# Minimum Fuel Tank Vent Line Diameter

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#### HomeBuilt101

##### Well-Known Member
What is the recommended diameter for a fuel tank vent line that will supply a 350 HP engine?

Is the diameter requirement of the fuel tank vent pipe dependent on whether the end of the fuel vent pipe is a simple cut off pipe end sticking straight down from the belly of the wing, or if the pipe end is arranged with a NACA scoop, a pitot tube design (like C-172), or the usual 45 degree cut end with the beveled edge pointed into the air stream?

Lets say the airplane will have one vent line will provide the vent for two 45 gallon fuel tanks and the configuration of the tanks is such that the top surface area of the fuel in both tanks will be about 2000 square inches. I cite the surface area of the top surface of the fuel because if the fuel vent end is pointed forward like a pitot tube, I can imagine that a pitot tube sticking out in the air stream at 180 MPH can create a fair amount of positive air pressure and this pressure being ported into the fuel tanks will spread out across the 1000 square inches of fuel surface area in each tank as well as the side walls and the top inside surface of the fuel tank...however the walls and top of the tank will not move so the actual force is going to be pushing down on the surface area of the fuel... so I can imagine that the diameter of the pipe can be a concern.

Kind of like a hydraulic floor jack...a small diameter squirt of hydraulic fluid pushing against the large diameter of the lift cylinder piston and inside the actuator there is a great deal of force on the actuator cylinder walls but since they do not move the actual force is being exerted down onto the piston surface area...so in a fuel tank the air pressure forcing its way into a small diameter pitot tube would ported to the fuel tanks and push against the very large surface area of the fuel creating a lot of force and unlike the floor jack needing to get stroked to exert pressure...the air pressure from the pitot tube would be continuously forced into the fuel tank as long as the airplane is moving...no?!?!?

To think of this in extremes can be helpful...A super small pitot tube opening would be way too small to effectively move the air through the pipes and on the other hand a super large diameter pitot tube opening like 2000 square inches would only push down on the fuel surface area on a 1 to 1 ratio (so that would not work for more reasons than that)...but there has got to be a sweet spot of diameter to create an efficient air flow plus sufficient pressure...

In this design, there will be a one way check valve plumbed into the system out in a void in the wing where the check valve could still draw air from this dry vented area in the event that the pitot tube portion becomes blocked with debris or ice the fuel pumps will be sucking away and the check valve will open up against its slight spring pressure.

Any ideas?!?!?

#### Dana

##### Super Moderator
Staff member
Air flow through the vent is minimal; it's equal to the fuel flow. The surface area of the fuel in the tank is irrelevant, the pressure doesn't "spread out" but is constant, other than due to flow losses, which are minimal.

#### BJC

##### Well-Known Member
HBA Supporter
As a 20-something year old engineer, I eyeballed a 6” diameter “vent” (actually a connection to a nitrogen blanket system) for a tank farm of 36’ tall ~184,000 gallon thin wall SS food grade tanks. The outlet was 6” diameter that fed into a manifold with 4” diameter connections. Designed product flow rates in / out were less than 50 GPM. The thixotropic product had a Specific Gravity of 1.6.. Tank wall thicknesses, which varied, had been given lots of scrutiny, but I had not considered negative pressures.

When draining the first completed tank following a leak test with water, (allowing the water to flow through the manifold and piping to the next tank without mechanical pumping) the vent proved to be inadequate; the negative pressure crumpled the side of the tank. We repaired the tank by carefully pressurizing it with water.

That was the last time I eyeballed a design for any engineering that I was paid for.

BJC

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
I have repaired tanks that was damaged from plugged vents when the tank was drained. Also repaired tanks where someone pressured it with plant compressed air of 350 psi. Lots of crazy things happen.

BJC

#### HomeBuilt101

##### Well-Known Member
So it seems that I am talking to the right crowd (as usual around here).

BJC and Pops. it seems that you have been around a tank or two in your time...

Does anyone know the proper diameter of the fuel vent pipe for 350 HP?

If the fuel vent is a pitot tube then the airplane flying at 180 knots will produce some positive stagnant pressure inside the fuel tank and that pressure will be pushing out on everything and assuming that the fuel tank walls and top do not burst the only thing left to move is the surface area of the fuel so I presume that will be a great deal of pressure...I certainly will not step out in flight and stick my 350 PSI compressed air into the pitot tube vent however I am interested in what is the best size of pitot tube diameter to use for this installation.

