Plantoil/diesel conversion basics
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PabloCasa

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Reply with quote  #16 
http://pexsupply.com/CategoryPre.asp?cID=310&brandid=This page has 4 examples.  I think you're thinking of popoff valves which are notorious for leaking.  But there may be instances of these devices failing, too.  But they work in much higher pressure systems with similar 190-210F fluid. 

The reasons I don't like the idea of a return line to the tank are:
1. it would need to be heated or it would clog up in winter (Montana resident),
2. it's more complicated - more fuel line and another port in the tank.

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danalinscott

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Reply with quote  #17 
Quote:
The reasons I don't like the idea of a return line to the tank are:
1. it would need to be heated or it would clog up in winter (Montana resident),
2. it's more complicated - more fuel line and another port in the tank.


1. Not very hard or expensive to do.

2. Not very complicated.

The reason I like it is it eliminates the most common problem people with looped VO returns have...air in the fuel line.

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danalinscott

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Reply with quote  #18 
Quote:
I had spoken to some greasers recently who had converted an international schoolbus. Their experience was that when they tried to loop their system, they kept blowing hoses or return lines, supposedly at low RPM's. They speculated that mercedes and smaller diesels could get away with this because the volume of fuel sent to the IP constantly was much less, as even at wide open throttle, these small engines demanded much less diesel. They reasoned that with the larger engines came larger lift pumps and fuel flow rates, and thus were less tolerant of being looped. It sounds reasonable in my head, but honestly I don't know enough about fuel systems to know whether or not it was a reason for their problems.

My concern with the loop is that it eliminates the flowing pattern of the fuel system. Instead of flowing tank > lift pump > IP > tank in a continuous motion, the "loop" is basically just teeing the return line back into the supply line before the filter. Doesn't this then create a pressurized "lasso" of fuel, where the return and supply sides are both equal pressure?


Yes..but its' pressure is only what the lift pump provides (if there is not a combination lift pump/IP like VWs have) Blown hoses usually indicate the loop is blocked somewhere though you do have to have to deal with a small amount of pressure (in most cases) in the return fuel lines.  So the (often) loosely fit return lines and "jumper lines" need to be replaced with tighter fitting hoses and secured with hose clamps or zip-ties.

Full returns are mainly a way to avoid problems with any air leaks into the system in a simple and "invisible" manner.


Quote:
Hard to explain how I have it in my head, but I'll try:

If there was a water tower with a pump and hose attached to it, and we turned it on, water would flow out the hose and onto the ground. If we then put the end of the hose back into the tank and turned on the pump, the water would cirulate through the hose, being retunred into the tank. But if we cut the hose halfway down and inserted a T, and hooked the end of the hose into this, the water cease to circulate, the pump would simply be creating pressure because there is nowhere else for it to go, exactly as if you had capped the end of the hose. Is this basically what's happening when you loop a return, you just create a pressure zone that the IP can draw off of, instead of adequately circulating fuel?


What reasons other than avoiding problems with air bubbles do you think are avoided with full return to tank? The fuel still circulates in the loop back to the IP.

Quote:
What are the disadvantages of doing a full return to tank system? Obviously there will be heat loss because all oil going to the IP will have to be heated, but if I'm running a heated pickup, HOH, FPHE, coolant wrapped filter, and line heaters in 90 degree temps, shouldn't I be able to circulate all the oil I want, and still have it be hot enough?


Maybe...but you will also be heating the VO in your tank much hotter than is desirable.

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TheLandYacht

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Reply with quote  #19 
Quote:
Originally Posted by danalinscott
Quote:
What are the disadvantages of doing a full return to tank system? Obviously there will be heat loss because all oil going to the IP will have to be heated, but if I'm running a heated pickup, HOH, FPHE, coolant wrapped filter, and line heaters in 90 degree temps, shouldn't I be able to circulate all the oil I want, and still have it be hot enough?
Maybe...but you will also be heating the VO in your tank much hotter than is desirable.


Can you tell me what the max desirable VO temperature in the tank is, and would that be exceeded after the "hot" fuel was returned to a tank at the rear of a large van (a fairly long travel path, right?)

Thank you in advance
danalinscott

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Reply with quote  #20 
Quote:
Can you tell me what the max desirable VO temperature in the tank is, and would that be exceeded after the "hot" fuel was returned to a tank at the rear of a large van (a fairly long travel path, right?)


I prefer to keep VO tank temps to under 100°F. This is plenty warm enough for even high fat/hydrogenated wvo to be in a flow-able state.

Since most VO fuel lines need to be heated to allow initial flow..and this includes return lines...a long return path would not in itself significantly cool the returning VO. But...since return flow varies significantly from one type of diesel engine to then next there is really no stock answer for this. On some the return flow is so low a full VO return to tank may not significantly raise VO tank temps even on very long trips.  On others...it can.

