Introduction
Steel pressure vessel fabricated to United States Department of Transportation (U.S. DOT) and American Society for Mechanical Engineer Boiler and Pressure Vessel Code (ASME BPVC) required periodic recertification based on local, state, and federal regulations. This article focuses on the nuances involved in rebuilding and recertifying tube trailers for continued safe use.
Overview of Tube Trailer Retesting
Tube trailer high pressure cylinders may be retested under DOT special permits 14584 and 15610. These permits allow for retesting steel cylinders using acoustic emission (AE) in lieu of internal visual inspection and hydrostatic testing. The main advantages of AE testing over hydrostatic testing are lower test pressure and no disassembly of the trailer is required depending on the time elapsed from the last cycle of tests. The typical special permit authorizes retesting of DOT 3A, 3AA, 3AX, 3AAX, 3T[1] cylinders using AE. 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
The fill history must be recorded and the number of fills between tests is limited to 600
Hydrogen and hydrogen bearing gases are not authorized for DOT 3T tube
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 and 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 the recertification contractor 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.
An electronic pressure transducer will be 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 ASTM E1419. TKS will notify the client when it is permitted to begin filling the tubes. The tubes will be filled continuously from 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.
Cylinder Rehabilitation
TKS rehabilitates tube components by changing worn components before they cause a release or damage. It is recommended to change out mechanical components the 5- and 10-year recertification dates.:
Valves: TKS removes old worn out valves by disconnecting the manifold pigtails from each valve and unscrewing the valves. Once removed the internal threads are cleaned and inspected. A new valve is installed after applying thread sealant. The manifold pigtails are then reattached and a leak test is performed. |  |
Relief devices: TKS removes old worn out relief devices by disconnecting vent tubing and unscrewing the relief bodies from the tubes. Once removed the internal threads are cleaned and inspected. A new relief device is installed after applying thread sealant. The vent tubing is then reattached and a leak test is performed. |  |
Rupture discs and shear rings: TKS replaces rupture discs and shear rings by disassembling the relief device body and removing the old components. The new components are assembled in the relief body using hydrocarbon grease. The relief body is then reassembled according to manufacturer specifications and torques. |  |
Vent tubing: TKS replaces vent tubing by unscrewing the old vent tubing from the relief device and unthreading it from the support structure. New vent tubing and caps are then installed on the relief body. |  |
Bull plugs: The first step in bull plug replacement is removing the valves or relief devices. After valves and relief devices are removed a striking wrench is used to remove the bull plug. The internal threads and tube face are cleaned and inspected. The new bull plug is installed by greasing the seals with hydrocarbon grease and threading the bull plug back in place according to manufacturer specifications and torques. A leak test is the final step. |  |
Manifolds: TKS replaces manifolds by taking detailed measurements of valve spacing and manufacturing a new manifold. The pigtails to all valves are then disconnected and the manifold unbolted from the bulkhead. The pigtails from the new manifold are then connected to the valves and the manifold is bolted to the bulkhead. A leak test is the final step. |  |
The quality of compressed gas used to pressurize the tubes during testing and afterwards for purging are important characteristics. The need for a very high level of cleanliness and purity of the residual nitrogen after testing is often critical as it related to moisture content, oxygen content and total residual hydrocarbons levels in the nitrogen. A typical requirement is 1 part per million moisture content following the use of traceable liquid nitrogen for the AE testing and a series of vacuum fill cycles needed to reach that moisture level. First a load of load of liquid nitrogen is dedicated to the pressurization of the tube trailer directly or via an intermediate holding tank. This nitrogen source is used to pressurize the DOT or ISO cylinders to 110% the maximum operating working pressure.
After completion of the pressurization sequence for the acoustic emission testing, the cylinders are prepared for their high purity gas application as required. A typical process includes venting the tested cylinders and pulling a small vacuum using a high capacity vacuum pump. The cylinders are then filled to 100 psig with traceable high purity nitrogen and evacuated up to 5 times. The residual nitrogen from each cylinder is sampled until the moisture and other contaminants are below the desired PPM level. This purging process is repeated until the desired purity levels are attained. If the trailer is placed into helium or hydrogen service, the process is repeated using the specialty gas.
Summary
High pressure DOT, ISO, and ASME steel vessels required properly qualified DOT and/or ASME qualified contractors. The recertification process may include at least one of the following test methods: acoustic emission, hydrostatic, ultrasonic, and visual testing. In addition to the non-destructive testing required for certification maintenance on cylinder valves, relief devices, rupture disks, vent tubing, bull plugs and manifolds are required.
References
ASTM E1419/E1419M − 15a (Reapproved 2020) Standard Practice for Examination of Seamless, Gas-Filled, Pressure Vessels Using Acoustic Emission
ISO 16148:2016 Gas cylinders — Refillable seamless steel gas cylinders and tubes — Acoustic emission examination (AT) and follow-up ultrasonic examination (UT) for periodic inspection and testing
CGA/GAS - CGA C-6 2019 Standard for Visual Inspection of Steel Compressed Gas Cylinders
CGA C-23: Standard for Inspection of DOT/TC 3 Series and ISO 11120 Tube Neck Mounting Surfaces
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