#### rick9mjn

##### Member
here is my 2 cent idea,,,,,, go to the airport and see how the big boys with " big engines" did it, with it being crop dusting season, there should be a large number of dusters parked outside to look at......
quote of the day; one good test is worth 10,000 good ideas and 100,000 theories.
/\ good day / rick|

#### Dana

##### Super Moderator
Staff member
Ram air pressure at 180kts is about 3/4 psi. That pressure will be the same regardless of the pitot tube size or the size of the tank.

#### Victor Bravo

##### Well-Known Member
If you start with a vent tube the same inside diameter as your fuel supply tube, you will likely be way on the conservative side of safe. If you have a 1/2 inch diameter line going to the gascolator, and you use a 1/2 inch vent tube, you should have more than enough vent capacity. This is because air will move through the vent tube a lot more easily than fuel will move through the supply line (air being significantly less dense and resistant to flow than fuel). Any and all effort in keeping the vent tube free and un-clogged is probably far more important than the actual diameter of the tube though.

#### Dan Thomas

##### Well-Known Member
Cessna used, on their 300-HP 185 and 206, a half-inch OD aluminum tube as a vent inlet, and it was located mostly behind the wing strut in a precisely specified position to prevent both excessive pressure and impact ice formation. That vent supplied both tanks via a 3/8" crossover tube between the tanks. A vent check valve was in the line just inside the let tank where the main vent entered, and that valve was designed to let air in rapidly and out slowly. Some of Cessna's airplanes had only a left-right fuel selector, no "Both" position, and some of those airplanes had separate vents for each tank, typically 3/8" tube.

Now, a vent tube sticking out the bottom of the wing and cut at 45° is an unpredictable thing, besides being a real scalp-ripper. Being under the tank puts in in a higher-pressure zone, but the airflow will still try to generate suction if that angle isn't good enough. It's why Cessna used a forward-facing vent.

There's an AD on a lot of Cessnas requiring a vented fuel cap on at least the right wing in case that vent inlet is plugged, with ice or with a big bug or by a mud-wasp nest. The unvented caps haven't been made for a long time so most now have two vented caps. The cap has a check valve in it to prevent air or fuel leaving through it.

There is some really good stuff in an AC that used to be in FAR 23. I can't find it. However, exactly the same stuff is found in the Canadian Aviation regulations, fuel systems starting here: https://tc.canada.ca/en/corporate-services/acts-regulations/list-regulations/canadian-aviation-regulations-sor-96-433/standards/part-v-airworthiness-manual-chapter-523-normal-utility-aerobatic-commuter-category-aeroplanes-25#523_951

The part on venting is at 523.975. The whole thing on fuel systems is worth reading and thinking about. Many homebuilts have crashed because of fuel system inadequacies, and venting systems are a big part of that. What works well on the ground sometimes doesn't work at all in flight, and you can get negative pressures holding the fuel back, or pressure differentials between the tanks that force all the fuel out of one and into the other and overboard.

##### Well-Known Member
The quick and dirty response is the old FAR23 requires the fuel system to deliver a flow rate of 1.5 x the engines maximum fuel consumption, which may set the required airflow into the tank, assuming you don't do anything odd with it (pressure refueling, subject it to extreme thermal cycles with full tanks, inter-tank fuel transfer pumps, etc etc).
Dan Thomas has got the detailed answer above, except that one can refer to FAR27 if one wants a higher design standard for fuel systems.

#### HomeBuilt101

##### Well-Known Member
Thanks good information and very grateful for your help!!!

#### pictsidhe

##### Well-Known Member
BJCs point about the vent being big enough to allow draining of the tank for maintenance may be the limiting factor, depending on the tank construction.

#### Dan Thomas

##### Well-Known Member
BJCs point about the vent being big enough to allow draining of the tank for maintenance may be the limiting factor, depending on the tank construction.
I used to just take the caps off and leave them sitting a little cockeyed and loose on the filler to achieve rapid draining. Leaving them loose there discourages bug entry.

