Pressure reduction valve is a control valve that lowers the inlet pressure of the fluid and gives the desired pressure at the output. The pressure reduction valve maintains a constant pressure in a part of the system that operates at a pressure lower than the normal system pressure. Often we use a variable pressure reduction valve to set it to any desired downstream pressure within its design range. Once the valve is set, it will maintain reduced pressure regardless of changes in supply pressure and system load variations.

Types of Pressure Reduction Valve

There are numerous designs and types of pressure reducing valves. But most common are:

• The spring-loaded pressure reducing valve
• The pilot-operated pressure reducing valve

Spring Loaded Pressure Reducing Valves

Spring-loaded pressure reducing valve structure and function

The spring-loaded pressure-reducing valve is most commonly used in hydraulic and pneumatic systems. It is generally referred to as a pressure regulator. The valve is normally open all the time. The adjusting spring pressure acts against a diaphragm to open the valve.

When the pressure at the outlet is less than the spring pressure, the spring pressure overcomes the outlet pressure and pulls the valve stem downward. Thus the valve will remain open.

When the outlet pressure is greater than the spring pressure, outlet pressure overcomes the spring pressure and pushes diaphragm and stem upward. Thus the valve will be close.

We can adjust the outlet pressure of the valve by adjusting spring pressure on the diaphragm. Normally, when we turn the adjusting screw clockwise, we are increasing the downstream pressure.

Pilot-Operated Pressure Reducing Valve

The simple diagram above illustrates a pilot operated pressure reducing valve. This valve is similar to the spring-loaded valve in addition to the pilot-controlled valve. High pressure will come from the pump to the inlet port. Normally due to the open condition, the oil will flow to the outlet port. Actually, the pressure at the outlet depends on the spring pressure of the reducing valve.

When pressure builds up at the outlet it will act on the bottom of the main spool. Simultaneously, a pilot pressure acts on the ball against a spring-2 and spring chamber through an orifice. This pressure keeps the main spool open until the pressure at the outlet port is less than the pressure at Spring-2. If the outlet pressure is above the set pressure of spring 2, the ball will move forward and the pilot line will connect with the tank. This will empty the spring chamber and the main spool will move upward and close the inlet port. Thus, a pilot operated pressure reducing valve works.

Pressure reducing valve Symbols

References

• ScienceDirect

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In brief, the hydraulic direction control valve is a device to direct the flow of fluid in a hydraulic or pneumatic system. It makes the hydraulic system to work repetitive.

As shown in figure 1. the pump will make the fluid to flow and extend the piston and does work. But, after complete extension, we can not retract the piston which is not intended. Therefore, we need something which makes our system repetitive. This is Direction control valve or in short DCV.

In the above figure 2. a hydraulic direction control valve (DCV) is used. In the central position, the cylinder will stay in its place and valve will be called in 0 positions or in the rest position. However, if it changes the position from 0 to b cylinder rod will extend. Similarly, when direction control valve changes its position from b to a cylinder rod will retract. Thus Hydraulic direction control valve makes the system repetitive and useful and we can get benefited to continue work.

Graphical symbol

Although the graphical symbol of the direction control valve depends on the number of port and number of position, however, figure 3 shows a general idea of the symbols. Here the number of square boxes indicates the number of positions whereas arrows indicate the direction of flow through the valve. Similarly, Closed paths show close ports. A and B are the working ports. P is the port connecting with the pump while T is the port connecting to the tank. So, this is a 4/3 direction control valve with 4 ports and 3 positions.

Here are some symbols of various hydraulic direction control valve

2/2 Direction Control Valve

3/2 Direction Control Valve

3/3 Direction Control Valve

2/2 Direction Control Valve

4/3 Direction Control Valve

5/2 Direction Control Valve

5/3 Direction Control Valve

6/3 Direction Control Valve

Controlling the hydraulic direction control valve

The hydraulic direction control valve can be controlled in various ways. However here are some mentioned.

1. Mechanically controlled.
2. Electrically controlled.
3. Pneumatically controlled.
4. Hydraulically controlled.
5. Controlled by combination of any of above.

The direction control valve either can be directl controlled or can be pilot controlled.

Illustrated figure. 4 is the example of 4/3 spring centred lever operated direction control valve.

