Corrosion of Linings
191
In an actual process, the polymer may absorb the process fluids, and
repeated temperature or pressure cycling can cause blisters. Eventually, the
corrodent may find its way to the substrate.
Related effects can occur when process chemicals are absorbed that may
later react, decompose, or solidify within the structure of the plastic. Prolonged
retention of the chemicals may lead to their decomposition within the polymer. Although unusual, it is possible for absorbed monomers to polymerize.
Several steps can be taken to reduce absorption. For example, thermal
insulation of the substrate will reduce the temperature gradient across the
vessel, thereby preventing condensation and subsequent expansion of the
absorbed fluids. This also reduces the rate and magnitude of temperature
changes, keeping blisters to a minimum. The use of operating procedures
or devices that limit the ratio of process pressure reductions or temperature
increases will provide additional protection.
6.2.4 inspection of the Lining
Scheduling the inspection of the liner and installation in the vessel should
be arranged so that timely inspections can be made at various stages. The
following items should be inspected:
1. Liner fit-up and preparation for welding. Fusion welded seams may
be made prior to application of the liner to the mold in the case of an
RTP shell.
2. In the case of an RTP structural shell, the liner welds should be
inspected on the mold prior to application of the RTP shell. All welds
should be checked visually and with a spark tester. If the mold is not
of metal construction, conductive back-up material must be placed
on the mold behind the seams.
3. If the structural shell is to be of RTP construction, it is necessary that
conductive resin ground strips be placed over the back side of the
seams so that the liner can be spark tested after the structural laminate has been applied. The resin ground strips are made from resin,
filled with carbon powder applied as a putty, or are resin reinforced
with carbon cloth laid up over the seams. This provides sufficient
electrical conductivity to create a ground for the spark. When a steel
shell is to be used, the strips are unnecessary because the steel shell
will act as a ground for the spark.
4. Application of the prime coat to the back of the liner.
5. With an RTP shell, fit-up of the components of the vessel. This
includes head-to-shell, nozzles to heads and shell, and shell joints.
Alignment is more critical in a lined vessel than in an unlined one.
191
In an actual process, the polymer may absorb the process fluids, and
repeated temperature or pressure cycling can cause blisters. Eventually, the
corrodent may find its way to the substrate.
Related effects can occur when process chemicals are absorbed that may
later react, decompose, or solidify within the structure of the plastic. Prolonged
retention of the chemicals may lead to their decomposition within the polymer. Although unusual, it is possible for absorbed monomers to polymerize.
Several steps can be taken to reduce absorption. For example, thermal
insulation of the substrate will reduce the temperature gradient across the
vessel, thereby preventing condensation and subsequent expansion of the
absorbed fluids. This also reduces the rate and magnitude of temperature
changes, keeping blisters to a minimum. The use of operating procedures
or devices that limit the ratio of process pressure reductions or temperature
increases will provide additional protection.
6.2.4 inspection of the Lining
Scheduling the inspection of the liner and installation in the vessel should
be arranged so that timely inspections can be made at various stages. The
following items should be inspected:
1. Liner fit-up and preparation for welding. Fusion welded seams may
be made prior to application of the liner to the mold in the case of an
RTP shell.
2. In the case of an RTP structural shell, the liner welds should be
inspected on the mold prior to application of the RTP shell. All welds
should be checked visually and with a spark tester. If the mold is not
of metal construction, conductive back-up material must be placed
on the mold behind the seams.
3. If the structural shell is to be of RTP construction, it is necessary that
conductive resin ground strips be placed over the back side of the
seams so that the liner can be spark tested after the structural laminate has been applied. The resin ground strips are made from resin,
filled with carbon powder applied as a putty, or are resin reinforced
with carbon cloth laid up over the seams. This provides sufficient
electrical conductivity to create a ground for the spark. When a steel
shell is to be used, the strips are unnecessary because the steel shell
will act as a ground for the spark.
4. Application of the prime coat to the back of the liner.
5. With an RTP shell, fit-up of the components of the vessel. This
includes head-to-shell, nozzles to heads and shell, and shell joints.
Alignment is more critical in a lined vessel than in an unlined one.