Festo on suction side of pump
 

Hello RCU,

I have been running my Cheetah SE and have not seen any major issues with it. I am considering another Jet Cental turbine purchase but have been wondering why Jet Central decided to use a festo connection on the PowerPack from the factory on the suction connection. I was under the impression that this is not recommended, that Festo fittings are designed ONLY for pressure, not vacuum.

Is there any testing which Jet Central performed to disprove the tribal knowledge? Any thoughts or comments would be appreciated.

Thanks in advance.

An urban myth. Festo fittings used with properly cut Festo tubing, work perfectly well on both pressure and suction. See the Festo web site for technical specs.

Glad to hear, thank you!

sc

Well....... I have to disagree. (it's not an urban myth)

Yes, a push-in festo, with a new hose, cut nice, works just fine on the suction side. FOR A WHILE. You may be fine for a season or two... but then... you may have a flight where you don't seem to get full power. Or even a flame out !! As your hoses age, they will sink in a little where the O-ring mates, and the O-ring it self may become hard and flatten as Kero soaks it. Then... if you get a little slime in your fuel, or just normal junk starts to plug your bubble tank... that will drive the vac level higher than when you first put it together. Also... If you look on Festo's info... push-in connectors are not rated for vac... only pressure. Either way... it COULD end in a dead model. (so why even chance it?)


This has been a debate for a while... but in general... it works for a while, and then causes issues as the model ages.

This is kind of like using the festo fuel filters. Some guys like them... but a few of us have had them fail in a catastrophic way.

The choice is up to you, but personally... I will stick with barbed fittings, with safety wire on the suction side. I have too much time/money in my models to be one of the 2% that finds issue.

Look a few posts down. A K45 was having flame out issues... guess what caused the issue???

Hi Tony,

Thank you for your feedback. This is kind of what I've always heard and personally thought was the case. That said, after posting this thread, a few guys have sent me the Festo documentation and apparently they are rated for vacuum. I was under the impression they weren't too. You do make a good point about peace of mind though, I'll have to give it some more thought.

sc

Honda has an opinion, I think he is misleading.

Festo QS series ARE rated at -.95 bar and work perfectly well, even using a medium, kerosene for which they are not designed, unless mistreated. Reread the Festo data sheet, it's quite clear.

I have 12 jets in my fleet, ALL have Festos in the suction side, none leak. My BVM F4 is in its 17th season, still with the original fittings. My 2 BobCats are about 15 years old, ditto! My FC Mig 29 has a myriad of Festos for the twin engine installation and in the hydraulic system, no problems.

The main fuel outlet of the main tanks on my two Airworld Hawks are fitted with Festos. I often remove the tube for de fuelling. Each season , I cut the tubing end to refresh it to ensure a fuel tight connection, never a problem.

It it does not make sense to condemn these fitting s because one or two guys have had problems. Bit like those who condemn Tygon, which, when correctly used, works perfectly!

A visual check for the amount of air in a hopper tank is a good, but not infallible, function check for leaking connectors.

David.

Hello David,

Thank you for sharing your experiences. I actually just received a fuel and smoke tank from CM Jets. I had the option to use barbs or Festo and I requested Festo. I'm curious about this subject and intend to test this with this jet. I figured if my turbine has Festo fittings on the powerpack, let me keep it simple and I'll also use Festo on the tank(s). Anyway, thanks again sir!

regards,

sc

OK... yes... they call it -0.95 bar vacuum. but that's not much. (about 4~5 inHg gage?) on the pressure side... they are rated at 14 bar. (so you can see what they are really meant for)

As I said... some are OK with them... but it's not really the proper use. And, I'm sure if you take a model that is a few years old, and pull a festo apart... you will see for yourself how the hoses deform. I'm not trying to be negative or anything... but I'm just trying to get you to think about the use.


This is a debate like any other... so pick a side, and go with it.

1
 

1 bar is 14.7 psi, 760 mm of Hg or 29.92 ins Hg, so - .95 bar is- 13.9 psi etc, almost as close as it is possible to get to a total vacuum, ie - 1 bar. So either your maths or mine are wrong ! Perhaps I have never understood pressures or vacuums!

(just looked it up. -.95 bar is 95% vacuum or -13.97 psi or 95% of 760 mm of Hg or of 29.92 inches of Hg. ) Can't change physics !

