HYDRAULIC SYSTEMS IN TURBOMACHINERY
Hydraulic systems can deliver the high force and torque necessary to operate large and powerful mechanisms within turbomachinery. Hydraulic fluid is practically considered to be an incompressible liquid even in high working pressures. Hydraulic systems offer stiffness in stroke and in operation. They are compact and lightweight.
Hydraulic systems are widely used in steam and gas turbines as well as systems, such as hydraulic actuated valves. They are comprised of hydraulic pumps and actuators, piping, valves and auxiliary components (to generate, transmit, control, and use hydraulic power). The hydraulic fluid is usually a petroleum or synthetic oil with additives to enhance performance. In some applications, an alternative liquid or a mixture of liquids may be used. For example, properly formulated fire resistance requires a certain kind of synthetic oil to be employed for hydraulics.
Most hydraulic applications employ positive-displacement pumps of the gear, screw, or piston type. Piston pumps can be axial, radial, or reciprocating. These pumps are widely used in high pressure hydraulic systems in many turbomachinery applications. A pump set, usually operating and standby, acts as the pressure and flow-rate generator. In other words, hydraulic liquid is pressurized in a pump set and pressurized oil is delivered for the required tasks. The main job of the pump set is high working pressure generation with as high efficiency as possible.
The higher the working pressure, the greater the stream density of the transported energy, therefore the higher the efficiency of the hydraulic system. On this basis, many systems use high working pressure and high-pressure piston pumps. Many medium-pressure hydraulic systems use gear pumps. For small, low power hydraulic systems, a constant flow pump set might be used with bypass or other means of flow control. However, more advanced hydraulic systems have variable stroke hydraulic pumps that can control flow rates without a bypass system. Power is transmitted from the hydraulic pump set to hydraulic piping, and actuators in the form of pressurized hydraulic fluid. This passes through a combination of tubing, fittings, valves and control items.
Flow characteristics of hydraulic circuits should take into account fluid properties, pressure drop, flow rate, and pressure-surging tendencies. The fluid transmission system should be designed to minimize changes in flow velocity, velocity distribution, and random fluid eddies, all of which dissipate energy and result in pressure drops in the hydraulic circuit. Pipe, tubing, and flexible hose are used for the hydraulic power circuits; suitable fittings are available for all types and to transition from one type to another. Stainless steel tubing is commonly used for many hydraulic systems.
Other tubing materials such as aluminium tubing and others have been used in some applications, but stainless steel is preferred for turbomachinery. Flexible hoses are needed in some movable mechanisms or machinery as rigid piping or tubing cannot serve such applications. Flexible hose selection Such hoses require great care for material selection and long life. These should only be used when really needed. Otherwise, rigid piping or tubing should be used. Tubing is more easily bent into neat forms to fit equipment and facilities and to transport pressurized hydraulic fluid to different actuators located in various locations of a large machine or facility. Hydraulic controls help to monitor pressure, flow rate and flow direction.
Valves in many different forms can be used such as safety/relief valves, pressure reducing valves, directional control valves and flow control valves. Directional control valves serve primarily to direct fluid to the actuators. These valves may have rotary and sliding spools. Important design parameters are flow rate, operating pressure, working temperature, fluid viscosity characteristics, and compatibility of the fluid with wetted materials. Flow velocity in suction lines is generally in the range of 0.5 to 1.5 m/s; in discharge lines, it ranges from 2.5 to 6 m/s. A lower velocity range (2.5 to 3.5 m/s) is preferable to limit pressure drops. Selection of material, wall thickness, supporting span and others require a safety margin. Safety factors range from 5 to 10% or even higher, depending on the severity of the application.
Hydraulic fluid may be considered incompressible, which results in stability and stiffness during operation. However, for large and high-pressure systems, some compressibility might be observed. Remote locations will require long runs of piping or tubing. Pressure drops in some cases will cause some operational issues. Tubing and piping can be expensive. Therefore, it is smart to optimize actuator location, sizing and selection.
