Compounding and Governing of Steam Turbines (With Methods) | Thermal Engineering

In this article we will discuss about the methods of compounding and governing of steam turbines.

Compounding of Steam Turbine:

The steam turbines operate at very high speed in the range 30000 r.p.m. The devices as a load operate at slow speeds in range of 1000 to 4000 r.p.m. It becomes necessary to reduce the speed of steam turbine using the compounding technique.

The passes through the nozzle then the velocity of steam increases in the nozzle and pressure reduces in the nozzle, then steam from the nozzle goes to the blades of the steam turbine, where the pressure remains constant and velocity is reduced to negligible value. The kinetic energy of steam is absorbed by the blades and converted to mechanical energy.

The velocity reduction in the steam turbine is very high and the velocity of the steam and blades is relatively higher which results into high rotational speeds of the rotor. This makes steam turbine to be impossible to work as prime mover and then to achieve the high reduction of speed is very difficult to obtain.

Several methods are used the speed of high rotor speed. All of these methods consist of a multiple system of rotors in series, keyed on a common shaft, and the steam pressure or the steam velocity is absorbed in stages as it flows over the rotor blades. This is known as compounding of steam turbine.

There are three methods of compounding of steam turbine:

Method # 1. Velocity-Compounded Impulse Turbine:

This type of turbine consists of a nozzle or sets of nozzles and a wheel fitted with two or more rows of moving blades. It has two-rings of moving blades on the rotor and such a wheel. There are also a number of guides or stationary blades arranged between the moving blades and set in the reverse manner.

The steam entering the nozzle expands from the initial pressure down to the exhaust pressure, and resulting steam velocity is then utilized by number of sets of rotor blades rings. The steam after passing through the first ring of moving blades rings gives up only a part of its kinetic energy and fairly high velocity.

It then enters the guide blades (stationary blades) and is redirected by them into the second ring of moving blades. There is a slight drop in velocity in the fixed guide blades due to friction. In passing through the second ring of moving blades the steam suffers a change of momentum and gives up another portion of its kinetic energy to the rotor.

In case of three-row rotor, steam further passes through the next ring of stationary blades and then through the third ring of moving blades and subsequently leaves the wheel and enters the condenser. It may be noted that a two-row wheel is more efficient than the three-row wheel.

It will be noticed from the pressure curve that all the pressure drop takes place in the nozzle ring, and the pressure remains constant as the steam flows over the blades.

Method # 2. Pressure-Compounded Impulse Turbine:

A number of simple impulse turbines in series on the same shaft, allowing the exhaust steam from one turbine to enter the nozzles of the succeeding turbine. Each of the simple impulse turbines would then be termed a stage of the turbine, each stage containing a set of nozzles and blades. This is equivalent to splitting up the whole pressure drop into a series of smaller pressure drop which is known as hence the term “Pressure compounding”.

The nozzles are usually fitted into partitions, termed as diaphragms, which separate one wheel chamber from the next. Expansion of takes place wholly in the nozzles, the space between any two diaphragms being filled with steam at constant pressure.

The pressures on either side of any diaphragm are therefore different. Hence, steam will tend to leak through the space between the bore of the diaphragm and the surface of the shaft. Special devices are fitted to minimise these leakages.

It will be noticed that the total pressure drop of the steam does not take place in the first nozzle ring, but is divided equally between the two nozzle rings, and the pressure remains constant during the flow over the moving blades.

The advantages of pressure compounding are:

i. It is most efficient

ii. It is expensive turbine.

Method # 3. Pressure-Velocity Compounded Impulse Turbine:

Another type of impulse turbine is the pressure-velocity compounded turbine. In this turbine both two methods are utilized. Total pressure drop of the steam is divided into stages and “the velocity in each stage is also compounded.

In this turbine each stage has two or more rows of moving blades and one or more rows of stationary blades, the moving and stationary blades being placed alternately. Each stage is separated from the adjacent stage by a diaphragm containing nozzle. A ring of nozzles is fitted at the commencement of each stage. It is turbine compounded both for pressure and velocity.

This method has the advantage of allowing a bigger pressure drop in each stage and consequently less stage are necessary. Hence, a shorter or more compact turbine will be obtained for a given pressure drop.

The pressure-velocity compounded turbine is comparatively simple in construction and is more compact than the multi-stage pressure compounded impulse turbine. But its efficiency is not high.

This method of pressure-velocity compounding is used in the Curtis turbine.

Governing Steam Turbines:

The steam turbine is used for operating the load which fluctuates with time and the steam turbine is subjected to varying load conditions. This results in fluctuation in speed of turbine. The speed of turbine can be maintained constant and the function of the governor is to regulate the supply of steam to the turbine so that the speed of rotation shall remain almost constant at all loads.

