23 replies to this topic

[GTE]

Bonjour,

The site engineers are telling me that the pump is cavitating.

I have done NPSH available calculation and I have been extremely conservative on the calculation of the dynamic pressure and calculation shows that there should not be any cavitation.

 

Has anyone experienced this before!?

[breizh]

GTE ,
To get a proper answer please submit a sketch and the calculation for forums members to review.
Breizh

[ElSid]

Also, look at pictures or as builts of the system. Many a system where the engineer assumed/designed a straight line and there were "lots" of fittings or bends. Also, verify the pipe size, I just did a project where the calcs and P&ID showed 18" pipe and 20 inch was installed. suffice it to say that the check valve and metering devices were not working properly (in my case bigger was worse).

[GTE]

THANK YOU . I DID NOT KNOW YOU HAD REPLIES.

I HAVE CARRIED OUT THE CALCULATIONS AND FOUND THERE WAS NOT CALCULATIONS. I HAD THE AS BUILT DRAWING.

I THEN WENT ON SITE AND FOUND OUT THAT THE SYSTEM IS SUFFERING FROM "AIR ENTRAINMENT".

THE WATER FALLING IN THE SUMP OF THE COOLING TOWER (AFTER THE PACKING), FALLS WITH HIGH VELOCITIES, WHICH GENERATE BUBBLES OF AIR WHICH ARE ENTRAINED IN THE SYSTEM AND CREATE AN AIR LOCK.

MOST AMAZING PHENOMENON TO OBSERVE.

SO THANKS AGAIN FOR YOUR REPLIES. NOW I AM TRYING TO READ ON HOW TO ABATE "AIR ENTRAINMENT". IF YOU COULD HELP, I WILL BE MOST GRATEFULL

[katmar]

"Air Lock" is one of those wonderful catch-all terms that plant operators use to explain pipe flow problems that they do not fully understand. Like breizh said above, you will have to supply a sketch if you want useful comments.

[GTE]

Ok, I will send sketch.
Have you come accross "air entrainment"? (meanwhile)
Thanks,

[katmar]

Yes, air entrainment is common. Sometimes it is even deliberately allowed to happen. It is sometimes used as a method of level control - you simply oversize the pump that is pumping water out of the tank and when the level gets low the pump draws air and the pumping rate decreases. The pump gets quite noisy and sounds just like cavitation but is usually much less damaging to the pump than real cavitation. But check with you pump supplier if you want to run your pumps this way.

If have air in the lines you need to design for it, so let us see the sketches or photos and then we can comment.

[ElSid]

All cooling towers have filters. Most filters for cooling towers are sized as a dual function filter/air separator. Do a search for Air Separator or Air Purger. See Spirotherm, B&G, LAKOS (cann't remember if they are filters only), Wessels Vessels etc.

Good article on what you are experiencing : http://completewater...wer-pump-noise/

Without pictures or a drawing, a best guess would be that you probably have to high a velocity (need bigger pipe) as the air is vaporizing or too high a pressure loss (filter) before the pump

Edited by ElSid, 18 December 2012 - 12:48 PM.

[kkala]

Editing note: Following started before post No 8 by ElSid, but sent after it due to interruptions. During writing, post No 8 has not been considered yet, although article thereby referred explains the case (possible air in cooling water 4-6%, apparently v/v).
Yet air entrainment can result in quite heavy maintenance cost, even production loss if online spare is not installed, http://www.cheresources.com/invision/topic/11192-dry-run-of-pump - post No 20. Measures to prevent in http://www.cheresources.com/invision/topic/12124-submergence-required-for-suction-pipe/; a vortex breaker at beginning of suction line also helps.
Nevertheless post No 5 seems to concern not a vortex swirling into the suction, but air bubbles created in the water and entrained into the suction with it. It is not a consequence of low level, seeing that the cooling tower basin normally operates at maximum level, with continuous overflow serving as blow down.
GTE asks for this matter specifically on http://www.cheresources.com/invision/topic/16821-cooling-tower-air-entrainment/, with no response so far.
Having not seen a similar case in practice, it is not easy to trace some explanation. Layout details of cooling tower basin would be useful, along with whole suction line position in it, just in a simple sketch. Has the problem appeared recently? Can it be related to change of cooling water additives? The latter could lower water surface tension, thus promoting bubble formation. But this is just "wild" thought. Observation could reveal whether bubbling occurs on the whole cross sectional surface of the basin (or locally) and to what depth.

