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:
- Stop solvent load
- Turn off heating (to reduce evaporation rate and sources of ignition)
- Warn the operator of condition
Because the process might already contain a substantial amount of unvaporized solvent, the concentration can continue to build rapidly following loss of ventilation, making this a very serious type of accident.
When a continuous solvent vapor concentration indicator and controller is provided, the safety ventilation rate shall be permitted to operate at a safety ventilation rate lower than that specified, in accordance with NFPA Standards.