Centrifugal Pumps and Suction Specs

what do you mean by suction specific speed (Nss) and suction lift capacity

kindly clarify to receive clear answer for your question

Suction specific speed means :: Nss = (N.Q^1/2 ) / (NPSH)^3/4, where N =RPM,Q= Flow,& NPSH is net positive suction head.(Ref : Perrys-10.58).

And if we say specific speed Ns=(N.Q^1/2 ) / (H)^3/4, where H = Total Head.

Suction lift capacity means - Theoretical lift of fluid below datum line (negative suction capacity).

Thanks

My opinion is, select the pump having NPSHr less than NPSHr of second pump. In this way, the pump will have a large operating limits.

Interesting subject and suggested reading would be "Centrifugal Pumps. Design & Application. Lobanoff & Ross(The figure 8-7 is from this book).

Nss is related to the suction conditions of the impeller and is mostly about comparing the cavitation performance of different pumps for the user and for the designer it is used for predicting the NPSH required by the pump.

Maybe this figure will explain the effect of varying Nss on the operation of the pump.

The designer chooses the Nss based on the available suction conditions and varies the geometry of the inlet eye of the impeller to achieve this. As Nss increases, vane entry angles decrease and velocity in the suction eye increases with a greater chance of cavitation.

In reply of very good answer/comment given by The prof,As understood pump having greater Nss is less preferable for the same duty point and range of operation, I would like to differ because of following understanding :

As, Nss = (N.Q^1/2 ) / (NPSH)^3/4

for given N & Q. pump having greater suction specific speed should require less NPSH, it means negative suction capacity is more and less chances of cavitation.

From a maths point of view your statement is correct, but there are many more factors to be taken into account as pointed out by Bill H.

The suction specific speed (Nss or S in Europe) is used to describe an impeller in regard to its NPSH requirement.

In other words the Nss is chosen by the impeller designer in order to achieve a safe margin between the NPSH available and the NPSH required by his impeller. If he had a choice he would probably prefer a higher NPSHa so that he did not have to reduce the angle Beta too much and increase the NSS as this makes the impeller more susceptible to internal recirculation.

In order to select your vendor you first need to ensure that you have adequate margin between NPSHa and NPSHr.

If both pumps are acceptable from an NPSH perspective then all other things being equal you would probably select the pump with the lower Nss. Remember that NPSH margins and Nss levels are inter-related and both must be considered when chosing a pump.

Some reasons for this:

• For a given specific speed the lower the Nss the safer the pump will be from intenal recircualtion/cavitation
• The lower the Nss the lower the minimum stable flow of the pump. This increases the operating range and allows flexibility to the operational people.
• Pump vibration increases when running away from the best efficiency point, this increase in vibration is higher the higher the Nss.
• High NSS impellers are usually more expensive due to casting difficulties with the change in vane angle.

I am taking this opportinity to add further that while refering Perry chapter 10 regarding pumping of lequid & gases under centrifugal pump section and pump selection subsection which read as - Typically for single suction pump, suction specific speed above 11000 is considered excellent.Below 7000 is poor and 7000 to 9000 is of an average design.

It is also a matter of fact and interesting to note that different enginners/designers may have different logic, but technically there is only one correct understanding, which i am trying to conclude with valid technical/mathemetical reason and standard text book or hand book to support that understanding.

It is just a technical discussion without any intension to question any individual technical knowledge.

Guest, are you the original poster?

You need to read all the posts again carefully, and also read Perry in context. The first sentence of what is a very brief handling of the topic is "Another parameter that helps in evaluating the pump suction limitations, such as cavitation, is suction specific speed'..... it then goes on with your quote. "Above 11000 (using US gpm, rpm, ft as the units) is excellent." The question is, "excellent" for what. Low NPSHr certainly, for almost every other reason as given in my last post, not excellent at all. "below 7000 is poor" again is referring to suction limitations, 6000 is considered a design criteria for some high capacity pumps so is certainly not poor in a general sense.

One of the "standard" text books on pumps is Karassik et al.

To quote "..pumps lie in the range 7500<S(Nss)<10000. S (Nss) 8000 is an average value frequently used for estimating purposes although much higher values may apply to special designs or service conditions such as an inducer ahead of the first stage impeller." He goes on to say "For a given Ns the lower the value of S (Nss) the safer the pump will be against cavitation. Experience with a large European pumped storage installation has shown that cavitation began at S approximately 6000 and this value is recommended for large pumps." (Stepanoff, Lobanof and many others all agree).

