Pump Problems: Newsletter Challenge (12/13/05)

Suction pumps only work when the atmospheric pressure causes a pressure differential against gravity. She knows this pump will never work because even if the pump has worked so hard to create a complete vacuum at the top, the pipe filled with water will have created a gravity induced pressure greater than the 14.7psi of the air around it at the bottom of the well. A pump at the bottom of the well can pump water up to much greater pressures than that of atmosphere, creating a drastically more efficient system.

The density of water is .036lb/in^3, if divided by 14.7lb/in^2, then inverted, you will get approximately 407", or 33'10". Any height greater than this will not be able to use suction to pull water to the top. A very analogous condition to mercury barometers that have a vacuum cavity proportional to atmospheric pressure on the open end of the tube.

Excellent. You described the reason why it will not work perfectly. However, how did she know that?

My guess is she saw the length of the original pump's pipe and wire and probably stepped off the distance to arrive at a rough length. That length was probably much longer than 33', which is, as you correctly described, a greater height than can be supported with one atmosphere.

At least this is what I would have observed if I came across the same scene.

Now, here is a thought. How is it that trees can move water 100's of feet above their roots with just capillary action?

capilary action works like many small pumps. Each part only has to do a small amount of work, but the collective effort, distributed along the tree, is quite immense. I have heard of large trees pumping more water than a fire truck. That's a little hard to believe, but when you consider that the fire truck pumps water at 100-200psi, whereas the tree is probably on the order of a few psi, I can believe it. P.S. That is quite a sassy little girl; fat lot of good she did explaining her statement to the poor lake dwellers. She basically insulted their intelligence and walked away. Sounds like you need to do a little mending of fences with the neighbors and give daughter a talking to.

I wouldn't want them trying to mend a fence, either. 8^) Sounds like the installers need to get some intelligence to insult. Walking away from a "show-them-up" situation is often the safest thing to do. A wise little girl.

I agree with the suction head being too negative. I agree with the girl stepping off the distance also. But she didn't step off the total distance to the pump, but only from the elbow where it left the well casing to the water line on the original pipe (ever looked at a pipe from a well... yuk). That is the distance that can not be greater than ~34' but in all reality is less due to elevation obove sea level and friction loss in the piping.

She probably read the pump O&M manual and reviewed the performance curve and realized pump NPSHR was greater than system NPSHA. All pumps require a certain amount of net positive suction head (NPSH) to maintain prime to operate. Most pump curves indicate NPSHR determined by its design. At sea level you have approximately 34 feet on a 60 degree F day. She probably determined NPSHA was insufficient after making the following adjustments for: 1. Altitude. 2. Suction pipe and fittings friction loss. 3. Suction lift (top of water source to center line of pump). 4. Suction entrance loss. Since NPSHA was less than NPSHR she knew there was no way the pump would hold prime or ever work.

The old pump was an immersion pump. It worked at the bottom of the well and forced water up the well pipe. The replacement pump lives at the top of the well and sucks water up a non-collapsable pipe. It isn't an issue of priming the pump, I believe. I think it is an issue that the well water level is significantly below the height of the replacement pump; so much so that it exceeds the 34' foot mark by a wide margine.

'It isn't an issue of priming the pump, I believe. I think it is an issue that the well water level is significantly below the height of the replacement pump; so much so that it exceeds the 34' foot mark by a wide margine.'.... Same thing my friend, however, even if the required lift was less than 34 feet, there's no guarantee that she'd prime. Depends on the characteristics of the pump.....

There is one thing about pumps that I have learnt and that is nothing is ever simple. • Generally it requires a very good centrifugal pump to have a suction lift over 28 feet. (At this lift special provision is needed for self priming.) • Water vapour pressure is a consideration. If the water warms up the suction lift reduces. • The problem did not discuss whether the pump was powered by single phase or three phase electric motor or other sort of motor. If a three phase electric motor was used, the pump could have been spinning in the wrong direction. Did they check the direction of rotation or exchange two of the phase wires? • It did not say what sort of pump was being used. It was assumed that a standard centrifugal pump was used. A jet pump mounted on the surface will pump the specified height, so a 500HP pump will pump a lot of water. • Was a foot valve fitted and was the suction line primed? • Did the suction line have a pin hole in it. A very small hole will prevent a pump from working. • Did the girl know any of this or was it just beginners luck?

