There has been a lot written in the papers recently about the problem of leaking diesel pumps. There are many reasons why seals fail on diesel fuel pumps. I remember a case in England where an unscrupulous supplier was buying agricultural red diesel and stripping out the dye with sulphuric acid. You can imagine the effect that would have on the rubber components! Somehow I can’t see this being the case with Enemalta Corporation though, as it would be unlikely that a main supplier like this would do such a thing.

So what else could be the problem? I think Enemalta Corporation provided the answer when it said that it has been selling ‘sulphur free’ diesel since January. So what difference is there between sulphur free and normal diesel? The older diesel fuel contained in the region of 500 parts per million sulphur compounds whereas the newer ultra-low sulphur petroleum diesel (ULSD) contains less than 15 parts per million of sulphur compounds.

The older diesel contained a number of aromatic compounds, which migrated into the fuel pump seals and some types of fuel lines, swelling and softening them. The hydro-treating process used to reduce the fuel’s sulphur content had the unintended effect of stripping out some of these aromatic compounds, reducing the fuel’s energy content slightly, reducing its lubricity moderately, and providing some mild solvent properties. It is these solvent properties, combined with the absence of the aromatic elements, which cause rubber seals, O-rings, and lines to shrink and/or lose integrity when exposed to ultra low sulphur diesel.

The older diesel fuel was a good lubricant, which is very good for engines, fuel-lubricated injector pumps, and any other moving parts that come into contact with the fuel. It will also tend to lubricate and expand any seals that it comes into contact with. Ultra-low sulphur petroleum diesel (ULSD), on the other hand, tend to shrink and harden seals.

Some mechanics have suggested that going back to a 100% fill of the older diesel or indeed bio diesel might (or might not) allow a leaking injector pump to re-seal. Some people suggest running some bio diesel through, and then allowing it to absorb for at least 24 hours before giving up and committing to an expensive injector pump repair or replacement.

What then is the explanation for the delay in some cases of these leaks occurring? Well, it may be a case of motorists filling up with diesel from different petrol stations some of which are using older supplies of diesel rather than the newer ULSD. Or indeed during the changeover period stations may have added ULCD to whatever was left in their storage tanks of the older diesel as the two can be mixed. This means that the rubber seals would have softened and expanded again making a better seal.

So what is the answer to the problem? Well, to begin with the refinery should be adding some form of lubricating additive to replace what is stripped out during the manufacture of ULCD. This is a low cost solution costing in the region of one cent per litre. Having said that, these problems may still occur even after such additions to the fuel. So a better solution would be the replacement of rubber fuel lines and other rubber seals with newer, synthetic Viton material, or A1-grade marine fuel components.

James A. Tyrrell,

COUNTY ANTRIM

N. IRELAND