Guidance
Note 01/07 – Inspection and Gauging of 3/4" NPSM Threads.
Introduction
In response to
requests for clarification, from members in the Far East and Australasia, the
following guidance is issued for SCUBA cylinders and valves that utilise the ¾"
NPSM thread form.
ASSET
recommends that technicians, involved in the assembly and maintenance of SCUBA
cylinders and their valves, use the appropriate, calibrated, thread gauges as a
means to ensure that they are both within tolerances and compatible with each
other. Additionally, the technician should have received training in their use,
as provided in the ASSET Dive Industry Technician’s and Cylinder Tester’s
Courses and Manuals.
Thread Gauges
It is
established practice to use a set of full-form ring and plug gauges, when
checking threads.
Note: In the EC, there is a
statutory requirement for cylinder inspection and testing. In the UK, in order
to gain IDEST approval, the technician must be ASSET certified and have access
to calibrated thread gauges for all thread forms to be inspected.
The following
thread gauges are recommended:
For internal
threads a double ended screw plug gauge (Style E) should be used. This gauge
comprises a “Go” element which tests that the Major and Effective diameters are
not too small. It also tests for pitch and flank errors in the thread. The “Not
Go” element tests only that the Effective diameter is not too large.
3/4” x 14
Parallel Threaded NPSM Male Screw “Plug” gauge to HS28 part 2, double ended, Go
and Not Go, medium fit, Class 2B.
For external
threads a pair of full form ring gauges should be used. These gauges comprise a
“Go” element which tests that the Major and Effective diameters are not too
large. It also tests for pitch and flank errors in the thread. The “Not Go”
element tests only that the Effective diameter is not too small.
3/4” x 14
Two Parallel Threaded NPSM Female Screw “Ring” gauges to HS28 part 2, solid, Go
and Not Go, medium fit, Class 2A
For other
thread forms, see either the ASSET Technicians Handbook Appendix 3 or the ASSET
Cylinder Testing Handbook Pt 1 Appendix I)
Gauges should
be checked and their accuracy certified on an annual basis.
The names for
these gauges indicate their purpose. A “Go” gauge should screw all the way to
the full extent of the number of threads to be engaged. A “Not Go” gauge should
engage with the thread but it should not be possible to screw it in / on at all.
However, in many industries, a “custom and practice” allowance of 1½ - 2 threads
engaged on the “Not Go” gauge is often considered acceptable to account for post
manufacture wear and tear. Such additional “tolerances” are not found in
thread manufacturing standards.
Precedent
In the mid
1980’s Luxfer UK addressed the question of how many threads could be allowed to
be engaged on a “not go” gauge when used on an aluminium cylinder containing a
G¾" (ISO 228) neck thread. At that time, by experimentation, a determination was
made that, if the valve stem in use comprised 11 threads and all other factors
were satisfactory, then the “Not Go” gauge could be allowed to “go” by up to a
maximum of four full turns leaving sufficient strength in the connection to
adequately withstand the forces acting on the threads.
This became the
accepted tolerance, particularly in the UK’s IDEST approved cylinder test
stations, and has formed the basis of ASSET guidance to its members ever since.
Guidance
Although not
identical, the ¾" NPSM thread is very similar to the G¾" and has a similar
strength. It is therefore reasonable to assume that a similar allowance can be
made:
Firstly, the
threads of both the cylinder and valve must be visually inspected to ensure that
they are free from significant corrosion and mechanical defects.
Secondly the
“Go” gauge must screw all the way in /on for the number of threads to be
engaged.
Finally, when a
valve with 11 threads is fitted, the “Not Go” gauge may be allowed to go by a
total of not more than four full threads. When a valve with less than 11 threads
is fitted, the “Not Go” gauge may be allowed to go by a total of not more than
two full threads
Note that this
is the total number for the combination of both cylinder and valve threads;
therefore, for example, if the valve has two threads engaged on the “Not Go”
gauge then only two threads could be allowed on the neck thread of the
corresponding cylinder. If there are three threads engaged on the “Not Go” gauge
in the neck, the valve stem would be allowed one.
Even new
cylinders and valves should be checked. Never assume that a new product will
automatically be correctly manufactured; experience shows that manufacturing
defects can and do occur. Defective products should be returned to the supplier
for replacement or refund.
Always inform
manufacturers of such defects stating, wherever possible, the serial number and
place of purchase. A conscientious manufacturer will want to investigate why
defects are getting past their quality control.
Safety Warning
It is entirely
possible to incorrectly assemble a G¾" (ISO 228) valve stem thread into a
cylinder containing a ¾" NPSM thread and vice versa. These two threads look
similar and have the same pitch (14 tpi) and are similar in diameter.
However, the angle of these threads differs by 5 degrees and if incorrectly
paired, produces a weak assembly, which can cause a catastrophic failure of the
thread, resulting in the valve being ejected when pressurised.
Do not
assemble cylinders and valves unless you are certain of the compatibility,
condition and tolerance of the threads.
The above
guidance is offered in good faith and is believed to be correct to the best of
our knowledge. Nothing contained therein shall be deemed to override the
manufacturer's policy, National / International Standards and the Law.
Enquiries about
ASSET Guidance Notes, Standards, Codes of Practice and Technician Qualifications
to: ASSET Admin Office, 12 Coulston Rd., Lancaster. LA1 3AE. Tel: +44 (0) 1524
381831 E-mail:
admin@scubatechnician.com
www.scubatechnician.com
Copies of ASSET
guidance notes are free of charge to members and can be obtained by email
request to ASSET, or by sending a large SAE to the above address. ASSET Codes of
Practice and Workshop Procedures can also be purchased from this source.
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