In manufacturing processes with flowing fluids, control valves are among the most important equipment to keep the process operating smoothly and reliably. For those not experienced in their design and operation, AIChE’s CEP magazine has a great article, Evaluate Control Valve Performance (AIChE membership required to view full article.)
Emerson’s James Beall authored the article and highlights the importance of focusing on control valves for improved operating and economic performance.
He opens describing what composes a control valve, including:
…a valve body, a stem or shaft, a valve trim and closure member (e.g., plug, ball, or disc), an actuator, and related accessories (e.g., a current-to-pressure transducer, a positioner, an air booster relay, and an air set).
Given all that goes into a control valve, this presents:
…an opportunity for problems. Proper control valve engineering and selection can help to avoid or minimize such issues.
James cites one refinery project, where the project team:
…identified poorly performing control valves as the root cause of process variability in 45% of the control loops analyzed.
What happens in many cases is that:
…the flow and… process variable [PV] do not change by the expected amount in the expected time, [and] the controller makes additional adjustments in its output to induce the needed flow and PV change. This often causes an overreaction and cycling, known as limit cycling.
James describes important concepts to understand in control valve static and dynamic performance responses. Static responses include travel gain, dead band and resolution. Dynamic responses include dead time, step response time and overshoot.
Read the article for more detail on these concepts as well as their impact on process control. James also shares examples illustrating control performance improvements made in an LNG main-feed loop, a steam flow valve, and a distillation column reboiler application.
If you’re new to process automation and a chemical engineer and active member of AIChE, this article is well worth your time to read and absorb.
Another great source for learning more about control valves is the Control Valve Handbook and connecting and interacting with other control valve experts in the Valves, Actuators & Regulators group in the Emerson Exchange 365 community.