Having read these endless threads regarding Festo and negative pressure I carried out out some physical tests requiring the disassembly of a number of 4mm fittings. Here are my findings hoping to clear up this issue (though I doubt it lol)

All of the 4mm "small" fittings use an "0" ring of Festo design. This ring is of flat design containing a moulded in o ring this design makes the fitting of the tube easy as the tube enters the flat part of the seal then the extra pressure engages the tube into the o ring portion. This fitting is able to sustain leak free use in both positive and negative pressure.

The second type of 4mm fitting is slightly larger and has a different feel when fitting your tubing, the reason for this is simply the seals are of the "lip" design, and are totally unsuitable for the suction side of any application.

m

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m

From my experience having got the "leaking" issue over the years in diferent customers, there is one detail that no one commented, and it is that the 4mm tube and 5/32 tube are apparently similar, same 1/4" tube and 6mm tube. I have seen festos "leaking" just because the customer mixed imperial and metric sizes. This, plus the badly cut ends, plus some brands of fast fittings or tubing that degrade quickly when used in Kero, have created the "urban mith". Festo brand with quality Festo tube, installed correctly is OK, other brands not.

Gaspar

Thanks. This may explain some of the outcomes.

As I have already pointed out, 1 negative Bar air pressure rating is not much compared to the hydraulic suction force a pump can deliver. Admittedly, if you have high suction, there may be another fuel system issue. But I agree with Dr. Honda and stick to using Festo pressure side only. Why introduce more bubble risk when you don't have to.

I have to take issue with that statement, As David has pointed out, you only have 14.7lbs sq ins whatever the application.

m

David, this is an AIR rating. Has nothing to do with the hydraulic force a pump's impeller delivers. The pumps used in our jets are pushing a hydraulic column at working pressures of 40psi and up (3Bar+). Fuel is incompressible so the same force can potentially apply on the suction side of the pump if there is flow restriction. This could occur momentarily because of a Lin restriction (clogged UAT filter, for instance) or a change in the commanded pump voltage from the throttle. This suction force can be enough to collapse the tubing just enough that the barbs inside the fitting no longer grip and then air gets pulled. Again, this is hydraulic force and has nothing to do with air vacuum. Apples and oranges.

I am not suggesting the suction side forces are this high in a properly flowing fuel system, but it can easily approach a 1bar air rating in a large turbine and sequential plumbing under certain conditions. After the pump, fine. Before the pump, not so good. Again, why introduce any more risk than one has to? The only routine disconnect needed anyway should be at the turbine.

Mick, please see my comments regarding hydraulic pressure (force) vs air pressure. Totally different.

Again this statement demonstrates a complete misunderstanding of what's going on with regard to the delivery side of an oil pump. The working pressure available to force liquid into the pump can only ever be 14.7psi. There is no more, unless you either pressurise your tank or you live on another planet. lol...

Another thing that may interest you, these Hausl pumps can deliver up to 280psi!!

n

This is a quick breakdown for Fetso'sThe Festo connectors are designed for gas pressure and not for liquid suction, however they are designed to handle petrol based lubricants and they are pressure rated from -0.95 to +6 bar-0.95 bar is very high, and fuel will start to evaporate at this low pressure, so it is safe to use themThe only problem by using them in the suction side is that they are more susceptible to a leak if there is something wrong like dirt, the tubing not cut squared, or the tubing not completely in etc.Regards

I am not sure what you mean by an "oil pump" or what this has to do with what we are discussing here. I can assure you that hydraulic negative pressure in a pump inlet (closed system) far exceeds 1bar. This is how all hydraulics work from bulldozers to automobile brakes. One cannot use an air vacuum rating to determine resistance to hydraulic suction pressure.

Again, I am not suggesting these suction forces exist in a properly vented fuel system, but momentary HYDRAULIC suction forces CAN easily exceed a Festo fitting's 1bar vacuum AIR rating. Not saying they will, but they could. If a fuel system is exerting enough suction to collapse the sides of a UAT, then it could pull a bubble at a suction side festo. It's that simple. Why take the risk?

​​​​​​​Exactly.

Spot on Gaspar, and with respect I did mention the issue of correct tubing in an earlier post. Festo tubing for Festo QS and QSM fittings,or there may be a slight mismatch.

The doubters should read page 9 of the Festo data sheets for QS and QSM push in connectors, all are rated at MINUS .95 bar.

Incidentaly my Merlin 100 is running just superbly, including the Festo on the suction side of the fuel system, brilliant engine!

David.

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