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Flow Scan Transducer calls for no sealant

Michael Burbidge

Well Known Member
I'm installing a Flow Scan Fuel Flow transducer. It takes 1/4 " NPT. The instructions say that absolutely no thread sealant of any kind should be used.

I did not know that pipe thread could be liquid tight without sealant.

Am I wrong?

Michael-
 
Yes they are. Do not use sealant. Sealant can, and most likely will migrate over time and fowl the impeller. Ask me how I know.
 
I'm installing a Flow Scan Fuel Flow transducer. It takes 1/4 " NPT. The instructions say that absolutely no thread sealant of any kind should be used.

I did not know that pipe thread could be liquid tight without sealant.

Am I wrong?

Michael-

It depends - read this, it addresses the sealing specification of tapered pipe threads, start about post #30.

Sorry, I am a lay-zee typist.
 
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No, it can not be liquid tight without sealant. The tapered threads still have helical leak paths at the roots and crests of the threads, because you can not cut a thread precisely enough that will contact in those corners at the same time that the thread faces meet.

Exceptions: plastic fittings (nylon, etc) can undergo enough plastic deformation to extrude into the thread roots to seal. A brass fitting is ductile enough that if it is way over-tightened, it may deform enough to achieve a seal. In doing so, you run the risk of splitting open the casing of the mating part.

The challenge here is that the floscan sensor has a very small internal passage with a little turbine in it. Any debris at all can obstruct the fuel flow, and/or interfere with the free rotation of that tiny turbine.

There has been at least one documented case of a tragic outcome from fuel obstruction in a fuel flow sensor, ( I believe from teflon tape, but I could be wrong)

What I did, and everyone can take this or leave it, it is simply what I did, was to use a very spare amount of teflon paste on the upper part of the threads, leaving a couple of threads completely clean. The hope was that whatever squeeze out of paste would mostly migrate outward, not inward, and even if a little bit did, it would move into those few bare threads and no farther.
After 500 hrs, no issues. I have not/can not look inside to see the actual state of intrusion of sealant into the sensor, but it works fine.

If the manufacturers of the sensor really want a liquid-tight connection with no sealant, they would machine a female "straight thread plus o-ring" boss, often called a JIC straight thread or SAE straight thread fitting. There is an AN spec for this and fittings are readily available. The fittings would have AN 37-degree flare male attachment on the other end, to receive a flare and B-nut. The cost difference between cutting a female pipe thread and a female AN o-ring boss thread is pretty small. It's just good engineering practice. Look at many many high pressure hydraulic devices to see how good connections are made that avoid use of sealant that can foul the internals.
 
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Mine are fine 750 hours. I?ll research the threads attached and see what others are doing. Steel fittings into the Floscan, no issues.
 
I'm installing a Flow Scan Fuel Flow transducer. It takes 1/4 " NPT. The instructions say that absolutely no thread sealant of any kind should be used.

I did not know that pipe thread could be liquid tight without sealant.

Am I wrong?

Michael-

Yes, you are wrong. Pipe threads are designed to self-seal. In the early days it was called pipe dope, not sealant because it is not designed as a sealant, but lubricant. In order to obtain a leak free NPT connection, it must be tight and requres a lot of force. There is a reason pipe wrenches are so long. The lubricant helps to achieve that at a much lower torque applied by the user and therefore increasing the likelihood of getting it tight enough. Most leakage is due to not understanding this and under-tightening joints.

Sealants are somewhat common in the plumbing trade because most/many threads are cut in the field with varying qualities of taps and dies. Sealants help when the threads have cracks and chips in them. That is not necessary with quality threads from a quality component.
 
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No, it can not be liquid tight without sealant. The tapered threads still have helical leak paths at the roots and crests of the threads, because you can not cut a thread precisely enough that will contact in those corners at the same time that the thread faces meet.

Exceptions: plastic fittings (nylon, etc) can undergo enough plastic deformation to extrude into the thread roots to seal. A brass fitting is ductile enough that if it is way over-tightened, it may deform enough to achieve a seal. In doing so, you run the risk of splitting open the casing of the mating part.

The challenge here is that the floscan sensor has a very small internal passage with a little turbine in it. Any debris at all can obstruct the fuel flow, and/or interfere with the free rotation of that tiny turbine.

