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 However, although this standard was re-confirmed in 2000, it appears that it was withdrawn in 2003.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 But before doing so, I want to understand why it was withdrawn.

If anyone can explain why it was withdrawn or advise of any other applicable standard I would be grateful.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

 

[wimple]

athomas236,

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

 

[MJCronin]

athomas,

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

 

[athomas236]

MJC,

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My recollection is that the Bank's Engineers requirements are based on ANSI/HI 9.6.1 even though it has been withdrawn.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

 

[Artisi]

My response to the client would, "That standard has been withdrawn for good reason as it was unworkable etc etc .....", the client should argue this point with the bank based on a realistic suggestion from yourself.

 

[BigInch]

They'll just withdraw the money.

The standard was obviously withdrawn after their lawyers saw it.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.

**********************
"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/

 

[JoeySoap]

That's a good perspective, BigInch and illustrates perhaps why rotating equipment engineering is a different kind of discpline to (say) pressure vessel design engineering.

 

[BigInch]

I wouldn't exactly say PV engineering is sooooo easy, just that some vessel designs would easily qualify for a canned solution, just as would a typical nonpotable cold water low head pump application. We all just have to realize when we are looking at one of those applications that lends itself to a canned solution and when its a high SS boiler feed water pump.

"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/

 

[GPRnD]

With all due respect, claiming the standard was withdrawn because of the lawyers is pretty speculative. It is also incorrect IMO.

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.

 

[BigInch]

But you yourself agree the standards don't always do a good job either, so I'd much sooner put my faith in someone with eyes and a half a brain, rather than just a flat piece of paper.

**********************
"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/

 

[GPRnD]

To answer the original thread question, 1m or 10% margin is way too low for a BFW service.

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

 

[GPRnD]

I agree the old HI standard wasn't a thing of beauty either.

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.