How To Understand Hydraulic Circuit

[Rode Strawther]

Witha variety of applications, hydraulic systems are used in all kinds of large and small industrial settings, as well as buildings, construction equipment, and vehicles. Paper mills, logging, manufacturing, robotics, and steel processing are leading users of hydraulic equipment.

The purpose of a specific hydraulic system may vary, but all hydraulic systems work through the same basic concept. Defined simply, hydraulic systems function and perform tasks through using a fluid that is pressurized. Another way to put this is the pressurized fluid makes things work.

Transporting liquid through a set of interconnected discrete components, a hydraulic circuit is a system that can control where fluid flows (such as thermodynamic systems), as well as control fluid pressure (such as hydraulic amplifiers).

Mechanical power is converted into hydraulic energy using the flow and pressure of a hydraulic pump. Hydraulic pumps operate by creating a vacuum at a pump inlet, forcing liquid from a reservoir into an inlet line, and to the pump. Mechanical action sends the liquid to the pump outlet, and as it does, forces it into the hydraulic system.

A hydraulic cylinder is a mechanism that converts energy stored in the hydraulic fluid into a force used to move the cylinder in a linear direction. It too has many applications and can be either single acting or double acting. As part of the complete hydraulic system, the cylinders initiate the pressure of the fluid, the flow of which is regulated by a hydraulic motor.

If neglected in procedures or forgotten when servicing equipment, uncontrolled hydraulic energy can have devastating results. Failure to control hydraulic energy frequently causes crushing events, amputations, and lacerations to exposed workers.

Therefore, like other energy sources, hydraulic energy must be controlled, using an appropriate energy isolating device that prevents a physical release of energy. There are also systems that require the release of stored hydraulic energy to relieve pressure. And also, those engaged in lockout/tagout, must also verify the release of stored hydraulic energy/pressure (usually indicated by zero pressure on gauges) prior to working on equipment.

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.

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.

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.

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.

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 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.

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.

The function of every hydraulic circuit can be described by a language of individual component symbols that are combined together to explain how the circuit works. They should conform to the Internation Standard ISO 1219 parts 1 and 2. This following section will look at some common symbols for the components used in our basic circuit. More detailed information on a wider range of components can be found in our symbols section.

Directional valves control the direction of fluid flow. They may be operated a number of different ways including electrically, manually, or mechanically. They may also switch between 2 to 4 different pipelines and come with many different switching connection options. We will discuss the many different versions that are available in later modules but from a fundamental design side, they are either open or shut and simply change the pipework connections.

Directional valves are normally constructed using a spool that moves up and down a very high tolerance bore. Even though spool to bore clearances are very small there will still be some leakage over time, which will allow heavy loads to creep, if they are not isolated.

It may be useful to compare hydraulic control with a motor car, where either the engine throttle or brake pedal can control the speed, but the brake is considerably smaller and cheaper for the same power capacity. It is similar with hydraulics components, the pumps tend to be larger, more expensive and less robust than control valves, so generally, it's preferable and safer to brake the load with a valve rather than control its speed with a pump.

Modern, high-quality hydraulic valves have very tight tolerances. But spool clearances provide small holes that allows fluid to leak through and therefore let the actuators creep while systems should be stationary.

Some valves use poppets instead of spools. Poppets generally have very low to practically zero leakage so they can be used to seal the flow and hold actuators in position for longer periods. The symbols shown here are of pilot operated check valves. These allow free flow in one direction but require and external pilot pressure to open the valve and allow flow in the other direction.

A major benefit of hydraulic fluid power systems is that the power source can be located remotely from where the power output is required. For example, on a large mobile excavator it may be easier to have a cooling fan driven directly from the engine drive shaft, however, this limits the fan location and blade clearance required to compensate for engine vibration. The small loss in system efficiency from using a hydraulic fan drive is more than compensated for by the increase in overall cooling efficiency. There is rarely space for an electric motor to drive the fan.

This course is designed to be the first step in rigorous hydraulics training for coiled tubing operations and maintenance personnel. Participants come away from this highly concentrated course with a solid understanding of hydraulic principles, as well as component design and functions. System analysis and troubleshooting skills are taught using the circuits and schematics on a coiled tubing unit.

The Ten Step Troubleshooting Process is taught with the goal of eliminating the "hit or miss" methods that can be so costly in terms of time and materials. Safety is stressed along with sound maintenance practices. This three to five-day course is taught with circuit simulation software and through instructor guided hands-on study of individual hydraulic components.

I learned about how to understand charge pressure and the true function of the R1E valve. The content was specific to my job and made me aware of what is going on with the hydraulics.Chilton M., Sanjel

Hydraulics for Coiled Tubing is a specialty class, along with Hydraulics for Slickline and Wireline Rigs, and Fundamentals of Crane Hydraulics. If you enjoyed this class, consider taking your mobile hydraulic education father, with our general hydraulics course, How to Maintain and Troubleshoot Hydraulics Systems.

In the USA we usually measure flow in gallons per minute. In a fixed displacement pump system, the flow rate is directly related to the speed of the pump. The higher the flow rate the faster the cylinder or motor will move.

Fixed displacement hydraulic motors require a fixed volume of oil to cause the shaft to turn 1 revolution. This volume is referred to the motors displacement, usually measured in cubic inch displacement (CID) or cubic centimeter (CC). If you supply the motor with 100 times its CID every minute, it will turn 100 RPM. Speed up the flow rate and motor will go faster, slow it down and the motor will turn slower.

What size hose should I use for the 13 GPM flow from the earlier motor example? There are many ways to evaluate hose diameter for a given flow rate. I prefer to use oil velocity. As you push the oil through a smaller and smaller hose the oil must flow faster and faster to maintain the flow rate. As you force the oil to move faster the back pressure increases because of the increased friction.

Hydraulic pumps generate flow and tolerate pressure. The pressure comes from resistance to the oil flow. For example, a hydraulic cylinder that is not connected to anything will extend and retract a cylinder at low pressure. The pressure measured at the pump is what is required to overcome the seal friction of the cylinder and back pressure from the oil flowing through the hoses and valves.

Hydraulic components need to be protected from pressures above there designed capability. It is very important that a hydraulic system has a way of relieving the pressure should it go higher than the components are designed to tolerate. In a simple circuit the device that does this is typically a relief valve. It allows oil to flow back to tank if the maximum pressure setting is exceeded. This is done to protect the components. Without a relief valve the components in the system will attempt operate at the higher pressure, resulting in damage or failure of the component.

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