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THICKNESS
TEST
It is important to note that
thickness testing is only required
when a Specification cargo tank has
been transporting ladings (or
commodities) that are corrosive to
that cargo tank's shell and head
material, or if corrosion is
discovered during any inspection of
the cargo tank.
Many commodities that are classed as
"corrosives" do not corrode
stainless steel or even steel tanks
and there may be no need to perform
thickness tests. Conversely, there
are some products not classed as
"corrosives" or are even listed as a
hazardous commodity that will
corrode stainless steel, aluminum or
steel. Specification tanks that have
transported these commodities need
to be thickness tested. An example
is an aluminum Specification cargo
tank transporting certain crude oils
that are corrosive to aluminum or a
Specification stainless steel tank
transporting certain fatty acids
containing minute quantities of free
chlorines. Even some waters, like
well water, may cause biological
corrosion and cause severe pitting
in stainless steels. Corrosion is
seldom of the uniform type that eats
evenly at the shell material. Most
corrosion found in cargo tanks is
selective or causes pitting. If a
commodity corrodes the shell,
usually it will attack the following
areas initially: The heat affected
zone of welds which occurs in a
narrow band along each side of the
weld; highly stressed areas like
over frame members or bolsters; ring
stiffeners; head stiffeners; around
sumps and nozzles and in integral
heat panels.
Pitting corrosion will occur
selectively (as above) including the
liquid level line of the product. It
can also occur generally through out
the vessel, depending on the
severity of the attack. Pitting
will not necessarily show up as
thinning when using a thickness
tester and continued use of a pitted
tank will have to be a judgment call
of the owner and/or inspector.
Small amounts of shallow pitting may
be able to be repaired, however, a
severely pitted tank can not be
economically repaired. Worse, once
severe pitting is started the
process is irreversible as the
contaminant that caused pitting to
start, cannot be removed from the
bottom of the pits and the pitting
continues until it goes completely
through the shell. Corrosion of
cargo tanks has been, and can be
very expensive for cargo tank
owners. If unexpected corrosion is
discovered in a tank, the employment
of a qualified metallurgist will
usually be a wise choice.
(i) Thickness testing.
(1) The shell
and head thickness of all unlined
cargo tanks used for the
transportation of materials
corrosive to the tank must be
measured at least once every 2
years, except that cargo tanks
measuring less than the sum of the
minimum prescribed thickness, plus
one-fifth of the original corrosion
allowance, must be tested annually.
(2)
Measurements must be made using a
device capable of accurately
measuring thickness to within ±0.002
of an inch.
(3) Any
person performing thickness testing
must be trained in the proper use of
the thickness testing device used in
accordance with the manufacturer's
instruction.
(4)
Thickness testing must be performed
in the following areas of the cargo
tank wall, as a minimum:
(i) Areas
of the tank shell and heads and
shell and head area around any
piping that retains lading;
(ii) Areas
of high shell stress such as the
bottom center of the tank;
(iii) Areas
near openings;
(iv) Areas
around weld joints;
(v) Areas
around shell reinforcements;
(vi) Areas
around appurtenance attachments;
(vii) Areas
near upper coupler (fifth wheel)
assembly attachments;
(viii)
Areas near suspension system
attachments and connecting
structures;
(ix) Known
thin areas in the tank shell and
nominal liquid level lines; and
(x)
Connecting structures joining
multiple cargo tanks of carbon steel
in a self-supporting cargo tank
motor vehicle.
(5) Minimum
thicknesses for MC 300, MC 301, MC
302, MC 303, MC 304, MC 305, MC 306,
MC 307, MC 310, MC 311, and MC 312
cargo tanks are determined based on
the definition of minimum thickness
found in §178.320(a) of this
subchapter. The following Tables I
and II identify the “In-Service
Minimum Thickness” values to be used
to determine the minimum thickness
for the referenced cargo tanks. The
column headed “Minimum Manufactured
Thickness” indicates the minimum
values required for new construction
of DOT 400 series cargo tanks, found
in Tables I and II of §§178.346–2,
178.347–2, and 178.348–2 of this
subchapter. In-Service Minimum
Thicknesses for MC 300, MC 301, MC
302, MC 303, MC 304, MC 305, MC 306,
MC 307, MC 310, MC 311, and MC 312
cargo tanks are based on 90 percent
of the manufactured thickness
specified in the DOT specification,
rounded to three places.
