Remember the Far Side? Of course you do, but do you remember the one depicting the boy who leans his whole body forward as he pushes against a door that clearly reads “Pull”? Although the sign leading up to the building shows “Midvale School for the Gifted” this particular student doesn’t seem to get it. Any possibility of progress is blocked by a physical constraint. The signs are there – literally – but the boy just ignores them. As a practitioner, don’t you wish that signs of physical constraints and performance issues were so obvious?
Many of the signs practitioners want are contained in a production facility’s existing process data and they’re readily available for those equipped with the right tools. Output Distribution, Output Travel, and Output Reversals are among them. They’re key performance indices (KPIs) used to assess control loop performance in general and FCE health in particular. Control loop monitoring software uses those KPIs to determine where and how a given FCE spends its time which can reveal quite a bit about its health. For sure control loop performance monitoring is much more than finding opportunities for control loop tuning. The metrics within most CLPM solutions facilitate both PID optimization and improved asset reliability.
If you think it’s odd to monitor where and how valves and dampers spend their time, then consider the following:
Output Distribution
Operating at the extremes often corresponds with an FCE sizing issue – whether too large or too small for the corresponding process application. If an FCE is sized properly, then a decreasing Output Distribution value may point to an eroding valve seat or other maintenance-related issue.
Output Travel
Output Travel accounts for the work performed by an FCE relative to its ability to track Set Point. When the Output Travel value rises while Set Point tracking remains relatively constant, the FCE is at increased risk of failure as it is working harder to maintain a consistent level of control.
Output Reversals
Output Reversals reflects the amount of volatility within a given process. Large and/or increasing reversals in output suggest instability as the CO hunts for a stable condition. Such behavior increases the need for valve maintenance as well as the potential for unplanned equipment failure. Many CLPM software solutions provide valuable insight into asset reliability issues and opportunities for control loop tuning. For practitioners who are focused on FCE health there are a variety of other loop performance metrics that are generally available to monitor stiction and to identify oscillatory process behavior. These KPIs are explored in greater detail in a publication entitled “No More Downtime”. What’s clear is the benefit of leveraging a facility’s existing process data and the need to read the associated signs.
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