Barbed fittings vs push-to-connect fittings.
 

Following the discussion on the D-Cat thread, I wanted to bring some concepts on a new thread and bring the discussion to everyone.

http://www.rcuniverse.com/forum/rc-j...ir-trap-2.html

I have seen an increasing number of people using push-to-connect fittings on the suction side of the fuel pump.
Most people do this because it is very practical and straightforward.
However I would like to bring some important cons to people's awareness and emphasize on technique.

First of all what is the pros and the cons of using barbed fittings vs push-to-connect fittings?

Barbed fittings:

Pros ( if safety wired ):
Secure
Leak proof
Durable
Several types of tube ( material ) can be used.

Cons:
Requires safety wiring
The tube will be somewhat difficult to pull when comes the time to remove it.

Push-to-connect fittings:

Pros:
Fast to insert
Fast to extract ( if done properly )

Cons:
Can only be used with PU tubing of shore A60 and above ( no tygons ! )
Can promote leaks after a while ( see below )
Can exhibit reduced life ( see below )

So what about the risk of leak and shorter life of push-to-connect fittings?
We this is mostly due to one fact:

The plastic push-to-connect fittings most people and manufacturers use ( QS type Festos fittings) are not designed to be used with fuels.

They are rated by Festo to mostly accept air and water or air and a mist of oil ( a few % of oil in the air for machine lubrication ).
What's the problem here?

1. The body of the plastic connector is made of polybutylene therephthalate ( PBT ).
https://en.wikipedia.org/wiki/Polybu..._terephthalate
Although this polymer exhibits good resistance to diesel/ kerosene/ gas, it does get affected by it on the long run. The material swells slightly and gets more brittle.

2. The o'ring that is sealing the tube inside the fitting is made of nitrile ( NBR ). Although nitrile also exhibits good resistance to fuels, it does harden slowly when exposed to them and get brittle as well.

Here is a cross section of a push-to-connect fitting:

https://cdn.shopify.com/s/files/1/01...g?v=1472214223

The o'ring is fitted inside the polymer body, close to the tube end.
The retention ring is a spring type nut that prevents the tube from sliding out of the fitting. it pushes against the pube and is only effective if the tube is rigid enough.
After a long exposure to fuels, the PBT body will swell a bit and the nitril o'ring will harden. The sealing of the tube will not be as good as before. In positive pressure, the slight deflection of the tube should be sufficient enough to prevent leaks in most cases. However in case of suction, thinner air is more likely to leak inside the system.

Push-to-connect fittings associated to soft tubes on the suction side of the pump might leak air.
Here is what can happen: A soft tube could be deflected inwards under the action of vacuum.
This could lead to a deficit in tube pressure against the seal and get some tiny bubbles of air get sucked inside the fuel system. Here is a schematics showing what could happen:

https://cdn.shopify.com/s/files/1/01...g?v=1472214721

However, if the fitting is on the pressure side of the pump, the tube will be pushed against the o'ring and sealing will be improved:

https://cdn.shopify.com/s/files/1/01...g?v=1472214825

Altogether, I don't recommend using push-to-connect fittings on the suction side of the pump for these reasons. Barbed fittings are much more reliable and relatively easy to setup. They require no servicing.

If anyway you are willing to go for push-to-connect fittings, please consider the following:
1. Inspect the fitting/ tube on the pressure side of the pump for wet areas. Change the fitting when the tube interface becomes wet ( fuel leak )
2. Change the fitting on the suction side of the pump every two years ( unless using Legris fittings ). Leaks cannot be easily detected. The first sign of leak could be small bubbles in the lines/ unexplained flameouts!

Now, let's discuss that matter.

Well written, Oli - like a chapter in a student's text book to turbines...Thanks for taking so much time and providing this.

Oli,

Appreciate this information and research as its very important to understand why different types of fitting are used for any given application.

CAT-V2 and the new D CAT series will be supplied with barbs as standard, the owner can if so chooses fit alternative connectors but it's not something I would endorse for all the reasons you state.

marcs

Absolutely correct. The push in fittings are NOT suitable on suction side.
A more severe issue is that the retaining ring, which is heavily spring loaded like a barb, eventually collapses the tubing as well, restricting flow even on pressure side. Regular trimming does overcome this. Must be due to fuel depolymerization of the tube materials as you do not see this using air.
The best of the festo's is the ones with the locking cap, almost like a gland fitting. AMT used to use these on their engines in the early days.( NPCK type)
Andre
https://www.festo.com/cat/en-za_za/d...DA5D04A96F.jpg

Thanks Oli,

Never used push fittings on the suction side for the same reason but I never had any data to support it. It was just what I was told when I started.

Thanks
Gary

Oli, here is a question for you, I might be totally wrong but internally in these push fit connectors is the bore reduced or less then the ID of the pipe which fits into it?

I know the illustrations show it is not but I'm sure I've seen fittings that have a reduction through the middle section - it might just be the on/off valves........

marcs

The Festo website has hundreds of fittings. They have cad models and drawings for probably all of them. No need to wonder.

This is why we have to service our planes. We have to change QS Festo couplings now and Then. My only hope is that those who supply products with barbed fittings, will make them fit the tubing. There should not be any reason for expanding the tubes with plyers to make it fit.

