Materials of Construction - Insulation
June 1, 2003
Introduction
Equipment may be insulated for one or more of the following reasons:
- Personnel Protection
- Heat Conservation
- Cold Conservation
- Process Temperature Maintenance
- Prevention of Condensation
- Fire Protection
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:
- Flame resistant
- Non-corrosive, whether wet of dry
- Asbestos bearing materials are not allowed
- Low chloride content (used on stainless steel
surfaces)
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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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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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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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
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
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