NPSH margin and ANSI/HI 9.6.1
NPSH margin and ANSI/HI 9.6.1
(OP)
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 However, although this standard was re-confirmed in 2000, it appears that it was withdrawn in 2003.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 But before doing so, I want to understand why it was withdrawn.
If anyone can explain why it was withdrawn or advise of any other applicable standard I would be grateful.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
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 However, although this standard was re-confirmed in 2000, it appears that it was withdrawn in 2003.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 But before doing so, I want to understand why it was withdrawn.
If anyone can explain why it was withdrawn or advise of any other applicable standard I would be grateful.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





RE: NPSH margin and ANSI/HI 9.6.1
RE: NPSH margin and ANSI/HI 9.6.1
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I am trying to obtain a copy of standard ANSI/HI 9.6.1 even though it has been withdrawn.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
RE: NPSH margin and ANSI/HI 9.6.1
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.
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
I2I
RE: NPSH margin and ANSI/HI 9.6.1
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.
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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.
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
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.
RE: NPSH margin and ANSI/HI 9.6.1
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
RE: NPSH margin and ANSI/HI 9.6.1
I was working before in a pump company, I was searching in my archive to come with some figures;
I found an pplication :
N° Stages : 1
capacity 212 m3/h
working temperature 109°C
psuction .955 bar
pdisch 124 bar
NPSHa : 6.1 m (0.5692 bar)
NPSHr : 3.324m (0.3102 bar)
fluid : feed water for boiler.
This a figure that could give you preliminary orientation; pls let us know your findings from your side
hope it helps
Rgds
Wimple
RE: NPSH margin and ANSI/HI 9.6.1
You stated that the "bank's engineer" demanded higher NPSHa...
Was the technical design basis for this figure requested from him or did it just somehow come flying out of a "banker's orifice" ?
I have been in many similar situations where the client makes spurious unrealistic demands and when asked why they are necessary, the typical response is ...."oh, I don't know....I am the client and I just want it"
Be tough......
-MJC
RE: NPSH margin and ANSI/HI 9.6.1
The client needed the acceptance of the Bank's Engineer on the NPSH margin to get funding for the project so has accepted the Bank's Engineer's requirements and paid extra for a revised design of boiler feed pump.
My recollection is that the Bank's Engineers requirements are based on ANSI/HI 9.6.1 even though it has been withdrawn.
Best regards,
athomas236
RE: NPSH margin and ANSI/HI 9.6.1
RE: NPSH margin and ANSI/HI 9.6.1
The standard was obviously withdrawn after their lawyers saw it. Logically its simply a no win situation for HI. Too little NPSHA = potential liability for any pump damage; too much, and capital cost increases resulting in nobody using it in either case.
Why do we try to dumb these things down all the time by trying to make up some "standard". Especially where standards don't fit. Standards only fit standard situations. NPSHA isn't one of those. Look at all the variables listed above. If there ever was a place for a NO standard, NSHA + a "margin" has gotta be one of them. HI realized exactly that. My hat's off to them. Somebody finally let engineers be engineers ... instead of standard writers and standard followers.
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSH margin and ANSI/HI 9.6.1
RE: NPSH margin and ANSI/HI 9.6.1
"A man's gotta know his limitations." Clint Eastwood in "Dirty Harry", ie. following a standard is no substitute for hands-on experience. Put it down and pick up the phone.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSH margin and ANSI/HI 9.6.1
As I mentioned earlier, I attended a number of the HI NPSH standards meetings over a space of about 3 years.
The old standard was withdrawn because nobody among the manufacturers and users could agree on what constituted acceptable margin. The only thing they mostly agreed on was that they didn't like the standard so out it went.
Unfortunately last time I checked the new standard was mired in arguments about the basic approach. Some people wanted simple margins, some wanted a Nss based approach etc.
I also disagree that there is no place for a standard on NPSH or if you prefer, some basic guidelines.
Look at the example of API 610. That talks about Suction Specific Speed (Nss) but leaves it open to the engineer to define the maximum allowable. As a consequence you have a horrible free for all where specifying engineers (who are not usually pump experts remember), require all manner of odd things. I've seen requirements for Nss <12,000, Nss <11,000, Nss <9500 all with no consideration of the type of pump or the pumped fluid.
In some applications this results in a grossly oversized pump. In others I've seen it result in dangerously small NPSH margins.
One can have too much faith in engineers knowing how to do the right thing.
RE: NPSH margin and ANSI/HI 9.6.1
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSH margin and ANSI/HI 9.6.1
Using the internal standard I work to and assuming the following:
Boiler Feed Water
13/4 Chrome impellers
Design Temperature 248 deg F
Impeller suction specific speed < 9300 US units
NPSHr : 30ft
Operation between 80 and 110% of BEP
40,000 hour impeller life required
The NPSH margin for these circumstances is 1.88
If you don't want a 40,000 hour impeller life it is 1.57
Sorry I can't publish the whole standard. But if you give me details of the duty conditions I can suggest what NPSH margin I would recommend.
Typically you see rarely see margins for critical BFW pumps of less than 1.5
Regards
RE: NPSH margin and ANSI/HI 9.6.1
The problem I think we would both agree on is the issue of experience that you mentioned in one of your posts.
On a recent tour of engineering contractors in Europe I was struck by how young many of them were. The majority looked like they had come straight out of university and taken up residence there.
Now I admit this might be my prejudice or my increasing years (or both). So I apologize in advance to the experienced engineering contractors.
Unfortunately experience takes time and being given the chance to break a few things. These days everyone seems to be in too much a hurry for that.