Accessories of Hydraulic System
Introduction :
● In the oil hydraulic system the main components which we have discussed are pump, control valves and actuators. These elements are primary elements which are must for every hydraulic system.
● The other elements which are required in the system are of secondary importance, hence known as accessories of hydraulic system.
● The system can work without accessories but will not give better performance and efficiency, hence to enhance performance and operational efficiency it is necessary to use various accessories.
Accessories of Pneumatic System
Introduction :
● In pneumatic system, the compressor, valves and actuators are the primary and essential elements without them system cannot work or perform.
● The auxiliary or secondary elements are mounted along with main elements to improve performance and reliability of system is known as accessories.
● In pneumatic system following accessories are used:
(a) Moisture separator : It helps to remove water particles from the air by following methods:
(i) Air dryer : It helps to reduce moisture in the air with the help of dryers.
(ii) Water trap : It is the device to separate out water particles.
(b) FRL Unit: It is combination of filter, regulator and lubricator which performs filtration, pressure regulation and Lubrication of air.
(c) Pipes and hoses : They are used to carry out compressed air from compressor to the various components of the pneumatic system.
(d) Pipe fittings : They are used for making connections during plumbing of pipe lines in the pneumatic system.
(e) Mufflers/silencer : They are noise reducing devices used in the pneumatic system.
Moisture Separator :
The air which contains moisture in atmospheric condition, contains the same even after compression and hence before this air is fed to any pneumatic system, it requires separation of moisture.
Effects of moisture on pneumatic system :
1. Corrosion of components:
Water particles are increasing the tendency of corrosion of components like valves, actuators which are made from ferrous metals.
2. Poor performance :
Moisture also reduce the system performance due to corrosion, noisy operation and increased wear and tear of components.
3. Reduced life of components:
It decreases life of the components due to corrosion poor surface finish, etc.
Methods of moisture separation :
1. Using air dryers :
● The air dryer is a device which removes moisture by dehydration using mechanical or chemical principle.
● The commonly used dryer is refrigerated dryer. These dryers use a refrigeration circuit to chill the compressed air to a specific temperature (dew point).
● Water vapour in excess of this temperature condenses and separates at the dryer.
Chemical dryer :
● In this dryer, the compressed air is passed through some kind of desiccant (chemical with an affinity for water) for a certain period of time to remove a portion of water vapour.
● Desiccant are usually crystalline substances and are of two types:
(a) Absorb type : Which changes physically when it will absorb moisture. It is salt base or urea base chemicals. It is known as deliquescent dryers. They have tendency to melt away in contact with the moisture. e.g. salt, sugar.
(b) Adsorption type : These air dryers use desiccants that do not change physically as the water vapour is absorbed.
They can be regenerated and used over and over again. The most common chemicals are activated alumina and silica gel.
2. Using water trap :
Principle :
When flow of air is diverted to abrupt angle or passed through number of abrupt changes in flow direction, the heavy moisture particles, get dropped at bottom due to inertia.
Construction:
● The water trap consists of (1) water trap drum which is attached with inlet pipe for entry of compressed air. The outlet pipe is also connected at the top of the drum to supply water free air.
● At the bottom of the drum, valve is provided to drain out moisture particles.
● The compressed air pass through the inlet and enters ina water trap drum. It is diverted to the flow path at 90° towards the outlet pipe. Due to sudden change in bottom of the drum and collected at the drain valve.
● The drain valve is pulled out to drain out the water particles.
Reservoir (Oil Storage Element)
Definition : It is a tank which stores the hydraulic oil for recirculation through the hydraulic system.
Importance of reservoir :
It has following functions:
(a) Storage of oil: It is main oil container which has capacity in liters.
(b) Cooling of oil : It acts as a cooling device for dissipation of heat in the oil through large surface area of the tank.
(c) Expansion of fluid : It allows thermal expansion of oil.
(d) Seperation of impurities : The dirt, dust,wear particles settled down in the reservoir.
(e) Structural support : It acts as a base for pump-motor assembly, relief valve, etc.
Construction of Reservoir:
It consists of:
(a) Sight glass (oil level indicator): It is the level indicator to show the quantity of oil available in the tank.
(b) Check valve : It is provided in the return pipe to prevent back flow.
(c) Baffles : They are provided to reduce foaming of oil.
(d) Breather: It is provided for air circulation in the tank.
(e) Filter : It removes dirt from the air passing through the breather.
(f) Drain plug : It is provided to drain out used oil.
(g) Sampling tap : It is provided to collect sample of for testing of viscosity, etc.
(h) The inlet and outlet of reservoir should not close each other so that heat can be dissipated easily during recirculation.
(i) The reservoir capacity should be three times the flow rate of the pump.