These questions are designed with a simple answer built in. The simple answer is that it was higher than the 33 some feet, so high that even if you had all of the elements together, that was the one hangup. It didn't mention the motor because it was not important. This is a little girl, assumably having no practical or college education in motors, so if she presents the answer, it would have nothing to do with the motor or the wiring.
The specifics are always more complex than the principle, but this girl did have inductive reasoning on her side. She realized that if the realm of possibility did not include this single portion of the system specified in the challenge question, then the whole system would not work, no matter what, even if the pump was so powerful to be able to pump a non-primed pipe (would create a complete vacuum, 0psi) then it would still not work. That was the point of the "500hp" figure.
To answer your questions, all of these are possible, but there is no way to say if these were the issue. We do know one issue and that it is a critical issue, the suction head height. After we establish that, the system does not fuction and the other details don't matter.

I realize that you are trying to teach people about atmospheric pressure and suction lift and so on but you cannot ignore the logic of the whole situation. It was immediately assumed that the little girl was correct because she was cute or perhaps better educated than the poor frustrated guys trying to fix the pump. The length of the old electric cable does not tell a great deal except the maximum distance the pump can discharge the water. The real issue which has been ignored is the distance from the surface of the water in the well to the new pumps suction inlet. The initial pump was a submersible pump designed to work below the surface of the water. You said that the suction lift was higher than thirty three feet but there is no evidence to suggest that this is correct. The old pump's inlet has to be below the surface, but how far?? Since it was a well it could have been one foot or twenty feet ----who knows. If the old pump was sitting twenty feet below the surface, the suction lift height would have been well within the capability of a normal centrifugal pump. Also, a common practice is to coil the electrical cable to cater for future maintenance…..who knows without asking the frustrated workmen. Also, why would anyone measure the electrical cables and not the old delivery hose?? What really would have impressed me if the girl had actually understood the problem fully and tied a stick to a piece of string and threw it down the well to determine how far down was the water surface. The girl was very lucky that she did not accidentally find herself at the bottom of the well. I realise you want to keep the principles simple but the differences in operating characteristics between a submersible pump and other available pumps has been overlooked as well as the important issue of interpersonal relationships.

I was under the impression that this was more of a riddle than a question presented for general discussion. The point of many of these challenge questions is to realize the unique arrangement of components presented in the situation and discover the single answer. It becomes believable that a little girl who could wander off and come back with the conclusion that it was too high of a well, but if the challenge question talked about her dropping a stick down the well, it wouldn't be much of a challenge figuring out what she was doing.
As for the general discussion, you're fairly on point in my opinion, but I figured I had the question nailed.

I understand where you are coming from but I think that part of the problem is that there is a definite culture difference here…..Australia vs. US. If you didn't wish to discuss reality then why ask people to tender their input. I find that the application of these fundamental laws of nature much more appealing. It is often the other laws we leave out that makes things come unstuck. A little more thought in the formulation of the initial question would have helped. Anyway, I hope other readers were enlightened by this little debate. Cheers!!

your child realized non of the taps are open so there can be no flow."There's nothing wrong with your pump. You could double the pump power and it's not going to matter. Even use 500 hp if you want. Won't work." because you are just building presure.

The first pump still works, the just need to throw the breaker switch.

Once again the answer is intuitively obvious. The well pipe is 7920 miles long and sucking air on the other side of the world.

This is the most entertaining answer yet, however I have to go with the closed valve theory.

wouldn't a 500hp pump burst a pipe if there was a closed valve? (assuming it was not a HVLP pump) I'm sticking to my story.

Why are we assumimg the original pump was deeper then 33'? It does not say that. What if the original pump was only 15'!!!! Maybe there is no water left in the well.

I am making that assumption based on the wording of the text. The clue to me is that statement that even a 500 HP pump will fail to work.

While it is true that there are a number of possibilities as to why the system would fail, some subtle (or not so subtle) clues would be the nature of the previous challenges from GlobalSpec. Typically, everything to solve the puzzle in word games will be presented in the story. Since the old pump and the new pump are of two different types (one pumps water from the bottom of the well, the other sucks water from the top of the well). I read this as a significant clue and derived that it was the logical source of the story's problem. Adding more power to a surface mounted pump will not help it suck water that is lower than one atmosphere of pressure can "push". A surface mounted pump relies on a vacuum to draw water up from a well. The maximum depth such a pump can work is about 34', so it is logical to assume that the well water level is deeper than that since the new pump did not work.

