A valve bonnet acts as a casing on the valve body. It is nearly always screwed permanently into the valve body. During the manufacturing of the valve, the interior parts are put into the body and then the bonnet is fastened to contain all the parts inside. To get into the internal parts of a valve, the bonnet must be removed. There are a lot of valves that don’t have bonnets like for example Plug valves.

The Bonnets offer a leak proof fastening for the valve body. The bolted bonnet design will not leak through the packing chamber, whereas, knife-gate valves will. A typical leak proof stem seal with graphite packing rings can offer a much greater lifespan than a standard knife gate valve and there is a very low risk of contamination of the environment.

Gate valves can have the following different types of bonnets:

• A screw-in bonnet is the simplest form of bonnet, and offers a long lasting seal.
• A union bonnet is appropriate for appliances that require regular inspection and maintenance and offers added reliability.
• A bolted bonnet is used for larger valves and valves used in high pressure appliances such as those used in the Oil refining, petroleum, Natural gas and refining industries.
• A pressure seal bonnet is used for really for high pressure situations as those used in many industries like the Oil refining, petroleum, Natural gas and refining industries. The exclusive characteristic of the pressure seal bonnet is that the seal improves as the pressure in the valve increases as opposed to other types where the increase in internal pressure is likely to cause leaks in the body-bonnet joint.