(j) Square/rectangular shape helps for better heat dissipation per unit volume.
Air Receiver:
Construction:
● The reciprocating compressors and other intermittent discharge compressors will require an air receiver to discharge compressed air in the tank for storage and further use.
● Air receiver is a cylindrical vessel mounted vertically or horizontally with inlet and outlet connections of compressed air.
● It has following functions :
1. It stores the compressed air.
2. It smoothness out the pulsations in flow of compressed air intermittent discharge) from reciprocating machine.
3. It helps to cool the air.
4. It condensate moisture of compressed air.
● The size of air vessel is specified by the volume of air in m cu stored under pressure. The capacity of air receiver should be at least equal to hold air delivery of compressor in 1 min. Generally, the receiver capacity should be about 1/10 of free air delivery per minute.
● Air receiver is provided with various mountings
1. Water separator :
It removes the water particles from the compressed air.
2. Air inlet pipe:
The compressor delivers high pressure air and allow enter in the inlet pipe of receiver.
3. Pressure gauge :
It is provided to indicate the pressure of compressed air which is stored.
4. Air outlet pipe:
It is provided with outlet valve to supply compressed air to the pneumatic system.
5. Safety valve :
It is the safety against the excess pressure generated due to some failure in case of emergency situation.
6. Water drain:
It is provided to drain out condensate water particles.
Pipes and Hoses
Introduction to Pipes
● In a hydraulic system, it is necessary to carry out the pressurized oil from one point to the another such as tank to pump, pump to actuator, actuator to the tank, etc.
● Hence pipes are the closed conduits which are used to carry or supply the pressurized oil through various connection points in the hydraulic system. It is similar to domestic pipe fitting supplying water at various points.
● A pipe can be defined as a functional connection for fluid flow in the fluid power system. It may termed as pipe, tube and hoses each one has specific characteristics.
● The pipes must be of sufficient size to pass the fluid without excessive pressure loss.
Piping layout:
When pipes and fittings are arranged to supply oil to the various desired points. It is known as piping layout of the oil hydraulic system.
Important Factors for Pipes and Hoses :
1. Types of pipes - Rigid, Semi-rigid ant flexible
2. Material of the pipes
3. Pipe specification
4. Pipe fittings
5. Energy loss
1. Types of pipes:
(a) Rigid :
The pipes are straight and can not be easily change their shape. They are suitable for longer connections which are permanent in the system. E.g. pump and suction strainer connection. They are made of steel, cast iron, galvanised iron, etc.
(b) Semi-rigid :
It has flexibility up to some extent hence known as semi-rigid. It includes tubes of aluminium, copper and other soft materials.
(c) Flexible :
At some points of connection. It is required to change the shape of pipe and rigid / semi-rigid pipes are to no use. The flexible pipes known as hoses are used for such application.
2. Pipe materials:
● The pipes are manufactured by cold drawing in the form of seamless (without joint) tubes to the required size. They are also coated or subjected to corrosion resistive treatment to improve its life.
● The materials used for pipes are :
(1) Steel :
Steel is an alloy or iron and carbon with alloying elements. It has good surface appearance and corrosion resistance. Steel tubes are made of annealed mild steel or soft ductile carbon steel. Stainless steel is also used. They are costly and suitable for marine application.
(ii) Copper :
Copper is a non ferrous metal with high ductility. It has good corrosion resistance. Copper is suitable for low pressure application as it has tendency to get harden at high temperature. It is manufactured for small bore sizes only.
(iii) Cuprous nickel alloy :
It is the alloy of copper and nickel which has good corrosion resistance. It is more costly hence limited and specific use.
(iv) Aluminium :
It is ductile non ferrous low density metal which can be easily drawn in the form of tubes. They are preferred where low weight of piping is important. E.g. Aerospace applications.
(v) Galvanised iron :
The iron pipes are easily get corroded hence zinc coating is done and the material is known as zinc coated galvanized iron. They are suitable only when the oils are treated with antioxidant additives.They may promote the chemical deterioration of certain oils.
(vi) Cast iron :
They are suitable for domestic water supply at 2.5 bar. In oil hydraulic system, they are used in return line connections where pressure is low in the range of 15 to 20 bar.
Factors to be considered while selecting the pipe fo pneumatic system
● Pressure of compressed air in the line.
● Total flow rate per unit time through the line.
● Permissible pressure drop in the line.
● Type of tube material and type of line fittings.
● Length and diameter of tube or other pipelines.
● Working environment.
3. Pipe specification :
● In general, pipes are specified by:
(i) Inside Diameter (ID)
(ii) Wall thickness
(iii) Pressure rating
(iv) Manufacturing method
● Inside diameter is expressed in terms of inches (old method) like 1/2,3/4,2, etc, but metric size such as 15mm, 18mm, 25mm is commercially used.
