Turbine bypass valves - TBPVs

In thermal power stations, the whole process of generating energy revolves around the turbine. At the turbine, the kinetic energy of the gas/steam is converted into rotational energy and then, in the generator, in electrical power.

Effectively protecting these complex components from mechanical damage is a key concern of power station operators and for this reason a bypass valve BPV is installed at every turbine stage (HP, IP & LP).

TBPVs (turbine bypasss valves) route pressure and temperature steam around the turbines, conditioning the steam and protecting the turbines, and discharging the conditioned steam from the main steam line typically to the cold reheat line.

In doing so, TBPVs must perform both pressure reduction as well as temperature control.

Pressure reduction is accomplished with the trim within the valve body. The inlet pressure is controlled by an upstream pressure controller, signalling the valve to modulate and maintain the pressure at the required set point. Alternatively it can be sent a digital signal to the valve to quick open or close the control system as required.

Temperature control is accomplished through the addition of water (coolant) to the steam, reducing the specific enthalpy of the steam (a process called desuperheating). A separate water control valve supplies the correct amount of water to the desuperheating mechanism (typically spray nozzles) within the steam conditioning valve. A downstream temperature transmitter typically operated in conjunction with a feed-forward algorithm within DCS will dictate the amount of water to be injected into the steam by the water control valve or WCV.

Valve inlet and outlet connections (commonly buttweld or flanged for that specific application) should be provided to suit customers piping and pressure requirements, and to maintain inlet and outlet steam velocities to reasonable levels (<80m/s –250ft/s).

Engineering of materials

Mechanical properties

The mechanical properties of materials depend on their composition and microstructure. Material's composition, nature of bonding, crystal structure and defects (dislocations, grain boundaries...) have a profound influence on the strenght and ductility of metallic materials and other factors, such as how lower temperatures can cause many metals and plastics to become brittle.

There are diferent types of forces that are encountered in dealing with mechanical properties of materials. In general, we define stress as the force acting per unit area over which the force is applied. Compressive, tensile and shear stresses are illustrated.
 

Strain is defined as the change in dimension per unit length. Stress is expressed in Pa (Pascal). Strain has no dimensions and is often expressed as cm/cm.

Elastic strain is defined as fully recoverable strain resulting from an applied stress. A material subjected to an elastic strain does not show any permanent deformation, it returns to its original shape after the force or stress is removed.

In many materials, elastic stress and elastic strain are linearly related. The slope of a tensile stress-strain curve in the linear regime defines the Young´s modulus or modulus of elasticity (E) of a material.

When a material deforms elastically, the amount of deformation likewise depends on the size of the material, but the strain for a given stress is always the same and the two are related by Hooke´s Law (stress is directly proportional to strain):  σ = . E ε

A viscous material is one in which the strain develops over a period of time and the material does not return to its original shape after the stress is removed. A viscoelastic or anelastic material can be thought of as a material with a response between that of a viscous material and an elastic material. The term anelastic is typically used for metals, while the term viscoelastic is usually associated with polimeric materials.

Steam Cycles

The Rankine cycle

Although the Carnot cycle is the most efficient cycle, its work ratio is low. Further, there are practical difficulties in following it.

Consider the Carnot cycle for steam as shown:

2 to 3: heat is supplied at constant temperature and pressure.
3 to 4: the vapour expands isentropically from the high pressure and temperature to the low pressure. In doing so, it does work on the surroundings, which is the purpose of the cycle.
4 to 1: the vapour, which is wet at 4, has to be cooled to state point 1 such that isentropic compression from 1 will return the vapour to its original state at 2. From 2 the cycle is repeated.

The four processes are physically very different from each other and they each require particular equipment:
- The heat supply, 2-3, can be made in a boiler.
- The work output, 3-4, can be obtained by expanding vapour thorough a turbine.
- The vapour is condensed, 4-1, in a condenser,
- and to raise the pressure of the wet vapour, 1-2, requires a pump or compressor.

At state 1 the steam is wet but it is difficult to stop condensation at this point and the compress it just to state 2. It is more convenient to allow the condensation process to proceed to completion, as shown:

The working fluid is water at the new state point 3, and it can be conveniently pumped to boiler pressure at state 5. The pump has much smaller dimensions than it would have if it had to pump a wet vapour, the compression process is carried out more efficiently, and the equipment required is simpler and less expensive.

This ideal cycle, which is more suitable as a criterion for actual steam cycles than the Carnot cycle, is called the Rankine cycle.

Rankine cycle with superheat

The average temperature at which heat is supplied in the boiler can be increased by superheating the steam. Usually the dry saturated steam from the boiler drum is passed through a second bank of smaller bore tubes within the boiler. This bank is situated such that it is heated by the hot gases from the furnace until the steam reaches the required temperature.

The Rankine cycle with superheat includes a steam receiver which can receive steam from other boilers. In modern plants a receiver is used with one boiler and is placed between the boiler and the turbine. Since the quantity of feedwater varies with different demands on the boiler, it is necessary to provide a storage of condensate between the condensate and boiler feed pumps. This storage may be either a surge tank or hot well.

The reheat cycle

It is desirable to increase the average temperature at which heat is supplied to the steam, and also to keep the steam as dry as possible in the lower pressure stages of the turbine.

The wetness at exhaust should be no greater than 10%. High boiler pressures are required for high efficiency, but that expansion in one stage can result in exhaust steam which is wet.

This is a condition which is improved by superheating the steam. The exhaust steam condition can be improved most effectively by reheating the steam, the expansions being carried out in two stages.

1´-2 represents isentropic expansion in the high-pressure turbine, and 2´-3 represents isentropic expansion in the low-pressure turbine. The steam is reheated at constant pressure in process 2-2´. The reheat can be carried out by returning the steam to the boiler, and passing it through a special bank of tubes. Alternatively, the reheat may take place in a separate reheater situated near the turbine; this arrangement reduces the amount of pipe work required.

The regenerative cycle

In order to achieve the Carnot efficiency it is necessary to supply and reject heat at single fixed temperatures. One method of doing this, and at the same time having a work ratio comparable to the Rankine cycle, is by raising the feedwater to the saturation temperature corresponding to the boiler pressure before it enters the boiler.

This method is not a practical proposition but this regenerative cycle has an efficiency equal to the Carnot cycle, since the heat supplied and rejected externally is done at constant temperature.

However, the Rankine efficiency can be improved upon in practice by bleeding off some of the steam at an intermediate pressure during the expansion, and mixing this steam with feedwater which has been pumped to the same pressure.

This mixing process is carried out in a open feed heater. The steam expands from condition 5 through the turbine. At the pressure corresponding to point 6, a quantity of steam, is bled off for feed heating purposes. The rest of the steam completes the expansion and is exhausted at state 7. This amount of steam is then condensed and pumped to the same pressure as the bleed steam. The bleed steam and the feedwater are mixed in the feed heater and then being pumped in a second feed pump.

Because of the number of feed pumps required, the heating of the feed water by mixing is dispensed with and closed heaters are used.