NEMA Force Allowed on the flange

[farhad2474]

Hi

I recently involved with a compressor it has 16’’ flange at output. I am going to do stress analyze the pipe is connecting to this compressor. The vender’s catalogue mentioned that this compressor’s flange can outstand 10x NEMA forces . I would be appreciated if some body tell me what mean it is (10 x NEMA).

Best regard
farhad

 

[DSB123]

farhad2474,
If what you write is correct then the Vendor is saying that the allowable loads acting on his flange can be 10 times the NEMA quoted values. This seems to be a very robust compressor if that is the case. I would check and ask for definative allowables from the Vendor rather than accepting catalogue information.

 

[farhad2474]

hi ;DSB123
thank you for you answer . Really i am right. They wrote like this in the meeting with my company .I wasnt involve that time.so please tell me if you know a bout 10x NEMA .
best regard farhad

 

[JohnBreen]

farhad2474

You will need to get a copy of NEMA Standard SM-23. PLEASE NOTE THAT THIS IS A STANDARD FOR STEAM TURBINES. On steam turbines, the inlet is the smaller and hotter nozzle and on compressors the outlet is the hotter and smaller nozzle. You will need to read and understand this Standard. Then using SM-23 you will have to calculate the loadings (a function of nozzle size and temperature)that are within the allowable limits of the Standard. Your compressor is said to be able to accommodate loadings that are an order of magnitude greater than the NEMA Standard maximum allowable loadings. This is a surprisingly robust compressor.

You should also read and understand API Standard 617, Centrifugal Compressors.

I concur with DSB13, get the actual allowable loadings in writing from the compressor manufacturer to cover your employer's liabilities.

 

[LSThill]

Piping Stress Analysis Design Criteria for Compressor with Steel Nozzles and Castings

Steel Centrifugal Compressor Force, Moment and Stress Limitations

1. With Limitations are not provided by vendors, the following limits are guidelines which are “reasonable” for design.
2. At operating temperature, using the hot modulus “E” resultant bending moments are permissible up to a value which would cause a bending stress of “Sh/4” in a connection having a section modulus equal to the connecting piping for the same size where the connection is 4” NPS or larger. On smaller size connections a stress of Sh/3 shall be permitted. (Sh is as defined by ASME B31.1 or B31.3 current issue, for the material of construction.)

3. NEMA SM 21 & 22 X 1.85
a. Most compressor vendors specify 1.85 times the NEMA SM 21 & 22 standards as their allowable load limits. Generally, this indicates that do not have test data to substantiate allowable loads. There are several methods of circumventing this problem. The best method is to present the piping loads to the vendor prior to purchase and making the loading part of the contract. If the equipment has been purchased, the loads should be presented to the vendor for review for possible acceptances. Reference to API 617 demonstrates.

From Korea


L S THILL

 

[MJCronin]

farhad,

John Breen gives good advice....get the allowable loads in writing from the compressor vendor.

You may be surprised at how many operating problems are blamed on "excesive piping loads" during the startup phase, when the true fault usually lies with the compressor vendor.

With regard to the point about NEMA SM-23 being STEAM TURBINE loadings, yes this is true, but my experience has been that the major compressor vendors will not develop thier own set of acceptable loads.

They, as a group, prefer to play the game of "submit to us your loadings, then we will charge you a extra to review and approve them"

Over the past 30 years, they see this as being a "seperate profit center" and a trap that all AEs seem to fall into.

My opinion only..

-MJC

 

[farhad2474]

thank for answering to my question
i am going to use following Equation for allowable load on the flang of compressoer according to NEMA-23

Fx=50Dc
Fy=125 Dc
Fz=100 Dc
Mx= 250Dc
My= 125Dc
Mz=125Dc

Dc= (18+equivalent dimeter)/3

Am i right ?

best regard
farhad

 

[tigny]

Farhad2474, dear all,

I would like to input a little of my own experience in this field.

