In this section we'll present information that will help in the successful design and implementation of Fluid Power Solutions

ALL ABOUT AIR – WHITE PAPER #9, Air Valves by Thomas W. Kreher, Applied Pneumatic Controls ©

Valves used to block or pass compressed air and then direct it to an outlet or exhaust port are typical ‘directional control valves’. These are used to direct compressed air, on demand, to or from pneumatic actuators, cylinders, motors, process’s and to pilot other devices. Directional control valves are a crucial component in pneumatic process and controls, including automation, robotics, manufacturing, production and more.

REVIEW

The Pneumatic Directional Control Valve is DIFFERENTIATED BY:

1. PORT SIZE AND FORM  - eg. ¼” NPT

2. FLOW PATH OR NUMBER OF PORTS

   1. Two Way -Two port Valve, Normally Open or Normally Closed. No exhaust.

   2. Three Way -Three port Valve, Normally Open or Normally Closed with Exhaust.

   3. Three Way Selector -  Three port multi directional flow.

   4. Four Way, Four Port – Common exhaust port. - often a Poppet Valve.

   5. Four Way, Five Port -  Separate Exhaust ports - usually a spool valve.

*Most Valves are Two Position, Actuated or Released.

This Four Way Five Port configuration is also used as a three position valve, stopped in the center, Detented or Spring Centered. Three flow patterns maybe used for the center position,  A) All ports blocked.  B) Cylinder ports open to Exhaust. C)  Cylinder ports open to Pressure.

3. ACTUATION METHOD

1. Solenoid – Electrically operated either directly by an electromagnet or with compressed air directed by the solenoid pilot section

2. Manual -  Lever, peddle, treadle, knob, toggle, tappet, lever roller

3. Pilot  - Compressed air (gas), liquid

4. RESET OR RETURN TO NORMAL POSITION

1. Solenoid – Possibly a double solenoid actuated valve.

2. Spring – For example; Solenoid actuated, spring return, including optional air assisted spring return.

3. Pilot – For example; Double air pilot.

4. Manual – Push – Pull
    

5. FORM

   1. INLINE – Least expensive, most compact, highest flow

   2. SUBBASE – A serious upgrade for tidy, serviceable

Verbally or with a correct symbol for a pneumatic schematic identify the valve requirement as; (see figure 1, valve symbol)

“One half inch, Three Way, Normally Closed, Solenoid Operated, Spring return”. Specify the details that assure this valve will meet all requirements for this specific application : (1) solenoid voltage (120 VAC)  (2) Manual override. (Locking or Non-Locking)  (3)Mounting Style (In line)  (4) Port Thread, size and form (1/2” ISO G).

*The terminology for a Three Position valves includes terms such as “Three Position, Spring Centered, All Ports Blocked Center.”

Specific descriptions and ISO symbols (which are universal) help assure the same correct results in Oslo, Okinawa and Omaha.

The Cv or amount of flow this valve will pass should always be specified. It is often possible to make a two way function with a three way valve by plugging a port. In the same way a three way function might be obtained by plugging a port on a four way valve. However it is improbable to match a flow requirement that is not specified.  A  valve with ½” ports may have a Cv from 1.5 to 6 or more.

As a rule of thumb for a Cv of 1,  remember 90, 10, 30. With 90 PSIG supply,  a 10 PSI  drop across the valve, the flow is 30 SCFM. For a Cv of 2 the flow is 60 SCFM.  Just FYI a well rounded orifice (full flow) of .183” Diameter has the same flow as a valve with Cv of 1.

Many valves will have less flow (smaller Cv) with a smaller body that is overported than the nominal capacity listed. A  valve with a Cv of 2 selected to replace a valves with a Cv of 4 will surely give HALF FAST results.  Most manufacturers today use similar but not necessarily identical formulas to determine the Cv numbers published for their valves. A few list flow to atmosphere which is roughly twice the flows listed above. When you encounter product information with flow rather than Cv listed and the flow numbers are very high by comparison, divide by 60 SCFM for an approximate Cv number for that particular valve.        