There has been at least one documented case of a tragic outcome from fuel obstruction in a fuel flow sensor, ( I believe from teflon tape, but I could be wrong)

What I did, and everyone can take this or leave it, it is simply what I did, was to use a very spare amount of teflon paste on the upper part of the threads, leaving a couple of threads completely clean. The hope was that whatever squeeze out of paste would mostly migrate outward, not inward, and even if a little bit did, it would move into those few bare threads and no farther.
After 500 hrs, no issues. I have not/can not look inside to see the actual state of intrusion of sealant into the sensor, but it works fine.

If the manufacturers of the sensor really want a liquid-tight connection with no sealant, they would machine a female "straight thread plus o-ring" boss, often called a JIC straight thread or SAE straight thread fitting. There is an AN spec for this and fittings are readily available. The fittings would have AN 37-degree flare male attachment on the other end, to receive a flare and B-nut. The cost difference between cutting a female pipe thread and a female AN o-ring boss thread is pretty small. It's just good engineering practice. Look at many many high pressure hydraulic devices to see how good connections are made that avoid use of sealant that can foul the internals.

I would assume the thread are NPTF, which is a unique cut with a very thin tip that easily deforms. I do believe that an NPT joint can seal liquid tight and have seen many examples of it. It does require proper intallation technique though. I also believe that slight deformation is typical, even in steel, and that is part of the design of the NPT thread. Have you noticed how it gets progressively tighter as you go? Then you back it out and run it in again, but it is much looser (i.e. easier to turn) until it hits the point where it was last resting? This is because the threads formed to one another via deformation. It is also why you need to go past the last tighten point if you disassemble the joint.

Also, NPT thread joints do seal at the roots. This is what the taper does. And what makes it so elegant is that all of that force eliminates the need for high precision of the thread. It forces the outside of the male thread into the root of the female thread and vice versa. The taper is forcing too big of a part into too small of a hole. All of that force you put on the joint is smashing that tip into the root and it deforms the tip until it fully complies. It is not elegant like a flare or ORB. It uses brute force to achieve it's goal. It is, however, time tested and proven when proper installation techniques are used.

All of that said, I always used permatex #2 on my NPT joints. I am far from perfect and this gives me some margin of protection from error as well as lubricates my threads. I also used at light coat on my red cube, but did also avoid the first 2 threads to be sure it didn't get into the sensor. #2 doesn't harden, so little bits can't break off and clog things.

Larry
 
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While it is true that there are pipe threads designed to seal without using any thread sealant, regular NPT threaded fittings that we use on light aircraft are not one of them.

There is a lot of info available on line if someone is interested in learning more.
One resource I found doing a quick sear can be accessed HERE
 
I would assume the thread are NPTF, which is a unique cut with a very thin tip that easily deforms. I do believe that an NPT joint can seal liquid tight and have seen many examples of it. It does require proper intallation technique though. I also believe that slight deformation is typical, even in steel, and that is part of the design of the NPT thread. Have you noticed how it gets progressively tighter as you go? Then you back it out and run it in again, but it is much looser (i.e. easier to turn) until it hits the point where it was last resting? This is because the threads formed to one another via deformation. It is also why you need to go past the last tighten point if you disassemble the joint.

Also, NPT thread joints do seal at the roots. This is what the taper does. And what makes it so elegant is that all of that force eliminates the need for high precision of the thread. It forces the outside of the male thread into the root of the female thread and vice versa. The taper is forcing too big of a part into too small of a hole. All of that force you put on the joint is smashing that tip into the root and it deforms the tip until it fully complies. It is not elegant like a flare or ORB. It uses brute force to achieve it's goal. It is, however, time tested and proven when proper installation techniques are used.

All of that said, I always used permatex #2 on my NPT joints. I am far from perfect and this gives me some margin of protection from error as well as lubricates my threads. I also used at light coat on my red cube, but did also avoid the first 2 threads to be sure it didn't get into the sensor. #2 doesn't harden, so little bits can't break off and clog things.

Larry

I'm sorry Larry, but this is simply false. The taper forces the faces of the threads into intimate contact, and at that point, when the thread faces seat against each other, the roots and tips of the threads do not contact. All you have to do is look up the dimensional specs on the threads and draw them out to see that the root radius and tip radius are different so that when the thread faces meet, the tip of the male thread does not contact the root of the female thread.

