olnico

The first page of the Festo technical paper also states:

"Pneumatic components must be supplied with properly prepared compressed air
which doesn’t contain any aggressive media"

"You should contact Festo's advisors if one of the following applies to your application:
The ambient conditions and conditions
of use or the operating medium differ
from the specified technical data."

roger.alli

I am with Maryr on this topic.. The Festo connector is rated to -0.95 bar. Our fuel pumps could never create this kind of negative pressure.. (Or more correctly, the fuel tank/piping system could never create -0.95 bar at the flow required.).

And of course there is the empirical evidence. There must be many of thousands of turbine models around the world with festos under suction operating perfectly happily.
I know that l my turbine equipped models (three off) have at least two festo connections under vacuum (Shut off valve). I have never experienced a fuel problem..

I don’t deny that festos need to be used correctly, but there is a mountain of evidence out there that they can be used under suction, in our application.

olnico

Once again guys. Festo specs are for dry air or pure air mixed with a very small quantity of atomised oil.
Not for kerosene/ diesel.
The value of -0.95 bar is irrelevant if use with kerosene/ diesel.
Please read the beginning of this thread.

darryltarr

My Feedback: (2)

A Negative Pressure (suction) of 0.95 Bar = 13.775 PSI

The suction caused by the fuel pump demand may well exceed this.

There are several Jets flying at our club with FESTO (push-on connectors) on the suction side but these turbines are small (max rating 100 Newtons). We have seen tubing collapse inwards on the bigger turbines, at full thrust however, our climate is seriously WARM. Shade temperature 40 degrees centigrade (104F), and this does soften the PU tubing just enough to allow air.

Just saying.

WimB

My Feedback: (1)

I have been using Festo's on the suction side, even in my Tomcat.

Problem there is that it is impossible to get the tubing on original barbs on the central tank and safety wiring it. This is why I chose to replace these with 6MM/M5 fittings. I do agree that they need care and correct installation, but to me the "maintenance ease" is also an important factor. replacing fuel lines or other maintenance is a much easier this way... I a barbed connection safer? probably yes, but, it's more difficult in maintenance...
And, if you generate that much vacuum on a fuel system to make a festo fail, there's some other issue in your fuel system......and you're likely to get cavitation.

I'm planning to use festo on all connections in my new jet, from tank to turbine.only exceptions will be the pump and the valves.

Also, not 1cm of tygon (except in the tanks/pickup line) on any of my jets.....

Wim

mauryr

How?

Henke Torphammar

Suction of well over 0.95bar? LOL

Just saying....

TTRotary

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TTRotary

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TTRotary

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Craig B.

I don't post much on RCU nowadays but......wow. There are some spectacular assumptions being posted on this thread as facts. If you took this all at face value you would have to assume that the fittings on almost every brand of turbine being sold are a disaster waiting to happen.

There are many variables that contribute to or influence the vacuum on the suction side of our fuel systems. Too many generalisations on here to comment. Anyone care to explain how a well vented system can generate so much negative pressure?

I have a mate who is a fluid dynamics engineer that would find this whole discussion quite amusing.

Wisest thing I have read on here so far.

TTRotary

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darryltarr

My Feedback: (2)

Agreed, but to be fair I don't think anyone is blaming FESTO, nor a well ventilated system.

The issues normally occur between the AIR TRAP and the FUEL PUMP.

Some devices (AIR TRAP / Hopper Tank / UAT etc), by design, will generate way more vacuum pressure (again between the device and fuel pump) then others. Cavitation, collapse of tubing, or leakage from a connector can occur, and for several reasons (restrictions in the device, low grade tubing, incorrect cutting method of tubing, small I.D. connectors, to name a few).

Most of these issues will go unnoticed on small turbines BUT run a B300F or JetCat P-400 and you will see for yourself.

Nowadays most manufactures have upgraded AIR TRAP / UAT systems to HIGH FLOW in order to accommodate larger turbines, but just go back 10 years - there were very few devices on the market. The original JetCat P-200 fuel demand would almost collapse some UAT's, and this suction, in turn, would contribute to other failures.

roger.alli

Hmmmm.. Interesting discussion..

My understanding is;
The negative pressure inside a chamber can never be less than –1 bar. Once you get to -1 bar, you cannot pump anything else out. This is called a hard vacuum.

Festo fittings, with the correct tube, are rated to 95% of this..

I suggest that if you were ever to take the pressure inside your tank system down to .095 bar, you would have much bigger problems then air leaks in the Festos. I reckon the tanks would collapse way before the Festos failed.


Festo fittings on the suction side of a pump work. As proven every day by thousands of modellers who have this item up stream of the pump. Successfully..
.

I also understand that Festos, by their design, have larger orifices than barbed fittings.. So, (provided the fittings are not letting air in), would not the larger orifice size help to reduce cavitation issues in hi flow/large engine installs?

This is a serious question

roger.alli

By "serious question", I mean i am not trying to be a smart *****. I do seriously wish to know why the "Festo on suction side" is so often advised against.

David Gladwin

[QUOTE

Bottom line: don't use Festo fittings on the suction side of a turbine fuel system.[/QUOTE]

Bottom line, don't believe everything you read on RCU. Festo QS fittings are perfectly suitable for pressure AND suction sides of fuel systems when correctly used.

With a fleet of 12 gas turbine powered models all with about 4 or 5 Festos in the fuel system, suction and pressure, engines from Wren 44 to Olympus, I have not had a single problem. I have zero experience with model engines bigger than the Olympus.

Some Festos have been in situ for over 15 years , BVM F4 and Bobcats, hundreds of flights without flameouts, various header tanks, BVM, PST , Intairco and CAT plus home made devices using JPX oil tanks, NO problems with Festo fittings.


On on some of my models defuelling is achieved by removing the suction feed from the main tank to insert the defuelling line. The feed tubing into the Festo is replaced each season but the Festo stays, doesn't leak according to ge minimal air in the hopper tank after each flight!

The big acid test?

My FC Mig 29 has two engines, extensive use of Festos in dual fuel system, AND a complex hydraulic system operating at 10 Bar with much higher spikes on landing due to system layout.

The hyd system has around 20 Festo fittings(26 actually) on pressure and Suction, the recent small leak was due to scratched aluminium tube where inserted into the Festo, easily fixed by replacing with Festo high pressure tube.

As for Tygon, great stuff, I have Tygon on almost all models on barbed fittings , never a problem here either. The Tygon connecting all 4 of my F4 tanks has been in place for 17 seasons!

Long and extensive PRACTICAl experience has given every confidence in Festo.

Dont blame Festo, blame the users, (some of whom STILL rubbish the Festo filter when it fails when being used under pressure, the filter is designed for gaseous vacuums according to Festo spec. Sheets ! )

Just my hands on, practical experience !

darryltarr

My Feedback: (2)

This is not always the case Roger.

Please also understand that you are lucky enough to live in a "First World Country" where the highest grade products are easily available, you have access to the FESTO stores so its easy for you to select good quality tubing with absolute precision, the best of the best.

Some model enthusiasts here may get there hands on what looks like FESTO PU tubing, but it is NOT, its FAKE, and therefore the tolerances are slightly out, this in itself may cause some air leaks on the suction side. Therefore one "workaround" that became bullet proof was to "SUGGEST" the use of barbed connectors (on the suction side), and with a safety wire lock.

Only recently have we been allowed the luxury of internet ordering and this now allows us to acquire TOP quality products.

roger.alli

Ahhh. Of course you are correct on that point Daryl. Festos do need to be used with the correct tube. They rely on a close tolerance between seal and tube OD. Where as a barbed fitting seals over a much wider range of tube diameters, and materials.

I still think many people advise againt thier use, without really understanding them. If they were that bad, there would be a lot more reports of flame outs around.

And yes, we are very lucky to live in a first world country, where we can afford to play with these amazing machines as a hobby.
Sometimes it is worth pausing to reflect on just that..

darryltarr

My Feedback: (2)

Well put Roger (and David), and as I mentioned previously in post # 38.

