Hydraulic Valves

Hydraulic valves contain and transfer the flow and pressure of hydraulic fluid in hydraulic power systems. They range from simple shutoff valves to precision control valves. Common types of hydraulic valves include angle, ball, block and bleed, check, control, cartridge, directional, drain, needle, poppet, pressure relief, safety, shut off, solenoid, spool, and stack mounted valves. In their unpowered state, hydraulic valves can be normally open (open center) or normally closed (closed center). Hydraulic valves with a tandem center connect the pressure and tank ports, but block the service ports to allow system unloading and isolation of the service lines. Devices with a float center allow the supply to be shut off and enable the load to move to other services.
There are many types of hydraulic valves. Angle valves admit media at an angle and permit maximum flow. Ball valves provide tight shut-off and reliable control. Block and bleed valves use a small port to depressurize the space between the inlet and outlet. Check valves prevent flow reversal. Control valves modify fluid flow. Directional valves steer process media through selected passages. Drain valves are used to remove surplus fluid from a system or container. Needle valves have a slender, tapered point at the end of a valve stem. Poppet valves open and close ports with a sealing device that includes a disk, cone, or sphere. Pressure relief valves remove excess upstream pressure. By contrast, regulators maintain a constant outlet pressure. Safety valves contain a thermal sensing component that opens or closes in response to temperature changes. Spool valves are actuated by a rotary or piston-like spool that slides back and forth to cover and uncover ports in the housing. Shut off valves close a line to stop flow when a pre-set condition occurs. Stack mounted, sandwich, or modular valves can be assembled to create a valve block.
Performance specifications for hydraulic valves include valve size, pressure rating, ports, media temperature, flow coefficient, and connection type. Valve sizes are typically measured in inches (in) while pressure ratings are usually measured in pounds per square inch (psi). Both double-port and multi-port hydraulic valves are available. Media temperature is a maximum amount expressed in degrees Fahrenheit. The flow coefficient equals the number of gallons of 60กใ F water that flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. There are several connection types for hydraulic valves. Some devices have internal or external threads for inlet or outlet connections. Others use a bolt flange, clamp flange, union connection, tube fitting, butt weld, or socket weld. With compression fittings, tightening a nut on one fitting compresses a washer around the other. With metal face seals, a gasket is sandwiched between two fittings.

Hydraulic Power Units

Hydraulic power units provide pressurized flow to hydraulic motors, cylinders, and other hydraulic components. Hydraulic power units (HPUs) differ from pumps, as an HPU contains a fluid reservoir, multiple pump stages and coolers to keep fluid at a safe working temperature. Performance specifications, physical characteristics, and features are all important parameters to consider when searching for hydraulic power units.
Performance specifications to consider when selecting hydraulic power units include operating pressure, flow, total power, and reservoir capacity. The operating pressure is the pressure the power unit can deliver at the outlet. The power unit may be expressed as a single pressure rating, or it can be rated to operate over a range of pressure. The fluid flow through the power unit may be a single rating, or have low and high rating points. The total amount of power the motor / pump can draw or is rated to operate. Some power units have multiple power sources, and this specification is the power available from all sources. Power is measured in horsepower, or similar units. The capacity of the power unit reservoir is measured in gallons or similar units. Some units may have interchangeable or modular reservoirs with a range of capacities.
Physical specifications to consider for hydraulic power units include the pump type, power source, cooling method, and unit weight. All hydraulic power units have some type of integrated pump. Some units are available with multi-stage pumps, which perform like multiple pumps connected in series. Pump types available for hydraulic power units include single stage, double stage, three or more pump stages, and multiple pump units. Power sources include electric motor, diesel engine, gasoline engine, and pneumatic compressor. The cooling method can be none, heat exchanger, or fan-driven oil cooler. The unit weight is also important to consider.