Fuel Tank Sending Units - Dangerous?

[capngrog]

I've recently installed new fuel tanks in a couple of cars: one is a 1947 Chevrolet, and the other is a 1950 Crosley.  Both of the new sending units feature an exposed rheostat.  I would suspect that, in operation, a rheostat would produce sparks, igniting the fuel tank ... but obviously, this doesn't happen for some reason.

 

I have two questions:

1. Does a rheostat produce sparks in the course of its normal operation?
2. Is the vapor space (ullage) within the typical fuel tank too rich to support combustion?

Curious, I am,

Grog

[ply33]

I think that the rheostat is unlikely to spark as the wiper moves across it as the wiper typically makes contact with multiple wires at once so there is no actual make/break of the circuit to cause sparks.

 

And even if there was a spark, the vapor in the tank is way too rich to properly ignite: You need both fuel and an oxidizer and in that case you are low on oxidizer.

 

But the above is just my opinion as I've not actually worked on the design of any such systems, just wondered about the same as you.

[joe_padavano]

I think that the rheostat is unlikely to spark as the wiper moves across it as the wiper typically makes contact with multiple wires at once so there is no actual make/break of the circuit to cause sparks.

 

And even if there was a spark, the vapor in the tank is way too rich to properly ignite: You need both fuel and an oxidizer and in that case you are low on oxidizer.

 

^^^THIS! 

[trickydicky43richard]

Ok, What is the correct and safest way to install a replacement fuel sender with the tank still in the car w/- fuel ??

R

[Larry Schramm]

I've recently installed new fuel tanks in a couple of cars: one is a 1947 Chevrolet, and the other is a 1950 Crosley.  Both of the new sending units feature an exposed rheostat.  I would suspect that, in operation, a rheostat would produce sparks, igniting the fuel tank ... but obviously, this doesn't happen for some reason.

 

I have two questions:

1. Does a rheostat produce sparks in the course of its normal operation?
2. Is the vapor space (ullage) within the typical fuel tank too rich to support combustion?

Curious, I am,

Grog

 

 

1. The rheostat is does not give off any sparks that I would know of.

2.  The concentration of vapor in the gasoline tank is too high for any explosion to occur.  There is what is known as a lower explosive limit and upper explosive limit for gasses to be able to combust.  For if the concentration is outside that range then the gas will not burn.  For gasoline the LEL & UEL limits are   Gasoline 1.2 -- 7.1.  Fumes in the tank are much higher. 

 

Look here for more information.  The first listing has a chart for different gasses.   https://www.google.com/?gws_rd=ssl#q=lel+gasoline

 

 

Ok, What is the correct and safest way to install a replacement fuel sender with the tank still in the car w/- fuel ??

R

 

 

The safest way to install a fuel sending unit is disconnect the battery.

Edited January 31, 2016 by Larry Schramm (see edit history)

[Bhigdog]

An explosion should never occur because of a too rich vapor/air mix. Then again there is TWA flight 800.................Bob

[Stude Light]

Keep in mind that late model cars all have electric fuel pumps in the tanks.  You would be amazed at how much sparking goes on, especially from static electricity coming from the fuel return lines (before return-less fuel systems).  But as correctly stated above, the vapor in fuel tanks are way above the UEL of gasoline. 

 

Actually, the LEL and UEL limits for gasoline create a very narrow window of opportunity for combustion which is why we hardly have any accidents....think about how many vehicles, lawn mowers, generators, chain saws, etc. are fueled each day. In my lifetime around cars I have only known of one fuel tank explosion....it was inside a cold test chamber, immediately after the fuel pump in the tank was replaced and at -40 deg F the vapor in the tank didn't have enough time to saturate all the air that was brought in the tank due to the pump replace. The car was started and the tank went thoowump and split a seam above the liquid level so it didn't even leak. There was no sustained combustion.  It was rather uneventful - thank God.

 

Most folks think it is easy to ignite gasoline because their experience is with a flame that they move from a distance away from the liquid fuel, towards the fuel and always end up passing through the LEL/UEL zone.  Confine gasoline in a container and it's another story in trying to get the vapor mixture to ignite.  I recall a Mythbusters episode using a spark plug in a fuel tank with boring results.

 

So, don't worry about your fuel tank rheostat. Even if you shorted it out by applying full power directly across it, it would only boil the fuel.

Scott

[Rusty_OToole]

Ok, What is the correct and safest way to install a replacement fuel sender with the tank still in the car w/- fuel ??

R

First check if you can get at the sender from inside the trunk (common on older Chrysler products) or from under the car (some GM)

 

Run the gas tank empty or near empty. If the car is not in service drain the tank by syphon. Remove old sender, put in new one.

 

In most cases you will have to drop the tank to get at the sender. It should be empty, don't forget to disconnect the fuel line. Put a floor jack with a board under the tank, remove the straps and let it down a few inches. Disconnect the sender wire and remove tank.

 

Change the sender and put the tank back in.

Edited February 1, 2016 by Rusty_OToole (see edit history)

[JACK M]

Ok, What is the correct and safest way to install a replacement fuel sender with the tank still in the car w/- fuel ??

R

I am quite certain that on your Plymouth you can do it from under the car. Just run the tank down to a quarter or less.

[TerryB]

Interesting question on the fuel sending unit and the comments on the electric fuel pumps in the tank. I had to replace a failing electrical fuel pump in a 2002 Suzuki motorcycle I owned. I never really thought much about the mechanics of how it operated electrically in the tank or the action of the fuel sender gage that was part of the assembly. Everything was changed out properly and worked fine afterwards.

Considering the proximity of the tank with some of my important body parts I guess I should have at least given it a little more thought! I was just just happy my new $600 pump did what it was supposed to do.

Terry

[capngrog]

An explosion should never occur because of a too rich vapor/air mix. Then again there is TWA flight 800.................Bob

 

Bob,

 

It's interesting to note that the respondents to this thread understand that the airspace (ullage) in a fuel tank is almost always too rich to sustain combustion. 

 

  In their report of the TWA 800 crash, the NTSB (with whom I've been honored to work in the past) discuss at length possible ignition sources for the fuel in the "empty" Center Wing Tank (CWT) of the TWA B-747; however, there is no discussion of the quality of the vapor space within the CWT.  This was probably due to the fact that, in the past, both the FAA and manufacturers of transport category aircraft just assumed that aircraft fuel tanks continuously contain flammable fuel/air mixtures.  While this makes for conservative engineering, it is hardly a reflection of actual conditions within a fuel tank.  The bottom line is that a Flammability Exposure Analysis was never done of the conditions within the Center Wing Tank of TWA 800.  Apparently, the assumption was that the CWT volume was flammable and that no further study was needed.

 

Don't get me wrong, because, in my opinion, the NTSB is the best in the world at what they do.  It just seems to me that a significant component of the investigation into the loss of TWA 800 was not performed.

 

I apologize for the off-topic comments about TWA 800, but it is somewhat tied into the proven expertise of the participants in the various AACA Forums.

 

Cheers,

Grog

[cheezestaak2000]

just for fun, put a gallon of gas in a normal size bucket. lite a match and throw in in (while standing a couple feet away). 99 times out of 100, the gas will put out the flame. i know it sounds crazy, but i've done it.  also if you ever need to solder a gas tank, simply run a hose from the tailpipe to the gas filler and start the engine. after 4 or 5 minutes it will be perfectly safe to use a soldering iron on the tank.

[Larry Schramm]

One important item when removing the tank, DO NOT USE a trouble / drop light that has an exposed incandescent light bulb or even a compact fluorescent bulb that if broken can start a fire from spilled gas.

 

I know of a dealership that burned to the ground when a tech was using one of these while pulling a gas tank and dropped the light which cause a major fire and I think killed the tech.

[Bhigdog]

Grog. There was/is a lot of speculation on the cause of the CWT vapor ignition. Everything from faulty fuel probes to a missle strike. Prevailing speculation is that the hot AC packs and a near empty tank produced the correct mixture and an unknown ignition source started the explosion.

I guess the take away for us on the forum is that there is no such thing as a "safe" fuel tank, be it empty full or in between.

I remember a number of years back the was a news story of a couple of kids playing on a long abandoned fuel storage tank. Kids being kids one of them dropped a lighted match into the tank. The explosion killed both of them...........Bob

[Stude Light]

I would agree that any fuel tank that is opened to atmosphere, like while removing the sender assembly, can create the right fuel/air mixture during the servicing so take every precaution. Drained tanks are the worst.

The original post seemed concerned with normal operation in which case the tank will be over-rich so no concern from having something inside the tank causing an issue (pump or sender). But having a source of ignition near an open fuel filler may have bad consequences - typically sustained cumbustion at the filler.

[JACK M]

One important item when removing the tank, DO NOT USE a trouble / drop light that has an exposed incandescent light bulb or even a compact fluorescent bulb that if broken can start a fire from spilled gas.

 

I know of a dealership that burned to the ground when a tech was using one of these while pulling a gas tank and dropped the light which cause a major fire and I think killed the tech.

 

Larry, That is exactly how the Ford dealership in my town went away. Its been twenty years or so and everybody got out.

I wish I had my camera with me when I saw it. All the new cars in the showroom were toast. and every car in the service department as well.

It was an old building with HUGE wood beams and all wood construction from back in the day.

At the new dealership there was an impact wrench framed on the wall that was melted flat. Might have even been the culprit tool.