Removing the bonnet gives access to the trim. Bolted bonnets with a gasket are the common type; pressure-seal bonnets, used in high-pressure steam, rely on internal pressure to squeeze a seal ring tighter; small forged valves often use welded or threaded bonnets. Extended bonnets move the packing away from cryogenic or high-temperature fluid so the packing stays within its working temperature.
Bonnet gasket is blowing on the feedwater valve, we'll need a clearance and new bolting to pull it.
Also heard as
- cover
Related terms
Body
The main pressure-containing shell of a valve, carrying the flow passage and the end connections.
Stem
The rod that carries motion from the handwheel, lever or actuator through the bonnet to the closure member.
Packing
Compressible rings packed around a valve stem to stop process fluid leaking out to atmosphere.
Yoke
The frame on top of a valve bonnet that holds the stem nut or mounts the actuator above the packing.
Backseat
A seat on the underside of the bonnet that a stem shoulder seals against at full open, isolating the packing.