Air control valvesAir control valves can be divided into a certain categories accordingto the function Directional control valves – Input elements – Processing elements – Control elements Non-return valves Flow control valves Pressure control valves Shut-off valves
Directional control valvesThe directional control valves control the passage of air signals. Theyfollow the following standardization.The valve is described by:Number of ports or openings: 2-way, 3-way, 4-way, etc.Number of positions: 2 positions, 3 positions, etc.Methods of actuation: manually actuated, mechanically actuated, pneumatically actuated, electrically actuated.Methods of return: spring return, air return, etc.
Influence of mid-positionDirectional control valves with two positions (such as 3/2-way or 5/2-wayvalves) allow the extension or retraction of a cylinder. Directional controlvalves with three positions (such as 5/3-way valves) have a mid-positionoffering additional options for cylinder actuation. This can be demonstratedusing the example of three 5/3-way valves with different midpositions. Wewill look at the behavior of the cylinder drive when the directional controlvalve is in mid-position. If a 5/3-way valve is used in which the working ports are exhausted, the piston of the cylinder drive does not exert any force on the piston rod. The piston rod can be moved freely (Fig. a). If a 5/3-way valve is used in which the all ports are closed, the piston of the cylinder drive is held in position. This also applies if the piston rod is not at a stop (Fig. b) If a 5/3-way valve is used in which the working ports are pressurized, the piston rod extends with reduced force (Fig. c).
Port DesignationA numbering system is used to designate directional control valves andis in accordance with DIN ISO 5599-3. Prior to this a lettering systemwas utilized and both systems of designation are presented here:
Constructional Principle:Designs are categorised as followsPoppet valves:With poppet valves the connections are opened and closed by means ofballs, discs, plates or cones. The valve seats are usually sealed simplyusing flexible seals.Poppet valves have fewparts which are subject towear and hence they havea long service life. They :way valve-3/2are insensitive to dirt and disc seatare robust. The actuating normally openforce, however, isrelatively high as it isnecessary to overcome theforce of the built-in resetspring and the airpressure.
way roller lever-3/2valve, internal pilot, normallyopen
Slide valves:In slide valves, the individual connections are linked together or closedby means of spools, flat slide or plate slide valves. 5/2-way double pilot valve, suspended disc seat
(Non-return valve (check valveNon-return valves are devices which preferentially stop the flow inone direction and permit flow in the opposite direction. Blocking of theone direction can be effected by cones, balls, plates or diaphragms.
Processing ElementsThese are processing elements whereby two signals are processedinternally and the resulting signal is output. The elements are: Dual pressure valve (AND function) Shuttle valve (OR function)Dual pressure valveThe dual pressure valve requires two signals(AND function) to produce an output. The dualpressure valve has two inlets 1 and 1(3) and oneoutlet 2. Compressed air flows through the valveonly if signals are applied to both inlets. One inputsignal at 1 or 1(3) blocks the flow due to the diffe-rential forces at the piston slide. If signals areapplied to both 1 and 1(3), the signal which is lastapplied passes to the outlet.
Shuttle valveShuttle valve requires at least one signal input(OR function) to produce an output. This non-return element has two inlets 1 and 1(3) andone outlet 2. If compressed air is applied tothe first inlet 1, the valve seat seals theopposing inlet 1(3), the air flows from 1 to 2. Ifcompressed air is applied to1(3), inlet 1 closesand the air flows from 1(3) to 2.
Quick-exhaust valveQuick-exhaust valves are used to increase the piston speed ofcylinders. This enables lengthy return times to be avoided, particularlywith single acting cylinders. The principle of operation is to allow thecylinder to retract at its near maximum speed by reducing theresistance to flow of the exhausting air during motion of the cylinder. Toreduce resistance, the air is expelled to atmosphere close to thecylinder via a large orifice opening.
If pressure is applied at port 1, then the sealing disc covers the exhaust3, whereby the compressed air passes from 1 to 2. If pressure is nolonger applied at 1, then the air from 2, moves the sealing disc againstport 1 and closes this, whereby the exhaust air immediately vents toatmosphere. There is no need for the air to pass through a long andpossibly restricted path to the directional control valve via the connectinglines.
Flow control valvesFlow control valves influence the volumetric flow of the compressedair. These valves are used for speed regulation of actuators.There are two types: two-way flow control valves and one-way flowcontrol valves.
In the case of the one-way flow control valve, the air flow is throttled inone direction only. A check valve blocks the flow of air in the bypassleg and the air can flow only through the regulated cross-section. Inthe opposite direction, the air can flow freely through the openedcheck valve.
