Many pools are undersized on the suction side, bigger is better. On the other hand pumps need backpressure. A high head pump with an open ended discharge will draw too many amps, shotening motor life. Ther is a lot of science here but I dont have time to write a book.

So, it's sorta like the exhaust on a car where you need some backpressure. If the exhaust pipes are too big, the motor won't run as efficiently?

Pumps perform better when there is lower head loss on the suction side than the return side which is why the suction side pipe is usually larger than the return side.

I would agree with some of the comments about the return side although with filters, heaters, valves and fittings on the return side, there is usually more than enough back pressure on the pump no matter what size return pipe.

Also, having multiple parallel runs has the same effect as upsizing a pipe. So 2x2" pipes or 3x1.5" pipes have about the same head loss as 1x2.5" pipe.

For a little physics lesson. Water can not be sucked, only pushed. The pump creates a vacuum (low pressure) gravity pushes on the pool, therefore pushing the water through the pipe. The centrifical force of the pumps impeller pushes the water through the pipe back to the pool.
So looking at it that way you can see why you would need a larger pipe feeding the motor, and a smaller pipe to return the water.

So vacuum != suction??????????????????????????????????

Racket,
Are you being funny,
or just thinking about it.
So a pump doesnt really suck!!
It just blows!!!

I'm speechless.

"For a little physics lesson. Water can not be sucked, only pushed."

Actually, there is some debate about that. It is often a subject which is discussed in Physics forums usually around how siphons work.

Does a siphon suck or push the water over the hump?

Encyclopedia Britannica: "The action [of a siphon] depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure--a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight." In other words, the water isn't being pushed over the hump by atmospheric pressure behind it, it's being pulled by the water ahead, as though it were (excuse me, but this is how I conceived of it) a giant stringy booger.

Reference: http://www.straightdope.com/columns/read/2372/how-does-a-siphon-work

Here is a good article explaining the physics of a siphon by a PHD from Pentair.

http://www.pumped101.com/siphons.pdf

OR

http://www.pump-zone.com/piping/piping/siphons-and-the-siphon-effect-that-sucking-sound.html

It seems both pressure and cohesion (tensile strength) may be at work. At least that seems to be the consensus.

Cohesion should also be at play in a mechanical pumping systems, so it is quite likely that a pump both pulls and pushes the water. Air pressure difference helps move things along.

Which came first? The chicken or the egg? Hehe

Why are suction lines larger than return lines? Recommended/allowable water velocity is higher for returns. Flow in GPM is not the only consideration in hydraulic design.

Well mas,
I guess there is one in every forum.
Nothing you wrote has anything to do with PUMPS, and is really of no use.
Yes there is more then one property (law) that effects a siphon. A siphon (also spelled syphon) is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher, or lower, than the reservoir, the flow being driven only by the difference in hydrostatic pressure without any need for pumping. It is necessary that the final end of the tube be lower than the liquid surface in the reservoir. Thanks Wikipedia.

Yes under "perfect conditions" a siphon will work in a vacuum, after all you cant keep out the gravity, try that experiment in space.
Yes different liquids have different tensile strenth, and cohesion, so if I filled your pool with SNOT I am sure it would rise up and push its self out of there.
Because after all itsnot water. :)

Cascade,
you are absolutly correct, and brought up a very good point, recomended velocity is generally lower on suction side then return.
APSP recomends not more then 10FPS on return, and 8FPS on suction side.
But that is for the pool penatration only.
I could run 1.5" suction, and T off and penatrate the pool with 3" and be perfectly legal.
Now with VGB the Suction Velocity is determined by the drain cover you are using. Most are 7FPS.
So if you are running a 2hp pump capable of over 120 gpm, a 2.5" pipe could do the job (short run), but you would exceed 8FPS, so you change pipe size to 3" from the last T to the main drains, now you are below 5FPS with one drain blocked, and under 2.5FPS running normal this makes for a safer drain system as well.
I know just run 3" the whole way and call it good, well we are in a recession.

"Nothing you wrote has anything to do with PUMPS, and is really of no use."

