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Centrifugal Pump Cavitating on tank changeover
3

Centrifugal Pump Cavitating on tank changeover

Centrifugal Pump Cavitating on tank changeover

(OP)
Hi,

I have searched this topic and found some useful info, but I thought I would write a post to see if anyone can offer any more specific info that might be useful.

I work in a bakery and we have two tanks of liquid yeast (consistency of single cream or slightly thinner). This yeast is stored in chilled stainless cone bottomed tanks, each holding 25,000 litres.

We have both tanks connected to a 'ring main' however we only use one tank at a time, with the other on hold. The contents are agitated at a speed of around 75rpm to prevent settling out of the product.

The yeast flows from the tank in use down to a centrifugal pump approx 8 metres away and approx 1.5m lower(NPSH) distance is ok according to Packo (pump manufacturer), it is then circulated around a 'ring main' and back into the tank through the side of the tank.

As the yeast is required it is drawn from the ring in small quantities at four points (mixers).

The problem we have is on tank changeover. At around 700 litres certain valves close on the tank in use and others open on the other tank which allow for a seamless transition.

However we find that when we reach around 600 litres remaining, the pump starts to cavitate and we lose all pressure in the ring main (normally 3 bar). We then have to manually switch to the new tank and dump the 600 Litres (which is not good obviously - approx £500 each time).

We are scratching our heads as to the cause. like I said, the pump size was calculated for the system and so the NPSH is ok. The pump people have no further ideas.

The yeast is circulating at around 4degrees.

Do you think the agitators could be creating a vortex perhaps and allowing air into the pump? The tanks are fitted with vortex breakers according to the drawings.

We have the pump running at full speed (2900rpm) it is designed for 7m3/h although we only need 4.5m3/h with pressure drops around the system of around 3m3/h this works out about right.

I really look forward to any advice you might have.

Thanks in advance

 

RE: Centrifugal Pump Cavitating on tank changeover

There may very well be a vortex in your tank. The vortex could be caused by your agitator but it is more likely due to how the recirculated fluid returns to your tank. Can you not see any vortex? I imagine that the yeast tank has plenty of dissolved CO2 which will promote cavitation.

RE: Centrifugal Pump Cavitating on tank changeover

When you ask a pump salesman to size a pump, you ALWAYS get a pump sized for water.  Okay, maybe not always, but _I_ have never met one who was able to deal with SpGr or viscosity other than the default in their catalogs and/or software.  Actually, I've never met a pump salesman who even understood the concept of kinetic pumps' performance being fluid- dependent.  So you have to double-check the pump sizing yourself, or engage someone who knows how.

I don't care what the drawings say, you have to verify the presence and condition of the vortex breakers for yourself.

If the problem has always been present, i.e. nothing has been modified from the design state, and the stated overcapacity actually exists, you might be able to run the pumps a little slower.





 

Mike Halloran
Pembroke Pines, FL, USA

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
hi Thanks,

compositepro, I cant see evidence of a vortex, but to be honest my view into the tank is through a 4" vent at the top. The tanks are approx 7 metres high. By the time the tank changes the liquid is at the top of the cone part of the tank and the agitator blades are just about covered.

You are right in thinking the tank is full of CO2.

Mike: The pump was specced for a viscosity of liquid similar to yeast approx 100cp. But I know what you mean.

 

RE: Centrifugal Pump Cavitating on tank changeover

possible to post a drawing of the piping setup
Q:if tank is switched over @ 700l, how can you reach 600l?
pressure drops in pipesystem results in consumption of delta pressure the pump is producing, not in flow reduction or in other words, reduce the flow in the return line by introducing a restriction (orifice)if you're able to reduce the flow of the pump, it will require less NPSHa to operate without trouble.

RE: Centrifugal Pump Cavitating on tank changeover

I would in some ways tend to agree with Compositepro with the added thought it is possible that the agitator is beating air / gas into the pumped liquor. Also if the return flow is  "free-falling" from a high level into the tank it could also be driving air/ gas into the liquor.

It is more than likely the pump IS NOT cavitating just going on and off prime due to entrained air/gas.

Can you raise the level of change over a little and see what happens.

My experience of tanks /agitators/ pumps is one of seeing many problems with entrained air.

