Simple differential vacuum question

[TommyCee]

This conundrum will sound hypothetical, but it represents a real-world problem:

You have a vacuum pump sucking water through the same diameter line from the same level in 2 different places in a pool. One line is 30' long, while the other is 70' long. The lines meet at a diversion valve just upstream of the pump which allows the vacuum to be accurately measured by isolating each line.

The steady state vacuum would not be expected to be equal but which line should give the higher reading?

It's been many years since I studied physics but the following competing theories emerge:

Theory 1. The shorter line should provide the greater vacuum reading because it's, well, shorter and thus less resistance.

Theory 2. The longer line should provide the greater vacuum reading because, since it's longer, it provides the greater resistance and therefore the pump must suck harder to move the water.

Before I announce what vacuum differential I measured, I'd like to see if anyone can please confirm Theory 1, Theory 2, or perhaps some other one.

Thanks in advance for your help.

 

[Pumpsonly]

Are you talking about a centrifugal pump or a vacuum pump?
If it is centrifugal pump, I would go for theory 2 if you are talking about the same flow rate. However if the losses in the shorter line is much lesser, the shorter line can also give higher vacuum simply due to the pump will be pulling more water and hence higher losses due to higher velocity.

 

[TommyCee]

As I said in the setup,

" You have a vacuum pump[/color red] sucking water ..."

See (for example) Hayward Super Pump Series Model SP2607X102S:

http://www.hayward-pool.com/prd/In-Ground-Pool-Pumps-Super-Pump-_10201_10551_13004_-1_14002__I.htm

 

[MikeHalloran]

That's not a vacuum pump.

It's a self-priming centrifugal pump with a built-in basket strainer.





Mike Halloran
Pembroke Pines, FL, USA

 

[Pumpsonly]

Hahah !Now I see how the OP mistaken the pump as vacuum pump. It is used like a house vacuum cleaner to suck out the debris at the pool floor.The pool cleaners attached a flex-hose to the pump suction and push the other end of the suction hose which has a fixture very much similar to the house vacuum cleaner with a long stick along the pool floor to suck out the debris.

 

[BigInch]

Let's go for theory #3.

Measuring the pressure drop in an isolated line with no flow in it means nothing to how liquid flows in a network. It could only give you an idea of the "potential" of how much could flow in one isolated pipe without any consideration of how much of that fluid isn't flowing in that pipe, but might be flowing in another pipe, if some of that fluid could find an easier alternate path to run.

You are completely ignoring the effects of the change of flowrate in each pipe that could happen if you connect each pipe together.

Think of overall potential as Pressure Drop x flow, which has to be equal in all pipe orutes describing alternate paths through the sytem. If one path still has a higher overall potential after connecting into a network, it will just increase its flowrate to balance the other, while the other path reduces its flowrate accordingly.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[DubMac]

pumps do not suck

 

[Pumpsonly]

DubMac,
So is vacuum cleaners.

BigInch

Agreed with your theory #3. But I think the OP is talking about measuring the vacuum with only 1 suction line flowing at a time.

The lines meet at a diversion valve just upstream of the pump which allows the vacuum to be accurately measured by isolating each line

.

 

[BigInch]

The OP question can be read many different ways without trying too hard. I will wait further clarification by the OP before discussing any further suppositions.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[TommyCee]

Several of the forums on Eng-Tips I have used and found quite helpful. I regret stumbling into this one - Pump Engineering - as it appears to be dominated by a fraternity of pinheads who are more bent on pouncing on technical "gotchas" - pinheads who can't see the forest for the trees. Pinheads who seem to seem determined to get off in the weeds. Pious pinheads who enjoy piling on and making a newbie feel like a dumb s**t.

What kind of pump I'm using is academic to the problem posed. Functionally, all pumps - of any ilk - suck fluids on one side (creating vacuum) and push on the other (creating pressure). This is related to Newton's 3rd Law of Motion. And of course, I was "talking about measuring the vacuum with only 1 suction line flowing at a time" - that's what an isolation valve allows one to do. That's inherent in the concept of differential vacuum: the vacuum in one suction line (in isolation) vs. that in the other.

Now I'll cut to the chase and answer my own question. The answer is Theory 2: the vacuum measured in the longer line would be expected to be greater (coupled w/ less flow). In fact, I measured 14.5"Hg in the longer line, and 9.3"Hg in the shorter one.

Q.E.D.

 

[BigInch]

Vacuum on one side of a fluid never ever moved that bit of fluid. The pressure differential, the net positive pressure, considreing both sides of the fluid particle, pushed that bit of fluid, nothing else. Nothing was sucked out of anywhere.

Of course if you had two identical pipes, diameter, roughness, elevations, etc. etc., except for length, flow would be less given the same inlet and outlet pressures, OR differential pressure, given the same flow, would be more in the longer line. No mystery there.

PS. you didn't answer the fundamentals of the question, you only posted your observed results. Not a big intellectual score for that, so I wouldn't be calling anybody any names if I were you. Please try to behave and maybe you still won't get tossed.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer