Some questions about fuel line fittings

[dovekie]

Thank you all for the information and I am sorry to have precipitated such heated debate. I shall have to use flexible hose for some of the pipe run but plan to buy 10metres and replace it every few years. For the rest of it I shall use copper pipe and olive compression joints where possible. In the light of all the discussion - if I have to - I shall use parallel to parallel threads with copper washers.

By the by, a quick search about liquid PTFE (which I will not need) turns up 1) lots of suppliers (e.g. screwfix), 2) various types, differing in the solvents and speed of cure I think. ROCOL's technical datasheet indicate it is suitable for
Air
Water
Steam
Mineral & Vegetable oils
Paraffin & White Spirit
Ammonia refrigerant
Presumably it is chemically compatible with diesel but I take the point about fragments breaking off.

Many thanks again.
Jon

 

[boatmike]

For the benefit of others I attach a link to what I consider to be the "best practice" fittings for both hydraulic and fuel lines.

http://www.sspfittings.com/products/brand_home.asp?brand=35

These fittings are specified for the highest duty on aircraft and other applications where there is high pressure and the need to reduce stress and fatigue. It can be seen that these fittings have parallel threads and seal on an o ring. If you read the brief notes you will see that they were designed to avoid the need to use tapered threads and gives the reasons that tapered threads are non-preferred in these applications all of which are equally valid for fuel lines.

I do not believe however that the expense of these fittings is justified in marine diesel fuel lines where a straight threaded fitting can be sealed in a similar fashion with a copper washer. Also, straight threaded brass fittings are easily obtainable in many forms and have been used for many years so that is the best way to go.

Both Kemps and Machinery's Handbook do indeed describe tapered threads which are still extensively used in many applications, usually on water, gas and other fluids where PTFE tape can be used so of course you will find them referred to. You will not find any detailed recommendations therein for best practice in using them however as both publications are aimed principally at manufacturing rather than design and application. There is indeed also a tight tolerance version of the American standard taper called a dry thread taper that is designed to be used in low pressure applications where PTFE tape cannot be used or is non-preferred.
I do not decry the use of tapered threads overall, and can see that they have many applications in general engineering. I would personally never use them in a marine fuel line however for three reasons.
1. They cause a bursting stress on the female part
2. They cannot be sealed well without PTFE tape
3. They do not lend themselves to easy dissassembly and re-use.
In fact the only applications where they are really useful is where a part is permanently fitted and will never need to be disassembled.
As far as fitting a male taper into a female parallel is concerned... yes it is done, in some cases it creates a seal too but spiral leakage is much more of a danger as it only mates on the first few threads of the hole. It also represents a higher risk of splitting the female part than taper to taper does

So getting back to the original question and avoiding tangential arguments, my contention is that the easiest and cheapest way of assembling a fuel line is using parallel fittings sealing on copper washers where the components such as ball valves have internal threads and either flared or brazed pipe ends sealing on mating faces. Where the tools are not available to flare or braze the end, compression fittings are OK. Thats it folks! Anyone who dissagrees is welcome to do so but don't expect any more debate from me as I have nothing else to add apart from A HAPPY NEW YEAR TO ALL MY READERS!

 

[vyv_cox]

Have a look at this page for useful information on the subject. Maybe it will cool off the thread a little!

 

[catmandoo]

As a distant amused viewer , also an Engineer, I note that it only takes a couple of men to get heated up about screwing and that if you have a tapered thread chaps go easy on the females.

ie screw with TLC

 

[gjeffery]

I was going to keep clear of this one. After an apprenticeship, and three universities trying to learn to be an engineer, I am confused about what the word means. But, I found the following:

"Threaded connections. These are made, as a rule, with taper or parallel female threads which screw over tapered male pipe threads. Because a joint made up in this way contains large leakage pagees, a sealant or filler is used to close the leakage passages." Zappe, R.W. Valve Selection Handbook, Gulf Publishing Texas.

That said, I would not mix taper and parallel threads, and can see disadvantages and no good reason for doing so. Personal preference is to use Blue Hylomar on threads (upstream the filter).