Discharge nozzle material is specified to be corrosion-resistant and compatible with the extinguishing agent, so the nozzle remains durable and functional under discharge conditions. Brass and stainless steel meet these needs well; they resist corrosion from the wet chemical agents used in Class K systems and hold up to the pressure and wear of discharge, preserving the correct spray pattern. Plastic, aluminum, and wood don’t fit because plastic can degrade or soften, aluminum may corrode with the agent, and wood is neither durable nor fire-safe for a discharge path.

Sealing penetrations must not weaken the hood or duct. The requirement is that any device used to seal openings—when properly listed for that purpose—keeps the hood’s or duct’s structural integrity intact. If a sealing device compromised the framing, support, or overall strength of the system, it could fail under heat or airflow conditions, defeating the protection the system is meant to provide. So the focus is on preserving structural integrity. Weight, appearance, and thermal conductivity aren’t the controlling safety concerns in this context. A listed seal should preserve strength and fire resistance, not just look or weigh less or more, nor primarily affect insulation.

Interior welding is allowed for seams, joints, and penetrations inside the hood, as long as the welds are smooth and cleanable. The emphasis is on keeping the interior surfaces continuous and easy to wipe down, which is essential in a grease-laden kitchen hood environment. When welds are inside, the exterior surfaces remain flush and free of protrusions, reducing places where grease can collect and making exterior cleaning easier, while the interior welds provide a sealed, durable seam. External welds would leave ridges on the outside that are hard to clean and could trap grease. Riveted joints create gaps and crevices that are difficult to sanitize, and soldered joints may not hold up to the heat and grease exposure. So, internal welding that results in a smooth, cleanable interior seam best supports the hood’s maintenance and fire-protection requirements.

Unmaintained grease buildup creates a real fire hazard and can weaken the protection system that cleans and vents the kitchen space. Grease is highly flammable, so when it accumulates on hoods, filters, and inside ductwork, a heat source or a small spark can ignite it. Once burning, flames can travel through the ductwork and spread quickly to other areas. The grease also restricts airflow, making the exhaust system work harder, which can overheat components, reduce overall ventilation efficiency, and potentially impair the performance of any automatic extinguishing or detection devices tied to the system. Because of these dangers, neglecting grease removal increases both the chance of a kitchen fire and the likelihood that the safety system won’t function as designed. The other options imply benefits that grease buildup does not provide, so they don’t fit the reality of the risk.