Having read through some other posts on this subject, it seems there is indeed a point which forces and moments are to be resolved about. The wording of the NEMA code section 8.4.6.2 is:

"The combined resultants of the forces and moments of the inlet, extraction, and exhaust connections, resolved at the centerlines of the exhaust connection should not exceed the values per limit 2."

I have reasons to be skeptical of how to interpret the code seeing that the following example after the above quote does not follow this process. The example only sums the forces and moments. There is no datum or resolution point to solve for. I do believe the wording is correct and example following it is incorrect. If I only sum the forces and moments for qualification against my allowable, there is no consideration for moments caused by the forces which are off axis (i.e. at the nozzles not at the point of resolution). I see this difference as being quite significant. So I am 90% sure that I am correct in following 8.4.6.2 but their example puts me in doubt. Considering how important this is and the consequences of getting this wrong, I figured I'd ask y'all.

Thanks in advance.


S

Dear boise-nick, The example given in NEMA is correct. The values of force and moments given in the example are after taking into consideration resolution of forces and moments at resolution point.
These are not the independent values of force and moment due to force and moments acting on that nozzle alone. It does include the additional moments caused due to forces acting on the resoltion point.

Regards,
Sush

I recall questioning your quotation several years ago. It looks like a sentence is missing. What does "resolved at the centerlines" mean? What's that "s" doing there at the end of centerline? I think the "resolution point" is the intersection of shaft and exhaust centerline. But what if these two lines do not intersect? We asked for an interpretation from the NEMA people. The issue was never "resolved" (ha).
Not only is the resolution point not settled but even the term "resolution". When I think of resolution I think of tranposing forces and moments to a new point where forces create moments. If you look at the example, that is not the case. But when you think of it, it's all artificial anyway. There is no load path to this arbitrary point inside the turbine. It's all math and rules. Just follow the rules and no one will (should?) get hurt.

I may not be totally understanding your statement. The forces acting at the resolution point (i.e. the exhaust nozzle) can't cause moments. There is not lever arm to create a moment so the forces taken at that point do not create moments.

"Summing" and "resolving" about the exhaust flange both invlove adding the component forces and moments. The descrepency I see is that to truly resolve the forces and moments, you need to add in the moments from those forces not on the resolution point. I do not believe the NEMA-23 example does this.

Oh, come on Boise Nick. Think.

All forces and moments acting on any point in the univers can be translated to forces and moments acting on any other point in the universe, using relatively simple algebra. And if a set of forces and moments act at the resolution point, then they are simply summed "as is." There is no requirement in NEMA SM-23 to create nonexistent moments from forces acting on a line passing through the resolution point.

I agree with Dave Diehl - if the centerline of the exhaust and the turbine rotor intersect, use that point.

If they do not intersect, I have always simply resolved the forces and moments to the point on the centerline of the exhaust nozzle nearest the turbine rotor centerline.

Sigh...I believe CraigB is missing the point. Yes I know all about the parallel axis therom and summation of components. I will try to make this simple by using an example: Pick 3 points in 3D space, A,B,and C. All points have x,y,z forces and mx,my,mz moments. So we have a total of...

Point A-> Fxa,Fya,Fza & Mxa,Mya,Mza.
Point B-> Fxb,Fyb,Fzb & Mxb,Myb,Mzb.
Point C-> Fxc,Fyc,Fzc & Mxc,Myc,Mzc.

So the following are true...

Total forces
X->Fxa+Fxb+Fxc
Y->Fya+Fyb+Fyc
Z->Fza+Fzb+Fzc

Total moments
MX->Mxa+Mxb+Mxc
MY->Mya+Myb+Myc
MZ->Mza+Mzb+Mzc

This is what the example says to do and what some of you would agree with. NOW, this is where I see a difference if you are going to resolve them around a point, I think it should be this...

Total forces
X->Fxa+Fxb+Fxc
Y->Fya+Fyb+Fyc
Z->Fza+Fzb+Fzc

Total moments
MX->Mxa+Mxb+Mxc+Fya(L)+Fyb(L)+Fyc(L)+Fza(L)+Fzb(L)+Fzc(L)
MY->Mya+Myb+Myc+Fxa(L)+Fxb(L)+Fxc(L)+Fza(L)+Fzb(L)+Fzc(L)
MZ->Mza+Mzb+Mzc+Fxa(L)+Fxb(L)+Fxc(L)+Fya(L)+Fyb(L)+Fyc(L)

Where 'L' is the distance to whatever axis the force is perp. to.
^This should be the correct way and is how every freebody diagram is set up.

I am arguing that the code implies an incorrect way of doing this.

Also, I called NEMA and was told that NEMA SM-23 is getting dropped because there is no product group to support it anymore. The person who took my call mentioned there were others with the same inqury.

Case 1
Total forces
X->Fxa+Fxb+Fxc
Y->Fya+Fyb+Fyc
Z->Fza+Fzb+Fzc

Total moments
MX->Mxa+Mxb+Mxc
MY->Mya+Myb+Myc
MZ->Mza+Mzb+Mzc

Case 2
Total forces
X->Fxa+Fxb+Fxc
Y->Fya+Fyb+Fyc
Z->Fza+Fzb+Fzc

Total moments
MX->Mxa+Mxb+Mxc+Fya(L)+Fyb(L)+Fyc(L)+Fza(L)+Fzb(L)+Fzc(L)
MY->Mya+Myb+Myc+Fxa(L)+Fxb(L)+Fxc(L)+Fza(L)+Fzb(L)+Fzc(L)
MZ->Mza+Mzb+Mzc+Fxa(L)+Fxb(L)+Fxc(L)+Fya(L)+Fyb(L)+Fyc(L)

Yes, you are correct in interpreting the resolved forces and moments at resolution point should be as per CASE 2. But the values mentioned in the given example in NEMA are as per CASE 2 and not as per CASE 1.
The nozzles may not be oriented in global directions always. So the example is just showing the Total resolved forces and moments acting due to overall piping configuration of all nozzles. These are not the forces and moments components acting at individual nozzle.

In the example, NEMA does not mention the components of force and moments caused by individual nozzle say Suction, