Fuel Hose Cracking 1

[nlj]

I've been chasing a fuel hose failure for some time now. We switched suppliers in April 2009 and the new hoses are cracking whereas we have no report of the old supplier hoses cracking. The hoses are stored in a covered area while in service, so likely wouldn't see significant UV. I've been investigating what potential fluids/solvents may come in contact with the hoses, but there are no fluid tanks near these hoses. The application for the hoses involves diesel fuel.

I've ran FTIR, TGA and XRF on the hose materials.

The hoses have two layers and testing was conducted on both plies. I've labeled them OD for the outer ply and ID for the inner ply.

Results are:

New hose:
FTIR results:
OD: nitrile rubber, an ester-based plasticizer and talc
ID: Nitrile rubber, phthalate based plasticizer
Methylene chloride extraction spectra:
OD: ester-based oil
ID: phthalate based plasticizer


Old Hose:
FTIR:
OD: Nitrile rubber, ester based plasticizer
ID: Nitrile rubber, phthalate based plasticizer and a silicate filler
Methylene chloride extraction spectra:
ODhthalate based plasticizer and a polyvinyl ester
ID: Phthalate based plasticizer

There don't seem to be any major differences between these materials that would result in one significantly out performing the other.

Does anyone have any thoughts to offer on why the new hose is not performing as well as the old hose? These hoses are made per API 1529. Should we be adding more to our print in terms of hose specifications?

Any advice would be greatly appreciated.

Thanks!

 

[GrahamBennett]

Nlj - you don't say whether the cracking is on the outside surface or on the inside surface of the bore of the hose.

If it's on the outside, check for any sources of ozone in the vicinity (sparking electric motors and discharge lamps are great sources of ozone for example). Next, check that the hoses are not moving or flexing due to pulsing of the fluid being transported. There are other things to consider: is the cracking on or very close to where the hose is coupled to any metal pipework? Are there any tight bends or kinks in the hoses as installed? And so on.

If the cracking is on the inside of the bore, it is likely that the fluid is stripping antiflex cracking agents such as 6PPD and its like out of the compound leaving it vulnerable to flex cracking.

 

[nlj]

Thanks. I'll check the items mentioned above.

The cracking is only on the outer layer. Thus far, no leaking has occured so the inner layer remains intact.

The majority of the cracking occurs close to one of the couplings. I didn't think that was the root cause since the hoses from one supplier have been in the same position for years and never cracked?

 

[GrahamBennett]

Beleive me it happens. If the couplings are fixed on too tightly, or the ID is very slightly smaller, the rubber immediately beyond the end of the metalwork increases in volume and causes different stresses - great for starting cracks.

 

[nlj]

Ahh. Good point. I never thought of that. I'll also verify that the fitting have not changed.

Thanks!

 

[GrahamBennett]

Nlj - thanks for the star.

 

[Ltormento1]

Nlj,
if you are using diesel with biodiesel blend it is the cause of your problem.

Biodiesel extract the plasticizer of the walls of the rubber compound and it became hard and break easily.

Solution for the problem change the plasticizer for a polyesther based - high MW with low extraction by biodiesel.

 

[GrahamBennett]

Ltormento, thanks for your input but we've already established the cracking is in the outer cover close to metal connections, therefore solvent extract of the inner is not relevent in this case.