Edited by kkala, 18 December 2012 - 04:57 PM.

[katmar]

The comments by kkala have awoken old memories of problems I experienced with cooling tower pumps many years ago, and I think that he is probably right in identifying vortices as the source of the air. There is an interesting and well-written case study at http://turbolab.tamu.../P16/P161-8.pdf

[GTE]

Hi,
I am still waiting for the cad guys to give me the isometrics (schematics) and I should give it as soon as I have it. (IT problems...)

Kkala is correct, I have moved away from thinking that the pump cavitates. Calculations shows that there is no cavitation. I have check NPSHR and NPSHA. in theory it is all clear.

Now the problem is "air entrainment", I believe, and I have read substantialy and it seems that there isn't much you can do against "air entrainment" in an open cooling tower system, apart, of course, from the odd strainer pipe and antivortex items.

Now my question.
Do you expect the designer of an open water system to take into account the reduction in thermal conductivity of the water due to the amount of air in it? As we all know, water conductivity is 0.6 and air is 0.025 W/m2/K.

Because the bottom line is that the "air entrainment" phenomenon impedes the heat exchanger of heat rejection system, which leads to shutdown on "over heating alarm".

Should I just ask to resize the heat exchangers and forget about trying to get the air out? (that's another question)

Thanks btw for your sharing experience and knowlege.

[ElSid]

<sub>The system still needs to be "bled" (remove the air). AntiVortex items help, however, in my experience, slowing down the suction velocity (no higher than 3 to 5 ft/s) and having provissions for air release work best (slope piping). If you look at air release tanks in hydronic systems, they work on the principle of "slowing" the velocity such that the entrained air has time to "bubble" out of solution. This is why I keep my velocities relatively low on the suction side of a cooling tower (and slope the pipe back to the tower when possible (avoids buying an air separator and or valves. )Also, I have had this problem when the original design "elbowed" directly from the tower to the pump.

Looking forward to the drawing. Remember, always more than one way to solve the problem.</sub>

[S.AHMAD]

GTE

The best method to solve problem is determine the root cause and remove or eliminate the root cause. Since the root cause is air entrainment, you should focus on the root cause but not heat exchangers or the thermal conductivity. Those are symptoms.

[GTE]

Hi,
Just have been given an idea. Quite simplistic but that works apparently.
They put floating devices on the sump. So water just run (don't fall) on the floating device and into the sump, avoiding bubble generated.

Does anybody see something wrong with that?

[ElSid]

Never heard of it. Can you provide a link to a manufacturer?

[GTE]

I am struggling to find a manufacturer but I guess, I am going to go in the local swimming pool to check where they buy their large floating board for kids. I don't see why it should be more complicated.

Anybody foresee a problem, I am overlooking by using this?

[S.AHMAD]

GTE

I believe it is going to work. The device is similar/analogous to anti-vortex device.

Prevent the air from entering... yes it is the right way... eliminate the root cause!

Edited by S.AHMAD, 02 January 2013 - 08:38 PM.

[breizh]

Hi ,
Reading this document may help you to fix the problem.

Breizh

[S.AHMAD]

Thanks Breizh, good article.

[ElSid]

Breizh, good article

GTE, if operating like a vaccum breaker ... it might work. Consider this a stop gap solution to upsizing the pipe.

