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

[mpeck1982]

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?

 

[ApplicationENG]

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?"

 

[1gibson]

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.

 

[ApplicationENG]

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.

 

[1gibson]

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

 

[ApplicationENG]

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.

 

[1gibson]

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.) 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?

 

[DubMac]

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.