- 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
Table for determining Compression Factor (C) and Pressure Drop Factor (B)
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.
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