Relation of (NPSHA-NPSHR) and Pump Life Criteria

" NPSHA shall exceed NPSHR by at least 1 meter throughout the entire range from Minimum stable flow to 120 % of BEP (Best efficiency point)"

The above is a reasonable and sensible requirement.

"For all Centrifugal pumps, NPSHA at 80 % of Normal Operating flow rate shall be equal or be greater than NPSHR required to meet the 40,000 hours life criteria" Surely this statement is not from the same client as the first one, because it would be unlikely to achieve four and half years life using this as the standard.

If one uses this criteria the NPSHr and NPSHa can be equal at 80% of normal operating point which means that at normal operating point (which one assumes is closer to BEP) the NPSH required will be greater than the NPSH available and the pump will cavitate continuously.

To get assurance from vendor you simply have them perform an NPSH required test at the same as the performance testing.

I agree with Prof #1, and 4.5+ years running 24/7 is good going by any standards. Is the 40000 hours mentioned elsewhere in the contract? It might have been slipped in where you might not notice, and then they tell you (much later) it applies to everything!

Cheers..........Codey

Requirement of 5-year un-interrupted operation (8000 x 5 = 40,000) is very much clear from the contract document.

This is more than API 610 10TH Edition requirement of 3-year.

By the way, the bearing selection shall be as per API 610 10TH Edition to full-fill 40,000 Hrs. continuous operation.

Now, with proper bearing design / selection and with NPSHA-NPSHR >/= 1 UP TO 120 % BEST EFFICIENY POINT IS sufficient to finalize the Pump selection. We can avoid NPSH Test.

The requirement of NPSHA & R valube at 80 % of normal operating point to full-fill 40,000 operating hrs, I can not digest so easily.

I agree with "The Proff" response that It will cavitate at Normal perating point.

But, If Vendor comply with the 1st requirement no, need to follow NPSH Test and we can save Money & time.

Please share your exept view on this.

with regards,

Nilesh.

I think we all agree with " NPSHA shall exceed NPSHR by at least 1 meter throughout the entire range from Minimum stable flow to 120 % of BEP (Best efficiency point)". If I comply with this requirement, I understand, requirement highlighted in "BOLD-BLACK" above is automatically fulfilled..........It's the reference to 40000 hours in the "BOLD-BLACK" spec that seems a red herring.

Also, for a satisfactory design, I would ensure there is the 1m margin at maximum flow which could occur in practice. This might be higher than specified duty flow, and higher than 120% x flow at BEP. Pumps can be run at considerably higher flow than BEP.

But you say, If Vendor comply with the 1st requirement, no need to follow NPSH Test and we can save Money & time. - though it may be somewhere in the contract, this is the 1st mention of NPSH test, or any other pump tests. So can't judge whether it will save money and time.

Cheers.........Codey

Dear All,

thanks for your accurate / detail and very much up to the point reply.

I like this discussion. I will claeify this matter based on the concusion I have from your replies.

About the NPSH Test, our standard practice is to perform a witness NPSH Test if margin between NPSHA and NPSHR is less than 1 meter. so, I thought, If I comply with NPSHA-NPSHR margin of atleast 1 meter up to 120 % of BEP Flow rate will avoid NPSH Test but, I agree with THE PROFF reply that It will unnecessary increase NSS.

If we modify the original statement

"For all Centrifugal pumps, NPSHA at 120 % (Instead of 80%) of Normal Operating flow rate shall be equal or be greater than NPSHR required to meet the 40,000 hours life criteria". Please tell me if my understanding is correct or not ?

If yes, than the BOLD-BLACK (Updated statement) is better solution than the one described in 1st para above. In this case, I can check case by case NPSH Test requirement.

I am in engineering consultancy and my job is basically to read and comply with Client requirement. Diff. Client has Diff. spec. with diff. requirements and some time due to typing or other reason this kind of problem we face.

With best regards,

Nilesh.

"For all Centrifugal pumps, NPSHA at 80% of Normal Operating flow rate shall be equal or be greater than NPSHR required to meet the 40,000 hours life criteria"

Strange spec likely from someone who doesn't understand the exact meaning of the words and the abbreviations.

I agree with the responses that support 120% instead of 80%. And one meter more NPSHA is a nice margin in a system for future problems that could increase suction pipe friction losses or handle other losses in expected head on the suction side.

It is important to note that the contractual requirement for sufficient NPSHA is normally imposed on the piping system designer.

The requirement for maximum NPSHR is usually imposed on the pump supplier and he responds by offering a pump that has lower NPSHR than the requirement to meet the terms of the contract he accepts.

And it is reasonable for the specifications written to cover a major facility to include requirements that the system designer specify pumps that have NPSHR capabilities that meet specified maximums. The wording of the such specs might well include words lifted directly out of some pump supplier's literature. Where the spec writer gets in trouble is trying to edit those words without a clear understanding of the engineering.

I can't help but suspect that some difficulties with English language translations might be a culprit here.

Ed Weldon

The pump manufacturer/vendor ordinarily has no control over the NPSHA, which is part of the overall system design, not a characteristic of the pump. I can't see how the bold-faced specification is useful, but the other spec makes good sense (NPSHA exceeding NPSHR by at least one meter over the given range of flow.)

To satisfy this criterion, one must pay attention to the elevation of liquid in whatever receiver/tank/sump/body-of-water is feeding the pump; to the size and geometry of the piping to the pump; and to the vapor pressure of the liquid.

<...For all Centrifugal pumps, NPSHA at 80% of Normal Operating flow rate shall be equal or be greater than NPSHR required to meet the 40,000 hours life criteria...>

Though grammatically correct, the statement conveys very little meaning.