Happy holidays to all

[kkala]

1. Article attached to post No 18 (by breizh) has been also referred in http://www.cheresources.com/invision/topic/14826-criteria-for-the-usage-of-vortex-breakers-in-drums.
Article attached to post No 10 (by katmar) explains how ambient air intrusion was eliminated in a rectangular basin, also noting that major modifications on it can be cost prohibitive.
Just to complete this matter a bit, basin as per attached "CT.xls" did not develop air intrusion or pockets in suction / discharge piping (it depends on operating parameters too), yet its horizontal CW pump was hard to prime.
Link in post No 8 (by ElSid) is a useful article, attached as "CTnoise.doc" here, since a lot of links are not valid after some time.
2. Nevertheless post No 4 (by GTE) and CTnoise.doc indicate excessive cooling water aeration as (main) cause of noise in the pump (non classical cavitation) and of air lock in downstream plate heat exchanger. This can be verified by throttling pump discharge. Potential remedies mentioned in CTnoise.doc could be:
α. Small quantity of air injected into suction line; but this would increase air pockets in the exchanger.
β. Increase suction diameter to result in 3 ft/s (maximum), make it "smooth", slope it so that formed air bubbles escape to the surface of the basin (post No 12 by ElSid). Depending on suction line, this may be easy or not; extent of success is not known in advance.
3. The "floating plane" mentioned in recent posts may hit the target, yet extent of success is not known either. It is understood as a floating plastic surface (without depth). Falling water bounces on it, then flows along it to all horizontal directions into the basin. Splashing is anticipated. Channeling of horizontal flow might occur.
It may be useful to measure air content of cooling water (*) to check assumption of excessive aeration. Even better, use a small "floating plane" as a pilot for experiments.
(*) Fill a lab bottle with "aerated" cooling water and place a rubber cap on it. Shake and keep at constant low temperature (thermostat). 4% v/v air will make a difference over water vapor pressure of a blind sample in an identical bottle.
This is just an thought, there may be better ways to measure air content.
A "floating pan" with internal baffles may be more efficient, but quite complicated.
4. Irrespectively of the above, it may be worth while looking into following.
(α) Place the end of suction line far from the visible place of air bubbles in the basin..
(β) Place automatic air vent on plate heat exchanger (like that on house radiators) or just upsteam of it, to mitigate air lock. Experienced opinion on it is welcomed.

Attached Files

•  CT.xls   31KB   42 downloads
•  CTnoise.doc   42KB   36 downloads

Edited by kkala, 30 December 2012 - 09:48 AM.

[breizh]

Hi,
let me add one more reference where gas dissolution is taken account , with a specific example ( cooling tower pump )

Hope this helps you

Breizh

[GTE]

thank you all,
I should let you know in the new year
all the best for new year.
hope you had a good break for those it applies.
GTE

[ElSid]

GTE,
There a few more items that can cause air entrapment:

• the suction line itself. If any portion of the line is under a vacuum, air will leak into the line through gaskets and fittings that are not absolutely tight.
• a valve in a suction line, under a vacuum, can leak air into the line though the stem packing.
• the pump itself is also another source of gas. If the stuffing box, or seal chamber, is under a vacuum at any time while starting or running, air can leak through the packing or mechanical seal.
• the effective vapor pressure was incorrectly applied.

breiz
Thank you for the article. In my notes, I have that Hydraulic Institute recomments we add 3 psi to the vapor pressue (I was taught 5 feet); however, in my career, I read a a design guide that referenced Chen without a source. Wondering if your attached article is the same one referenced on the guide as the guide stipulated to add 14 to 16 feet of head to the Vapor Pressure which corresponds to a) the articles author and the added head whithin this article.

Follow up:
http://www.cheresour...ained-nitrogen/
Also, for cooling towers, the proper working water level is critical. If the water level is too low (say i inch off) the entrained air from the cooling fan does not have enough time to float up to the surface before it gets sucked into the pump suction. Double check with the manufacturer. I've seen documents from BAC (a reputed company) where their working water level was different in drawings and publications, in their submittal. Do NOT under estimate the power of that water level.

Edited by ElSid, 02 January 2013 - 06:20 PM.