Does Suction Specific Speed Vary with Actual Operating Speed

[feo1966]

Hi,

I had been under the assumption that the calculation of Suction Specific Speed used the actual operating speed, not the maximum design speed. Last week, I noticed that the Goulds E-Prism program shows the same Nss regardless of speed. i.e... they show the same Nss for 3600 rpm, 1800 rpm and 1200 rpm. I can come up with the same Nss as long as I use 3600 rpm.

I contacted the local Goulds/ITT rep and mentioned it to him. He forwarded an article from Alan Budris, that indicated Nss was an index that doesn't vary with speed. Below is s quote from a book he wrote with Heinz Bloch, saying the same thing.

"It should be noted that suction specific speed and specific speed values are constant for a given pump geometry, regardless of the pump speed. This is why one of the authors (Mr. Budris)developed the concept of “Suction Energy,” which includes both the pump suction specific speed and impeller inlet tip speed (speed × diameter)"
Bloch, Heinz; Budris, Allan. PUMP USER'S HANDBOOK: LIFE EXTENSION, 4th Edition (Kindle Locations 3086-3087). The Fairmont Press, Inc.

However, API-610 uses the words "at a given rotative speed" It doesn't say to use the maximum design speed.

This make more sense to me. The impeller doesn't know what it's maximum design speed is. So Nss calculated at the actual operating speed (using Q and NPSH at the BEP for that speed) seems to make sense.

At least I understand why Alan Budris added "N" in the suction energy calculation. Since he was under the belief that Nss (or S) is based on the maximum design speed of the pump. It does not make sense to say Nss is "constant for a fixed geometry".....since there are no terms in Nss that directly indicate geometry.

What do you guys think? Should NSS vary with the "given rotative speed". It is certainly makes it more useful. It can be used to estimate the improvement from speed reduction, for example.

Thanks in advance for your thoughts and help

 

[1503-44]

You can calculate suction specific speed at any RPM, flow rate or head. What is maximum speed, given infinite energy? Angular velocity of light? (Obviously) It must be maximum useful speed.

Since N is a performance index, thus it is best tied to the reference point where the pump is designed to perform, its BEP. If it were to be given at just any ole flow rate, or head, or speed, it would no longer serve as a useful performance reference, as all references would be lost, or they would each need to be stated with the calculation.

It's like miles/gallon. That number is always given at cruising speed. You might be able to increase your MPG by 50%, up to 55 MPG, if you drove at 17.5 MPH, but who does that? Grand Ma goes faster than that. So, would you want the dealer to give you that 55 MPG number when you wanted to buy a car? No, you want to know what MPG is when it is taking you somewhere at a useful speed. Same with a pump. I couldn't care less what N is at 15.7 RPM or when it's ready to vibrate out of the casing. I want to know what it is at the speed I should be thinking about using it. That's at BEP, the pump's cruising speed.

The terms are different, however stating N at BEP does indeed relate N to the pump geometry, as BEP scales geometrically.

 

[feo1966]

Not quite following you. For every operating speed, there is a different Q (BEP), and a different NPSH3 (BEP). Below is a sample Goulds 3196. The BEP flow moves is direct proportion with pump speed.

API 610 does indicate "at a given rotative speed". Therefore, I believe it is appropriate to the BEP flow and BEP NPSH3....at a given rotative speed. Make sense

 

[1503-44]

GOULDS 3100 series are centrifugal pumps, so all those curves scale with RPM.
Flow is proportional to speed, head is proportional to RPM squared, power to RPM cubed.
22ft/1200rpm^2×1800rpm^2 = 50ft

SEE pump affinity laws.

 

[feo1966]

Yes. I know all that. Thanks.

My point is that there is no such thing as a BEP speed. Maybe I misunderstood your 1st post, but it sounds like you were indicating there there was a BEP speed.

What I am trying to ask is.....like API-610 indicates. Can you use "a given operating speed"....and use the BEP flow and BEP NPSH3 at that speed to calculate the suction specific speed for that "given operating speed"

 

[1503-44]

Right. There is a BEP for each speed.

Got it. You were talking about the N relationship so much, I forgot the question was about Nss.

Yes you can determine if NPHS is OK at different speeds.
Scale NPSHr with RPM
Check this link.

https://www.engineeringtoolbox.com/suction-speed-pumps-d_638.html

I think that's what you want, right?

 

[LittleInch]

You need to be careful about the difference between Specific Speed and Suction specific speed.

Your quote from API 610 is Specific Speed.
Nss is the same formula but the head is now the NPSH to the power 0.75.

Generally as speed goes down so does NPSH and hence the Nss tends to stay normally quite similar even though the RPM changes.

you don't show the NPSH curves on your picture unfortunately.

Eng toolbox says the same thing, i.e. Nss doesn't change with speed.

https://www.engineeringtoolbox.com/suction-speed-pumps-d_638.html

This is an informative article

https://www.pumpsandsystems.com/basics-suction-specific-speed

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

 

[feo1966]

Ah...I see what you are saying. I did take a particular Goulds 3196 and tried two different speed. I get close to the same Nss. See below. In the ball park anyway, I probably wasn't picking the exact spot on their software.

Now I understand what Alan Budris thought the term "Suction Energy" was needed. Since suction specific speed stays the same regardless of speed, it is not as useful of a reliability indicator as it seems. A 900-rpm pump with an NSS of 12,000 (US units) will be much more reliable than a 3600-rpm pump with an Nss of 12,000

So he added pump operating speed and eye diameter...and specific gravity to come up with Suction Energy. Unfortunately, as he slips into retirement, "Suction Energy" seems like it is going with him. It has been dropped from ANSI 9.6.1, Guideline for NPSH Margin. Doesn't seem to be as much chatter about it in recent article related to NPSH and cavitation.

 

[1503-44]

I think reliability of anything (given similar equipment) that moves must be inversly proportional to speed squared.

 

[feo1966]

So I took a closer look. Using the BEP flow, and the BEP NPSHR for each speed. Nss does come out to a different value for each speed?

 

[feo1966]

You are correct. I did grab the wrong quote from API-610.

But right below it says the same thing for Suction Specific Speed. "....at a given rotative speed..."

 

[pierreick]

Hi,
Consider these links to support your query :

https://www.michael-smith-engineers.co.uk/resources/useful-info/specific-speed

https://www.pumpsandsystems.com/suction-specific-speed-part-one

https://pumpsdesign.com/suction-specific-speed-limit/

Pierre

 

[feo1966]

Thanks Pierre,

That's a good article. He doesn't specifically clarify whether or or not it is a pump index that doesn't change with speed....or if it can be calculated at any given speed (with BEP flow and BEP NPSHR)

My two reference points are Mr Budris, who was the product development manager for ITT for several years....

And API-610, which makes it sound like it isn't an index, and varies with operating speed. It definitely DOES vary with speed when you do the calculation.

I think I go with the API definition.