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Field Report - How to read fluids circuit diagrams, Part 1 symbols

Welcome to Field Reports - Please note that this content is based on field service engineer experience during hundreds of in-mill service calls. Your experience may vary.

Below are some common illustrations of equipment located on fluids circuit diagrams, followed by descriptions of the most common elements. Later in this article series we will describe some simple hydraulic and pneumatic circuits composed of these circuit elements.

Common groups of fluids circuit elements

Miscellaneous fluids drawing elements

Flow control valves

Actuators

Pressure control valves

Directional control valves

 

Specific fluids circuit elements

Needle valves

Needle valves are used to throttle or shut-off flow of fluids. They usually will vary flow with pressure or viscosity change. Some valves can be pressure and/or temperature compensating.

Needle valves

Check valvesCheck valves

Check valves are one way valves, allowing flow in only one direction.

 

 

 

 

GaugesGauges

Gauges are used to measure the oil pressure at a given point in the system. This is usually measured in PSI or bars. One bar = 14.5 PSI.

 

Flow control valves

Flow control valves are used to control oil flow in one direction and unrestricted in the opposite direction. "Metered in" control means that the flow controls are controlling the fluid into the actuator, "metered out" is controlling the fluid out of the actuator. Some valves can be pressure and/or temperature compensating.Flow control valves

 

Pilot operated check valve, pilot-to-openPilot operated check valves, pilot to open

When the pilot line to a pilot-operated check valve is not pressurized, flow is allowed in one direction but blocked in the opposite direction. When the pilot line in a pilot-to-open valve is pressurized, the check valve is open, allowing flow in either direction.

 

 

Pilot operated check valve, pilot-to-closePilot operated check valves, pilot-to-close

When the pilot line to a pilot-operated check valve is not pressurized, flow is allowed in one direction but blocked in the opposite direction. When the pilot line in a pilot-to-close valve is pressurized, the check valve is closed, blocking flow in both directions.

 

 

Shut-off valveShut-off valves

Shut-off valves are used to isolate one part of a fluid system from another.

 

Air bleed valveAir bleed valves

Air bleed valves are used to automatically eliminate air bubbles from pressurized hydraulic systems.

 

 

 

Level switchLevel switches

One use of a level switch is to detect when the oil in a reservoir is reduced to a minimum operating level.

 

 

 

 

Temperature switchTemperature switches

One use of a temperature switch is to detect when the oil in a reservoir reaches the maximum operating temperature.

 

 

 

Pressure switchPressure switches

Pressure switches are use to detect a pressure rise or fall through a set pressure point. These switches may or may not be adjustable.

 

Pressure reducing valvePressure reducing valves

Pressure reducing valves are used to reduce the pressure in individual circuits.

 

 

 

Pressure relief valve

Pressure relief valves

Pressure relief valves are used to limit the maximum pressure in all or part of the hydraulic system.

 

 

 

Counterbalance valveCounterbalance valves

Counterbalance valves are used to control overrunning loads and to support loads should a function be stopped at any point throughout its travel. NOTE: this valve is typically preset and should not be tampered with.

 

 

 

 

 

Flow fuse

Flow fuses

Flow fuses are normally open valves which close if the pressure difference between the inlet and outlet valves is too high compared to the design setting. The valve can be reset by reversing the direction of flow. When placed inline with an actuator (for example, a cylinder), flow fuses limit the maximum speed of that actuator.

 

AccumulatorAccumulators

Accumulators are used to store hydraulic energy and to absorb shock in a hydraulic system.

WARNING:

Make sure to discharge all hydraulic energy before working on any components.

 

Directional control valveDirectional control valves

Directional control valves are used to direct fluid flow into the appropriate lines for the designated operation. These valves are usually electrically controlled.

 

 

 

Hydraulic pumpsHydraulic pumps

Hydraulic pumps are used to pump oil from the power unit to other parts of the hydraulic system. Some pumps have control options such as pressure or flow compensators.

 

Filters

Filters are used to remove contaminants from fluid.

Filters

 

StrainerStrainers

Strainers are used to remove large solid from water or oil supply. They may have a check valve bypass.

 

 

 

Water modulating valveWater modulating valves

Water modulating valves are used for controlling the oil temperature in the reservoir automatically by controlling the volume of water going through the heat exchanger.

 

 

 

 

 

Heat exchanger (cooler)Heat exchangers (cooler)

Heat exchangers are used to remove heat from the circulating oil in the hydraulic system. The most common heat exchanger is water-to-oil but some times air-to-oil units are used. Coolers will cool the fluid.

 

 

Heat exchanger (heater)Heat exchangers (heater)

Heaters are used to warm the fluid.

 

 

 

CylinderCylinders

Cylinders are used to convert fluid energy into mechanical linear motion.

 

 

Fluid motorFluid motors

Fluid motors are used to convert hydraulic energy into mechanical rotary motion.

 

 

 

 

Quick disconnectQuick disconnects

Quick disconnects are used to disconnect a line to separate one piece of equipment from another.

 

 

Proportional (servo) valveProportional (servo) valves

Proportional valves are electrically controlled hydraulic valves. These valves proportionally control the hydraulic pressure and/or flow based on an electrical input signal.

 

 

MufflerMufflers

Mufflers are used to reduce the noise of exhausting air.

 

Air blowsAir blows

Air blows are represented as shown below. The number of blows varies.

 

Pneumatic actuatorPneumatic actuators

Fluid actuators are used to convert fluid energy into mechanical linear motion.

For more information about reading hydraulic and pneumatic circuit diagrams, read the next article in this series which describes sample hydraulic circuits, or contact your Valmet representative.