– Integral vent valve – needle kind, screwed bonnet or flanged bonnet (OS&Y) – Screwed vent valve – ball valve This kind of Double Block and Bleed Valves Possess a patented design and style which incorporates two ball valves in addition to a bleed valve into one particular compact cartridge http://valves-and-fittings28383.topbloghub.com/227972/5-simple-statements-about-dbb-valve-explained