Waste Heat Boiler Erection Job

Waste Heat Boiler Erection Job

A waste heat boiler is a special type of boiler that generates steam by removing the heat from a process that would have otherwise been wasted. Waste heat boilers are therefore able to improve energy indices of process plants. In most ammonia plants fire tube waste heat boilers are installed downstream of secondary reformer, high temperature shift converter & ammonia synthesis converter to cool down the effluent of these reactors and for energy recovery. Normally these boilers are high pressure operating around 120bar pressure.

Waste heat boiler has always been an integral part of petrochemical industry just like primary and secondary reformers. The erection job of waste heat boiler along with steam drum is tricky and demands for extreme care as very severe temperature conditions are involved during service.
This article is about erection job of waste heat boiler, risers, downcomers and steam drum especially all hot work involved like welding, heat treatment and NDTs. The typical detail of waste heat boiler and steam drum arrangement has been shown in Fig. 1.  The most difficult aspect of this job is the installation / welding and post weld heat treatment of piping. For the case taken here, the typical piping material is of 15NiCuMoNb5 and nozzles of 13CrMo4-5. These materials falls in low alloy family typically can be taken equivalent to P1 low alloy piping (P.No. 3). High strength low alloy piping is considered to be the most economical option when dealing with temperatures as faced in waste heat boiler and the steam drum.

Fig. 1 Arrangement of WHB & Steam Drum

Before placing steam drum over WHB, the first thing needs to be done is the placement of temporary supports underneath steam drum so that there must be adequate spacing between WHB nozzles are steam drum nozzles where riser pipes have to be provided, see Fig. 2(a)(b). Fabrication job of risers’ pipe as per the actual dimension is to be made accurately, see Fig. 3. Flame cutting is normally adopted for the same and proper beveling is done after verifying the actual dimensions in field. After bringing the cut lengths of risers to the field, they are to be installed in between the steam drum and WHB nozzles and the root gap along with alignment has to be verified by QC inspectors. After OK marking by QC inspectors, tacking shall be done. No root gap was to be given in upper weld joints with steam drum and a gap of 3-4 mm was to be provided for the bottom welds with WHB. And after welding of bottom joints, the root gap of upper weld joints was to be set at 3-4 mm.

Fig. 2(a)(b) Temporary Supports beneath Steam Drum

Fig. 3 Flame Cutting of Risers to required length

The welding of risers with steam drum and WHB nozzles was to be made in parallel for all the risers so that expansion due to welding heat must be evenly distributed to the steam drum. It was done in two steps; initially weld joints of risers to WHB were to be made and then with the steam drum. There were total 12 weld joints to be made. First 06 weld joints of risers to WHB nozzles and then remaining upper 06 weld joints of risers to steam drum.

A team of 12 proficient welders at site was constituted, 06 welders for GTAW and 06 for SMAW. All the pre-heating elements were installed prior to welding. Proper pre-heating machines were used with graphs for this activity in order to monitor the pre-heating temperature which was 180 deg C in this case. After attaining the required pre-heating temperature, it was confirmed with the sticks and welders were allowed to start the welding. All the 06 GTAW welders started their job at the same time in parallel on all the 06 risers. Two passes, one root and one hot pass were made with 2.4 mm filler wire Tenacito 65R (OERLIKON).  After completion of these passes, electric welders started their job in the same way with 3.2 mm electrode DMO-1G (Bohler).

Fig. 4 Pre-heating of Riser Joints

The welding of all the bottom welds was completed and a post heating to 280-320 deg C was applied with 04 hours of holding time. It follows cooling to ambient temperature in air. Radiography was the next step and 02 weld joints were found with repair in root. The procedure followed for getting rid of these repairs was to pre-heat all the welds at the same time and then to carry out the repair job of two defected weld joints. This was done in order to have an equal thermal expansion and reduced strain induction.

Fig. 5 Welding job (root & hot) in progress

The same procedure was adopted for the welding of upper weld joints. A root gap of 3-4 mm was provided by grinding off the beveled ends. Temporary support is to be removed after completing two passes of upper weld joints with SMAW. In some of the weld joints, a high low of 5 mm was detected and it was decided to grind the inside diameter from steam drum inside after completion of grinding (see Fig. 6) and then radiography is to be made.

Welding on downcomers was rather simpler than risers but precise fabrication, fit-up proved to be the key to repair free welding. Typical repairs encountered were in the root due to improper fit-up (changed inside diameter of beveled ends). Slag was the second problematic repair type. SMAW welders were advised to properly clean the weld before making a subsequent pass.

Fig. 6 Alignment Issue rectified after Welding

After all the weld joints were declared OK in radiography, post weld heat treatment was to be carried out on all the 12 welds simultaneously. Alternatively, 06 upper welds and 06 bottom welds could also be post weld heat treated in parallel. Later methodology was adopted and post weld heat treatment was done at 610 deg C for one hour with 200 deg C as heating and cooling rate.

Fig. 7 QC Inspector performing Inspection

Necessary non-destructive testing was to be performed after PWHT operation. It involved ultrasonic and magnetic particle testing. Ultrasonic would be checking any defect / crack from the root area to the filling and magnetic particle testing would be taking care of any defect especially crack in the capping area which may propagate when taken in service. Manual pulse-echo ultrasonic was performed and all the weld joints were declared OK. On performing magnetic particle testing, cracks were found at one of the weld joints in the capping area. They were ground off to the sound metal and the weld joint was left as it is.

Hydrostatic testing of the whole loop including risers, downcomers, WHB shell side and steam drum was the final step to ensure mechanical integrity of the whole system. Hydrostatic test pressure is to be set after careful working on design pressures of all the components in the test limits. All the welding was perfectly fine except two valves which were found leaking from body. They were replaced after the hydrostatic test was declared OK.

Important Notes
1. All the hot work including pre-heating, welding, post heating, repair job and PWHT needs to be carried out in parallel on all the joints to avoid uneven thermal expansion and excessive strain induction.

2. Proficient welders must be assigned on this job just to avoid repairs as repair activity involves pre-heating of all the welds in parallel which consumes a lot of time.

3. It is recommended that the welding of any joint must be completed in a continuous shift and then post heated instantly.

4. At least three QC inspectors must be assigned on this job for visual testing and monitoring all the necessary inspection checks especially before and during welding job. VT eliminates many defects.

5. Inspectors performing NDTs must be skilled and well qualified. Especially UT & MPT checks after PWHT are very critical.

6. PWHT machines for this job should be the best of the lot available. They must be calibrated properly and their integrity must be checked before moving them to this job. Elements breakage during PWHT of these critical joints can cause adverse effects.

7. NDT after PWHT is of utmost importance as the material being used was susceptible to crack initiation. It is advisable to perform UT check for root, hot, filling passes and MPT for capping region. TOFD inspection would save much of time.

8. Hydrostatic testing of whole loop including risers, downcomers, WHB shell side and steam drum is to be carried out after all the hot work and NDT.