1.6 Hydraulic fluid system - part 3
In the case of the submersible screw-pump, a coarse suction filter only is fitted and there is very little scope for creation of low pressure conditions at the pump suction. For the separately-mounted pump, it is normal to provide a separate boost pump of the centrifugal type, having an outlet pressure of about 7 bar.
This type of pump is not susceptible to cavitation damage and has the additional advantage of being able to circulate viscous fluid on initial start-up from cold. However, the pump must be provided with relief valve facilities and LP filters are needed, following the boost pump, to protect the main pump from possible damage by the boost pump.
The main screw-pump shown in Fig 2.26 is a constant flow device when driven at constant speed. In these systems it is designed to deliver the maximum flow requirements of the system, plus a margin. Under normal conditions, the system flow will be a fraction of this, so the outlet pressure is controlled by a spring-loaded pressure control valve and the excess flow is returned to the reservoir. Again, a separate pressure relief valve must be provided as an overall protection. A non-return valve is fitted in the outlet flow of the pump line, so that two pumping lines can be coupled together. Although only one line is needed for operation, the standby line can be brought into operation either manually or automatically by sensing loss of pressure in the duty line.
Pumping generates heat, and circulation of the fluid through the valve relays adjacent to hot steam pipes causes a further rise in fluid temperature. Phosphate-ester fluids have a high temperature gradient of viscosity. At 40°C, the fluid has optimum conditions for the main pump. At higher temperatures, the viscosity will have a lower value and excessive pump and system leakages may occur. At still higher temperatures the fluid begins to break down. Conversely, at low temperatures, the fluid starts out as extremely viscous and difficult to pump, system pressure drops may be excessive and it may not be possible to achieve the required rate of response from the steam valve relays. For these reasons, coolers are necessary in all systems and, in some systems where cold-starting is essential, heaters may also be required. Coolers are of the cross-flow, double-pass shell and tube type, using demin-eralised water as the cooling medium. Temperature control to 40°C is achieved by an electrical controller and pneumatic actuator. Heaters are of the electrical immersion type, fitted either in the reservoir or in a flow-line. A schematic diagram of a typical pumping system is shown in Fig 2.27.
Systems employing axial-piston variable-swashplate pumps have similar arrangements for boost pumps, LP filters, coolers and pressure relief valves, but their variable-delivery capability makes the use of a spill-type pressure control valve unnecessary. A cross-section of a variable-swashplate pump is shown in Fig 2.28. It will be immediately apparent that this is a much more complex device than the screw-pump shown in Fig 2.25. However, since the flow can be made to exactly match the system requirements, the reservoir, pipework, coolers and system filters can be scaled down accordingly. From Fig 2.28 it can be seen that the axial-pistons are forced to reciprocate in their cylinders by rotation of the pump barrel at constant speed and the movement of the slippers against the fixed angle of the swashplate. The piston displacement and hence the pump output can be varied by tilting the swashplate. Fixed inlet and outlet ports supply and deliver the hydraulic fluid. The fluid is pumped against the system resistance and a pressure controller senses the outlet pressure and modulates the swashplate to regulate it to the desired value. Some pressure oscillation is produced at a frequency equal to rotation frequency times number of pistons, but this is damped out by a local accumulator.