 

RE: Centrifugal Pump Cavitating on tank changeover

Cavitation means there is lack of NPSHa.  It is becasue of the lack of NPSHa you are getting bubbles forming and collapsing in the pump.  Air or gas in the liquid, not due to lack of NPSHa, will create the same noise as you would hear if it was cavitating because you are collapsing the bubbles in the pump.  It may sound the same as cavitation but the solution to fix it is different.  By the way, the damage to impeller is at the same location for both problems.he gas or wate

RE: Centrifugal Pump Cavitating on tank changeover

QualityTime, will have to disagree with you on the effects on air entrainment as you have noted.
It is very unlikely that entrained air will cause any noticeable noise and also extremely unlikely to cause any damage, the main effect is entrained air is to expand in the eye of the impeller and restrict flow while sufficient entrained air will cause a complete lack of prime. Entrained air entering an impeller eye will expand due to the lower pressure but will return to its earlier state after passing thru the impeller eye - there would be insufficient energy transfer to cause damage unlike a cavition bubble imploding from its gaseous state back to its earlier state.

One interesting fact is to purposely entain air into the inlet of a cavitating pump to reduce the noise and damaging effective of caviation resulting from NPSHa/r problems, this reduces damage not increase it.  

RE: Centrifugal Pump Cavitating on tank changeover

Seems today I have problems typing - cavition - caviation - Oh bother!  cavitation  

RE: Centrifugal Pump Cavitating on tank changeover

I tend to agree with the consensus that ingested gas is likely causing the pump to loose prime.  Vortexing is a good possibility. But there is one other.  You state that at the transition, certain valves close and others open.  The order and timing for the valve moves is very important.  Is the system set up to close off the tank coming out of service before opening up the tank coming into service?  Is there any point in this transition where the suction conditions to the pump change significantly?  If you have a liquid that is saturated with CO2, any drop in suction pressure will result in gas flashing out of solution in the piping and pump.  Even if the steady state calculations show adequate NPSH margin, this transition may not qualify as steady state.  We have seen this on the pumps that draw streams off of a fractionator.  In theory, it is impossible to loose NPSH margin since the elevation head is greater than the NPSH required.  But, during transient conditions, the temperature or pressure of the stream in the suction piping can change fast enough for vapor to form in the piping.  In one case, the sudden drop of pressure caused product to vaporize in the piping with enough violent force to cause a leak with a resulting fire.  I would explore the timing of the valves and closely monitor the suction pressure right before the pump during this transient.  Any drop in pressure with a saturated liquid will result in vapor formation.

Johnny Pellin

RE: Centrifugal Pump Cavitating on tank changeover

Could you describe the sequence of your valves opening and closing in a bit more detail?  Or maybe post a diagram?  Just off the top of my head, solving the problem is going to require either (1) changing the sequence of the valves opening and closing or (2) starting the process at a higher tank level.

Are you trying to completely drain the tank?  That might not be possible.

Patricia Lougheed

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RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Hi,

Thanks for the advice so far. Some more info might be useful.

I attended site this morning and the tank that previously was causing cavitation I switched off the agitator, flooded the line to the pump and started the pump without any problem. I then circulated the yeast before returning back to the tank, so I wonder if it is due to agitation that perhaps should be switched off at a pre determined level.

Anyway I have attached a drawing of the system, so here is a brief explanation of the sequence.

Under normal circumstances the yeast path follows the yellow line shown in the diagram.

When the tank level of the tank in use reaches a predetermined level of around 1000 Litres the other tank valve (in this case AV2) opens to flood the line in preperation for changeover

When Level reaches "Open New Return Route " 500 litres then the Tank Inlet Valve (in this case AV4)for the new tank is opened to allow yeast to return to the new tank from the old.

When the Empty Pipe Sensor (ES1) or the Flow Sensor (FS1) or the Pressure Sensor (PS1)detect a loss of yeast, then the final automatic valve (in this case AV10) is opened to allow yeast from the new tank into the flow plate.

Please note ES1, FS1 and PS1 are not shown on the diagram but ES1 is situated just before the pump after AV11 and AV12, FS1 is a flow switch situated just after the pump and PS1 is also situated just after the pump.

A very short time later the Old Tank Valves (AV1 and AV9 or AV2 and AV10) are closed and the changeover is completed



I hope this info helps you to help me.

