Components of hydraulic systems

Virtually, all-hydraulic circuits are essentially the same regardless of the application. There are six basic components required for setting up a hydraulic system:

1. A reservoir to hold the liquid (usually hydraulic oil)
2. A pump to force the liquid through the system
3. An electric motor or other power source to drive the pump
4. Valves to control the liquid direction, pressure and flow rate
5. An actuator to convert the energy of the liquid into mechanical force or torque,
to do useful work. Actuators can either be cylinders which provide linear
motion or motors which provide rotary motion and
6. Piping to convey the liquid from one location to another.

Figure 9.1 illustrates the essential features of a basic hydraulic system with a linear hydraulic actuator.

The extent of sophistication and complexity of hydraulic systems vary depending on the specific application.

Each unit is a complete packaged power system containing its own electric motor, pump, shaft coupling, reservoir and miscellaneous piping, pressure gages, valves and other components required for operation.

hydraulic-system

Water-cooled hydraulic heat exchanger

Figure 7.26 is an illustration of a common type of water-cooled heat exchanger used in hydraulic systems.

This is typically a shell and tube-type heat exchanger. The cooling water is pumped into the heat exchanger and flows around the tube bank. The hydraulic fluid, which is to be cooled, flows through the tubes. While flowing through the tubes, the fluid gives away heat to the water, thereby reducing its temperature.

Advantages of water-cooled heat exchangers are:
1. They are very compact and cost-effective
2. They do not make noise
3. They are good in dirty environments.

Disadvantages associated with water-cooled heat exchangers are:
1. Water costs can be expensive
2. Possibility of mixing of oil and water in the event of rupture
3. Necessity for regular maintenance to clear mineral deposits.

water-cooled-hx

Hydraulic heat exchangers

Heat is generated in a hydraulic system because of the simple reason that no component can operate at 100% efficiency. Significant sources of heat include pumps, pressure relief valves and flow control valves. This can cause a rise in temperature of the hydraulic fluid above the normal operating range. Heat is continuously generated whenever the fluid flows from a high-pressure region to a low-pressure region, without producing mechanical work. Excessive temperatures hasten oxidation of the hydraulic fluid and also reduce its viscosity. This promotes deterioration of seals and packings and accelerates wear and tear of hydraulic components such as valves, pumps and actuators. This is the reason why temperature control is a must in hydraulic systems.

The steady-state temperature of the fluid depends on the rate of heat generation and the rate of heat dissipation. If the fluid-operating temperature is excessive, it means that the rate of heat dissipation is inadequate for the system. Assuming that the system is reasonably efficient, the solution is to increase the rate of heat dissipation. This is accomplished by the use of ‘coolers’, which are commonly known as heat exchangers. In certain applications, the fluid needs to be heated in order to achieve the required viscosity of the fluid in the system. For example, if a mobile hydraulic equipment is required to operate in sub-zero conditions, the fluid needs to be heated. In such cases, heat exchangers are termed as heaters.

The factors to be considered when sizing a heat exchanger are:
• The required drop in temperature of the hydraulic fluid
• The flow of the hydraulic fluid in the system
• The time required to cool the fluid.

There are two main types of heat dissipation heat exchangers:
1. Air-cooled heat exchangers and
2. Water-cooled heat exchangers.

Diaphragm-type separator gas-loaded hydraulic accumulator

The diaphragm-type accumulator consists of a diaphragm secured in a shell and serving as an elastic barrier between the oil and the gas. The cross-sectional view of a diaphragm type accumulator is shown in Figure 7.18.

A shut off button which is secured at the base of the diaphragm, covers the inlet of the Hne connection when the diaphragm is fully stretched. This prevents the diaphragm from being pressed into the opening during the precharge period. On the gas side, the screw plug allows control of the charge pressure and the charging of the accumulator by means of a charging and testing device.

diaphragm-accumulator

With the help of the following figures (Figures 7.19(a)-(f)), let us now see how exactly a diaphragm-type accumulator works.
Figure 7.19(a) shows the accumulator without the nitrogen charge in it or in other words in a precharged condition. The diaphragm can be seen in a non-pressurized condition.
Figure 7.19(b) shows the accumulator in charged condition. Here nitrogen is charged into the accumulator, to the precharged pressure.
Figure. 7.19(c) shows how the hydrauhc pump delivers oil to the accumulator and how this process leads to the deformation of the diaphragm.
As seen from Figure 7.19(d), when the fluid delivered reaches the maximum required pressure, the gas is compressed. This leads to a decrease in gas volume and subsequent storage of hydraulic energy.
Figure 7.19(e) shows the discharge of the oil back to the system when the system pressure drops, indicating requirement of oil to build back the system pressure.
Figure 7.19(f) shows the accumulator attaining its original precharged pressure condition.

diaphragm-accumulator-operation

The primary advantage of the diaphragm-type accumulator is the small weight-tovolume ratio, which makes it highly suitable for airborne applications.

Gas-loaded-type hydraulic accumulators

These types of accumulators (frequently referred to as hydro-pneumatic accumulators) have been found to be more practically viable as compared with the weight and springloaded types. The gas-loaded type operates in accordance with Boyle’s law of gases, according to which the pressure of a gas is found to vary inversely with its volume for a constant temperature process.

