What is an Insert Flange?
By Millard Fitzgerald, Jr.
Reprinted from Flow Control magazine, May 1997
©1997 Witter Publishing Corporation

Insert Flanges have been used on piping and pressure vessels for almost a half-century and offer many advantages. They have a proven track record in the food, chemical, petrochemical and pharmaceutical industries. Various styles, materials and pressure ratings are available to satisfy the varied requirements of the end user and his piping contractor.

What is an Insert Flange?
Insert Flanges are a specialty in the arena of pipe size flanges and consist of two parts - the insert and the flange ring. The flange ring is the outer part of the insert flange assembly, containing the bolt holes. Except perhaps for the thickness, it is similar to the standard B16.5 flange beyond the raised face or gasket surface. It's function, along with the bolting, has but one major purpose, and that is to be the clamp that applies the necessary axial force against the insert to seat the gasket and to withstand the pressure and other piping forces and moments. Material selection is relatively simple, considering only atmospheric corrosion, strength, temperature and economy.

The insert, on the other hand, has several duties to perform and for that reason is offered in many different styles. In some ways it is similar to a pipe lap joint stub end. The front portion of the insert contains the gasket-seating surface, which must be machined true and with the proper finish to seat and seal against a variety of different gasket materials and styles. The rear portion contains a suitable surface or hub for the attachment fillet or butt welds to the process and jacket pipes. Depending upon the style, the OD (or shoulder) at the rear may be may be sized to accept the bore of the jacket pipe. Again, depending on the style, this shoulder may even contain a step to simulate a buttweld with integral backing. The machined bore is sized to allow the process pipe to slide inside as a slip-on or socket, or to match the ID of the pipe when provided with a butt welding hub or even threaded to accept the threaded end of the pipe. The materials from which the insert is made must be compatible with both process fluids and jacket fluids and provide a reasonable level of corrosion resistance.

As an alternative to clam-shell construction, a transit hub allows the jacket pipe to slide further along the hub, thus allowing access to the welding area of the process pipe at the other end of the spool.

Since insert flanges are a specialty item, they are manufactured in much smaller lot sizes than the B16.5 flanges. And because they are not a commodity item like their B16.5 flange relatives, they are manufactured to closer tolerances. They also offer an economical means of fabricating piping spools.

Insert styles are offered as butt weld, slip-on, socket, thread-on and even blind. They can be obtained with the same gasket finishes and facings as B16.5 flanges, including male and female, tongue and groove, and even grooves for O-ring and RTJ gaskets. Some manufacturer offer custom styles to meet almost any requirement. Polished surfaces are also available when required by the process conditions.

The ability of the flange ring to rotate is a feature appreciated by piping installers and maintenance personnel for maintaining bolt hole alignment since field conditions are far less precise than the fabrication shop. The two piece construction of the insert flange also offers the economy of matching the insert material to the process pipe (usually some corrosion resistant alloy) while the outer flange ring may be manufactured from steel. When the environment requires the flange ring to be made of some alloy the rotating feature is still maintained. The insert and flanges can be made of just about any alloy used in process piping.

Reference to "slip-on" as a type of jacket insert flange indicates that the insert will slip over the process pipe to allow front and back fillet welds.

Development of Insert Flanges
Insert flanges were initially developed in the 1950s as a way of fabricating pipe without welding, using roller expansion technology. Roller expansion, even without a seal weld, was and is a proven and highly reliable method. When performed properly the roller expansion method, similar to sealing tubes into a tubesheet of a shell and a tube heat exchanger, provides a strong mechanical lock and offers an alternative where welding is not permitted.

Insert flanges then incorporated the idea of the rotating flange ring. This contribution has saved time and money in the fabrication of the piping systems, by making the alignment of insert flanges for bolting especially easy.

As the insert flange was being improved upon and changed to meet new applications, an insert flange designed for jacketed or containment pipe cations became an industry standard. Engineers began specifying this insert flange because it was designed specifically for jacketed piping. Fabrication of jacketed pipe spools was made easier and more efficient.

Insert Flanges and Jacketed Piping
The use of insert flanges, made specifically to make jacketed piping assembly easier, also avoids the cost of alternate jacket closure methods such as closure rings, swaging the ends of the jacket pipe and custom machining the backs of standard B16.5 flanges. Instead the inserts are machined to offer easy and proven attachment methods to the process pipe and jacket pipe via slip-on type fillet welds, butt welds, or full penetration corner joints.

Reference to butt weld as a type of jacket insert flange indicates that the insert will butt weld to the process pipe, allowing a full penetration V-groove weld.

Fabrication
Clam-shell construction (see Figure 1) is the most commonly used method of fabricating jacketed or containment piping using insert flanges. In this method a section of the jacket pipe, closest to the flange, is split to allow an attachment weld to the back of the insert and the process pipe. An alternate method provides a means for the jacket pipe to be alternately slid back and forth during assembly to allow clearance for the welder to make the attachment weld between the back of the insert and the process pipe. They provide a very economical approach to jacketed pipe assembly when the specifications or codes require the back-of-the-insert hub to be welded to the inner pipe. This is also true when the specifications require a butt weld between the end of the inner pipe and the back of the insert. In addition, it allows inspection and non-destructive examination of the same weld prior to welding the two ends of the jacket pipe and avoids the necessity of clam-shelling the jacket. Insert flanges are offered with flanges matching B16.5 flange bolting for either the process pipe size or the jacket pipe size. The former are referred to as non-reducing or line size and the latter as reducing or jacket size.

