352
Fundamentals of Corrosion
be specifically required over a vapor barrier mastic primary finish.
The metal itself is not intended to be a vapor barrier. Proper precautions should be taken in those instances where metal jacketing is used
so that metal edges do not promote a break in the mastic barrier.
To keep insulation systems dry, the following practices are given for
cold service:
1. Insulation of attachments to cold equipment for some distance
away from the cold surface is necessary to allow sufficient heat
gain in the attachment to permit the metal to warm up to the
dew point. A distance equal to the following factors times the
insulating thickness has been used with reasonable success. This
distance can be reduced by using specially designed thermal
insulators or similar techniques.
−51°F to −100ºF 2X or 2 times
150ºF to −50ºF
1X or 1 time
2. The requirement to insulate attachments includes cold equipment supports. The method used to support the equipment
should be thought out carefully. Skirt-supported vertical equipment is usually not a problem because the skirts are generally
long enough to allow sufficient insulation to be installed without
covering the anchor bolts or base plates. Saddle-mounted equipment can be a problem. Saddles should be high enough so that
the insulation can be terminated above the anchor bolts. Also,
because the temperature of the base plates will be below the dew
point, the expansion end should be equipped with noncorrosive
expansion surfaces. Lug-mounted equipment should be avoided
whenever possible because the need to insulate the supporting
beams may make platform and wellway design complicated.
3. Leg-supported equipment should use pipe sections instead of
structural shapes to simplify insulation sealing problems.
4. Where possible, equipment should be located in structures to
minimize the need for structural attachments, such as ladders
and platforms, to be attached directly to the equipment. Where
such attachments are used, the metallurgy of the attachment
inside the insulation must be reviewed to be certain it is suitable
for the cold temperatures.
5. In addition to the above design practices aimed at keeping the
insulation dry and consistent with reports of corrosion under
insulation in NACE publication GH 189, carbon steel and austenitic stainless steel between –50 and 150ºF should receive a protective coating prior to insulation.
Fundamentals of Corrosion
be specifically required over a vapor barrier mastic primary finish.
The metal itself is not intended to be a vapor barrier. Proper precautions should be taken in those instances where metal jacketing is used
so that metal edges do not promote a break in the mastic barrier.
To keep insulation systems dry, the following practices are given for
cold service:
1. Insulation of attachments to cold equipment for some distance
away from the cold surface is necessary to allow sufficient heat
gain in the attachment to permit the metal to warm up to the
dew point. A distance equal to the following factors times the
insulating thickness has been used with reasonable success. This
distance can be reduced by using specially designed thermal
insulators or similar techniques.
−51°F to −100ºF 2X or 2 times
150ºF to −50ºF
1X or 1 time
2. The requirement to insulate attachments includes cold equipment supports. The method used to support the equipment
should be thought out carefully. Skirt-supported vertical equipment is usually not a problem because the skirts are generally
long enough to allow sufficient insulation to be installed without
covering the anchor bolts or base plates. Saddle-mounted equipment can be a problem. Saddles should be high enough so that
the insulation can be terminated above the anchor bolts. Also,
because the temperature of the base plates will be below the dew
point, the expansion end should be equipped with noncorrosive
expansion surfaces. Lug-mounted equipment should be avoided
whenever possible because the need to insulate the supporting
beams may make platform and wellway design complicated.
3. Leg-supported equipment should use pipe sections instead of
structural shapes to simplify insulation sealing problems.
4. Where possible, equipment should be located in structures to
minimize the need for structural attachments, such as ladders
and platforms, to be attached directly to the equipment. Where
such attachments are used, the metallurgy of the attachment
inside the insulation must be reviewed to be certain it is suitable
for the cold temperatures.
5. In addition to the above design practices aimed at keeping the
insulation dry and consistent with reports of corrosion under
insulation in NACE publication GH 189, carbon steel and austenitic stainless steel between –50 and 150ºF should receive a protective coating prior to insulation.