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Working on the diesel tank - a triumph for the theory


Keeping Up
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There was a drain tap sticking straight out by several inches at the bottom of my diesel tank. The problems were (1) that it was very vulnerable, being next to where I step down into the engine bay so I was frightened that one day I'd kick it and snap it off and (2) when opened, the diesel spurted straight out so it was hard to catch it without any spillage; however it was extremely convenient for letting out any water from the bottom of the tank (or just checking for it).

 

I asked at a couple of boatyards and they both said that yes it would be easy to swap the straight adaptor for a right-angled one, which would cure both problems; but the job could only be done by first draining all the diesel from the tank as otherwise too much would gush out in the short time between unscrewing the tap and then screwing in the adaptor and then the tap again. However, thinking hard about the problem, I calculated that a mere 5% reduction in the air pressure at the top of the tank should support a 2ft head of diesel, and that this could be achieved by sealing the breather and then draining enough diesel to increase the volume of air above it by 5%. I reckoned that starting from a reasonably full tank, it would need only about 1.5 litres of diesel from the sealed tank to give this result.

 

Today I found the courage to try it. I bound the breather up tightly with cling film and opened the tap (after removing the bung in the end) to fill an old milk carton with diesel. Sure enough, before the 2-litre bottle was full the flow of diesel from the tap ceased. I unscrewed the complete tap assembly, and replaced its straight adaptor with a right-angle one before replacing the tap (using PTFE tape to seal the threads).

 

In total I reckon that maybe just 5cc of diesel escaped from the hole, to land on my absorbent sheet below, and then the job was complete. The diesel in the bottle was clean apart from a tiny bit of crud that had washed out with it, so I carefully decanted it back into the tank then removed the cling film. Job done, in about 10 minutes.

 

SO once again it proves, never listen to the experts who tell you something can't be done!

Green thing for your post,most informative.

 

CT

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Exellent result! A technique regularly used by moi when replacing motorised valves on heating systems.

 

An enhancement I've often thought about but never tried is to attach a Pela pump to the vent and use that to ceate the slight vacuum needed, to avoid the draining of a litre or so of mucky fluid...

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There was a drain tap sticking straight out by several inches at the bottom of my diesel tank. The problems were (1) that it was very vulnerable, being next to where I step down into the engine bay so I was frightened that one day I'd kick it and snap it off and (2) when opened, the diesel spurted straight out so it was hard to catch it without any spillage; however it was extremely convenient for letting out any water from the bottom of the tank (or just checking for it).

 

I asked at a couple of boatyards and they both said that yes it would be easy to swap the straight adaptor for a right-angled one, which would cure both problems; but the job could only be done by first draining all the diesel from the tank as otherwise too much would gush out in the short time between unscrewing the tap and then screwing in the adaptor and then the tap again. However, thinking hard about the problem, I calculated that a mere 5% reduction in the air pressure at the top of the tank should support a 2ft head of diesel, and that this could be achieved by sealing the breather and then draining enough diesel to increase the volume of air above it by 5%. I reckoned that starting from a reasonably full tank, it would need only about 1.5 litres of diesel from the sealed tank to give this result.

 

Today I found the courage to try it. I bound the breather up tightly with cling film and opened the tap (after removing the bung in the end) to fill an old milk carton with diesel. Sure enough, before the 2-litre bottle was full the flow of diesel from the tap ceased. I unscrewed the complete tap assembly, and replaced its straight adaptor with a right-angle one before replacing the tap (using PTFE tape to seal the threads).

 

In total I reckon that maybe just 5cc of diesel escaped from the hole, to land on my absorbent sheet below, and then the job was complete. The diesel in the bottle was clean apart from a tiny bit of crud that had washed out with it, so I carefully decanted it back into the tank then removed the cling film. Job done, in about 10 minutes.

 

SO once again it proves, never listen to the experts who tell you something can't be done!

