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:

Back to the Basics: Safe reduction of ventilation

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? 

Back to the Basics: Limits to solvent vapor concentration

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:

  1. The fundamental safety limit
  2. The limit when an analyzer and control system are used

This week let's take a look & expand upon Case #1: Fundamental ventilation requirements.

Back to the Basics: Temperature Effects on LFL

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

Back to the Basics: Variation in Published LFL 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. 

It is likely that the following test conditions account for the deviations in these published values:

Back to the Basics: LFL

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: 

Pages