Hydraulic Valve
As we continue speaking about hydraulic systems, let us focus on the elements of secondary importance after pumps and pressure filters. Typically one pump in an open system has several consumers. Various hydraulic valves are used to control them. Valves can have mono-unit or sectional structure and act as hydraulic circuit loading devices, provide protection against excessive pressure and distribute the operating fluid flow to deliver it to the corresponding consumers. Hydraulic valves differ from each other depending on the method of influence on the spool. In older circuits the spool was moved manually, by means of levers, while in modern circuits the spool movement is performed by means of control, leading pressure or electric signal.
Spool design allows unimpeded flow of the operating fluid from the pressure section to drain while in neutral position. When any of the spools is in operating position, the operating fluid is delivered to the actuator. So when the engine is started, operating fluid flows in a circle from the tank via distributing pump back to the tank with no pressure.
The main safety valve is located in the pressure section of the distribution pump. This valve protects the hydraulic circuit elements against excessive loads. For this section of the circuit the valve shall be adjusted at maximum. Some consumers can operate at a lower pressure, so their individual safety valves, intended to protect the specified actuator cavities are typically provided in every section of the distribution pump.
Depending on consumer (cylinder or hydraulic motor), different neutral position circuits of distribution pump spools are used. For a cylinder it is typically a switch circuit which is, in some important cases, is enhanced by hydraulic locks. When a hydraulic motor is connected in the neutral position of the spool, cavities are interconnected and coupled with the drain.
Operation experience shows that hydraulic distributors are reliable units which can be operated for years without any notable faults. Failsafe operation is achieved by cleanliness of the hydraulic system. Otherwise, presence of dirt or water may lead to unpredictable consequences. When an engine is started without any interference of the maintenance personnel, various equipment units may move spontaneously. Typically, a spool becomes blocked by solid particles or, in case of winter operation, by ice pieces. Clearance between a distribution pump housing and a spool shall be considered and shall not exceed 0.02-0.03 mm. Here is one more curious moment. During start-up, a machine which has not been prepared for winter operation starts to operate spontaneously. It was revealed that frozen water in the drain filter created an obstacle for operating fluid drainage. Increased pressure in the drain circuit allowed for inadvertent operation of the machine.
Lately hydraulic circuits with a feedback between pump and distributor are more commonly used. The distributor creates control pressure which is called XL or LS for different circuits. This pressure shall be 20 bar less than the operating pressure. It is delivered to the differential pump spool, compared to the operating pressure and the hydraulic pump is loaded depending on this difference. Usage of such circuits results in substantial energy saving.
Let us investigate a certain case. A pump with a capacity of 240 l/min or 4 l/sec can be operated at a pressure up to 200 bar or 20 MP. Our consumer, let it be a cylinder, demands 60 l/min or 1 l/sec at a pressure of 100 bar or 10 MP. Standard option uses either an adjustable restriction, or a flow-control device. Excessive oil will be drained. Power consumed by the pump from the engine will be 4*20 80 kV, while effective power will be 1*10 10 kV. More than 87% of engine power will be taken for operating fluid heating and only 12% will be used as intended. When the new circuit is used, pump control unit receives the control pressure and deflects the pump cage so that pump capacity will constitute 60 l/min only with a pressure of 100 bar. Hydraulic distributor with feedback allows to simplify the hydraulic circuit and save lifetime of pump and engine as well as fuel consumption, as while the engine is operating there is no operating fluid flow at deactivated equipment. The pump is in the zero output mode and the delivery is minimal. At excessive loads and in case of actuation pressure exceedance pump output will be also reduced to minimal.
