Back to the Basics: Catalytic Combustion

The catalytic detector consists of two small electrically heated beads having a finely divided platinum or palladium coating on the surface. A reference bead ONLY responds to changes in:

  • temperature
  • pressure
  • humidity

The active bead ALSO responds to:

Back to the Basics: LFL Analyzers and Control Systems

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

  1. catalytic
  2. infrared
  3. flame ionization
  4. flame temperature

Although several different types of sensors are employed as LFL monitors, each has an appropriate application to which it is best suited. Other types of detectors, for example electrochemical and tin-oxide semiconductor types, are generally inappropriate. 

Back to the Basics: Recipe Controls

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.

Back to the Basics: Variable Ventilation

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: 

Back to the Basics: Elimination of “pockets” of high concentration

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. 

Back to the Basics: Detection of Ventilation Loss

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: 

Back to the Basics: Secure Damper Positions

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.

Back to the Basics: Minimum Ventilation Rate

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

These requirements call for a minimum below which the ventilation rate is never reduced. This predetermined, fixed value is calculated from solvent input and flammability characteristics. Fundamental safety does not require use of an analyzer but, if none is used, the LFL limit is usually decreased to about one half the value allowed when an analyzer is installed.

Back to the Basics: Transient conditions

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: 

Back to the Basics: The Steady State

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: 

  1. The steady-state (approximately time invariant) 
  2. 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: