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Fundamentals of Corrosion
4. Hot service:
Normally, the thickness of the insulation must result in a heat loss
of 4 ± 1% of the heat loss for bare pipe or equipment. For hot systems,
the surface temperature should be less than 140ºF for nonmetallic
covers and less than 130ºF for metallic covers over insulation.
Refer to Table 11.1 and the characteristics of the different insulations. Absorptive insulations (e.g., fiberglass, mineral wool, calcium silicate) will initiate corrosion on the underlying surfaces.
Alternatively, wet and dry cycles within the insulation will induce
corrosion of carbon steel and chloride stress corrosion cracking of
austenitic stainless steels. To ensure against corrosion under insulation, nonabsorptive insulation materials should be considered.
Coating systems applied before insulation are protective to metal
surfaces when conditions are severe. The comments under cold service for protective coating apply in the intermediate temperature
range between 140 and 230ºF.
For elevated temperature service, the vapor barrier system must
be sufficiently permeable to vent moisture vapors yet resist the passage of liquid water inward.
See Section 11.3.2.3 on jacketing and Table 11.2 for the outer
covers.
5. Special environments:
Corrosive chemicals, including caustic soda (NaOH) vapor, dust,
or liquid spillage, will chemically attack galvanized surfaces. For
this reason, stainless steel (rather than galvanized) tie wires have
been specified in all installation systems.
6. Piping systems:
Because piping systems need to be enclosed completely by the
insulation system in order to keep out any moisture, the piping
designer must envision the insulated system. All supports should be
outside the insulation to minimize the sealing problem. Insulated
pipe supports are available that use cellular glass as the insulating
material. In some cases, inserts of higher compressive strength materials must be used, such as calcium silicate. This is acceptable when
necessary if precautions are taken to preclude the entrance of water
into the insert material.
Valves should be specified with extended bonnets so that the
sealing surface is above freezing at average winter temperatures.
When possible, anchors and guides need special attention and
may require additional pipeway spacing, depending on the design.
Vents, drains, and other connections should be minimized, particularly in very cold services. Piping manifolds should be laid out so
Fundamentals of Corrosion
4. Hot service:
Normally, the thickness of the insulation must result in a heat loss
of 4 ± 1% of the heat loss for bare pipe or equipment. For hot systems,
the surface temperature should be less than 140ºF for nonmetallic
covers and less than 130ºF for metallic covers over insulation.
Refer to Table 11.1 and the characteristics of the different insulations. Absorptive insulations (e.g., fiberglass, mineral wool, calcium silicate) will initiate corrosion on the underlying surfaces.
Alternatively, wet and dry cycles within the insulation will induce
corrosion of carbon steel and chloride stress corrosion cracking of
austenitic stainless steels. To ensure against corrosion under insulation, nonabsorptive insulation materials should be considered.
Coating systems applied before insulation are protective to metal
surfaces when conditions are severe. The comments under cold service for protective coating apply in the intermediate temperature
range between 140 and 230ºF.
For elevated temperature service, the vapor barrier system must
be sufficiently permeable to vent moisture vapors yet resist the passage of liquid water inward.
See Section 11.3.2.3 on jacketing and Table 11.2 for the outer
covers.
5. Special environments:
Corrosive chemicals, including caustic soda (NaOH) vapor, dust,
or liquid spillage, will chemically attack galvanized surfaces. For
this reason, stainless steel (rather than galvanized) tie wires have
been specified in all installation systems.
6. Piping systems:
Because piping systems need to be enclosed completely by the
insulation system in order to keep out any moisture, the piping
designer must envision the insulated system. All supports should be
outside the insulation to minimize the sealing problem. Insulated
pipe supports are available that use cellular glass as the insulating
material. In some cases, inserts of higher compressive strength materials must be used, such as calcium silicate. This is acceptable when
necessary if precautions are taken to preclude the entrance of water
into the insert material.
Valves should be specified with extended bonnets so that the
sealing surface is above freezing at average winter temperatures.
When possible, anchors and guides need special attention and
may require additional pipeway spacing, depending on the design.
Vents, drains, and other connections should be minimized, particularly in very cold services. Piping manifolds should be laid out so