Corrosion of Linings
181
excessive softening of the film and makes recovery of the lining more difficult, and thus prone to early failure.
Certain tank coating systems may have excellent resistance to specific
chemicals for a given period of time, after which they must be cleaned
and allowed to recover for a designated period of time in order to return
to their original resistance level. Thirty days is a common period of time
for this process between chemicals, such as acrylonitrile and solvents such
as methanol.
In some cases, the density of cure can be increased by loading a hot, mildly
aggressive solvent at a later date. Ketamine epoxy is such an example. There
have been cases where three or four consecutive hot, mild cargoes have
increased the density of the lining to such an extent that the ketamine epoxy
lining was resistant to methanol. Under normal circumstances, ketamine is
not compatible with methanol.
When case histories are not available, or manufacturers are unable to make
a recommendation, it will be necessary to conduct tests. This can occur in
the case of a proprietary material being handled or if a solution might contain unknown chemicals. Sample panels of several coating systems should
be tested for a minimum of 90 days, with a 6-month test being preferable.
Because of normal time requirements, 90 days is standard.
The test must be conducted at the maximum operating temperature to
which the coating will be subjected and should simulate actual operating
conditions, including washing cycles, cold wall, and effects of insulation.
Other factors to consider in coating selection are service life, maintenance
cycles, operating cycles, and the reliability of the coating. Different protective coatings provide different degrees of protection for different periods of
time at a variety of costs. Allowable downtime of the facility for inspection
and maintenance must also be considered, in terms of frequency and length
of time.
Once the coating system has been selected, recommendations from the
manufacturer as to a competent applicator should be requested and contact
made with previous customers.
6.1.3 Lining application
The primary concern in applying a lining to a vessel is to deposit a void-free
film of a specific thickness on the surface. Any area that is considerably less
than the specified thickness may leave a noncontinuous film. Additionally,
pinholes in the coating may cause premature failure.
Films that exceed the specified thickness always pose the danger of
entrapping solvents, which can lead to poor adhesion, excessive brittleness,
improper cure, and subsequent poor performance. Avoid dry-spraying of the
coating material, as this causes the coating to be porous. If thinners, other
than those recommended by the manufacturer are used, poor film formation
181
excessive softening of the film and makes recovery of the lining more difficult, and thus prone to early failure.
Certain tank coating systems may have excellent resistance to specific
chemicals for a given period of time, after which they must be cleaned
and allowed to recover for a designated period of time in order to return
to their original resistance level. Thirty days is a common period of time
for this process between chemicals, such as acrylonitrile and solvents such
as methanol.
In some cases, the density of cure can be increased by loading a hot, mildly
aggressive solvent at a later date. Ketamine epoxy is such an example. There
have been cases where three or four consecutive hot, mild cargoes have
increased the density of the lining to such an extent that the ketamine epoxy
lining was resistant to methanol. Under normal circumstances, ketamine is
not compatible with methanol.
When case histories are not available, or manufacturers are unable to make
a recommendation, it will be necessary to conduct tests. This can occur in
the case of a proprietary material being handled or if a solution might contain unknown chemicals. Sample panels of several coating systems should
be tested for a minimum of 90 days, with a 6-month test being preferable.
Because of normal time requirements, 90 days is standard.
The test must be conducted at the maximum operating temperature to
which the coating will be subjected and should simulate actual operating
conditions, including washing cycles, cold wall, and effects of insulation.
Other factors to consider in coating selection are service life, maintenance
cycles, operating cycles, and the reliability of the coating. Different protective coatings provide different degrees of protection for different periods of
time at a variety of costs. Allowable downtime of the facility for inspection
and maintenance must also be considered, in terms of frequency and length
of time.
Once the coating system has been selected, recommendations from the
manufacturer as to a competent applicator should be requested and contact
made with previous customers.
6.1.3 Lining application
The primary concern in applying a lining to a vessel is to deposit a void-free
film of a specific thickness on the surface. Any area that is considerably less
than the specified thickness may leave a noncontinuous film. Additionally,
pinholes in the coating may cause premature failure.
Films that exceed the specified thickness always pose the danger of
entrapping solvents, which can lead to poor adhesion, excessive brittleness,
improper cure, and subsequent poor performance. Avoid dry-spraying of the
coating material, as this causes the coating to be porous. If thinners, other
than those recommended by the manufacturer are used, poor film formation