Cooling tower pumps impellers getting destroyed 3

[amer015]

I have 5 condenser pumps where each one of them is connected to a cooling tower, the discharge of the pumps are connected to a main header and then to individual chillers.
I have removed the pressure gauge before the pump and i noticed that the pump is sucking air if we open the valve instead of water coming out.
I have noticed the damege on the impellers of all the pumps, one of them is already gone 2 years ago and we are changing the impeller every year, the rest of the other pumps is getting dameged slowley and afew days ago when doing the maintenance of the pumps i have noticed that one of the impellers is starting to have a small hole in the dameged area so it's gone even this one.
water level inside the cooling tower basin is 1 meter above the impeller axes.
The distance between the pump and the cooling tower is only 5 meters, there is a butterfly valve 1.5 meters befor the pump and an elastic connection.
on the suction side of the pipe inside the basin i have discoverd that we have a grid connected to the pipe and we have a cage with 3 cubic meters volume around the pipe.

I have tried the following solutions:

1) remove the grid connected to the pipe and to cut the pipe with some inclination to increase the suction area but it didn't work.
2) close the discharge valve little bit but it didn't work.

the only solution that it worked was by increasing the water level inside the basin by almost 40 cm and the water start coming out from the pressure gauge valve before the pump, but this solution is useless cause the water start passing from one basin to another and i cannot increase the basin level cause it's very expensive.

I'm working on another solution now, we have a side stream filtration system that we are not using, so i'm trying to use the side stream filtration pump between the basin and the condenser pump to increase the pressure head before the condenser pump.

Questions:

Do u think this solution will work?

any other solutions you can help me with? and please don't give solutions like ( lower the pump level or increase the pipe diameter..)

you think that the atmospheric pressure above the cooling tower basin IS NOT 101325 pa do to the presence of the cooling tower fan so that's why in the NPSH(a) i cannot use that number?

you think that the cooling tower water contain to much dissolved air that is getting released inside the suction pipe and causing me reduced area inside the pipe that cause the fluid to increase it's speed?

thx

 

[katmar]

This case history of a very similar problem may give you some pointers.

http://turbolab.tamu.edu/proc/pumpproc/P16/P161-8.pdf

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[amer015]

i forgot to mention that it's a centrifugal pump and please find below the pictures of the impellers, the one that we repair every year and the other one that is getting dameged more every year.

http://files.engineering.com/getfile.aspx?folder=d3c0d2c0-efc1-43a7-9b9d-e10e1e4c12dc&file=pump1.jpg

http://files.engineering.com/getfil...0-7120-4fb3-ba99-cb9e9a924fbe&file=pump_2.png

http://files.engineering.com/getfil...6-11d9-49f5-80d0-646128f30905&file=pump_5.jpg

 

[gr2vessels]

Obviously, there is a classic case of not enough NPSH available. Like it or not, you have to give the pump enough NPSH. How you do it, it's up to you, there are many simple solutions, provided you learn first to not exclude anything, like you said, "please don't give solutions like ( lower the pump level or increase the pipe diameter..)".
How did you figure out the side stream to be higher NPSH than the available one, without interconnecting the storage vessels, seems interesting. It won't work.
Dissolved air in the water, yes, it could be a significant problem. Can you quantify that and put it in the NPSHA formula? Can you talk to the pump manufacturer asking for his professional advice? The pump seems to be severely cavitating, but I am surprised you never mentioned the horrible noise of cavitation and the pump vibrations during the worst case of operation.
The atmospheric pressure will never vary to the extent of offsetting the available NPSH, so that shouldn't be a problem. Most likely, the pump was incorrectly selected first place, you might need to decrease the impeller diameter to reduce the required NPSH below the available NPSH.

 

[amer015]

i had to esclude those options because it won't be possibile to do them( very expensive).
the side stream filtration pump is connected to the same cooling tower basin and it's 1 meter under the condenser pump so it's axes is 2 meters under the water level of the basin.
thx again

 

[Artisi]

At first look it doesn't look like classic cavitation from an NPSH perspective, are the 1st and 2nd pics.the same impeller? It looks like erosion and there could be discharge recirculation involved. A bit hard to tell from 3 pictures exactly what the problem is. By raising the water level 40cm you could be overcoming an air entrainment problem which is effecting the pump inlet conditions and putting the pump off-prime or resulting in poor entry of the flow onto the impeller blades, again hard to tell from pics. and not being onsite.
Removing the inlet pressure gauge and "sucking air" into the inlet may just be the result of low pressure at this point due to the velocity of water passing the inlet tapping. Then the gauge is in place what is the reading + or -.

