Control valve and its effect on process performance

Control valves are the final link within a processing system. Often, these devices are expected to perform at levels consistent with highly accurate instrumentation in order to control process variability with 0.5% or less. The dilemma is that many control valves cannot meet that expectation, which increases the potential for off-spec product, increased chemical usage, high energy usage, plus a host of other profit-robbing control ills. Just how serious is this problem? The findings of process control analysis experts, which are based upon audits of thousands of flow control loops, show that as high as 40% of all variability is caused by poorly performing control valves. This paper will review the reasons why a control valve may not control with 0.5% accuracy or better, and it will present the design features that a valve user should require in order to achieve the desired level of system performance.