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Doubt on Pump Suction
4

Doubt on Pump Suction

Doubt on Pump Suction

(OP)
We are using a 6/4AH Warman slurry pump for transporting slurry sand. Our pump lost most of its load taking capicity and on inspection found that the Giboult joint has devloped a leak at the suction side.
    But in one of the posts the top guns in this forum stated that pumps will not Suck but just devlop pressure. So how can we explain the phenomenae.
      The suction head is 2mts above pump(flooded suction)open to atmosphere the duty is sand and water mixture of about 20 -25 % solids by weight

RE: Doubt on Pump Suction

I do not have a lot of time this morning, but will quickly conjecture at what you might be asking.

Newtonian fluids (free flowing) cannot be pulled, they can only be pushed.

So how does water get into the pump?
It is pushed to the pump.
By what?
In your case atmospheric pressure plus elevation above the pump (head).

The pump impeller spins throwing water outward, pushing water out of itself, pressure is thereby lowered at the eye of the impeller, water is then pushed into the pump.

If you still do not understand or if I did not understand what you were looking for, let us know.

PUMPDESIGNER

RE: Doubt on Pump Suction

Last thought,
Non-Newtonian fluids can sometimes be pulled in theory,
They are viscous or "sticky".
Put your hand into a batch of taffy or molasses and lift it out, some will pull up with your hand.

PUMPDESIGNER

RE: Doubt on Pump Suction

Let me understand: are you assuming that the hole developed at the Gibault joint is a source of air been sucked by -and into- the pump ?

As for myself I think the "top guns" are right.
A centrifugal pump doesn't suck, being a dynamic machine it even doesn't develop pressures but velocities which are eventually converted to heads and pressures by the volute.

Slurries may cause erosion/corrosion depending on many factors. And the hole in the suction line may be coincidental with other mechanical or hydraulic problems.

As a result dissolved air may be released from the air-saturated water inside the pump provoking "gas binding" and loss of capacity.

Even though dissolved air is 1.7% by vol at 30oC at atmospheric pressure, and is small at the pump-suction-source conditions, it can become significant if the pressure at the pump's eye is much lower.

If for any reason (hydraulic or mechanical) the pressure drop in the suction line changes with time (slurries behave as non-newtonian fluids), air volumes released in the pump from saturated water may become much more than the 2-3% pumps can generally tolerate without encountering serious mechanical problems.

The following estimation will show the maximum % vol of air that can be developed. The minimum acceptable pressure at the pump's eye would be water's vp= 0.61 psi at 30o. The corresponding maximum volume and vol % of the dissolved air at this pressure would be, respectively:

1.7*(14.7/0.61) = 41 cc/100 cc H2O
(41/141.5)100 = 29.1% !

I'm sure the experts will comment on this subject for us to learn, in particular when dealing with slurry pumps.

RE: Doubt on Pump Suction

As far as the pump is concerned, the slurry is treated as a Newtonian Fluid.  Force must be applied to move the slurry into the pump.

PUMPDESIGNER

RE: Doubt on Pump Suction

(OP)
I will reframe the question . We are having about 35 feet of suction head (Atm head + 2mts bin height) So if the pump does not suck then the water should leak out through the suction hole even when the pump is running, since the suction side will have about 16 psi (aprox) pressure, But in actual practise it is not happening WHY ??
If as pump designer says liquids can only be pushed in pump naturally it takes the path of least resistance in our case the suction hole Am I right?

RE: Doubt on Pump Suction

Centrifugal pumps can handle also non-Newtonian fluids. All the fluids aren't sucked but brought to the pump by the available pressure. The problem with dilatant (shear thickening) fluids is that they can become too viscous (almost solify) inside the pump by the shearing action of the impeller.

May I ask about the location of the hole ? On the vertical or the horizontal sections, and if the latter, on what part of the pipe ?

If the friction drop along the line becomes larger than the 2 m of the bin height, for example by sheer turbulence or as a result of a reduction of the pipe cross-section because of solids' deposition, one can envisage a drop in static pressure (Bernoulli) just equal or below atmospheric, because of the increase in linear velocity. Thus an inward air leak could appear. Arsmith, please comment.

RE: Doubt on Pump Suction

I assume that you have a situation where air is entering the pipe.
If that is true then the following applies.

Fluid pressure at the exact entry point of the air, is less than atmospheric pressure, so air is pushed into the hole and into the fluid, which by observation appears as the pipe "sucking" the air in.

The following are some ideas about how this can happen:

1
The overall pressure in the fluid is lower than atmosphere.

2
Obstructions, sudden increases or decreases in pipe diameter.  In short, anything causing turbulence so that there is a localized pressure drop in the fluid.  The overall pressure in the conduit may be higher than atmospheric, but turbulence in the line can cause lower pressure at a specific point in that fluid.

3 There is a Tribal Knowledge thingy that could easily apply.  If a vertical riser is installed into a fluid transmission line where the fluid pressure is not too much higher than atmospheric pressure, and the vertical riser is installed such that there is no turbulence in the line as the fluid passes by the opening to the riser, then velocity of the fluid will carry the fluid by the riser without pushing up into the riser.  If conditions are approximately correct, the fluid will pull air into the line through the riser even if fluid pressure is greater than atmospheric pressure.  I do this on my pool spa to “inject” air into the spa jets.  Those lines carry water higher than atmospheric, but I get the line to pull air into the fluid by putting a riser into that pressurized line in a clean straight run with a smooth Tee.

PUMPDESIGNER

RE: Doubt on Pump Suction

2
I think a simple summary of the above is - if the pressure in the pipeline / joint is at a lower pressure than atmospheric pressure then it is more than likely that air will be forced into the suction line, and once sufficient air is entrained in the pumped product you then have the problem of the pump losing capacity or going "off-prime" altogether for the reasons already explained in other replies.

RE: Doubt on Pump Suction

I agree with your final analyses.

PUMPDESIGNER

RE: Doubt on Pump Suction

Yes pumps do suck, in laymans terms.
Its just a word thing. The inlet to a pump is often called the suction and there you have it.

If the pressure is below atmospheric we have suction.
Pump inlets are sometimes arranged to do this.

A word of caution. If your 6/4 is rubber lined and you are sucking, there is a danger that the rubber liners will be sucked in and will rub on the impeller. This does happen on some Warmans. If the feed level is 2m above the pump suction and the piping is reasonably short, then some attention to the suction pipe is in order. Warman have some very good information to show you how to arrange things.

Cheers

Steve

 

RE: Doubt on Pump Suction

Steve is right on the semantics. Lower than atmospheric suction pressure is sometimes called "lift".

RE: Doubt on Pump Suction

I think it is important in this forum that "We" try and educate as well as solve problems where-ever possible. Statements like pump do suck whether playing with semantics or not does not help those who do not understand.
What also doesn't help is calling the pump inlet the "pump suction" and the pipe work to the pump inlet "suction pipe"
althought we all mostly do it unless we are being academic.

But what ever you want to call the inlet to a pump and its pipe work, pumps don't suck.
"We" know that pumps are capable of reducing their internal pressure to below atmosphereic pressure which then allows atmospheric pressure to force the pumped medium into the eye of the impeller.
The physics is no different to being "sucked" out of a plane door at high altitude or "sucked' under a train while standing close to the edge of the platform, which in both cases it is not "sucked" but "pushed" by the transfer of high pressure to low pressure.
So let's always use correct terminolgy and also point out incorrect understanding when "we" come across it.

RE: Doubt on Pump Suction

Great stuff Artisi.
Some of those words really suck don't they?
But then some people are just suckers for this stuff.
Ok, Ok, but I do agree.

PUMPDESIGNER

RE: Doubt on Pump Suction

All good points - May I add "Just Fix The Leak!"
Entrained air in a slurry pump (regardless of how it is ingested) can accelerate wear and appear to be cavitation pitting on the impeller. It also reduces your efficiency. Suction leak repairs could be as simple as wrapping the joint with plastic kitchen wrap or good old Duct Tape since the atomspheric pressure will help seal it. The do it right at your next planned maintenance shutdown.

