High pressure cylinder threads require careful examination on a regular basis to ensure their integrity has not been impaired by corrosion, damage, or cracking. While inspection is required for both steel and aluminum cylinders threads, aluminum cylinders are more likely to sustain thread damage.
The increased potential for 3AL cylinder thread damage exists because the metal is relatively soft, is susceptible to galvanic corrosion and, in some cases, subject to sustained load cracking. Threads on steel SCUBA and SCBA may corrode from the presence of moisture, particularly salt water, but are seldom impacted by dissimilar metal corrosion or cross-threading by the valve during insertion.
Sustained Load Cracking occasionally occurs in the threaded area of older aluminum cylinders made from 6351 alloy. Stress Corrosion Cracking, although rare, might be found in cylinders made from either 6351 alloy or the 6061 alloy now commonly used for making cylinders. The discovery of cracking potential prompted the Department of Transportation (DOT) to determine suitable methods for examining threads during the visual inspection of 6351cylinders. In addition, industry, working in cooperation with cylinder manufacturers and PSI, Inc., the inspector training agency, developed tools to increase the precision of thread inspection. These tools have been integrated into the well- established protocol of annual cylinder visual inspections.
The DOT determined that a very small dental mirror capable of reaching into both SCUBA and smaller SCBA necked cylinders was the minimum necessary to adequately view thread integrity. Fortunately, although those 7/16 inch diameter magnifying mirrors are no longer used in the dental field, PSI, Inc. is able to special order those mirrors from international sources.
Eddy current technology was also incorporated into the aluminum thread assessment process. First developed by Flare Technology, Inc., the Visual Plus device provided excellent detection of otherwise difficult to see thread area cracks. It is sold by Advanced Inspection Technology. Currently, eddy current machines are also sold by Flare Technology under the brand name of Visual Eddy. Various models of these two brands cost between $700. and $1,000.
In 2000, Forest City Scuba began marketing a viewing tube that illuminates and magnifies the cylinder thread area. Called the Thread Inspection Pipe (TIP), the tool was well received by visual inspectors because it is simple to operate and clearly shows each thread during a careful evaluation. The device allows businesses with very small cylinder inventories to have a device more effective than the standard magnifying mirror and at less cost ($310.) than the eddy current devices.
In the fall of 2002, Flare Technology introduced a device similar to the TIP tool but is smaller in size. The Flare Optic Viewer (FOV) develops 4.3 power of magnification in the cylinder thread area. Its cost is $140.00.
The DOT requires its registered hydrostatic requalifiers to have at least one of the tools described here for enhancing the view of cylinder threads. The PSI, Inc. visual inspection protocol likewise incorporates a magnifying mirror or optionally, one of the more precise tools to examine the threads of all cylinders being inspected.
Neither a cylinder hydrostatic requalification nor formal visual inspection is complete without examining threads, whether steel or aluminum. The minimum number of complete, continuous threads varies among different cylinder designs and pressure ratings. PSI, Inc. trained visual inspectors receive thread standards during formal training. Also, cylinder manufacturers may be contacted for minimum thread requirements. Remember, the thread count must begin at the exterior (cylinder top) and proceed interiorly. Only those threads that will be in contact with the valve may be counted.
For further information or to purchase magnifying inspection mirrors, Opti-Viewer or Optical Plus for illuminating thread viewing tools, contact PSI, Inc. at email:
. or phone PSI in the US at 425.398.4300.
Contact AIT for Visual Plus Eddy Current device at 951-776-9994 or Visual Eddy for the Visual Eddy device at 630-736-5500