At present it seems the pump is cavitating as the tank contents expire, however at one point with 900 litres in the tank, a tank change had to be forced as the pump was cavitating.

At present the agitator runs until it is clear of yeast (approx 300 litres)

 

RE: Centrifugal Pump Cavitating on tank changeover

Is it an option to kill, or slow down, the agitators during this transition?

May or may not be a long term solution, but it would be good information and seems like an easy troubleshoot.  

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Hi 1gibson,

yes this is my intended plan for the next changeover due on Sunday, I am going to shut off the agitator around 1500 Litres and see what happens. If it works then I will re write the program to incorporate this into sequence.

if it doesn't work, I don't know what the next step will be!

RE: Centrifugal Pump Cavitating on tank changeover

awfull lot of valve for the functionality you describe (could do with only four)
one more question:the square plate in the midle between the tanks, what is it for?(i assume you named it a flowplate.
a few things I would check:
what kind of level measurement is used, can it be misled by foaming of yeast (if agitator is running on high rpm)?
can there be any blockage in the pipesystem,caused by solids?

RE: Centrifugal Pump Cavitating on tank changeover

Artisi, I think you are incorrect about damage to pump impellers when there is air in the liquid.  If you do s search on the internet you will see a number of articles on it.....especially with cooling tower pumps.  I had a recent experience with this and i documented on this website

RE: Centrifugal Pump Cavitating on tank changeover

QualityTime, Thanks for the articles - unfortunately I can not open the second one.

Actually nothing new in the first article, however I would point out that the article does really zero in on cooling tower pumps renown for air entrainment - which is not an unexpected problem in such an application. This problem is also common in the mineral dressing handling liquors from froth flotation (special pumps required) and also in the paper industry where most paper stock is agitated in the stock chests, in some cases special impellers or modifications to the pump are required.

My thinking is that air collection in the approach pipework or inlet / eye of the impeller can/does in fact result in true cavitation, this is due to flow being restricted by the expanded air, in reality this is no different than closing down on an inlet valve to restrict flow which results in a change NPSHa to the point where cavitation results.
As this is problem is not usually encountered in day-to-day pump applications I stand by what I said earlier, entrained air is unlikely to cause noise or damage but now with an over rider "except in specific circumstances".

Would appreciate it if you could resend the second article for review.

RE: Centrifugal Pump Cavitating on tank changeover

Just to add a bit more discussion to the problem, a mis-match of flow onto the impeller blade from pre-rotation, operating well left or ever well to the right of BEP can cause uneven operation and considerable noise sometimes mistaken for cavitation.

RE: Centrifugal Pump Cavitating on tank changeover

Hi Artisi:

For some reason the article I wanted to download does not seem to open.  The following is the actual web link:

http://www.practicalpumping.com/articles/pdfs/Cavitation%20or%20Not.pdf


If you do a search elsewhere, you will see that air can cause damage to a pump.  If you think about it...you have a mixture of air and water in the pump...air is compressible and water is not...and because of that, the pump pumps easy (because of the air) and then it pumps hard (because it hits water).  It is not good for the pump, motor and coupling.  If you do have problems with air  and you put your ear to the pump discharge you will probably hear that the flowing water in the piping does not sound right.  

The damage to the impeller due to cavitation is due to the little air bubbles being compressed.  If you have air coming into the suction of the pump (not due to cavitation)...you get the same result in that the bubbles get compressed and that is why you get the same sound as cavitation.  

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Hi,

Just an update, the pump is definately cavitating, not just sounding like it is.

It is getting extremely hot and we lose all pressure on the discharge.

What do people think of running the agitator in reverse so that the blades due to the angle in which they are mounted, in effect push the liquid down? Would this work or might it put greater pressure on the gearbox perhaps?
 

RE: Centrifugal Pump Cavitating on tank changeover

As per the article I posted....if it is cavitating the true test is to throttle the pump discharge.  The cavitation noise should get quieter and quieter the more you throttle.  You will reach a point where there is no cavitation and there is no noise

If the "cavitation" is due to entrained air....no matter how much you throttle the pump the "cavitation" noise will not disappear

In order to back the above practical field test have you done a calculation to determine whether or not it should be cavitating??

