Smoothing Out Pressure Pulsation of a Diaphragm Fuel Pump

Discussion in 'Furnaces and their construction' started by Melterskelter, Mar 20, 2019.

  1. Melterskelter

    Melterskelter Gold Banner Member

    I have been using a generic 12V automotive fuel pump to provide 8 to 9 pounds of diesel fuel pressure to my Hago Siphon Burner. I have never like the pulsation of the fuel pressure as I felt that if I had the furnace tuned for an average rate of fuel delivery over time, because there is significant pulsation of the fuel, there must be a portion of the time when more than desirable fuel is instantaneously delivered and also a period of time within each pulse when too little is delivered. And I could hear the furnace pulsating in time with the pump frequency which lead me to believe there might be some validity to my concern. Figuring the fix would be pretty easy I went ahead with making a an air-over-fuel accumulator.

    FurnaceFuelAccumulator.JPG

    Here is the fuel gauge pre-accumulator in slo-mo


    Here is the gauge post accumulator in slo-mo


    And as a surprise the furnace was 5 dB quieter. That I can not fully explain. But, I like it. As hoped the furnace does not pulse in sync with the pump.

    Denis
     
  2. PatJ

    PatJ Silver

    I get a significant low frequency rumble when my furnace is running, but it does not really pulsate like I see some furnaces doing, but all oil burning furnaces seem to make the low frequency rumble.
    One thing that seemed to help make my furnace significantly more quiet is when I added pieces of rubber walk pad under each wheel.
    This seemed to break the coupling of the low frequency that was being transmitted through the ground.

    You could probably check this by forcing a rod into the ground, and putting the dB meter microphone hard up against the rod, with the furnace wheels on and off rubber pads.

    But just like the V-1 rocket engine, any changes in the fuel flow rate is going to create sound pressure differences inside and outside the furnace in addition to the rumble that is created when a constant fuel tank pressure is used.

    I have a high frequency sound being made by the leaf blower.

     
  3. Petee716

    Petee716 Gold Banner Member

    It occurred to me watching some of your more recent videos how quietly your furnace runs as you open and close your lid. Your modifications really seem to have helped a lot. But as I watched this I couldn't help thinking that although the roar from your furnace may have been audible to your neighbors, it may have been the pulsing that sealed the deal.

    You referred to this as an air-over-fuel accumulator. Could you explain the "air" part? I assume that the pipe would somehow maintain a column of air inside but I don't see how. Is the back pressure from the hago enough?

    Pete
     
  4. Great fix!

    A longer accumulator would help reduce what fluctuation you still have, but it sure looks good.

    Do you drain the accumulator occasionally? Might be nice to have a vend valve on the top cap to facilitate draining and replace any air which has left with the fuel.

    Pete: He labeled the in and out connections. It's just an empty pipe and the fluid compresses the air as it builds up pressure, back pressure from the Hago, as you said.
     
  5. PatJ

    PatJ Silver

    Its the old plumbing surge tank trick; they work great with water to stop water hammer, and I would assume with fuel too as long as you have some air in the upper part of the chamber to compress.

    Electrical equivalent is a capacitor.

    .
     
  6. Melterskelter

    Melterskelter Gold Banner Member

    I think the miniscule residual fluctuation is completely insignificant so the 6” pipe is adequate. Agree, Longer or greater diameter pipe would cause more dampening but would be bulkier for questionable practical gain.

    It self-drains at the end of each session.

    Pete, the vertical pipe remains about 2/3 full of air as fuel pressure is 8 pounds and ambient air pressure is 15. The air column acts like a spring to absorb the impulses of pressure.

    The action of the accumulator is analogous to placing a large capacitor in a DC electrical circuit. I wish I had videod the pressure smoothly increasing from 0 to 8 pounds over a few seconds time with the accumulator vs the immediate pressure pulsation that occurs without it—-just as would occur in a “noisy” DC electrical line.

    Denis
     
  7. Zapins

    Zapins Gold

    Theres a lot here that is new info to me so forgive my noobie questions about what's going on.

    The 12V automotive fuel pump you have. Is that basically just a pump that sucks up fuel from a container and pushes it out the other tube? Like an aquarium power head but for fuel?

    How do you control the amount of pressure entering the siphon nozzle? I have that same type and haven't been able to get it working. Does the small orifice of the siphon nozzle choke down the psi to manageable levels? Do you use needle valves to adjust?

    So the pipe bomb like thing in your photo traps air in the top and fuel sort of sits inside the bottom? How do you ensure the air stays there and doesn't dissolve into the fuel and get filled up? Also, the function of that pipe is to make sure the pressure is supplied evenly to the burner nozzle right?
     
  8. Melterskelter

    Melterskelter Gold Banner Member

    Pipe bomb, Huh?! ;-) I agree with you and I know that is the first thing that comes to mind. I am hopeful that this accumulator will never make a kaboom. I think that is most unlikely.

    But the rest of it you got right.

    The pump is just a generic fuel pump intended for cars or trucks and used to supply fuel to carburetors. It is needed to get the fuel from the tank of a vehicle to the carb as the tanks typically sit lower than the engine. It only supplies 8 pounds of pressure---just enough to fill the float bowl. Injected engines require much higher pressure pumps which are also far more expensive. I use it to boost the amount of fuel I can deliver to my furnace as siphoning alone will only suck up about 1.5 gal per hour with the fuel line wide open and the fuel tank level with the furnace. By increasing the pressure I can deliver about 2.5 to almost 3 gal per hour. The same thing could be accomplished by raising the tank ten or twelve feet (I forget the actual amount) in the air. That solution is cheap, pulsation free, and simple. But it is also very inconvenient for me. The pump cost only around ten to fifteen dollars and is dead simple to use. In practice I run my needle valve wide open always though I use it as a shut off when turning off the furnace. A simpler on/off valve could have been used.

    I wish the pipe bomb were transparent. Then you could see that when the system is off the entire pipe is filled with air. But when I turn on the pump the fuel needle valve at the furnace is off so the only place for fuel to go is into the bomb. It does so until the air pressure is equal to the pump pressure---8 pounds. At that point the pipe is about 1/3 to 1/2 full of fuel with air sitting on top of the fuel. When I turn on the fuel flow to the nozzle, the pump has no trouble keeping up with the fuel consumed by the furnace so the air/fuel level is essentially unchanging throughout the burn. In theory if that condition went on for a long time, the air could dissolve slowly into the fuel and eventually the pipe would be full of fuel and the dampening effect would be gone. In practice air dissolves very slowly into the fuel and in the hour or two of operation the air level does not vary appreciably and dampeneing of pulsation is maintained.

    dampening of the fuel pulsation is probably not a real high priority feature. But, I like it and I like the reduced furnace noise level. I think the average furnace would function just fine without it. Mine did. I just did this because I was making a more convenient mount/carrying setup for my fuel pump and fuel lines and adding in the bomb took very little additional time and money. It is a nicety but far from a necessity.

    I wish you could come visit and see how I use my siphon. I am sure yours will work nicely, too. Like a lot of things, a second set of eyes can help uncover the not so obvious.

    Denis
     

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