Use of Banjo Fittings in Diesel Fuel systems 1

[KiwiME]

I would have posted this in the automotive forum but I thought I would get a more-considered answer here.

As we know, a banjo fitting normally has the threads wetted by the media passing through the fitting. My concern is that in a very clean system, e.g diesel fuel before the high-pressure pump but after the filter(s), there is some possibility that debris from the threads may contaminate the system. In this particular case the female half is a tapped thread in 6061-T6 aluminum, while the male is a conventional hollow banjo-type screw, presumably a machined thread.

In particular, if the connection is loosened and then tighened without flushing, what are the chances debris larger than 2 micron will be present?

Also, are there any general hydraulic design guidlines that discourage the use of fittings with "wet" threads?

Thanks,

-Paul

 

[danhelgerson]

Paul,

Thank you for using this forum. The issues would seem to be the quality of the machined parts and the knowledge of the person doing the assembly/re-assembly. The sealing is all done by the O-rings and there is no metal-to-metal contact in the flow path of the fluid. There is a tendency to over-tighten parts that seal with O-rings, especially if someone is used to NPT threads. Over-tightening the banjo fitting could cause some thread damage and dislodging of debris. This would then be a problem the next time the fitting is removed and replaced. I do not think anyone would guarantee that there would by no chance of a 2 micron particle getting down stream but the banjo fitting should be fine if properly machined and fitted with O-rings of the proper material.

I hope this is helpful.




Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[KiwiME]

Thanks for your answer Dan. Perhaps there are other types of banjo fittings that I'm not aware of.

I'm refering to the type that have a copper washer on each side of the steel banjo, and the assembly is clamped together with a hollow screw against a tapped port. No O-rings here. The fluid passes through the end of the screw at which point it is exposed to the threads.

When the joint is disturbed, especially when aluminum female threads and higher tightening torques are involved, I'm assuming that a small amount of aluminum material is shed.

Because of the risk, I would assume that as a general design guideline this type of fluid connection would be considered a risky choice for a "clean" system where an unskilled person is likely to disturb the connection for maintenance, etc.

There are so many couplings on the market that locate the threads outside the fluid-tight joint that I'm surprised banjos haven't dissappeared.

 

[aviat]

Woodward has been making governors and fuel control units for many years using banjo fittings, but they don’t screw the steel fitting directly into aluminium. They use an insert that prevents the pick up you are concerned about. Steel screwed into aluminium acts like a tap and cuts its own threads, and after a few uses there often is nothing left to the threads. The use of inserts prevents this problem.

They also use copper or aluminium seals. Also, if you are designing from scratch you could consider putting a small screen in the inlet like they use in carburetors etc.

 

[KiwiME]

Thanks, aviat, that's the sort of info I was looking for!