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Deltaflex Performance, Steel Stub-ends

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IPP Deltaflex® Flange Performance on Steel Stub-ends

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Improved Piping Products (IPP) markets a patented engineered cross section flange for piping under the trade name IPP Deltaflex® flange. The IPP Deltaflex® flange engineered principles have been incorporated into a unique line of modern high performance flange products. IPP Deltaflex® flanges and back-up rings reduce the cost and weight of a flange without reducing its pressure performance rating. Two critical factors have influenced the growth and acceptance of the IPP Deltaflex® designed flanges in the piping industry:

1. National standards for flanges such as ANSI B16.5 and AWWA C207 were developed many years ago. This was long before the sophisticated computer analytical tools available today, which can be utilized to design flanges. As a result IPP has shown that the standard flanges called out were either over designed in the case of ANSI or not applicable for modern piping materials such as thermoplastics as is the case for AWWA.

2. Historically engineers have been under the impression that the only method to reduce the cost of standard flanges was to make them thinner and thinner. As a result flange pressure and sealing performance has suffered, especially in the stainless steel industry.

The design problems listed above and their solutions utilizing IPP Deltaflex® flange engineering methods are the basis of the discussion that follows.

The IPP Deltaflex® back-up flange offered by Improved Piping Products, Inc. has been popular with piping system designers and fabricators for several reasons. It is lighter in weight (about 50% of the weight of conventional flanges) and therefore less expensive. In addition, it is easier to handle in the field. The reduced cost of a two piece stub-end back-up ring design instead of the conventional weldneck flange also eliminates the field hassle of flange alignment or "two holing" for about the same cost as the weldneck. Now, a sophisticated engineering analysis has shown that there are also strong structural engineering preferences for this design with its reduced stress level, especially when thermal stresses or corrosive conditions exist that could lead to stress corrosion or cracking, either from within or from ambient environments.

The IPP Deltaflex® pipe flange design is shown in Figure 1. Note the reduced cross section. The unique geometry provides a structure that is lighter in weight and superior in performance than conventional flanges with greater material content. IPP engineers utilize a variation on the I-Beam Principle to create this unique approach to weight reduction. The flange design allows the flange to store energy like a spring and maintain a constant load on the flange faces with changes in system pressure and temperature.

Figure 1:  Lap Joint Flanges

Recognizing that new designs are not immediately integrated into well established engineering practices such as piping system designs, IPP knew that to be successful they would have to provide solid engineering analysis of stress levels in the flanges to piping design engineers. IPP initiated a rigorous stress analysis in addition to performing a series of physical hydrostatic tests. The analysis was performed using Auto Design (version 3.1), finite element analysis software that calculates and plots all principal stress levels including the von Mises based on the test pressures and bolt loads on defined elements of the flange and cross sectional elements. Bolt loads were assumed to be uniform in accordance with ASME Section VIII, Appendix 2 and test pressures were used for internal pipe loads.

In the following, we will examine the stress level in a series of standard flange designs and compare them to the IPP Deltaflex® flanges at the same physical conditions. The results of these stress analyses were of interest for what they revealed about the stress level in commonly used flanges. They confirm the IPP design stress levels are equal or below that obtained with conventional flanges with major material savings. The examples include comparisons for common flanges such as:
  • Weldneck Flanges
  • Slip-on Welded flanges
  • Lap joint Flanges (with butt-welded and slip-on stub-ends)
Please see Figure 2, which is a finite element analysis of a 24" standard weldneck flange designed for 275psi working pressure (415psi test pressure) and IPP Deltaflex at the same conditions. The results indicated that because of the conservative design, conventional flanges are extraordinarily stiff members as compared to the process pipe they are welded to.

This results in high stress levels (25,000 psi) in the thinner and more critical pipe wall just aft of the flange hub-to-pipe weld for the conventional flange (FIGURE 2A) versus 13,000psi in the IPP Deltaflex® flange (FIGURE 2B). The flange body itself is a mass of metal with hardly any stress associated.

Figure 2:  IPP Reducing Pipe Wall Stress

Figure 3A shows a conventional slip-on flange with a similar stress pattern. IPP's engineers believe the high stress concentrated just aft of the hub is a result of the high bending loads that are transferred to the pipe from the flange, with little of the energy absorbed by deformation of the flange. Note that the stress at this location for the example shown is approximately 65% higher than the mean pipe stress. While the stress level is still below yield for most materials used, it could be cause for concern in corrosive applications. Corrosive agents such as chlorides or caustics can contribute to stress corrosion, cracking and eventual pipe failure in this location of high stress level in the pipe wall.

Figure 3A:  IPP Reducing Pipe Wall Stress

Figure 4 shows there is still a high stress level in the pipe aft of the slip-on stub-end with the IPP Deltaflex® back-up ring design, but it is lower than that of the conventional flange design. Thus, a lighter member actually reduces the maximum stress in a pipe flange area for a given process pressure and bolt load.

Figure 4:  IPP Reducing Pipe Wall Stress

Another interesting result of the IPP finite element analysis is that pipe and flange stress is virtually independent of bolt loading (except for local compressive stress) because of higher compression of the gasket and bending of the flange. Another advantage of the IPP Deltaflex® flange is that excessive bolt tightening leads to flange flexure rather than bolt elongation. This can help solve a classic problem of flange gasket leaks or crushing of the gasket material when a piping system cools down.

If there is no flexibility in the flange, the bolts can be stressed beyond their yield point during high thermal excursions. When the system cools down, the bolts return to an elongated state, resulting in potential leaks at the flange connection. The IPP Deltaflex®, on the other hand, stores elastic energy like a Belleville washer and maintains constant tension on the bolts thereby maintaining the moisture seal and eliminating the need to retightenthe bolts.

Results from proof tests conducted by IPP in accordance with the United Pressure Vessel Code ASME Section VIII, Division 1 and ANSI B31.3 confirm the superiority of the IPP Deltaflex® flange design.

Fatigue tests were also performed in which a 4" flange mounted to a 50" pipe section was subjected to 90,000 cycles of 0.6" amplitude without failure.

Conclusions

  • IPP Deltaflex incorporates the I-Beam principle in its design to reduce weight without reducing performance. Mass is removed and re-allocated for greatest efficiency and safety.
  • The lighter weight IPP Deltaflex® flange conforms to vital ANSI B16.5 dimensions and offers advantages of corrosion resistance, higher yield resistance and total dimensional interchangeability with forged, plate and cast iron flanges.
  • The IPP Deltaflex® flange stores energy like a Belleville washer and is better able to tolerate thermal stresses and pipe vibration. This feature maintains the original bolt torque values when flange joints are tightened. Re-tightening is not necessary and costly spiral-wound gaskets are not required.
  • The IPP Deltaflex® flange/stub-end combination results in a lower pipe wall stress level. Stress is transferred to the more massive forged stub-end. The wide bearing surface and large radius cast in the flange bore minimizes stress risers.
Note: Minor modifications can be made to the inside diameter of the IPP Deltaflex® flange without derating the operating pressure of the design. This feature allows the IPP Deltaflex® inside diameter to be modified to accommodate slip-on welded stub-ends, butt-welded stub-ends and metric pipe sizes with ANSI bolt circles at an additional charge. Major modifications should be referred to the factory prior to ordering.

Industry References

IPP Deltaflex® flange designs have been incorporated by major engineering companies into the world's largest construction projects and have proven their worth in some of the most critical applications.
View major organizations and Engineering companies who specify and use IPP Deltaflex®.

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