What is the maximum allowed grade of smoke for boiler operation?

The guideline that states "No darker than grade 1 for more than 5 minutes per hour" is correct as it establishes a standard for acceptable smoke emissions from boiler operations, emphasizing the importance of air quality and pollution control. Grade 1 smoke corresponds to a light gray color with minimal soot, indicating efficient combustion and lower emissions. Allowing it for a limited duration—specifically, up to 5 minutes per hour—acknowledges that while maintaining efficiency is crucial, the operation may experience brief fluctuations due to operational issues, fuel quality differences, or other factors.

This provision balances the need for compliance with environmental regulations while still allowing for practical operation in real-world conditions, where perfection is not always achievable at all times without some allowances. It reflects the understanding that some variability in emissions may occur without significantly impacting overall compliance or environmental standards.