Inline valves are probably the most common type of valve used in industrial automation systems; as their name suggests they fit directly in the individual line of each application.
A manifold has a definition, “a pipe or chamber branching into several openings”. For pneumatics applications this means that several valves can be fitted onto a single base, or manifold, and all enjoy a common air supply and exhaust. This, clearly, simplifies the pipework and centralises valves used for a particular process.
In-line valves can also be known as directional control valves and manifolds as bases.
A valve is a pneumatic switching device that either blocks or changes the direction of air flow. Before we consider how they work, it is important to explain some of the terminology. You will see terms such as 2/2, 3/2, etc associated with valves. The first number specifies the number of ports; that is, the number of connections in and out of the main part of the body - not including ports in the end caps. The second number identifies the number of positions the valve spool can occupy. So, a 5/3 valve has 5 ports and 3 possible positions for the valve spool.
The spool slides inside a bore within the valve; a suitable machined profile on the spool means that, as the spool slides in the bore, body ports are correctly blocked or opened to allow the flow of air as required. The diagrams show how the valve can be operated to enable the out and in motion of a double acting cylinder. The different construction of spool valves is discussed below.
Clearly, the spool needs to be moved, or operated in order for the valve to work. There are various types of operator available (see below) but in the picture above a solenoid would be placed at either end, the electro magnetism creating movement of the spool.
The picture above shows a typical manifold. The black gaskets on the top show that this unit can take four individual valves; units are available capable of taking ten valves. The ports on the right allow flow common to all valves; in this way, valves used for a particular process can be mounted together and the pipework is simplified. The port footprint of each valve must be compatible with the outlets on the manifold.
The key factors in the selection of an inline valve are:
flow considerations - the rated flow through the valve must be sufficient to meet downstream process requirements and this will be related to the port size. It is worth noting that modern valve construction and technology allows greater flow than valves of the past; a golden rule that must still be applied is to calculate the greatest instantaneous flow required and size the valve to this, rather than averaged values.
valve function - does the application require, for example, a 3/2 or 5/2 valve function.
valve operation - what is the best mode of operation of the valve. Can it be electrically operated by a solenoid or is an air supply (pilot) better suited to the application. What return mechanism is required – is mechanical spring or electrical/air operation best for the application. Should the valve be normally open or normally closed (see below)
system considerations - is cycle rate a critical consideration and the ability to work at higher operating pressure. This might affect the type of valve construction chosen as outlined below
environmental considerations - at what ambient temperature must the valve operate and are there any process deviations from this. Does the process involve a hostile environment, for example the ability to work in an explosive atmosphere.
The following should be considered in choice of the manifold:
valve compatibility - clearly, the manifold must be matched to the choice of valve, ensuring port alignment and no restriction to flow
number of ways - how many valves need to be co-located for a given process. Should some ‘redundancy’ be built in for future expansion
environment - as for the valve, the manifold might need to operate, or be protected from, a hostile working environment
As mentioned above, there are different types of spool valves available to suit different application requirements. A spool valve is part of a more general family called shear-action valves; they are called this because the spool mechanism slides across the inlets and outlets, its position controlling the routing of the air flow.
The profiled spool fits inside a machined metal bore and, clearly, the seal between the surfaces is critical in minimising leakage. In one system (soft seal) resilient but softer (compared to metal) sealing rings are employed. The seals are fixed to the moving spool in one option and this is called dynamic sealing; in static sealing, the seals are held in the valve body and the spool moves through them.
A second system (glandless) relies on the tolerance between the metal spool and metal bore surfaces being so precise that softer seals are unnecessary (an "air bearing" principle).
The soft-seal option offers higher flow and lower leak rates compared to glandless; however, glandless has a significantly longer life - soft-seal typically 30 to 50 million cycles compared to 250 to 300 million cycles for glandless - the latter also able to work at faster cycle rates and higher operating pressures.
We also referred to normally closed or normally open. The 'normal' refers to the function of the valve with no external influence acting on it. So, a normally closed (N/C) will block flow in it's rest position; the opposite is true for a normally open (N/O) valve (note, this is the opposite for the electrical switch).
The 'normal' for a 5/3 valve is in the middle position; here, three options are possible. APB means All Ports Blocked - all 5 ports are sealed; COE (Centre Open Exhaust) means the supply is blocked and outlet ports are exhausted. Finally, COP (Centre Open Pressure) means the supply port is connected to both outlets with the exhausts blocked.
Valves are employed in a multitude of industrial applications. As said, the valves function is to control downstream components, usually an actuator. For example, a 3/2 valve would be used to control a single acting cylinder and a 5/2 valve a double acting cylinder. In the event where a process only requires infrequent operation of the actuator, a normally closed valve would be employed; this ensures that the system remains unpressurised other than when force is required.
If a process requires multiple valves, it is efficient to use a manifold to mount the number of valves required; this ‘centralisation’ is particularly useful if the valves need to be housed in an enclosure due to environmental considerations.
Fittings and tubing will be required consistent with the port size of the valve used. Silencers on exhaust ports are recommended. Where a new system design is concerned, there is a wide variety of actuators and air preparation equipment available.
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