Pipe Flanges: Key Considerations for Metal Hoses and Expansion Joints

By: Frank Caprio | On: May 28, 2024

While corrugated metal hoses and expansion joints can accommodate many different end fitting types, pipe flanges are one of the most common connections specified by customers. However, we find there is a good deal of confusion in our industry as to the proper use of pipe flanges on metal hoses and expansion joints. Here are a few things to keep in mind when helping customers with flanged assemblies.

Defining Pipe Flanges

Pipe flanges allow for easy installation of the component into a piping system. Proper flange usage avoids many potential problems associated with threaded connections (space limitations, thread allowances, numerous thread standards, thread sealing challenges, etc.). Flanges must be manufactured in accordance with well-defined specifications, which are then referenced in piping codes that are used in various industries. Industry relies on these codes and standards to ensure the safety and consistency of a properly designed piping system.

Fixed Flanges

Pipe flanges can be attached to flexible connectors (metal hoses and expansion joints) in a number of ways, and it is important to recognize the advantages of each attachment style. First, a flange may be directly welded to the end of the hose or expansion joint, which is referred to as a “fixed” flange. This is the most basic method of attachment. It is important to note that a flange welded directly to a flexible metal hose may require a section of pipe (a “pipe spacer”) to be welded between the hose and the flange, in order for the assembly to conform to various piping code requirements.

If a hose or expansion joint has fixed flanges on both ends, it is critical that these flanges align properly with the mating connections. Otherwise, the connector may be twisted in an attempt to align the bolt holes during installation, creating torsional stresses that can damage the connector. The NAHAD Hose Safety Institute lists recommendations for proper bolt-hole alignment, which follows sound design practices and conforms to ASME’s Pressure Piping Code B31. If non-standard bolt-hole orientations are present, a good way to ensure proper alignment is to wait until the hose or expansion joint has been installed before welding the mating flange onto the adjoining piping system. This will ensure that the flange on the pipe will properly align with the flange on the hose end.

Floating Flanges

Another way to avoid flange misalignment is by installing a flange that rotates on the end of the flexible connector. These are referred to as “floating” flanges and come in various configurations. Floating flanges rotate without twisting the flexible connector, allowing the installer to align the bolt holes to the flange on the mating piping system. Once these floating flanges are in place and bolted to the mating flange, they no longer swivel. This means they cannot accommodate rotational movements after installation. If a rotating movement is required during operation, a properly designed swivel joint should be used to prevent twisting the flexible connector. For flexible metal hoses, flanges are installed onto a stub end, which is then welded to the hose. The face of the stub end becomes the sealing surface for a floating flange. Some stub ends are made by flaring a piece of pipe on one end; others are made by machining the stub end out of solid material. They can be made in various alloys and pipe schedules, and have different configurations that accommodate different flange types.

C-Stub with Floating Flange

Vanstone Flanges

While metal expansion joints may also contain floating flanges on stub ends, there is an alternative that provides a floating flange with a much more compact profile: Vanstone flanges. Vanstone flanges are attached directly to a metal bellows by slipping the flange over the unconvoluted skirt of the bellows (also called the “cuff” or “tangent”), and then flaring out the skirt to form a sealing surface over the face of the flange.

Vanstone flange connections take up much less length than stub ends, while still allowing the flange to rotate during installation. Because no welding is required for a Vanstone connection, it is a very economical means of flange attachment. However, the Vanstone connection may not be suitable for high-pressure applications, and require a special gasket, so due diligence is needed before these should be recommended.

Vanstone Flange Diagram

Not only do floating flanges compensate for bolt-hole misalignment in a piping system, they also isolate the flange from the media being conveyed. This means that a flange of a less expensive alloy may be allowed, as it does not come into direct contact with the material being conveyed.

Trust the Experts

There are many different flange specifications out there, and knowing which one to use can be confusing. It is not merely a “good idea” for any piping design to be reviewed by a qualified piping engineer or designer; failure to do so can lead to a catastrophic failure of the system. Once the proper flange style has been identified, the pressure class, alloy, method of attachment, gasket selection, nut/bolt specifications, and other factors must then be considered. This will ensure the flanged connections are safe and appropriate for the intended service.

Still unsure? Reach out to Hose Master today and let our team of experts help you design the safest and most reliable assembly for your application.