Another green thingy for bravery in facing the animosity of inanimate objects and winning!

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Exellent result! A technique regularly used by moi when replacing motorised valves on heating systems.

 

An enhancement I've often thought about but never tried is to attach a Pela pump to the vent and use that to ceate the slight vacuum needed, to avoid the draining of a litre or so of mucky fluid...

A Pela Pump?thought you would have deployed a Bicycle Pump with the Washer reversed?

 

CT

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No NRV. A Pela pump would just do the job with no feckin' about. It's not as if they are expensive compared to what I charge for my time wink.png

But he would have had to seal the pump to the diesel tank somehow, his sounded a much better way of doing things.

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Brillant idea! Thanky ou for sharing.

 

Only thing is, keep an eye on the threads as PTFE and diesel dont mix... ive seen it used on many an application and within a year they've failed/started failing (weeping).

Plus bits of it can come off and block an injector etc. It's usually suggested to use liquid fuel safe thread sealer.

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Plus bits of it can come off and block an injector etc. It's usually suggested to use liquid fuel safe thread sealer.

Let's hope the filters would catch it, although as it's on the drain tap I reckon it's more likely that it would just drop out when I next opened the tap. All the same I'll get me some of that sealer, can you point me towards it please?

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Let's hope the filters would catch it, although as it's on the drain tap I reckon it's more likely that it would just drop out when I next opened the tap. All the same I'll get me some of that sealer, can you point me towards it please?

It's nothing special, loads of them around, like this:

https://www.amazon.co.uk/Permatex-59214-Temperature-Thread-Sealant/dp/B0002UEOP0

 

Or this is probably a better choice: http://m.ebay.co.uk/itm/Heldite-Jointing-Compound-15ml-/111543635843?hash=item19f884cf83%3Ag%3AaToAAOSwkNZUhwn7&_trkparms=pageci%253A0214db25-f5f6-11e6-a531-74dbd18097bc%257Cparentrq%253A520549b915a0a606257d2556fff3bb86%257Ciid%253A2

 

Larger bottles of Heldite are available of course, and can be applied by brush.

Edited by WotEver
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There was a drain tap sticking straight out by several inches at the bottom of my diesel tank. The problems were (1) that it was very vulnerable, being next to where I step down into the engine bay so I was frightened that one day I'd kick it and snap it off and (2) when opened, the diesel spurted straight out so it was hard to catch it without any spillage; however it was extremely convenient for letting out any water from the bottom of the tank (or just checking for it).

 

I asked at a couple of boatyards and they both said that yes it would be easy to swap the straight adaptor for a right-angled one, which would cure both problems; but the job could only be done by first draining all the diesel from the tank as otherwise too much would gush out in the short time between unscrewing the tap and then screwing in the adaptor and then the tap again. However, thinking hard about the problem, I calculated that a mere 5% reduction in the air pressure at the top of the tank should support a 2ft head of diesel, and that this could be achieved by sealing the breather and then draining enough diesel to increase the volume of air above it by 5%. I reckoned that starting from a reasonably full tank, it would need only about 1.5 litres of diesel from the sealed tank to give this result.

 

Today I found the courage to try it. I bound the breather up tightly with cling film and opened the tap (after removing the bung in the end) to fill an old milk carton with diesel. Sure enough, before the 2-litre bottle was full the flow of diesel from the tap ceased. I unscrewed the complete tap assembly, and replaced its straight adaptor with a right-angle one before replacing the tap (using PTFE tape to seal the threads).

 

In total I reckon that maybe just 5cc of diesel escaped from the hole, to land on my absorbent sheet below, and then the job was complete. The diesel in the bottle was clean apart from a tiny bit of crud that had washed out with it, so I carefully decanted it back into the tank then removed the cling film. Job done, in about 10 minutes.

 

SO once again it proves, never listen to the experts who tell you something can't be done!

Thanks for posting this. I need to tackle a similar job, so your experience is very useful.