Furthermore, the question posed by the story was "How did she know?" So, the correct answer should be phrased or bounded in the context of how she discovered that the new pump would not work. A logical deduction would be that she "measured" the depth of the water in the well by some means. If it were me, the simplest method would be to step off the distance of the old pump's pipe from where the water line is evident to the top of the pipe where the new pump would sit. It is logical to assume this happened when she "wandered off" and nobody was paying attention to a child walking along a dirty old pipe.

Q.E.D.

The clue is "It seems the original pump failed down in the well". Why? Did it fail, or only "seem" to fail? My daughter may have noticed the power was off in a nearby cabin and a power truck was positioned under a nearby pole. Another possibility is that she knows water weighs 8lbs a gallon.

Then why all of the other clues? Why does the story make a point that the new pump is a surface mounted pump versus the submersable pump? Why doe the daughter mention horse power is not a factor? Time will tell.

Just like the question how many engineers does it take to change a light bulb, no-one remembered that they have to prime the pump first (remove the air from the tube and replace with water) also there is no mention of a foot valve .......

She found a sign that said, Pump no longer works due to the current Global Warming....well it affects everything else...

There's no such thing as Global Warming. Just a steady and synchronized increase in average local temperatures globally....hang on, I think another icesheet fell off Antartica.

The pump, and line down to the water in the well needs to be primed with water in order for the pump to do it's job.

She, clever girl has found the old pump and discharge pipe. She then measured the distance from the waterline on the pipe to the elbow at the surface and found it was above the suction capability of the pump (32feet theoretical or about 25feet practically).

I would have thought it had to do with the length of the extension cord involved. As we know, the K value of a standard copper extension cord is 10.4, and her pacing off the length of the cord would indicate the internal resistance involved. Knowing that a standard 12 gauge extension cord could only safely handle 20 amps without burning up, she would know that you could never pull enough power through the cord to power a 5 or 500 hp motor. But I guess that wouldn't matter in this case. Interesting.

Maybe it's a gas (fuel) powered surface pump, no wiring required.

She found a sign that saying that "Do NOT Switch "ON"" sign at the Power Distribution Board Maybe the camp owner never pay the electricity bill...

Katy; all sassy little, too smart, girls are named Katy, realized that the water from the bottom of the well got there, somwhow, from the lake. Just like at her house. She noticed the old well pipe and paced it off wondering what it meant. They had talked about barometers in school and she thought she remembered that water could only stand about 10 meters in an evacuated cylinder but that didn't seem to be the problem here even though she thought the pump numbers meant it probably could only produce enough vacuum to obtain only about 75% or 80% of that, and besides it couldn't be more than ten or fifteen feet vertically from the edge of the water to the ground over by the pump. Shoot, 20 or thirty feet was taller than her entire house. But since the pump shaft was sealed to the atmosphere there was no way a suction pump would work, but if these guys had done it before then the pressure to make it work must come from the air pressing on the lake. And it did seem, however, that there was a lot more beach this year than she remembered from previous visits. Probably due to the drought. Then she saw it. The well inlet pipe was right by the edge of the water sticking out of the sand four feet back from the water's edge. She straightened her glasses and miss smarty pants high tailed it back to the pump mechanics and did her little song and dance before skipping back to her house feeling very, very smart.

The Foot Valve is missing!!

For each centrifugal pump, there is minimum NPSH required and if NPSH available is less than required, it is not posible to pump the fluid no metter how efficient motor it is. NPSH is net positive suction head that is water head available at the suction eye of the impeller - head loss due to vaporization of water at atmosphereic temperature-head loss in the piping due to friction loss. The girl must have seen NPSH required on the pump tag and by simple gudgement she could have determined that available NPSH is insufficient

How about if the daughter looked at the old submersible pump and realized that the pump mechanics didn't take into account the height of the submersible pump. Most submersible pumps mount the outlet piping on the top and the inlet is the base of the pump, due to all the NPSH reasons stated previously. If they just removed the outlet pipe from the old pump and cut the new one to match it would be shorter and if they did put a foot valve on (required as we all know) that wouldn't make up the difference. Who is gonna crawl down in a well and measure the water line? or if its an artesian well your not gonna be able to.