They have a lot more to do with each other than you realize. Siphons and pumps are bound by the same laws of hydraulic physics.

The examples I gave were to show that water has cohesive (i.e. tensile strength) properties which means that it can be pulled or sucked as is demonstrated by the siphon. However, it doesn't matter if it is by a siphon or a pump the point is that water can be sucked and all water movement in a pipe is due to both pressure differential AND cohesion.

In fact, if water was not cohesive, a pump would never work. Cavitation, vaporization of a liquid due to low pressure, is directly related to tensile strength. So if water had no cohesion, once the pump was turned on the water would simply pull apart and vaporize.

It is the water cohesion which creates the lower pressure in the pump to begin with. So when a pump is running, water is being thrown away from the impeller via centrifugal force toward the outlet. The water cohesion bonds the water molecules together so the water molecules are pulling on one another creating a chain effect which pulls more water into the impeller. So the water is essentially sucked out of the impeller which creates low pressure and then the pressure differential in the rest of the pipe compensates for the dynamic head loss in the pipe creating the overall flow of water.

So yes, believe it or not, the pump sucks.

In addition to cohesion, there is compressibility. Most liquids cannot be squeezed into a smaller space with the addition external pressures. Hydraulic pistons, such as those on the hoe that dug your pools, leverage the fact that liquids are neither compressible nor do they change to vapor when a moderate vacuum or negative pressure is applied.

Like I said, which came first, the chicken or the egg?

I can't wait for spring break.

Poolguynj's point on compressibility and made me think about when water is under pressure, cohesion doesn't really have a role in the water movement.

If the pump were well below the water level of the pool, there may not be a partial vacuum in the pump so pressure is always above 0 PSI. In that case, cohesion does not play a significant role and all of the water movement is due to the positive pressure differential. In that case, a pump does not suck.

Sorry bobcattracks, spring break is still a ways off.

Not quite mas985,

A pump that is below the water level will usually pull more water through the suction line than if it was just via suction. However, the lower the pump, the greater the head and eventually it will even out. As you continue lower, the head pressure will eventually become too much, the output will slow and the system will eventually dead head.

The same thing happens when you elevate the pump except the head pressure is coming from the suction side.

Now were getting somewhere.

Wake up in the back!

With pump below pool, you not only have atmospheric pressure, but the weight of the water.

Ok its quiz time.

If I have the identical plumbing system.
One in Sandiego pumping 100 degree water, and one on top of Mt McKinley pumping 50 degree water. Which has a better flow rate and why?

"A pump that is below the water level will usually pull more water through the suction line than if it was just via suction. However, the lower the pump, the greater the head and eventually it will even out. As you continue lower, the head pressure will eventually become too much, the output will slow and the system will eventually dead head. "

I disagree. You will only see an increase in flow rate if the return side is at a lower elevation than the suction side. Then you will have a net head gain and the pump will deliver more flow.

As long as the suction and return paths are to the same body of water (i.e. the pool), the flow rate in a pump (GPM) will be the same no matter what the elevation of the pump is relative to the pool. As you pointed out the the static head gain on the suction side is cancelled out by the static head loss on the return side so total head loss seen by the pump remains the same and thus flow rate does not change. There is no fundamental limit to this except for the pressure ratings of the equipment on the return side.

There is a limit on the height above the water level that a pump can operate and this is related to the net positive suction head (NPSH) of the pump. Once the pump gets above this level, usually around 8-12 feet, it will not be able to pump properly and will start to cavitate.

However, what does change is the pressure in the plumbing. Both the suction pressure at the pump and return pressure at the pump will increase by the height of the pool above the pump in feet divided by 2.31.

So my point which I think was completely missed was that the pump basket could actually have positive pressure but it would still be less than the return side. Under this condition, cohesion does not play a roll.

Hmmm, High tide or low tide?

Mas,
you missed your quiz question.

What effects will water temp have on your pump 10' below the pool?

And Definatly high tide.

