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RR736 - Comparing subsurface migration of LPG with natural gas

A programme of experiments assessed differences in behaviour between leaks of natural gas and Liquefied Petroleum Gas (LPG) from buried pipes, to inform opinion on whether an existing (Advantica) model of the risks from buried natural gas pipework could be useful in assessing the risks associated with buried metallic LPG pipes.

In experiments with two different soil types, LPG or natural gas was injected at a fixed position and samples were then taken from a number of points below and at the surface. The effect of covering the surface was also assessed. Both LPG and natural gas migrated away from the leak into the surrounding soil, gas being detected both above and below the release point and reaching the surface quite quickly. This showed the importance of pressure differences in driving the flow.

In sand, with uniform porosity, LPG was seen to migrate more quickly than natural gas, needing a substantially shorter time to reach a hazardous concentration. In soil with larger, less regular pores, differences in buoyancy were more apparent, and the results showed much more variability.

It was concluded that the Advantica model included the appropriate processes, but that the model would need to be carefully modified to account for the differences in LPG and natural gas behaviour before being used in any prioritisation of LPG pipe replacement.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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Updated 2010-01-27