Bonnet and Bonnet Joint Characteristics

Bonnet and Bonnet Joint Characteristics of gate, globe, and angle valves

Which is best?

In choosing valves, the service characteristics of the bonnet joint should not be overlooked. Bonnets and bonnet joints must provide a leakproof closure for the body. There are many modifications, but the three most common types are screwed-in bonnet, screwed union ring bonnet, and bolted bonnet.

Screwed-in Bonnet

The simplest and least expensive construction, frequently used on bronze gate, globe, and angle valves, and recommended where frequent dismantling is not needed.

When properly designed with running threads, and carefully assembled, the screwed-in bonnet makes a durable pressure-tight seal, suited for many services. On modified steel valve designs such as the lip-seal valve with a weld around the periphery of the body-bonnet juncture, the screwed-in bonnet withstands even high pressures and temperatures.

Screwed Union Ring Bonnet

A good choice for quick dismantling and reassembly - yet a strong, well-reinforced joint.

Convenient where valves need frequent inspection or cleaning - also for quick renewal or changeover of disc in composition disc valves.

Separate union ring applies direct load on bonnet to hold a pressure-tight joint with body. Turning motion used to tighten ring is spent between shoulders of the ring and bonnet. Hence, the point of seal contact between bonnet and body is less subject to wear from frequent opening of the joint. Contact faces are less likely to be injured in handling. Union ring gives the body added strength and rigidity against internal pressure and distortion. While ideal on smaller-size valves, it is impractical on large sizes.

Bolted Bonnet Joint

A practical and commonly used joint for larger-size valves or for higher pressure applications.

Adaptable to all types of gasketing.

Multiple bolting, with small diameter bolts, permits equalized sealing pressure without the excessive torque needed to make large threaded joints. Only small wrenches are needed.

Has practically no limitation for size. Only the highest pressures and temperatures tax its capacity to permanently hold tight.

Lip-Seal Bonnet Joint

lip seal design features simplicity. The body and bonnet are screwed together until a firm metal-to-metal contact is made between the smoothly machined, flat surfaces on the shoulder of the bonnet and the top of the body. The shoulder of the bonnet is smaller in diameter than the mating area of the body, thus permitting the use of a fillet form of seal weld around the periphery of the connection. Dismantling is accomplished by grinding off the fillet weld and unscrewing the bonnet. The design makes possible compact, relatively lightweight valves ideal for high pressure-temperature services. Absolute tightness, full seating area, and freedom from bonnet joint maintenance are other advantages.

Pressure-Seal Bonnet Joint

Newest and most effective bonnet joint for sealing the highest pressures and temperatures, especially in steam service.

Tightness of seal does not depend on nuts, bolts, and threads as in conventional bonnet joints. Instead, Pressure-Seal bonnet joint utilizes line fluid pressure to seal the joint. The greater the pressure, the tighter the seal.

The actual joint is inside the valve, and is sealed with a wedge-shaped seal ring. Internal fluid pressure acting on the entire underside area of the bonnet, is concentrated at the smaller contacting area of the wedge-shaped ring to make a pressure-tight metal-to-metal joint.

Available in gate, globe, angle, check and stop-check valves.