Mast arms bare the load of the electrodes, holders, and cables. Arms endure heat, cyclic loading, high stress, and corrosion. Maintenance on arms prevents against very costly and dangerous failures. Arms can also be sent into Erie Copper Works to have faults documented and repaired. Routine inspection of mast arms aids in safety and efficiency.
Understanding Your Arms
Arm Type: Mast arms can be categorized into two different types – traditional arms and hot arms. Traditional arms utilize water cooled bus tube to transfer power from the cables to the electrode holders. If your mast arms have copper tube running along the arms’ tops or sides, your system uses traditional arms. Hot arms are constructed from, generally, highly conductive plated clad to transfer power from cables to an electrode holders. They don’t use bus tube and tend to have lower maintenance costs at the expense of a higher initial costs.
External Arm Inspection
Visual inspection of your arms can prevent dangerous and costly situations. Arms have a tendency to crack, leak, overheat, and arc during failure. We highly advise scheduling in-house maintenance inspections for your arms and to familiarize yourself with when arms need sent out for further repair and inspection.
Leaks may begin where gaskets have failed, where cracks have formed, or around poor incomplete welds. Gasket leaks may be stopped by tightening loose fasteners, or by cleaning and replacing failed gaskets. Leaks from cracks must be fixed by field welds or sending the arm out for repair. Cracks initially form due to oscillatory stresses, corrosion, thermal expansion and contraction, and improper weld penetration. Field welds have a tendency to re-crack due to lack of chipping, cleanliness difficulties, and unavailable proper tooling. Longitudinal cracks generally form on the edges of mast arms and along weld seems where plates conjoin. Check Image 4 for a visual of the most common visible crack (longitudinal crack).
B) Field Weld Cracking
We’ve received many arm repairs that’ve required the removal and re-welding of field welds. Field welds are necessary to prevent unnecessary downtime by patching up leaks and surface cracks. However, good judgement should be made to determine if a field weld is suitable for a job. Further routine inspection should be made to ensure field welds are maintaining strength. Image 4 shows a field weld that’d been performed on an arm’s lifting lug. A longitudinal crack formed along the full length of the weld causing the lug to be incredibly dangerous. We chipped, cleaned, heated, and repaired all welds for further use after a customer sent us their arm for inspection. Field welding would not have been suitable. Be sure to check field welds for strength, especially in areas that endure stress.
C) Arcs and Overheating
Arcing on a mast arm requires immediate attention. Arcs are often visible by a luminous discharge, but can also be located in tight hidden locations that may not be noticeable at first. Dust buildup, moisture, humidity, failed insulation, loose fasteners, and broken/bent parts can result in arching. Pitting, overheating, extreme corrosion, and leaks are a biproduct of arcing. Arced contact surfaces tend to produce a higher chance of arcing due to mating inconsistencies and insulative barriers created by pitting. Arced arms typically have to be sent in to be serviced.
While more common in hot arms, blocked waterways mar result in overheating of the arm body. Electrical current generates heat, heat generates resistance, and resistance generates more heat with passing current. The phenomenon is known as thermal runaway, and it can cause the arm and various other components to overheat and eventually melt down. Monitor temperature sensors and check for discoloration in your arm to determine if it’s overheating. Check for head loss (higher pressure needed for average flow) to determine if your arm has become clogged. Note the internals of an arm can also become coated in an insulative barrier of debris, and may not be detectable with pump pressure. Windows may be needed to remove foreign material.
D) Window Cracks
We’ve fixed a many window cracks as a result of competitors’ repairs being insufficient. Window cracks form from improper welds, plate sizing, and window placement. If your arms have been clogged or thoroughly cleaned, look for a cutout similar to the shape of D. If your company requires NDTs, be sure that these windows are included in tests! Leaks may be visible upon failure of these locations after cracking has broken the welding surface.
Routine arm cleaning is greatly advised. Many arcing incidents are caused by dust buildup. Dust conductivity is high enough for electricity to pass around insulation, across gaps, and over nonconductive surfaces. All around inspection of the arms for cleanliness should be one of the primary concerns of arm maintenance.
F) Plate Seem Cracks
Although much less common, we’ve witnessed seem cracks from poor arm manufacturing. Seem cracks are located where steel plates are joined together. Companies that require NDTs should also include seem locations in tests.
Insulation between the mast columns & arms, arms & bus tubes, and electrode holders & arms are replaced every time we receive arms for repair. Insulation absorbs moisture, separates from heat, and fatigues from cyclic loading. Ensure fasteners in locations with insulation remain tightened. Check for insulation darkening, layer separation, and expansion (signs that insulation has failed or is about to fail).
H) Electrode Misalignment
Electrodes’ arc circle must be maintained for power efficiency, steady electrode consumption, and ease of arcing. We have received a repair from an across seas arm manufacture that neglected the treatment stage of manufacturing a mill’s mast arms. Their arms began to warp after installation and use. If you’ve installed new arms and your arc circle begins to misalign, there’s a chance that your arms weren’t properly treated. Be sure to check your arc circle after arm installation and as time progresses.
Many mills keep spare arms in the event that the arms that are being used need to be repaired. Here are a few minor tips for arm storage that may save your company hefty expenses:
1) Dry Storage
We’ve spoken with companies that’ve had their arms completely destroyed by leaving them out in weather. Water gathers within the arms from rain and usage, and winter temperatures causes water to expand by roughly 10%. The pressure from freezing water expansion is enough to bulge and tear 3/4″ welded plate at the seams. Arms should be completely dry and free from rain when place into storage.
2) Arm Coverings
Uncovered arms typically result in missing components when the arms are placed back into service. Workers may grab various parts from a finished arm to use as spare parts on an arm in use. If you want to have spare arms remain intact for future use, be sure to cover them and make part removal difficult while the arms are in storage.