Northeast Pressure Vessel Testing Ultrasonic Testing

Quality inspection practices ensure cylinder safety.

In accordance with DOT specifications, NEPVT inspects the neck region of your aluminum cylinder with electrical eddy current looking for small changes caused by cracks and folds, thus, detecting any and all imperfections. The visual eddy inspection is able to detect flaws that can be missed by ordinary visual techniques.

For this reason, the Department of Transportation (DOT) requires all cylinders made of 6351-T6 aluminum alloy materials must be carefully inspected by trained inspectors with special attention given to the shoulders and thread neck area. The industry practice is to assume that any aluminum cylinder manufactured prior to 1990 is manufactured from the 6351-T6 alloy.

Conditions occurring in cylinders made of aluminum alloy 6351-T6 may lead to sustained load cracking (SLC), depending upon the material sensitivity, cylinder design, defects and/or uncontrolled operational stresses. Sustained load cracking is a slow creep failure mechanism occurring in aluminum alloy (AA) 6351-T6 material. Coupled with sufficient internal stresses, any pre-existing flaw can reach a critical crack size leading to leakage. Sustained load cracking (SLC) is metallurgical anomaly that may develop in 6351 aluminum alloy high-pressure cylinders.

As the name implies, SLC is a stress-related and progressive process that may take 8-10 years for a crack to propagate through the neck. Stresses leading to this phenomenon are primarily introduced in the valving process and can be worsened by over-torqueing of the cylinder valve.