Valve Selection

Valves are typically sized based on how much air they can move. The flow coefficient or 'C_v of the valve' is critical to determining if a cylinder or other actuator can move with sufficient speed for the application. A valve with a higher C_v can move more air. To account for various losses in all pneumatic systems, remember to oversize by at least 25%.

How to Calculate C_v

The formula below can be used to calculate the MINIMUM C_v or 'coefficient of velocity' required for operating a particular size air cylinder in a specific time period. There are many other factors that affect cylinder speed, such as tubing size and length; fitting size and shape (ninety-degree bends are especially disruptive to air flow), etc. This formula should be used as a guideline ONLY to determine a MINIMUM C_v for a given system.

Interactive C_V Calculator:

Select
Bore
Size:
7/16" 9/16" 3/4" 7/8" 1-1/16" 1-1/4" 1-1/2" 2" 2-1/2" 3" 3-1/4" 4"

b

Enter
Stroke
Distance(in)


s

Enter
Time(sec)


t

Select Inlet
Pressure
10 PSI 20 PSI 30 PSI 40 PSI 50 PSI 60 PSI 70 PSI 80 PSI 90 PSI 100 PSI 110 PSI 120 PSI

P_i

Select % Expected
Pressure Drop
Across Valve:
 2% ΔP
 5% ΔP
10% ΔP

Select Air
Temperature (F°):
40 F 50 F 60 F 70 F 80 F 90 F 100 F

T

Select Atmospheric Pressure (PSI):
10.1 PSI (10000' above SL) 10.9 PSI (8000' above SL) 11.8 PSI (6000' above SL) 12.2 PSI (5000' above SL) 12.7 PSI (4000' above SL) 13.2 PSI (3000' above SL) 13.7 PSI (2000' above SL) 14.2 PSI (1000' above SL) 14.7 PSI (Sea Level)

P_a

CV =				= Cv
	b² s (Pi + Pa)		T + 460
	824.33 t Pa		ΔP(Pi - ΔP + Pa)

General Rules of thumb for Fast Cylinder Action

The first general rule of thumb is choose a cylinder which will allow for at least 25% more force than what is required. For extremely fast operations, choose a cylinder which will allow for 50% more force than what is required. This will leave 25% or 50% of inlet pressure to satisfy system losses.

The second rule of thumb is to select a directional control valve which has the same port size as the cylinder which it will be operating. For large valves, internal flow capacity is typically the same or greater than the capacity of the intended connection fittings. On smaller valves, the internal flow capacity is typically much less than that of the connection fittings. But always be sure to check the valves flow rate, and do not rely solely on the port size.

Lastly: use large diameter tubing (relative to cylinder bore), keep the tubing length as short as possible and use as few fittings as possible.

Achieving a Specific Speed

If a specific cylinder speed is desired, it's advisable to size the valve, tubing, fittings, and cylinder for a faster stroke. And then use flow control valves to slow the cylinder down to the desired speed. If extremely fast action is desired, some trial and error testing will probably be required. Consider the use of cushioned cylinders to prolong the life of the components in high speed systems.

More about cylinder speed here.

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