The available designs of body joints are screwed, union, bolted-bonnet. pressure-seal, and welded-bonnet joints.
- The screwed joint is shown in Figure 2.2. The male thread on the bottom of the bonnet screws into the female thread in the top of the body. It is used on small bronze valves.
- The union joint is shown in Figure 2.3. As in a pipe union, a female-threaded nut locks the unthreaded bonnet against the male-threaded body. It has an advantage over the screwed joint because the body and bonnet sealing surfaces do not rub together and wear when the joint is being tightened. This makes it preferable to the screwed joint when disassembly is necessary. Because of the high torque required to assemble or remove the union nut, the union joint is limited to use on small valves.
- The bolted-bonnet joint shown in Figure 2.1 has mating flanges on the body and bonnet that are held together with studs or bolts and nuts, On small valves, it is common to have the bonnet held on : bofts that screw directly into threaded holes in the top of the body. A gasket is placed between the flanges to prevent leakage. Depending on the pressure capability of the valve, the gasket can be flat, solid material: filled spiral-wound material; or a metal ring. Flat gaskets are found on low-pressure valves; whereas spiral-wound gaskets and metal rings are used on medium- and high-pressure valves. Bolted-bonnet joints can •be readily disassembled for repair and are used on all sizes of cast iron and steel valves.
- The pressure-seal joint is shown in Figure 2.4. In this design, internal pressure against the bottom of the bonnet causes it to press against a relatively soft seal ring. Because of its wedge-shaped cross-section, the seal ring deforms to press tightly against both the outside surface of the bonnet and the bore of the body, sealing the joint. The segmented thrust ring, which is held in a circular groove at the top of the body, absorbs all the thrust applied by the internal pressure. The hardened spacer ring between the seal ring and the thrust ring prevents deformation of the top of the seal ring. The retaining studs of the bonnet hold it in place and apply a preload to the seal ring before the valve is put into service. Pressure-seal joints are used on steel valves for high and very high pressure service.
- A welded joint is shown in Figure 2.5. Welding the bonnet to the body ensures against leakage at the joint but makes disassembly of the valve much more difficult. The welded bonnet produces a lighter valve than does the bolted bonnet or the pressure seal. This joint is usually found only on small steel valves.
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