RE: Centrifugal Pump Cavitating on tank changeover

If the pump is getting hot, sounds like it is running un-primed due to air or low NPSHa by restriced valve opening.

I would suggest a systematic approach to the problem, look at each likely problem in turn and eliminate them one-by-one,is it entrained air, is it restricted NPSHa due to valve closing sequence, is it operating too far left of BEP etc.    

Reversing the direction of the agitator is a fix sometimes used and sometimes successful in the paper industry when it is causing air entrainment problem.
 

RE: Centrifugal Pump Cavitating on tank changeover

QualityTime,  Thans for the link to the second article, seems to me to be a rehash of the earlier article like many others which just restate the same information over and over.

Have attached a link which you may find of interest.

http://www.waterworld.com/index/display/article-display/363196/articles/waterworld/volume-25/issue-6/departments/pump-tips-techniques/impact-of-cavitation-air-on-centrifugal-pump-performance.html

RE: Centrifugal Pump Cavitating on tank changeover

Artisi, I beleive the discussion was about damage whether or not air in the pump will do damage to the pump impeller.  you seemed to have doubted it and the article I referred you to talked about the potential damage that could occur.

thanks

RE: Centrifugal Pump Cavitating on tank changeover

Postbeattsmjk,
Probably has been taken into account during the design that most yeasts are seriously non-Newtonian in their behavior. They normally show pseudoplastic behavior, which is quite different from how water behaves. This is especially visible at start of flow and filling of spaces. Another aspect is the carbon dioxide which is normally substantially present in reacting yeast.

You state that: "When the Empty Pipe Sensor (ES1) or the Flow Sensor (FS1) or the Pressure Sensor (PS1)detect a loss of yeast, then the final automatic valve (in this case AV10) is opened to allow yeast from the new tank into the flow plate."

This includes that you have already a too low suction pressure at the moment the valve is opened. Only by a rapid and full supply of yeast from the newly opened tank will the pump be able to continue normal operation. To my opinion following points could cause your problem:

(1)    I do not know whether the primed/flooded part of the pipe below AV2 is without gas. After opening the valve AV2 a substantial gas bubble could be present below the newly entered yeast.

(2)    As yeast shows pseudoplastic behavior, it will have a large initial viscosity until it starts to flow substantially. This combined with the required acceleration of the newly connected yeast, could cause a substantial gas bubble in the suction line.

(3)    I assume you have a kind of non-return valve installed, as at the moment you have both tanks in parallel on the pump, the yeast from the full tank will flow to the nearly empty tank. At that moment the pressure in the suction line will stay that of the nearly empty tank. As far as I know, no return valves should not be used in a hygienic application, due to difficulty in cleaning.

(4)    In order to cope with the gas should the pump be self venting.

Looking at your system, it appears that the Empty Pipe Sensor (ES1), the Flow Sensor (FS1) and the Pressure Sensor (PS1) should be located in the legs between the valves AV2 – AV10 and also a set between the valves AV1 –AV9. This would allow switch over at the moment the pipe below the valves AV9 and AV10 is still full off liquid.
 

RE: Centrifugal Pump Cavitating on tank changeover

I think what is happening is that when the TANK IS FULL and the mixer is running there IS ENOUGH depth and residence time to allow the air that is introduced by the mixer to rise to the surface.  When the LEVEL IS TOO LOW there is NOT ENOUGH TIME FOR THE AIR TO RISE TO THE SURFACE BEYOND THE ZONE OF INFLUENCE before it gets sucked into the pump.

I have run across this type of problem before in a cooling tower installation and it is documented on this site.  The cooling tower manufacturer's rep set the working water level as per the levels they have always been doing it.  The rep advised that they have always set the working water levels to this level.  We were getting air being introduced into the cooling tower pumps and it was causing a lot of problems.  I could see the effect of the air on magmeters that I installed on the cooling water pump discharge.  I then reviewed the working water levels DIRECTLY with the cooling tower manufacturer and found that THEY HAD historical conflicting published data showing what the water levels should be.  The manufacturer looked into it and advised I was right.  At the end of the day, the manufacturer had the rep raise the working water levels and the air problem disappeared....and we were only talking about raising the water level about 1.5 - 2 inches!!

Getting back to the problem...some potential solutions:

1   ...if you can live with shutting off the mixer when the level reaches a certain point you that is the easiest solution.

