Which item provides easy assembly and dismantling of screwed piping connections?

The item that provides easy assembly and dismantling of screwed piping connections is unions. Unions are specifically designed to allow for quick connections and disconnections in piping systems. They consist of three pieces: two end fittings and a central nut that, when tightened, holds the pieces together. This design allows for a straightforward method of connecting and disconnecting pipes without the need to unscrew the entire pipe, making maintenance and repairs much more convenient.

In contrast, while couplings can join pipes and may facilitate some level of disassembly, they typically require the entire connection to be unscrewed to separate the pipes. Flanges also enable the joining and detaching of pipes but are more complex, requiring bolts and nuts which can complicate the process and may not be as quick as using a union. Bolts are merely fastening elements and do not serve the purpose of connecting pipes themselves; they are usually needed in conjunction with flanges. Thus, unions stand out as the most effective solution for achieving quick and easy repairs or adjustments in piping systems.