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Once the cage is inspected and approved, the Purchaser’s representative should stay and witness at least the initial stages of tube loading. Typically, if there are going to be any problems with tube loading, they will manifest themselves in the first couple of rows. This is where the bend radii are the tightest and there is the least amount of flexibility in the U-bends. Therefore, the differences in tolerances between the tubes and the supports are more critical. It also is important to ensure that the crew loading the tubes are being conscientious and handling the tubing properly – i.e., wearing gloves, not pushing the tubes in too fast or with too much force that could result in scratching or damage to the tubes.
The bundle is inserted into the shell shortly after the completion of tube loading. The shell is typically fabricated in parallel with the tube bundle. Similar to the baffles and supports, this inspection is typically conducted concurrently with another inspection and typically does not require a separate trip to the manufacturer’s facility.
- Nozzle location, size and weld prep
- Shell diameter, wall thickness and overall length
- Support and other miscellaneous lug location
- All welds have been properly NDE tested as required by the specification or code
Prior to the bundle insertion to the shell, both the tube bundle and the shell should be checked for any foreign material.
Following the bundle insertion into the shell, the tubes are welded to the tubesheet if required by the specification. Prior to expansion of the tubes to the tubesheet, these joint welds should be tested to ensure they do not leak. This is typically done by pressurizing the shell side with air or helium and then using either a soap bubble solution or helium detector to check for any leak paths from the shell side to the tube side. Additionally, all of the joint welds should be dye penetrant tested before and after expansion to ensure they did not crack during the expansion process.
Regardless of the type of expansion process used, a percentage of the tubesheet holes should be identified as “control holes” in order to quantify the amount of tube expansion and ensure that the resultant tube wall reduction is in accordance with the Vendor’s procedures or the specification.
When witnessing the above processes, the inspector should:
- Verify pressure gages are with calibration
- Shell pressure is maintained for the required period of time
- All welds are free from cracks, splits and leakage at tube ends
- Range of required tube wall reduction is achieved during expansion process
- Hydrostatic testing
Following tube expansion, final assembly of the channel occurs. This includes installation of the hemi-head (based on the design), channel internal pass partition plates and the channel cover.
Following assembly, the heater is subject to a final hydrostatic test on both the shell side and the tube side. While the ASME Authorized Inspector is required to witness these tests, many utilities decide to send their representative as well since it is the final test that the heater is subjected to prior to shipment. When witnessing a hydrostatic test, the inspector should verify:
- All pressure gages are within calibration
- The pressure gage reading is consistent with the specification/code requirements
- The pressure is maintained for the appropriate amount of time/number of cycles
- Hydro water temperature is well above the Nil-Ductility temperature for the materials of construction