Illustrated figure. 5 is the example of 4/3 spring centred electrically operated direction control valve.

Illustrated figure. 6 is the example of 4/3 spring centred electro-pneumatic operated direction control valve.

Illustrated figure. 7 is the example of 4/3 spring centred electro-hydraulic operated direction control valve.

Illustrated figure. 8 is the example of 4/3 spring centred electro-pneumatic operated direction control valve.

Hydraulic direction control valve types

1. Spool type
2. rotary type

Spool type

Rotary type

Spooling Position of 4/3 hydraulic direction control valve

1. Open centre: In this 4/3 DCV all the ports are connecting with each other in centre position.

2. Close Center: In this 4/3 DCV all the ports are blocked in center position.

3. Float center: In this 4/3 DCV pressure port is block but working ports are connected with the tank in cente position.

4. Tandem Center: In this 4/3 DCV, pressure port and tank port are connected in center position but working ports are blocked.

So, this is the brief description of Hydraulic direction control valve. Hope you learned something.

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You have read in article Hydraulic basic principle-part I how hydraulic jack works. Now consider the following example

Pressure will build up when pumping the lever on a car jack. The load will lift. But what would happen here if you let go off the lever? The piston will come again to its initial position. But your requirement is to lift the load and hold it in lifted position. For this you need to maintain the built-up pressure.

The solution is non-return valve or check valve. It will allow the flow in only one direction but in opposite direction it will restrict the flow. In the above jack a flap valve is used as a non-return valve.

Similarly, in hydraulic system we use the check valve or non-return valve to maintain pressure and to allow the flow in only one direction.

Spring Loaded Check valve

Most of the check valve is coming with a spring. This check valve type is called spring loaded check valve. The function of a spring-loaded check valve is to open and allow flow in one direction only when pump-side pressure is more than the spring pressure.

Here in figure 5, it is a ball type check valve. However, there are various design of check valve. Some are poppet type, disc type and diaphragm type.

Pilot operated check valve

Function of pilot operated check valve or reversible check valve is quite interesting. As the check valve does not allow reverse flow but reversible check valve will allow only when there is a sufficient pressure in control line to open the ball. See figure 6.

In figure 6 when pump is on and direction control valve in position b then cylinder will extend against the load. The pump will built pressure. First check valve behaves like normal check valve but second check valve will restrict the return line flow from rod side. But the pressure line will control the pilot line and open the ball to allow return line flow to tank. Hence, a control pressure is required to allow flow in reverse direction.

Hope You enjoy this artical and learnt somthing. Keep reading and stay in touch with us.

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In this article we will learn about the hydraulic tank basic concept, its design, tasks, components, the importance of pressure ratio in a hydraulic tank, which component is responsible for which task & hydraulic tank types. So, be ready for an adventurous journey of knowledge.

Hydraulic fluid is the media that transfers force with the help of pressure & flow. Therefore, the amount of hydraulic fluid is very important in a hydraulic system and it depends on force multiplication and motion multiplication. For example,

We have read in How hydraulic works: Basic- Part-I that how simple hydraulic jack works and help to lift heavy load such as car’s wheels. For example, in figure 2 this amount of hydraulic oil is not sufficient for lifting tire enough to change. So, what we can do?

Here is some solution for that situation.

1. Either lengthen the stroke of the pump. Means increase the pump’s cylinder length.
2. Either reduce the area of the lifting cylinder.
3. Top up the hydraulic fluid and pump again.

By the first method, you can make available more hydraulic fluid to pump, however, it is not convenient because it makes the jack bulkier, bigger and heavier. So, it is not an applicable daily practice.

The second answer, reducing the area of lifting cylinder will reduce the consumption of hydraulic fluid and cause long-stroke, however, it is not convenient because now force multiplication is less and more force required.

The third solution is best and suitable. we can top up the fluid from a separate supply of fluid and pump again as much we need.

Introduction of the hydraulic tank in the system

In this diagram, we are seeing a hydraulic tank that provides fluid to the system whenever needed and directed back to the hydraulic tank when it is no longer needed. In this diagram, we also see two flap gate (check valve)
To lift the vehicle, the hydraulic fluid is pumped out of the hydraulic tank and into the lifting cylinder while these flap gates prevent the fluid to push back into the pump cylinder or to the tank. And for lowering the lifting cylinder there is another valve in a separate return line call the coke valve that needs to be open which will direct the fluid back to the tank.