TABLE
I—IN-SERVICE MINIMUM
THICKNESS FOR MC 300, MC
303, MC 304, MC 306, MC 307,
MC 310, MC 311, AND MC 312
SPECIFICATION CARGO TANKS
CONSTRUCTED OF STEEL AND
STEEL ALLOYS
|
Minimum manufactured
thickness (US gauge
or inches) |
Nominal decimal
equivalent for
(inches) |
In-service minimum
thickness reference
(inches) |
|
19 |
0.0418 |
0.038 |
|
18 |
0.0478 |
0.043 |
|
17 |
0.0538 |
0.048 |
|
16 |
0.0598 |
0.054 |
|
15 |
0.0673 |
0.061 |
|
14 |
0.0747 |
0.067 |
|
13 |
0.0897 |
0.081 |
|
12 |
0.1046 |
0.094 |
|
11 |
0.1196 |
0.108 |
|
10 |
0.1345 |
0.121 |
|
9 |
0.1495 |
0.135 |
|
8 |
0.1644 |
0.148 |
|
7 |
0.1793 |
0.161 |
|
3/16 |
0.1875 |
0.169 |
|
1/4 |
0.2500 |
0.225 |
|
5/16 |
0.3125 |
0.281 |
|
3/8 |
0.3750 |
0.338 |
TABLE
II—IN-SERVICE MINIMUM
THICKNESS FOR MC 301, MC
302, MC 304, MC 305, MC 306,
MC 307, MC 311, AND MC 312
SPECIFICATION CARGO TANKS
CONSTRUCTED OF ALUMINUM AND
ALUMINUM ALLOYS
|
Minimum manufactured
thickness |
In-service minimum
thickness (inches) |
|
0.078 |
0.070 |
|
0.087 |
0.078 |
|
0.096 |
0.086 |
|
0.109 |
0.098 |
|
0.130 |
0.117 |
|
0.141 |
0.127 |
|
0.151 |
0.136 |
|
0.172 |
0.155 |
|
0.173 |
0.156 |
|
0.194 |
0.175 |
|
0.216 |
0.194 |
|
0.237 |
0.213 |
|
0.270 |
0.243 |
|
0.360 |
0.324 |
|
0.450 |
0.405 |
|
0.540 |
0.486 |
(6) An
owner of a cargo tank that no longer
conforms to the minimum thickness
prescribed for the design as
manufactured may use the cargo tank
to transport authorized materials at
reduced maximum weight of lading or
reduced maximum working pressure, or
combinations thereof, provided the
following conditions are met:
(i) A
Design Certifying Engineer must
certify that the cargo tank design
and thickness are appropriate for
the reduced loading conditions by
issuance of a revised manufacturer's
certificate, and
(ii) The
cargo tank motor vehicle's nameplate
must reflect the revised service
limits.
(7) An
owner of a cargo tank that no longer
conforms with the minimum thickness
prescribed for the specification may
not return the cargo tank to
hazardous materials service. The
tank's specification plate must be
removed, obliterated or covered in a
secure manner.
(8) The
inspector must record the results of
the thickness test as specified in
§180.417(b).
(9) For MC
331 cargo tanks constructed before
October 1, 2003, minimum thickness
shall be determined by the thickness
indicated on the U1A form minus any
corrosion allowance. For MC 331
cargo tanks constructed after
October 1, 2003, the minimum
thickness will be the value
indicated on the specification
plate. If no corrosion allowance is
indicated on the U1A form then the
thickness of the tank shall be the
thickness of the material of
construction indicated on the UIA
form with no corrosion allowance.
(10) For
400-series cargo tanks, minimum
thickness is calculated according to
tables in each applicable section of
this subchapter for that
specification: §178.346–2 for DOT
406 cargo tanks, §178.347–2 for DOT
407 cargo tanks, and §178.348–2 for
DOT 412 cargo tanks.
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CONDUCTING THICKNESS TESTS DETERMINING
CORRECT MINIMUM THICKNESS:
Prior to performing a thickness test it is
necessary to determine the minimum thickness
acceptable for the cargo tank being
inspected. Many cargo tanks are constructed
with material thickness significantly
thicker than the Specifications require.