Don't blame FESTO, but rather the understanding (or lack of), used on some installations.

WimB

My Feedback: (1)

Where do you get viton tubing, and what size do you use then?

Wim

Jgwright

David

Absolutely agree, I was going to write a similar post to yours but you beat me to it.

Never had a Festo fitting leak in 15 years and I have had many of all sizes. Pipes need trimming from time to time especially if regularly plugged and unplugged. Although not rated for Kero who cares.. they are ideal for our purposes and work fine.

Mine are only used on pressure side so cannot comment on suction though if tanks and pipes are being sucked flat then there is something seriously wrong with the installation.

Never used anything except Tygon inside tanks and often on the suction side from the tank to pump and never had Tygon go hard.

I can beat you on the pressure side. I have been using them at 180 psi on the Boulton Paul both in a test rig and in the plane with no leaks. I use Hydraulic oil.

It would be very interesting to find at what point they failed in practice on both pressure and suction, but bet they have a safety margin. Maybe someone could test them, if they can generate enough pressure and suction!

John

Craig B.

I concur with David's and John's comments. 16 turbine powered models and 18 years of turbine model operation (up to 200 class) has given me a small amount of practical experience to draw on. I frequently observe installation errors with both barbed and festo connectors. The classics are zip ties on barbed fittings, Tygon inserted into festo fittings and poor entry angles into festo fittings. I have never had a leaking fitting on either suction or pressure sides.

If my negative pressures were even close to approaching the level that exceeded festo spec, I would be taking a critical look at the rest of my fuel system installation. My comments do need to be taken in the context of the engine sizes and corresponding fuel draw rates within my experience.

mauryr

Please excuse my infinite ignorance, but as I understand it, vacuum = the lack of matter.
Vacuum (negative) pressure thus becomes the pressure exercised by the earth's atmosphere to 'fill' the space left by this lack of matter.
As a pump pulls a fluid from a container, the fluid moves because the atmosphere pushes it to fill the space (vacuum) created by the fluid that has been sucked by the pump. Once the drag of the fluid on the suction side of the pump causes resistance to raise enough, the pump will start to cavitate because the fluid cannot 'catch up' fast enough, so the pump basically works in vacuum moving absolutely nothing and creating no additional pressure on the pressure side.
I have not installed a vacuumeter in any of my fuel systems, but I am willing to bet it would never show any number even close to -1, even if the pressure to the engine ever reached 10 bar.
A quick google search: https://www.google.it/search?q=vacuu...ZlDsQQ_AUICCgB
shows how vacuumeters also only reach -1....

I am also willing to bet the 'boiling' effect described by oli in his fuel systems blog would start showing up long before reaching -1 bar, showing your fuel system already failed miserably at that point.

So yes, the -0.95 rating on festo is as good as it can be.

I think in your usage of 'basic physics' you have confused 'flow' with 'pressure'. Sorry, but they are very different things...

Please correct me if any of what I said is wrong.
Thanks

David Gladwin

Absolutely wrong. Tygon is perfect for internal tank useage.

Last week I looked inside a home made header tank (JPX oil tank ) where I had used Tygon. The Tygon had been immersed in fuel, Jet A, for over 10 YEARS . it was still fine, discoloured but still flexible and with a tight grip on the non barbed metal tubing !

Several other models including my AirWorld Hawks have Tygon internal tank tubing, work perfectly even after 9 years, in some cases more, of use !

By any standards , worthwhile tests !

However due to the slight natural stiffness and bouyancy of Tygon I do add a small wrap of lead at the pickup point of the Tygon to overcome that slight "bouyancy" and stiffness and ensure it remains immersed in the fuel reducing the possibility of it sucking air.

Daryll too, has a good point. I use ONLY Festo tubing on Festo connectors, SMC tubing on SMC units, and caution there are items out there which LOOK like Festos but are copies. I believe there are units called "Singa" amongst others, which may or may not be made to the exact standards of Festo, the world leader in this aspect of production.

John, I must take a look at your hyd. system sounds good !

TTRotary

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