7 replies to this topic

[pasquale]

Goodmorning gents!

A distinctive number for a centrifugal pump is the SUCTION SPECIFIC SPEED:
this number is defined by the following equation:

Nss=(RPM*(Q^0.5))/(NPSHreq)^0.75

So a pump with a high Nss has a low NPSHreq and this is good to avoid cavitation.

If the pump runs at reduced flowrate, and Nss is constant for the pump, the required NPSH will decrease. One of the reason because a pump has a minimum flowrate is that if the flow drops below the minimum there is a high risk of cavitation, impeller recirculation, but there is cavitation when there is an increase of NPSH req.

My question is: is correct what I have said? The NSS is constant for a pump and what about NPSH req at low flowrate?

Thanks in advance

Pasquale

[Art Montemayor]

Pasquale:

First of all, let’s be specific. The type of pump you are referring to is an induction motor-driven CENTRIFUGAL type. This must be established in order to address the subject of Suction Specific Speed.

Your given equation is exactly the mathematical definition of Suction Specific Speed (Nss). By employing reasonable empirical values of the Nss you can estimate the NPSHR in a suction line layout that must be provided to the pump:

NPSHR = [ (rpm) (Q)^1/2 /Nss]^1.33

Suction specific speeds can range between 3,000 and 20,000 – depending on the impeller design, rpm, capacity, nature of liquid and conditions of the service and degree of cavitation.

From experience, reasonable values for Nss for estimating purposes have been in the range of 7,000 to 12,000 for water, depending on the pump rpm and type of service under which the pump operates. Pumps handling hydrocarbons may operate satisfactorily with values of Nss ranging up to 15,000 or higher.

A high Nss may indicate the impeller eye is somewhat larger than normal and, consequently, the efficiency may be compromised to obtain a low NPSHR. Higher values of Nss may also require special designs and may also operate with some degree of cavitation; to avoid marginal designs on the pump’s suction side, it is advisable for the user to consult with the pump manufacturer to obtain suggested design criteria and to make certain that the suction conditions will meet the requirements of the selected pump.

Note that in the equation, the Nss is dependent on 2 variables that also are related: the flow rate (Q) and the NPSHR. The higher the flow rate, the higher the fluid friction (& pressure drop) for a given pipe diameter; and the higher the pressure drop, the higher the NPSH required! Therefore, your statement: “So a pump with a high Nss has a low NPSHreq and this is good to avoid cavitation” is not true nor logical.

Since we are talking about an induction motor driver, then the rpm is fixed and constant. The only things we can vary (either mutually and/or separately) are the flow rate (Q) and the NPSHR. When you state “But if the pump runs at reduced flowrate” you are insinuating that you are throttling the discharge of the centrifugal pump – that is the only way you can accomplish this with a constant rpm and a constant, given impeller. The moment you reduce the flow rate you are also reducing the NPSHR --- refer to your pump’s performance curve(s).

I believe you are confusing the issue(s) here because of an underlying fear of pump “cavitation”. For example, you state: “This is one of the reason because a pump has a minimum flowrate, if the flow drops below the minimum there is a high risk of cavitation, impeller recirculation due to the increase if NPSH req”. This statement is not true as written. A centrifugal pump has a minimum flow rate requirement because of need to maintain a positive NPSHA, internal lubrication, and cooling. If it doesn’t receive these items, it may cavitate and lose pumping action. I assume that we all understand well the phenomena of “cavitation”: it is the creation of vapor bubbles from a liquid (mainly due to heat and/or vapor pressure) combined with the subsequent implosion of the same bubbles back to liquid immediately downstream due to increase in pressure.

Contrary to what you state, if the flow rate through a centrifugal pump is varying, the Nss is not constant in a centrifugal pump from a practical point of view. It can’t be by pure mathematical definition as expressed in your equation!

In order for the Nss to be (& remain) constant, the pump’s flow rate and NPSHR must also remain constant – and these values often are not constant, but vary.

