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? 

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. 

In general, there are two cases:

So we know what the LFL is and how it's determined, but what about the factors that can effect these values?

Now that we understand what the Lower Flammable Limit is, let's look at how these values are determined and what that means for your application. 

LFL is determined empirically. This means that there are variations in the values published by different authorities at different times. 

For each flammable substance there is a level of concentration in air, usually expressed as a percent by volume, that is known as its Lower Flammable Limit, LFL, or Lower Explosive Limit, LEL. Below the LFL, the mixture of fuel and air is too lean to support combustion.

There are 3 important factors to consider when measuring flammability: 

The next topic in our "back to the basics" series is flammable hazards. Its important to understand what causes a fire or explosion to occur in a process before you can even think about preventing one.