Shut-off valvesShut-off valves are non-adjustable valves, which release or shut offflow in both directions.
Pressure control valvesPressure control valves are elements which predominantly influencethe pressure or are controlled by the magnitude of the pressure. Theyare divided into the three groups: Pressure regulating valve Pressure limiting valve Pressure sequence valve
Pressure regulating valvesThe pressure regulating valve is dealt with in air preparation section“Service unit”. The role of this unit is to maintain constant pressure. Theinput pressure must be greater than the required output pressure.
Pressure limiting valveThe pressure limiting valves are used mainly as safety valves (pressurerelief valves). They prevent the maximum permissible pressure in asystem from being exceeded. If the maximum pressure has beenreached at the valve inlet, the valve outlet is opened and the excess airpressure exhausts to atmosphere. The valve remains open until it isclosed by the built-in spring after reaching the preset system pressure.
Pressure sequence valvesThe principle on which this valve acts is the same as for the pressurelimiting valve. If the pressure exceeds that set on the spring, thevalve opens.The flow from 1 to 2 is closed.Outlet 2 is opened only if a presetpressure has built up in pilot line12. A pilot spool opens thepassage 1 to 2.
Combinational valvesComponents of different control groups can be combined into the body ofone unit with the features, characteristics and construction of a combin-ation of valves. These are referred to as combinational valves and theirsymbols represent the various components that make up the combinedunit. The following units can be defined as combinational valves: Time delay valves: for the delay of signals Pulse generator: for the execution of rapid cylinder movements Stepper modules: for sequential control tasks
Time delay valveThe time delay valve is a combined 3/2-way valve, one way flow controlvalve and air reservoir. The 3/2-way valve can be a valve with normalposition open or closed. The delay time is generally 0-30 seconds forboth types of valves. By using additional reservoirs, the time can beextended.
The following operational principle applies for a time delay valve with a3/2-way valve in normally closed position: The compressed air is suppliedto the valve at connection 1. The control air flows into the valve at 12through a one-way flow control valve and depending on the setting of thethrottling screw, a greater or lesser amount of air flows per unit of timeinto the air reservoir. When the necessary control pressure has built up inthe air reservoir, the pilot spool of the 3/2-way valve is moved downwards.This blocks the passage from 2 to 3. The valve disc is lifted from its seatand thus air can flow from 1 to 2. The time required for pressure to buildup in the air reservoir is equal to the control time delay of the valve.If the time delay valve is to switch to its initial position, the pilot line 12must be exhausted. The air flows from the air reservoir to atmospherethrough the bypass of the one-way flow control valve and then to theexhaust line. The valve spring returns the pilot spool and the valve discseat to their initial positions. Working line 2 exhausts to 3 and 1 is.blocked
The following illustrations show the time behavior of circuits with timedelay valves.
Flow Rate Through an OrificeSince a valve is a variable orifice, it is important to evaluate the flow rateof air through an orifice. The following gas dynamic equations providefor the calculation of air volume flow rates through orifices using Englishand metric units, respectively.
The preceding equations are valid when p2 is more than 0.53p1. Beyondthis region, the flow through the orifice is said to be choked. Thus, thevolume flow rate through the orifice increases as the pressure drop (p 1 -p2 ) increases until p2 becomes equal to 0.53 p1 . Any lowering of p2 tovalues below 0.53 p1 does not produce any increase in volume flow ratebecause the downstream fluid velocity reaches the speed of sound. Also,raising the value of p1 beyond 1.89 p2 will not increase the volume flowrate as the orifice remains at the choked state. Thus, increasing the ratioof (p1 / p2) beyond 1/0.53 (or 1.89) does not produce any increase involume flow rate.From a practical point of view, this means that a downstream pressureof 53% of the upstream pressure is the limiting factor for passing airthrough a valve to an actuator.
Sizing of Valves Based on Flow RateValues of the flow capacity constant Cv are determined experimentally byvalve manufacturers and are usually given in table form for various sizesof valves. The proper-size valve can be selected from manufacturerscatalogs for a given application. Knowing the system flow rate (Q), theupstream air temperature (T1), the required pressure downstream of thevalve for driving an actuator, and the maximum acceptable pressure dropacross the valve (p1 - p2 ), and, the corresponding flow capacity constant(Cv) can be calculated using the previous equations.Selecting a valve with a Cv greater than or equal to that calculated fromthe equations will provide a valve of adequate size for the applicationinvolved.If valves are too small, excessive pressure drops will occur, leading tohigh operating cost and system malfunction. Similarly, oversized valvesare undesirable due to high component costs and space requirements.