A bolted bonnet valve attaches its bonnet to the body with a gasketed, stud-bolted flange joint — the standard construction of API 600 gate valves and most cast globe and check valves. It allows full in-line access to the valve internals for maintenance and re-gasketing with ordinary tools.
The bolted bonnet is the default answer to a question every valve must solve: how to close the second opening that lets the machinery in. Body and bonnet meet at machined flanges with a gasket between — spiral-wound graphite/316 is today's standard for steel valves — clamped by a ring of stud bolts. Everything inside (wedge or disc, seats, stem, packing) can be reached by unbolting that joint with the valve in the line, which is why plants with maintenance workshops favour the construction.
Its limits appear at the extremes. Every gasketed flange is a potential leak path, counted in fugitive-emission audits and eyed suspiciously in lethal service, where a welded bonnet removes the joint entirely. And at Class 900+ in large bores, flange and bolting mass grows uneconomic — the territory of the pressure-seal bonnet. Between those extremes, roughly Class 150–600 across all sizes, the bolted bonnet is unchallenged.
API 600 specifies the construction in detail: male-female or tongue-groove flange facings, minimum bolting, gasket contact stress. All VTXFLOW cast gate, globe and check valves use API 600-compliant bolted bonnets unless pressure-seal or welded construction is ordered.