Let us start by having a closer look at Cv (ANSI T3.21.3) and the implications we encounter when using this standard to determine the size of a pneumatic valve. Coefficient of flow, or simply Cv, is a standard applied to determine the flow capacity of components using water as the test fluid. It also defines a pressure drop of 1 psi (1-2%) across the test object. To eventually get an airflow rate (Q), it is necessary to apply a conversion factor. Based on its inherent properties, it is apparent that this method will not provide an accurate result. In particular, the low-pressure drop is a problem, since pneumatic circuits frequently operate with a pressure drop across the valve seat up to 15% of the inlet pressure. I know there have been countless valves selected based on the Cv data, and they probably worked just fine. The truth is, there are critical applications out there that require a more refined selection procedure, rather than chronically over-sizing components "just to be on the safe side." When it comes to sizing, I like to say: "As small as possible and as big as necessary!" This also makes sense from an economical and ecological point of view.
展开▼
