Directional control valves start, stop or change the direction of flow in compressed air applications. They are one of the most fundamental parts in hydraulic machinery as well and pneumatic machinery. They allow fluid flow into different paths from one or more sources. They usually consist of a spool inside a cylinder which is mechanically or electrically controlled. The movement of the spool restricts or permits the flow, thus it controls the fluid flow. Many manufacturers apply compressed air as the power to operate tools and equipment which are used to make the products they offer. To understand the different applications of compressed air and how valves are used, one must first have a knowledge of the kinds and types of valves used by industries.T ypes of pneumatic directional control valvesWhen choose a pneumatic directional control valves, below factors need to be considered:' Working environment' Acting type (3/2 way, 5/2 way, 5/3 way)' Working pressure ' Working temperature' Fluid type ( air, water, oil, etc)' Size (thread size)' Approvals needed (for example: Department of Transportation, Underwriters Laboratory, FDA)
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Miniature pilot valvesMiniature pilot valves are one of the most fundamental parts in hydraulic machinery as well and pneumatic machinery. They allow fluid flow into different paths from one or more sources. It not only has a micro size, but it's very quiet when it works.Features:1. Fluid type: filtered air2. Working pressure: 0-7 bar3. Acting type: 3/2 N.C.; 3/2 N.O.; 2/2 N.C.4. Working pressure: -10-80 '5. Pipe size: M56. Advantage: smaller volum, low power, wide application, quick response, long life.7. Disadvantage: The biggest orifice is 1mm, and the flow rate is 100L/min, maybe it's limited in some areas.
Directional solenoid control valves Directional solenoid control valves are common in pneumatic systems to control fluid direction within a pneumatic circuit. They are controlled by the action of the solenoid and typically control the compressed-air to cylinders, rotary actuators, grippers, and other mechanisms in packaging, handling, assembly, and countless other applications. The four main functions for directional-control valves are 2/2 (two-port, two-way), 3/2, 5/2, and 5/3.Features:1. Fluid: Compressed air2. Working pressure: 1.5-8 bar3. Acting type: 2/2 way, 3/2 way, 5/2 way, 5/3 way4. Joint size: M5, 1/8', 1/4', 3/8', 1/2'5. Advantage: Compressed air can result in huge forces available to act on another valve and be recycled after its done its job, so it doesn't add very much weight to the mechanism.
TIPS
When choose the directional solenoid valve, the action required by your application will determine the type of valve needed. How much flow do you need at the output port of the valve? It is important to know the required flow rate of your application in order to correctly size a directional control valve. If the valve is too big for the application, you will be wasting air & money. If it's too small, the actuator will not function properly.
Drectional pneumatic control valvesDirectional pneumatic control valve controls at a predetermined pressure setting, the switching pressure is generally a variable setting on the valve itself. A change in position of the directional pneumatic control valve (i.e. open or closed) occurs either gradually (by control) or suddenly (by switch). Directional pneumatic control valves can be subdivided into control and switching task.Features:1. Fluid type: compressed air2. Working pressure: 1.5-8 bar3. Acting type: 2/2 way, 3/2 way, 5/2 way, 5/3 way4. Pipe size: 1/8', 1/4', 3/8', 1/2'5. Advantage: The valve being tripped through the use of compressed air (gas). At a particular point in an industrial or manufacturing process, compressed air is released, causing a valve to open or close.
Mechanical valve Mechanical valve relies on a lever, push button, or knob to shift a valve. Manually actuated valves are often used when a machine requires human intervention, or when a machine needs pneumatic output to start another process.Features:1. Operating Pressure: 0-8 bar2. Operating Temperature: -5~60 '3. Application medium: compressed air4 Joint size: 1/8', 1/4'5. Advantage: Fluid entering the valve from any working port does not affect spool movement. Low-force solenoids can be used because the most they need to overcome is mechanical friction and light springs.
TIPS
Mechanic directional valve is widely used for pneumatic industries, when you choose it, one important point is to confirm if the fluid will bypass the seal. When using any media other than air, it is important to review the materials of construction to ensure that the internal components and seals are compatible with the fluid passing through the valve.
Hand lever/ Foot pedal valveHand/Foot valve is ideal for manual and direct control of pneumatic actuators. Sturdy, lockable, position detection, different outlet port orientation and a practical labeling feature characterize this advanced manual hand lever/ foot pedal valve.Features1. Fluid Type: compressed air2. Working Pressure: 0-8 bar.3. Working Temperature: -5~60 '4. Joint size: 1/8', 1/4', 3/8', 1/2'5. Advantage: Ideal choice for the direct manual control of stand-alone actuators and can be used in clamping, pressing, conveying or feeding operations.
TIPS
When choose the hand lever valve / foot pedal valve, you need to know the method to actuate the valve, by hand or by foot, it can help you save much energy and time. At last, the price also a key point to choose the valve, compete many suppliers and choose the suitale one.
However, how much thought has been given to whether the specification is right, the valves will be suitable, and if they will be right for the job?
There are many assumptions around valve selection, the most common of which are:
'this is what we used before' ' on an entirely different process
'contractor x said this is what we need to use' ' based on what?
'the site standard is x' ' but what if that type of valve wont work on this application
'this is what we have always used' ' but have been experiencing downtime and regular failures
Getting your valve selection right from the get go is really important and in this article I will walk you through a simple 10 step process to help you ensure your valve selection is based on facts rather than assumptions.
1. What is the purpose of the valve?
Valves can be used on various different duties and perform a variety of different functions. The best starting point when selecting any valve for industrial use is to decide what purpose it will serve. Some things to consider might be:
Will it be used to control or isolate media?
Will it be used many times a minute or every few months for maintenance?
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How critical is it to your process and what would happen if it failed?
Is it a valve that will need to work in an emergency?
Having this clear in your mind will make it a lot easier to select the right valve for the job!!
2. What is the pressure of the media going through the valve?
In deciding what valve will be most suitable for your application you should always ascertain the pressure. On high pressure applications, specialist valves will be the norm, and these types of valves typically have thick wall diameters to cope with the pressure. They also have specialist sealing arrangements like you would find on Trunnion Mount Ball Valves. On low pressure applications where bubble tight shut off is not imperative, a valve may not be the best solution at all. A good example of this is on low pressure air/exhaust applications where a 'damper' would be more cost efficient and lighter in weight.
3. What is the temperature of the media going through the valve?
Temperature is also a minimum requirement for selecting Valves. Valves that are seeing extremes of temperature will often be 'bespoke' as opposed to 'off the shelf'. The two main considerations with temperature, are (a) whether the materials of construction will be suitable and (b) whether the valve will seal against the temperature. Your supplier can advise you on the most suitable body material and sealing design. A good reference point here is the use of pressure/temperature charts which show how the temperature limitation of a valve will decrease as the pressure rises. These are available for most types of valves.
4. What media will be going through the valve?
Some things to consider when it comes to media:
There are a lot of questions around media but ascertaining this at the outset and communicating this to your supplier will mean the right valve is selected and minimise downtime as a result.
5. What are the environmental conditions where the valve will be situated?
Not all valves are going to be situated indoors in an ambient, dry and dust free environment. Therefore think about where your valve will be fitted. Could it be on the end of a Jetty in a saline environment? In the middle of a dry, sandy desert in extreme heat for most of the day? Submerged in water? It isn't the end of the world if the valve location is not hospitable. It just means that the materials and particularly the exterior coatings and internal seals will need to be suitable to withstand the environment.
6. Is the valve likely to be noisy?
Sometimes valves can be noisy and a good example of this would be a Control Valve on high pressure steam. This doesn't matter if the valve will be located inside in an area that is well noise insulated, but could be an issue outside when in close proximity to houses and other buildings. Low noise trims are available on such valves to mitigate some of this noise but its best to get this clarified at design stage as it can be something that catches engineers by surprise.
7. Will the valve be manually operated or automated?
Most modern plants are moving towards automation, but in some instances valves that are rarely used, or just used for maintenance or isolation are kept as manual valves. Where you are looking to automate a valve, there are a few points worth considering:
8. Do you need to use the valve for control?
Control is easily achieved in an actuated valve and the easiest way of achieving crude control on a ball or butterfly valve is with a 4-20mA Electro Pneumatic Positioner or a 4-20mA Potentiometer Card (in the case of an Electric Actuator). Another great option for greater control is a V-Ball or Ball Sector control valve. Where very accurate control is required, it will be worth considering a rotary control valve, or linear globe style control valve which is sized on a bespoke basis. These will generally be more costly but where control is critical they are generally the best option.
9. Valve dimensions.
Sometimes valves arrive on site for fitting and at that point the fitters realise that there is not enough space. The hand-wheel fouls on adjacent pipework, the valve actuator is hitting a tank, or the lever is hitting some steelwork. The majority of valves have a dimensional datasheet available, so always get a copy of this and double check that the valve will fit into the space you have available. There is nothing more disheartening than that call from a frustrated fitter with a hefty quotation for altering pipework.
10. Weight & Supports
Valves by their very nature can be heavy ' very heavy in fact. Large bore valves with actuators and additional controls can weigh over 500kg. This puts quite a lot of stress on pipework, so always factor this in when you are designing a system that is likely to have large valves installed. A common place where this can become an issue is ducting and HVAC type installations where the ducting is quite lightweight. As with valve dimensions your supplier should be able to supply you a datasheet with valve weights in kilograms. With actuated assemblies they will calculate for you a weight with actuator, valve, switchboxes and other ancillaries combined.
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