enginameers is this true, yes / no

afterburn 549 · 06:47 PM

It is true a larger diameter water column equals more weight and thus more suction required on the vertical. However, we are usually siphoning more than drawing vertically as the pump inlet is at the bottom of below the bottom of the tank. (Depending on the specific install)

We do see a point of diminishing returns with regards to draw straw diameter. We have built custom units in the 3/4" and larger range and they work with varying results. 5/8" is about the practical maximum with really large pumps.

So, using the siphon as the general rule, it takes less suction to do the same job as gravity helps us draw the fuel by using the weight of the fuel. Yes, if the system was bone dry, it would force a longer draw, but that's not the typical case with these trucks.

The bottom line is the factory restrictive modules makes for far more work for the pump than does a larger inlet fuel line.

I am still not in favor of 1/2" lines on the pressure side of the pump as I am an old time drag racer and know the effects of acceleration on that larger fuel diameter, not to mention the fuel has to pass through the restrictive 12mm fuel filter and or VP inlet anyway.

But one thing is for certain, if you have a restrictive module and you are consuming enough fuel, you WILL make the pump work harder and it can and will create aeration/ cavitation in the fuel from the pressure drop. With regards to the non bypass setups, stock, Airtex, my 12V lift pump, Raptor, HPFP or whatever, inlet restrictions are a very real cause of this issue. And you can still have full fuel pressure while pumping mostly air. I have proven it in my own rig.

I am not telling you to buy a Draw Straw. I am telling you that you need to be sure you are fulfilling all the requirements of the complete fuel system. It starts in the filler neck and ends at the injector tip.

Dave. End of said Quote.

This is about a dodge Diesel pick up.
I say people are adding loads of work to the fuel pump (electric) when they go a bigger draw tube in the tank..........gets all emotional over there @ DTR,

unclebilly · unclebilly's Garage

For static pressure, column diameter makes no difference. Pressure = density * gravity * depth.

If you start circulating fluid, then friction pressure becomes a factor - this is where column (pipe) diameter becomes a factor.

ruf-porsche · ruf-porsche's Garage

Sounds like Fluid Mechanic.

afterburn 549 · 01-15-2009, 06:48 PM

Yes, but to draw it out of a tube....... the larger the harder right ?

Danimal16 · 01-15-2009, 08:06 PM

Head loss is not a algebraic relationship to pipe diameter. I need to find my King's handbook.

Head is head it is based upon the height of the column. The diameter of the tank would only give you the rate of which you would draw that water surface down, in other words how long you could sustain that column or fluid surface.

As far as cavitation, and the hydraulics guys would need to look at the system geometry, it depends on how far down the line from the change in direction or constriction in the line before the flow returns to laminar, if at all. This could assist with the pump impeller not cavetating. Placement of the pump could impact its efficiency.

BK911 · BK911's Garage

Quote:

Originally Posted by afterburn 549

Yes, but to draw it out of a tube....... the larger the harder right ?

Not really. A pumps energy usage increases with flow rate. If the flow rate is the same, then the energy increases with pressure drop. If this is a fuel pump with a constant flow, then the smaller the straw the more energy required to pump the fuel.

afterburn 549 · 01-15-2009, 10:25 PM

OK , I base my little knowledge on this example= One time a old friend had a well that would not pump...we tried everything...did I say every thing ??
So we went to or genius friend about the problem, he said take a smaller pipe cap both ends and drop it dwn the bigger pipe (ie making the dia smaller )
It fixed the problem !!!

911Rob · 911Rob's Garage

Let me ask my wife for you; she's the expert at drawing fluid from the large tube

afterburn 549 · 01-15-2009, 10:44 PM

Czars do not count

turbo6bar · 01-16-2009, 05:32 AM

Suction head for a pump is expressed in feet. Never mentions anything about pipe diameter. I think the only time the pump would suffer is during priming. The pump would have to lift more water with a larger pipe and would take longer to start pumping. If the pump isn't self-priming, I think you could have issues.

FWIW, I finally went to a mechanical fuel pump driven by the crankshaft on my Dodge.It and a Racor heated filter are my current favorite. The Racor heater has allowed biodiesel to be run later in the season without power loss.

sammyg2 · 01-16-2009, 07:33 AM

No.
pounds per square inch remain the same no matter what the diameter of the pipe.
Also, said pumps are positive displacement, not centrifugal. Cavitation is not an issue. Neither is aeration after the restrictive filter because the filter is not a true orifice unless it is nearly completely clogged or is grossly undersized. The restriction of flow is after the filter, whether it be the float valve on a carb or the injectors.

Pumps are rated several ways, one of which is NPSHR. Net positive suction head required and it is absolute, not gauge.
That means given a certain flow rate, the absolute pressure at the suction eye must be above minimum or the pump will lose efficiency. The diameter of the suction tube is not an issue, unless it is TOO SMALL and restricts the flow. A larger suction pipe or tube is advantageous in nearly all pump applications.

It is true that on a positive displacement pump, more restriction on either the suction or discharge equates to more amp draw. Flow is not a factor in load on a positive displacement pump. The pump is designed to move a given volume per stroke. No matter what. Most of these pumps have a relieve valve or pressure regulating valve that spills back excess volume ot hte tank so regardless of how much fuel the engine is using, the pump doesn't move any more fuel or build any more pressure.

