Process Trip Limits: Identifying Causes and Implementing Effective Solutions

The Problem

Each process element has to operate within safe limits as part of the normal operation of a plant. The overall plant and processes are designed such that trip limits kick in as a protection mechanism only during abnormal plant operating conditions.

Plant control strategies should be designed such that the basic process control system can moderate plant upsets to bring the process conditions back within tolerances. However, there are times when process upsets are too large for the control system to have a meaningful impact. This is when trip limits are breached, and the plant shuts down as a protection mechanism. While plant trips due to unforeseen circumstances are inevitable, any other trips should be viewed as wasteful and their root causes eliminated.

The Consequence

The inability of the control system to deal with process upsets may be a persistent problem and will not go away unless changes are made to the control system or to the plant. The likelihood of unnecessary repeat trips is high.

The Causes & Solutions

There are several potential root causes of this problem, with the main ones being that control loops are not tuned properly and are under-performing.

Root cause #1: Control loops are not tuned properly and are under-performing.

The Solution: Deploy advanced loop-tuning software that operates at both a plant and a control-loop level. This way, you are able to tune loops individually and tune the plant as a whole, taking into account any interaction between control loops.

Setting up the software is fast, and good results can be achieved within hours. By simply providing historical data or a snapshot of a particularly problematic loop, theoretical optimum tuning settings can be calculated.

Root cause #2: Trip limits have been calculated and/or set incorrectly.

The Solution: Identify the most likely culprits by monitoring the appropriate parameters within the control system (e.g. persistent alarms) using advanced diagnostic tools. Armed with this information, recommendations on trip settings can be made as well as any changes that should be made in the control logic.

Root cause #3: The plant and processes have been physically under-designed and are unlikely to operate within tolerances at all times.

The Solution: Heat and material balance models can assist with identifying under-designed and over-designed plant elements. The biggest challenge in this area is to convince plant design engineers that the problem is not with the control system and is inherent in the plant or process design.