TUBE TO TUBE SHEET JOINTS WITH STRENGTH DATA

ROLLER EXPANSION
The most commonly used method of making tube/tube sheet joints is roller expansion.

Roller expansion procedures for titanium tubes into tube sheets are similar to those used for other materials. For best results, the tube sheet holes should be within the limits specified by TEMA (Tubular Exchanger Manufacturers Association) for shell and tube heat exchangers or within the limits specified by HEI (Heat Exchange Institute) Standards for steam surface condensers (see Page 19).

There are three commonly used methods of determining the correct amount of expansion:

1. Measuring wall reduction
2. Simulating wall reduction by interference
3. Pull-out strength versus torque curve

The above pull-out test is preferable.

The suggested wall reduction for titanium tubes is 10%. Thus, in a .028" tube or a .020" tube, the required reduction is .0028" or .0020". A small error in measurement can result in a large deficiency in pull-out strength.

Using the interference fit method, four measurements are required:

1. Tube I.D.
2. Tube wall thickness
3. Tube sheet hole diameter
4. Tube O.D.

In this method, the increase in the I.D. of the tube is used to determine the theoretical decrease in wall thickness.

For example, consider a 1" O.D. x .020" wall tube:

1. Tube I.D. measurement .964"
2. Tube wall thickness .020"
3. Tube sheet hole diameter 1.010"
4. Tube O.D. measurement 1.004"

The necessary inside expanded diameter for a 10% wall reduction is:

Tube I.D. .964"
Clearance between tube O.D. and hole .006"
10% wall reduction (.2x wall thickness) .004"
-----
Expaned Tube I.D. .974"

This method is not preferred because it requires precise measurement and a perfectly round tube. It also depends on the tube sheet hole not enlarging.

The use of torque vs. pull-out strength curve for determining the necessary amount of roller expansion is the preferred method and its use is increasing. This is done by using a sample tube sheet of the same material and thickness as the full-sized condenser and with drilled holes with the same spacing and tolerance as the full-sized condenser or heat exchanger.

Tube samples about 12" long are sealed at one end either by crimping and welding or by welding plugs in one end of the tube. The tubes are then roller expanded into holes at varying torques. The usual range for this test is 7 to 12 ft. lbs.

When using a torque controlled air motor to drive the expander, it is important that it be calibrated correctly. This can be done either in the field or in the laboratory by using a portable prony brake such as the Coleco Power Tube Analyzer Model P-15 manufactured by Dresser Laboratories. For thin wall tubes, a five-roller expander with a thin gauge collar, such as the Wilson 72D, is recommended. Pull-out strengths are determined using a hydraulic tensile tester. The tubes can be either pulled from the plugged end or pushed from the expanded end. If the push-out method is used, a filler material such as sand should be placed inside the tube for the push rod to contact. It is important that the level of the filler material in the tube be below the roller expanded area and that the push rod is kept vertical and not in contact with the tube wall. If this is not done, erroneously high values will result. When the ratio O.D./t is greater than 25, five roller expanders should be used. For lower ratios, three roller expanders are generally satisfactory.

When doing the roller expansion, both the tube ends and the tube sheet holes should be wiped clean of debris and soil. The presence of lubricating oil in the tube holes or on the tube's outside surface can result in reduced pull-out strength and leak tightness of the finished roller expanded joint. It is preferable that the tube ends be wiped also with a solvent such as acetone. Do not use chlorinated solvents or methanol.

The expander should also be inspected for cleanliness and should be free of dirt or other foreign matter. The rolls and mandrel should be free to move and should be in good condition. Before inserting the expander into a tube, the shut-off torque should be confirmed by using the prony brake, then the expander should be properly lubricated with a water soluble lubricant, such as Wilson Expander Lube. A suitable air-powered expander drive is Wilson Series 3A Torque-Air-Matic, Catalog No. 40511, having a speed of 450 rpm and a maximum torque of 18 foot pounds.

The suggested torque for different tube sheet materials is shown in Figure 4. The torque should be checked hourly using the prony brake. Additional lubricant should be used on the expander as required. The rolls and mandrel should be inspected periodically for chipped rolls and/or mandrel damage. Damaged pieces should be replaced immediately.

Roller expanded joints only are not recommended for use with explosively bonded tube sheets. These joints should be both roller expanded and welded. The material used as the cladding is usually relatively thin, 3/16" - 1/4" max, and is almost always softer and lower in strength than the tube material. Such roller-expanded joints have a history of leakage after short periods of time. If the thickness of the cladding is increased so that leak integrity is not a problem, the economics usually favor a solid titanium plate.