When a brass valve is stained blue, what does it indicate?

When a brass valve is stained blue, it typically indicates that the valve has been used with anhydrous ammonia. Anhydrous ammonia is a compound that has a specific chemical reaction with brass, leading to the formation of a blue-green patina on the metal surface. This reaction is a result of the corrosive nature of ammonia, which can cause stress corrosion cracking in brass and lead to visible discoloration.

Understanding this association helps in identifying potential issues with the valve and the equipment it is connected to. Recognizing the blue staining as a sign of previous exposure to anhydrous ammonia also serves as a crucial reminder to handle the equipment with care and to inspect for any possible damage or operational failures that may have occurred as a result of its use.