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Recertification of Tube Trailers using Acoustic Emission Testing

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

Overall Procedure

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:

Neck thread inspection must be performed every 10 years on tubes over 18” diameter. A non-disassembly external inspection must be performed every 5 years. The service product must not have corrosive effects on the tubes. The dew point of the service product must be less than or equal to -52°F at 1 atmosphere.

Before being used under the terms of SP 14584, each DOT 3T cylinder (tube) must be subjected to shear wave ultrasonic examination (UE). The UE must be performed by an authorized DOT UE special permit holder or the tube manufacturer. Maximum acceptance defect (sidewall crack) for a tube that is tested by UE shear wave is 5% of the minimum design wall thickness of the tube. The UE shear wave must cover 100% sidewall of the cylinder.

Prior to TKS on-site arrival, the tubes to be tested shall be filled to 50% MAWP. Upon TKS arrival, Acoustic Emission (AE) sensors will be installed to the painted surface of the tubes near the shoulder. The sensors will be held in place with magnetic hold downs. The sensors will be connected to the acquisition system using long BNC cables.

General Procedure

An electronic pressure transducer is installed in the manifold (with a ¼” NPT fitting). The electronic pressure transducer actively collects pressure data during pressurization. Calibration of the AE system will be carried out as required by DOT Special Permits. TKS will notify the client when it is permitted to begin filling the tubes. The tubes will be filled continuously from 1000 PSI or 50% MAWP to 110% MAWP. After reaching 110% MAWP the sensors will be moved to the next test batch and the process will be repeated. Typically one trailer can be completely tested each day.

Any areas on the tubes that generate event clusters will be further investigated using angle beam ultrasonics. A comprehensive visual inspection will be carried out on all visible portions of the tubes.

Figure 2: Typical setup for an ultrasonic retest under a U.S. DOT special permit

A follow-up ultrasonic inspection is required only if the AE inspection detects an active flaw during the loading period. Our inspection procedure follows ASTM E 2223 Standard Practice for Examination of Seamless, Gas-Filled, Steel Pressure Vessels Using Angle Beam Ultrasonics.

These follow-up tests use 45 deg. and 70 deg.o shear waves to locate and size detected flaws accurately. Ultrasonic transducers are scanned longitudinally and circumferentially along the cylinder’s OD during this test.

Leaders in the Acoustic Emission Testing

TKS’ inspection procedures have been developed by the company’s president Dr. Thomas R. Hay (Ph.D. Engineering Mechanics), Professional Engineer, and American Society for Non-destructive Testing Acoustic Emission Inspector Level 3 Certification # 107162. Our team has tested thousands of cylinders in the United States and worldwide since 1998. We have a proven track record for high quality nondestructive testing services and safety. For more information, or to schedule inspections, contact us at:

2134 Sandy Drive Ste 14

State College PA 16803


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