Construction and Working of Bladder Accumulator

Figure 1: Bladder Accumulator.

Fig. 1 shows static position of accumulator. Oil comes in and also goes out through port (A). We can fill the gas through port (B) so that flexible bag will expand.

Figure 2: Bladder Accumulator fully precharged.

To operate this accumulator first we have to fully precharge the gas into the gas bag. The position of the gas bag after fully precharging the gas (This gas is generally Nitrogen gas or any inert gas) is shown Figure 2. When we fill the gas in the bag through port (B), the gas bag fully expands. During this step oil is under normal pressure. Now the oil under pressure will start accumulating into the accumulator through port A.

Figure 3: Bladder Accumulator with compressed gas and bag.

The pressure of oil is higher than the pressure of gas filled in the gas bag. Due to this pressure difference the oil will compress the gas bag. This is shown Figure 3. The compressed gas and bag are clearly seen. Now oil under pressure is stored in the shell. This is ‘fully charged’ position of this accumulator.

Now when system in which this accumulator is connected, if demands hydraulic oil under pressure, then oil will start flowing out through port A. As oil goes out, the pressure on gas bag will reduce slowly and the bag will expand. As bladder goes on expanding, the pressure of outgoing oil will go on reducing.

Advantages of Bladder Accumulator

1. The accumulator is compact and light in weight.
2. A gas bag is of flexible material like rubber. Hence, bag gives quick response to minute changes in expansion and compression.
3. It is having very few functional parts and hence cheap.

Disadvantages of Bladder Accumulator

1. The pressure of outgoing oil will not be constant. As gas bag (bladder) goes on expanding, the pressure of oil reduces.
2. The volume of oil stored in the accumulator is small.
3. We have to change the gas bag after specific period of service.
4. We cannot handle high temperature fluids in this accumulator.

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Types of Hydraulic Accumulator

There are three basic types of hydraulic accumulators:

1. Dead weight accumulator.
2. Spring loaded accumulator.
3. Gas pressurised accumulator.

Dead Weight Accumulator

Figure 1: Dead Weight Accumulator.

This accumulator consists of a sliding piston in a cylinder. The piston rod diameter is much bigger. The oil under pressure usually from pump enters into the cylinder through port P (see Figure 1). The pressurised oil forces the piston upwards, until it reaches position B. Dead weight W kept on top of piston rod is in the form of concrete/steel/other heavy material. This weight is chosen to exert a predetermined pressure on the fluid entering the cylinder. When piston reaches at point B, the accumulator is said to be ‘fully charged’. Now the oil in the cylinder is having ‘pressure energy’ due to dead weight W acting on it.

The energy is stored by oil in cylinder. Now when the system in which this accumulator is connected, it demands hydraulic oil under pressure, then pressurised oil starts flowing out of port P. When oil starts going out the piston will move down. Due to dead weight pressure of oil will be maintained.

Advantages of Dead Weight Accumulator

1. Pressure remains constant for full stroke due to dead weight.
2. This accumulator can supply large amount of oil under pressure.

Disadvantages of Dead Weight Accumulator

1. These accumulators are bigger in size and occupy more space and are bulky.

Spring Loaded Accumulator

Figure 2: Spring Loaded Accumulator

This is a modified version of dead weight accumulator. In this accumulator, there is a spring loaded piston which moves up and down in cylinder. The oil under pressure usually from pump enters into the cylinder through port P. This oil forces the piston upwards causing the spring to compress. The top dead position of the piston shown by point ‘B’ in the Fig. 2 will be decided by full compression length of spring.

When piston reaches point B by fully compressing the spring, the accumulator is said to be ‘fully charged’. Now the oil in the cylinder will be having ‘pressure energy’ due to resilience of the spring.

Now when system in which this accumulator is connected, if demands hydraulic oil under pressure, then spring starts expanding thereby pushing the piston downwards and pressurised oil will come out of the port ‘P’. When spring will be fully expanded (reaches to its free length), the pressure of oil coming out of the accumulator will be almost minimum.

Advantages of Spring Loaded Accumulator

1. All moving parts are enclosed in the cylinder. No parts outside.
2. Compact design i.e. if we use compound spring, the design will be more compact and handy.

