Oil burner Pump" Lift"

[Hube]

Does anyone know how many feet an residential oil burner's pump will actually "lift" with a single 3/8" (supply) feed line of approx 25 feet in length?

Beckett Afg burner .065 nozzle,# 2 oil, Danfoss fuel unit ,3450 rpm, suction port 1/4" nptf,

[LazyPup]

There are far too many variables to offer a definitive answer but I would estimate that it probably would be much less than 25ft.

While oil is slightly lighter than water for the purposes of explanation I will use water, given that i have water specs handy and would have to research oil.

The absolute maximum that water can be lifted by means of suction or vacuum is 32.76ft.

If you have an open vessel (or a vessel which is vented to atmospheric pressure) an you attach a vetical column to that vessel, then create a negative pressure in the column the atmospheric pressure pushing down on the surface of the liquid will push the liquid up the column until the static head (physical weight) of the liquid equals atmospheric pressure.

Standard atmospheric pressure at sealevel is 14psi. If the vertical riser were to have a perfect vacuum (-14psi) water would rise in the tube until the static weight of the water equaled 14lbs. The static head of water is 0.434 lbs/vertical ft.

Therefore the absolute maximum water could rise would be:

14psi/ 0.434/lb= 32.76 ft.

While 32.76ft is the theoretical maximum at sealevel, we must also consider that atmospheric pressure drops with increase in altitude or changes in weather conditions and as the atmospheric pressure drops the static head drops proportionally.

The next parameter we must consider is the physical resistance of the pipe or line. Pipe specifications will show the physical linear resistance of the line in lbs/ft. You must compute the length of the line to find the total resistance for the line and deduct that pressure from the vertical column. Fittings on a line offer a substantial increase in resistance, by example a 90 would have the same resistance as an additional 10 feet of line. A 45 would be in the order of 6 ft of line. As you can see, if one were to erroneously attempt to use two 45's to make a radius fitting, it would actually increase the resistance as you would have to allow 6 ft for each 45 deg fitting for a combined resistance of another 12ft as opposed to 10 ft for the 90.

The next problem you encounter is "Pump Cavitation". We are all aware of the fact that water boils at 212degF, but that is an incomplete statement. Water actually boils at 212degF at standard atmospheric pressure at sealevel. As the pressure is decreased the boiling temperature of a liquid decreases dramatically. In a vacuum (-14psi or -29.9in/hg) water boils at 40degF. That is why refrigeration techs pull a vacuum on a system to dehydrate it. As the pressure drops in the system any residual water will boil and be drawn out at standard room temp. Once the vacuum reaches 29.9in/hg is can be assumed all remaining water has boiled out of the system at room temperature.)
In a pump, as the pressure drops below the boiling pressure and temperature for the liquid, the liquid in the impeller section flashes to a gasseous state (steam) and the impellar is no longer able to push the liquid. For that reason pump specifications will often list a minimum static head pressure, which would mean the minimum pressure of the liquid at the inlet of the pump.

For illustration purposes let us assume the minimum "inlet static head" was listed as 10psi. Water weighs 2.34lb/ft vertical, therefore 10psi would be:

10psi/ 2.34psi/vert = 4.27ft.

We would then have to deduct the inlet pressure from the static head pressure of the vertical line.

With zero line resistance the static head at standard atmospheric pressure at sealevel was 32.76 ft, therefore the maximum elevation of the pump would be:

32.76ft - 4.27ft = 28.49ft to prevent cavitation.

Oil is slightly lighter than water therefore the static head of oil would be slightly higher but this would probably be only a fraction of a foot difference, and in no case should a pump be expected to work efficiently at or above 80% of the static head compensated for line resistance.

Also keep in mind that the line resistance is for new, clean pipe. As residual debris collects on the pipe wall line resistance will go up dramatically.

[Hube]

lazyPup; thanks for the very informative reply.

I do know these pumps (fuel units) will only lift anywhere from 2'-8' maximum range ,depending on the length of the line (head)from tank to burner. Mine is 25ft.... Some manufacturers will sometimes give a lift for a certain head, but I cant seem to obtain a listing for the Danfoss fuel unit
But I do know that the maximum lift for this type of fuel unit is only 8 ft. at a maximum head. But as of yet I cannot find that calculation .
The one calculation I do have from Danfoss is; Line length in feet including vertical and horizontal length; as shown below.
H = Head in feet. (Approx 25 ft)
Q = Firing rate (nozzle size) in GPH (.65 gpm)

3/8" line length = 6- 0.075 H divided by 0.0086 Q = feet
I cannot fathom this calculation,[:I] can you?

Thanks again for any help.

