which in turn closes the steam ports partially. It throttles the steam and the steam pressure at inlet to the turbine is reduced. When the power developed by the turbine equals to the load on the turbine, the oil ports C – D are covered and the relay piston is locked.
It should be noted that this method of governing is through simple but it reduces the efficiency of power plants at part loads because a part of the available energy is lost in the irreversible throttling process. The Willan’s straight line relationship between load and steam consumption is also followed as discussed in case of steam engines.
Method # 2. Nozzle-Control Governing:
In this method, the first stage nozzles are split-up groups which are controlled by individual throttle valves. Various arrangements of valves and group of nozzles are employed. Three such arrangements are shown diagrammatically in the Fig. 20.8. An arrangement often adopted with large steam turbines using high-pressure steam is shown in Fig. 20.8(a). The number of nozzle groups may vary from three to five or more.
Figure 20.8(b) shown an arrangement where the nozzle control valves are arranged in a casting forming part of the cylinder of bolted thereto, and containing passes loading to individual nozzle groups. The nozzles are confined to upper half of the cylinder and are of admission is limited to 180° or less. The number of nozzle groups may vary from four to twelve.
Figure 20.8(c) shows an arrangement in which the group of nozzles Ni is under the control of valve V1 through which all the steam entering the turbine passes. Further admission of stem is through the valves V2 V3 etc. in turn.
The nozzle-control is necessarily restricted to first stage and the absolute pressure of steam in front of the second stage nozzles will be directly proportional to the rate of steam flow through the turbine.
There is greater heat drop available when nozzle control is employed but the efficiency decreases when load is reduced. When there is a comparatively large heat drop in the first stage, nozzle control leads to reduction in steam consumption.
Method # 3. By-Pass Governing’s:
Modern steam power plants employing impulse turbines at very high pressure of steam at admission are usually designed to operate usually at an economical load which is bout 75-80% of the maximum load. In such cases -it is desirable to have full admission of steam in first few stages of the turbine operating at very high pressure since the enthalpy drop is very small in the initial stages and the nozzle governing cannot be used effectively.
Therefore the load regulation is achieved by throttle governing upto the stage of economical loads. However at the maximum loads the additional amount of steam required cannot be passed through the first-stages since the required additional number of nozzles is not available. This difficulty of steam regulation is overcome by employing by-pass governing.
Steam required upto economical loads are passed through the inlet valve and it is collected in the nozzle-chest. The governing is affected by throttle valve. However at loads more than economical load, the by-pass valve lifts and a part of the steam is by-passed into the steam belt and this steam mixes with the steam of high pressure turbine stages and it is supplied to lower stages of the turbine. This increases the power output of the turbine. The movement of the by-pass valve is controlled by the turbine governor.
Method # 4. Extraction Pressure Regulating System:
Extraction pressure regulating system is used where steam at constant pressure is extracted for heating or process requirements from a condensing turbine which meets the power requirement. The control gear must be designed to maintain speed and pressure of extracted steam regardless of the variation in power and heating loads.
These requirements are met by using a speed governor which controls the admission of h.p. steam, to the turbine and a pressure regulator controlling the admission of steam to the L.P. stages. A change of either power load or heating load will cause both governors to operate.
When the generator load increases the throttle valve opens causing a pressure rise in the extracted steam. The pressure regulator acts to open the regulating valve to maintain the steam pressure. If the heating load increases, the regulating valve closes and consequently the speed falls due to less flow of steam through L.P. stages. The speed governor acts to open the throttle valve.
When the speed and pressure regulators operate independently, there is a tendency to hunting and this is overcome by interconnecting the governor gear and pressure regulating system. A drop in speed causes both the valves to lift whereas an increase in heat load and fall in heating steam pressure causes pressure, regulating valve to close particularly and the throttle valve to open further. In order to simplify the diagram oil relays have been omitted.
Method # 5. Over-Speed Tripping System/Emergency Governor:
If a turbine has a not too satisfactory speed regulating system the sudden increase in the shaft speed at times of load tripping may reach dangerous figures. The usual over-speed limits is taken as about 10 to 12% of the normal operating speed. Hence every turbine is provided with one or two over-speed trips which shut off the supply of steam to the turbine if the r.p.m. exceeds a certain limit.
The over-speed tripping device consists of an unstable centrifugal governor (static regularity). Figure 20.11 shows the main details of construction of such a ring-type static regulator. The eccentric ring 1 is directly mounted on the turbine shaft. The eccentric right is held in the position shown in Fig. 20.11 spring 2. The eccentricity (e) of the regulator is given by the distance between the axis of the turbine shaft and the center 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. Over-speed regulators are made of various different constructions as well.
Regulator pin 1, when over-speeding, 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 operations have to be carried out. The hand wheel of the stop valve is rotated in the direction of closing. During this operation sleeve 4 is displaced upward. Levers 7, 2 and segments 3 are now brought back to their original position. The stop valve is opened by means of the hand-wheel.