Disadvantages of Spring Loaded Accumulator

1. When spring starts expanding it gives off the stored energy very quickly. And hence as spring starts expanding the pressure of oil below the piston goes on decreasing. So we cannot get uniform pressure in a stroke.
2. When spring is fully compressed, its length is called solid length. Due to this solid length, the stroke of piston becomes limited, as compared to dead weight accumulator

Symbols of Hydraulic Accumulator

Figure 3: Symbol of Hydraulic Accumulator

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Figure 1: Double Acting Hydraulic Cylinder.

In double acting Hydraulic cylinder, the pressurized liquid is admitted on both sides of the piston alternately. Work is performed during forward motion as well as backward motion of the piston (see Fig. 1). Double acting Hydraulic cylinders are most commonly used hydraulic cylinders in industrial applications. In these cylinders, the pressure is applied on both end ports to obtain power in both directions. They are also termed differential cylinders, as they have different extension and retraction areas.

Construction of Double acting Hydraulic Cylinder

1. The main parts of double acting cylinder are,

Two end cover plates (or) Two end caps with port connections

• Base Cap
• Bearing cap

2. Cylinder barrel
3. Piston
4. Piston rod

The end caps are welded to barrel (or) joined by the tie rods (or) by fasteners. The end caps are fixed to the barrel by means of tie rods or thread sections by weld joint. For any lengths of cylinder barrel, the size of end caps and piston remains same. The barrel should be wear resistant and leak proof. It is made of seamless drawn steel tube, which is precisely finished. Piston is made of cast iron, transmits force to the rod. It acts as sliding bearing in the cylinder barrel and also, as seal, in order to reduce the leakages and thus, avoids the use of piston seals. A bronze bearing surface is deposited on piston surface, which is accurately finished.

Working of Double acting Hydraulic Cylinder

During retraction process, the reduction in piston area, due to the cross-sectional area of rod inside piston, causes the effective area difference. But the extension process takes place slowly as the fluid has to be filled in more area compared to retraction process. As pressure is given by the force per unit area, it implies that more effective area results in the greater force in extension process. Also, less force acts during retraction process, due to reduced fluid volume displaced by the rod inside the piston.

During the extraction operation, the cylinder rod comes in contact with the atmosphere and thus, undergoes corrosion because of dust and impurities in the air. These impurities enter the barrel during retraction and damage the cylinder. Hence, heat-treated chromium alloy steel is added to prevent corrosion. While the dusty particles can be eliminated by fixing the wipers or scraper seal fixed to the end caps.

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In a centrifugal pump, the fluid is rotated with high speed, to rise beyond the restricted walls of the casing by means of centrifugal action of impeller. Then the liquid flow out.

Centrifugal Action in Centrifugal Pump

Figure 1: Centrifugal Action.

The pressure difference is caused due to spiral forced vertex flow with the help of centrifugal action. ‘Whirling action is added to fluid by means of blades (generally backward curved blades) of impeller of the pump. The liquid enters at centre of impeller (i.e., eye) and discharges into casing surrounding the impeller. The potential energy head developed by centrifugal action is entirely due to velocity of rotating impeller and not due to any impact or displacement. Hence, the pump is named after this action on liquids.

Parts of Centrifugal Pump

Figure 2: Centrifugal Pump

The main components of a centrifugal pump are as follows,

1. Impeller
2. Casing
3. Suction pipes
4. Delivery pipes
5. Foot valve and strainer
6. Delivery valve
7. Pressure gauges
8. Foundation base
9. Foundation locking bolts
10. Air valve, funnel etc.

Working of Centrifugal Pump

The first step in the operation of centrifugal pump is “priming”, it means removing all the air or gas or vapour from pump portion by filling water in suction pipe, casing and portion of delivery pipe up to delivery valve. Afier priming, close the delivery valve and start electric motor to rotate the impeller. Rotation of impeller in casing receives mechanical torque and produces forced vortex, which imparts a centrifugal head to water and causes increase in pressure. When delivery valve opens, the water is made to flow rapidly in outward direction. At the eye of the impeller, vacuum is produced due to centrifugal action, this helps to suck the water from sump (source of water) through suction pipe. The higher pressure water leaves the impeller and flows through delivery pipe to required height.

