A double union ball valve, as the name suggests, has two unions – one on each side. The unions allow the valve to be unscrewed and withdrawn from the pipeline, where it can be serviced or examined for blockages, and then quickly reinstalled in the pipeline with the unions screwed back on. A double union ball valve’s key advantage is that it provides full-bore flow all the way through. For more info visit here: https://www.joinarticles.com/u....sage-of-union-ball-v