[LazyPup]

I think we may be confusing Static Lift for Dynamic lift.

Static lift is the height a liquid may be raised if the pump is physically located above the top level of the liquid, such as a water shallow well pump where the liquid is being sucked into the pump.

Dynamic lift is the maximum height a liquid can be raised by the discharge pressure of the pump.

To compute maximum DYNAMIC LIFT you deduct the static head of vertical rise and the total line resistance from the discharge pressure of the pump. By example, if you had a well that was 100ft deep and you required 30psi line pressure at the house you would first compute the static head, 100ft x ,434lbs/ft = 43.4lbs.

With a static head of 43.4lbs and a desired pressure of 30lbs the pump discharge pressure would have to be 73.4psi plus whatever the actual line resistance is for 100 ft of line.

In this instance the atmospheric pressure is the same at the top of the liquid as it is at the point of discharge, therefore atmospheric pressure is in equilibrium and negates itself.

IN the case of residential oil burners, the pump is normally mounted on or very near the burner unit and well below the level of the oil in the service tank therefore the dynamic head would only be the distance from the pump to the burner nozzle which should place it well below the 8ft max.

[HayZee518]

I read on a site that uses a danfoss fuel pump most installations are gravity feed. If the pump distance is more than ten feet vertical then a two stage pump with a return line is necessary. This is considering a 3/8" feed line.

[LazyPup]

Hazee is absolutely correct.

When a pump is installed below the top level of the liquid in the storage tank the inlet pressure at the pump is equal to the static head from the inlet of the pump to the top surface of the liquid. Thus the inlet pressure may vary greatly between a full tank and one that is nearly empty.

Some pumps have a method of regulating either the inlet pressure or the discharge pressure to keep the Dynamic pressure equal. In instances where there is no regulation of inlet or discharge pressure the dynamic pressure (discharge pressure) of the pump will be the storage vessel static head plus the mechanical advantage of the pump.

In those instances it is best to design a pump system that will supply full DYNAMIC PRESSURE to the demand when the storage vessel STATIC HEAD is minimal (when the tank is nearly empty)

You then install a metering device (an orifice) at the demand to regulate the amount of flow that will go to the demand and the excess is diverted to a return line where it will go back to the storage vessel.

[Hube]

Hayzee518;
My situation is this;
tank's BOTTOM is 2 ft lower than the burner's centerline. Tank is 4 ft high overall. Tank is approx 35ft from burner.

This means that when the tank is 1/2 full, the oil level is EVEN with the burner. This means that the flow from being Full to 1/2 full is more-less a gravity flow, assisted by the burner's pump. When the level of oil goes down to lower than a 1/2 tank, there will be no gravity assisted flow ,and the pump will have to "lift" any remaining oil.
The question is; will the pump lift this remaining 2 ft of oil ?
We have never let the level of oil get any lower than half at any time because of this unknown "lift" limit.
You suggest the lift is 10 ft, I thought maybe 8 ft.
Hopefully if this is the answer I have nothing to worry about when the oil gets down to less than a half.

Thanks.

[LazyPup]

Hube..

the Syphon principal states that a liquid will maintain the same level at all points in a vessel or the associated piping thus you will still have static head flow to the lift riser at the pump even when the tank is nearly empty.

You state that the pump is two feet higher than the bottom of the vessel therefore your maximum "Pump Lift" in the worst case scenario, a nearly empty tank, would be only 2' plus the line resistance of that 2' of line.

[Hube]

thanks lazy pup for the info.
I think i've got this "syphon effect" principal doing it's thing, and with the added suction of the pump it should be ok, especially with the low line resistance.

And just to ease my mind, I'm going to let the tank get down to the last gallon or so to prove it.
Thanks again.

[LazyPup]

If line resistance were to prove a problem you can overcome that by increasing the line diameter to 5/16 or 1/2"

[Hube]

Lazy pup. The feed line is already a 3/8" copper line, within a 1" abs conduit that is run under the poured concrete basement floor.
I think the line is quite adequate, no change necessary.[^](Thank God)
Thanks for the advice anyway.

[HayZee518]

OK - try this. Go and get a 1/2" horizontal check valve. Bush the ends so it will accept 3/8" flare fittings which is what the oil burner line should have. Install it right at the tank filter and point the arrow towards the burner (away from the tank) In essence when the tank is full or 3/4 full the flow will be gravity feed. When it's at 1/2 or below the pump will have to suck the fuel out. When the burner and pump shut down the check valve will slam closed, keeping the inlet line feed full of fuel so the pump won't have to work as hard to prime and pump. Most pumps are gear pumps anyways.