Why steel boiler fire tubes should not be welded to the tube sheets?


As you know, there are many fire tubes in hot water and fire-tube boilers. If welding is used to seal fire tubes and connect them to the beginning and the end of the sheets and tubes of boiler, the tube sheet will become rigid and due to the imposed tension, it will crack at welding points. Thus, the sealing will be ruined and the boiler will have to be repaired.

All authentic manufacturers of hot water and steel boilers, therefore, use riveting or tube cold expansion, through standard automatic machines instead of welding the fire tubes to the tube sheets.

One of the features of quality and standard ST35.8 fire tubes is that they are capable of being riveted and as mentioned earlier, only riveting should be used to connect the tubes to the tube sheets in broilers according to engineering standards, except for resistant tubes and inside the secondary furnace where standard cold expansion and welding is used (using ESAB machinery for network welding).

boiler figure 1

In addition, riveting should be conducted using automatic and electronic machinery. For this purpose, first the required torque should be calculated relative to the diameter and thickness of fire tube and tube sheet, and using fully electronic torque gauge, the tube riveting should continue to reach the calculated and adapted torque and then it will be halted automatically; because excessive riveting damages the tubes and tube sheets and inadequate riveting also make the tubes unsealed after a while.

In sealing the tubes using rivets, quality and standard ST35.8 fire tubes should be used and the thickness of tube sheets should be 20 mm to maximize sealing lifespan of the tubes by expanding their surface contact with the tube sheet.

steel boiler

Another advantage of riveting tubes is their possible replacement in the long run when the tubes are replaced without any damage to the tube sheets with the least cost and time.

In summary, when riveting steam and steel hot water boilers to the tube sheets using manual or automatic welding, tube sheets with low thickness of 10 to 12 mm are used to lower the costs. First, the buyer would not notice any problems since they are sealed. But, it will definitely have a negative influence of the boiler's lifespan because it causes irreparable damages such as creating cracks in the gap between the tubes in the tube sheet, twisting the tube sheets, etc. In addition, when replacing welded tubes, removing the welding points will cause serious damages to the tube sheets and turn it into a timely and costly process.