Different physical damages due to:

• Scrap impact
• Arcing
• Exposition to radiation of electric arc.
• Burner flame rebound
• Corrosion
• Thermal fatigue of the pipe base material

When prevention is not possible, the option is to periodically verify the integrity of the panels in order to detect the problems on time.

Periodical inspection of the panel’s exposed surface

To determine if there’s any existing damage that might result in a failure and repair it before it causes a major issue in the furnace. In order to perform a good inspection it is important understand how the different types of issues are manifested.

A good cleaning of slag and steel on the panel is mandatory.

Easiest to identify

Minor damage: Light marks on the pipe’s surface.

Coat with welding to restore the original form.

Severe damage: Deformation which can obstruct the flow of cooling water.

Replace the pipe section between the nearest returns or elbows.

ARCING

The current between the electrodes finds a path of lower resistance and flows through the panel rather than through the scrap.

Minor damage: The pipe surface is partially melted in one or several areas without perforating the pipe.

Coat with welding in order to restore the original shape, if it’s a small area.

Replace the entire section of pipe if the area of damage is greater.

Severe damage: The pipe is perforated and there is instant leak which does not allow to continue operating.

A good ground system helps to minimize arcing problems.

EXPOSED ARC

The radiation of the electrode is not completely covered by the slag. This energy melts the surface of the tubes.

Similar to the arcing appearance, however the damaged surface is greater.

It affects both the steel and copper panels.

The most common outcome is a leak that prevents the furnace to continue operating.

Normally, the affected area is so large that the panel repair is no longer affordable.

BURNER FLAME REBOUND

A heavy scrap section in front of the burner flame makes the flame rebound/deflect while it melts the heavy scrap piece.

The flame partially melts the surface of the pipes of the steel panel, thinning the walls of the pipes.

Normally, the flame does not melt completely the pipe’s wall and the panel continues operating.

You may have a leak if the thinned area receives more scrap impacts.

If the panel reaches the inspection, replace the thinned pipes.

(Usually affects more than 1 pipe)

THERMAL FATIGUE

Caused by temperature changes in the exposed surface of the panel pipes.

It appears sooner or later in the lifetime of the panel, depending on the temperature variation to which it is exposed.

Produces transverse cracks on the surface of the tube, however it takes time for the crack to develop a leak.

With a good inspection, it can be early detected before it causes major problems in the furnace.

Replace the entire section of pipe between the returns or nearest elbows.

A pipe with thermal fatigue in an early stage, show a rough surface that can be detected during the inspection

A penetrant test on the exposed surface of the pipes helps to detect cracks in an early stage.

CORROSION

It is caused by acid vapors in the gases, which under certain conditions are condensed on the surface of the tubes.

Corrosion symptoms:

A thin layer of iron oxide on the surface of the panel pipe after removing the slag.

Non uniform high roughness on the surface of the tube.

Good water treatment control to avoid:

• Thinning of the pipe walls due to acid pH in the water.
• Salts adhered to the inner surface which block heat transfer and cooling flow due to a basic pH.

Distortion of the panel

 The panels receive heat only in a section of pipe; this sometimes causes distortion of the panel.

Use a template to check whether the panel has deviated significantly from its original form and if so, correct the shape to prevent further damage by mechanical impact of scrap.

Thickness measurement

The thickness measurement of the exposed surface of the pipe, determines the remaining useful thickness of the tube.

Depending on the thickness measurement and the frequency of inspection of the panels it is determined if the thickness is acceptable to operate another period or if the pipe should be replaced

Melter considers 0.100” as an acceptable minimum thickness to operate.

Flow test

This test helps to detect obstructions in the cooling flow and / or fouling inside the pipes of the panel.

Expel internal obstructions circulating water in both directions of the circuit.

Wels seam inspection

Visual inspection of Weld seams exposed to heat.

If needed perform a penetrant test to check for cracks

Inlet and Outlet coupling threads

Verify the integrity of the thread of the couplings or nipples of each circuit.

Replace couplings or nipples with damaged threads.

Hydrostatic test

After the visual inspection, an hydrostatic test is performed to a minimum of 200% of the work pressure to detect any additional problems.

Once the defects are repaired and/or the damaged pipes and connections are replaced, a final hydrostatic test is performed.

Damages in round bar and plate

 Check the integrity of the round bar or plate used to seal the gaps between pipes, structural support plates and mounting brackets on the panel.

Replace any damaged section detected.

Slag retainers

If the design of the panel includes them, it is possible that they have been partially worn out, or completely melt.

Replace the damaged slag retainers, caring to apply the appropriate weld to avoid early     damages for thermal fatigue.

Recommendations

• Have a complete spare upper Shell and roof.
• Periodically replace the panels in operation for inspection and maintenance.
• The recommended inspection frequency to have good control and minimize problems during furnace operation is every 3-6 months. This period may be extended depending on the outcome of inspections.
• In case of an accident in operation or abnormal operating conditions, schedule a special inspection as soon as possible and not wait until the next scheduled inspection.

 A proper maintenance of panels must include the following:

• Cleaning the exposed surface of the pipes.
• Visual inspection of the surface.
• Penetrant test inspection.
• Thickness measurement with ultra-sound equipment.
• Review of the threads of the couplings and nipples.
• Flow Test
• Interior cleaning of the pipes.
• Hydrostatic test
• Necessary repair of pipes and components for assembly and sealing
• Repair deformation of the panels.