Process control Essay Example | Topics and Well Written Essays - 2500 words

Process control - Essay Example 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 rat e 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. A control valve is said to be equal percentage if equal increase in valve travel or opening m (or Z) produces or results in equal percentage increase in flow F. If the situation emerges at the optimum condition of constant drop in valve pressure (DPv), the valve is said to be inherently equal percentage. The valve gain is not constant in this case but increase with an increase in flow rate. Therefore, when the control valve is just beginning to open at low flow rate, the valve gain is quite low and the change in flow is quite insensitive. At high flow during the upper portion of the travel, the valve gain is high which translates to rapid flow or sensitively for the same increase in valve travel. When plotting the flow rate against valve travel or position, the slope is flat near the initial travel position, but the slope increases drastically with the flow as the valve opens on the upper section of the travel. It can be shown by a straight line plotted on a semi-logarithmic paper for log F against m. The Quick open control valve characteristic is flattened in shape as opposed to the contoured valve plug. The flow of this valve increases rapidly up to the highest flow level with the least initial opening of the valve. In the lower section of travel position, the valve gain is so high for application and utilization in modulating control. The slope is then flat where the flow rate hardly increases with valve openings. Therefore, the Quick open control valve is restricted to a service of ON and OFF and in a specified application that needs discharge of flow. Applications of Control Valves Linear plugs are mostly used on the systems where the valve pressure drop in a major section of the total system pressure drop , where the valve is the major pressu re loss mechanism. Linear control valves are used in about 10-15 % of all applications. Linear control valves are also used in gas process. In the gas process, there is a large volume, decreasing DP with an increasing load DP at maximum load. The maximum load DP is greater than 20% of minimum load DP. It is also applied in the gas compressor recycle control valve. Also, the linear valves are