PU hoses expanded with pliers, as long as you don't break the material, go back to their original shape quite quickly and do not lose their strength. That's not the case when the PU is heated and deformed. Standard alu filters supplied with some turbines (i.e. kingtech) are to be used with 4mm pu, and it's pretty much impossible for me to put the tube on the barb without using pliers to expand it a little first. Once it's on, though, it doesnt even need safety wire: I once had an ECU failure which ran the pump at full power against a closed valve and the hose ballooned up without detaching from any barb, until the valve exploded before I could unplug the battery.

Edit- deleted due to RCU stupidity

OK so I have a question. I own a turbine (45 Lbf) where all the solenoids and the fuel pump are inside the case. Regulations require a shut off valve which can only be placed on the suction side. What other options besides a Festo 6mm 1/4 valve are there?

Thanks

Hi Oli,

Ref your statement above: The plastic push-to-connect fittings most people and manufacturers use ( QStype Festos fittings) are not designed to be used with fuels.


All of the turbines I have ever owned have festo fittings for fuel input. Is this a problem over time on the engines themselves? I have never had an issue but am surprised the turbine mfg. use them based on your statement.

Gary

Deleted

That is a very good point.
I am making specific brass tubes for this purpose.
In that case, the tube cannot deflect under vacuum and since it is inserted once in the pump/ valve and not removed, the o'ring is not exposed to kerosene/ diesel.
In this case the fitting will last a very long time.

http://www.ultimate-jets.net/products/behotec-high-flow-pump-fitting

Good question.
Historically, Jetcat came with the Festos because they were the only German made connectors available 20 years ago.
They never considered Legris because they were French made at this time ( now USA made, Legris got bought over by Parker over 10 years ago ).

Legris fittings are much more resistant to fuel because their body is made of reinforced nylon ( this is what we use for all our military/ industrial setups ). however, even Legris does not certify their fittings for fuels as the internal o'ring is also nitrile.

For 100% reliability, the way to go is metal fittings. However they are heavy.
Note that I am designing a new line of Al606s barbed fittings that are feather lights.
They will be available very soon on our store.

Here is the link to our Legris line;

http://www.ultimate-jets.net/collect...ush-on-fitting

Great post Oli, its a nice timely reminder to us all.

In-fact, the only time I have used FESTO press-on connectors "on the suction side" was for my Turbine powered EC135 with the two-stage JetCat SPM5-HM.

Due to the internal heat inside the cabin the Polyurethane became way too soft for my liking. I then modified the entire fuel system to 4mm Brass/Aluminum (from K&S), and Teflon.


http://www.rcuniverse.com/forum/atta...mentid=2178573

Not entirely. The Festo fittings are designed to work with petroleum based lubricants that are used to lubricate air cylinders and keep seals from drying out. If we had used pure kerosene which depolymerizes many plastics and rubbers then maybe. But we run oil as well. The 2 main issues are the fact that the inward pressure from the retaining barbs and low pressure does not mach the design intent of positive pressure which allows the seal to collapse. They are perfectly OK for handling pressure up to 10 Bar (145psi). Also the seal in not an o-ring but a flanged seal, certainly not suitable for negative pressure at all.

http://www.rcuniverse.com/forum/atta...mentid=2178601http://www.rcuniverse.com/forum/atta...mentid=2178602
These images are taken from the festo QS specifications and clearly state these connectors are designed for vacuum as well as pressure.
-.95 bar and up.

While it's true the tube must be cut well, rigid and especially must not make sharp turns right after exiting the connector, I don't see how it can be claimed these are unsafe for the suction side.
I've been using one before the UAT and one between UAT and pump on each airplane for the past 4 years and so far never had any issues (up to 180N).

Wow!! that must have been freaking exciting!! :eek:

"reo" has had a similar experience with the pump running away in the air. Full throttle all the time I believe he said.. Since then he has added a separate power switch to the fuel pump that can be turned on/off via the Tx instead of the ECU.

Sorry for being off topic.

Bad image...

You answered the question. There are a few preconditions for it to work well and consistently so. It is not fit and forget. Because you are aware of this and take the required precautions it works for you. Unfortunately there are others out there without that insight. Also Festo does not spec the leak rate for that fitting. All seals leak depending on medium and application.
These are really light and reliable. Easier to remove/fit than regular barbs but with lockable gland. Goes to zero Bar. The diff between zero and 0.05 is an airleak...

http://www.rcuniverse.com/forum/atta...mentid=2178614

An enjoyable couple of seconds, yes...
What happened was the ecu controller for the pump failed close, which pretty much causes the pump to operate at full battery power regardless, potentially over full throttle even.
Ecu no longer had any control. This happened because I forgot the fuel shutoff valve (which was after the pump) closed while attempting a gas start. Ecu pushed and pushed but there was no rpm increase, then the circuit overheated and failed. While I know exactly why this happened, I also had friends experience this in flight. A cutoff switch is certainly a wise thing to install should this happen.
And yes, sorry for the OT :)

This actually looks to NOT be suitable for vacuum at all?
I assume vacuum to be between -.9999 and 0 bar?

If you use the correct size tube on a barbed fitting there is no need to wire them on. I have 14 turbine jets and have never wired a single fitting, Just use the correct sized tube

Not exactly. Festo requirements for the QS line is air, dry or mixed with a very limited amount of atomised oil. Anything else is out of specs. The max amount of oil is very low.
"Bio oils, max 0.1 mg/m3. Larger amounts may result in premature product failure"
"Mineral oils: 5 mg/m3. A higher residual oil content is not permitted. This can lead to malfunction"