To address the "To suck or not to suck" question I think this may be better explained by the fact that centrifugal slurry pumps won't self prime and require positive suction head to prevent cavitation. The impeller vanes create a lower pressure region and the available suction head pushes the slurry into to fill the void.

Happy Holidays!

Keep the wheels on the ground
Bob
showshine@aol.com

RE: Doubt on Pump Suction

sprintcar,
Good post, But I have one question.
How can entrained air do what you said?:
"appear to be cavitation damage on the impeller"

PUMPDESIGNER

RE: Doubt on Pump Suction

One suggestion to clarify the point: since centrifugal pumps don't suck, their "suction" passages should better be called "intake". Please comment. Season greetings to all.


RE: Doubt on Pump Suction

Prisoner 25362,
I agree, intake (and also inlet) are most correct.
Intake and Inlet are accurate descriptions of a simple thing, fluid enters the pump at this connection or through this line.
That is all the words imply, no more and no less.
No implied reasons about why the fluid is moving into the pump, it is just moving into the pump.

Suction is not good because it has too much loaded into it. Suction is a description not of what is occurring in reality, but what the observer may be seeing or hearing.
Suction is inaccurate when NPSHa is higher than atmosphere.

PUMPDESIGNER

RE: Doubt on Pump Suction

Using the terms "pump suction", "suction pipeline", and "suction lift" really "sucks" as it implies the opposite to what is in actual fact happening at the pump inlet (read impeller eye),inlet pipe work and when the pump is higher than the supply source.

Although "we" know it is incorrect terminology we seem to use it as a day-to-day short-hand way of saying what we are implying - hence the confusion for many people who are not  pump literate.

As PUMPDESIGNER has pointed out, the correct terms are - inlet or intake, these are neutral terms and do not imply the inlet conditions at the impeller eye.  

My new year resolution is to use correct terminology in the future.

RE: Doubt on Pump Suction

Artisi,
I also am making sure that I discipline myself to only use the most correct terms.  I just reviewed my writings to make sure nothing was incorrect in them.  I have used these terms in the past (suction, suction inlet, suction line, etc.)

One term I am going to be stuck with however, I am afraid.
"End Suction Centrifugal Pump"

Not knowing any substitute I will be forced to use that term.  However, I think using that term will not cause any real problems.

PUMPDESIGNER

RE: Doubt on Pump Suction

PUMPDESIGNER, it's no wonder the layman can be totally confused, when we use terms like: end suction - double suction - split suction - vertical suction - positive suction - negative suction - suction head - suction lift  etc.
Unfortunately we are stuck with some of them so we can only do our best to educate where and when we can.
 

International College
Naresuan University
Phitsanulok
Thailand

RE: Doubt on Pump Suction

Artisi,
Wow, forgot about all those *.suction pumps.
I thought I had two brain cells left.
One must have died, now I am in trouble.

PUMPDESIGNER

RE: Doubt on Pump Suction

It all boils down to using the word suction as an adjective or noun for a spot's location or position or geometry, rather than for an action or the result of an action. Right ?

To PUMPDESIGNER from "prisoner 25362" :

"Once we are destined to live out our lives in the prison of our mind, our own duty is to furnish it well."
Peter Ustinov

RE: Doubt on Pump Suction

25362, guess that's a reasonable comment -but doesn't help the layman who is confused by the conflict in pump terms as well as deciding whether it's a noun or adjective we are using - all we need to do is use correct terms and keep telling everyone "PUMPS DON"T SUCK" unless of course the pump is giving us a problem - then the whole situation might "suck"

International College
Naresuan University
Phitsanulok
Thailand

RE: Doubt on Pump Suction

25362,
Good Quote.

PUMPDESIGNER

RE: Doubt on Pump Suction

PUMPDESIGNER, looks like we should put our 2 remaining brain cells together - seems we both missed the root of all this evil language we have been using for all these years -
NPSH - Net Positive SUCTION head.  We now know the reason why.
Dictionaries don't help either - some are confusing and some imply the wrong sense.