Pic of pump 5 looks to me like a severe case on corrosion / erosion.

Have the pumps been flow tested against design conditions, how long have they been installed - has this always been a problem or something new resulting from an operation change.
Are the pumps noisy / vibrating in operation - what sort of noise ? it is hard not to distinguish cavitation noise over other noises, if no crackling / banging / gravel thru' the pump noise then it's unlikely that NPSH is the problem.

How far did you close the discharge valve, a little bit may not do anything - I would close the valve down in stages, letting the flow settle between each setting to a point where the pump started to protest and then back-off a little while taking flow and pressure readings, these can be compared to the design parameters.

If the 5 pumps all discharge into the same header, as performance falls off on any one pump it will be forced to operate at other than its selected point, flow will fall well off its best point due to the higher head it has to deliver to keep up with the other pumps forcing left on its performance curve - if you have non-return valves on each discharge the NRV will close if the head from the pump falls below the system head and the pump will then run against a closed valve condition unless it is monitored.

A few things to think about.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SteveWag]

What and how much, if any, chemicals are added before pumping?
Steve

 

[1gibson]

Is there an alternate impeller design (fewer vanes) that has lower NPSHR requirements? You need new impellers anyway...

You may be able to improve NPSHR by taking some material off of the eye side of the vanes (try a 1" strip of material removed from the leading edge of each vane.) You can't take off much because it will reduce your vane overlap and can affect performance, but 1" should be safe.

If you've repaired these impellers multiple times and made the vanes too much longer, that will block the inlet area and exascerbate the problem. If the vanes were originally curved with respect to the "floor" of the impeller and they were made straight when repaired, that would reduce the inlet area and exascerbate the problem.

 

[JJPellin]

I have a few comments and suggestions based on problems we have had with cooling tower services.

First, you need to do the math. Calculate the actual NPSH available for your system. Measure the water temperature and calculate an accurate vapor pressure. Get an accurate number for the head loss through the butterfly valve. If you do not have an NPSH margin of at least 5 feet, you are likely to have problems with cavitation. Just because you have negative gauge pressure on the suction does not suggest that you have inadequate NPSH available. What is the NPSH required at the normal flow rate?

Calculate the specific speed and suction specific speed. If the suction specific speed is greater than 10,000 (US units), you are likely to have more problems. The pump will be more sensitive to flow rate and more prone to recirculation modes.

Calculate the minimum required submergence for your system. This can be done using methods defined by the Hydraulic Institute. If you do not have enough submergence, you are likely to have problems with air ingestion.

These analyses may lead you to a solution. If low NPSH margin is the problem, you have limited options. If you can’t lower the pump, raise the level or increase the line size, you are going to have a hard time solving this one. There is one good option left on the table. A butterfly valve is arguable the worse option for a service with limited NPSH margin. It produces turbulence and it has a larger pressure drop than other valves. Replace the butterfly valve with a full port gate or ball valve. Otherwise, you could redesign the impeller inlet vanes with a parabolic profile to optimize the NPSHr for the pump.

If submergence is the problem, you can redesign the inlet to break up the vortices. The Hydraulic Institute defines a number of ways to accomplish this.

If neither of these is the problem, then I would suspect pre-rotation or low flow suction recirculation. If the flow is pre-rotating into the impeller, you can cavitate even if the NPSH margin suggests that you shouldn’t. The pre-rotation can be broken up with flow straightening vanes just before the pump suction.

If suction recirculation is the problem, you may be able to redesign the impeller to reduce the eye area. As long as you have sufficient NPSH margin, this could be as simple as a choke ring in the impeller eye to pinch down the open area. Some designs can experience suction recirculation even running at or above Best Efficiency Point flow. Don’t assume that this is not the problem just because the flow is well above minimum flow.