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danalinscott

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Reply with quote  #21 

Quote:
Originally Posted by crdpoker

I was wondering what size orifice you recommend.

As small as is practical.
1/64th is usually sufficient but it it tricky drilling through thick metal with such a small diameter drill bit. If you have thick metal to drill though for this I suggest drilling a 1/8" pilot most of the way through and only drilling the last 1/6" with a 1/64" bit.

Quote:
I assume you would install it immediately after the IP return, in an inline T, at a high point in the system, with the one side of the T pointing upwards with the orifice mounted in at the high point. This would allow any air in the system to bleed out, and it looks like the amount of oil that would pass through the orifice would be minimal (if any, depending on the size of the orifice). Is the amount of oil that would pass through really worth plumbing a return to tank line under the truck, or could it just flow into a small under-hood container that I could empty occasionally back into my WVO tank?


Either option should work OK.





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crazycronshey

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Reply with quote  #22 

If I am understanding correctly, the pressure in the system prevents air from entering the partial return,  then wouldn't it keep it from entering a leak. And wouldn't that leak as mentioned above present itself while engine is running? thnx.

danalinscott

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Reply with quote  #23 
Quote:
Originally Posted by crazycronshey

If I am understanding correctly, the pressure in the system prevents air from entering the partial return,  then wouldn't it keep it from entering a leak. And wouldn't that leak as mentioned above present itself while engine is running? thnx.

Fuel systems (or sections of fuel systems) which are under positive pressure would tend to leak fuel outward. This is not true of fuel systems (or sections of fuel systems) which are under negative pressure (vacuum).

Air leaks into a fuel system are particularly vexing since they may only occur intermittently. This is because the leak may not occur until the leak point reaches a certain temperature (in heated line), the VO fuel is particularly thick due to low ambient temperature, or vacuum rises due to a partially clogged filter...or a combination of the above. 

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DoubleNickel

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Reply with quote  #24 
Hi:  I have a question.  Is there any pressure in the return line to the fuel tank where it has to escape through the vent in the fuel cap.  I have been posting here about my diesel engine backhoe.

http://www.heavyequipmentforums.com/Forum/showthread.php?t=12481


If someone could read my posts and get to understand my problem, I would appreciate it.  I think I have it resolved but not for sure.

Any input would be appreciated.

Sincerely,

DoubleNickel
danalinscott

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Reply with quote  #25 
Short answer.
No.

If you have problems with a VO conversion I suggest posting them with as much detail as possible in the Huston, I have a problem section.

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food_yum

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Reply with quote  #26 
Could someone post either a link to one of those needle valves or post a photo of one?  The only one I can find either in local shops or at Home Depot is this:
http://www.homedepot.ca/webapp/wcs/stores/servlet/SearchView?catalogId=10051&storeId=10051&langId=-15&N=0&Ntt=needle+valve&Nty=1&D=needle+valve&Ntx=mode+matchallpartial&Dx=mode+matchallpartial&s=true
(geez, that's a big URL!  Sorry about that.  And I see the U.S. branch doesn't have it on the web, just the Canadian stores; I wonder why?)
I've never used these things, but the salesman figures that model would just collapse the fuel line I'm "T"ing into, as it is actually made for rigid copper pipe.  Am I to use this on a hard fitting, or on a hose?  I'm sure there is some clever work-around that is so obvious I'm missing it, but to cut down on all the time I'm spending already, I thought I'd see what those with experience have used.
Any photos of real-world applications would be great!  (Or just clarify how to use this one needle valve I HAVE located.)\

Another idea I had was to use a universal radiator drain cock, which I could open just a tiny bit, set in place with some silicone or other sealant, and attach a 5/16 hose onto the other end, running back to veg tank.  The one I've spotted fits a 5/16 nicely on the drain side, although it isn't truly barbed.  (Thus another potential screw-up spot; but I figure it won't be under much pressure there, so shouldn't be an issue.)

Thanks gang
marc

danalinscott

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Reply with quote  #27 

The type of needle valve you want for a limited return is a "pass through" rather than a bleed type.

Here's a pic and a cutaway schematic of one.



Just add a short section of tube on each side and adding this after a tee to the loop circuit you can limit the amount of wvo returned to the tank.  If you add the valve at the top of a downward section of the return loop you can capture any air bubbles that would otherwise circulate in the loop and send them to the tank.

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ytk

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Reply with quote  #28 
Does this look like a good simple needle valve already with the barbed fittings? 
http://www.ebay.com/itm/NEW-GAS-AIR-LPG-3-way-hose-Connector-BRASS-needle-valve-/261271397001?pt=UK_Campervan_Caravan_Accessories&hash=item3cd4fd2289
danalinscott

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Reply with quote  #29 
It might work if mounted properly. I can't say for sure as I havent used a pass through type needle valve for this.
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