used for throttling service20, and are especially useful on gate, plug, and ball-type shut-o valves where seat engagement is substantial for tight shut-o .

27.11Split-ranging

There are many process control applications in industry where it is desirable to have multiple control valves respond to the output of a common controller. Control valves configured to follow the command of the same controller are said to be split-ranged, or sequenced.

Split-ranged control valves may take di erent forms of sequencing. A few di erent modes of control valve sequencing are commonly seen in industry: complementary, exclusive, and progressive21.

20Prior to the advent of motor-actuated valves, practically all shuto valves in industrial facilities were manually operated. While this is an inconvenience for operations personnel, it did carry one advantage: the human operators tasked with closing these valves by hand could feel how each valve seated. The amount of e ort and the onset of closing torque sensed while turning the valve handle shut gave operators tactile feedback on the condition of each valve seat. Motor-powered valve actuators eliminated the need for this routine manual labor, but also eliminated the routine collection of this valuable diagnostic information. Modern electric valve actuators now provide the best of both worlds: convenient and fast valve operation with accurate self-diagnostic assessment of valve seating.

21I have searched in vain for standardized names to categorize di erent forms of control valve sequencing. The names “complementary,” “exclusive,” and “progressive” are my own invention. If I have missed someone else’s categorization of split-ranging in my research, I sincerely apologize.

An alternative expression for this split-range valve behavior is a graph showing each valve opening as a colored stripe of varying width (wider representing further open). For this particular mode of split-ranging, the graph would look like this:

Controller

output

With this form of split-ranging, there is never a condition in the controller’s output range where both valves are fully open or fully shut. Rather, each valve complements the other’s position22.

An alternative to complementary valve sequencing in a process where two fluid streams mix (or diverge) is to use one three-way valve23 rather than a pair of two-way valves:

22In mathematics, a “complement” is a value whose sum with another quantity always results in a fixed total. Complementary angles, for instance, always add to 90o (a right angle).

23Also known as a mixing valve or a diverting valve, depending on how it is applied to process service.