Updated: Mar 11, 2020
There are many classes of DOT steel cylinders used for medical, industrial, power generation, and gas storage, such as nitrogen, oxygen, helium, hydrogen, etc. It is common to mount these cylinders on a trailer so that the desired gas may be delivered to the customer’s site and refilled at bulk gas storage facilities as required. Since these tube trailers transport certain gases, compliance with U.S. DOT Pipeline and Hazardous Material Safety regulations are required.
Part of this requirement includes a minimum 5-year re-certification process that assures the tubes or cylinders are fit for continued service. There are two competing methods for tube trailer recertification. Hydrotesting requires that the tubes be disassembled from the trailer at significant cost to the owner. Acoustic emission testing can be performed while the cylinders are mounted on the trailer at tremendous cost benefit to the owner.
Dr. Thomas R. Hay is the President at TechKnowServ. “TKS is one of the select few companies in the U.S. that has been granted Special Permits by the U.S. Department of Transportation (DOT) to retest compressed gasses cylinders with Acoustic Emission (AE) in lieu of hydrostatic testing,” Hay said. According to Hay, acoustic emission (AE) technology has been applied to steel pressure vessel testing, including DOT cylinders, for over 20 years. Over the last decade, the technology has also been applied to composite pressure vessels.
“Acoustic emission testing is unique because it detects active flaws in pressure vessels,” Hay says. The concept is shown in Figure 1 for DOT steel cylinders.
Figure 1: Typical setup for an acoustic emission retest under a U.S. DOT special permit
How does acoustic emission testing work?
“Two sensors are placed on both ends of the cylinder and connected to AE instrumentation using coaxial cable. The tube is pressurized from 50% to 110% of its maximum allowable working pressure (MAWP),” Hay says. “If a fatigue crack is present in the pressure vessel and grows during pressurization, it will emit a sound wave, or acoustic emission, which is detected by the sensors. The fatigue crack is located through analysis of the relative time-of-arrivals of the acoustic emission at each sensor. A cylinder passes or fails the test based on how much AE is detected over the course of the test.” This is a summary of the basic procedure used in all U.S. DOT Special Permits that allows for acoustic emission testing in lieu of hydrostatic testing.
Hay says that acoustic emission was developed as an alternative retest method to hydrostatic testing for steel cylinders due to the following reasons:
In-service testing: The cylinders can be tested in-service on the trailer using the gas product.No contamination: The water used during hydrostatic testing contaminates the cylinder and accelerates corrosion on the inside. Remote inspection: The entire cylinder is inspected using only two sensors. Sensitive to active flaws that grow under pressureInspection is fast and economical
TechKnowServ retests tube trailer cylinders under DOT special permits 14584, 15610, and 15536. “Our special permits allow us to retest cylinders using Acoustic Emission (AE) in lieu of internal visual inspection and hydrostatic testing,” Hay said. “The main advantages of AE testing over hydrostatic testing are lower test pressure and no disassembly of the trailer is required.” According to Hay, TKS is authorized to retest DOT cylinders of type 3A, 3AA, 3AX, 3AAX, 3T, UN/ISO 11120 and non-DOT cylinders made under special permits SP 13230 and 13258. Retests performed under special permit 14584 are valid for a period of 10 years if the following conditions are met: