Doubt on Pump Suction 4

[arsmith]

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

 

[Artisi]

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

 

[PUMPDESIGNER]

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

 

[25362]

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

 

[Artisi]

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

 

[PUMPDESIGNER]

25362,
Good Quote.


PUMPDESIGNER

 

[Artisi]

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

 

[PUMPDESIGNER]

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

 

[arsmith]

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

 

[PUMPDESIGNER]

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

 

[aviat]

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

 

[25362]

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.

 

[quark]

Aviat!

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

http://www.mcnallyinstitute.com

and

http://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,

 

[25362]

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".

 

[quark]

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

 

[25362]

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.

 

[PUMPDESIGNER]

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

 

[Artisi]

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

 

[smckennz]

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