She knows because while she wandered off, she found a tree where many elves live who are operating a quite impressive cookie and snack operation. It seems the magical head elf in charge (MHEIC), E.L.Fudge, has very magically turned the entire camp communiy's ground water supply into thick delicious chocolate for use at the tree manufacturing plant. To prevent contamination and discovery of what he's done, E.L. has put a spell on all the pumps in all the wells to impair their function. However, for good measure and to ensure her silence, E.L. has arranged for the daughter to receive 3 stripped shortbread cookies each night under her pillow to keep or eat at her discretion.

She saw the lake was dry, therefore knowing the water table had dropped below the previous water line, the well was obviously too shallow.

That many elves living in one tree? No running camp water? Come on! That ain't chocolate! Everyone knows that there is no such thing as magic elves living in trees creating delightful treats unless they have an elaborate network of sanitary plumbing which is impossible without running water.

First, although your daughter is a member of Mensa, she is very observant as well. After walking away in boredom she wanders down to the lake and notices that the lake is low (drought in many parts of the country and wasteful water management practices have contributed). She looks back up the hill to were you are trying to be a hero and ponders...hmmm... I believe the bottom of that well they are working on is higher then this lake. Water finds the lowest point... I know they will never get water out of the well because it is dry and I better let dad know before he gets too involved with solving the problew and Mom starts yelling at him for not spending any time with the family..

The pump at ground level won't work because the well depth is 34 feet or deeper: no matter how much vacuum the pump provides, the atmospheric pressure will only push the water to a maximum height of 34 feet.

The girl threw a rock into the well, as kids do, and discoverd that there was no water in the well. No pump will work if it has nothing to pump.

The girl walked to the edge of the lake and noticed the pipe that feeds the well from the lake is above the water because the lake has been drained to build another Walmart.

Contrary to a lot of statements that I've read, pumps do not create vacuum or suck, they pump by virtue of atomospheric pressure that actually pushes liquid into the pump inlet once primed.

Okay, but don't they do that by lowing the pressure inside the inlet side of the pipe?

I think your technically right, but it is a little like arguing if a person gets sucked out into space when the airlock door opens or is pushed out into space. Technically it is pushed, but either way for the poor SOB that is forced out the airlock it still SUCKS. ;-)

Oh, she noticed the discharge valve closed.

Several things actually; -the old pump was dry and showed signs of having been dry for a long length of time - she knew about vacuum/presure and how water can't be held by a vacuum beyond about 34' (what with altitude and all - she dropped a stone down the well, and counted to about 10 before she heard a very faint sound of a rock hitting mud - she saw that the neighbors didn't know much about electricity and that the wiring had been damaged to the point of only the grounded conductor having continuity to the power source - she saw that the local utility also lost confidence in them and had disconnected additional wiring from the power grid - she saw that the WalMart bags that had blown across the mudflats were being sucked down a deep sink hole not too far away - she spotted another sink-hole between the cabin and the pump location with only rattlesnake skins left behind and what appeared to be ferrous oxides like those left by pieces of old rusted pipes around the back of the scrapyard she'd seen on the Discovery Channel.

The lake is low, water table low, so no water in the well.

Gravity sucks and so does the snippy little girl, but only so much. Too bad, she may be right. A vacuum,(in which her behavior indicates she belongs,) will only lift a column of water about 33.5 feet. If the well is deeper than that, it doesn't matter what size suction pump is used. Positive displacement is needed to lift the well water higher than that. Negative reinforcement, combined with behavior modification, is required to correct the girls behavior. If she wanted to be a truly better person she should have told her victims how to fix their problem.

A postitive displacemnt pump on its own will not due the job. The only way a positive displacement pump will lift the water higher then 34', is to add an air inlet at the bottom of the pick up. This can be done with a simple air line along side the down tube.

The suction pump is a kind of centrifugal aparatus, which works by dispensing enclosed water out. Out-flowing water creates a negative pressure and thus sucks additional water in. Without enclosed water, the pump blades can only rotates and throwing air outward. It does create negative pressure by this means, but it is too tiny and is always insufficient to pull water up.

