A welded bonnet valve has its bonnet seal-welded or fully welded to the body, eliminating the gasketed flange joint and its leak path entirely. The construction is standard on small forged API 602 valves and specified wherever zero body-joint leakage outweighs the loss of in-line maintainability.
On a 1/2" Class 800 forged gate valve, a bolted flange joint would be half the valve's cost and its most likely failure point. Welding the bonnet on solves both: a single circumferential seal weld (over screwed-in threads) or a full-strength butt weld makes body and bonnet effectively one piece of steel. There is no gasket to creep, no bolts to relax with thermal cycling, no flange to leak — which is why welded bonnets dominate small-bore steam service, hydrogen units and anywhere fugitive-emission counts are audited.
The trade is maintainability. A welded-bonnet valve cannot be opened in the line; repacking the stem is usually possible, but seat or wedge repair means cutting the weld or replacing the valve. Since small forged valves cost less than the labour of overhauling them, the economics almost always favour replacement — the construction simply acknowledges reality.
API 602 recognises bolted, welded and threaded-with-seal-weld bonnet options; specify "welded bonnet" where the service is severe or emission-monitored. VTXFLOW forged gate, globe and check valves from 1/4" to 2" are available with seal-welded bonnets in A105, F304, F316, F11 and F22.