Hello Fernley,
An interesting proposal!
Certainly the cooling effect that occurs when a gas is decompressed is well known to me. If any water has collected on the atmospheric side of the regulator diagram, there have been instances where the heat absorption of the expanding gas has frozen the water and thus the diaphragm can no longer function correctly, and pressure regulation is compromised.
The ability of the body and needle valve to conduct heat too or from the site had not occurred to me, and if there are aerosols of oil or other detritus in the vapour flow, then any that are sensitive to condensing onto cooler surfaces, this must be considered.
However there is other evidence that suggests your theory is not the whole storey. When the regulators are sited higher than the surrounding pipe work the problem seems not to occur. Thus if it were aerosols the deposition would occur regardless of the relative height of the regulator as they are in the Vapour flow. As height seems to be a factor then gravity is indicated, and that will only affect fluids that are formed outside of the regulator and are able to run back to it.
I would also cite other incidents, where gas appliances that are located at the lowest point of the pipe work have been found to have oily deposits occluding gas ways and injectors, yet the regulator and other elevated appliances have not been affected.
This is one of those situations where it is difficult to prove anything, but there is no evidence that suggests any of the metal fittings or pipe work producing any fluid from their structures, so that leaves the LPG or flexible 'rubberised' components.
The LPG producers are adamant that their product has not changed in its chemical composition, but they cannot be totally sure of what is inside their bottles in the way of contaminants. As most will be denser than the LPG, they will collect at the bottom of the tanks, the farthest point from the valve. As these tend to stay in their liquid for even at normal atmospheric pressure, the chances of them vaporising and being delivered with the LPG vapour are pretty remote. - but not impossible if the bottle has been stored on its side or even upside down.
On the other hand the flexible pipe will often have been through manufacturing processes, that dose the materials with various plasticisers. If the process has not fully cured the product, some of the agents may remain lodged or embedded in the molecular structures of the pipe wall but not chemically bonded. When they are exposed to the dissolving and lubricating properties of LPG's (and other petro-checmical based solvents) the agents may become sufficiently free to escape the walls and collect as a fluid in the pipework. Gravity does the rest.
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