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: [email protected] 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.