ROTFLMAO

I still don't know which is heavier, a ton of bricks or a ton of feathers. "Just a bp" would you mind commenting on my "out with the old, need new equipment" post. Sorry to hijack. I will go back to being a spectator on this post.

"One in Sandiego pumping 100 degree water, and one on top of Mt McKinley pumping 50 degree water. Which has a better flow rate and why?"

Water's viscosity decreases with increasing temperature. So the higher the temperature, the lower the head loss and thus the higher the flow rate. From 50 deg F to 100 deg F, my pool might experience about a 4% increase in flow rate based upon my head calc model.

As for altitude, water viscosity is not dependent upon air pressure so altitude should not have any effect on it.

So San Diego would have a better flow rate.

One more thing, the NPSHa will drop with altitude so at 20k ft, it would drop by over 21 feet. Depending on the head loss in the pipe, NPSHa most likely drop below NPSHr so the pump would just cavitate.

Ah Grasshopper, I mean mas.

First are you a politician, you seem to be doing a little flip flopping.

Lets look at your first answer.
"Water's viscosity decreases with increasing temperature. So the higher the temperature, the lower the head loss and thus the higher the flow rate. From 50 deg F to 100 deg F, my pool might experience about a 4% increase in flow rate based upon my head calc model."

You had better check your head again.
The hotter the water the higher the Hvp(vapor head) the lower the NPSHa. So this would relate to a loss in flow rate.

For your second part: "As for altitude, water viscosity is not dependent upon air pressure so altitude should not have any effect on it."

Well this is where you flopped a little. I guess you had a little time to sleep on it or look it up.

Altitude has a lot to do with it. Remember my first general comment that started all this. " pumps blow"
Well the higher the atmosperic pressure the more push on the pump.

And Grasshopper,
After your flop you came up with a fact that helps prove that.

NPSHa or(Net Positive Suction Head required)
Lets look at that formula:
NPSHa=Ha+Hs-Hvp-Hf-Hi.
For those following along
Ha = atmospheric pressure
Hs = Static Head
Hvp = Vapor head
Hf = Friction head
Hi = Inlet head.
Lets plug some numbers into this.
Remember my quiz question.
"One in Sandiego pumping 100 degree water, and one on top of Mt McKinley pumping 50 degree water. Which has a better flow rate and why?"
Well we will assume that Hs,Hf, and Hi are the same with both system so we can see the effects of elevation and temp.

For Sandiego:
NPSHr = Ha + Hs - Hvp - Hf - Hi
26.697=33.9 +(-1) -2.203 - 2 - 2
For Mt Mckinley
11.58 = 17 + (-1) - .411 - 2 - 2

So yes Sandiego would have better flow, and Mt Mckinley would more then likely be in a state of cavitation.

Oh yes your last comment.
"so the pump would just cavitate"
Pumps dont cavitate, systems create cavitation.

One more little detail about altitude. Obviously we are not going to build a pool on top of Mt McKinley but we do build pools in ski resorts atop large mountains, at high altitudes, and then heat them up.
Low pressure created by high altutude, combined with the low pressure created by the impeller greatly reduces the boiling temp of water, the closer you get to that point the higher your chances for cavitation.

This ends the lesson for today.

Remind me not to make any more generalizations on this forum that is meant for homeowners.
How dare I.

just-a-pb,

I think that people should be able to have an intellectual discussion without resorting to juvenile comments. Don't you?

So let's get back to the facts and stick with the science.

First, there was no flip flopping only your failure to understand what I was explaining so I will try again. You are mixing NPSHa with flow rates when they really are somewhat independent.

Look in any text book or here: http://www.engineeringtoolbox.com/water-dynamic-kinematic-viscosity-d_596.html, you will see that kinematic viscosity decreases with increasing temperature and is completely INDEPENDENT of altitude. Kinematic viscosity is the resistance to flow of a fluid, in this case water. The lower the value, the less friction loss there will be in the pipes. So, if there is less friction in the pipes, the head loss will drop and flow rates will increase according to the pump's head curve. This is a well known principle in hydraulics so please take the time to do a little research before so easily dismissing facts.

