Bank tubes are used to connect the upper drum and lower drum in bi-drum boilers. These bank tubes not only act as raisers and down comers between the drums but also carry the weight of the bottom drum and the down comer of the bottom drum. Any failure of these tubes results in damage to the nearby tubes or drum or both, depending on the location of the leak. Attending to these failures require special care and methods.
These joints are mechanically held by the expansion of the tube in the drum holes. The tube deforms to a plastic condition and the drum hole, being in elastic condition, exerts the required sealing force for the joint against the drum operating pressure. It is seen that an expansion resulting in about 7 to 11 % thinning of tube thickness results in a good joint. Normally these joints are more effective for pressures up to 120 kg/cm2. Tube wall thinning can be calculated by knowing the tube ID before and after expansion.
D1 = Tube ID before rolling when the tube OD just touches the drum hole. This is achieved after light rolling and is called touch rolling; not much torque will be required for this. Once the tube OD touches the drum hole ID, the expander torque will increase indicating the tube has touched the drum hole ID.
D2 = Tube ID after rolling
T1 = Tube wall thickness before rolling. Note: Even at touch roll point the thickness of the tube is the same as that before starting touch expansion.
Percentage thinning = 50 x (D2 - D1) ÷ T1
There is also a practice to measure the thickness before and after expansion and calculate the percentage of thinning. However it is my experience and opinion that measuring tube ID is more reliable after expansion. Hence I always prefer to use the diameter method.
Reasons for bank tube failure
- The main reason for bank tube failure due to expanded joint failure is inadequate expansion. Many times it is very difficult to measure each tube ID for checking the percentage thinning, hence it is a practice to measure a few tubes in every 100 or 200 tubes and fix the expander torque to get the required thinning. This torque is applied to the tubes and is taken for granted that the expansion is achieved. This is true only if proper lubrication is done on the expander rollers so that minimal torque is lot in friction and the thinning is achieved.
- Expanded joint also fails due to thermal shock created due to large number of start-ups and shut downs in a short span; say 50 start-ups and shutdowns in about ten to twenty days.
- Internal deposits leading to long-term overheating failures
- Blockage leading to short term overheating failure
- Erosion and corrosion of tube surface