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 Tube Outside diameter (OD), Inside Diameter (ID),Tube wall thickness and the thickness of the tube sheet. Also, you will have to take into consideration if a tube projection is required out side of the tube sheet and any procedure required if step rolling is necessary.
principles of tube expanding
Tube Expanding is the art of reducing a tube wall by compressing the O.D. of the tube against a fixed container, such as rolling tubes into tube sheets, drums, ferrules or flanges. To assure a proper tube joint, the tube wall must be reduced by a predetermined percentage. The following chart can be used for determining the correct tube wall reduction.
The chart below shows a typical 3/4” – 16 gauge tube. Before rolling this tube you would find the proper rolling dimension as shown.
- A. First determine the tube hole size.
- B. Then determine the tubes outside diameter.
- C. Subtract the tube outside diameter from the tube hole dimension.
- D. With a tube gauge, determine the inside diameter of the tube before rolling.
- E. By adding the dimension found in “D” to the clearance between the tube O.D. and the tube hole, you will then know the tube’s inside diameter at metal to metal contact.
- F. Roll
the tube to what you feel is a
good tube joint. This example
was rolled and then the I.D. of
the tube was checked
with an accurate Tube Gauge.
- G. By subtracting “E” from the rolled diameter you determine the actual amount of expansion (tube wall reduction) on the inside diameter of your tube. 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 of the tube will result in actual wall reduction. Since the amount of wall reduction greatly determines the quality of the tube joint, you should arrive at the % required for your application prior to tube rolling.
By subtracting the tube 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 of the tube after rolling (.636”).
The complete document on tube 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
A tube 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 tube is made to fill the gap between the tube and the tube hole. After the tube touches the tube hole, tube is further thinned to ensure a leak tight joint is established.
LITERATURE ON TUBE EXPANDING
Tube expanding is 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.
Tubes may be attached to drums or headers by welding, expanding or a combination of both. Welded connections may be made by joining the tube directly to the header, as in the case of membrane wall panels or by welding the tube to a stub (short length of tube) that is attached to the drum or header in the shop. This type of construction is used for the majority of connections. In some applications, however tube expanding is a practical method of tube connections. Tube expanding or tube rolling is a process of cold working the end of the tube into contact with the metal of the drum or header containing the tube holes or seats. When a tube is expanded, the outside diameter, inside diameter and length increase and wall thickness decreases (see figure 1). The increase in length, called extrusion, occurs in both directions from some section X-X. The residual radial pressure between the tube and tube seat resulting from a properly expanded tube will provide a pressure tight joint of great strength and stability.
A typical roller expander contains rolls set a slight angle to the body of the expander causing the tapered mandrel to feed inward when it is rotated in a clockwise direction. As the mandrel feeds inward the rolls develop the internal force, which expands the tube.
The expanded joint presents a simple and economical way of fastening tubes into low-pressure boilers. Under axial loading, the expanded joint is as strong as the tube itself. However for conditions of widely fluctuating temperatures and bending loads, the expanded joint must be seal welded (see figure 4) or replaced by a shop attached tube stub. Generally, tubes above 1500–psi (105 kg/cm2g) range are either expanded and seal welded or attached to shop welded stubs. Selection of the type of tube end connection to be used is dictated by design, assembly, and operating characteristics.
Seal welds are used to make mechanical joints fluid tight. The strength of the connection is developed by the expanded joint, pipe threads or by the configuration as in the case of hand hole fittings. The throat dimension of seal weld is limited to 9.5 mm. Maximum, and post weld heat treatment is not required.
When fitting tubes into drums or headers, be sure each tube 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. Tubes 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 the tube and hole measurements can be obtained, the correct amount of expansion can be found by using the following formula For tubes in drums: diameter of the tube hole minus OD of the tube, plus 0.012 inch (0.3048 mm) per inch OD of tube. For tubes in headers for boiler design pressure less than 500 Psi (35 kg/cm2g): Diameter of tube hole minus OD of tube plus 0.015 inch (0.381mm) per inch OD of tube. For tubes 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.