Pump Cavitation

[WilliamTz]

Hi

I have installed 2 x large pumps 75KW (111L/s) under 2 x open circuit cooling towers
The pumps are on VSD's
If I switch on and ramp up the pumps slowly 1 pump or 2 pumps at the same time everything works very well...no noise, with no apparent cavitation
If however I ramp up a pump to full speed fast, the pump make noise and it appears there is air in the pipes and pump impeller (the gauges shake)
There is also noise and shaking if 2 pumps are running with no issues and suddenly I ramp down and switch off one of the pump, the one remaining ON makes noise, and appears to have cavitation

I have checked the NPSH and all looks good according to calculations.

Why is it an issue when the pumps are stopped/started suddenly and not when it ramps up or down slowly

Regards

B

 

[Artisi]

It's unlikeky cavitation.
How about a sketch of the installation, pipe lengths, elevations etc.
Common inlet pipe, common discharge pipe?
Pump curve and duty point.

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.)

 

[rotw]

There is also noise and shaking if 2 pumps are running with no issues and suddenly I ramp down and switch off one of the pump, the one remaining ON makes noise, and appears to have cavitation

Water hammer??
If you switch off one pump, and keep the other pump running at constant speed, you would have half the total flow going to be delivered to the downstream system. This means the system curve requires less delivery pressure for a single pump running at constant speed and it looks like it occurs as a sudden disturbance.

https://translate.google.fr/transla...n-angulaire-au-joint/coups-belier&prev=search

 

[LittleInch]

Needs a sketch and section to show levels, pipe submergence etc

Also how are pumps arranged? parallel? series?

Do you have flow measurement anywhere? If so what are the readings?

DO you have current measurement of the motors during these events?

What are VFDs controlled on ? level or pressure/flow?

Cooling tower ponds can be quite aerated and also suffer from vortexing.

When turning one pump off if your system curve remains unchanged you might be running off the end of the pump curve, hence my question about flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WilliamTz]

Hi
Schematic attached

We have automatic balancing valves on each tower..mechanical devices to give you the flow you need....set a 111L/s each... (FLOWCON Valves) I have to check the DP across them next week..to verofynot too high or too low.
The 2 pump motors are running fine today..no issue 47 Hz 1400rpm, 100Amps each... but i did start them up together slowly... No vortexing in the basins...just a slight swirl they you would expect as the tower water leaves.

Regards

b

 

[LittleInch]

Won't download for me - looks like its missing a .pdf or .jpg

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

The filename has an "&" in it. It works if you copy the link and manually replace "&" with %26.

https://files.engineering.com/getfi...a12a23bef72&file=Schematic_&_side_profile.pdf

 

[WilliamTz]

Hello

Schematic here again...thanks for tips above

 

[WilliamTz]

...just another word on the hydraulics of the system... 2 towers and 2 pumps have been added to the existing system of 2 towers and 2 pumps

When all pumps are switched off the towers overflow... the volume of water in the big 600dia headers at 5m drives on the water over the smaller 7m hihg pipes into the top of the towers...I think the actual weight volume and siphoning effect drive it on...Could this be at play when 2 pumps are running and one is switched off suddenly

Today I ramped up both pups slowly, result no issues. Then I ramped down 1 pump slowly... and the remaining pump wasent perfect but it was better...then when I closed 1 tower it then reverted back to being nice and quite

Your thoughts?

 

[LittleInch]

So where are these mysterious flowcon valves?

If they are set to 111l/sec, but you have four towers but only one or two pumps runnnig??

What controls the VFD?
What is the downstream pressure and flow?
does it vary?

If you turn the pumps off where is the water coming from?

How is the level in the towers controlled?

How do you maintain equal flow into the towers.

Does it make a difference if you use different pumps.

I think you might be having issues with equal flow into the pumps or through the tanks.

Ramping too fast might be pulling one tank down too fast and then sucking air. Turing a pump off I think you might be going off the end of the curve.

Very difficult to assess this from a distance with no operating information.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WilliamTz]

I have figured this out.... the problem was the pump(s) was running at full speed with all towers and chillers available so it was doing too much and running towards the end of the pump range causing the guage to hop all over the place. I have figured out where to run the VSD (40hz) and everyting is now good.

Regards

b

 

[LittleInch]

Good, Thanks for letting us know and hope we helped. Has this stopped the towers from overflowing as well?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[WilliamTz]

Hi LittleInch
Yes thanks for all your help + others....The towers still overflow when pumps turned off and the inlet and outlet valves are still open. I have yet to figure out if its filling from the inlet or filling from the outlet.... Ill be back when I figure it out.

Regards

Billy