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Determining the lowest speed you can operate a pump without cavitation 2
- Thread starter mpeck1982
- Start date
electricpete
Electrical
did you mean lowest speed or highest speed?
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(2B)+(2B)' ?
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(2B)+(2B)' ?
- Thread starter
- #3
btrueblood
Mechanical
Most pumps (I'm trying to think of exceptions and can't) don't have cavitation issues at lower speeds, thus e-pete's question. In other words, if the pump is not cavitating at some speed, flow and inlet pressure condition, then it will not cavitate at any lower speed.
electricpete
Electrical
I guess that would be zero rpm.
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(2B)+(2B)' ?
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(2B)+(2B)' ?
Irrespective of speed, a centrifugal pump will cavitate if there is insufficient NPSHa.
As already asked- send curve -- and some application data for a useful answer.
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.)
As already asked- send curve -- and some application data for a useful answer.
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.)
zerosequence
Electrical
MCSF (Minimum Continuous Stable Flow)
Running below MCSF does promote a form of cavitation called recirculation. This sounds like and can be just as destructive as vapour pressure cavitation. Often curves have MCSF actually displayed, it should at least be in the manual. Sometimes the curve is dashed below MCSF
Running below MCSF does promote a form of cavitation called recirculation. This sounds like and can be just as destructive as vapour pressure cavitation. Often curves have MCSF actually displayed, it should at least be in the manual. Sometimes the curve is dashed below MCSF
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1
- #9
MCSF has nothing to do with the lowest speed a pump can operate.
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.)
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.)
- Thread starter
- #10
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1
- #11
ApplicationENG
Mechanical
The lower your speed the lower your NPSHr. Look at the 1750 curve compared to 1150.
As another user stated, you only need to be concerned with the NPSH at your worst case scenario (high speed) and ensure that there is sufficient margin/ratio. Speeding down should not matter.
Typically the lowest speed I've seen in operation is 300 RPM for VFD applications - specifically for your pump size and configuration. I would contact Aurora and verify as their min. speed is probably higher compared to the upper tier Pump OEM's.
As another user stated, you only need to be concerned with the NPSH at your worst case scenario (high speed) and ensure that there is sufficient margin/ratio. Speeding down should not matter.
Typically the lowest speed I've seen in operation is 300 RPM for VFD applications - specifically for your pump size and configuration. I would contact Aurora and verify as their min. speed is probably higher compared to the upper tier Pump OEM's.
Since NPSHa is a function of flow losses in the suction piping system, the lower the speed, sometimes that results in lower flow and hence there would be lower losses in the suction piping and hence a higher NSPHa. So you not only have a lower NPSHr according to the curves, you will have a higher NPSHa with lower speed and lower flow.
rmw
rmw
I'm with rmw. I don't how you could have any trouble with reducing speed. You would have to have a very unusual set of hydraulic conditions to run into trouble doing that.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
It would be nice to know exactly what the OP has in mind, like why / what does the minimum speed question have to do with the application.
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.)
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.)
- Thread starter
- #15
Well we have recently demo'ed/replaced a CHW pump. The pump has a VFD. The DP setpoint in the building is 15psi and the actual is 29psi. The VFD will back down to its minimum output setting of 50%. I would like to bring that minimum output setting to 20 to 25%. This is how the VFD is set per Honeywell. Because currently, the actual DP in the building never fluctuates and the VFD is always at 50%. There is just to much CHW for the building. I was just curious how far left of the BEP on the pump curve I can go... I don't want to go to far where the pump might be damaged/cavitation, etc.
First thing I would look at is reducing the impeller diameter to reduce flow and head.
Plus you should ask the supplier for a variable speed curve for this pump - then there is no guess work.
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.)
Plus you should ask the supplier for a variable speed curve for this pump - then there is no guess work.
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.)
btrueblood
Mechanical
"This is how the VFD is set per Honeywell."
The lower speed limit might not have diddly to do with the pump. The VFD may be limited (by its design) in the lowest speed it can drive, or the pump motor may reach limits in which it can no longer be properly cooled at lower speeds. Have you talked to the Honeywell guys, or reviewed the VFD and motor manuals? The latter limit can be overcome with an auxilliary cooling fan if necessary.
The lower speed limit might not have diddly to do with the pump. The VFD may be limited (by its design) in the lowest speed it can drive, or the pump motor may reach limits in which it can no longer be properly cooled at lower speeds. Have you talked to the Honeywell guys, or reviewed the VFD and motor manuals? The latter limit can be overcome with an auxilliary cooling fan if necessary.
You need to be aware that when reducing speed, flow is reduced linearly and head is reduced by the square of the speed. At 20% speed, will you have enough head to move water, or will the pump just spin and heat up? Depends on your system curve.
I second the notion that an impeller trim is in order.
I second the notion that an impeller trim is in order.
Or just replace the pump. It obviously is not the right one for the flow you need. Impeller trim may be too large a cut to reduce head by 50%.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
I agree with 1gibson. Head is reduced by the square of the RPM. The pump needs to produce enough head to deliver the flow required to keep it from overheating. I think this is what limits the RPM, and is more important in this case than NPSH.
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