1.6 Hydraulic fluid system - part 2
The fluid system is designed to permit either on-line monitoring or manual sampling of the fluid condition. The parameters to be monitored, their limiting values and the frequency of monitoring depend on the performance requirements of the most critical items in the system (usually the servo-valves). This monitoring programme is set out by the turbine supplier to suit the system components. Deviations outside limits should be investigated promptly, otherwise component life will be very seriously affected.
In basic form, the fluid supply system comprises a fluid reservoir and a pump to deliver high pressure fluid. To ensure high availability two pumping lines are needed, one being the duty line and the other on standby. Pumps are either of the screw-type (for use in systems up to 80 bar) or of the axial piston variable-delivery type (for use up to 150 bar). Additional system components are necessary to ensure that the fluid is supplied to the relays in good condition. These will be described as the needs of the basic components are considered.
Screw-type pumps may be of the submersible kind, mounted on the reservoir top plate, or they may be separately mounted. Axial-piston pumps and screw-type pumps require that the pump suction pressure is adequate to provide a lengthy service life. Failure to meet the suction pressure requirements will result in cavitation-erosion of the pump components and eventual break-up of the pump, with the release of numerous metallic particles. This would necessitate shutdown of the whole turbine-generator to replace the faulty pump and to flush the entire hydraulic system.
The phenomenon of cavitation results from the characteristics of air in the fluid. The solubility of air is proportional to the fluid pressure, so that air bubbles present in the fluid at low pressure become dissolved into the fluid at high pressure. Consider a reservoir open to atmosphere and full of fluid connected by a pipe to a pump, so that the pump suction is flooded under static conditions (Fig 2.25). When the pump is started, the flow through the pipe causes a friction loss and the pump suction pressure may well become sub-atmospheric. This will cause air, normally dissolved in the fluid at atmospheric pressure, to come out of solution in the suction pipe and form bubbles. At some point within the pump, the pressure increases to the extent that the air returns into solution with an abruptness which causes collapse of the bubble and damage to any metal in the vicinity. Although such cavitation is usually avoided by good design, the user must be aware that blockage of any suction filters could lead to a similar effect. Also, the reservoir is provided to give a definite fluid dwell time to allow any entrained air to be released. Should the fluid be allowed to deteriorate for any reason or should there be a fault causing excess air to be entrained in the fluid returned to the reservoir, the air release process may be affected and excess air could be supplied to the pump in this way.