As a rule of thumb for a Cv of 1,  remember 90, 10, 30. With 90 PSIG supply,  a 10 PSI  drop across the valve, the flow is 30 SCFM. For a Cv of 2 the flow is 60 SCFM.  Just FYI a well rounded orifice (full flow) of .183” Diameter has the same flow as a valve with Cv of 1.

The flow path through a valve, depending on size and shape, will pass a specific amount of gas or fluid depending on the inlet pressure and pressure drop. Someone once told me “I don’t want any pressure drop.”   Sorry, no pressure drop no Flow.  Remember,  the pressure on the cylinder air motor or other device will be approximately the outlet pressure from the valve less pressure drop in the plumbing between them.

As an example, if catalog literature states that the air motor will consume 45 SCFM  at 90 PSIG.  Using the 90-10-30 rule of thumb, our pressure to the motor would be 80 PSIG. Divide 45 SCFM  by 30 SCFM  to obtain the Cv of the valve at 90 PSIG = 1.5. Then correct for the pressure difference (90 + 14.7) / (80 + 14.7) = 1.11. Next multiply the Cv calculated, [1.5]  x 1.11 = 1.67 [Min Cv required.]

The Cv for a valve that will stroke a cylinder in T seconds is  Cv = .002 x Bore” (Dia x Dia) x Stroke” L x SqrRt [(P1+14.7)/Delta P]

Please remember this is a single stroke. If you require a cylinder cycle be completed within the same time double this Cv.

TYPES OF VALVES

SOLENOID OPERATED:

1. SOLENOID PILOT: A small 3 way valve in the solenoid operator is actuated by electromagnetic force from the solenoid coil when excited with electrical energy. Compressed “pilot” air then forces the main valve assembly to the alternate position. When the electrical energy is removed the pilot valve blocks the pilot air and exhausts the compressed air that shifted the main valve assembly. The attractive features are small, reliable, low energy. Negatives might be capturing the exhaust in sensitive areas, a small orifice in cold conditions and the need for a minimum pilot pressure for low pressure and vacuum applicatons.

2. DIRECT SOLENOID: A force great enough to move the main valve assembly is produced by a modest sized solenoid assembly. This is often faster than using pilot air. The operator does not exhaust and does not require small orifices or minimum pressure. Draw backs might be size, expense, energy and rejected heat.

POPPET: A simple, rugged design that seals like a stopper against  a valve seat blocking or allowing flow. For example the majority of solenoid pilot operators for all pneumatic valves use a direct acting poppet valve assembly.  Poppet valves are functional two and three way assemblies. For 4 way functions two poppet valves might be used together in one body.  They are fast, high flow, short stroke, long life and forgiving of air quality. An industrial work horse. Draw backs might be large size, cost, unrestricted exhaust.

SPOOL: Spool valves may seal with elastomers on the spool, elastomers held in position in the bore and ultra close clearance. The less expensive spool valves use elastomer seals. Some close tolerance spools with an air bearing effect are renowned for their longevity.

Good air quality, sometimes coalescing filtration, is important to prevent coating the spool with goo. Spool valves come in all sizes and shapes especially three way and four way, five port. The flow path of spool valves lends itself to subbase and manifold mounting with electrical raceways for wires. Controlled flow from the exhaust ports on spool valves is convenient and effective.

SPECIAL PURPOSE:

PROFI-BUS, ET AL: Manifolded valves with special electrical modules allow central control and coordination of valves between sectors of production operations. The ability to transmit adaptive intelligence to multiple valves and multiple machines is leading pneumatic valve applications to new horizons of automation and productivity.

BALL VALVES with manual levers are ubiquitous. For high flow automated applications ball valves are available with pneumatic and electric operators. With a full flow ball valve the flow path is short, strait and the size of the pipe I.D., almost no pressure drop. They are used for “water,oil, gas(air)” with  common brass or bronze bodies and more aggressive fluids in stainless steel. 

 HOW TO READ ISO VALVE SYMBOLS

See Figure 1