Classification of Centrifugal Pumps

The types of centrifugal pumps as,

According to the type of casing,

1. Volute pump
2. Diffuser or turbine pump.

According to the number of impellers per shaft,

1. Single stage centrifugal pump
2. Multi stage centrifugal pump.

According to the direction of fluid flow;

1. Radial flow pump
2. Axial flow pump
3. Mixed flow pump.

According to the number of entrances to the impeller,

1. Single suction pump
2. Double suction pump

According to the disposition of shafts,

1. Pumps with horizontal shafts
2. Pumps with vertical shafts.

According to the head developed,

1. Low head
2. Medium head
3. High head.

Advantages of Centrifugal Pumps Over Reciprocating Pumps

1. Initial cost for installation of centrifugal pump is low.
2. Installation and maintenance of centrifugal pump is easier and economical as it has small number of components.
3. It is very compact and requires less space.
4. It is a high speed pump and can be directly coupled to electric motor or oil engine.
5. Discharging capacity of centrifUgal pump is higher than reciprocating pump.
6. Discharge through the centrifugal pump is uniform.
7. Performance of centrifugal pump is superior than the reciprocating pump.
8. Torque is uniform for centrifugal pump.
9. The efficiency of centrifugal pump at low heads is quite high.
10. It can be used for lifling high viscous fluids like oil, paper pulp, sugar molasses, etc.
11. There is no danger of separation and cavitation of liquid in the pump.
12. For same capacity and energy transfer the gross weight of centrifugal pump is less than the reciprocating pump.

Applications of Centrifugal Pumps

1. For agricultural and irrigation purposes.
2. Thermal power plants.
3. For drainage and drinking water system.
4. Process industries such as paper pulp, chemicals, petrochemicals, pharmaceuticals to convey raw material and finished products from one place to another place.
5. Food industries
6. Water Purification plants.
7. Oil and gas industries for purification purposes.
8. Metal treatment processes like electroplating, anodizing etc.
9. Textile industries for bleaching of fabrics.

Volute Centrifugal Pump

Figure 3: Volute Pump.

In ‘volute type’ of centrifugal pump, the casing is of spiral shape that gradually increases the area of flow (Figure 3). As shown in figure, the liquid is sucked through the centre of the impeller, which is rotated by motor.

When the impeller starts rotating, liquid present at eye of the impeller is pushed towards the periphery. The liquid flows through a gradually widening channel. As a result of it, its velocity reduces with corresponding increase in pressure. Thus, volute chambers are the ducts alTanged around the impeller that reduce the exit velocity and convert velocity head into pressure head.

Diffusion Centrifugal Pump

Figure 4: Diffusion Pump.

In this type of pump, a diffusion ring surrounds the impeller (Figure 4). The ring consists of series of guide blades or diffusers. Liquid from the impeller eye is forced towards the periphery through the openings between the guide blades. When the liquid flows through the diverging passages between the guide blades, its velocity reduces with corresponding increase in the pressure. As a result, the velocity head is converted to pressure head. Diffusion pumps are also known as turbine pumps as these resemble Francis turbine with inlet guide blades

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Construction of Hydraulic Dynamometer

A hydraulic dynamometer contains a rotating disk that is attached to the driving shaft of the test machine. The disk has semi elliptical grooves which allow water steam to flow through them. A stationary casing mounted on antifriction bearings (also known as trunnion). It has a braking arm and a balance system is fixed to it so as to make the casing rotate freely. However, this movement is restricted by the braking arm. Same as the rotating disk the casing also contains semi-elliptical grooves (recesses) on it. These two elements are arranged such that the rotating disk rotates inside the casing. The schematic of hydraulic dynamometer is shown in figure 1.

Figure 1: Hydraulic Dynamometer

The semi-elliptical grooves of the disk match with the corresponding semi-elliptical recesses of the casing. Therefore, a chamber is formed through which liquid flow is maintained. As the driving shaft of the prime mover rotates the liquid follows a helical path in the chamber which causes vortices and eddy currents to develop in the liquid which inturn causes the casing of the dynamometer to rotate in the direction of the shaft.

The braking action is varied and controlled either by changing the distance between the casing and disk or by changing the amount of water and its pressure. The power absorption is maximum if the casing is full and is minimiun if the amount of liquid is minimum. The total power absorption of this device changes approximately as,

1. Cube of rotational speed
2. Fifth power of rotating disk diameter.

The absorbing element contains a force sensing element such as load cell placed at the end of the arm/whose radius is ‘r’. Therefore the exerted torque is given by,

Torque (T) = F × r

Where,

F — Force measured at radius r and

Power is given by

P=2πT

Advantages of Hydraulic Dynamometer

The hydraulic or water vortex dynamometer is more popular because of the following reasons Advantages).