If you use a software like CAEPIPE or CEASAR II, you should use the analysis tools for NEMA SM23, and enter all relevant details: equipement centerline axis, !for all nozzles: nozzle nominal diameter, distance from resolution point (= biggest nozzle) to nozzle, calculated forces and moments, and if needed, the factor for allowable increase.


If the vendor's catalog gives you an allowable of 10 time NEMA SM-23 on this nozzle, it means a check of only limit 1:

chapter 8.4.6.1 gives limit 1 on total resultant force and total resultant at any connection

formulas are given for resultant forces and resultant moments, as well as equivalent connection size

according to your manufacturer spec., you can multiply 500xDe by 10
and check 3FR+MR <= 10x500De

According to your vendor's catalog, this should be the only thing to check.

But if you want to comply to NEMA, you shall check also the following, which will in turn mean much lower loads on the machine:

chapter 8.4.6.2 gives limit 2, a on combined resultants of the forces and moments of the inlet, outlet, etc... resolved at the centerlines of the biggest nozzle of all your nozzles (typically, for your compressor, the inlet nozzle)

There you need to know the relative dimensions from one nozzle to the other.

according to your manufacturer spec., you can multiply 250xDc by 10 and check 2Fc+Mc <= 10x250Dc (limit 2a)

and limit 2 ,b, which you gave, limits the componants of these resultants.

according to your manufacturer spec., you can multiply the second terms of the six formulas by 10 and check:
Fx= 10x50Dc, Fy = 10x125Dc, etc.

If you check only the limit 1, the overall compliance would not be OK, as far as my experience tells me.

The NEMA, as it is presented, is misleading because many people seem to check only the nozzles one by one, whereas the intent of the NEMA is to take into account the dimensions of the machine (nozzle to nozzle).

And I don't like having fixed values from the vendor, since with the NEMA formulas, you could increase one componant, reduce another, and still be within the limits. I'd rather have him certify the allowable coefficient.

yours truly,

tigny

 

[tigny]

Hi all,

I forgot to mention that all calculations to NEMA should be done with english units (pounds for forces, foot pounds for moments), then the results can be transfered to SI or others.
Otherwise, because on non dimension factors in the formulas, you'll get into trouble.


In API 617 you have formulas for SI units (Newtons and Newton.meters), but a 1.85 factor is already included.

yours truly,

tigny

 

[robinc]

Hi all,

Regarding the "resolution point" for the combined moments in API 617 and/or NEMA SM23:

I understand the forces/moments are "resolved at the centreline of the largest connection ..." (API 617 2002 pg 2-42)

Where along this centreline is this force resolved ... the nozzle face ... in the vertical plane of the shaft ... random location?

Caesar II implies that it is the nozzle face but I can't find this explicitly stated in either code. In my mind the resolved forces should be about the shaft. If these loadings deflect the shaft, then you end up with mechanical issues.

Looking forward to either a one sentence clarification, reference, or long discussion.

Thanks,

Robin

 

[C2it]

Robinc,

The resolution point is not too clear. It should always be specified and agreed with the vendor. In a 2 nozzle machine there is logic in using the largest nozzle face and c.l., since that is where major pipe loads are imposed. Using that for combined loads means that the moment effect of forces acting there is zero.

 

[tigny]

Hello Robinc,

you state "If these loadings deflect the shaft, then you end up with mechanical issues."

I would dare to say "if loadings deflects the pressure casing, the bearing casing and/or the gearbox casing, then you may end up with mechanical issue". I suppose that if the shaft bends, you shurely have already rubbed everyplace in your machine!

About the reference point: At first we took some reference point at the level of the fixed point of the machine (between the fixed legs i.e. for compressors-expanders) but to be more consistent with NEMA SM23, API 617, and after reading the help file from CAESAR II, and also because of the request of one of our clients, we now use the biggest nozzle center as the reference point.

yours truly,

tigny