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Pipeline and Hazardous Materials Safety Administration Inspection Procedure for Inspection of DOT/TC cylinder

U.S. Department

400 Seventh
Street, S.W.

of Transportation

Washington,
D.C. 20590

Pipeline and Hazardous
Materials Safety Administration



Inspection Procedure for Inspection of DOT/TC cylinder
Mounting Threads

Scope

This document provides basic guidelines for the inspection
and evaluation of DOT/TC cylinder (tube) mounting threads
with outside diameter greater than or equal 18.

Definitions -

Bulkhead-a vertical steel plate located at one or both
ends of the tube bundle on a tube trailer, ISO module
and tube bundle that provides structural support for the
mounting of the tubes.

Mounting Threads- external threads cut on the outside
surface of the necks of a tube that are used to attach
the tube to the support structure.

Mounting Flange- a circular disk that is threaded onto
the mounting threads of a tube and subsequently bolted
to the bulkhead in order to attach the tube to the
trailer, ISO modules, or tube bundles. This flange
arrangement is used primarily on tubes.

Major Diameter- The diameter of a thread measured across
the crests of the threads for an external thread or
across the roots of the threads for an internal thread.
This value typically corresponds to the thread size
designation for the thread.

Saddle- A clamp device used to provide structural
support of a tube on the straight cylindrical portion,
rather than by a mounting flange securing the tube to a
bulkhead by means of mounting threads.


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Sleeve (collar) -An intermediate threaded component
between the flange and the tube that engages all
available threads on a tube neck

Tube- a seamless compressed gas cylinder longer than 6.5
feet (2 meters) which is authorized for transportation
only when horizontally mounted on a motor vehicle or in
an ISO framework or other framework of equivalent
structural integrity (Definition from TB-25)


Inspection Guidelines -

To prevent the wear on the mounting threads of a DOT cylinder
(tube) from weakening the threaded connection to a point
where safety may be compromised. The mounting threads on the
tube must be inspected once every 10 years. This inspection
will be performed by visual inspection after disassembly and
by using Thread Pitch Gauge for measurement of the thread
wear. Visual inspection requires removal of the mounting
flanges. When evaluating the mounting threads on tubes,
there are two basic categories of thread degradation that
should be considered: generalized thread wear and isolated
thread loss.

While inspecting mounting threads on tubes, care should also
be taken to follow the requirements of CGA C-6, Standards for
Visual Inspection of Steel Compressed Gas Cylinders, such as
examination for welds between the tube flange and the neck
threads, which are not allowed.

Generalized Thread Wear -

Generalized thread wear is the
erosion of the mounting threads over a significant area of
the neck thread due to the relative motion between the tube
and the mounting flange and is characterized by a measurably
shorter height of the threads in the area engaged by the
mounting flange. In cases of extreme wear, such as
illustrated in Figure 1, this wear has progressed to the
point where the threads are completely eroded. In less
extreme cases, a straightedge placed across the crests of the
threads, as shown in Figure 2, can help to identify less
severe general wear.



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Figure 1- Example of Generalized Thread Wear


Figure 2 Visualization of Thread Wear Using a
Straightedge (Note the gap between the straightedge and the
peak of the damaged threads)


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To properly evaluate the thread wear, two pieces of
information are required: the amount of the original thread
crest that has been worn away and the number of threads
affected by the wear. The effective remaining thread height
can be evaluated by measuring the diameter across the crests
of the worn threads using calipers, a thread gauge (see
Figure 3) or a micrometer. This measurement can be compared
to the major and minor diameters of the threads in the unworn
condition to determine the degree of erosion that has taken
place.


Figure 3- Thread Pitch Gauge

If this remaining thread height measurement is greater than
75 %, the threads with a new mounting flange should be
adequate for continued service without further evaluation or
modification.

Isolated Thread Wear (Deep Cuts and Gouges in Threads)

Isolated thread wear is localized damage to the mounting
threads caused by setscrews or anti-rotation pins. Anti-
rotation pins are cylindrical pins that are installed by
drilling a hole at the interface between the threads on the
tube and the threads on the mounting flange. The pin
essentially locks the mounting flange in place and prevents
it from rotating on the tube. Once installed in the
bulkheads, the pins prevent rotation of the tube in the
mounting flanges that could cause damage to the manifold
piping. Since these pins resist the torsion loads imparted
on the tube during transportation, it is not uncommon for the
pin site to become worn after years of use. In some cases, a
tube may be re-pinned several times in its lifetime prior to
when the flanges are replaced (See Figure 4). The depth and
number of pin sites on a tube can vary greatly depending on
the methods used to drill the tube, the age of the tube and
the design of the equipment. The cumulative effect of these

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multiple pinning locations might significantly reduce the
shear strength of the threaded connection.



Figure 4 - Example of Isolated Damage from Anti-rotation
Pins

Some older trailer designs utilized setscrews to lock the
mounting flange on the tube instead of the pins described
above. These setscrews were threaded through the mounting
flange in a radial direction and were tightened against the
tube threads to prevent rotation. Again, years of over-the
road transportation and repeated tightening of the setscrews
can result in localized erosion best characterized as an
isolated pit in the mounting threads on the tube (See Figure
5).


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Figure 5 Example of Isolated damage from to Setscrew

When evaluating areas of isolated thread wear, it is
important to consider both the depth of the erosion that
causes a reduction in mechanical strength at that point on
the tube neck and the number of isolated locations around the
neck of the tubes as these voids in the mounting threads can
weaken the shear strength of the threaded joint. The
allowable depth of isolated pits in the necks of the tubes
due to setscrews or pins is dependent on the geometry of the
tube neck and the design of the equipment.

The flaw, resulting from placement of setscrews or pins,
shall be measured and assessed by the retester to assure the
remaining bending moment is adequate for the weight of the
tube and to justify the continued use of the tube.

Inspection Procedure for Measurement of OD neck threads
This procedure applies on ICC / DOT / TC / CTC cylinders
(tubes) with 3AX, 3A, 3AA, 3AAX and 3T specifications with OD
greater than or equal to 18 the threads are made to 8-UN
class 2A thread specification (See Appendix B).


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A.

Pre Measurement Process -
1.

Remove the existing flange, collar/sleeve & any other
mounting hardware with care and ensuring that the tube
is not damaged.

2.

Clean the OD neck threads with a wire brush or any other
means that will not cause damage to the threads (see
Figure 6). There exists a good possibility for the
thread to be out of form due to impact from the flange
(the play resulting from tolerance available in the
thread classification). A die of the same thread
specification (same size (maximum major diameter) or
0.001 oversized) may be required to correct the threads
to give them their proper form (profile / contour).
Adequate precaution needs to be taken to ensure that the
lubricant used during this chasing operation is not
allowed to get inside the tube.

Note2:

The requalifier must prepare a written procedure and
document training personnel to perform these functions.






Figure 6- Typical steps in a thread cleaning process

B.

Threads Measurement tools -

1.

Thread Pitch gauges of the same pitch & minor diameter
as the thread specification of OD neck threads (e.g. 8
UN Class 2A)