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.
This method is implemented with a computerized control system that interfaces with the ventilation controls and the analyzer. The safety of the system is based upon a comparison of the measured LFL vs an expected LFL value calculated from process variables.
The potential errors that can be detected include:
- incorrect solvent type
- incorrect solvent application weight
- analyzer failures not otherwise detectable
- ventilation failure
For each configuration of the process intended to produce a certain product, several important variables are input to the control system. These include:
- the minimum and maximum speeds
- the minimum and maximum expected ratios of LFL reading to process speed (solvent load)
- the maximum expected ratio
- the ventilation settings (as predetermined for this product)
- the process temperature setting, alarms levels, etc.
By using predetermined ventilation settings the analyzer can be removed from the control loop and can now be recognized as a safety device. The control system generates three important deviation alarms from the recipe data:
- % LFL too high
- % LFL too low
- % LFL increasing at too high a rate (rate-of-rise alarm)
The system can compare the analyzer's measurement of LFL, compare it to the expected values of LFL based upon the process speed and solvent load, and generate an alarm if the difference between these two values are outside of the programmed limits. Therefore a variety of faults can be detected as a deviation in reading. It is this detection of unacceptable deviations that can be the basis for not only a detection of analyzer faults, but for an improvement in product quality through the elimination of variation.
It should be noted that a rate-of-rise alarm often has a general enough setting that it may be possible to implement this alarm regardless of the use of recipe control.