In order to determine the suitability of a particular analyzer system, it is useful to study potential process upsets.
There are two main types of process upsets that could present a hazardous condition:
- The steady-state (approximately time invariant)
- The transient (time sensitive)
Of these, the transient upset condition poses the greatest difficulties in detection and correction. But before we get to that let's look at the steady-state conditions:
In the steady-state condition, a hazard is produced when the process is relatively stable, but the solvent concentration unintentionally exceeds the safety margin.
- Excessive line speed
- Coating weight or web width
- Change in solvent mixture
- Slow loss of ventilation
For example, filters on the exhaust ventilation system could collect particulates over many days, causing a slow increase in solvent concentration. Detection of these conditions is made easier by the fact that the changes occur relatively slowly, and the time in which there is an opportunity to detect the change and correct it is usually sufficient. In addition, the ventilation decrease might occur slowly enough that routine monthly or quarterly maintenance of the filter is frequent enough to prevent a hazardous condition.
Therefore the process’ margin of safety, can often accommodate a large steady-state error before a hazardous condition exists.