Valve End Connection Concerns

Once the valve has been sized, but before it can be selected, the end connections mating the control valve to the adjacent piping system must be specified. The end connections are the configurations provided to make a pressure tight joint to the pipe carrying the fluid to be controlled. End connections on valves must match piping design and specifications. In addition to the piping specifications, other factors to be considered are:

•  The piping forces that will act on the valve body.
•  The forces generated in the piping by valve action.

There are three common methods of installing control valves in pipelines:

1. Threaded end connections
2. Flanged end connections (including flangeless or wafer style)
3. Welded end connections

For many general service applications, valve end connection selection is a simple question of whether the desired end connection style is available for the type of valve under consideration. However, for demanding services, it is a matter of ensuring rigid and leak free pipe to valve connections. In the severe services, proper valve end connections must be ordered when specifying the valve, which may add to the cost. An example of this is when special nipples, reducers and/or expanders must be ordered with the valve to accommodate dissimilar metal welds between the valve and connecting pipe (see later section on stress relieving requirements).

End connections on valves are usually specified by the piping design engineer and it is preferable to match the piping design specifications. However, the piping designer does not determine the forces generated in the piping by valve action and must be informed of these by the valve engineer. Proper valve end connections can then be selected according to the strength required to maintain gasket sealing stress under all operating conditions and the degree of gasket seal tightness considering:

• Toxicity
• Amount of product contamination allowed on vacuum service
• Fire hazard
• Potential for human burn hazard
• Erosion and crevice corrosion
  

Stress Relieving Requirements for Dissimilar Metal Welds

Weld ends on chrome-moly steel bodies are often welded to carbon steel piping. Chrome-moly bodies are specified for high temperature service and also to prevent erosion for flashing service. Welding on Chrome-moly valve bodies, or any piping wall thicker than 1 inch, requires stress relieving. This should be localized to prevent body bridge wall distortions in globe valves (produces an ovalized seat joint).

For valves requiring dissimilar metal welds between the valve body and the adjoining pipe, it is advisable to specify pipe nipples with the valve when ordered. The purpose is to have the valve manufacturer perform stress relief locally at the factory, on the nipple attached to the valve, rather than having to perform stress relieving in the field. The same rule applies for valve applications smaller than the adjacent piping where pipe reducers and/or expanders (downstream from steam or gas service) are required. It is best practice to procure the reducers or expanders with the valve body and let the valve manufacturer fabricate joints for dissimilar metal welds and perform any required stress relieving.

This practice is also strongly recommended for installations where the piping is Chrome-moly material. This permits furnace stress relief of the all chrome-moly body-nipple or reducer/expander assembly. It also produces true alignment as the body is factory machined for the bonnet gasket and seat joint.

Joint Connections for Valve Bodies Lined with TFE or Elastomers

In corrosion resistant service involving lined valve bodies (lined with TFE or elastomers), care must be exercised to ensure that the protected end of the valve body connects to the protected end of the piping with adequate gasket loading. It is also important that this loading is limited so that bolt tightening does not extrude the liner. Piping engineers should give adequate instructions to field personnel to make the installation successful.