Flanged Hose Considerations

According to a recent report by the U.S. Chemical Safety and Hazard Investigation Board (CSB), 37% of incidents they investigated occurred in chemical and manufacturing facilities prior to, during, or immediately following maintenance activities; of the investigated incidents, 410 resulted in injuries while 86 led to fatalities. Flexible hoses play an important role in conveying media that has the potential to present significant safety hazards, based on their operating conditions including: elevated temperatures, corrosive chemicals, or high operating pressures. For hoses where threaded connections are not practical (especially in larger diameters), flanged connections are preferred.

Whenever flanged assemblies are required, there are a number of considerations that can be taken into account to improve both the safety and the service life of hoses with pipe flanges on one, or both ends.

By Frank Caprio, Corporate Trainer and Major Market Specialist, Hose Master

Understand the Application Requirements

An important first step, when considering hose safety, is to gain an understanding all of the operating parameters and applicable specifications to which the hose and fittings must comply. There are many different styles of flanges, with a variety of sealing faces, pressure classes, alloys, and sizes that vary according to industry specifications. The most common specification for flanges on hoses is ANSI/ASME B16.5, which dictates all allowable operating parameters for steel pipe flanges and flanged fittings through 24” Nominal Pipe Size (NPS). Other flange specifications include various international standards (JIS, DIN, et al.), and industry-specific standards such as the Tank Trailer Manufacturers Association (TTMA) or the American Water Works Association (AWWA). It is important to understand all aspects of the intended use of the assembly, as well as its required standards, to ensure that the flanged hose ends also conform to these requirements.

Diagram showing the proper ‘two-hole’ flange alignment technique (courtesy of NAHAD).

Flange Sealing Faces

Understanding the various sealing surface options is critical to ensuring a safe, leak-free connection on a flanged hose assembly. The type of face a pipe flange has – either a raised face or a flat face – indicates the type of gasket required for that sealing surface. Raised face pipe flanges are commonly used on flexible hose assemblies and are either slip-on or weld neck flange styles. Raised face slip-on (RFSO) flanges are designed to be slipped on to the end of a pipe and welded in place; they can also be welded onto the end of a pipe that has been attached to a hose. For example, a corrugated metal hose can have a pipe spacer welded to the hose, and a slip-on flange welded to that pipe. RFSO flanges may even be directly welded to the end of the hose. Weld neck flanges also have a raised face, but the back of the flange terminates in a pipe section designed to be butt-welded onto a pipe end; weld neck flanges may also be welded to a corrugated metal hose.

Plate flanges typically have a flat face with no raised surface, and therefore require a full-face gasket, which covers the entire flange face, to achieve a proper seal. A flat face flange should never be connected to a raised-face flange, as the different surfaces will create uneven sealing forces on the gasket between them. This can result in leakage or damage to the flanges.

As there are many other flange facing styles available, including O-Ring Face Seal (ORFS) flanges, Ring Type Joint (RTJ), and various male/female flange sets, getting sufficient information is critical. For both flat face and raised face flanges, the sealing surface must be free from dents or scratches that could prevent the gasket from sealing properly.

A floating flange must use a compatible stub end.

Fixed vs. Floating Flanges

Once the proper flange has been selected, the attachment method must be determined. Flanges can be ‘fixed’ to the hose end, or they can be ‘floating’ on a pipe stub end, which allows the flange to rotate in order to facilitate a connection to a mating flange. If fixed flanges are attached to both ends of a hose assembly, they must be carefully aligned to each other in order to meet standard piping code requirements and to prevent twisting the assembly during its installation. According to most standards (ASME et al), the proper way to align flanges on opposite ends of a pipe or hose is referred to as the ‘two-hole’ alignment rule. This states that the flange must be aligned so that a pair of bolt holes on each flange can evenly straddle a common centerline on both ends of the hose.

A common way to prevent problems with proper flange alignment is by using floating flanges. Floating flanges incorporate a pipe stub end with a captive freely rotating flange on the end of the hose. A stub end can be envisioned as a piece of pipe with one end flared outward, which serves as the sealing surface of the flanged connection. The stub end is attached to the hose, allowing the flange to rotate freely on the stub end which permits proper alignment of the flange when it is connected to the mating flange on the piping system.

Floating flanges are not ‘swivels’, as they are no longer able to rotate once they are connected to a mating flange. When installing a hose with a fixed flange on one end and a floating flange on the other, it is important to connect the fixed flange before connecting the floating flange to prevent twisting the hose. For longer hose assemblies, floating flanges may be desirable on both ends of the hose in order to minimize handling and facilitate installation. Floating RFSO flanges are typically used with a type ‘C’ stub end, and are commonly made using Schedule 10 pipe.

Lap Joint flanges are similar to RFSO flanges, except that they do not contain a sealing surface on the flange itself. Lap joint flanges can therefore only be used as a floating flange on a Type ‘A’ stub end, which are typically machined to Schedule 40 or heavier wall thickness. The machined face of the Type ‘A’ stub end serves as the sealing surface. Lap joint flanges are common in chemical plants, refineries, and steam systems wherever higher pressures or hazardous media are present. Manufacturers of these flanges and stub ends offer detailed information regarding flange and stub end compatibility.

Hose assemblies with a fixed flange on one end and a floating (rotating) flange on the other help eliminate alignment issues by preventing any twisting of the assembly during installation.

Look for Markings

If a user or operator is replacing a hose assembly that has been in service, it is important to inspect the outside surface of the flange for any stamped markings. The stamp will typically contain information to help identify the flange such as: size, alloy, pressure class, and the standard to which it conforms (e.g. B16.5, AWWA, et al). On floating flanges, the stub end may also be marked with the size, alloy, and pipe schedule as well as the stub end style. Looking for these markings is especially helpful if there is no documentation available regarding the assembly specifications, or if the hose is still in service, which makes flange identification more difficult.

Proper flange identification is only part of the challenge. Selecting the proper gaskets, hardware, and use of sound fabrication practices, are all important considerations, as they all have a dramatic effect on the ultimate safety and service life of flanged assemblies.

Look for flange markings to assist with proper identification.

Routine Inspections

Dock hoses, water cooling lines, and connections to various process equipment are just a few of the applications where flanged hose assemblies are common. Once the hose assemblies are placed in service, it is essential to inspect hose assemblies before each use, drain and store the hoses properly, and perform testing at regular intervals. It is recommended that users work with vendors who can recommend proper guidelines for hose installation, inspection, testing, storage, and replacement. It is also important to write detailed descriptions of all flexible hoses, including details on the end fittings. As incomplete descriptions can result in improper flange selection, simply writing “150# carbon steel flanges” is not specific enough.

By asking suppliers to conduct a hose survey to identify areas where improvements can be made, and by assisting with the writing complete descriptions of each hose assembly, one can increase safety and productivity while reducing unplanned downtime and maintenance.


Frank Caprio, Corporate Trainer and Major Market Specialist at Hose Master, LLC, has more than 35 years of experience in hydraulic, industrial, metal hose, and expansion joint products and applications, and is the ‘Dean’ of Hose Master’s training program, Hose Master University.

He is nationally recognized as a leading authority in metal hose, has authored various articles for industry publications, and has become a sought-after source for industry facts and trends.

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