1. Major findings
- While refurbishing this old EPM3 LRS on a shredder, severe arcing damage was found where the busbars pass through the shorting contactor floor into the tank.
- The arcing was between the busbars and the metalwork of the busbar floor, hidden by the top slit-plate and a bottom pipe flange. It only became apparent when the busbars were disassembled in the process of fitting refurbished electrodes.
- To change out the electrodes on this model of LRS is not trivial. The design makes it very difficult to get the old electrodes out, and the nuts and bolts are often very corroded. Even our best technician could not guarantee a change-out time of 8h, with our planner specifying a minimum time of 16h for this job to allow for contingencies. The work included busbar modifications to allow fitting of the new electrodes and doing the top-flange modification for eliminating flash risks.
2. Summary of work
- On this shredder LRS, evidence of flashing was found on the Blue and White phase busbars where the busbars enter through the floor of the shorting contactor panel.
- This was due to a vulnerability in the design of the LRS where the busbar passes through the floor only protected by heat shrink sleeving.
- Although it passes through the water in an isolating pipe, this pipe terminates on a flange under the floor.
- If the heat shrink is damaged, the busbar can flash to the metal floor.
- This is not self-evident, as it is hidden by flange from the bottom, and a clamping plate from the top.
- The problem was addressed as follows:
- All three busbars on this LRS were re-sleeved with new heat shrink.
- On Red and Blue phases, a modification was implemented where the isolating pipes were inserted with the welded flange on the top of the panel floor, therefore the pipe now protects the busbar where it passes through the floor. (Top-flange modification)
- On White phase another modification was implemented where a longer pipe was fitted going through the panel floor up to the busbar joint. This was necessary due to the fact that the flange broke off from the pipe and would therefore not have provided a high level of insulation as was the case for Red and Blue. (Long-pipe modification)
- New gaskets were fitted at the clamping plates, while sponge rubber gaskets were fitted into pipes where the busbars exit the pipes, to block steam from entering into the shorting contactor panel.
- To facilitate easy swopping of electrodes during services, a joining bridge was fitted on the busbar next to each electrode of the shredder LRS.
- This was also necessary to facilitate the pipe modifications described above.
3. Busbar Arcing Damage, Shredder LRS
- During the process of changing out the electrodes on this LRS, it was discovered that arcing had been occurring between the Blue phase rotor circuit and earth, as well as to a lesser extent between White phase rotor circuit and earth, as evident from damage on the busbars.
- Both Blue and White phase busbars had extensive arcing damage where it passes through the floor of the shorting contactor panel to the tank below.
4. Heatshrink sleeving damage
The heatshrink sleeving around the busbars was damaged where the busbars pass through the floor. On the blue phase the sleeving was cut before a photo was taken, but the extensive damage is still clearly visible on the photo taken after the cut was made.
5. Hidden Points of flashing at the bottom and top
- The bottom flange is plastic-welded to the isolating pipe going down to the bottom electrodes. This flange was originally mounted underneath the floor of the contactor panel, which is the roof of the tank.
- The busbar then passed up through a hole in the metal floor, only protected with the heat shrink sleeving.
- In the shorting contactor panel, the busbar emerged from the floor through a slit plate.
- When the slit plate was removed, arcing damage was evident on the plate as shown in the photograph.
- The area where the arc occurred was therefore effectively hidden from view between the bottom flange and the top slit plate.
6.Top-Flange modification (Red and Blue phase)
- The LRS design vulnerability of the busbars passing through the floor without protection of the insulating pipe is only present on very old LRSs.
- The RWW factory has since changed the design to insert the insulating pipe from the top, so that the flange is on top of the panel floor instead of at the bottom.Therefore, the busbar is protected by the pipe as it passes through the metal panel floor.
- SDG therefore executed this modification on the shredder LRS Red phase and Blue phase busbars, taking out the pipe from the tank and inserting it from the top.
All busbars were re-sleeved with thick wall heat shrink sleeving.
New rubber gaskets were cut to seal around the pipe flange and the contactor floor.
The next phase of the Top-Flange modification is to block the pipe around the busbar with a foam rubber gasket material, to block steam or water from rising between the busbar and the pipe.
The gasket was then clamped from the top with the slit plates which were previously removed.
