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Rolling Boiler Tubes

Boiler Tubes - Formulae to work out a Finished ID.

Rolling Boiler Tubes with the combination beading expander

Rolling boiler tubes is the art of reducing a tube wall by compressing the OD of the tube against a fixed container, therefore boiler tubes into a tube sheet. To achieve a proper tube joint, causing a gas-tight seal. The tube wall must be reduced by a predetermined percentage. The chart below can help determine the correct tube wall reduction needed in tube rolling. 

This Chart shows a typical 2.0"OD x 10 BWG (0.134")

Principals of Rolling Boiler Tubes - Determine the following:

  1. (A) Determine the Hole size.
  2. (B) Determine the tube outside diameter.
  3. (C) Subtract the tube outside diameter from the tube hole dimension.
  4. (D) Accurately measure the ID of the tube before rolling.
  5. (E) Add the dimension found in “D” to the clearance between the tube OD and the tube hole, you will then know the tube’s inside diameter at metal to metal contact.
  6. (F) Roll the tube to what you feel is a good tube joint. This example was rolled and then the ID of the tube was checked with the tube gauge.
  7. (G) By subtracting “E” from the rolled diameter you determine the actual amount of expansion (tube wall reduction) on the inside diameter of the tube. This can be converted to a % of wall reduction by dividing the actual wall thickness (“B minus D”)  in the amount of the roll.

Follow the link above for Expander specifications found in individual pdf's for Rolling Boiler Tubes

Tube Sheet Hole
Tube Outer Diameter OD
Clearance (A-B)
Tube Inner Diameter ID
Clearance
Inner Diameter at Metal to Metal
13% of 0.134 x 2
Inner Diameter at Metal to Metal
Expanded Tube Inner Diameter

2.010″

2.000″

0.010″

1.732″

+0.010″

1.742″

0.035″

1.742″

1.777″

Tube Wall Reduction Percentages based on Material

Application

Non Ferrous tubes in surface condensers.

Steel tubes in heat exchangers.

Soft Copper, Cupro Nickel and aluminum tubes in heat exchangers.

Boiler tubes.

Tube Wall Reduction

3% to 4%

5% to 10%

8% to 12%

12% to 14%

Major Causes of Tube Leaks

Tube rolling leakage can happen because of:  under rolling, over rolling, improper preparation of tube sheets and differential thermal expansion. Improper expansion can lead to serious difficulties with the manufacturer and the repair serviceman.

Under Rolling

Under rolling is when the tube is not expanded to fill the tube sheet hole and the proper amount of wall reduction is not obtained. It is better to under roll than to over roll.

Over Rolling

Over rolling is when the expansion of the inside diameter of the tube surpasses the expansion required for the proper percentage of wall reduction for the ultimate tube joint. This can do considerable damage a tubular vessel. Excessive rolling will decrease the dimensions of the ligament between tube holes and weaken the bridge. Once a ligament is weakened, it will cause a reaction in all ligaments surrounding that weak ligament. If we decrease the strength of the ligament the tube to the tube being rolled will be weak.

Also, it causes distortion in tube sheets or drums, such as egg-shaped holes. It will also cause diametrical expansion which is the overall increase of the tube sheet or drum. Over rolling has also been known to cause a tube sheet to bow or warp to the point where the standard length tube could not be used in the vessel until bowing or warpage is returned to normal. This is usually corrected by placing stay rods in the vessel and pulling the tube sheets back to their original position.

Preparation of Tube Sheets

Preparation of tube seats in drums, tube sheets, and headers is as follows: Tube holes are normally drilled and reamed to approximately 1/32” larger than the nominal outside diameter of the tubes. It is extremely important during this operation that there are no longitudinal scorings left in the tube seat. In cases where out-of-roundness is extreme, predrilling of the tube is advised. Be certain that the tube hole walls and the grooves in the tube walls are cleaned down to bare metal before the tubes are inserted. Be certain all foreign material such as oil, grease rust or just plain dirt is removed. Special attention during this cleaning will prevent serious trouble later.

After tube holes have been prepared they are usually coated with a rust preventative compound.  Before inserting any tube, it is important to remove all traces of this coating.  It is extremely important that great care be taken in handling the tubes for insertion in all of the vessels discussed above.  Be certain that the tube ends are clear of any foreign material.  Be especially certain that there are no chips on the tubing which may gouge the tube sheet or tube seat when the tube is placed in the vessel. 

In some cases, it will be necessary to force a tube into a tube hole.  This should be done with extreme care.  It is better to spring the tube than to try to force it with a hammer.  If a tube end is kinked or damaged before rolling, the expanded end will be damaged, and a leaky roll joint will result.  Attention at this time to the tube ends and the tube alignment will prevent future troubles.

tube sheet holes

Improper Preparation of Tube Holes

Improper preparation of tube holes is another major cause of tube leakage.  If the tube sheet or drum is gouged, it is extremely hard to expand the tube to fill these gouges or tears without over-rolling.  The smoother the tube seat or tube hole the easier it is to roll an optimum tube joint.  The ligaments and light tube walls make it more important that the finish of the tube hole be in the low micro range.  We find many manufacturers today are drilling, reaming and sizing or burnishing to get the micro finish desired for tube holes.

Preparation of Tube Holes

The preparation of tube holes in heat exchangers and condensers is as follows.  Drill and ream tube sheet holes to .007” to .010” over the outside diameter of the tube to be used.  Be certain the ligaments are enough to guarantee a safe and permanent tube joint.  When conditions permit, utilize a sizing or burnishing tool to further assure a good finish in the tube hole.  This will also increase the tensile strength of the ligament.  The serrations or grooves to be used will determine the holding power of the tube.  It is extremely important when retubing that the grooves be cleared of all metals or any foreign material.