Control Loop Monitoring Solutions Provide Valuable Early Detection of Valve Stiction and Valve Deadband
It’s generally known that the behavior of final control elements (FCEs) (valves, air handlers, etc.) change over time. Like most things the dynamics of FCEs are different from the time they’re first installed to the time they’re serviced and ultimately replaced. Sometimes the change in dynamic behavior is subtle. Other times the change is dramatic. Unfortunately that’s the nature of things, and that’s the primary reason why monitoring for valve issues is beneficial.
Stiction and Deadband (also known as “slop”) are conditions that frequently hamper the performance of FCE’s like valves. They prevent PID control loops from maintaining steady process operation at a designated Set Point. Stiction and Deadband result in oscillatory behavior and negatively affect downstream processes. A process’ often delayed, generally inconsistent reaction to changes in Controller Output (CO) are due to the valve’s inability to effect a response. Simply put the controller is confused as the valve seemingly doesn’t obey the controller’s commands. That standoff is often costly and sometimes dangerous.
Control loop performance monitoring solutions utilize a variety of metrics to detect a wide range of issues that hamper production and lead to unplanned downtime. A pair of metrics used by CLPM technologies specifically for identifying valve issues are the following:
- Valve Stiction
Most valves are affected by some amount of Stiction. The condition frequently results from the use of excessive packing during either initial installation or maintenance, and it prevents the free or unrestricted movement of a valve. Stiction often reveals itself in trend data. Data that is jagged or saw-toothed in appearance provides a clue.
Stiction metrics available in most CLPM solutions evaluate changes in the Process Variable (PV), taking the size and frequency of changes into account. This enables them to assess the probability that Stiction exists. Select CLPM tools not only calculate the probability of Stiction they also determine the relative amount of Stiction present. That additional detail helps with prioritizing maintenance projects.
- Valve Deadband
Valves and other FCEs can have a zone of operation where changes in their position effect no change on the process. That zone is generally referred to as the Deadband. The valve must traverse the full range of Deadband before a change to the process can be effected. This presents challenges for most control loops in general and conservatively tuned loops in particular. In contrast with aggressively tuned loops, conservative loops allow for a more lax response. That “laxness” is magnified by Deadband.
Deadband is usually the result of mechanical wear and it can be uncovered by monitoring Output Travel. This metric quantifies the amount of movement performed over time by a given valve. A control loop that exhibits increased Output Travel may be affected by Deadband. If the increase corresponds with normal production levels, then it suggests that the valve is working harder in order to effect the same amount of change, the definition of Deadband.
Stiction and Deadband are among the more common issues affecting valve performance. They are also correctable. With the introduction of controller monitoring software solutions these troublesome issues can be identified and corrected before they become significant problems. CLPM technologies offer another set of eyes on the process. It’s worth noting that the eyes of a monitoring solution see the little differences in control loop performance that can affect performance and production, and they’re open 24 x 7.