The selection process
This page is just a general look at the different sizes and configurations available. On the right side of this page there are links to the various models and the selection will be based on the Outside diameter (OD), Inside Diameter (ID), wall thickness and the thickness of the tubesheet. Also, you will have to take into consideration if a projection is required out side of the tubesheet and any procedure required if step rolling is necessary.
principles of tube expanding
Expanding is the art of reducing the wall by compressing the O.D. against a fixed container, such as rolling into tubesheets, drums, ferrules or flanges. To assure a proper joint, the wall must be reduced by a predetermined percentage. The following chart can be used for determining the correct wall reduction.
The chart below shows a typical 3/4” – 16 gauge tube. Before rolling you would find the proper rolling dimension as shown.
- A. First determine the tubesheet hole size.
- B. Then determine the tubes outside diameter.
- C. Subtract the outside diameter from the sheet hole dimension.
- D. With a tube gauge, determine the inside diameter of the before rolling.
- E. By adding the dimension found in “D” to the clearance between the O.D. and the sheet hole, you will then know the inside diameter at metal to metal contact.
- F. Roll to what you feel is a
good joint. This example
was rolled and then the I.D. was checked
with an accurate Tube Gauge.
- G. By subtracting “E” from the rolled diameter you determine the actual amount of expansion (wall reduction) on the inside diameter. This can be converted to a % of wall reduction by dividing the actual wall thickness (“B minus D”) .130” into the amount of roll .009.
You can use this chart to your advantage by predetermining both the % of wall reduction required and the actual inside diameter which should be rolled. After the completion of “E” you realize any additional increase of the inside diameter will result in actual wall reduction. Since the amount of wall reduction greatly determines the quality of the joint, you should arrive at the % required for your application prior to rolling.
By subtracting the inside diameter “D” from “B”, you determine actual wall thickness. This example would therefore be .130”. If you then take the 7% wall reduction times the wall thickness, you arrive at .0091”. Adding .0091” (“G”) to .627” (“E”) we get “F” the inside diameter after rolling (.636”).
The complete document on rolling procedures can be found here:
Boiler tubes are assembled by welding or by expanding. Expanding is mainly adopted in Boiler bank assembly. Expanding is resorted to, as it is the simplest way to fit the tubes in boiler bank. In this subject of tube expanding one may not find good literature regarding the % thinning to be adopted and the sequence to be adopted. Many have doubts whether a leaky joint can be seal welded or not. This article is brought out to clear the doubts of boiler users.
A TUBE EXPANDER
The expander can be a roller type, which is widely used in Industry even today. It has a taper mandrel, when driven in, pushes out the three rolls. The wall is made to fill the gap between the tube and the sheet hole. After the wall touches the sheet hole, the wall is further thinned to ensure a leak tight joint is established.
LITERATURE ON TUBE EXPANDING
Expanding has been in vogue for many years. Yet code regulations do not spell out enough on this subject. It has become necessary to reproduce the paragraphs of code regulations on this subject, for reader’s interest.
When fitting into drums or headers, be sure each extends far enough in to the header or drum. Tubes up to (but not including) 2 inches in OD should project 3/16 inch to 5/16 inch (4.7 mm to 7.9 mm) in to the drum or header. Ones of 2 inches OD and larger should project 5/16 inch to 7/16 inch (7.9 mm to 11.1 mm) in to the drum or header. In tube joints where measurements can be obtained, the correct amount of expansion can be found by using the following formula For in drums: diameter of the hole minus OD, plus 0.012 inch (0.3048 mm) per inch OD. For in headers for boiler design pressure less than 500 Psi (35 kg/cm2g): Diameter of hole minus OD plus 0.015 inch (0.381mm) per inch OD. For in headers for boiler design pressure over 500 Psi (35 kg/cm2g): Diameter of tube hole minus OD of the tube, plus 0.02 (0.508mm) inch per inch OD of tube. The figure arrived by the using above formula should be added to OD of the tube as measured to give the required OD of the tube after rolling. If it is impossible to reach OD of the tubes in drums to gauge them, the inside diameter of the tube must be measured. Since plastic deformation of the tube wall varies with the tube wall thickness, the ID of the tube for different wall thickness will vary. Where the outside of the tube is not accessible the following formula is used to in the expansion of the tube. The ID of the tube, plus the tube hole diameter minus the OD of the tube plus the expansion increase factor. Tube expanding (also referred to as tube rolling) is the cold working of the metal of the tube ends into contact with the tube sheet holes to achieve a pressure tight joint. A tube expander is used to increase the circumference of the tube ends until a proper joint is produced. This can be related to the rolling of steel, since the steel sheet is made thinner and longer. The tube is an endless sheet and the tube expander enlarges the outside and inside diameter of the tube. A completed tube joint must have the tube larger than the containing metal in the tube sheet hole. The expanded tube joint contains a force trying to make the tube smaller and a force trying to make the tube hole in the tube sheet larger. As the tube is expanded, the tube wall is thinned and the tube circumference is increased. The tightness at the tube joint will be measured by the increase of the inside diameter or the tube. The following variables determine the proper expansion of a tube. Clearance between the OD of the tube and tube sheet hole Original tube I.D. Amount of expansion after tube-to-tube sheet contact Pre-thinning of tube wall before tube-to-tube sheet contact Each of these factors can be measured and determined before the rolling operation begins.