Reducing Temperature Changes in Jacketed Piping Systems
A frequent concern of process engineers and piping designers is how much temperature drop will occur from the back of the insert to the gasket surface in jacketed piping. A definitive answer is not possible since there is an infinite possibility of combinations and conditions. However, the following guidelines can be offered. (see figure 6)

     Reducing Insert Flange          Non-Reducing Insert Flange           Slip-On Flange

Getting the jacketed medium (usually heating) as close to the gasket surface of the flange is the obvious first choice. As mentioned above there are several methods for terminating jackets at flanged connections. The most common method is to end the jacket at a closure ring short of the back of a non-reducing flange so that attachment welds to the pipe can be made to both the hub of the flange and the closure ring and there is space for nuts and bolt heads. Another method is to terminate the jacket right at the back of a reducing flange hub.

Insert flanges simulate this second method, bringing the jacket medium directly against the back of the insert portion and providing similar thermal performance to that of solid B16.5 slip-on flanges while offering assembly and rotational advantages. However, many times it is not possible to use reducing flanges.

When process pipe size flanges are required, the jacket medium cannot be brought as close to the gasket surface because of wrench clearances. In the case of insert flanges, the insert length must be extended to provide room for nuts and bolt heads. The amounts to about the same distance as a jacket closure ring would have to be placed for the same reasons. Here the advantage in thermal performance clearly goes to the insert flange. The mass of metal in the insert acts as a heat conduit up to the gasket surface, drastically reducing the temperature drop along the process pipe wall. One study indicated that the temperature drop with piping using a closure bar was three times as much as than using a non-reducing insert flange.

Piping and piping components are subjected to various forces created by thermal expansion, the weight of equipment and vibration. Insert flanges are able to deal with these forces just as well as standard B16.5 flanges.

To accomodate the jacket pipe there is a 1/4 to 3/8-inch hub, machined 1/2 inch above the hub of the lap joint flange, for the specified jacket schedule.

Engineering Considerations
Deliberations for the need of expansion joints, pipe hangers, restraints, etc. are the same as with any other method of jacketing piping. It must be noted, however, that it is the pipe designer, or the end user's responsibility to analyze the various conditions to which the insert flanges will be exposed and to specify appropriate material and attachment style as well as the pressure class required. Beyond providing pressure vs. temperature ratings, it's not practical for the insert manufacturers, just as this is true for the B16.5 flange manufacturer, to anticipate all of the possible piping loads and stresses which might be imposed on their products. When unusual conditions exist and the end user is concerned about their effects on the insert flange, they may wish to conduct their own finite element analysis and judge the suitability of insert flange accordingly.

Insert flanges are generally designed to the SAME code, Section VIII, Div. 1, and specifically Appendix 2. This means that in order to match the B16.5 pressure/temperature ratings, the thickness of the flange rings are generally greater than the B16.5 flanges themselves. The reason for this is twofold. Normally the flange rings of the insert flanges are hubless, and secondly, the B16.5 flanges geometry including their thicknesses were established prior to the inclusion of the present day version of Appendix 2 into the Code. For instance, 3O-150# B16.5 slip-on flanges, when used with spiral wound gaskets and high strength bolting such as SA-193 B7, would be considered over-stressed when subjected to Appendix 2 analysis. As a matter of fact, even the bolting would be considered over-stressed. However, since there had been many years of successful service with the use of the B16.5 flange geometry, they have been accepted as an industry standard. However, insert flanges are a hybrid of B16.5 and the pressure/temperature ratings must be established by calculations and/or proof testing.

Usually insert flanges are designed for use with standard gasket dimensions such as found in SAME B16.20 and SAME 16.21 for metallic and non-metallic gaskets. To minimize leakage at the gaskets, it is important that the piping designer or specifying engineer carefully carefully consider suitable gasket and bolting materials based on the anticipated and operating and upset conditions which the piping system will experience in service. Just as it is true with B16.5 flanges and lap joint stud ends, a gasket surface finish should be specified to match the type of service gasket used. When in doubt, these requirements should be discussed with the insert flange supplier or manufacturer to insure that the particular style selected is suitable for use with these conditions.

As mentioned above, certain B16.5 150# flange size have marginal reserve strength in themselves and their bolting, when used with gaskets with high seating stress fractures. Since insert flanges must co-exist with the same bolt sizes as their B16.5 cousins, the same considerations should be applied. When using 150# class flanges, if the actual pressure and temperature conditions are near the maximum P/T ratings and there are a high number of pressure and/or thermal cycles, the designer may wish to specify a high pressure class flange. This would provide more clamping force from increased bolting area. However, even when the total bolting area is not marginal, but the number of thermal cycles and/or magnitude of the thermal expansion is high, or mechanical shock loading is a concern, the designer may wish to specify conical disk springs under the bolt heads or nuts. This offers more reliable clamping forces on the gasket that merely pre stressing the bolts. Even in cases where cycling and shock are not an issue, properly sized conical disk springs can be used to insure a more uniform method than torque wrenches for pre stressing bolts.

Since insert flanges are a specialty item, it is important for the designer to become familiar with the available product styles, features, advantages, and limitations.

About the Author
Millard C. Fitzgerald, Jr., is an engineering and computer consultant with 30 years experience working for metal fabricators designing ASME code pressure vessels, heat exchangers, storage tanks and related equipment. Responsibilities included the management of engineering and estimating departments, establishing standards and procedures, implementing CAD, as well as writing weld procedures and quality assurance procedures. Mr. Fitgerald also has 30 years experience in programming and administering minicomputers and PCs. He consulted with Stainless Insert Flange Co. (Media, PA) for this article.