1.6 Hydraulic fluid system - part 1
Universal use is made on large turbine-generators of hydraulic fluid at high pressure to provide the large opening forces and rapid stroking times. This section deals with the design of the pumping system for supplying fluid at high pressure to the steam valve relays and to the protection system.
The design and construction of the system is closely related to the choice of operating fluid and to the use of small-clearance components in the valve relay operating systems. System pressure is chosen to allow the use of an economically-sized relay capable of meeting the required opening forces. For small turbines (<500 MW), system pressures may be 35 bar or less. For larger turbines, system pressures of 70-150 bar are needed.
These high pressures represent a significant fire risk, as a small leakage can result in a jet of oil being sprayed over a considerable distance. The proximity of steam pipes at high temperatures (typically 565°C on coal-fired plant) makes it necessary either to provide for double enclosure of the pipework and components or to use a fire-resistant fluid (FRF). All CEGB systems falling into the latter category use phosphate ester fire-resistant fluids. These systems are completely separate from lubricating oil systems and one pumping unit is supplied for each main turbine-generator unit. Where a boiler feed pump turbine is associated with the main unit, its control and protection systems are powered by the same supply unit for convenience.
Phosphate-ester fluids are used because of their superior thermal stability compared with water-based fire-resistant fluids which would be unsuitable for the high temperatures normally found in the vicinity of steam valves. They also have the advantage that their viscosity and lubricating characteristics are similar to lubricating oil, so that conversion of earlier units running on high pressure oil is possible with some care, subject to the fitting of compatible sealing materials. These fluids also have the necessary fire resistance and provide for a long service life from the system components. They can be maintained in good condition for an indefinite period without the need for replacement.