#### Dan Thomas

##### Well-Known Member
I should mention that the 177 and 210, having no struts, have their vents terminating in the aft end of the wingtip rib, right next to the aileron. The end of the tube is squished shut and holes are crossdrilled in the tube. That gives pretty much ambient pressure and is almost ice- and bug-proof. The vents are arranged somewhat differently, though. The 210's:

The 177's:

See that? the 210 has vent lines coming out of the top outboard of each tank and going to the respective wingtips. That means fuel dribbles out when the airplane is parked with one wing low and the tanks are full. The check valves have small bleed holes in them to prevent tank pressurization, and that lets some fuel out. The 177 has a much better system, with the vent lines leaving the tanks and running around the backside of the tanks and all the way over to the opposite wingtip. No fuel can drain out with a system like this. The drain plugs allow the mechanic to check the lines for water or other junk that might plug the vent system. Like water that freezes in there in cold weather.

#### Old Koreelah

##### Member
Crickey Dan, that's a lot of plumbing! My vents exit the inner top edge of my wing tanks and go to the wingtip, so it can't drain the tank, even if parked on a slope (unless I've been dumb enough to leave the cross-feed valve from the other wing's tank open).

Another function of the fuel vent is to cope with the pressure shock if you prang the aeroplane. I imagine that pulse would be considerable with a mass of fuel suddenly sloshing across the confined space. If that pressure pulse rips the vent lines free of the tank, you could be upside down and drenched in AvGas. In the hope that it might prevent this, my vent lines have a large internal diameter.

#### Dan Thomas

##### Well-Known Member
Crickey Dan, that's a lot of plumbing! My vents exit the inner top edge of my wing tanks and go to the wingtip, so it can't drain the tank, even if parked on a slope (unless I've been dumb enough to leave the cross-feed valve from the other wing's tank open).

Another function of the fuel vent is to cope with the pressure shock if you prang the aeroplane. I imagine that pulse would be considerable with a mass of fuel suddenly sloshing across the confined space. If that pressure pulse rips the vent lines free of the tank, you could be upside down and drenched in AvGas. In the hope that it might prevent this, my vent lines have a large internal diameter.
No vent line is going to be large enough to relieve the tank pressure in a crash. The air just won't get out fast enough, and once fuel gets into the line things get real slow. In crashes, the fuel often surges forward so hard that it peels the tank and wing wide open.

#### Richard Roller

##### Member
I should mention that the 177 and 210, having no struts, have their vents terminating in the aft end of the wingtip rib, right next to the aileron. The end of the tube is squished shut and holes are crossdrilled in the tube. That gives pretty much ambient pressure and is almost ice- and bug-proof. The vents are arranged somewhat differently, though. The 210's:
View attachment 100029

The 177's:
View attachment 100028

See that? the 210 has vent lines coming out of the top outboard of each tank and going to the respective wingtips. That means fuel dribbles out when the airplane is parked with one wing low and the tanks are full. The check valves have small bleed holes in them to prevent tank pressurization, and that lets some fuel out. The 177 has a much better system, with the vent lines leaving the tanks and running around the backside of the tanks and all the way over to the opposite wingtip. No fuel can drain out with a system like this. The drain plugs allow the mechanic to check the lines for water or other junk that might plug the vent system. Like water that freezes in there in cold weather.
MD-80's have cross vented tanks also. I'm sure everyone has seen fuel streaming from a 210 when making a tight turn on the ramp.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
... I'm sure everyone has seen fuel streaming from a 210 when making a tight turn on the ramp.
...and I've seen it from the cockpit of a 177 as well.

#### Dan Thomas

##### Well-Known Member
Fuel streaming from Cessna fuel vents is caused by a sticking or incorrectly positioned vent check valve. That valve has a flapper that is hinged at its top to let air in quickly, and seals when fuel surges against it. There is a tiny hole in the flapper to let pressure out (such as in a tank sitting in the sun and heating and vaporizing the fuel). The flapper has a really light spring holding it shut. If the valve, which is threaded onto an offset tube attached to the inside end of a bulkhead fitting in the tank wall, isn't clocked with that hinge at the top, it's not encouraged by gravity to close; if the zigzag tube it's on isn't clocked so that the valve is as high as it can be in the tank, it's sunken into the fuel and fuel will run out readily. Lots of it. If the flapper hinge is gummed up by dried fuel residue, it gets sticky and wants to stay open.

It's all about maintenance and fixing stuff right and keeping it fixed. A big percentage of the fleet is undermaintained.