The bonnet or bonnet assembly is that portion of the valve pressure retaining boundary which may guide the stem and contains the packing box and stem seal. The bonnet may be integral to the valve body or bolted or screwed.
The bonnet if it is detachable, will generally provide the opening to the valve body cavity for removal and replacement of the internal TRIM. The bonnet is generally the means by which the actuator is connected to the valve body.