On a recent startup, a plant process engineer requested a level controller tuned to eliminate nuisance alarms. This was a simple and straight forward request so we started tuning the loop. The tank in this case was a small buffering tank which smoothed out the pulsing flow from the upstream piston-style filtration unit. As the level controller tuning started to hold a more consistent level, we noticed the downstream flow was now pulsating. This flow fed the rest of the process; therefore, a pulsing flow was not an acceptable result. After bringing this to the attention of the requesting process engineer, he agreed this was not what plant operations wanted, and we reverted back to the original tuning on the level controller.
This experience shows the importance of looking at the entire process, not just an individual unit or loop. A quick solution to a seemingly simple request would have created a larger problem. The ultimate question that needed to be answered was "did this level need to be controlled, and did the level alarm matter?" A secondary question was "would this be an issue after startup when the plant was running normally?"
There are several options to solving this particular issue. Alarm rationalization could be used to determine which alarms are actually needed and the appropriate severity of those alarms. Also, it is possible that variable alarm limits or alarm inhibiting may be required for different modes of plant operation. And, of course, looking at the total process is a key point when considering if a fluctuating level may actually be exactly how normal operations are intended. In some cases, a cascaded level to flow controller may be the best solution to achieve the desired results.
Whether integrating new processes or improving current processes, we can help you determine which solution is the best for your situation. We can help you understand each piece of the process, as well as the process as a whole so that all interactions can be understood. And, of course, we can help you weed out the pesky nuisance alarms. Give us a call, we will be happy to schedule a site visit to further discuss our expertise and how we can help you.