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There are four main detector types used for combustible gas analyzers:

The control of variable ventilation rates can undergo an additional improvement that can result in the detection of certain system faults with a greater margin of safety than the previously discussed methods.

Recipe controls use modeling or prototyping or historical records of process variables to determine limits for detection of unacceptable deviations in the LFL control system.

Variable ventilation controls allow the maximum reduction in ventilation, and thus maximum economy. It also allows some extra corrective action but also one important safety concern: Variable ventilation is based upon the measurement of the L.F.L. concentration and adjustment to ventilation rates through damper or blower controls. 

There are two main types: 

Places where pockets of high concentration might occur through uneven ventilation flow or obstruction by the product being coated should be sought out and eliminated. 

In general, the concentration immediately at the product being coated is very high, as the liquid solvent evaporates and only just begins to mix with air. 

The minimum ventilation calculations should be made for the “worst case.” A direct measurement of the ventilation rate - using properly situated low-pressure switches - should be connected to the control logic so that the process shuts down immediately if ventilation falls below the minimum required.

In such cases the following shut down logic should be applied: 

Once the minimum ventilation requirement is known, dampers must be secured so that ventilation is never reduced below the minimum. The best method is to cut away the damper so that it is not possible under any condition to reduce the ventilation rate to an unsafe level. Manually adjusted stops, and sometimes even welded stops, have been found to be insufficient to prevent accidental loss of ventilation due to incorrect damper settings.

There are several fundamental safety precautions which should be applied to all dryer designs.

Last week we discussed the first type of process upset that could present a hazardous condition, the steady-state, this week let's focus on on the condition that poses the greatest difficulties in detection and correction.

Process upsets from transient conditions produce a hazard from an unstable process that is changing relatively quickly. Causes include: 

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: 

Last week we discussed the 2 cases at which the authorities have determined that certain industrial processes can proceed at solvent vapor concentrations up to, but not exceeding, some percentage of the Lower Flammable Limit.

Let’s focus on case #2: What happens when an analyzer and control system are used?