Centrifugal Pump Cavitating on tank changeover 3

[beattsmjk]

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

 

[Compositepro]

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.

 

[MikeHalloran]

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

 

[beattsmjk]

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.

 

[CH5OH]

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.

 

[Artisi]

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.

 

[QualityTime]

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

 

[Artisi]

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.

 

[Artisi]

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

 

[JJPellin]

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

 

[vpl]

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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[beattsmjk]

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)

 

[beattsmjk]

i dont know why that link doesnt work, will try again below

http://files.engineering.com/getfil...3c-45ee-8641-e8f6ed784adc&file=P&ID-Model.pdf

 

[beattsmjk]

aaah crock of s**t, upload software.

please use this link

http://i32.tinypic.com/294tauo.jpg

 

[1gibson]

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.

 

[beattsmjk]

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!

 

[CH5OH]

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?

 

[QualityTime]

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

 

[QualityTime]

Artisi, i am appending another pdf file that talks about damage to the pump impeller due to cavitation and due to air

 

[Artisi]

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.