The compressibility of the gas accounts for the storage of potential energy in these accumulators. This energy forces the oil out of the accumulator when the gas expands, due to a reduction in system pressure.

Gas-loaded accumulators fall under two main categories:
1. Non-separator type
2. Separator type.

Spring-loaded-type hydraulic accumulators

A spring-loaded accumulator is similar to the weight-loaded type except that the piston is preloaded with a spring. A typical cross-section of this type of accumulator has been illustrated in Figure 7.15.

spring-loaded-accumulator

The spring is a source of energy, acting against the piston and forcing the fluid into the hydrauhc system. The pressure generated by this accumulator depends on the size and preloading of the spring. In addition, the pressure exerted on the fluid is not constant. They typically deliver small volumes of oil at low pressures and therefore tend to be heavy and large for high-pressure, large volume systems.

A spring-loaded accumulator should not be used for applications requiring high cycle rates as the spring may lose its elasticity and render the accumulator useless.

Weight-loaded-type hydraulic accumulators

The weight-loaded type is historically the oldest type of accumulator. It consists of a vertical heavy wall steel cylinder, which incorporates a piston with packing to prevent leakage (Figure 7.14).

weight-loaded-accumulator

A dead weight is attached to the top of the piston. The gravitational force of the dead weight provides the potential energy to the accumulator. This type of accumulator creates a constant fluid pressure throughout the full volume output of the unit, irrespective of the rate and quantity. The main disadvantage of this accumulator is its extremely large size and heavy weight.

Hydraulic Accumulators

Accumulators are devices, which simply store energy in the form of fluid under pressure. This energy is in the form of potential energy of an incompressible fluid, held under pressure by an external source against some dynamic force. This dynamic force can come from three different sources: gravity, mechanical springs or compressed gases. The stored potential energy in the accumulator is the quick secondary source of fluid power capable of doing work as required by the system. This ability of the accumulators to store excess energy and release it when required, makes them useful tools for improving hydraulic efficiency, whenever needed. To understand this better, let us consider the following example.

A system operates intermittently at a pressure ranging between 150 bar (2175 psi) and 200 bar (2900 psi), and needing a flow rate of 100 1pm for 10 s at a frequency of one every minute. With a simple system consisting of a pump, pressure regulator and loading valves, this requires a 200 bar (2900 psi), 100-lpm pump driven by a 50 hp (37 kW) motor, which spends around 85% of its time, unloading to the tank. When an accumulator is installed in the system as shown in Figure 7.12, it can store and release a quantity of fluid at the required system pressure.

accumulator-circuit-diagram

The operation of the system with accumulator is illustrated by Figure 7.13;

accumulator-operation

At time A, the system is turned on and the pump loads, causing pressure to rise as the fluid is delivered to the accumulator via a non-return valve V3. At time B, the working pressure is reached and a pressure switch on the accumulator causes the pump to unload. This state is maintained as the non-return valve holds the system pressure.

The actuator operates between time C and D. This draws the fluid from the accumulator causing a fall in the system pressure. The pressure switch on the accumulator puts the pump on load again, to recharge the accumulator for the next cycle.

With the accumulator in the system, the pump now only needs to provide 170 1pm and also requires reduced motor hp. Thus it can be seen how an accumulator helps in reducing the power requirements in a hydraulic system.

There are three basic types of accumulators used extensively in hydraulic systems. They are:

1. Weight-loaded or gravity-type accumulator
2. Spring-loaded-type accumulator
3. Gas-loaded-type accumulator.

Tell-tale Hydraulic filters

This is a versatile filter, which can be directly welded into a reservoir’s suction and return line or conversely installed in pipes with a maximum pressure of 10 kg/cm2 (142 psi). This filter can remove particles as small as 3 u. It consists of an indicating element, which indicates the time when cleaning is required. That is why this filter is referred to as a telltale filter (Figure 7.8).

tell-tale-filter

The operation of a tell-tale filter is dependent on the fluid passing through the porous media, which traps the contaminants. The tell-tale indicator monitors the pressure differential buildup due to dirt, which gives an indication of the condition of the filter element.

Hydraulic filter installation location

Filter location in a hydraulic system is critical to ensuring acceptable levels of fluid cleanliness and adequate component protection. Figure 7.5 shows the location of the various filters in a hydraulic system.

hydraulic-filter-location

The function of breathers in a hydraulic system is to prevent entry of airborne particles which are drawn into the system due to changes in the fluid level of the reservoir. They are usually mounted on the reservoir. Components such as servo valves which are located immediately downstream of the filter are protected from wear and silting-related problems by pressure filters. These pressure filters are designed to withstand high pump pulsations and the system pressure. Return hne filters provide protection against entry of particulate matter when the fluid is returned to the tank. An off-line filter also known as kidney loop is often provided in a hydraulic system especially when fluid circulation through the return-line filter is minimal. Off-line filters operate on a continuous basis. The chief advantage associated with these filters is the flexibility they offer with regard to their placement. Since these filters are independent of the main system, their location in a hydraulic circuit can be chosen in such a way that easy serviceability is ensured.