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.