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.
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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.
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.
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
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.
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.
Corrugated metal hoses come in many configurations. These tools can help ensure you select the right corrugated metal hose for your application.
There are many components in a metal hose assembly and care should be taken when selecting each of them. Moreover, the components have their own unique technical limitations so it is important to make sure each of the components is compatible with your application. In much the same way as a “chain is as strong as its weakest link”, a metal hose assembly will only perform to the limits of its weakest component.
Once the components have been selected, the quality and skill of the fabricator assembling the components becomes important. The procedures and care used when fabricating assemblies also has a dramatic effect on the assembly’s overall performance.
Hose Master has invested a considerable amount of resources to researching metal hose fabrication and developing a state-of-the-art fabricating center. In addition, we share the welding technology we have developed with our fabricating distributors. All of this is done to promote quality and consistency in fabricating metal hose assemblies.
In this section, we will discuss the various components that make up a corrugated metal hose assembly, and what information a metal hose fabricator will need in order to make an assembly for your application. If you need assistance determining the information, we have also included an explanation of how to analyze the application and make the appropriate selections.
In order to make an assembly, the fabricator will need answers to the following five questions. For more information about any of these questions, or for a list of available options, click the topic heading for a more detailed explanation.
If you have the answers to these questions, a metal hose fabricator will be able to make the assembly. If you do not know the answers to all five questions, you will need to obtain them. The next section is designed to help you determine the answers.
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To properly design a metal hose assembly for a particular application, the following design parameters must be determined. To help remember them, they have been arranged to form the acronym “S.T.A.M.P.E.D.”
Corrugated metal hose is used in a very broad spectrum of applications. Just as the hose, fittings, and other assembly parts must be tailored to suit the demands of the service, so must the methods of joining these components. While standard production joining methods work very well for the majority of service demands, the following extremes may dictate special joining or fabrication techniques:
Hose Master has developed specialized welding, brazing, joining, and fabrication procedures to assure the integrity and serviceability of metal hose assemblies in even the most extreme applications.
The fabrication options to be considered are:
In each of the following sections, the standard method and available options are explained. Select the options best suited for your application.
This method will be used unless another method is specified
Standard fabrication of an assembly generally consists of:
Standard fabrication sometimes leaves a portion of the cut corrugation, or corrugation “lip”, just under the base of the fitting. In specialized applications this residual lip may not be desirable. To prevent any exposed corrugation edges from causing damage, the hose can be specially prepared for welding by cutting the corrugation on the crest, rather than in the valley, thereby removing the lip.
For applications in which corrosion is a concern, all crevices and fissures must be minimized. Specialized hose and fitting preparation, in conjunction with proprietary welding techniques, is available to provide a full penetration hose-to-fitting weld that is smooth and crevice free.
Assemblies operating at the upper limits of their rated working pressure or in severe service may benefit from a braid-over construction. The fitting is first welded to the unbraided hose. Then a special metal reinforcing ring is installed over the fitting and next to the weld. Finally the braid is drawn over the end of the hose and the ring, and welded to the side of the fitting. This technique reduces the amount of heat introduced into the braid wires, nearly eliminates the heat effected zones of the cap and attachment welds, and maximizes the wire strength. Braid-over construction may also be used for specific high cycle applications.
Note: Always test an assembly with a medium that has a smaller molecular or atomic size than the service required.
Every corrugated hose assembly is leak tested prior to shipment. Standard testing consists of pressurizing the assembly with air and then submerging the entire assembly under water. This method is reliable and sufficient for the majority of applications.
While the standard test is designed to detect leaks, hydrostatic testing is designed to test the assembly’s strength. Testing of an assembly to its full permissible test pressure can be economically and accurately accomplished by filling the assembly with liquid while concurrently evacuating all air. The assembly is then hydrostatically pressurized using high pressure pumps. The test pressure is maintained for a predetermined period of time.
Testing with air under water, at pressures of up to psi, is available for specialized applications. For a more sensitive test, the use of gases such as nitrogen or helium can be requested.
Dye penetrant testing is available for both leak and for weld bead inspection, in accordance with Hose Master procedures or to customer specified standards.
This is the most sensitive leak detection method generally available. The standard test method is to attach the assembly to a mass spectrometer and generate a very high vacuum in the assembly. The exterior of the assembly is then flooded with helium. The relatively tiny helium atoms penetrate even very small openings and are drawn into the mass spectrometer where they are detected and the leak size quantified. Helium Mass Spectrometer testing can be modified to satisfy customer or regulatory agency requirements.
The hydroforming method of corrugated hose manufacturing inherently yields a very clean product. However, specialized cleaning for specific applications is available upon request. Contact Hose Master’s Customer Service Department for details.
All assemblies are shipped with protectors over sealing surfaces such as threads and flange faces. Spacer bars are installed on all shorter double-flanged assemblies to prevent compression of the assembly during shipping and handling. Special packaging is available to suit customer requirements, including crating, plastic bagging, labeling, and custom fitting protectors.
Hose Master places emphasis on continual improvements within our manufacturing processes, product offering and services. Hose Master reserves the right to make changes without further notice to any products contained herein.
Contact us to discuss your requirements of Flanged Flexible Metal Hoses. Our experienced sales team can help you identify the options that best suit your needs.