CORRELATION BETWEEN % WALL REDUCTION AND TUBE JOINT STRENGTH AND TIGHTNESS

TUBE EXPANDING - A VERY SUCCESSFUL PROCESS.
Tube expanding has been used successfully for joining tubes to tubesheets of boilers and tubular heat exchangers since the 19th century The history of tube expanding is long and interesting and covers methods such as Prossering, manually driving mandrels with hammer blows and hand rotation, electronic controls of rolling torque inputs, controls for air driven rolling equipment,hydraulically driven rollers with hydraulic insertion of mandrels, direct pressure applications such as explosive expansion, hydraulic expansion, hybrid expansion consisting of a first stage of hydraulic or explosive expansion and a second stage of roller expansion, hybrid expansion consisting of a first stage of either explosive or hydraulic expansion and a second stage of hydraulic after explosive or explosive after hydraulic expansion, inflating balloons inside tubes and compressing polymers inside the tubes.

PARAMETERS - INFORMATION LACKING
The strength and tightness of expanded tube-to-tubesheet connections depends upon the residual interfacial pressure, the tube-hole contact surface area and the coefficient of friction. There is no generally available means for calculating interfacial pressure created by direct application of hydraulic pressure, explosive expanding, roller expanding or combinations of these methods. There is no generally available information on coefficient of friction; it depends upon the surface finishes and the materials of construction of the tubes and tubesheets. Many of us who have worked in this field guess the coefficient of expansion to lie between 0.3 and 0.7 and the few papers on tube expanding that use values of coefficient of expansion generally guess at the mid point of the range or 0.5.

NO CORRELATION OF PERCENT WALL REDUCTION WITH EXPANDED JOINT STRENGTH OR TIGHTNESS
MGT Inc. has very extensive collection of papers on tube expansion including some by Stanley Yokell who also discussed it in his book "A Working Guide to Shell-and-Tube Heat Exchangers", that McGraw-Hill published in 1990. A thorough search of MGT's files and files published on the internet finds that there is no published correlation between percent wall reduction and expanded joint strength and tightness.

NO TECHNICAL BASIS FOR MANUFACTURERS' RECOMMENDATIONS OF PERCENT WALL REDUCTION.
Most roller expanding tool manufacturers publish recommended percent wall reductions for various tube metals, diameters and gages. But they do not and cannot offer a rational basis or bases for their recommendations. The responses to an email to six manufacturers of tube expanding equipment verified that they have no experimental basis for their recommendations. Possibly that explains some of the differences between some of the expanding tool manufacturers recommended percent wall reductions.

Q AND A. WHY ARE EXPANDED JOINTS SO SUCCESSFUL?
The question arises, "How then can we explain the widespread industry experience of consistently and regularly achieving strong tight expanded tube-to-tubesheet joints?"

The answer is that most exchanger manufacturers either formally or informally correlate tests of mockups or data garnered from manufacture of production equipment that produced joints tight enough and strong enough for the intended services.

ASME CODE TUBE JOINT EFFICIENCIES
With respect to Table A-1 of Appendix A of Section VIII Division 1 and Table 4.C.1 of Annex 4.C of Section VIII Division 2, it is generally recognized that the joint efficiencies shown in the tables were put together by people with long experience in tube joining. But there is no experimental work that underlies the joint efficiencies. There are some few papers that investigate such efficiencies but they do not form the bases of the efficiencies listed in these tables.

REQUEST FOR VIEWERS' INPUT
I have been searching diligently to see if there is any information in the literature that correlates percent expansion with joint strength and tightness. But so far with no success. Noting that there are substantial differences between the effects of explosive and hydraulic expansion and roller expansion and that rolling tools vary in the number of pins, shape of pins, slot orientation of mandrels, etc. I question whether the same correlation would apply for the uniform pressure percent wall reductions as for the roller expansion percent wall reductions or that it is possible to predict interfacial pressure produced by roller expanding.

If any reader of this blog has information that would even resemble such correlations I would appreciate your sharing it with me.