How to Properly Size a Pneumatic Valve to Actuate a Pneumatic Cylinder

In order to actuate a pneumatic cylinder, a valve is required to direct the flow of compressed air -- this is what makes the cylinder actuate. Because there are many different styles and sizes of pneumatic valves on the market, it is important to size the valve appropriately for the application. The measure of the amount of air a valve can flow is typically found as the Coefficient of Flow, or Cv, of the valve. This unitless quantity is derived from the number of gallons per minute that will pass through a valve with a 1psi differential between the valve's inlet and outlet. Generally, larger valves have higher Cv values, because they have larger ports and flowpaths. Using the Cv value, you can compare different valves from different manufacturers, and know that you will get a comparable flowrate. This guide will help you calculate the required Cv you will need from a valve when applying it to actuate a pneumatic cylinder.

Things You'll Need

• Calculator
• Pen/Pencil
• Paper

Instructions

1. First, determine the area of the cylinder, in square inches. This can be easily obtained by squaring the BORE of the cylinder (in inches), and multiplying by 0.7854. For example, if you have a 1.5" bore cylinder, your calculation would be: Area = 0.7854 x 1.5 x 1.5 = 1.767 square inches. We will label this Area as A.

2. 

   Use the included image to get your Compression Factor (C) and your Pressure Drop Factor (B) for the inlet pressure you will use, and the amount of acceptable pressure drop. If you are unsure what is acceptable, use the 2psi drop column.

3. So far, we have determined values for A, B, and C. We will enter them into a formula in just a moment, but first, we must also get a couple more parameters. The stroke length of the cylinder, in inches, will be defined as L. The time required for the cylinder to stroke, in seconds, will be defined as T.

4. Enter your values for A, B, C, L, and T, into the following formula to determine the Cv required:

   Cv = (A x L x C) / (B x T x 29)

5. Here is an example calculation:
   Cylinder bore = 4", stroke length = 10", time to extend = 2 seconds, inlet pressure = 90psi, allowable pressure drop = 5psi.

   A = 4*4*0.7854 = 12.57 square inches (area computed from cylinder bore)
   B = 21.6 (from table)
   C = 7.1 (from table)
   L = 10" (stroke length)
   T = 2 seconds (time to stroke)

   Cv = (12.57 x 10 x 7.1) / (21.6 x 2 x 29) = 0.71.
   So you would need to specify a valve with a Cv of 0.71 or higher for this application.

Tips & Warnings

• This is a simplified formula, based on air temperature at 68 degrees F. The required Cv will change with temperature, but this formula should give you a good starting point.