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

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Materials of Construction - Insulation
June 1, 2003

Introduction
        Personnel Protection
        Heat Conservation
        Cold Conservation/Prevention of Condensation
        Freeze Protection
Insulation Types

        Mineral Wool
        Calcium Silicate

        Fibreglass
        Foamglass
Insulation for Piping
        Acid Piping
        FRP Piping
        Utility Piping

Insulation for Equipment
        Acid Coolers
        Contact Section Equipment
        Converter
        Storage/Pump Tanks
Cladding
Hardware
Associated Links

Installation

Inspection and Maintenance


Introduction

Equipment may be insulated for one or more of the following reasons:

The type of insulation will depend on the intended service, temperature range and purpose of the insulation.  Insulation thickness will depend the purpose of the insulation, process temperature, type of insulation and ambient conditions.

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Personnel Protection

Piping and process equipment that do not need to be insulated to conserve energy but have a surface temperature greater than 60°C (140°F) and can be touched by personnel must be insulated to prevent personal injury.  Piping, ducting or equipment that is 3 m (10 ft) away from a platform, stair, ladder or area normally accessible by personnel may not need to be insulated for personnel protection since it cannot be contacted by accident.

The type of cladding applied to the insulation will affect the sensation of surface temperature.  Metal cladding will feel hotter than a plastic cladding t the same temperature.  An embossed metal cladding with a textured surface will feel different than a smooth metal cladding.

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Heat Conservation

If the reason for insulating is primarily heat conservation, then the economic thickness of insulation must be determined.  The economic thickness of insulation is that thickness of insulation which will save enough energy to pay for itself in a given time period.  This type of calculation is typical of insulation applied to power generation boilers or steam systems where fuel has been burned in order to heat up a fluid.  In an acid plant heat conservation is more for maintaining process temperatures, operability of the plant and protection of equipment.

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Cold Conservation/Prevention of Condensation

When air comes in contact with a surface whose temperature is lower than the dew point temperature of the air, condensation occurs.    The outside temperature of insulation on low temperature piping or equipment is below the ambient temperature.  Therefore, the possibility of condensation is always present and condensation will occur at some specific condition of temperature and humidity.

If condensation gets into insulation, it will cause an undesirably high heat gain.  Even if such entrance is stopped by a vapour barrier, a wet surface is undesirable and the subsequent drippage may cause damage and become a housekeeping problem.

The design conditions will vary climate and relative humidity depending on the location.  In humid climate more insulation is need for the same temperature to prevent condensation.  It is impossible to provide insulation of sufficient thickness to prevent condensation at 100% relative humidity as an infinite thickness would be necessary.

In low temperature insulation situations, the required thickness will usually be based on the prevention of surface condensation rather than cold conservation (i.e. heat gain).

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Freeze Protection

Where the ambient conditions are such that the contents of the piping or equipment can freeze, heat tracing and insulating for freeze protection should be considered. 

The first step should be to design all piping so that its contents drain back to the pump tank on shutdown of the circulation system.  Any dead legs must be provided with a drain.  In unavoidable situations acid lines can be electrically heat traced and insulated for the purpose of freeze protection (i.e. sample lines, drain lines and headers).

The insulation thickness should be based on the scenario where heat tracing has failed and flow has stopped.  The estimated time to restore heat tracing or flow should be determined and the insulation thickness determined to prevent freezing of the contents.

If heat tracing or flow cannot be re-established in the time it would take for the liquid to freeze than there should be sufficient time allowed to drain the piping.

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Insulation Types

Insulation used in acid plant service must have the following characteristics:

Insulation in contact with stainless steel shall conform to ASTM C795.

The following types of insulation are commonly used in acid plant service:

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materials_insul5.JPG (13696 bytes)Mineral Wool

Mineral wool is the standard material to be used in most applications throughout a sulphuric acid plant.  The mineral wool shall conform generally to the requirements of ASTM C612.

  • Bulk density (excluding metal facing and ties) 96 kg/m³ (6 lb/ft³) minimum
  • Maximum service temperature 650°C (1202°F)
  • Total sulphur compounds shall not exceed 1% measured in terms of sulphur.
  • Leachable chlorides shall be less than 65 ppm.
  • Thermal conductivity shall be less than 0.4 Btu in/h ft² °F at 204°C (400°F) mean temperature.
  • Resin bonded mineral wool is not acceptable
  • Individual mattresses shall not be more than 50 mm (2 in.) thick
  • The shot content shall be no greater than 15% by weight.   Shot is unfiberized particles in the material which add to the density of the material.

Bulk densities greater than 160 kg/m³ (10 lb/ft³) result in insulation boards that are too stiff to conform the curvature of most vessels except for the largest in diameter.  A higher bulk density does not necessarily imply a greater insulating ability so specifying higher bulk densities only adds cost and weight to the vessel.

In general, mineral wool insulation boards should not be applied to flat surface which will see traffic since the insulation will become compressed.

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materials_insul1.JPG (8646 bytes)Calcium Silicate

Calcium silicate will be used on surface where access is provided to the surface of a vessel and compression of the insulation cannot be tolerated.  The calcium silicate blocks shall conform generally with ASTM C533.

  • Bulk density after drying shall be approximately 240 kg/m³ (15 lb/ft³)
  • Maximum service temperature 650°C (1202°F)
  • Thermal conductivity shall be less than 0.48 Btu in/h ft² °F at 204°C (400°F) mean temperature
  • Chloride content in the insulation shall be minimized (maximum 25 ppm) and inhibited

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Fibreglass

Fibreglass insulation, both blanket and block form, may be economically preferable in certain operating conditions.  The insulation will generally conform to the following:

  • Bulk density 56 kg/m³ (3.5 lb/ft³)
  • Maximum service temperature 340°C (644°F)
  • Thermal conductivity shall be less than 0.24 Btu in/h ft² °F at 204°C (400°F) mean temperature

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materials_insul4.JPG (10920 bytes)Foamglass

Foamglass insulation offered by Pittsburgh Corning Corporation shall be used in low temperature application where condensation may be a problem.  Foamglass insulation is a lightweight, rigid insulating material composed of millions of completely seal glass cells.  Each cell is an insulating space.  The all-glass, closed-cell structure provides the following benefits:

  • Constant insulating efficiency
  • Fire protection
  • Corrosion resistance
  • Long term stability
  • Physical strength
  • Vermin proof

materials_insul3.JPG (8701 bytes)The insulation will generally conform to the following:

  • Bulk density 136 kg/cm³ (8.5 lb/ft³)
  • Temperature range -270°C to 24°C (-454°F to 75°F)
  • Moisture absorption 0%
  • Thermal conductivity shall be less than 0.38 Btu in/h ft² °F

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Insulation for Piping

Insulation for piping will generally be preformed insulation for the given pipe diameter.  Preformed insulation is also available for standard fitting such as tees and elbows.

Acid Piping

Acid piping regardless of material is generally not insulated for either personnel protection or heat conservation.  External insulation will contain any leaking acid resulting in external corrosion of the pipe.  This problem is made worse if moisture is present in the insulation resulting in the formation of weak acid.

All piping should drain back to the pump tank on shutdown of the circulation system.  Any dead legs must be provided with a drain and connected to the strong acid drain system.  In unavoidable situations acid lines can be electrically heat traced and insulated for the purpose of freeze protection (i.e. sample lines, drain lines and headers).

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FRP Piping

FRP lines generally do not require insulation for personnel protection even if the process temperature is above 60°C (140°F).  FRP has a relative low thermal conductivity compared to metals so the surface temperature generally does not exceed 60°C (140°F).  The surface of the pipe is also different from a metal pipe such that contact with a hot FRP pipe does not 'burn' in the same manner as a metal pipe at the same temperature.

All piping should drain back to the pump tank on shutdown of the circulation system.  Any dead legs must be provided with a drain.  In unavoidable situations lines can be electrically heat traced and insulated for the purpose of freeze protection (i.e. sample lines, drain lines and headers).

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Utility Piping

Insulation of utility piping (i.e. steam, condensate, process water, cooling water, etc.) is generally for heat conservation or maintenance of process temperature reasons.  Mineral wool or fibreglass insulation will generally be used depending on the service temperature. 

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Insulation for Equipment

Acid Coolers

Acid coolers whether shell and tube or plate and frame are generally not insulated for either personnel protection or heat conservation.  Freeze protection is provided by elevating the cooler and allowing the acid side to drain back to the pump tank or by installing the cooler in a heated building.  If possible, the cooling water side should drain back to the source, otherwise the cooler will need to be drained of water if freezing is a possibility.

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Contact Section Equipment

Generally, all equipment in the contact section of an acid plant is insulated for the purpose of heat conservation and process temperature maintenance.  Excessive heat losses in the plant may affect the performance of the converter and heat exchangers and raise the autothermal limit of the plant.

In most cases, it will not be necessary to perform detail heat loss calculations to determine the required insulation thicknesses for equipment.  It is acceptable to draw upon experience to quickly define insulation thicknesses for all equipment and process ducting.  The following table gives typical insulation thicknesses for the various pieces of equipment.

Equipment Insulation Thickness Remarks
Converter Typically 150 mm (6") although in cases where a greater degree of heat conservation is required 200 mm (8") has been specified. Mineral wool on sides and calcium silicate on roof. 
Hot Exchanger 150 mm (6") Mineral wool
Intermediate or Hot Interpass Exchanger 100 mm (4") Mineral wool
Cold or Cold Interpass Exchanger 100 mm (4") Mineral wool
Economizers 100 mm (4") Mineral wool
Superheaters 150 mm (6") Mineral wool
Interpass Absorber Towers 50 mm (2") Top mist eliminator housing only to prevent condensation of acid
Ducting
< 150°C (275°F)
150°C (275°F) < T < 300°C (572°F)
300°C (572°F) < T < 450°C (842°F)
> 450°C (842°F)

25 mm (1")
50 mm (2")
100 mm (4")
150 mm (6")
 

Mineral wool

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Converter

The installation of insulation on the converter is a special case due to the size of the vessel and the thermal expansion of the vessel shell.  The insulation system must allow the vessel to expand without significant radial movement.  The insulation must remain weather-tight and not open up as the vessel expands and contracts.   Damage to the cladding and insulation must be avoided as the vessel expands and the insulation contacts adjacent access structures.  Excessive compression of the insulation must be avoided as this will reduce its ability to meet the requirements for heat conservation.

A special specification for converter insulation is required. 

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Storage/Pump Tanks

Storage/pump tanks may require insulation for a number of different reasons (i.e. heat conservation, personnel protection, freeze protection, etc.).  The application of insulation to these vessel is similar to pipes.  A vessel can be treated as a very large pipe or as a flat surface if the vessel is large enough.

Bricklined pump tanks are generally not insulated since the internal lining is designed to maintain the shell temperature within acceptable limits.

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Cladding

Cladding or jacketing is the covering that protects the insulation from damage and the elements.  Cladding will normally be aluminum with 300 series stainless steel sheet specified as a premium upgrade.  Galvanized material is unacceptable.

Cladding on the vertical surfaces of vessels will be corrugated material, 0.020" thick with 3/16" corrugations.

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Hardware

materials_insulhardware.JPG (15428 bytes)

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