NPSH margin and ANSI/HI 9.6.1 2

[athomas236]

Gentlemen,

I have been asked to investigate what NPSH margin should be applied to power plant boiler feed pumps.

I have discovered that ANSI/HI 9.6.1 addressed this particular matter. However, although this standard was re-confirmed in 2000, it appears that it was withdrawn in 2003.

As far as I can tell this is the only standard that has addressed the issue of NPSH margins and I want to use the limits within this standard. But before doing so, I want to understand why it was withdrawn.

If anyone can explain why it was withdrawn or advise of any other applicable standard I would be grateful.

Best regards,

athomas236

 

[Artisi]

Seems logical to ask the Hydraulic Institute why they withdrew it.

 

[athomas236]

Artisi,

I agree with you and have emailed the Hydraulic Institute accordingly, awaiting a response.

I am trying to obtain a copy of standard ANSI/HI 9.6.1 even though it has been withdrawn.

I am also trying to obtain a copy of ANSI/HI 1.3 just in case it offers advise on NPSH margins. Looking at the contents list, I am not hopeful that this matter will be covered in the same detail as in ANSI/HI 9.6.1.

At the moment I understand that the sales of this standard ANSI/HI 1.3 have been suspended because of errors in the formulae in the standard.

The reason for posting my original question was to obtain advice from engineeers working in the pump field and I would still welcome any response to my original post.

Best Regards,

athomas236

 

[Artisi]

It will be interesting to know why this was withdrawn but anyway the NPSH margins seem to be acceptable for many people and manufacturers.

However, if you are talking about high flow / high head BFP's I would certainly opt for the higher figure that was in 9.6.1., of course there may well be others with actual field experience for the same equipment you are considering in similar installations and this could be your guide to establishing margins. A check with manufactureres and or users would be well worth while if you do not get any good response here.

 

[athomas236]

Artisi,

Thank you for that information. I have ordered 9.6.1 today and found that someone in our Dubai office has a copy of 1.3.

If I get a reply from HI, I will let you know.

Best regards,

athomas236

 

[athomas236]

Artisi,

Shown below is the complete response from HI.

Best Regards,

athomas236

QUOTE
Dear Mr. Thomas,

Thank you for contacting the Hydraulic Institute (HI) and for your inquiry regarding the NPSH Margin Standard, ANSI/HI 9.6.1 (1998).

This standard was approved and published in 1998 and has been distributed to a wide readership. You may be aware that ANSI requires that all standards be reviewed within a 5-year period and reaffirmed, updated, or withdrawn. In keeping with ANSI requirements, a review committee was convened in May 2002 for the purpose of reviewing this standard to determine what action should be taken. After thorough analysis and careful deliberation, the NPSH review committee recommended that a complete revision of the standard be undertaken and that the 1998 version be withdrawn until the new standard is approved. ANSI was advised that ANSI/HI 9.6.1-1998 should be withdrawn immediately and that HI could no longer distribute or support the document. The official public announcement appeared in the October 24, 2003 edition of ANSI Standards Action, a weekly ANSI publication.

A new, totally revised NSPH Standard is under development by the NPSH Committee and will be entering the review process soon. The committee is aware that the replacement document is urgently needed and has been working diligently since 2002 to develop a new document. They want to be sure that it is accurate, complete, and addresses the needs of the pump community. It will contain considerably more tutorial information than the 1998 standard and will respond to the questions most often posed by those involved with pumps and NPSH.

It is anticipated that it will require at least another year to complete the process and the new NPSH Margin is available for purchase. I wish that HI could offer you an equivalent or replacement standard, but to my knowledge no other NPSH Margin Standard exists.

I hope that this information is helpful. Thank you again for contacting the Hydraulic Institute.


Mary Silver
Director, Marketing & Membership
Hydraulic Institute
UNQUOTE

 

[insult2injury]

Don't forget that specifying too much NPSH margin may not be desirable either as it could have adverse affects on the pump/system.

I2I

 

[Artisi]

"The committee is aware that the replacement document is urgently needed and has been working diligently since 2002 to develop a new document."

Pleased to see they are treating this with some urgency it should be finished within another year, wow - only 7 years or so to revise a standard.

insult2injury makes a good point re specifying too much margin - hydraulically there shouldn't be a problem but it can put you into a bigger pump /motor operating away from BEP - hence my suggestion of checking with the manufacturers or operators of the same equipment in similar installations.

 

[athomas236]

Artisi,

I agree with your sentiments and I still do not really know why 9.6.1 was withdrawn.

My problem is that I am investigating the case of a 3x350MW coal fired sub-critical plant where the EPC contract says the BFP NPSH margin should be 1m or 10% whichever is greater and a bank's engineer who says it should be 80%.

At the moment I am proceeding on the basis that the EPC contract requirements apply to both continuous and transient conditions such as following a turbine trip and and the bank's engineer's requirement applies only to continuous operation.

I2I,

I am not exactly clear how "too much NPSH margin" can cause adverse effects except as mentioned by Artisi.

Best Regards,

athomas236

 

[Artisi]

athomas236
Guess you need to clarify that you are comparing apples with apples as to what/where the margin is to be applied.

For interest, what size units, type and configuration are you talking about and is this a new or existing installation?

There are a couple of regular posters who will have good first hand experience in this area - for your sake let's hope they see this post.

 

[insult2injury]

Without knowing the specifics of the plant, 10% is probably too low and 80% is most likely overkill on the margin. Most that I've been involved with operated in the 30%-50% range. You need to look at NPSH not only at design conditions but also at runout. Also confirm whether the margin is stated in terms of the 3% NPSH.

Excessive margins lead to larger pumps with larger suction passages. The larger suction passages allow for a lower NPSHR, but result in higher suction specific speeds. Since the suction is oversized, suction recirculation (and probably discharge too) will commence at much higher flowrates limiting the range of operation. With boiler feed pumps/deaerators you also have to consider transient operation during load swings/abnormal trips/etc.

I2I

 

[athomas236]

Artisi and I2I,

Once again thanks for your responses.

I received ANSI/HI 9.6.1 today and have to admit that for £70 I was disappointed with the standard. It looks as if the first draft could have been a useful document but it was watered down during the committee discussions.

I was particularly disappointed by the statement that Fig 3, which is used to determine if the suction energy is low, high or very high does not apply to multistage pumps such as used for boiler feed pumps.

I have today ordered the 2nd edition of Pump User's Handbook, Life Extension in the expectation that one of the authors (Allan Budris who was also chairman of 9.6.1 committee) will provide more information in the book than he was able to do in the standard.

I agree with the points about apples and apples, looking at run out conditions and 3% NPSH.

All I know about the project at the moment is that there will be 3 by 60% BFPs per unit and each unit will have an output of 350MW gross and the project is a new build.

Best Regards,

athomas236

 

[stanier]

Google "NPSH Grist". Grist's book comes up. He is the guru on NPSH.

 

[athomas236]

stanier,

I did Google "NPSH Grist" but most hits were about the book without giving a clue of what is said in the book.

I have looked at the contents list, which looks like the book will be really useful, and have today ordered a copy of the book on loan from the library.

Thanks for the information and regards,

athomas236

 

[sdennisf]

I'm a mechanical engineer with over 40 years experience, many of those in specifying and applying large, >20,000 horsepower pumps. Your concern over NPSH is a valid one but can be addressed very simply. You must specify that the pump must be stable over its entire operating range without any negative loss of head. HI specifies that NPSHR is reached with a 3% drop in head, most specifiers of large pumps reduce that value to 1%. Most knowledgeable engineers will not allow any head deviation, especially on large pumps. So don't try and design the pumps for the manufacturer, just specify that the NPSHR must be stable without any head loss. If you have any concerns about your system design you can substract a margin from your NPSHA design value but being overly cautious can increase the cost of the pump. I had an application many years ago that had specified a 3% head drop for NPSHR and later corrected it to 1%, which resulted in a 250' increase in the NPSHR!