I do not understand the solution you have proposed. If all you are doing is directing a slip stream of water at higher pressure into the existing suction flow, I would not expect this to do any good. If you are completely changing the suction stream to the pump to a stream with a higher NPSH available, it might help. But, this would be determined by the NPSH calculation.


Johnny Pellin

 

[Pumpsonly]

Does not look like cavitation. The impeller is a double suction impeller.Does the damage happened on one side of the impeller or both?
If only on one side, then you could have unbalanced flow to the impeller.
Do you have a bend in the same plane as the pump shaft too near to the pump suction flange?

 

[1gibson]

Is the butterfly valve installed where it opens "up and down" or "side to side" ? If asymettrical wear and the butterfly valve is "side to side" then I think the mystery may be solved (assuming original system design was satisfactory.) You still may need new impellers, or different type of rework, to minimize any remaining problems.

 

[amer015]

thank you all for your comments...

i understand now it's not 100% a cavitation problem.

i wanted to tell you that when i was rising the water level inside the basin, i noticed that by increasing the water level only 20 cm the water start coming out from the gauge valve ONLY in case there is only 1 pump running but in case 2 pumps are running i need to rise the water level to 40 cm to have the same result.

Regarding the noise, yeah sure there is too much noise coming from the pumps it sounds like there is a small stones or a coin moving with the water inside the pump (cavitation!).
there is too much vibration too, i can feel the vibration inside the office 10 meters away from the pumps.

i'll give you soon the answers for the questions you have made as i need to check so many things before i answer.

 

[TD2K]

You have just over 1m static head between the centerline of the pump and the water level in the basin per your original post. If you have negative pressure at the pump suction you are taking more than that in line losses which wouldn't be what I would suspect for 5m of suction piping. I typically assume about 0.5 psi line losses and at reasonable suction line velocities, I don't see anything near that when I check my initial allowance against the final drawings.

Is there by chance a suction strainer in the line upstream of the pressure point? I'd do a quick check for the line losses and see what you would expect with one and two pump operation. Any indication you are vortexing air into the inlet since it doesn't sound like there is a lot of vertical depth of water above the top of the suction line.

 

[DubMac]

Do you have any pump curves to share?? Also speed pump is running, and any pressure readings??

 

[Artisi]

Are replacement impellers original supply or knock-offs?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[racookpe1978]

Can you lower the speed of the pump?

That would lower the required suction pressure, but would decrease pump output as well. But it sounds like like most days you are not anywhere near limits on cooling flow.

The variable speed drive (higher expense) minus its lower electric cost might pay off on the pumps.

 

[Artisi]

If the pumps are over performing, no need to try and lower speed by using expensive, complicated VFD - just reduce impeller diameter.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DubMac]

Lots of excellent pointers above; I would reiterate the following:

Like JJPellin said: DO THE MATH. Get your pump curve, find the NPSHR and calculate your NPSHA. No sense shooting off in all directions trying to solve a problem before you properly diagnose. If it is an NPSH problem, check for a lower NPSH impeller available.

Could very well be vortexing air into suction as in the paper above. A "poor boy" solution is to tether a piece of plywood to float above where the pump suction is. This will break the vortex if that is the problem. You should be able to visibly check for vortexing.

Have you checked that your butterfly valve stem is not broken? The paddle could be stuck in partially closed position.

Make sure impeller materials and water chemistry are compatible.

 

[Artisi]

"
Regarding the noise, yeah sure there is too much noise coming from the pumps it sounds like there is a small stones or a coin moving with the water inside the pump (cavitation!).
there is too much vibration too, i can feel the vibration inside the office 10 meters away from the pumps."

Feeling the vibration 10 metres away sounds like a complete mis-match of water onto the impeller blades from air-entrainment, vortexing, pre-rotation, running well over or under capacity.

If the pumps ran ok previously (this info you need to provide - helps with our advice) then it's unlikely NPSH is a problem unless there has been changes to water levels, pipe work etc.

You need to assemble a complete check list of all likely causes and work your way thru each point - a shot-gun is not the way to approach these problems.

My first move would be to establish flow / head against design.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[amer015]

No vortex are present and they never been present !!!