The major methods of governing steam turbines are:

Method # 1. Throttle Governing:

This is used on small turbines, because its initial cost is less and mechanism is simple. In throttle governing, the pressure of steam is reduced before reaching the turbine at part loads. The flow of steam entering the turbine is restricted by a balance throttle valve which is controlled by the centrifugal governor.

In turbines of small power in which the valves are light and the forces on them due to steam flow are negligible, the governor may be arranged to actuate the relay. This is a device in which the relatively small force produced by the governor for a small change of speed is caused to produce a large force if necessary to actuate the throttle valve.

Fig. 11-54 shows a simple differential relay. This throttle valve is actuated by the relay piston sliding in the cylinder. A floating or differential lever is attached at one end to the governor sleeve and the other end to throttle valve spindle, and at some intermediate point to a point or piston valve which consists of two small piston valves covering ports without any lap, i.e. the length of the valve is just equal to the length of the ports. The operating medium is usually lubricating oil supplied by a pump at a pressure of 30 to 40 N/cm² ab. The pipes G are open to the oil drain tank.

The starting of turbine for full load, valve (A) is opened by operating hand lever. Consider that the turbine is running at a load less than full’ load. Then throttle valve (B) will- be opened to such an extent that the steam flow is just sufficient to maintain constant speed under the given load conditions.

Suppose the load on this turbine is reduced rather quickly, then there is now an excess of energy being supplied to the turbine and the surplus will accelerate the rotor. The turbine and governor speed will now rise and thus cause a lift of the governor sleeve.

For the time being, the throttle valve spindle is stationary and the relay piston valve is raised. The port is opened to the oil pressure and other port to drain. The relay piston is thus forced downwards and throttle valve partially closed.

The downward movement of the throttle valve lowers the piston valve and so closes the ports. As soon as the oil ports are covered, the relay piston is locked in position. This will occur only when the opening of the throttle valve is correct for the load on the turbine.

Since, for equilibrium of the governor mechanism, the piston valve must always be in its central position and covering both oil ports, the position of the governor sleeve will vary according to the position of the throttle valve. The position of the floating lever is indicated by chain dotted lines in fig. 11-55 for no-load and full load.

Although in throttling no energy is lost, the available energy or the enthalpy drop is decreased. The dry saturated steam which may be expanded isentropically from point 1 (pressure P₁) to point 2 (pressure P₂) isentropic total enthalpy drop (H₁ – H₂).

If the governor first decreases pressure from P₁ to P₃ by throttling (point 1 to point 3), the isentropic total enthalpy drop is far less than available isentropic enthalpy drop (H₁ – H₂) without throttling. This reduces the efficiency of the turbine at part load.

This relationship between load and steam consumption for a turbine governed by throttling, is given by the well-known Willan’s straight line fig. 11-55 shows the relationships. The steam consumption is related to load as shown in fig. 11-55.

This is specific steam consumption which reduces with increase in load. Therefore in throttle governing the efficiency increases with increase of load.

Method # 2. Nozzle Control Governing:

Fig. 11-56 shows nozzle control. The poppet-type valves are used steam passages to meet the load, each passage serving a group of nozzles. The control governor has the advantage of using steam at full boiler pressure.

In automatic governed turbines, various arrangements of valves and groups of nozzle are employed. An arrangement, often adopted with large steam turbines and with turbines using high-pressure steam, is shown in fig. 11-56. The number of nozzle groups may vary from three to five or more. There are three sets of nozzle N₁, N₂ having 10, 4 and 3 nozzles respectively are used.

Fig. 11-56 shows an arrangement sometimes employed. The group of nozzle N₁ is under the control of the valve V₁, through which all the steam entering the turbine passes. Further admission of steam is through the valves V₂, V₃ in turn.

In some instances, the nozzle group N₁ has been arranged in the lower half of the turbine and supplied with steam through a throttle valve V₁ up to, say, half load. For loads greater than half load, a further supply of steam is admitted through the valves V₂, V₃, etc.

Method # 3. By-Pass Governing:

In modern impulse turbines, which are operating at very high pressures this method is used. All the steam entering the turbine passes through the inlet valve. This valve is under the control of the speed governor and enters the nozzle box or steam chest. In certain cases, for example, this would suffice for all loads upto the economical load, the governing being effected by throttling.

For loads greater than the economical loads, a by-pass valve is opened allowing steam to pass from the first stage nozzle box into the steam belt and so into the nozzles of the fourth stage. The by-pass valve is not opened until the lift of the valve exceeds a certain amount; also as the load is diminishing the by-pass valve closes first. The by-pass valve is under the control of the speed governor for all loads within its range.