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process control - Essay Example
Author : stoltenbergbrau
Engineering and Construction
Pages 10 (2510 words)
Introduction: Flow characteristics of Valves Flow characteristics of valves refers to the relationship between the volumetric rate of flow (F) plotted in the Y axis through the valve and the valve travel or opening position denoted by m(X- axis),as the valve is seen through its various degree of opening right from opening position to the closing position…
This flow is usually expressed by plotting the valve travel on the (y-axis) against the flow percentage under constant pressure differential on the (X-axis). There are two main types of the inherent flow characteristics; these are linear and equal percentage. A control valve is said to be linear if its inherent flow characteristics can be represented on a graph by a straight line. The flow is usually directly proportional to the valve travel. It is a straight line when represented on a rectangular coordinate of flow rate F or % F or Cv or % Cv versus or against the percentage travel at the optimum condition of constant drop in valve pressure (DPv). It can be deduced that an equal increase in travel rate or opening position, m, results in an equal increase in flow F (or Cv) at constant valve pressure drop (DPv). If m represents travel and F represents the rate of flow, then, dm= k dF where K is a proportional constant. Therefore the valve gain or sensitivity is independent of flow rate and valve opening. Shown by; Kv = Df/dm = constant The valve gain (Kv) is the slope or tangent of the rectangular coordinate or plot drawn. The slope of the graph remains constant and independent of the flow rate. ...