2.  If you need mixing I would talk to your mixer manufacturer and look at a blade design that does not entrain air.  The other thing is that mixers are typically designed to work well when the tank is FULL.  When the tank level drops the mixer performance IS VERY DIFFERENT so you should look at this fact very carefully with your mixer supplier.

3.  You might want to look at some form of non violent jet mixing using a recirculation pump.

4.   As far as the valve sequencing issue you should be able to solve that by reprogramming the PLC

 

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Many thanks for the excellent comments, in particularthe last two.

FMJalink,  I appreciate your thoughts very much and you have some valid points there that I will definately look into.

In terms of the changeover, I switched off the agitators at 1500 Litres (they normally switch off automatically at around 100 Litres). I then watched the process over a period of 5 hours and am please to say the tank changeover took place without any problem.

There was a drop of pressure to zero when (as mentioned in FMJalinks post) the old tanks outlet valve closed and the new one opened. There is no check valve because the system waits for one valve to close before allowing the other to open, therefore for a second there is no flow.

I will look into adjusting this, perhaps by repositioning the sensors as you mentioned.

Many many thanks chaps, it is much appreciated. I will keep you posted on progress.

  

RE: Centrifugal Pump Cavitating on tank changeover

I really want to stress Point (2) I made in my last post:


THE MIXER MIXING ACTION IN A TANK IS COMPLETELY DIFFERENT WHEN THE TANK IS FULL COMPARED TO WHEN IT IS PARTIALLY FULL.  It could be that when the tank is full there is no air being introduced into the process and when it is partially full the action of the mixer blades is quite different and as a result you might be causing air to get into the process.  THE DEPTH OF SUBMERGENCE OF A MIXER BLADE IS REALLY IMPORTANT.  I WOULD TALK TO YOUR MIXER MANUFACTURER FOR POTENTIAL SOLUTIONS.  Maybe there should be two mixers..each placed at stratgic levels and they turn on or off when the tank reaches a certain level...just a suggestion

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Thanks Qualitytime,

I completely agree with you. When the mixer is fully into the liquid, there are no bubbles at the surface, yet when the blades are submerged by say 500mm, there is a dense layer of foam on top of the yeast.

I will definately look into this more

Many thanks again!

RE: Centrifugal Pump Cavitating on tank changeover

...where do I send my bill.....   smile

RE: Centrifugal Pump Cavitating on tank changeover

Hi Beattsmjk:

One other thing...and I don't want to sound belabor the point I made earlier in this post....BUT IF IT IS THE MIXER INTRODUCING AIR INTO THE SOLUTION AND EVEN THOUGH IT SOUNDS LIKE CAVITATION IS OCCURRING IT IS NOT CAVITATION BY DEFINITION.  In this case the potential solution is to fix the problem with the mixer so you don't get the foaming.

Cavitation, by definition, is because you have a lack of NPSHa and the solution to that is to, for example, reduce the friction losses in the suction piping by, for example increasing the suction pipe size.  

As I have said earlier in this post, THE TWO PROBLEMS PRODUCE THE SAME SIMILAR CAVITATION SOUNDS BUT THE SOLUTION IS ENTIRELY DIFFERENT.
 

RE: Centrifugal Pump Cavitating on tank changeover

It would seem that the first response from Compositepro and my first response were pretty well on the mark, the agitator beating air into the liquor.  

One of life's lessons (free of charge), if an agitator is involved in a process, it is the number 1 suspect when it comes to inlet problems.   

RE: Centrifugal Pump Cavitating on tank changeover

So....what has happended since?

RE: Centrifugal Pump Cavitating on tank changeover

(OP)
Hi Qualitytime,

we have had flawless changeovers since we stopped the agitator at 1500 litres. I have now written this into the program and all seems well.

Many thanks for your help.

RE: Centrifugal Pump Cavitating on tank changeover

Well, what a surprise!  

RE: Centrifugal Pump Cavitating on tank changeover

Hi...you are welcome!

RE: Centrifugal Pump Cavitating on tank changeover

Hi:

By switching off the agitator at 1500 L a side benefit would be that you are extending the life of the agitator.  Agitator bearings do not stand up well when the blades are not submerged to the level they work well at  smile

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