3.1 Control valve issues 3.1 Issues for Control Valves Demands for excellent process control places many requirements on the performance of control valves. Some of the more generally applicable requirements are presented briefly in this section as a basis for the subsequent sections on valve features. While reviewing these issues the reader should refer to Figure 8 which shows the typical behavior of a fluid when flowing through a restriction. The valve should have a capacity for the most limiting situations during plant operation. This is often reported as a ratio of the largest to the smallest flows and is usually in the range of 35 to 50.
Some situations require that the valve be able to reduce the flow to essentially zero.
Thus, some valves are designed for tight shutoff, while others do not achieve zero flow. Note that if zero flow is a high priority, an addition, special valve giving tight shutoff should be installed in series with a control valve. The non-recoverable pressure loss across a valve represents an energy loss that can result in a significant cost in some situations, especially gas flows from a compressor. Some control valve designs discussed below provide a lower pressure loss that results in lower operating costs. As a result considerable friction can occur, and this friction can inhibit the stem from moving precisely, especially for small changes in position.