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Can you show us how you calculated the 5%?

 

A nice thread. An inch of water is 250 Pa, Atmospheric pressure is about 1000 mbar (millibars) or 105 Pa, or about 400 inches of water head. Two feet is about 1/20th of 400 inches, hence the 5% quoted above. So if the pressure in the air above the diesel is less than this, then this means that the pressure in the diesel at the bottom of the tank is less than atmospheric, so it will suck air in rather than let diesel out.

 

To reduce the pressure by 5% (at constant temperature) one has to increase the air volume above the diesel by 5% or so. That's obviously easier if the tank is full.

 

http://www.sengpielaudio.com/ConvPress.htm

 

(ETA: of course the density of diesel is a bit less than that of water, so atmospheric pressure is more than 400 inches of diesel head).

Edited by Scholar Gypsy
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Brillant idea! Thanky ou for sharing.

 

Only thing is, keep an eye on the threads as PTFE and diesel dont mix... ive seen it used on many an application and within a year they've failed/started failing (weeping).

 

Curious. When I was an OFTEC oil service technician PTFE was approved for use with oil. I never had any problems using it.

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Can you show us how you calculated the 5%?

Scholar Gypsy has described it precisely. It is of course essential that the tank is as full as possible to minimise the amount of air.

 

I was also aware that it was possible that an air bubble might enter the open hole and that then there would be a brief spurt of diesel, so I kept a finger over the hole while I readied the replacement adaptor.

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I would like the OP to answer!

The OP has already answered!

 

I initially got my original idea from the fact that one atmosphere will support about 30" of mercury whose density is at least 16 times that of diesel, hence it should support at least 40ft of diesel and 2ft is 5% of that.

 

When moderately well topped up my tank still has about 3" of air in the top, and 5% of that would be 0.15" - while the tank which holds 250 litres is actually 28" from top to bottom so that's about 10 litres per inch. Thus the 0.15" should be 1.5 litres, plus a bit for good luck.

Edited by Keeping Up
  • Greenie 1
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This also worked well when we had to replace our main diesel tap. We created a slight vacuum in the tank first using a pela style oil extractor connected to vent hose, then clamped vent hose using G clamp. Only lost about a small takeaway container worth.

 

Again, several boatyards told us it wouldn't work and we'd end up with 150L of diesel in the bilge. We also used ptfe tape but with some heldite as well. No sign of leaking for us.

 

Our main concern was if the tap we bought was the correct thread or not!

 

Tom

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Parp parp. A large rubber horn bulb, squeezed first and shoved onto the breather stub and then let go of would maintain a steady vacuum. Or get some sucker to encompass the breather stub with their mouth, ensuring a good leak proof lip embouchure around it, exhale first and then keep up a gentle but constant suck until the job is done. A reed instrument player ''saxophonist'' would be the best person to ask to perform this task, or indeed a harmonica player too, as they suck as well as blow on their instrument and should be able to maintain a good strong and lengthy vacuum. Kiss method. closedeyes.gif

Edited by bizzard
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We'll that's pretty conclusive. It's like "Mythbusters" in here, innit! Good job too. wink.png

 

 

Yes, and consistent with my own personal experience of using PTFE with fuel oil.

 

In fact I find it perplexing how whenever someone suggests using PTFE on a diesel joint, someone else pops up saying PTFE is not suitable for diesel, when it so demonstrably is.

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Yes, and consistent with my own personal experience of using PTFE with fuel oil.

 

In fact I find it perplexing how whenever someone suggests using PTFE on a diesel joint, someone else pops up saying PTFE is not suitable for diesel, when it so demonstrably is.

I saw it being advised against for fuel filter joints.

 

IIRC the reason wasn't that the tape was chemically incompatible.

 

It was that small pieces of the tape, dislodged by the act of tightening the fittings, might end up in the injector pump or injectors, doing them a mischief

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