Centrifugal pumps are a different animal, pump curves show that as differential pressure from suction to discharge increases, flow decreases and amps decrease. As the differential pressure across the pump decreases, flow increases and amps increase. Again, automotive fuel pumps are typically NOT centrifugal in design, especially when used in a fuel injection application.

Danimal16 · 01-16-2009, 07:37 AM

Sammy,

Are you a mechanical guy?

sammyg2 · 01-16-2009, 08:02 AM

Yep, mechanical engineer specializing in pump application and reliability. 3 decades worth

Danimal16 · 01-16-2009, 08:08 AM

Mine was in civil, big difference. I gotta bone up on positive displacement pumps.

Dottore · 01-16-2009, 08:28 AM

Quote:

Originally Posted by Danimal16

Head is head...

You'll get no argument from me there...

sammyg2 · 01-16-2009, 08:31 AM

Quote:

Originally Posted by Danimal16

Mine was in civil, big difference. I gotta bone up on positive displacement pumps.

Everyone has a specialty. I know pumps but would be lost trying to figure a CE or EE question.

afterburn 549 · 01-16-2009, 09:12 AM

OK, for re dunce sake - the pump I am talking about is 12 volts - vane operated and has to suck or create a vacuum to deliver to a much bigger pump.
It has to lift about 4 feet to the top of Eng over a say 10 foot span
On a good day they deliver 9 to 15 lbs pressure.
The big pump delivers excess back to tank.
To my brain It says running a 5/8 hose into the tank or a 3/4 or a 1 inch will change nothing except make it harder on the pump to "draw" diesel fuel.
True, if the size is reduced, at some pt it will get difficult also

sammyg2 · 10:17 AM

I guess I could see how that might make sense, but it is without merit from a physics standpoint.

At sea level we have about 14.7 pounds per square inch pushing down on us, on the stuff around us, and on the fuel.
The vane pump (positive displacement) creates a lower than atmospheric pressure in the suction line.
If the pressure in the suction line is lower that 14.7 absolute (zero gauge), then the diesel fuel will be pushed up into the suction pipe in an effort to equalize. Higher pressure always wants to go to lower pressure.
If the pressure in the suction pipe is low enough, the fuel will be pushed all the way up to the pump where it will be pressurized and sent on it's way. We call that suction but there is no thing really, it's a layman's term used to describe head pressure below atmospheric. If the pressure at the suction of the pump is not low enough for the atmosphere to push the fuel all the way up to the pump it will not take suction. The ability of a pump to be able to create sufficient suction determines if it needs to be primed or not and that is inherent in the particular design.
If I understand your description, the suction line is 4 feet long and the pump has to create 4 feet of lift. That is not hard for a vane pump at all unless it is completely worn out and the internal clearances are shot.

The head pressure of water at sea level is about 33 feet or so, haven't looked that one up in a while. Might be 34 feet.
So if a pump was perfect and could create absolute vacuum (which they can't), and it could lift a column of water 33 feet. The most efficient pumps at doing that I work with coincidentally are vane pumps in vacuum service (again, not a scientific term but slang). Diesel fuel has a specific gravity lower than water but pump engineers always use water as a measure of performance characteristics and then divide by specific gravity for actual instead of theoretical. Don't ask me why, we just do. I guess it's easier to teach the students that way.

Pumps can build up enormous discharge pressure so they can push a column of liquid much higher than they can pull it. Similar to a Porsche engine (wink wink). A normally aspirated engine is reliant on atmospheric pressure to "push" air into the cylinders on the suction stroke. A turbo doesn't, because it can build positive gauge pressure which is cumulative with atmospheric. 1 bar of boost is actually two atmospheres of pressure or almost 30 PSIA.

You seem to be confusing two different principles. The drinking straw thing is all about surface tension. Totally different application of theory.

afterburn 549 · 01-16-2009, 10:26 AM

I may be be confused...trying to unconfuse my self. I do understand that is the absence of atmospheric P. that cause the pump to get the fuel. Also, I do know if I go to siphon fuel it is a lot harder to "draw" on a larger dia tube then a smaller one..hence I would think (guess) that it would be harder on these little pumps to add a bigger DIA straw in the tank ..??

sammyg2 · 01-16-2009, 10:28 AM

I just re-read your post:
One thing to add. If you have a large suction pipe with huge volume, it would take the pump longer to take suction because it would have to displace more air to create sufficient vacuum to take suction.
It would not be harder from a load standpoint, just take longer. During the time it was developing sufficient suction it would be trying to pump air and not be running in liquid so it wouldn't be lubricated or cooled as well and in extreme cases that would lead to premature failure, if that what you meant?

Not to muddy the waters any more but most in-tank pumps don't lose prime so they don't have to re-establish it every time they are turned on. That is usually due to them having a foot valve (a check valve at the suction screen to keep the suction line full at all times), or possibly I suppose in some applications it could be designed to take advantage of the drinking straw thing we discussed earlier. Problem is, most fuels don't have a very high surface tension when compared to water and would lose prime after a while unless the suction line was teeny tiny (technical jargon).