And, as I said, if the manufacturer of the fuel flow sensor would use a proper straight thread with o-ring connection, we wouldn't even be having this discussion. See, for example, MS21900 for the appropriate male fitting to fit into a female straight-thread boss.
 
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The fatal obstruction referred to was not teflon tape. It was a huge glob of RTV, never intended to be used a a thread sealer.
 
I would follow the manufactures instructions. If you use a sealant, you take a chance of fowling the impeller, or worse. I do not know what thread type or design Floscan uses in the body of their product but I can speak from experience that it will not leak dry, at least mine and others who posted in the past haven’t.
I always have used sealant on all pipe threads and did the same on the Floscan. It gave up the ghost and when I followed up with the engineer at the time with Floscan, they said “do not use a sealant of any type.”

So, great general discussion on pipe threads and fittings but it doesn’t necessarily apply to Floscan. I do not know exactly why, but following the manufactures instructions is usually good practice.
 
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I'm sorry Larry, but this is simply false. The taper forces the faces of the threads into intimate contact, and at that point, when the thread faces seat against each other, the roots and tips of the threads do not contact. All you have to do is look up the dimensional specs on the threads and draw them out to see that the root radius and tip radius are different so that when the thread faces meet, the tip of the male thread does not contact the root of the female thread.

And, as I said, if the manufacturer of the fuel flow sensor would use a proper straight thread with o-ring connection, we wouldn't even be having this discussion. See, for example, MS21900 for the appropriate male fitting to fit into a female straight-thread boss.

My apologies, you are correct. I was incorrectly applying what I learned about NPTF threads to NPT. NPTF is also called "Dryseal" and was designed to seal without pipe dope or teflon tape. I will stop arguiing the point and those interested can research NPTF threads.

Larry
 
I would follow the manufactures instructions. If you use a sealant, you take a chance of fowling the impeller, or worse. I do not know what thread type or design Floscan uses in the body of their product but I can speak from experience that it will not leak dry, at least mine and others who posted in the past haven?t.
I always have used sealant on all pipe threads and did the same on the Floscan. It gave up the ghost and when I followed up with the engineer at the time with Floscan, they said ?do not use a sealant of any type.?

So, great general discussion on pipe threads and fittings but it doesn?t necessarily apply to Floscan. I do not know exactly why, but following the manufactures instructions is usually good practice.

I know of cases where people have installed them dry and have had leaks.

I think floscan was named in the law suite for the RV-10 crash where the builder (incorrectly) used high temp RTV to seal fittings to the sensor which ended up plugging the sensor orifice and causing engine stoppage. I don't think their documentation specified a dry fit installation prior to that accident (but I could be wrong).

If sealant is used correctly, there should be no chance of it getting inside.
The problem is people fail to think how small of an amount of sealant will remain in the threads once the fitting is tightened to an interference fit. Everything else you apply has to go somewhere. If you look at builders photos, you will usually see a fat ring of exposed sealant around the perimeter of the fitting (signs of it being done incorrectly).

No one wants to have leaks so I fully understand the thought process to apply a heavy amount, but this is not an instance where more is better.
 
Oddly, the EI FT-60 "Red Cube" states that that one uses NPT threads, that they are not self-sealing, and that thread sealant *must* be used (but it warns not to use either Teflon tape or pipe dope), and (in all caps) do not exceed 12 ft-lbs of torque (presumably because exceeding that may crack the housing).
 
My floscan leaked when it was installed dry. I replaced it with a Redcube and used liquid lubrication to install the fitting and never leaked.
 
BE careful of overtighening a fitting in a Flo scan----you CAN crack the threaded boss and it will leak, with or with out sealant. Red cubes dont have that issue.

Tom
 
I?m glad mine doesn?t leak installed per manufactures instructions. Perhaps someone could call the Floscan folks, who we ever they are, and see what they say.( mines fine and I don?t need to prove a point). Tom helped me to replace all my hoses, including those from the servo to divider thru the Floscan. So, several times, it worked for me without sealant.
That?s good enough for me.
If you do use a sealant Do Not Use Fuel Lube. It doesn't cure or harden and may migrate! If you do use a sealant, or not, don?t cover it up with fire sleeve or you can?t see if it is leaking.
 
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