The DOT 400 series tanks will have the
minimum shell thickness stamped on their
certification plates and is, therefore, easy
to determine. Paragraph 178.340, the
general section for the MC 300 series tanks,
required that the certification plate for
these Specifications show only the shell and
head material, but not the manufactured or
minimum thickness. So, MC 300 series cargo
tanks, which were manufactured from 1967 to
1995, may not have any shell thickness
stamped on their certification plates, or
they may have the thickness of the material
of which they were constructed. If the
thickness is stamped it may be expressed as
a "decimal" thickness or as a "gauge"
thickness. This thickness is not necessarily
the minimum to which the tanks could have
been manufactured. There are several ways
to determine the minimum thickness for the
tank you are testing. The best is to contact
the manufacturer and provide them with the
tank's year of manufacture and serial
number. If contact with the manufacturer is
not an option, you may use the minimum shell
and head thickness tables which are a part
of MC 306,307 and 312. These tables require
that you have considerable design
information about the tank to use them
properly. TTMA, (Truck Trailer
Manufacturer's Association) has published a
Technical Bulletin # 113, on how to
determine the correct thickness. In some
cases it may also be necessary to employ the
services of a Design Certifying Engineer.
When you have determined the correct minimum
nominal thickness for the tank being
inspected, the thickness may be described
using a gauge thickness. Table I, Part
180.407 (i) provides the minimum decimal
thickness for that gauge number for steel
and stainless steel cargo tanks. If the
thickness is described using a nominal
decimal thickness, Table II shows the
minimum decimal thickness for aluminum tanks
giving decimal minimums for the decimal
nominal thickness' required in the MC 300
series Specifications.
MEASURING SHELL THICKNESS TAKING THICKNESS
READINGS:
Thickness readings are taken using an
Ultrasonic Thickness Tester available from
several manufacturers. These instruments are
capable of reading thickness within, plus or
minus .002" (when properly calibrated) and
that accuracy meets the requirements of
Paragraph 180.407 (1).
LOCATION AND NUMBER OF THICKNESS
MEASUREMENTS:
49CFR 180.407(i)(4) lists ten areas of the
vessel (cargo tank wall) which must be
tested. Although DOT is not more specific,
they have shown through audits of thickness
tests, that they expect more detail and
numerous readings are necessary to pass the
audit. As a practical matter, setting up to
take readings is time consuming, but
readings are made quickly, so it is
reasonable to take quite a few readings to
insure a thorough test.
TTMA Technical Bulletin No.113, dated
January 1995, suggests seven points on each
bulkhead, and six points around the
circumference at each circumferential weld
or shell stiffener location, such as a ring
stiffener or baffle. They also suggest
thickness readings at all tank openings. If
you are performing thickness tests, it will
be necessary for you to determine the
minimum number of readings with the cargo
tank owner, and/or the local DOT auditing
agency. If you cannot get that information,
use the TTMA procedure and document that
fact in your inspection report.
DOCUMENTATION OF THICKNESS TESTS:
The
Figure below shows the most typical format
used to report readings. It consists of a
diagram for each of the bulkheads, with the
locations and designations for each reading,
and a flat layout of the vessel. Clock
designations along the ends of the layout
are used for each circumferential reading
taken, with 12: 00 o' clock representing the
top centerline of the tank. Bands through
the pattern indicate longitudinal locations
for each set of circumferential readings.
The location of openings such as outlets and
manholes can also be shown with designations
for those readings. Readings may be
recorded directly on the layout if there is
room, or they may be tabulated in tables
which reference the designated points. See
Figure 4 (see graphics section).
CARGO
TANKS NOT MEETING THICKNESS REQUIREMENTS:
If
a cargo tank fails to pass the thickness
requirement for its specific designation, it
cannot be continued in service until changes
are made. Typical examples are MC 312 or DOT
412 cargo tanks which have been manufactured
with a corrosion allowance which has been
used up by the commodities carried, or MC
312 tanks manufactured for a high density
commodity which would still be authorized
for lower density commodities because the
shell and head thicknesses are within the
Specification's allowances for such
products. In the case of the MC 312 tanks,
they may be stamped for the lower density
commodity as determined by Table II in
Paragraph 178.343. However, it is important
to note that, under present DOT
interpretation, an MC 312 cargo tank MAY NOT
be recertified as (for example) an MC 307
cargo tank. An MC 312 may also qualify for
transportation of a variety of flammable
materials if its venting meets MC 306 or 307
requirements. The same would be true for a
DOT 412. It may qualify as a DOT 407 or 406
cargo tank. Again, a cargo tank
manufactured to one of the 300 series
Specifications may no longer be converted to
a another MC 300 Specification.
Please contact
us with any repair or service work you may
have:
Ed Mansell - General Manager
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