Note the Nss = 11000 maximum that is given in the GPSA Handbook below. This is pretty much an industry norm, though not applied to special cases such as very high speed low flow high head pumps for example.

Yes, I am the original poster.

As far as discussion is concerned regarding this suction specific speed topic,this seems to be a general confusion/discussion over which one is better technical option-Low suction specific speed or high suction specific speed for centrifugal pump ?

I have not this opportinty to go through text book authored by Karassik, but what is point of discussion here is- If two author (perry & karassik) using the same mathematical realtion for suction specific speed,why are the different conclusive opinion.

Only difference is noticable between karassik & perry mathemetical realtion is former analising at Q_bep & R_bep while perry is quite open and anaysing at any point,but again this may not be an influencing factor.

If find an opportinity to go through perry, refer centrifugal pump section and pump selection sub section in pumping of lequid and gases chapter (For reference exact extract from the perry written in post no #8) and then conclude .

I tried - I failed - I give up.

I can only suggest that you read the posts again carefully, I do not know how to make it simpler - there is no confusion amongst the specialists.

Good luck.

Hello, The Prof -Don't give up it is least expected and do feel,you are an specilist and specilist should not give-up. It may seems to be sily qusetion for specilist & Guru, but I am not convinced. Having said that at the same time quite open to absorb & change new understanding on centrifugal pump.

Honestly, I tried to find fineness what has been suggestet in your post but couldn't point out the differences.

In the mean time I have opportunity to go through "know and understand centrifugal pump - By LARRY BACHUS & ANGEL CUSTODIO '' which also conclude while ealborating suction specific speed Nss as written below :- (in reference to same well known mathemetical ralation as)

In the numerator we have the speed and the flow. If we were comparing similar pumps into an application, these multiplied numbers would mostly be a constant. In the denominator we have the NPSHr of the pump (or competing pumps under comparison for an application). As the NPSHr of the pump goes down, the Nss value rises. As the Nss value increases, the operating window of the pump narrows.

So,again this is quite contradictory with reference to perry and seems to be in line of The prof logic.

So,which centrifugal pump is better- having higher suction specific speed or having lower suction specific speed ?

OK, one more try. Please forget that you ever read Perry. They are talking about what is good for NPSHr. We are talking about whether a high Nss or a low Nss is "better". We assume that the NPSHr requirements are met by both pumps.

The question then is; In a given system, for a given specific speed pump, where the NPSHr is staisfied, would we prefer the higher or the lower Nss speed pump?

Is my understading of your question correct?

If so, then the below extract from an article by Dale Andrews of Lawrence pumps should make it clear. (As well as Bachus & Custodio, GPSA, Stepanoff, Lobanof, Ross, Karrasik etc quoted above who all say the same thing in different words.)

If the NPSH is satisfied, then as a general rule, lower Nss pumps are prefered over higher Nss Pumps as they have a wider stable flow envelope.

OK,understand & absorb the new concept on Nss.

In fact,why was the initial resistance because of conclusion written in general in perry which was seems to be correctly interpreted by the prof. And perry was more reffered by prodessionals in comparison to Bachus & Custodio, GPSA, Stepanoff, Lobanof, Ross, Karrasik etc and that's why i am trying to generalize the concept of Nss.

At last but not least it was really nice discussion.

For the same duty and operating range, if both pumps can meet your flow and discharge requirements, then you should consider the differing suction lift and specific speed perameters when you consider potential future operating conditions and adjustments to impeller diameters or operating speeds to meet potential future changes in the system.

The differences that you noted are due primarily to the differences in impeller design. Some factors in the shape of the Volute inlet and the way that water thus enters the eye of the impeller also affect the Suction Specific Speed, but are generally often neglegable in comparison.

Even though both pumps generate simular conditions of flow and overall head added, suction conditions can impact the size and length of suction piping, the elevation of the pump relative to the elevation of your suction free surface or suction static pressure requriements as well as size of solids capabilities of the pump.

This information is given for you to evaluate these factors and are VERY OFTEN overlooked by engineers in pump selection. For many years I have taught pumping station design and am always surprised at the number of engineers who never consider these factors and only look at the catalog pump discharge and NPSr curves and then wonder why thier designs end up with operational issues. Most Operational issues result in operations mechanics that make adjustments in system configuration that were not considered by the engineer.

Good luck.