International College
Naresuan University
Phitsanulok
Thailand

RE: Doubt on Pump Suction

Artisi,

I guess this is the Tower of Babel we live in.

I see you are in Thailand.
They have lots and lots of water just laying around everywhere.  Huge potential for water pumps huh?

PUMPDESIGNER

RE: Doubt on Pump Suction

(OP)
Hi guys Thankx for all that comments. Think Iam almost satisfied with all your posts. But one question still remain Is there any equipment in this world that can really suck ? If so please intimate me so that I can really differenciate between the pump and that equipment's principle. Hope U all enjoyed your X'mas. Have a nice holiday

RE: Doubt on Pump Suction

The problem arises from focusing on the pump when the fluid properties are the real issue.

Imagine a pail of water.  How will you move the water out of that pail?  You can find only two ways: gravity and pushing.

Soooo,
If the water can only be moved in two ways, then all pumps must work with those two methods only.

I left out the qualifier to keep the explanation simple.  The only qualifiier is that we are speaking herein of only Newtonian Fluids, which are most simply defined as free flowing.

PUMPDESIGNER

RE: Doubt on Pump Suction

There is little on the Internet about pumps except courses available, but I found one site that explains some. A centrifugal pump is a non-positive displacement pump. A pump creates flow, not pressure. It is the resistance to flow that creates pressure. For instance, it you disconnect your outlet line and install a pressure gauge and flow meter on pump, it would read zero pressure and as much flow as the pump is capable of producing at that RPM.  The reason they give a pressure rating for pumps is that is the amount of pressure that the pump is able to withstand before damage will occur.

Flow determines the speed and is often rated in some quantity per minute. It determines how fast a flooded basement may drained or how fast a hydraulic cylinder moves against a load. In order to get flow you must have a difference in pressure. In your inlet, as has been said before the atmospheric pressure pushes down on the fluid, and the pump creates a more negative pressure and fluid flows into pump. If you have a reservoir supplying fluid to the pump and the vent to it gets plugged the pump will stop pumping, unless the pump is strong enough to collapse the reservoir. You can also have positive pressure entering pump from a charge pump or pressurized reservoir.

A leak on the inlet side of a pump is as bad as on the outlet side, but it is harder to find or know it is occurring. To check for leaks on inlet you should make a fitting up so you can install a compound gauge close to pump inlet. You then read this gauge with the pump running and compare it to pump specs. or ask OEM if they are within limits.  This should be repeated periodically as a preventive maintenance program to ensure no leaks have developed.  If you suspect leaks you can take whatever you are pumping, in your case water, and either slowly pour or spray it on the intake lines. If there is a leak the fluid will be drawn into the line at that spot and if pump is cavitating it will become quieter.

http://www.tpub.com/fluid/ch1p.htm

RE: Doubt on Pump Suction

When we "suck" a liquid through a straw, it is in fact atmospheric pressure, acting on the liquid surface in the glass, that is "pushing" the liquid up the straw.

The movement of liquids in pipes is prompted by pressure differences, whether resulting by a drop of pressure at one end or by increasing it at the other, to overcome flow resistances.

Thus, concerning arsmith's last question the answer would be a definite no. In this line of thought, pumps do not suck.

Even for pumps that produce "vacuum" (reduce pressures below atmospheric) by displacement, it can be said that it is the created pressure difference that "pushes" the liquid into the pump.

BTW, displacement pumps, whether rotary or reciprocating, also need a net positive head to get the liquid to their intakes without problems.

Without trying to be philosophical, taking these thoughts to an extreme, it could be said, that from the movement of fluids point of view, one should speak only of absolute pressures, and neither "sucking" nor "vacuum producing" would represent physically correct wording.

Any comments will be appreciated.

 

RE: Doubt on Pump Suction

Aviat!

Your post is very good except the first line. There is good lot of info on pumps on the net. For example,

www.mcnallyinstitute.com and www.pumpworld.com will give you enough details.

