headerdrawing1.jpg (96365 bytes)

Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

Knowledge for the Sulphuric Acid Industry Line.jpg (1139 bytes)

Sulphuric Acid on the Web

Introduction
General
Equipment Suppliers
Contractor

Instrumentation
Industry News
Maintenance
Acid Traders
Organizations
Fabricators
Conferences

Used Plants
Intellectual Propoerty
Acid Plant Database
Market Information
Library

Technical Manual

Introduction
General

Definitions
Instrumentation
Plant Safety
Metallurgial Processes
Metallurgical
Sulphur Burning
Acid Regeneration
Lead Chamber
Technology
Gas Cleaning
Contact
Strong Acid
Acid Storage
Loading/Unloading

Transportation
Sulphur Systems
Liquid SO2
Boiler Feed Water
Steam Systems

Cooling Water
Effluent Treatment
Utilities
Construction
Maintenance
Inspection
Analytical Procedures
Materials of Construction
Corrosion
Properties
Vendor Data

DKL Engineering, Inc.

Handbook of Sulphuric Acid Manufacturing
Order Form
Preface
Contents
Feedback

Sulphuric Acid Decolourization
Order Form
Preface
Table of Contents

Process Engineering Data Sheets - PEDS
Order Form
Table of Contents

Introduction

Bibliography of Sulphuric Acid Technology
Order Form

Preface
Contents

Sulphuric Acid Plant Specifications
 

Google Search new2.gif (111 bytes)

 

 


Contact Section - Converters - Maintenance/Problems
January 10, 2011

Introduction
Converter Scaling
Screen Plugging
Deteriorated Brick Work
Collapsed Bed

Battle of the Bulge
Cast Iron Grid Separation
Damaged Refractory
Pregnant Converters
Cracked Grids
Converter Posts
Associated Links

Materials of Construction
Catalysts
Quartz
Converter Fabrication and Transportation


Introduction

The following are examples of some typical problems that have occurred with converters.  You may have experienced some of them yourselves while others may provide you with ideas on how to better maintain the converter(s) in your own plants.

Converter Scaling

Scaling of carbon steel converters is a well known phenomenon which can be addresses by metallizing the surface with aluminum.  Scaling or flaking represents a loss of material from the converter shell.  When the scale falls off they contaminate and plug the catalyst beds.   If the surface of the shell has been metallized, it is important to maintain the coating to prevent the metallizing from flaking off and to prevent the formation of scale.

With the advent of stainless steel converters, scaling was thought to be a thing of the past.   However, at least two isolated cases of scaling in stainless steel converters are known.  These scaling problems have occurred on metallurgical acid plants only.  Stainless steel converters in sulphur burning and acid regeneration plants do not seem to be affected.

Scale formation occurs by high temperature gas/metal chemical reactions between the sulphur (SO2) and oxygen in the gas and constituents of the steel.  Gas with high SO2, low O2 and high temperatures appear to be the most aggressive towards stainless steel.

The corrosion resistance of 304 stainless steel (0.040 to 0.080% carbon) (or 304H SS) is a result of the high chromium content of the metal (18% wt).  Under the operating conditions present in the converter, chromium will react to form a stable and dense layer of primarily chromium oxide (also chromium sulphide) at the surface that protects the underlying metal.  This layer prevents the diffusion of metal atoms outwards and prevents the migration of sulphur and oxygen inwards.  Further corrosion of the metal is effectively prevented as long as this chromium oxide layer remains in place.

The first step in the formation of the protective layer of chromium oxide is the concentration of chromium at the surface of the metal so that it can react with the oxygen in the process gas to form chromium oxide.  This is achieved through bulk material to surface diffusion of chromium and removal of other metals such as iron and nickel, through reactions with the gas components.

converterscale.JPG (17293 bytes)Iron and nickel will diffuse more quickly to the surface of the metal and react first to form and layer of scale.  Scale begins as a dark gray surface discolouration soon after startup.  Underneath the layer of scale, the protective layer of chromium oxide has formed in the chromium rich layer.  When fully developed scale appears as a black, hard brittle material approximately 0.05-1.0 mm in thickness.  Thermal cycling and gas velocity will cause the scale to break away from the parent material.  The scale will be broken up and be carried in the gas stream or fall onto the surface of the catalyst bed.

Although the amount of scale may appear to be significant, the amount of material actually loss from the shell is minimal.  Oxide and sulphide scales have a larger volume than the volume of the parent material consumed to form the scale.  Thickness measurements taken where scale has formed indicates little or no loss in metal thickness.

During a plant shutdown and catalyst bed screening, the scale can be removed but care must be taken not to damage that underlying protective chromium oxide layer.  A light brushing or vacuuming is recommended to remove loose scale.   If the chromium oxide layer is removed, the cycle of scale formation is repeated.

One proposed solution is to metallize the surface of the stainless steel with aluminum.  Metallizing of carbon steel has been used for years to prevent high temperature scaling of the surface.  As such, metallizing should be an effective means ofpreventing scaling of stainless steel.  At present it is not known if this has been attempted on the inside surface of a stainless steel converter.

Converter Scaling - 3.jpg (267499 bytes)  Converter Scaling - 4.jpg (314972 bytes)  Converter Scaling - 5.jpg (277696 bytes)

One plant owner has applied metallizing to the inside surface of the shell and is proceeding to replace converter internals with calorized components.  Read More...

Screen Plugging

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.

screenplug.jpg (42426 bytes)

Deteriorated Brick Work

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.

conv8.GIF (31429 bytes)

conv9.GIF (30669 bytes)

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.

conv6.GIF (33173 bytes)

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.

conv7.GIF (18072 bytes)conv5.GIF (12825 bytes)

Cast Iron Grid Separation

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.

conv4.gif (27284 bytes)

Damaged Refractory

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.

conv3.GIF (31329 bytes)

Pregnant Converters

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.

Convbulge.JPG (28463 bytes)

Cracked Grids

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.

crackedgrid.JPG (19567 bytes)

Converter Posts

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.

convpost.JPG (23035 bytes)