Materials, Tube Leaks, & Pre-Expansion Steps
|Material||TEMA Standard Target % Wall Reduction||API-660 Target % Wall Reduction|
|Carbon Steel and Low-Alloy Steel (max 9% Chromium)||5 – 8%||8% max*|
|Stainless Steel and High-Alloy Steel||5 – 8%||6% max*|
|Duplex Stainless Steels||4 – 6%||Not Specified**|
|Titanium (and other non-ferrous, work hardening materials)||4 – 6%||5% max*|
|Admiralty Brass (and other nonferrous, non-work hardening materials)||6 – 9%||8%*|
|Copper & Copper Alloys||7 – 10%||Not Specified|
*API-660 lists maximum tube wall thickness reduction values only. For some materials, API-660 allows the wall thickness reduction to be increased by an additional 2%, if approved by the purchaser.
**Maximum tube wall thickness reduction to be agreed between the purchaser and vendor.
These materials and percentages can be your guideline to rolling tubes of like materials. In general, you want to roll to the lowest wall reduction possible where a tight tube to tube sheet joint can be achieved.
Note: The harder the material the less wall reduction is required for a good mechanical joint.
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 6% – 9% 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 5% – 8%. 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 7 – 10% 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 4 – 6%. 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.
Major Causes Of Tube Leaks
Tube leaks are usually caused by under-rolling, over-rolling, improper preparation of tube sheets, and differential thermal expansion. Improperly rolled tube joints can cause several challenges, such as necessary reworking and loss of vessel efficiency.
Under-rolling occurs when the proper amount of wall reduction is not reached. If left uncorrected, this will cause leaks between the tube OD and tube sheet hole ID. Under-rolled tubes can usually be fixed by rolling a second time to obtain the proper amount of wall reduction. Be sure to use a tube gauge to periodically check the amount of wall reduction during rolling.
Over-rolling of tubes occurs when the expansion of the tube surpasses the expansion required for the proper wall reduction. Over-rolling can cause considerable damage to the tube sheet and adjacent tubes. Once a ligament is over-rolled, it will deform the ligaments of the tube joints surrounding it. This deformation can cause adjacent tubes to leak.
In addition to impacting tubes, over-rolling can cause distortion in tube sheets or drums. Overrolling can potentially cause a tube sheet to bow or warp to the point where the standard length tube could not be used in the vessel until the bowing is returned to normal. This can be corrected in some cases by placing stay rods in the vessel and pulling the tube sheets back to their original position.
Lastly, over-rolling can have a significant impact on expander tool life. It can cause the tube material to flake off, which can get trapped in between the rolls and mandrel. If this debris is not cleaned from the expander, it can reduce the life of the rolls and mandrel.
Over-rolling tubes is not something that can be easily corrected. Once ligaments become deformed, the entire tube sheet has an increased chance of leaks, higher maintenance costs, and decreased efficiency. As a result, it is important to avoid over-rolling by using an electronic or pneumatic torque control or assisted rolling system. These systems will ensure you roll to an exact wall reduction every time.
Improper Preparation Of Tube Sheet Holes
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. It is important that light ligaments and thin tube walls are mated to a tube hole that has a low micro range finish. Many manufacturers today are drilling, reaming and sizing or burnishing to get the micro-finish desired for tube holes.
Differential Thermal Expansion
When the expansion due to heat varies noticeably between the thinner tube and tube sheet, a shift of the tube results. One of the most important steps for guaranteeing 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, mill scale and pits or scratches. Note: It is extremely important to eliminate any longitudinal scratches or cracks in the tube sheet hole wall. These longitudinal lines will cause leaky tubes.
Preparation of tube holes in heat exchangers and condensers
- Drill and ream tube sheet holes to TEMA or ASME codes.
- Be certain the tube sheet material and ligaments are sufficient 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 slightly increase the tensile strength of the ligament.
- The serrations or grooves to be used will determine the joint strength of the tube joint. It is extremely important when retubing that the grooves be cleared of all metals or any foreign material.
Preparation of tube seats in drums, tube sheets, and headers
- Tube holes are normally drilled and reamed to comply with API, ASME, or National Board Inspection Code.
- It is extremely important during this operation that there are no longitudinal scratches or cracks left in the tube seat.
- In cases where out-of-roundness is extreme, pre-rolling of the tube holes is advised.
- Be certain that the tube hole walls and the grooves in the tube walls are cleaned down to bare metal before tubes are inserted. Be certain all-foreign material such as oil, grease, rust, or just plain dirt are 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. It may be helpful for this condition to employ the use of a Tube End Setting Tool to properly set the tube in the sheet.
In order to create a better mechanical joint, it is important that the appropriate pre-expansion tools are used.
Tube Hole Gauges make it easy to accurately measure tube IDs and tube sheet holes. Accurate measuring of tube holes ensures that expansion requirements are being met. Rolling to the required wall reduction is vital in creating a proper mechanical joint. Proper mechanical joints ensure that the vessel is built to applicable engineering standards and to maintain the efficiency and safety of the vessel.
Prior to rolling, it is important that tube hole surfaces are free of debris and minor scratches in order to have a properly prepared tube hole surface. A tube sheet hole brush can aid in the cleaning of these ID surfaces by removing dirt and deposits from tube sheets in surface condensers and heat exchangers. Clean tube sheet holes help to minimize the possibility of leak paths during expansion.
Grooving tools, also known as serrating tools, can be used in milling or drilling equipment for OEM tube sheet hole preparation. These tools provide consistent diameter and depth of grooves in the production of the machined tube sheet holes.
Tube pilots, or tube guides, are used to pilot replacement tubes through tube sheets and tube sheet support plates that are commonly found in heat exchangers or surface condensers. The tube guide head is designed with a shoulder for the tube to rest against, eliminating the possibility of bending the tube end as the guide pushes through the support plates.