Barbed fittings vs push-to-connect fittings
We have recently added a line of push-to-connect fittings suitable for the suction side of the fuel pump.
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:
Slower and more difficult to install.
Requires safety wiring.
The tube will be somewhat difficult to pull when comes the time to remove it.
Heavier ( Both Tygon and barb ).
Push-to-connect fittings:
Pros:
Fast to insert
Fast to extract ( if done properly )
Light
Cons:
Requires PU tubing of shore A60 and above ( no tygons ! )
Requires well calibrated tubes
Can exhibit reduced life when used with fuels ( 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:
Most 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 to only accept air and water.
What's the problem here?
1. The body of most plastic connector is made of polybutylene therephthalate ( PBT ).
https://en.wikipedia.org/wiki/Polybutylene_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 inside the fitting that is sealing the tube 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:
The o'ring is fitted inside the polymere 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 fitting body will swell a bit and the nitril o'ring will harden. the sealing of the tube might 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, the thinner air is more likely to leak inside the system.
Push-to-connect fittings associated with 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.
Coupled with a hardened o'ring, 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:
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:
Barbed fittings are much more reliable and although slower to setup, do not require servicing.
If you are willing to go for push-to-connect fittings, please consider the following:
1. Use our Pisco fittings which are made of fiber reinforced nylon ( very strong resistance to fuel ) and use NBR seals ( very good resistance to fuel ).
2. 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 )
3. Change the fitting on the suction side of the pump every two years ( unless using Pisco fittings ). Leaks cannot be detected other than by observing small bubbles/ unexplained flameouts!