The second thing to consider is altitude. NPSHa does NOT change the flow rate characteristics of a pump it only determines when the pump will cavitate. It is the impeller which vaporizes the water so I beg to differ with you, the pump cavitates. The plumbing system only creates the conditions for cavitation but not cavitation itself. If NPSHa is below NPSHr, the pump will caviate. NPSHr is determined by the pump alone and is usually measured in a lab.

Also, last time I checked Mt. McKinely is at an elevation of about 20000 ft. The pressure at 20000' is about 6.76 psia or an elevation head of 15.6' See:
http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

Also, most pools will have well over 2 ft of dynamic suction head. In fact, most are closer to 10' feet of head. I have measured data on several pools and the more well designed pools with lower HP pumps have lower suction head of around 5' and the older pools and higher pump HP have suction head closer to 10'. So I get a NPSHa closer to 4.2'. Most NPSHr is 8-10' so the pump would still likely cavitate.

Note too that if the pump was placed more than 4' below the surface of the water, the pump may be able to operate without cavitation. However, it doesn't change the answer. San Diego will have better flow rates due to water temperature alone. Altitude only comes into play with cavitation.

While you like to give the impression that you know everything and are the master of all, there is a lot you could learn if you would simply open your mine. You continue to look for errors in my statements where there are none. Don't get me wrong, I do make mistakes and I am happy to continue the banter but I warn you that I have quite a bit of experience in this area so I really do understand hydraulics quite well. So far, you have failed to prove anything that I have said to be wrong. But keep trying if you like, I'm happy to play along.

See class, learning is fun.

First I would like to say that I did open my "mine" and just found gold, thanks for that.

Yes I never said that increasing temp does not decrese viscosity. The very miniscule gain you get as water is moving through the system is lost at the impeller. The spinning thing, with water molecules farther apart. Think about it. And if you have a heater on your pool or spa, and a couple of gauges you can see for yourself.

As far as cavitation. It is more correct to say that the pump is in cavitation, or suffering from cavitation. Again it is the system that effects the pump, not other way around.

The numbers I through in were for demo purpose only to make a point, not a real system.

Altitude: Since water is pushed into the pump by,and adding pressure to the impeller, wouldnt it make sense that at a higher elevation, you would have less pressure thus more suction head.

Talk about opening a mind.

I'll try and keep my typos to a minimum.

The change in viscosity (~2:1 for a 50 deg temp change) results in a change of head of only about 4% as I had mentioned previously. Yes it is small but it is still a change none the less and isn't zero.

If you run through the Darcy-Weisbach hydraulic equations, you will see how temperature effects head loss. Here is a summary

Temperature increases by 100% (50 to 100 deg F)
Kinematic Viscosity decreases by about 50% (see table)
Reynolds number increases by 100% (inversely proportional to Kinematic Viscosity)
Friction factor from Moody diagram decreases by about 4%. (http://en.wikipedia.org/wiki/File:Moody_diagram.jpg)

The friction factor is directly proportional to dynamic head loss so it too decreases by 4%.(http://en.wikipedia.org/wiki/Darcy_Weisbach_equation)

Please take the time to run some numbers and you will convince yourself that head loss does change with temperature, albeit by very small amounts.

I have actually seen this effect in action. When I run my spa during the winter and the water is about 50 degrees, the filter pressure reads close to 19 PSIG before the heater starts. After about a half hour, the water temp is 102 degrees and the filter pressure reads close to 18 PSIG. No plumbing changes were made during the warm up period, only water temperature. The 1 PSIG change is not a big change but it was predicted by theory and the hydraulic equations.

If you don't have a spa and are not interested in wasting a lot of money heating your pool, you can do the calculations yourself. In addition to Wikipedia, here is a good starting point:

http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

Joe Evans from Pentair touches on viscosity in the following article but more from a point of different liquids. He also touches on altitude.

http://www.pumped101.com/pumpintro.pdf

That web site has wealth of information and I encourage anyone to read through the articles if you really want to understand hydraulics.

