VALVE Body-Bonnet Design

Gate valves are normally available in five different body/bonnet joint designs. They are: screwed, union, bolted-bonnet, welded-bonnet & pressure-seal.

* The screwed joint is the simplest design. However it is only used for inexpensive bronze valves that rarely if ever require disassembled.
* The union joint is also primarily used on bronze valves, but the union design allows for easier disassembly for repair and maintenance.
* The bolted-bonnet joint is the most popular joint and it is used on the vast majority of gate valves in industrial use today. Unlike threaded and union bonnet valves, the bolted-bonnet connection requires a gasket to seal the joint between the body and bonnet. On lower pressure valves, sheet gasket materials are used. ANSI Class 150 steel valves usually employ a corrugated soft iron or graphite/corrugated soft iron gasket. Valves of class 300 and higher employ either a spiral-wound or ring joint type gasket.
* The pressure-seal joint is energized by the fluid pressure in the valve body acting upon a wedge shaped, soft iron or graphite gasket wedged between the body and bonnet. On a pressure-seal valve, the higher the body cavity pressure, the greater the force on the gasket. Pressure-seal bonnets are used extensively for high-pressure high-temperature applications, such as the power industry. Pressure-seal valves are much lighter than their corresponding bolted bonnet designs. Due to the pressure energization of the seal ring, they are normally not used in pressure classes below ANSI class 600.
* Welded bonnets are a very popular body-bonnet joint for compact steel valves in sizes ½” through 2” and pressure classes 800 through 2500, where disassembly is not required. The higher pressure welded-bonnet type valves rely on threads to handle the force generated by the body cavity pressure, while a small peripheral weld bevel actually contains the pressure. Like pressure-seal valves, welded-bonnet valves are much lighter than their bolted-bonnet counterparts.