TUBE EXPANDING INFORMATION
ELLIOTT SCOTT LTD. TUBE & PIPE TOOLS
Basic Principles of Tube Expanding
Tube Expanding is the art of reducing a tube wall by compressing the outside
diameter of the tube against a fixed surface…..such as expanding tubes into
tube sheets, drums or flanges. To ensure a tube is correctly expanded, the tube wall must be reduced by a pre-determined percentage and such work must be repeated accurately across all tubes to be expanded.
To start this process it is important firstly, to obtain accurate dimensional data about the tube expanding job and secondly provide information about the materials being used. The easiest way to do this is to download our Tube Expander Enquiry Form, fill in the details and return it to us.
To understand the amount of tube wall reduction that will be necessary to achieve a 100% bond between both the tube and the tube sheet, we need to determine what the percentage of wall reduction will be and what are the characteristics of the tubing materials being used. A simple rule of thumb is the harder the material, the less wall reduction is required to achieve a tube joint. We should therefore know the amount of tube wall reduction we can apply to each metal. Below are some guidelines that can be used when expanding tubes of the same materials.
Copper & Cupro- Nickel 8-10%
Carbon Steel & Admiralty Brass 7-8%
Stainless Steel & Titanium 4-5%
Carbon steel is used widely today in pressure vessel construction. Tube wall reduction should be approximately 7-8% and lubrication is a must when expanding. If the tube end is flaking or cracking and the tooling shows excessive wear, then two factors need consideration. One is the amount of wall reduction being applied and the second is the tube material hardness.
Brass tubing is widely used in condensers. Use plenty of lubrication when expanding. The amount of tube wall reduction required is approx 7-8% for optimum results but when expanding into a serrated hole this can vary between 4 – 10% for a tight joint. Further wall reduction above 10% can cause splits, leaks or flaked tube ends.
Stainless steel, Titanium and other harder alloys require approx 4 – 5% tube wall reduction to ensure a good quality tube joint. Because these materials tend to work harden, the entire wall reduction needs to done quickly with minimal or no re- rolling undertaken. Rolling speed is very important and the use of multi-roll expanders ( 4 rolls or more) is recommended when rolling Titanium tubing.
Major Causes of Tube Leaks
Tube rolling leakage is usually caused by one of the following factors: under-rolling, over-rolling, improper preparation of tube sheets or differential thermal expansion. Such conditions can have serious implications for both the Manufacturer and Service Companies.
As the name suggests, this occurs when the tube is not expanded to fill the tube sheet hole and the correct amount of wall reduction is not achieved. It is however, better to under–roll than to over-roll.
Over-rolling occurs when the expansion of the inside diameter of the tube exceeds the expansion required for the correct amount of wall reduction to achieve an optimum tube joint. Over-rolling can do considerable damage to a vessel. Over-rolling will decrease the dimensions of the ligaments between tubes and weaken this bridge. Once this occurs it will cause a reaction in all ligaments surrounding that weak ligament. If we decrease the strength of the ligament, the tube next to the tube being expanded will leak.
Over-rolling also causes distortion in the tube sheets or drums, such as egg-shaped holes. It will also cause diametrical expansion which is the overall increase of tube sheet or drum. Over-rolling has been known to cause a tube sheet to bow or warp to the point where a standard tube length cannot be used in the vessel until the bowing or warpage has been returned to normal. Tube bundles can also be damaged in the same manner.
Improper Preparation of Tube Holes
Improper preparation of tube holes is another major cause of tube leaks. If the tube sheet or drum is gouged, it is extremely hard to expand the tube to fill these gouges or tears without over-expanding. The smoother the tube seat or tube hole, the easier it is to roll the optimum joint. The ligaments and light tube walls make it important that the surface finish of the tube hole be in the low micro range. Whilst CNC drilling equipment has improved the quality of the holes, we are still of the opinion that sizing or burnishing tools will further improve the surface microfinish of the tube holes.
Differential Thermal Expansion
Differential thermal expansion can result with thicker tube sheets. When the expansion, due to heat build-up varies noticeably between the thinner tube and tube sheet, a shift of the tube results. One of the most important steps to ensure a safe and permanent tube joint is to thoroughly clean the surfaces of the tube end and the tube hole wall. These two surfaces must be clean and free of all dust, dirt, mill scale, pits and scratches. It is extremely important to eliminate any longitudinal cracks in the tube hole wall. These longitudinal lines will cause tube leakage.
Elliott Scott Ltd turbo machinery + tube and pipe products