ACCESSORIES USED IN HYDRAULIC SYSTEM

CONTENTSPower Units

Reservoir

Accumulator

Intensifier

POWER UNIToThe Power unit:

oProvides energy for the operation of the hydraulic

oMoves fluid through the system

oProvides a safe maximum system operating pressure

oAssisting in maintaining correct system operating

temperature and fluid cleanliness

POWER UNITSA basic power unit consist of :

oPrime mover to power the system

oPump to move the fluid

oReservoir to store the fluid

oRelief valve or pump compensator to control maximum system

pressure

oFilter to clean the fluid

oPlumbing to transport fluid to components

POWER UNITS

Power Units are

often supplied by

manufacturers as

package.

CLASSIFICATION

1. Vertical power units

2. Horizontal power units

3. Portable power units

RESERVOIRThe function of a fluid reservoir in a power hydraulic system is:

To provide a chamber in which any volume of fluid can be

accommodated.

To provide a filling point for the system

To serve as a storage space for the hydraulic fluid

To provide a volume of fluid which is relatively stationary to allow

entrained air to separate and heavy contaminants to settle.

To accomplish the dissipation of heat by its proper design and to

provide a radiating and convective surface to allow the fluid to cool.

CONSTRUCTIONAL DETAILS OF A RESERVOIR

FEATURES OF HYDRAULIC RESERVOIR1. Filler Cap (breather cap):It should be air tight when closed but may contain the air vent that filters air entering the reservoir to provide gravity push for proper oil flow2. Oil level gauge:It shows the level of oil in the reservoir without having to open the reservoir.3. Intake filter:It is usually a screen that is attached to the suction pipe to filter the hydraulic oil.

4. Baffle Plate:The purpose of baffle plate is to separate the pump inlet line from the return line to prevent the same fluid from recirculating continuously within the tank.1. To permit the foreign substance to settle down to the

bottom2. To allow entrained air to escape the oil3. To prevent localized turbulence in the reservoir.4. To promote heat dissipation through the reservoir wall.

5. Suction & Return line:1. They are designed to enter the reservoir at the points

where the air turbulence is least.2. They can enter from top or sides but their ends should

always be at bottom3. If return line is above the oil, the return oil can foam and

draw in some air.6. Strainers and Filters:They are designed to remove foreign particles from the hydraulic fluid.7. Drain Plug:It allows oil to be drained from the reservoir. Some drain plugs are magnetic to help remove metal chips from the oil.

TYPES OF RESERVOIR1. Non- pressurized: These reservoir operates at the Atmospheric pressure. They are vented to atmosphere using an air filter or a

separating diaphragm. Most commonly used type in Industrial system, machine

tools. Since they are open to atmosphere, as such they are

prone to atmosphere. Hence they are not commonly used.

2. Pressurized: These reservoirs are pressurised inside between 0.35-1.4

bar and has to be provided with some methods of pressure control.

There may be a small air compressor to maintain set charge pressure.

Pressurising improves suction characteristic of pump by providing additional suction head.

It also helps in preventing contaminants from entering into the reservoir.

So it avoids a boost pressure to the main pump and prevents the ingress of dirt.

SIZING OF RESERVOIRThe reservoir capacity should be adequate enough to cater for changes in fluid volume within the system and sufficient area to provide system cooling.

CRITERIA:

1. The minimum reservoir capacity should be twice the pump delivery per minute.

2. The reservoir capacity should be three-four times the pump delivery per minute for a high volume mobile application.

3. It must make allowance for dirt and chips to settle at the bottom and air to escape.

4. It must be able to hold the oil.

5. It must maintain the oil level high enough to prevent the whirlpool effect.

6. It should have large surface area to dissipate heat generated in the system.

7. It should have an adequate air space to allow for the thermal expansion oil.

DESIGN OF RESERVOIR SURFACEFor reservoir performing primarily as heat exchangers, the estimation of heat transfer surface for rapid heat dissipation is of great significance,

H = heat transferred, watts h= overall heat transfer coefficient

A= surface area∆t = temperature differential

6.3*thAH

For a vertical plate of height L,

4/1

42.1

Lth

For horizontal plate of width, W

4/1

32.1

Wth

Best design for heat transfer Poor design for heat transfer

ACCUMULATORA hydraulic accumulator is a device in which the potential energy of an incompressible fluid is held under pressure by an external source against some dynamic force from sources like gravity, mechanical spring and compressed gas.

- The potential energy is stored when the demand of energy by the system is less than that available from the prime mover and is released to the system during its period of peak demand of energy which the prime mover alone cannot meet.

- Its function is analogous to that of the flywheel in a mechanical system and a capacitor in an electrical circuit

DESIGN OF ACCUMULATOR

CLASSIFICATION Accumulator

Gravity or dead weight type

Spring loaded type

Gas loaded type

Free contact type (Non-separator type)

Separator type

Piston type

Diaphragm type

Bag type

DEAD WEIGHT OR GRAVITY TYPE ACCUMULATOR

Dead Load Accumulator

DEAD WEIGHT OR GRAVITY TYPE ACCUMULATOR It consist of a vertical thick-walled steel cylinder housing a

piston. The piston rod extending out of cylinder supports the dead

weight by raising the piston on admitting high pressure. The force of gravity of the dead weight is used to store the

potential energy. This accumulator generates a constant pressure of fluid

through full volume output of fluid independent of rate and quantity of energy supplied from prime mover.

Whereas for other types, the fluid output pressure decreases as a function of the flow rate output of the accumulator.

The main drawback being is its very large size and heavy weight which renders it unsuitable for portable applications.

SPRING LOADED ACCUMULATOR

Spring Loaded Accumulator

SPRING LOADED ACCUMULATORIt is similar to a dead weight type except that piston is preloaded with a spring compression.The spring is a source of energy acting against the piston. The pressure created by this type of accumulator depends upon the stiffness and pre-loading of spring.The pressure exerted on the fluid is not constant.It typically delivers a low flow rate of oil at low pressures, so for high pressure situations, these type of accumulator are somewhat heavy.This are not suitable for application demanding high cycle rates as the spring may fail in fatigue and lose its elasticity.

GAS LOADED/CHARGED ACCUMULATORSThese are also known as hydro-pneumatic accumulators

because in this type the force is applied to the oil using

compressed air.

The gas loaded accumulators works on the principle of

Boyle’s law of ideal gas. The storage of potential energy is

due to the compressibility nature of the gas.

The expansion of the gas forces the oil out of the

accumulator.

NON SEPARATOR OR FREE CONTACT TYPE:The non separator type consist of an enclosed cylindrical shell having oil part at bottom and a gas charging part at the top.The oil below is in free contact with the gas above without any separator.It is always in a vertical position, so that the oil and gas always remain separated.It can handle large volumes of oil, which is in advantage.While the disadvantage is the ingress of gas into oil by absorption.This is not suitable for high speed pumps as the entrapped gas in the oil promotes cavitation.

NON SEPARATOR TYPE

Gas loaded accumulator

GOVERNING RELATIONSa) ISOTHERMAL: when the expansion/compression of the

gas is extremely slow, the process is isothermal following the relation,

PV=Constantb) Isentropic process(Adiabatic): this is a process which

occurs at such a fastest speed that there is no flow of heat into and out of the accumulator.

c) Polytropic process: in this process, the speed of the occurrence is neither too fast nor too slow.

Cpv

SEPARATOR TYPEHere the oil and gas are separated by an element or a diaphragmThe partition or separator allows the effective utilization of the compressibility of the gas.Depending on the type of element used to separate the oil and gas, they are further classified as,

Piston Type Accumulator.

Diaphragm Type Accumulator.

Bladder or Bag type Accumulator.

PISTON TYPE ACCUMULATOR

Piston type gas loaded

PISTON TYPE ACCUMULATORIt consist of a cylinder housing a freely floating piston.

The piston separates the gas and oil.

The friction between the piston and seals may adversely affect

the working of particularly low pressures system. Leakage past

the piston is yet another problem requiring frequent pre-

charging.

Due to greater inertia and seal friction, these accumulators are

not suitable for damping, pressure pulsation or shock

absorption.

DIAPHRAGM TYPE ACCUMULATOR

DIAPHRAGM TYPE ACCUMULATORIn this type the hydraulic fluid and nitrogen gas are separated by synthetic rubber diaphragm.At the bottom of the diaphragm a shut-off button is provided, this button covers the inlet of the line connection when the diaphragm is fully stretched.The advantage of the diaphragm accumulator over the piston accumulator is that it has no sliding surface that requires lubrication and can be therefore used with the fluid having poor lubricating qualities.It is less sensitive to contaminations due to lack of any close fitting components.

OPERATION OF DIAPHRAGM TYPE ACCUMULATOR

BLADDER TYPE ACCUMULATOR

BLADDER TYPE ACCUMULATORit functions in the same way as the other two accumulators.

Here the gas and the hydraulic fluid are being separated by

synthetic rubber bladder.

The bladder is filled with the nitrogen gas until the designed pre-

charged pressure is achieved.

The hydraulic fluid is then pumped into the accumulator

The poppet valve closes the inlet when the bladder is fully

expanded, thus the pressing of the bladder into the opening is

prevented.

SIGNIFICANCE OF NITROGEN GAS FILLED ACCUMULATOROnly nitrogen gas is used to charge a gas filled with accumulator.Nitrogen is chemically inert, non-flammable and does not combine easily with other elements.The ingress of nitrogen in to the oil is detectable in the form of foaming in the reservoir.Reduction of nitrogen volume also causes narrow limits of pump operation and overheating, thus reducing the speed of the cylinders and motors.The effectiveness can be compensated by using gas back-up bottle.