But the hydraulic tank is not only for storing the hydraulic fluid. Now we will find a more significant purpose of a hydraulic tank.

Hydraulic tanks do not just hold hydraulic fluid; they often full fill other tasks, such as:

• Calming the hydraulic fluid.
• Gas purging.
• Cooling Hydraulic Fluid.
• Cleaning the hydraulic fluid.

Calming hydraulic oil is one of the most important tasks for a hydraulic tank. The liquid might flow very quickly back into the tank. That can cause the formation of foam and air inclusions. The pump should not suck them in. That is why the tank has to be constructed so that the flow calms down well.

Gas purging is also one of the hydraulic tank’s tasks. The hydraulic fluid can contain air, for instance in the form of tiny air bubbles. This air in the tank must be able to escape from the liquid as much possible.

While we performing any hydraulic operation, hydraulic fluid gets heat up due to friction and it may also heat up due to working in high-temperature environments such as processing of hot red metal. Therefore, the hydraulic fluid must be cool down somewhere in a hydraulic system and this task is being done in a hydraulic tank.

In the hydraulic system, there are plenty of moving parts. Due to these moving parts wear and tear occurs. Also, seals will damage. These all unwanted particles are carried by the hydraulic fluid as dirt. So, the filtering of hydraulic fluids becomes much more important. Without filtering, if hydraulic fluid is used it may cause some catastrophic damage.

Now, we will see the basic design and component of hydraulic tank.

Basic Design of hydraulic tank or reservoir.

Suction Line:

The suction line connects the hydraulic tank to the pump. After that hydraulic fluid goes to the circuit. Suction pipes generally have a larger diameter as compared to return lines. Suction pipes have a smaller length so as to facilitate lesser loss and cavitation.

Return Line:

After performing the necessary task hydraulic fluid comes back to the tank by return line through the return manifold and return filter. The return line end should not be too high from the bottom of the reservoir as the too high an outlet will cause turbulence to the output flow. Normally the return line ends are taper cut to facilitate the flow direction towards the wall side. This will help the fluid to travel a larger distance so that it gets more time to cool.

Access Plate:

the access plate is for cleaning the tank.

Suction Strainer:

The strainer is placed in the suction line of the pump. This provides filtered hydraulic fluid to the pump. It typically has some filtering screens (for particles of a particular size) that filters the contaminants before going to the pump. This is periodically replaced to ensure uninterrupted flow.

Baffle Plate: 

Baffle plates are normally the steel plates that are incorporated in the reservoir to divide the fluid into different chambers so that the fluid has to travel through other ways to get to the suction chamber from the return flow chamber. As a result, oil gets cool and contaminants set at the bottom of the chamber. It gives them time to hydraulic oil for suppressing, calming, and gas purging.

Breather:

This is the opening for equalization of air pressure when fluid level changes.  A filter for a breather protects the fluid from dirt. It is important to understand that too small a breather cap or choked filter will cause a vacuum in the oil tank and cause cavitations in the pumps. So, it needs extra care.

Oil level gauge:

In a hydraulic tank, an oil level gauge shows the level of oil. commonly there are two lines in gauge. one for a high level and one for a low level.

Filler cap:

The filler is for top-up the hydraulic oil.

Next, we will see Processes in the hydraulic tank and combine all tasks of a hydraulic tank.

Processes in the hydraulic tank

The return-line filter first cleans dust particles from the recycled hydraulic fluid. The liquid still has a strong current upon entering the re-circulation area. The calming baffle suppresses the flow’s motion and prevents it from spreading to the suction area. Any air bubbles that might be present rise to the surface of the liquid and escape into the tank. Excess air escapes from the tank to the outside via a breather (ventilation and exhaust). The liquid flows into the suction area through openings in the calming baffle’s floor area. The liquid dissipates heat to the outer air over the hydraulic tank’s entire surface, thereby cooling it.

Types of hydraulic tank or reservoir

For the proper function of a pump, the pump needs a continuous feed of fluid. And therefore, the pressure at the suction line must be great enough. When the tank is full, the pressure at the inlet will be great enough (liquid pressure + atmospheric pressure). That is why much fluid is available for suction but if the fluid is just above the inlet then the pressure will be less enough and cause less fluid available for suction which can create a cavity in the pump.

