Expanding Fire Tube Boilers
When the tube sheet is welded into the shell or, in some designs, when the main fire tube is welded to the sheet, the tube holes have a tendency to become oval shaped. Since it is practically impossible to prevent this, it is advisable to pre-roll tube holes with a pre-rolling tool. This tool must be equipped with four expanding rolls to round out the tube hole and improve the seat. This rolling action also has a tendency to strengthen the ligament between the tube holes. NOTE: The three roll tube expander design is not recommended for this operation.
Tubes can be cut to length and inserted into the tube holes in smaller boilers. When cutting tubes to length for larger boilers it is usually best to cut all the tubes approximately 3/4” to 1” longer than required to allow for variance in the location of the tube sheet in the shell from one unit to the other, also to allow for a slight warpage in the tube sheet. Insert the tubes in the tube holes and set them uniformly at one end, preferably the back end of the boiler, so that the ends of the tubes project 3/16” to 1/4” beyond the face of the tube sheet for the beading operation. Each tube should then be held or pinned at one end while the other end (set to project 3/1 6” to 1/4”) is rolled and flared. Excess tube projection can be removed by using an internal cutter or trimming tool.
There are currently two processes that can be used for firetube boiler tube installation:
- Single roll beading expander
- Traditional expanding and percussion beading of the tube
Rolling With A Single Roll Beading Expander
Single roll-beading expanders simultaneously expand and bead the tube end in a single operation. The expander assures the creation of a joint, which is both pressure tight and has a bead in contact with the tube sheet. The expander achieves this objective by utilizing the natural feed forces built into the expander. When expansion begins it forces the beading roll against the end of the tube forming it into a bead in one continuous operation.
The guide roll and the beading roll must be the proper size for the tube wall being expanded and beaded. Guide rolls that are too large or too small will prevent proper bead formation. Beading rolls operate on a range of wall thicknesses, but the best results are obtained by using the beading roll
specifically designed for that tube gauge.
Tubes must have the correct amount of projection from the tube sheet before starting expansion. Tube projection refers to the distance between a tube end and the tube sheet. The amount of projection can vary depending on the tube diameter, wall thickness, tube sheet thickness, and weather or not the tube is square or has a flare due to use of an internal type tube cutter. The normal range of tube projection is 3/16” – 9/32”. If the tube sheet is bowed or warped and is not parallel to the end of the tube, the desired projection should be measured at the midpoint so that half of the tube will have greater than the desired projection and half will have less than the desired projection.
Roll Beading Fire Tube Boiler Tubes
Retract the mandrel and insert the tool into the tube until the bead roll is in contact with the end of the tube. Pushed the mandrel forward until the rolls make contact with the ID of the tube and adjust the mandrel stop to reflect the previously calculated expansion requirement. Position the tube projection from the face of the tube sheet using the guide in the operating instructions for the specific tube OD being expanded. Start the expanding operation and continue until the mandrel stop nut is against the thrust bearing, then allow three additional revolutions of the mandrel to “iron out” the tube bead. Reverse the mandrel rotation and remove the expander from the tube. Verify the target ID and adjust the mandrel stop if needed. Inspect the bead and adjust the tube projection to achieve a “tight to the sheet” tube bead.
Use of coolant on the tube and the roll beading expander is necessary in order to reduce the heat created during the expansion process. If coolant is not used, the expander can overheat. This may result in tube material flaking, premature tool failure, and poorly formed beads. In order to avoid these issues, it’s recommended that you dip the roll end of the expander in a bucket of coolant between expansions. This will prevent the tool from overheating while also keeping it clean, extending the life of the rolls and mandrel.
Upon completion of the expansion of one end of the boiler, the tube projection on the opposing end can be maintained by using a tube cutter to cut the excess tube from in front of the tube sheet.
Percussion Beading Of The Tube
t’s recommended that you use a combination rolling and flaring expander for the first operation when the tubes are prepared for beading. The tubes should be rolled sufficiently tight so it doesn’t move as a result of the beading operation. At this point, it’s preferred to have an under-rolled joint because the beading operation has a tendency to move the tube end by a few thousands. This will prevent any ligament damage during the beading process.
Once the beading has been completed, the tubes can be re-rolled with a straight expander. This final operation requires only a few seconds per tube, to ensure uniform leak-proof joints.
Hand Beading Tool