Best Practices For Expanding Water-Tube Boiler Tubes
The American Society Of Mechanical Engineers (ASME) develops standards to provide consistency and ensure public safety and reliability. While these codes are not always required, they have been widely adopted and implemented across many industries. As a result, it is important to be knowledgeable and aware of these standards prior to beginning a job.
When it comes to tube projection, the ASME boiler code states that the tube ends shall not be less than 1/4″ and not more than 1/2″. These are measured on the high side of the drum shell and before the tube end is flared. The code further specifies that the flare of the tube end must not be less than 1/8″ greater than the diameter of the tube hole. It is important not to exceed these specifications, in order to prevent splitting.
It is extremely important that the flare roll does not travel into the drum. Otherwise, it will create a sharp edge on the I.D. of the tube and a surface crack on the O.D. of the tube. The expander must be one suitable for the outside diameter and gauge or wall thickness of the tube being rolled. Tube hole tolerances should be considered (figure plus 1/32″). If a condition exists where the tube hole tolerance is extremely large, it may be necessary to set the tube with a straight roll expander, flare the tube end, and, finally, re-roll with the straight roll tube expander. The roll length should allow you to expand a minimum of 3/16″ to a maximum of 1/2″ beyond the tube sheet or seat, as in a drum or counter-bored tube hole.
Using a flare roll expander, like Elliott’s 1500 Series, roll and flare the tube end in one pass, approximately ¾ tight. When setting the tube expander in the tube it is a must to determine how far into the tube the expanding portion of the rolls should be set. The rolls should be set and the mandrels tightened so that the space between the end of the tube and the front of the flare roll will allow for rolling the tube three-fourths tight and, at the same time, provide a suitable flare. There aren’t strict guidelines for this step because tube rolling is best performed through the experience and judgment of the operator.
If there is a situation where the tube hole tolerance is extremely large, it may be necessary to set the tube with a straight roll expander first, then flare, and finally, re-roll with the straight roll expander.
Once the tubes are flared, you want to use a straight roll tube expander, like Elliott’s 3300 Series, to form a leak-proof joint. This tool is equipped with a ball-bearing thrust ring, which bears against the tube end to reduce friction produced during expansion, allowing you to roll the tubes uniformly. When rolling, it is best to use a 3 roll style expander to get the best joint. This is because the internal surface of the tube has less blemish and more uniform extrusion of the tube metal into the seat groove. Re-rolling with the three roll expander aids in relieving stress built up in the tube joint from the original rolling and flaring operation.
Overall, it is important to be aware of ASME and other industry code prior to beginning a job. Additionally, when expanding boiler tubes it is critical that you follow the step rolling process and use both a flare roll and a straight roll expander. If you force the expander to flare and expand in one operation, it can result in tube leaks and premature failure of the vessel.
For more information on expanding boiler tubes, contact our sales team.