In this way, we can categories hydraulic into two types.

1. Atmospheric tank
2. Pressure tank.

Atmospheric Hydraulic Tank

Atmospheric Tank: Because of the atmospheric tank is open with the atmosphere via a breather so, pressure on the suction line depends only on the liquid level. Hence, we can not increase the pressure anymore. The total pressure will be the sum of pressure due to liquid level and atmospheric pressure

Pressurised Hydraulic Tank

Pressure tank: In a pressure tank firstly, pressurized air is fed from external to maintain the required pressure, and the second thing pressure regulator is added for regularising the pressure. As a result, if pressure exceeds the required pressure a spring-loaded check valve releases the pressure and if pressure is lowered below the atmospheric second check valve will allow atmospheric air into the tank.

We have completed all the basics of hydraulic tank and their types. Hope you learned something and will come soon for another interesting article. Till then bye-bye. Thank you.

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Pressure Multiplication

As we read in the previous article ‘How Hydraulic works: Basics Part-I‘ we knew how force is multiplying in hydraulic system. But its reverse is similarly correct. In other words we can multiply pressure according to our use i.e Pressure Multiplication.

Sometimes for industrial use we have to increase the pressure without increasing the force, like high pressure pump for fuel injection. Then we can use this method for increasing pressure with the help of low pressure.

We know that Pressure,

p = F/A

Let the force ‘F’ acts over an area ‘A1’ and then reduce the area by ‘A2’ it will increase the pressure or we can say we multiplication of pressure.

Let, p1 = 10 bar (Low pressure available)

Note: bar is the unit of pressure usually use in industrial application.

1 bar = 100000 N/m²

A1 = 100 cm²

A2 = 1 cm²

F1 = p1 X A1

F1 = 10 x 100 = 1000

Same force will act on area A2,

p2 = F/A2

p2 = 1000/1 = 1000 bar.

Hence, we multiplied the pressure 100 times with the available pressure. Similarly like we multiply the force.

Pressure multiplication in double acting cylinder.

Here is one of the reason for cylinder seal damage. As per the pressure multiplication theory you know even there is low pressure in larger area (piston end) pressure may be much higher in other end of the piston (rod end) due to less area available.

Now, suppose if system pressure for preventing the damage of cylinder seal is 250 bar. This mean maximum pressure should not increase 250 bar ever. But due to some reason your rod end is block and now hydraulic oil is not flowing from rod end. If you have set pressure relief valve on 250 bar and at your piston end you have maximum 250 bar then definitely due to pressure multiplication, the rod end pressure will much higher then 250 and ultimately damage the seal.

In conclusion, if we are using pressure multiplication according to our purpose then it is beneficial but however it has negative effects too. So, be care full and must consider this effects while setting maximum pressure of system.

This pressure multiplication is also known as Pressure Intensification.

Hope you enjoyed this article. Please do share and support.

Till then, Good Bye.

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Before going to understand Hydraulics one must have some knowledge of Physics. Because, it will help for better understanding of basic concepts, how hydraulic works and where to use it. Read this article and after that you will be able to understand some basic physics & basic hydraulics. So, have fun.

Some basic physics which we need for basic hydraulics are
Mass, Force, Pressure, Work, Energy, Pressure Ratio, Force multiplication.

Mass:- The amount of matter present in an object is call mass.

Force:- It is the push or pull which can change or tends to change the state of rest or uniform motion of a body, change the shape of the body or change the direction of the motion.

Pressure:- It is force applying perpendicular to a surface per unit area on a body.

Work:- Work is said to done if force is applied on an object and it gets some displacement in the direction of force.

Energy:- Energy is the ability of doing work.

Now suppose your vehicle’s tyre gets flat. what do you do? Of course the first thing you try to find a jack. If you found a hydraulic jack it would be more easier for you to lift the vehicle and change your flatted tyre.

Hydraulic Jack

In this simple Diagram you can see a small piston and a big piston. we apply a small force in small piston as in our hydraulic jack but it lift heavy load like car.

As we know pressure is equal to perpendicular force on a surface per unit area. i.e: P=F/A. means larger the area lesser the pressure or vice versa.

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