 

[athomas236]

sdennisf,

Thank you for sharing your experience.

The situation I am in, is that I am, as part of the Owner's Engineer team, currently involved with two power station projects for which the Bank's Engineer has said that the the specified margin on NPSHr is not adequate and should be of the order of 50 to 60%. I was asked to investigate this matter so that we could respond to the Bank's Engineer.

I did think that the Bank's Engineer was basing his opinion on the HI standard but this has been withdrawn and seems to be taking an excessive time to rewrite. I did ask why the HI standard was withdrawn but the response was not helpful; please see post above.

As the result of advice I have received on this site, I have bought the book "2nd edition of Pump User's Handbook, Life Extension". I have also borrowed the book by Grist from the lending library. I am still reading Grist and did enjoy the section on the history of the centrifugal pump and the work by the Frenchman Denis Papin in 1705.

As you can appreciate it is not my intention to the design the pumps for the manufacturer but I do want a creditable engineering solution to the matter of the margin on NPSHr. It does seem that the NPSHr margin we have specified is not adequate. We specified a margin of 1m for NPSHr in the range of 1 to 12m which seems to have come from an oil compny's standard.

Best regards,

athomas236

 

[RebelLamb]

athomas236,

I realize that I may be a bit behind the times, but I thought I would add a little to this thread.

NPSHr + 10% or 2-ft, whichever is greater, is the absolute minimum gap between NPSHr and NPSHa when used in water and wasterwater pumping applications. A more common standard is NPSHr + 35% or 5-ft (1.5m) whichever is greater. These standards work if you are pumping water/wastewater, the fluids have a temp between 50F and 90 F, and your pump is operating within 15% of BEC (flowrate at BEP).

Pumping Station Design, 3rd edition suggests that if you are operating your pump outside of 15% of the BEC, then you might consider using NPSHr + 80%. This, of course doesn't mention the effect of pumping "hot" water which would also require a larger gap between NPSHr and NPSHa.

This is where the 10% & 80% numbers come from, but both could be perfectly valid depending on your selected pump/turbine and the fluid properties.

RebelLamb

 

[wimple]

Hi,

For some booster pump, I used in the past to sum two NPSH margin, one that account for permanent cavitation phenomena (as per NPSH defined at 3% head losses, different impeller material will be impacted not in the same manner) then add a NPSH safety margin; The point is that working out of some API610 field like it is the case in water applications for example, the NPSH is estimated mainly relying on field experience of expertized people. That's a real difficulty.
Not unusual to have up to 3 meters total NPSH margin to be on safe side.

Rgds

 

[GPRnD]

The HI standard was withdrawn because of the disfunction nature of the HI committee dealing with the NPSH standard.

There were too many competing points of view which resulted in a stalemate what was the correct approach in determining NPSH margin. Having sat in on many of those discussions, I can tell you it was like having your teeth pulled.

I am not aware of any specific standard that fills the gap left by HI.

Sadly some of the advice on this thread regarding margin is quite dangerous.

NPSH margin for BFW service needs to be quite large. I have an internal methodology for determining it (as used by my pump company), which is decent. Depending on your criteria you end up with NPSHa/NPSHr margins of 1.5 to 3. The range is a result of the water temperature, Nss of the pump, operating point and desired impeller life.

BFW service is v aggressive and needs large margins. Apply 10% margins is a sure fire way to end up with cavitation damaged impellers.

 

[athomas236]

Thank you gentlemen for your responses.

As far as I am concerned the matter of NPSH margins is still a live issue. So any advice will be greatfully receieved especially if it in the public domain.

Best Regards,

athomas236