Proportional smart control valves play critical roles in process industries. Accurately assessing valve performance, however, can be a cumbersome routine.
Valves open and close proportionally, and vary the degree of travel depending on a variable 4 to 20 mA signal applied to their input. Many valves have a feedback signal that shows the actual position as a percentage of open/close. This output can be a 4 to 20 mA signal or a digital HART variable that represents 0 to 100 percent of control valve operating span.
Another key indicator of a valve’s performance is the pressure needed by the valve to move it to the desired position. For example, a valve may be programmed to know that if 12 mA is applied it should be 50% open. The smart electronics will control what is essentially a smart pressure regulator to increase or decrease the pressure as needed to move the control element to the desired position.
Applying a varying mA signal while also monitoring the output milliamp or percentage of travel signal will tell you whether a control valve is operating correctly over its range. Similarly, monitoring and recording the pressure applied to the final control element while varying the input 4 to 20 mA signal to the valve is a key test to identify whether a valve is sticking. The relationship between pressure and mA or position of the valve is normally linear if the valve is working correctly. If additional pressure is required, this can often be attributed to the valve sticking and will be shown if measurements are logged and graphed. By recording those signals, the valve's performance can be documented. This documented test and result is often called a valve's "signature."
Valves typically include simple manual indicators that give you an approximation of the percent of travel for a setting when in operation. However, that indicator does not show you how the valve will operate under dynamic and changing conditions, and its accuracy isn’t assured.