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Boiler Tube Applications

Tight Radius Bending - Boiler

Occasionally there is a need to bend boiler tubes to a tight CLR. Some boiler designs require a return bend with only a ¼ inch spacing between the tubes. For a 2 inch OD tube this would be a 0.56D bend. Pretty much impossible. The solution is to bend the tube to as tight a CLR as possible and then resize the part to the proper dimension. Usually this means cold bending the tube to around 1.1D. The bend can then be heated and run through a series of dies to swage the part to the desired shape.

During cold bending the tube will lose a great deal of strength on the outside wall. Typically ~30% for a 1D bend. This is not acceptable for boiler applications. Historically the only way to achieve the necessary wall thickness was to preheat the part prior to bending. This “Hot Bending” process can achieve exceptional results when properly utilized. Unfortunately the tooling costs for the part shape and specialized heating elements for each tube OD and sometimes each wall thickness and material type make this a less than desirable process.

In order to minimize the outside wall reduction during cold bending Pines can provide a Booster that reduces the wall thinning by pushing the tube forward into the bend area faster than the bend arm pulls the tube forward. This results in a substantially thicker wall than would be possible in a normal bending process. The inside wall also gains in thickness appreciably. Boosting the tube can often yield results of 15% wall reduction for a 1D bend.

Today’s boiler requirements often require as little as 10% wall reduction. In order to meet this more stringent standard Pines Engineering’s engineering team has developed special, shaped groove tooling. When combined with a booster mechanism less than 10% wall reduction can be achieved.

Bending Boiler Tubes

Heavy wall boiler tube is bent to various radii. Boiler tubes (not pipe) are sized according to OD and minimum wall thickness prior to bending. Boiler tubes can be quite long and heavy making material handling a large part of the overall production concerns. Boiler tubes are often ganged together and follow parallel paths through the boiler with a minimum spacing between the tubes. This requires high precision in bending to maintain the desired bend angles and rotations.

For years the standard bending machine for this industry has been the Pines Engineering #4NC bender. The single axis digital control has a proven track record for reliability in a hostile shop environment and provides the extreme accuracy required for critical boiler applications. However the distance between bends and rotation angles were still set manually, often resulting in expensive rework of the parts.

Recently there has been more interest in a CNC bending machine for this industry. A standard CNC machine will not suffice. The weight of the tubes and the torque required for plane of bend changes exceeds the capacity of machines designed to bend truck exhausts, or thin walled aircraft type parts.

Enter the Pines Engineering CNC150HD. The CNC150 is based on the Pines #4. It has all the bend and clamping force you will ever require for this application. The HD stands for Heavy Duty. The B-axis drive is an oversized motor and gearbox designed to handle extreme torque and easily manipulate the heaviest tube. The Carriage Y axis linear rails are beefed up to handle the extra weight and rotation forces. A booster can be added to provide extra longitudinal force to help reduce wall thinning on those tight radius bends.

Mandrel bending is almost never employed when bending boiler tubes. Mandrels are used to help maintain the ovality (roundness) of the tube in the bend area. With minimum wall tubing the inconsistency of the material prevents a good fit of the mandrel to the ID of the tube. The long lengths of boiler tubes would require a lot of handling to load an unload over tubes the mandrel rod. Possible but not practical. The better solution for ovality is either boosting or shaped groove tooling. Boosting, even slightly, can have a large impact on the ovality of most tubes.

Since material handling is such a large part of this production process, Pines has developed a series of automatic tube loaders. Each loader can remove a single tube form the load rack and place it in the carriage collet automatically. The carriage locates the end of the tube precisely and then begins the bending process. Tube loaders can handle any number of tubes when equipped with a bundle loader/separator and handle tube lengths up to 20 meters.

Special serpentine applications, often with 2 different radii in a part require a different approach. This bender handles tubes 50 feet in length (or more) and can perform multiple bends in different rotation planes automatically. An automatic loader and tube feeder are part of the overall system. Right and Left had bending, hot bending, and flip table are all available as required. Called a System Bender, this custom machine is designed around a customers specific part shape to ensure the best results.



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