# Are your control loops more restless than they need to be? > Twelve questions about the signs of restless PID control loops: manual correction, alarms, oscillation, running close to the trip limit. Your result and explanation right away. Most PID controllers are tuned once and then left alone for years, while the process does change. The twelve signs below are each small, but they often point to loops that operators keep correcting, that keep raising alarms or that come closer to the trip limit than necessary. Check them for your plant. One minute, and your result right away. This is the text version of an interactive checklist. On the page itself you answer each sign with yes, no or don’t know, and you immediately see your result and the explanation. ## Twelve signs ### 01. Manual intervention Are there control loops that operators regularly switch to manual, or where they nudge the setpoint to keep the process under control? *What this often means:* Operators do not trust the loop. In practice that means correcting all day long and a way of running the process that differs per operator. ### 02. Recurring alarms Are there control loops that keep giving the same alarm, even when nothing is really wrong? *What this often means:* A hunting process value keeps touching an alarm limit. After a while nobody pays attention any more, and then the alarm that does matter no longer stands out. ### 03. Close to the trip limit During disturbances, do process values come close to a trip limit, or has a control loop ever contributed to a trip or near-trip? *What this often means:* A restless loop leaves little margin. When a disturbance hits, the trip limit is reached sooner, and then one unfortunate coincidence is enough for an unplanned shutdown. ### 04. Overshoot or oscillation Do control loops regularly show overshoot or recurring oscillations around the SP? *What this often means:* The loop periodically heads towards the alarm or trip value. This is the silent cause behind many unplanned trips that nobody recognises as a control problem. ### 05. Slow recovery after a disturbance After a disturbance or setpoint change, does it often take longer than you would expect before the PV is stable around the SP again? *What this often means:* After every disturbance the process is out of spec for a long time and operators have to correct. That is why it is often run with extra margin. ### 06. Behaviour depends on load Does the control behaviour clearly change with a different load, production capacity, recipe or operating conditions? *What this often means:* One setting for all conditions only works at one operating point. Outside that point operators intervene, or the process runs with extra margin. ### 07. Output moves continuously Are there control loops whose output keeps moving up and down, while the SP and process load are fairly constant? *What this often means:* Unrest that wears the valve and never lets the process really settle. Often too sharp a P or I action, sometimes a valve that no longer follows cleanly. ### 08. Much correction, little effect Are there control loops that make many or large corrections, while that has hardly any visible effect on the PV? *What this often means:* Suspicious for the valve: stiction or oversizing. Then part of the problem is not in the tuning, but in the valve itself. ### 09. Output at minimum or maximum Do outputs regularly run into their minimum or maximum, so the controller temporarily has no control room left? *What this often means:* No control room means no grip on the process. This is where trips start, and it often points to a valve or range that no longer suits the current load. ### 10. Persistent offset from the SP Are there control loops where the PV stays above or below the SP for a long time, without the controller removing that offset? *What this often means:* The controller does not remove the error. Usually too weak an I action, sometimes a valve at its limit or a measurement that is wrong. ### 11. Reacts to measurement noise Are there control loops that react strongly to small fluctuations or measurement noise in the PV, so the output keeps correcting unnecessarily? *What this often means:* Unnecessary valve traffic and wear. Usually a missing filter or a D action that does not suit the signal. ### 12. Settings grown over time Have the P, I and any D settings of your control loops mainly grown over time or been adjusted by feel, rather than substantiated from process behaviour or trend data? *What this often means:* The knowledge is gone. Nobody can explain why the controller is set the way it is, so nobody dares to touch it. ## Result The number of signs you recognise determines the result: - **0 signs: No signs.** Your control loops seem to do what they should. If you have doubts about one loop in particular, check the signs for that loop. - **1 to 2 signs: One or two signs.** That happens in almost every plant and is usually no cause for concern. Keep a particular eye on overshoot and outputs that run into their limit: that is where trips start. - **3 to 6 signs: Enough signs to take a closer look.** This kind of unrest rarely stands out: no fault goes off, but operators correct and the process runs with extra margin. Because nobody tracks performance per control loop, it never makes it onto a list. Whether it is down to the tuning or to a valve or measurement, you can largely check yourself. - **7 or more signs: Strong indications that your control loops run more restlessly than necessary.** This many signs cost your operators attention every day, make the process run with a safety margin and bring it closer to the trip limit than you would like. The cause is not always the tuning: sometimes it is a valve or a measurement. Which of the three, you can largely check yourself. Prepared by Brent Zwart, Process Control Engineer. ## Your breakdown Above you can see what your signs mean. The breakdown tells you what to do about them, matched to your answers. You receive it by email straight away. - The checks for your answers, in order of urgency: for each sign what to look up in your own trend data. - Where the problem most likely lies for you: the tuning, the valve or the measurement. - Within one to two working days a short personal response from us to your answers. ## Would you rather talk straight away? Tell us which control loop is running restlessly. In a first conversation we often already know whether there is something to gain. [Get in touch](/en/contact) [About us](/en/about-us)