25362!

It's very true that differential pressure makes it all. Liquids can never be sucked, they are pushed rather.

BTW, can I have your e-mail id dropped at builblock1@yahoo.com

Regards,

RE: Doubt on Pump Suction

To quark, your PS notes have always attracted my attention, to the point I once suggested -based on these notes of yours- to add a new forum that could be called "unbelievable facts" or "engineers' fallacies" or "extraordinary facts" or something like that, and I felt sure contributions would abound. Somehow I let time pass feeling this may not be the right website for that; anyway the enthusiasm is still there. Let me know what do you think about this subject through the "round table" forum.

If you don't mind, I'll prefer momentarily to maintain my anonymity. To paraphrase and quote C.F.Jung:"...The finest and most significant conversations of my life were anonymous".

RE: Doubt on Pump Suction

It's ok 25362. Infact I have a gut feeling that I can meet you all in person one day.

...and you may not have to quote that ever again afterwards.

Best regards
and
Wishing you a Happy and Prosperous New Year

RE: Doubt on Pump Suction

One more point that I feel wouldn't do any harm to mention is: we agreed in saying that pumps don't suck liquids since these are "pushed" by the upstream pressure, however, when the waterways are filled with air they may be started by creating a "depression" by taking air (gas) out of the "suction" line exhausting it to the pressurized discharge side. In short, these pumps may even create a vacuum (a lift).
Positive displacement pumps do it with relative ease.

Centrifugal pumps, with differential heads expressed in feet of water, couldn't do that with air (800 times lighter than water).
There are ways for centrifugal pumps to get primed with liquid, and there are those designed to handle mixtures of gas and liquid; however, as a general rule a conventional centrifugal taking its suction from an atmospheric reservoir located below the pump wouldn't be able to remove this air and start pumping liquid.

My question is: could this air removal from the intake piping be called sucking or suction ? I think yes. Please comment.

RE: Doubt on Pump Suction

Suction and sucking are both descriptions of what we observe rather than what is happening.
We say the sun sets, because that is what we see happening.
But we all know that the sun does not really set, the earth turns away from the sun.

There is nothing wrong with using the terms suction or sucking in general, but because they can cause confusion some of us start being more accurate.

No problem.  I lectured my wife the other day when she said the sun was setting.  OK, I can handle the couch for a few days, its worth it if I can get her straightened out, but I worry that the couch is going wear out first.

PUMPDESIGNER

RE: Doubt on Pump Suction

The word suction is ok so long as we / you all understand its meaning. To suck is only the word we use to describe the action of lowering the pressure at a given point(in our case the impeller eye) so that atmospheric pressure can push (force) the fluid we are pumping into the eye of the impeller.

" He sucked on the straw to drink cold water from the glass". --- So what is happening, as he "sucks" on the straw, the pressure in his mouth is lowered causing a pressure inbalance between atmospheric (the external pressure)and his mouth (the internal pressure) this in turn forces (pushes) the water from the glass up the straw into the low pressure area - his mouth.

This is exactly the same physics as a pumping system when the supply source in below the level of the pump or when a pressure inbalance must be created so that flow to the pump can take place.

International College
Naresuan University
Phitsanulok
Thailand

RE: Doubt on Pump Suction

This is my last go at this one.

I have always wondered why certain engineering-inclined people have difficulty with the concept of suction. One of my first grade teachers tried to explain why suction could not exist while we were all drinking milk using straws.
The problem may be one of total energy gradient. As we all know, mass will (almost always)flow in the direction of lower total energy unless external energy is added.
In the case of suction, energy is removed from the lower energy sink in order to increase the total energy gradient. Increased suction causes the flow. This requires work to be done.
The more normal course of events results in energy being added to the energy source to increase flow.
Perhaps it is the concept of doing work to lower an energy level is the concept that some people have difficulty with.

Then there is the concept of centrifugal force, which is far more difficult, but agsin, everyone understands what is going on and words appear to be the cause of misunderstanding.

Cheers

Steve

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