[palusa]

Pasquale,

both pumps' SPecific Speed and Suction Specific Speed are evaluated at impeller optimum efficiency point.
As such Nss is essentially an index descriptive of the suction characteristic of a given impeller.
It can be used to compare impellers under the premises that at rated conditions the pump is operating at or near its optimum efficiency.

Ciao.

_Lf

[pasquale]

Hi gents;

when a centrifugal pump runs at low flow (respect to normal flow) there is an increase in vibration and cavitation and for this reason this kind of pumps must run always above a minimum flow.

If the pump has cavitation and vibrations problems I suppose that the NPSH req is
bigger respect the normal flowrate case.

This is the explanation of why a centrifugal pump has this kind of problems at low flow? I'm not sure....help me

A second thing: if I have to compare two pumps for the same service (and same rpm and Q) the best choice is the pump with higher suction specific speed because it requires a lower NPSH. It's right

[Art Montemayor]

Pasquale:

Now I’m starting to be convinced that there is confusion on this subject.

Again, I re-state the fact that the only way you obtain a reduced flow from a centrifugal pump that is operating at a constant rpm is by throttling the discharge. And it is this throttling that can cause pump vibration, not the “low flow”. There is no increase in cavitation characteristics within the pump when you reduce the flow. Read my definition of cavitation once again. If there is any “cavitation” (actually, probably flashing) taking place, it is AFTER the throttling discharge valve. This has no effect on the pump, its impeller, or its casing. This effect takes place in the discharge line or piping – well after the fluid has passed through the pump. This has to be clearly understood and accepted. Otherwise, there is going to be a lot of confusion in the employment of such terms as “cavitation”.

Yes, there is a probability that if a pump is actually and realistically undergoing cavitation and vibration problems it could be that it is undergoing a NPSH availability problem. This could be especially true in the case of a saturated liquid – or a liquid at “equilibrium” conditions.

Centrifugal pumps have, in my opinion, only ONE problem when pumping a low flow: the problem is that the flow may be less than the minimum required for hydraulic balance, lubrication, cooling, etc. as judged by the manufacturer.

If I had to choose between two pumps, for the same service (and same rpm and Q), I would select the pump with HIGHER suction specific speed. This implies that the pump has a LOWER NPSHR – and therefore will pump will less difficulty than one with a HIGHER NPSHR.

Does this help to alleviate your concerns?
There are some excellent pump articles within this Cheresources Website that explain a lot of necessary items - especially cavitation - regarding centrifugal pumps. These can be easily downloaded, read, and studied with ease.

Attached find some information on Specific Speed.

[pasquale]

Art thank for the good attachments,

I know all about caviatation.

Some days ago I read an article about pump minimum flow and something confuse me, (especially figure 3 and 5).

I've attached the article to the post....

Can you help me?

thanks in advance

Pasquale

[palusa]

Pasquale,

Although in general I agree with the statement that minflow does not directly impact cavitation characteristic within the pump --as Art states, in several articles published on the subject, kind of 'distress' on pump --cavitation like-- is indicated as (potentilally...) occurring at reduced flow because of internal recirculation (both on suction and discharge).

An example is attached: this is an excerpt taken from an article of I.Karassik, one of the most recognized authority on pumps system design: here the internal recirculation phenomenon is detailed and some guidelines how to take it into account on Suction Specific Speed are also given (you will notice here some graphs similar to those indicated in your article...).

Hope this can clear up at least some of your doubts and help in your work.

_Lf_

[pasquale]

Hi Palusa!

So it is the suction recirculation that causes at low flow cavitation-like damage!

Now I have understood the graph by Fraser, in fact a pump with high Suction Specific Speed has a lower NPSH req (because it has a larger impeller eye and a larger impeller area) but in this way there more risk of suction recirculation due to the impeller design.

In fact also in my company I've heard that a pump with a Suction specific Speed higher than 9000 must be regarded with suspicion because even if it has a lower NPSH req it can have some problem at low flow.

Grazie del file che mi hai inviato le tue "fonti" sono sempre molto interessanti

Ciao

Pasquale