Back to the Basics: Sampling System Issues

The speed and accuracy of the analyzer depend equally upon the sampling system and the detector. The final development of the worst accidents occurs in seconds. It is part of the last line of defense against fire and explosion. There’s precious little time between the rise in concentration and the explosion. That’s a very good reason to pay close attention to the analyzer’s response time.

Did you know? 10 Facts about the PrevEx

 

10 FACTS ABOUT PREVEX ANALYZERS

the PrevEx is our flammability analyzer for lower flammable limit monitoring (LFL/LEL)

Back to the Basics: Flame Temperature

The flame temperature detector measures the heat given off by a flame as it burns combustible gas that diffuses into the flame from the sample. 

The small, well-regulated flame heats the tip of a temperature sensor suspended directly above it. The signal produced by the sensor when no flammable vapors are present drives the LFL indicator up to 0% LFL. This failsafe technique is known as a "live" zero because a weakening or loss of flame caused by lack of fuel will generate a downscale malfunction alarm. 

Back to the Basics: Flame Ionization

Flame ionization is a well-established measurement technique.

Back to the Basics: Infrared Absorption

Combustible gases absorb infrared radiation at certain characteristic wavelengths.

A typical non-dispersive infrared detector passes a pulsed source of infrared energy through the sample, and measures the energy received by two detectors.

One “active” detector responds to wavelengths in the same band as the combustible gas, and the other detector responds to wavelengths in a reference band to compensate for changes in the source.

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. 

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