● Wall thickness of the pipe is the difference between inside and outside diameter. The outside diameter size remains constant while inside diameter may change to vary the wall thickness. The wall thickness is indicated by using schedule number. Schedule number has three grades:
(i) Schedule 40 : It is standard size
(ii) Schedule 80 : Extra strong for high pressure
(iii) Schedule 160 : Double extra strong for very pressure.
● Pressure rating is in terms of proof pressure/Burst Pressure (B.P.) labeled on pipe. It indicates maximum pressure of pipe.
● Seamless, cold drawn, etc. is also mentioned on pipe to give process used for pipe manufacturing.
4. Pipe fittings:
● Pipe fittings are needed in following cases:
(a) Ends of pipes or tubes is to be attached to the ports of the hydraulic components like actuators, valves,etc.
(b) Pipes are to be joined to form plumbing layout which includes flow diversion, flow reduction, etc.
● The pipe fittings are generally called as :
(i) Adaptors: End connectors of valve, actuator ports.
(ii) Connectors: For joining of pipes ends.
(iii) End fittings: End of pipes and hoses for easy assembly.
The various hydraulic fittings are :
● Selection of fitting depends on the size of the pipe or tubes. Flanges may be used for size up to 63.5 mm. Flare less compression fittings are used from 1/8" to 2" diameter tube. Flared fittings are used on 50 mm (2") and below. Compression fittings are easy for dismantle flanged joints are used above 2".
● It has end enlarged by flaring tool and fitted by using nut and flared fitting.
5. Energy losses in pipes:
● When hydraulic oil flows through a pipe or fitting, there is a energy loss due to:
(i) Pipe friction
(ii) Due to pipe fittings, valves
(iii) Leakage at joints
● The energy loss due to pipe friction is given by the Darcy's equation and Chezy's equation.
● The various losses due to pipe fittings are
(a) Sudden contraction : The flow area is reduced suddenly and loss of energy occurs.
(b) Sudden enlargement : The flow area increases, and causes energy loss.
(c) Bend : Loss of energy due to change in flow path or direction.
Hoses:
● There are number of applications in the hydraulic system where rigid piping is not suitable.
E.g.
(1) When pipe is to bend for short distance connection.
(2) Drive unit is to be moved with the pipe line.
Definition : Flexible pipes which can be easily bend to any shape are used in hydraulic system. These flexible pipes are known as hydraulic hoses.
Construction details of pneumatic/hydraulic hose :
● They are made of composite material in the form of rubber reinforced with fibre or steel wire braiding. Usually hoses are constructed in three layers.
(a) Inner tube :
It is rubber or other synthetic material tube through which the fluid is passed and it comes directly contact with the pressurised oil.
(b) Re-inforcement :
It gives strength to the hose in the form of wire arranged in grid design, it uses steel wire of high tensile strength to withstand very high working pressure, permit longer life, and more durability to the hose.
The steel wire may be either spirally woven or cross-woven.
(c) Outer protective layer :
● The reinforcement of the hose is protected by the outer cover from corrosion, abrasion and other damage.
● Various types of synthetic elastomers having excellent abrasion resistance and weather resistance materials are used for outer protective layer.
● Neoprene, synthetic GRS rubbers are commonly used for this cover.
● Sometimes sleeves are provided for protection of hoses from external rubbing, abrasion, etc.
Advantages of hoses:
1. Flexible in nature.
2. High capacity to absorb shocks/vibrations than metal tubes.
3. Easy for installation and dismantle.
4 They can be manufactured in long lengths.
5. They are capable to take high pressures.
Disadvantages of hoses :
1. Poor abrasion resistance.
2. Initial cost is higher.
3. They are damaged due to incompatible oil abrasion.
4. Poor in weathering resistance.
Factors for selection of hose/pipe:
1. Pressure: Pressure of fluid to be conveyed.
2. Flow capacity : To decide diameter and length of pipe.
3. Type of fluid.
4. Temperature.
5. Type of application.
Various Types of Hoses :
1. As per pressure rating
a. Low pressure hoses
b. Medium pressure hoses
c. High pressure hoses
2. As per no. of layers
a. Single layer
b. Double layer
c. Multiple layer
Requirement of good fluid power plumbing:
● Fluid power plumbing consists of arrangement of pipes and hoses in a particular designed manner for given application.
● Following are the requirements of good fluid power plumbing
(a) The pipe material and size should be properly selected.
(b) The pipe should have sufficient strength to withstand pressure of the system.
(c) There should be minimum joints fittings and diversion to reduce frictional losses.
(d) Pipe joints should be leak proof with proper seals.
