Thread sealant for diesel fuel lines?

Ian_Edwards

Ian_Edwards

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Hi
I've had consistent problems with sealing BSP screwed joints on 3/8 inch fittings.
I've had a combination of air leaking in and diesel leaking out, depending on where in the fuel line the fitting is, and the pump drawing the fuel out of the tank.
Part of the problem is getting the correct fitting. Where I can I use parallel threads and Dowty washers (washers with rubber seals attached). These work OK, but I've often finished up with a male tappered thread and a parallel female thread. These seem to be very difficult to seal, especially when trying to line up an elbow.
I've tried different jointing compounds, high temperature silicone and the sort of thing you use on an engine. But they all seem to leak eventually.
What do others suggest I try?
 
Ian_Edwards said:
Hi
I've had consistent problems with sealing BSP screwed joints on 3/8 inch fittings.
I've had a combination of air leaking in and diesel leaking out, depending on where in the fuel line the fitting is, and the pump drawing the fuel out of the tank.
Part of the problem is getting the correct fitting. Where I can I use parallel threads and Dowty washers (washers with rubber seals attached). These work OK, but I've often finished up with a male tappered thread and a parallel female thread. These seem to be very difficult to seal, especially when trying to line up an elbow.
I've tried different jointing compounds, high temperature silicone and the sort of thing you use on an engine. But they all seem to leak eventually.
What do others suggest I try?
Click to expand...
Loctite 243. How to choose the right LOCTITE® threadlocker
 
Daverw said:
loctite 55 ptfe cord or my preferred loctite 577 as it seals and sticks as well
Click to expand...
Cord and tape are better not used in fuel systems that have very small clearances and jets. Lost particles can obstruct them.

My best case history is a Frame 5 gas turbine in a German refinery. One jet in a burner blocked due to PTFE tape. The flame in the combustor can was diverted sideways, burnt out the combustor and many other parts. Repair cost was $1 million, plus lost production.
 
I just re-read your post and realised you are putting a BSPT into a BSPP. Surely there must be a way of changing to a BSPP? Trying to seal a taper thread into a parallel one is asking for trouble.
 
Graham_Wright said:
If you need to mate a male tapered thread with a female parallel one, you can create a pseudo taper with tape or cord by increasing the diameter of the binding progressively towards the hose end.
Click to expand...
That’s what I do with tape or cord, always start away from start of thread to keep it tidy
Gustywinds said:
I just re-read your post and realised you are putting a BSPT into a BSPP. Surely there must be a way of changing to a BSPP? Trying to seal a taper thread into a parallel one is asking for trouble.
Click to expand...
very common requirement and not normally any issues as long as sealant is used, it does allow for fittings to be oriented as needed whereas parallel threads need spacer seals or backing nuts
 
Most internal pipe threads are parallel and mail threads can be both and as long as the tapered thread don't bottom out in the internal thread sealing should be fine.

I do morally for use some liquid thread sealer of some type as extra sealant

The only issue I have had is reusing copper washers on banjo fittings. I now use Dowty washers on all banjo bolts
 
Daverw said:
very common requirement and not normally any issues as long as sealant is used, it does allow for fittings to be oriented as needed whereas parallel threads need spacer seals or backing nuts
Click to expand...
Very common bodge maybe but its not good engineering practice and you should never see it on a newbuild piece of equipment. If you need orientation of an angled fitting you use positional elbows or banjos. Having said that, the pressure in fuel systems like this is very low
 
I used what the guys in the plumbing shops called the green stuff to fix a difficult joint a few years ago. I had cross threaded a banjo bolt on the sump on the inaccessible side of the engine.....this year it fixed many bad joints in water and fuel systems.

I used it with a bit of horse hair...

VIKY-PLAST thread compound
 
You should not be sealing the thread but sealing the olive or whatever part of the pipe/fitting that is in contact with the other part/fitting.
Threads should possibly be lubricated.
 
Mister E said:
You should not be sealing the thread but sealing the olive or whatever part of the pipe/fitting that is in contact with the other part/fitting.
Threads should possibly be lubricated.
Click to expand...
A female thread (tapered or parallel) and male thread is a common type of joint connection alone . Not requiring olive or flanges. Think your getting confused with the
OPs questions.
 
wingcommander said:
A female thread (tapered or parallel) and male thread is a common type of joint connection alone . Not requiring olive or flanges. Think your getting confused with the
OPs questions.
Click to expand...
I don't think so there must be a mating surface or the pipe will pull out.
It is that mating surface that needs to be sealed not the thread.
 
We are talking about crimped hose fittings and valves etc not rigid pipe and compression fittings
 
wingcommander said:
A female thread (tapered or parallel) and male thread is a common type of joint connection alone . Not requiring olive or flanges. Think your getting confused with the
OPs questions.
Click to expand...
No, only a tapered thread can seal, a parallel thread must have some other method of sealing, usually an o ring, washer or gasket. In rare cases it may have fine tolerance flanges that metal to metal seal.
 
Gustywinds said:
No, only a tapered thread can seal, a parallel thread must have some other method of sealing, usually an o ring, washer or gasket. In rare cases it may have fine tolerance flanges that metal to metal seal.
Click to expand...
You can buy BSP tapered TAPs and BSP Parallel taps. BSP Parallel Holes are spot faced to create the seal at the face and not in the thread and flanged BSP fittings with appropriate washers should be used. BSP taper thread is used on pipework where the thread angle is the same on both male and female. The seal is thus formed along the full length of the thread and not the first turns if it was parallel.
 
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