Back to altitude:

You are correct in that an increase in altitude decreases the pressure in the suction line. But the decrease in pressure from altitude also decreases the pressure in the return line. Net net, there is NO change in TOTAL head loss experienced by the pump. If the total head loss does not change, the flow rate delivered by the pump will not change per the head curve. Therefore, there is no change in flow rates due to altitude. Only NPSHa changes with altitude as is shown in Evan's article.

This is a similar effect to what I wrote about earlier in this thread. Putting a pool pump above the water line or below the water line will not change total head due to the change in static head. The static head has the same effect on both the suction and return pressure so total head once again does not change and flow rates will not change.

Hey tsunami did we answer your question?

About your filter example.
You say that due to heat, viscosity lowers the friction, water flows easier (higher velocity), psi drops by one.
Hmmmmmmm
Could it be that due to heat, we have an increased suction head, which lowers the flow dropping the psi by one.

How can we prove this one way or the other?

By the way, I am not an engineer, I just stayed at a Holiday Inn last week.

"I just stayed at a Holiday Inn last week."

HA!

pb, I have no idea whether you're right or wrong here, but I've been noticing your many "show off" pictures lately - you build a lovely pool. Wish you were in my neck of the woods when I built mine.

Apparently some people do have time to write a book on the subject. I have never seen a pool on Mt.McKinley. I never really looked. What is the boiling point of water up there anyway? Just thought I'd give you something to do.

Having a bit of skepticism is usually a good thing but one should not ignore existing theories and experience either. The information I have presented in the thread is from existing hydraulic engineering theory and applications. I have tried not to stray from this and to present it as best as I can. Really, I am not making any of this up and I try to source all of the information that I have presented. Having said all that let me address your questions.

"Could it be that due to heat, we have an increased suction head, which lowers the flow dropping the psi by one."

Not according to any hydraulic engineering theory that I am aware of. As I had outlined before, friction loss will decrease with increasing temperature on both the suction and return lines. The laws of physics are the same for both the suction and return lines and so will behave the same way with temperature.

"How can we prove this one way or the other?"

I thought I had given enough proof already but if you need more convincing, here are a couple of ways:

First, assuming you donÂt dismiss hydraulic theory all together, run the calculations yourself and you should be able to convince yourself. The Darcy-Weisbach equation has been used for close to 200 years in hydraulic engineering. These equations have been verified time and time again and are currently used in civil engineering to design water treatment plants, hydroelectric dams, pumping stations and everything else that has to do with pumping fluids.

If you are interested in math or the above still doesnÂt convince you, the second method is to directly measure the effect. I had looked at only the return side but by placing a vacuum gauge on the suction side, one could actually measure the effect. Next time I warm up the spa, I will do this. If you have a spa, you could do it too. However, I am very confident that the hydraulic equations do not lie.

"By the way, I am not an engineer, I just stayed at a Holiday Inn last week."

Well, not only have I stayed at a Holiday Inn Express, but IÂm an engineer too. Not that this will convince you or anyone else of anything I say and more often than not solicits a negative response for mentioning it but I have close to 27 years of experience as an electrical engineer including an advanced degree. Hydraulic engineering has many things in common with electrical including many of the same required undergrad courses such as advanced calculus, differential equations and 3 years of physics. I have studied hydraulics now for close to 6 years and I think that I now have a pretty good grasp of the theory and concepts although there is still much to learn. One thing I have learned from engineering is that there are always opportunities for learning something new and to stay relevant and marketable, one can never stop learning.

No, seriously, what is the boiling point of water on top of Mt.McKinley? I know you are not too busy watching American Idol to answer that.

I don't know and I'm too cheap to send my wife up to find out.

Mike,
167-168.

Poolguy,
You could send your mother in law.

I Âam a mechanic for a large commercial airline and deal with engineers on regular bases. With all do respect to all the engineers here, it never ceases to amaze me that they give a 300 page detailed explanation and instructions on how to simply peel the tape off a placard and stick it to the side of the aircraft. ItÂs nice to see that all engineers think alike.