Knowing how to determine wall reduction is important; however it is equally important to know the characteristics of the popular tubing materials. A simple rule of thumb is the harder the material, the less wall reduction required to obtain a tube joint.
The following are guidelines that have been used in the industry throughout the years. These are in no way recommendations for all heat transfer equipment but are offered as general suggestions. Always consult the manufacturer of the heat transfer vessel for specific information before undertaking any maintenance procedures.
In general, you want to roll to the lowest wall reduction possible where a tight tube to tube sheet joint can be achieved. Additionally, to increase tool life and expansion quality, it is important to lubricate each tube end and make certain that the inside of the tubes and the tube expanders are kept clean.
Aluminum alloys are made up of a mixture of aluminum and other alloying elements, like manganese and silicon, primarily to increase strength. When rolling 3003 or 4004 Aluminum you should not reduce the walls over 5%. When rolling 6061-T Minimum, one of the most popular materials used in aircraft fittings, you can reduce the wall 10 to 12% for a mechanical joint.
When rolling Alonized tubing, abrasive particles are removed from the inside diameter of the tubing and gathered in the expander. It is recommended that two expanders be used. One should be cleaned and lubricated while the other is being used.
Brass, Carbon Steel, & Copper
Admiralty Brass is widely used in condensers. This material should be well lubricated. The tube wall is reduced by approximately 4% to 6% for optimum tube joints. Rolling to a higher wall reduction may cause leaking, splits or flaked tubes.
Carbon Steel is used in almost every type of pressure vessel built today. Tube wall reduction should be approximately 4% to 6%. Heavy lubrication is a must. If the tube is cracking or tooling shows excessive wear, tube hardness should be checked. Carbon Steel tubes should be 90 to 120 Brinnel hardness for rolling. It is possible to roll tubes up to 150 Brinnel; however, flaking and cracking is more likely to occur as the tube hardness increases.
When rolling Copper and Cupro Nickel, consider approximately 6 – 8% wall reduction for a proper tube joint. Since Copper is a softer material, it doesn’t take as much force to expand. However, it is important to use plenty of lubrication when rolling copper because it can be abrasive on tube expanders.
Titanium, Stainless Steel, & Exotics
Metals such as Titanium, Stainless Steel, and other exotics tend to work harden very quickly due to their elasticity. Elasticity refers to a material’s ability to stretch and return to its original state. Materials behave elastically until the force increases beyond the material’s elastic limit, meaning it cannot return to its original shape. During the expansion process, the tube material and tube sheet hole will expand until the tube reaches its plastic state and is contained by the tube’s sheet’s elastic properties.
When working with these types of materials, you want to roll quickly and to the lowest wall reduction you can, approximately 2-4%. Otherwise, you risk creating an uneven roll, known as “tenting”. Using a 4 or 5 roll expander can also be used to prevent this from occurring because it covers a larger surface area.