FRA looks at cryogenics by rail

CATCH A COLD

CRYOGENICS • HOW WILL TANK CARS WITH LNG OR CNG BEHAVE IN AN ACCIDENT? THE FEDERAL RAILROAD ADMINISTRATION IS TRYING TO FIND OUT BEFORE ONE HAPPENS

Increasing availability of liquefied gases in the US, primarily resulting from the exploitation of shale gas reserves and higher availability of natural gas and natural gas liquids (NGLs), is generating additional demand for cryogenic transport services. Ensuring safety in this developing business will be critical if political and public opinion is to allow its growth, which will be one way to help the country with its eventual transition away from traditional fuels to lower carbon fuels based on natural gas.

One area that has been explored is the movement of LNG by rail, not least since the railroad industry itself is interested in using gas-based fuels for its locomotives. The US Pipeline and Hazardous Materials Safety Administration (PHMSA) published a notice of proposed rulemaking (NPRM) in October 2019 proposing changes to the Hazardous Materials Regulations (HMR) in Title 49 of the Code of Federal Regulations (49 CFR) so as to permit the transport of LNG in rail tank cars. PHMSA has also issued special permits to allow the transport of compressed natural gas (CNG) by road, although there have been at least two incidents involving such transport, leading to calls for it to be restricted.

The Federal Railroad Administration (FRA) has also been doing some work. Noting that the railroad industry is actively working on plans to move LNG and CNG in bulk by rail, FRA says that the safety performance of the tank cars involved in such transport under derailment-induced fire conditions has not been verified. This is a cause for concern, FRA says. As such, it is interested in developing methods, both analytical and experimental, that might help evaluate the thermal safety performance of means of containment for LNG and CNG under fire conditions.

SET IT ON FIRE Research in this area began in September 2016, when FRA engaged the Southwest Research Institute (SwRI) to fire-test a portable tank filled with liquefied nitrogen and demonstrate the performance of the pressure relief valve (PRV) system installed on the tank. Tests showed that the PRVs opened and were able to relieve the pressure in the tank fast enough to avoid a boiling liquid expanding vapour explosion (BLEVE) event.

The fire test generated a great deal of data to help understand how the fire exposure affects the internal and external heating of the tank, which will inform future computer models to predict performance in different types of tank and different fire scenarios. The next phase of the project will include a fire text with a tank filled with LNG; this will provide additional validation data for modelling calculations.

FRA has published the initial report on the SwRI test project, which presents the following conclusions: -The temperature of a tank will greatly influence the failure pressure of that tank.

The failure pressure for a heated tank has

RAILROADS ARE ALREADY EXPERIMENTING WITH

been shown to be much less than that of a tank at ambient temperature. Additional effort should be applied to assessing the potential failure of a tank under elevated temperatures. - Preliminary modelling suggests that more work is needed to accurately characterise the thermal insulation properties as well as vacuum retention capabilities of the annular space under thermal loading. - The introduction of phase change into the modelling will provide a better understanding of the various mechanisms involved during the heating process. - Studying the effect of wind, fl atcar geometry and burner size on the heat fl ux to the tank should be extended so as to better understand the effects of conditions that may be present at the physical test. HIT IT HARD In a separate project, FRA has been working with the Transportation Technology Center (TTC) to conduct a full-scale shell impact test on a DOT-113C120W (DOT-113) tank car. This specifi cation is a tank-within-a-tank designed for the transport of certain cryogenic materials and comprises an outer tank of carbon steel and an inner tank of stainless steel, separated by insulation and vacuum to keep the lading cold.

In the test, carried out in November 2019, the shell of the outer tank was struck by a 297,000-lb (135-tonne) ram; the tank was fi lled with water to 82.4 per cent of its capacity, sealed and pressurised with air to an initial test pressure of some 50 psig (3.45 bar), representing the mid point of the allowable pressure and outage range proposed in PHMSA’s NPRM. The impact occurred at 16.7 mph and punctured both the outer and inner tanks.

Material samples have been taken from both the inner and outer tanks and will be subjected to fi nite element analysis (FEA). The test data, photos and videos are to be reviewed and compared with the FEA in an effort to validate the model. Test results will also be compared with corresponding measurements from earlier tank car impact tests to understand the similarities and differences between different tank cars under similar impact conditions. FRA also plans to carry out an FEA to investigate the impact response of the DOT-113 tank when loaded under service conditions, i.e with cryogenic lading and its infl uence on inner tank material properties.

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