1. It does not require any external cooling arrangement because water itself serves as coolant.
2. It has high absorption capacity even in a small space.
3. The instrument can be protected from hunting effects if it is provided with dashpot-damper system.
4. It is economical.

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Types of Unloading Valves

Unloading valves are of two types. They are,

1. Pilot operated unloading valve
2. Direct acting unloading valve

Pilot Operated Unloading Valve

Figure 1: Pilot operated unloading valve.

This type of valve cannot allow the pump flow directly to the tank like the direct acting type valve. It is siiiìilar to the pilot-operated relief valve. The only difference is that a movable spool is used in pilot-operated unloadmg valve as an addition part. It consists of an inlet port and the outlet port connected to pump and tank respectively. A movable spool and spring loaded ball is placed in pilot section. A piston with spring load is placed in the main chamber and a narrow orifice is provided through the piston. A remote-pilot line is arranged inside the valve, connected to the pilot section. A pilot line is connected to the outlet port. A pilot-operated unloading valve is shown in figure 1. When the pressure in the circuit exceeds the valve setting, high pressure signals reach the unloading spool through remote-pilot port. Due to lower back force, spool moves rightward thereby, spring loaded ball opens completely. A small amount of fluid starts to flow to the tank through the pilot line and remaining fluid flows through the piston orifice. When the flow starts in the pilot line and the orifice, the pressure drop takes place across the piston. Due to pressure drop, the piston moves up against the spring force and the outlet port will open. Then, fluid flows freely from pump to the tank with small or zero pressure.

Direct Acting Unloading Valve

It consists of an adjustable spring moves in the control chamber and a movable spool held in closed position in the main chamber by the spring force. The inlet and outlet ports are connected to the pump and reservoir (tank) respectively. A drain is provided at control chamber to return the leaked fluid past the spool into the reservoir. It is a normally closed type valve, i.e., the outlet port is closed by the spool in normal condition, so that the spool offers the resistance to flow from the pump to the tank. Initially, the valve restricts the flow through outlet. The fluid from pump enters the external-pilot port with high pressure where the fluid offers a force against the pilot piston. When the pressure at the pilot port reaches to the spring load, the fluid will deviate from pump to tank. If the fluid pressure exceeds the spring force, then the spool in the main chamber moves against the spring force, thus, opening the outlet port. Then, the fluid flows directly to the reservoir through the outlet port with small or zero pressure.

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Working of Pressure Sequence Valve

It consists of inlet and outlet ports which are connected to the pump and cylinder ‘A’ respectively. It also comprises of a secondary outlet port connected to cylinder ‘B’. The schematic arrangement of pressure sequence valve is shown in figure 1.

Figure 1: Pressure Sequence Valve.

Construction of Pressure Sequence Valve

When the valve is in its normal position as shown in figure 1, the fluid flows freely through the inlet port to outlet port. Flow to cylinder ‘A’ continues until its working stroke is completed. When the pressure in the system components connected reaches the preset value on the sequence valve, the control signal line transmit this pressure to the movable spool. Then, the spooi lifts against the spring force and secondary out port will open and outlet port will close (i.e., flow to the cylinder A is restricted). Thus, the fluid flows to cylinder B. When the outlet port is closed by spool, there is a possibility of leakage of fluid into the spring chamber. The leaked fluid will send out through the drain line. This type of valves are used in clamping and machining circuits.

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Figure 1: Compound Pressure Relief Valve.

Construction of Compound Pressure Relief Valve

The compound pressure relief valve consists of a small pilot operated relief valve with poppet arrangement and a main relief valve (Figure 1). It also consists of two ports, of which is one is pressure port, and the other is tank port. Pressure and tank ports are connected to the pressure line from the pump and the tank respectively.

In pilot relief valve, poppet is held in its seat by the spring action and an adjustment screw is provided to set the spring force at a value equal to desired operating pressure. The main relief valve consists of a piston with stem and piston has a drilled orifice. Both top and bottom surface areas of piston, on which the pressure acts is equal, hence it is in balanced condition by hydraulic forces in its closed position. To ensure that piston is closed properly, it is provided with light bias spring.

Working of Compound Pressure Relief Valve

The pressurized liquid flows to system when the system pressure is less than the preset spring force. When pressure in the system raises above set pressure, the pilot poppet moves away from its seat. As the pilot relief valve is opened, a small amount of flow go through the pilot line to the piston stem and back to the reservoir. A pressure difference across the piston occurs because of restricted flow at piston orifice. As a result, piston and stem move upwards from their seats and thereby, opening the tank port. Thus, the flow from the pump goes directly into the tank until the pressure of the system gets reduced to design pressure.

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Types of Servo Valve

It is classified into two types.

1. Electro hydraulic servo valve.
2. Mechanical hydraulic servo valve.

Electro hydraulic servo valve

Electro hydraulic Servo valves can be single stage or multistage, where the movement of valve spool is proportional to the input electric signal. The figure 1 below shows the multistage hydro electric servo valve which mainly consists of torque motor, double nozzle and a spool.

Figure 1: Electro hydraulic servo valve.

Construction of Electro hydraulic Servo valve

In the electric torque motor. the two coils are winded over the armature and the magnets are arranged below and above it. The flexure tube is used to prevent the fluid flow into the electromagnetic field. Flapper is connected at the armature centre and extended down to the nozzle area through flexure tube. Two nozzles are provided on either side of flapper and corresponding inlet orifices are provide at the ends of filter as shown in figure 1. The feedback wire is connected to the spool and armature such that the spool displacement can be controlled based on the required flow rate.

Working of Electro hydraulic Servo valve

The pressurized fluid is passed through the filter and supplied to the nozzles by any one of the inlet orifice. The clockwise or anticlockwise torque developed on the armature due to electric current causes the flapper displacement between the nozzles. The nozzles openings are varied by the motion of flapper. Thus, the variation in flapper motion produces pressure difference between two ends of spool which causes the spool displacement either to left or right direction. The fluid flows to the outlet load sections with required pressure and quantity, as directed by the spool movement. The feedback wire which is connected to the spool applies reverse torque (counter clockwise or clockwise) on the armature and flapper returns to its centre position. Thus, the spool remains in its position for the next input electrical signal.

Applications of Electro hydraulic Servo valve

The hydro electric servo valves are used in following various industrial and automobile applications

1. Farm machinery
2. Cargo handling cranes
3. Earth moving vehicles
4. Logging equipment
5. Fire and lift trucks.
6. Vehicle servicing equipment.
7. Pivoted arm devices.
8. Steel mill industrial control etc.

Mechanical Hydraulic Servo Valve

Mechanical-Hydraulic Servo Valve: In this type of servo valve, the required amount of force is applied to valve spool through force amplifier such that the spool movement opens the port A and shuts the port B. The fluid from the tank flows into the hydraulic cylinder through port A and pushes the piston towards right as shown in figure 2. The feedback link is arranged such that, it forces the sliding sleeve in right side direction to restrict the flow to the cylinder. Due to this, the desired amount of output motion is obtained for a given input force. It is a closed loop system and is used in automobiles hydraulic power steering system.

Figure 1: Mechanical hydraulic servo valve.

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Figure 1: Internal Gear Pump.

This gear pump consists of a stationary part called crescent shaped seal, which is used to separate the gears and also acts as a seal between the suction port and the discharge port (Figure 2).

Figure 2: External Gear Pump.

Working of Internal Gear Pump

Figure 2: Internal Gear Pump working.

Among the two gears, the internal gear is connected to the driving shaft and power is transmitted to it. Therefore, the internal gear is the driving gear and the external spur gear is the driven gear. During the rotation of the two gears, they appear to be coming out of mesh at the inlet port. This creates the suction and causes the fluid to enter into the pump. The fluid is completely filled around the crescent seal and the gear spaces. The rotation of gears transfers the fluid from inlet port to outlet port.

At the outlet port, the two gears comes into mesh. This decreases the space between the two gears and increases the pressure. which forces the fluid outside the pump system.

Advantages of Internal Gear Pump

1. It can be used for high viscosity fluids.
2. Less moving parts.
3. Uniform discharge irrespective of varying pressures.
4. It is a bi-directional pump.

Disadvantages of Internal Gear Pump

1. The parts used should have high dimensional accuracy.
2. Lubrication is required.
3. Shaft bearing experiences an overhung load.

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