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
Attending to the bank tube failure mainly depends on the type of failure and the intensity of the failure.
- In case of expanded joint leakage resulting in puncture of the tube, generally it is the practice to plug both ends of the bank tube in top and bottom drum. This is mainly due to the reason many a times it will not be possible to reach this tube location without removing large number of tubes. Plugging tubes should not be adopted if locally there are many tubes to be plugged. This can cause problem in other tubes nearby.
- Plugging is adapted to other type of failures when approach is not there without removing many bank tubes. No spool piece welding is to be done on bank tubes as this can cause failure in the expanded joint.
- When the failure reason is established as low expansion carried out, then re-expansion has to be done in all the tubes. It is always better to restrict the thinning of the tube thickness to a maximum of 15 to 18%. Above this you will see that the tube material start flaking and lead to brittle failures.
- Seal welding of bank tubes is permitted after expansion. All precautions are to be taken during welding not to deviate from the welding procedure as any welding on drum has to be done with the utmost care. It is required to lightly re-roll the seal welded tubes and do not flare the tube end.
Replacing all the bank tubes
There are times when all the bank tubes need changing. The procedure will have to be carried out by an expert group knowing all the implications while carrying out this.
Since the entire bank tubes are to be replaced,
- The bottom drum has to be properly supported before removing the old bank tubes
- After removing the bank tubes, each hole in the drums (steam and mud drums) has to be checked for its required dimension and for any damage before replacement of the tubes
- For enlargement/ovality in the holes, the normal tolerance limit for acceptance is (d+0.032" ) + 0" / – 0.015" ; where ‘d’ is the outside diameter of the bank tube
- If due to steam or water erosion the hole size is more than the limit, then weld build up has to be carried out to get the required size with suitable preheating and post heating.
- After getting the required hole size, the insertion and subsequent expansion of the bank tubes have to be carried out in proper sequence
- If seal welding is carried out with suitable electrode and with required preheating.
- Do not weld the bank tube directly to the drum without achieving required expansion on the bank tubes, and no strength weld is allowed in this area.
Boiler tube failures are inevitable. There are twenty-two primary reasons for tube failures in a boiler. It is true that being forewarned is being forearmed. Knowledge and good operating and maintenance practice reduce tube failures.
As carbon emission is a major concern today, more super critical pressure units are bound to be preferred due to the increase in plant cycle efficiency. This will make once through type boilers take over from drum type boilers.