What grade of smoke is permissible in boiler operation for no longer than five minutes?

In boiler operation, the permissible smoke grade is critical for maintaining environmental standards and ensuring efficient operation. Grade 1 smoke indicates a light smoke emission, which is generally viewed as acceptable for short durations. In the context of the question, having smoke no darker than grade 1 means that emissions are relatively minimal, contributing less to air pollution and signifying that the combustion process is efficient and producing fewer byproducts.

Accepting smoke emissions at grade 1 for no longer than five minutes suggests a temporary allowance for slightly higher emissions without compromising overall standards. This is particularly relevant in situations where a boiler may temporarily be adjusting to changes in fuel or operational state, such as initial startup or load changes.

In contrast, allowing darker smoke grades would indicate greater pollution and less efficient combustion, which may breach environmental regulations and indicate potential issues with the boiler operation. Therefore, choosing grade 1 represents adherence to best practices in stationary engineering, particularly when it comes to emissions control.