ECR Posted January 29, 2020 Share Posted January 29, 2020 Just replacing a fuel pump and a question that has vexed me before has re surfaced. The pump is totally immersed in fuel (when the tank is full of course) and that means that the pump wiring is submerged in fuel (see the attached photo – the black and red wires are the positive feed and earth. Why doesn’t that blow fuses? As a supplementary question, the pump motor potentially produces sparks (if of a certain type) isn’t that a problem in a fuel tank? Perhaps the motor design is “none sparking”. Electrical gurus please advise .... . Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted January 29, 2020 Member Share Posted January 29, 2020 Why doesn’t that blow fuses?I presume that it's because the electrical conductance of the petrol is too low.As a supplementary question, the pump motor potentially produces sparks (if of a certain type) isn’t that a problem in a fuel tank?A couple of guesses. Because there isn't enough oxygen to ignite the fuel? Or because there isn't a spark if whatever causes it in air doesn't happen in petrol. I prefer the first as we don't hear about ignition when fuel pumps are run in tanks containing very little liquid petrol.JonathanPS: Inflammable oil is commonly used in transformers etc as a coolant. And IIRC the petrol does cool the pump in ordinary cars.PPS: Ordinary bullets don't cause car fuel tanks to explode, pace Hollywood (Mythbusters episode 15). And just checking cigarette butts flicked in to the filler... Link to comment Share on other sites More sharing options...
SM25T Posted January 29, 2020 Share Posted January 29, 2020 Petrol vapour is the biggest danger. 1 litre of petrol can produce around 400 litres of vapour. Link to comment Share on other sites More sharing options...
ECR Posted January 29, 2020 Author Share Posted January 29, 2020 Just found this which confirms your suggestion Jonathan" "Petrol is not a conductor of electricity. Many of the modern cars have electrically operated fuel pumps. These pumps are driven by dc motors. These motors are located inside the fuel tank, submerged in petrol. The commutator and brush gear are also submerged in petrol. If petrol was a conductor of electricity, all the segments of the commutator would get shorted and the pump would cease to operate. The pump works only because petrol does not conduct electricity." As to the second question, the commutator and brushes will be working in petrol vapour when the tank is nearly empty so does the electric motor driving the pump have commutator and brushes or is there another design of electric motor that does away with this spark source? Just found this which confirms that brushless dc motors do exist, perhaps this is the answer....BLDC = brushless DC "BLDC motors are more versatile, mainly because of their savvy in the speed and torque departments. They also come in compact packages, making them viable for a variety of compact designs. Typical apps include computer hard drives, mechanical-based media players, electronic-component cooling fans, cordless power tools, HVAC and refrigeration, industrial and manufacturing systems, and direct-drive turntables." And now I think of it, my Milwaukee 18VDC impact wrench is a brushless design Link to comment Share on other sites More sharing options...
bjw Posted January 29, 2020 Share Posted January 29, 2020 The range of petrol vapour concentrations that will burn is very limited - from ~1.5% by volume (LEL - lower explosive limit) to ~8% (UEL - upper). A closed tank with even a small amount of liquid petrol will have a very much higher vapour concentration than the UEL. It simply can't be ignited - by sparking from a motor or the wiper of a fuel gauge sender or anything else.It's different to an open container of petrol where a flammable concentration will exist at some point between the liquid surface and the air above. If that ignites the liquid itself doesn't burn - the heat of combustion vapourises the liquid which in turn reaches a combustible concentration, again at some small distance from the surface.Cheers Link to comment Share on other sites More sharing options...
anthonym Posted January 29, 2020 Share Posted January 29, 2020 a flammable concentration will exist at some point between the liquid surface and the air above.is this the danger in petrol stations when refuelling? Link to comment Share on other sites More sharing options...
bjw Posted January 29, 2020 Share Posted January 29, 2020 I suppose you could say so - petrol in a pump or an underground tank isn't very dangerous - it's just when it gets out !Cheers Link to comment Share on other sites More sharing options...
Wrightpayne Posted January 29, 2020 Share Posted January 29, 2020 Natural gas is only dangerous/explosive at a concentration of between 5 and 15 percent gas in air. Its funny what you remember from training courses 35 years ago! Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted January 29, 2020 Member Share Posted January 29, 2020 Did they reinforce that with a practical?Jonathan Link to comment Share on other sites More sharing options...
ECR Posted January 29, 2020 Author Share Posted January 29, 2020 I forgot about the sender .....So if an open fuel sender wiper is ok I guess a brushed motor is also Link to comment Share on other sites More sharing options...
Wrightpayne Posted January 30, 2020 Share Posted January 30, 2020 Jonathon,No practical but lots of shocking aftermath photographs. The carbon monoxide produced by a cigarette was interesting!Just triggered another memory - engineers called out to reports of domestic gas leaks will never ring the bell! Ding-dong-boom!Ian Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted January 30, 2020 Member Share Posted January 30, 2020 The carbon monoxide produced by a cigarette was interesting!Everything about carbon monoxide poisoning is interesting! I teach medical students about it but there's less than there was. Anyone like to guess how many deaths each year?JonathanPS: It's still a common plot line. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted January 30, 2020 Member Share Posted January 30, 2020 I forgot about the sender .....So if an open fuel sender wiper is ok I guess a brushed motor is alsoYes, same line of explanation.Jonathan Link to comment Share on other sites More sharing options...
Area Representative Piers300 Posted January 30, 2020 Area Representative Share Posted January 30, 2020 Brushless alternators and motors have been around for a long time. No sparks!In the generator business, which I was in for 40 years plus, we have been using them since I stated in 1970.Piers Link to comment Share on other sites More sharing options...
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