A lot of people have criticized a fictional girl for her way of saying that the new pump will simply not work. The trouble with some of these people and some of the others who gave good technical answers are real but outrageous spelling mistakes. In some cases an inadequate word was spelled correctly. Write your text first in any word processor. Then check the spelling and get out your dictionary to check all important words and some of the unimportant ones. Then copy and paste the text. Finally, press the preview button. Literacy in any language goes hand-in-hand with good engineering.

Amen!

Seems to me that the "well too deep" answer doesn't explain why she wandered off for a few minutes. In my experience the old pump would be lying right there in a rusty heap where they disconnected it. I also think going to look at the lake level is a bit simplistic. Subterranean water follows it's own rules. I've had a property on a hill next to a river and we pulled water from well (sorry!) above river level whilst others lower down had problems. Usually wells pull from a fair bit below the water table level. Drought problerms are generally indicated by running dry after a few minutes not total non function. Downstream tap turned off or pipe blocked doesn't sound likely as you usually open a priming port to get all the air out initially. Now having rubbished all the ideas that I could come up with, I'm baffled! Any other suggestions?

Were they working on the correct well pump?

Q: what if you could fill the tube with water — would the suction pump work then? Why?

A: No! The reason is the same. If you fill the tube with water, assuming a one-way valve at the bottom of the well, you will only draw out a portion of the well tubes water. Once the pump's vacuum reaches maximum, the pumped out water will be replaced with a partial vacuum at the well top. Well water level in the draw pipe will stand at something less than 34 feet above the water level inside the well. Beyond that will be a partial vacuum inside the pipe, which represents the volumetric portion of the water that was pumped out for the short time the pump operated.

Considering most people don't understand the concept that there really is no such thing as "suction" only low to high pressure, it's unlikely that the daughter walked off for a minute and came back to say that, even if she measured the old pipe. Even if the daughter is 30 years old and has an engineering degree, a few minutes is too soon to be sure that 1 atm wouldn't do the trick. She must have just seen the closed valve. If it was a 1/4 inch diameter tube, would it work? I can't remember. If the old tube was too long, could she tell how far it was submerged? I agree with the person that said she found the closed valves. (Even though opening them might not solve the problem.)

-and yes, if the pump was primed with water before it started the hydrogen bonds would more then likely fix the problem.

Not likely, the bond market has been sluggish. ;-)

Well, High School science class did mention a fellow by the name of Torricelli... That was a clue.

Alright, she walked off and saw that the lake was bone dry, with a cracked lake bed 200 feet below. It was a drought, and the first pump wasn't broken at all. Dust was coming out of the outlets, not water vapor.

When you use centrifugal pumps, which are not submerged in water, you have to consider the NPSH value (Net Positive Suction Head) which for water is around 22 feet. This means that (ideally) you may locate a pump to a maximum height of 22 feet above the water surface. But due to the hydraulic losses (on the pipe and fittings which connect the pump to the water surface) these height distance is going to be reduced to a practical value close to 20 feet (6 meters) or less. Beside that, the pump should be primed before it is started (fill the water column between the pump and the end of the pipe) and should have a foot valve at the end of its connecting pipe. Actually when a centrifugal pump (which is primed just before) starts spinning, it creates a pressure drop in the intake (suction) side of the impeller. The positive pressure (in this case atmospheric pressure) above the water reservoir (well) will feed the water towards the impeller. But if the pressure inside the pipe falls below the vapor pressure value of the liquid this will evaporate and then the water column inside the pipe will brake and the pumping action will stop.

The smart girl that she was, she concluded since this was a surface mounted pump all the hp. in the world wont make it work without a foot valve to maintain prime.

Nobody has mentioned a Jet Pump. The girl remembered last year when the workmen fixed their well and they said that the jet was good to 200ft.

Under engineering point of view, my guess is agree with the well level water statment (pipe length, NPSHa and minimum suction head).

However, other reasons posted here are possibles under different considerations, such as the suction side pipe could be broken or plugged, and other simply guesses, like:

The electricity supply failed. No matter what pump they use, if there is not energy, will not work.

So, has been the question well formulated?

It seems that the aquifer is shared between the camp and the other camps near by. If other pumps are working simultaneously there will be not enough water permeating into the aquifer hence drying up the well.