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Determining the lowest speed you can operate a pump without cavitation
2

Determining the lowest speed you can operate a pump without cavitation

Determining the lowest speed you can operate a pump without cavitation

(OP)
From looking at a pump curve, how do you know if what is the minimum speed you can operate the pump without cavitation? Will the pump curve be useful?

RE: Determining the lowest speed you can operate a pump without cavitation

did you mean lowest speed or highest speed?

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(2B)+(2B)' ?

RE: Determining the lowest speed you can operate a pump without cavitation

(OP)
What is the lowest frequency I can operate a pump on a VSD avoiding cavitation.

RE: Determining the lowest speed you can operate a pump without cavitation

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.

RE: Determining the lowest speed you can operate a pump without cavitation

I guess that would be zero rpm.

=====================================
(2B)+(2B)' ?

RE: Determining the lowest speed you can operate a pump without cavitation

Could you send a copy of this pump curve you speak of??

RE: Determining the lowest speed you can operate a pump without cavitation

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.)

RE: Determining the lowest speed you can operate a pump without cavitation

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

RE: Determining the lowest speed you can operate a pump without cavitation

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.)

RE: Determining the lowest speed you can operate a pump without cavitation

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.

RE: Determining the lowest speed you can operate a pump without cavitation

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

RE: Determining the lowest speed you can operate a pump without cavitation

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

RE: Determining the lowest speed you can operate a pump without cavitation

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.)

RE: Determining the lowest speed you can operate a pump without cavitation

(OP)
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.

RE: Determining the lowest speed you can operate a pump without cavitation

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.)

RE: Determining the lowest speed you can operate a pump without cavitation

"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.

RE: Determining the lowest speed you can operate a pump without cavitation

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.

RE: Determining the lowest speed you can operate a pump without cavitation

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

RE: Determining the lowest speed you can operate a pump without cavitation

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.

RE: Determining the lowest speed you can operate a pump without cavitation

mpeck -

Ill remind you the point of this post was to determine the minimum speed. based on your description above, after numerous posts and discussions, it is clear that the minimum RPM is not the question, it is the minimum flow of the pump. This entirely has to do with flow and NOT cavitation.

in essence, aside from the info. recommending to contact the pump vendor and reducing the impeller diameter, all posts are off topic because of a poor initial description.

The min. flow is based on the pump design (recircluation of fluid, bearing vibration, etc.) from the OEM. Youll also notice the the min. flow isnt listed on the curve you provided us - these are the pitfalls of going with tier II/III pump companies.

Best advice - contact the OEM - first tell them the pump size, speed, and current impeller trim, the head and flow currently and the head and flow desired - ask them "what is the min. flow for this pump operating at a reduced speed to achieve the requested head?"

RE: Determining the lowest speed you can operate a pump without cavitation

The min flow of the pump isn't the real question either, it is the min flow that will work with the system.

Flow scales 1:1 with speed, so 1/2 original speed also means 1/2 original min flow for the pump. At some point, the pump will be making insufficient head to even move water through the system, that is the constraint.

RE: Determining the lowest speed you can operate a pump without cavitation

gibson -

you are speaking from a process perspective - i am talking from a pump perspective.

Determine and min flow of the system ----> see if that is feasible with the current pump ------> if not, buy new pump.

RE: Determining the lowest speed you can operate a pump without cavitation

I'm speaking from a general engineering perspective (despite the fact that I work as a centrifugal pump engineer) and I've answered the original question: "what is the minimum speed you can operate the pump without cavitation?"

Since cavitation is not relevant, the question is effectively "what is the minimum speed you can operate the pump." The answer is: the lowest speed that the pump will still move fluid through the system. If it doesn't move the fluid, it will just spin and heat up. A pump spinning but not producing flow, is not actually "operating" in the literal sense.

op·er·ate
[op-uh-reyt] Show IPA verb, op·er·at·ed, op·er·at·ing.
verb (used without object)
1.
to work, perform, or function, as a machine does

RE: Determining the lowest speed you can operate a pump without cavitation

gibson -

not to get into a battle - you sound knowledgeable. I am sure you can agree that there are instances where the min. flow of the system can be lower than the min. flow of the pump. I can name a number of them if you want to dispute it.

hence the process vs. pump perspectives.

regards.

RE: Determining the lowest speed you can operate a pump without cavitation

Of course, no hard feelings. I just think taking sides on "process vs pump" is fundamentally flawed no matter what the scenario (pointing fingers on warranty coverage being the only exception.dazed) I wouldn't want to project that philosophy in a technical discussion. Even re-reading my posts (which often changes my own opinion of what I was trying to say) I'm not sure what could be construed as a process perspective.

I think you may have mispoken: "there are instances where the min flow of the system can be lower than min flow of the pump" would mean limitation is with the pump. While I don't doubt the existence, but my only dispute to scenarios where min flow of the system is higher than the min flow of the pump, would be relevance to the original question.

In any event, I think we've covered all bases and then some. Always a good intellectual exercise to put such a general question under the magnifying glass, right?

RE: Determining the lowest speed you can operate a pump without cavitation

Before this topic is put to rest, I found some old rules of thumb concerning MCSF sittin' around. They are quite antiquated and both based upon pumping water in general service pumps. It has always been my understanding that the term MCSF = the minimum flow required to remove the heat generated by the pump's operation; nothing more, nothing less. The corresponding pump "minimum speed" can be back-calculated from this flow.

Temperature rise in centrifugal at reduced flow:
T = H/778 * (1/e - 1)
T = Temperature rise in Deg.F
H = total head in feet
e = pump efficiency at capacity corresponding to H

MCSF = (6 X BHP@Shutoff Head) / (permissible temperature rise, DegF)

Permissible T rise of course must come from the manufacturer or empirical data.

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