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

 

[Artisi]

A vortex doesn't always originate on the water surface.

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]

i have done some research on internet and i found some articles telling that the best way to identify if it's a cavitation problem or air entertainment problem is by throttling the discharge valve, if the the noise from the pump disappear it means it's a cavitation problem, cause by closing the discharge valve we will be lowering the NPSHr.
my pumps still making the same noise even by closing the valve up to 65%.
do u think it's an air entertainment problem? and if it is, any solutions?

 

[Artisi]

You seem to be ignoring some important questions that have been asked:
1. are the replacement you use OEM or pirate parts?
2. has this problem always existed?
3. suction side valve placement
4. curves and duty point plus current operating values.

When a refurbished pump is put into operation is it noisy from start-up?

When throttling the discharge are you recording discharge pressure?


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]

1. OEM
2.the problem always existed
3.what is suction side placement?
4. duty flow= 425 l/s operating flow= 430 l/s
duty head=60 ft operating head=43.49 ft
when throttiling the valve:

valve 100% open pressure=1.3 bar
valve 60% open pressure=1.5 bar
valve 50% open pressure=2 bar
valve 40% open pressure=2.2 bar


when removing the pressure gauge upstream the pump and opening little bit the valve and the air is getting sucked inside the pump vibration is reduced too much.

 

[JJPellin]

Just because the problem did not improve when throttling on the discharge does not eliminate cavitation as a potential cause. If the cavitation is caused by suction recirculation, reducing the flow could make it worse. If allowing air to enter the pump makes the noise and vibration better, that suggests that cavitation may be the problem.

Johnny Pellin

 

[Artisi]

Suction side valve placement = where is the valve in relation to the pump inlet, and what is its orientation.

Something doesn't make sense with your figures,

You say that the operating head is 43ft with a (discharge ?) valve throttled, yet you say that with the valve 100% open the head is 1.3 bar which is near enough to 43 ft.

Do you have a pump curve for the design conditions.

Agree with Johnny, if you have either suction or for that matter discharge recirculation when introducing air may well eliminate the noise.
However, looking at picture No 5 - that impeller is completely destroyed and unlikely caused by cavitation - why? as the impeller is being destroyed flow and head will fall-off - this would reduce cavitation - therefore I would think that something else is going on causing the majority of damage.

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]

when discharge valve is open 100% =operating condition (discharge valve is not throttled neither inlet valve) pressure=1.3 bar => head=43.49 ft, but from the pump curve design head must be 60 ft and 425 l/s flow.
suction side valce is 2 meters away from the pump and it's placed on the side.