Ivan Vegvary wrote:

If you fill the tank, a weak concrete slurry is typical. I don't remember
from my engineering days what the porosity is for sand, but, rest assured
that uniformly graded sand is probably 20% voids and can hold volatile
hydrocarbons making them dangerous.

If the tank is completely filled with sand it should be safe.


Gas fumes aren't dangerous, it's gas-air mixtures that are. You need a large
continuous volume too (large meaning a pint or so) for the flame front to
propagate and grow in size, speed and power. Start with a sphere and it's
really dangerous, spread the same volume out in pipes and it's much less so
- the pipes absorb some of the energy, and they also prevent the reaction
from going to completion in the small volumes closest to the walls, the gas
there can't get hot enough.


The tiny holes in bulk sand are sufficiently small and dispersed and
surrounded by sand so that even if one ignited* the sand would quickly damp
out the flame and it would not spread.

* this would actually be impossible, with the cold sand forming the walls of
the cavity so close



Hard to fill a tank with sand so that there are no large voids though, and
hard to be sure. Same with filling it with concrete, will there still be any
large voids? Because they _will_ fill with fumes, even through concrete, if
there are any fumes to be had (and if there aren't any, then it doesn't
matter anyway).



--
Peter Fairbrother