Materials of Construction
Catalysts
Quartz
Converter Fabrication and Transportation

Dust from the process gas and deterioration of the catalyst will accumulate in the catalyst bed.  Eventually, the dust may work its way down to the bottom of the bed where it accumulates and begins to restrict gas flow and increases pressure drop across the bed.  The bed support whether it is cast iron grids, stainless steel mesh, plates with holes, etc. will become blinded with this dust.   The only way to resolve the problem is to removed and screen the catalyst and carefully clean the support grids.

This converter is constructed of carbon steel lined with refractory brick.  Brick arches/domes divide the passes as well as support the grids on which the catalyst is placed.  The brick had become unstable so it was reinforced with steel bracing.

Collapsed Bed

The support bed of this stainless steel converter collapse after being in service for more than 10 years.  The problem was not failure of the material or a fault in the design but a error in fabrication.  The fabrication required continuous full penetration welds but the erector only did stitch welds.  This left a weak point at the all the joints between of the plate sections.   The error in fabrication was not discovered in the numerous levels of inspection while the converter was being erected. 

To its credit, the all welded stainless steel converter construction even though not fabricated properly managed to operate for over 10 years before it failed.  If fabricated properly, the converter probably would have lasted the life of the plant.

Battle of the Bulge

Due to its higher strength at elevated temperatures, stainless steel have become the standard for new converters.  Thinner plate can be used than if the converter was constructed from carbon steel.  However, the design must still be done properly to avoid problems such as shown in the picture to the left.  Here a core tube in a stainless steel converter is bulging from the weight of the shell, catalyst and internal heat exchanger.  As part of the design, cut-outs are required in the core tube to allow the gas to enter or exit the beds.  When such large amounts of metal are removed the entire structure is weakened.  The design should have allowed for thicker plate material, reinforcing pads or stiffeners.

A full collapse of the core tube is unlikely but such bulging should be inspected during every opportunity to determine if the bulging is getting worse.   Corrective action may be required if the problem is worsening.  In the photo on the right the same designer has corrected the fault in a later design and has provided stiffeners on either side of the cut-out.

The strength of carbon steel is reduced considerably at the normal operating temperatures of a converter.  Over time the shell tends to become permanently deformed and a bulge develops in the shell that is visible from the outside (see Pregnant Converters).  Inside the converter the movement of the shell causes the cast iron grids to move as well resulting in gaps between the castings.  If the gas become large enough the quartz and catalyst will begin to fall through.  If larger gaps develop the grids may move off the support post causing the grids to collapse.  In this photo a large enough gap has formed such that one end of the grid is no longer supported by the post.   Collapse of the bed would have occurred if the problems was not detected as it was in this case.  The solution is to reset the grids and post.  Post with larger support shoulders may be require to provide adequate support for the grids.  The grids should be repositioned so that any gaps are moved to the shell where additional support can be welded to the shell and the gaps filled in more easily.

In carbon steel/cast iron converters the inlet and outlet of bed one is bricklined to protect the carbon steel shell from the high operating temperature.  After many years of operation with numerous thermal cycles the bricklining may become damaged.  If this occurs the carbon steel shell is no longer protected and is exposed to the high temperatures.  Hot spots will occur on the shell which places additional stress on the shell.  If the damaged bricklining occurs in the vicinity of the gas outlet nozzle the added  stress on the transition nozzle usually results in cracks and gas leaks.  The damaged bricklining will generally not be discovered until an internal inspection of the converter is performed.  If damage is discovered it should be repaired immediately.

Pregnant converters refer to the bulging of the shell that is typical of older carbon steel converters.  At the high operating temperatures of beds 1 and 2, carbon steel does not possess much strength compared to at ambient temperatures.   Over time the shell of the converter begins to bulge as a result of the stress placed on the shell from the weight of the converter and the operating pressure.   Internal bricklinings on the interior walls of the hotter beds helps to the reduce the temperature of the shell but it is still insufficient to prevent bulging of the shell over extended periods of time.

The weight of the catalyst and high operating temperatures combine to weaken the catalyst support grids resulting in sagging of the grids.  The high stresses and possible defects in the casting may result in cracks forming in the webs of the support grids.  These cracks may eventually lead to failure of the grid and collapse of a portion of the bed.  The best remedy is to replaced the damaged grid.  A temporary solution is to place a temporary support post directly underneath the crack to prevent total collapse.

Thermal movements inside the converter can cause support grids and posts to shift out of position.  If the grids shift enough there will no longer be anything to restrain the post and it may shift out of position.  Once this happens there is nothing to support the grids and they will be in danger of collapse.

The photos illustrates this situation and it should be rectified before the converter is placed back in service.