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

 

[PUMPDESIGNER]

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

 

[PUMPDESIGNER]

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

 

[25362]

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 30^oC 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 30^o. 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.

 

[PUMPDESIGNER]

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

 

[arsmith]

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?

 

[25362]

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.

 

[PUMPDESIGNER]

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

 

[Artisi]

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.

 

[PUMPDESIGNER]

I agree with your final analyses.

PUMPDESIGNER

 

[smckennz]

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

 

[25362]

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

 

[Artisi]

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.

 

[PUMPDESIGNER]

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

 

[sprintcar]

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

 

[PUMPDESIGNER]

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

 

[25362]

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.

 

[PUMPDESIGNER]

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

 

[Artisi]

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.

 

[PUMPDESIGNER]

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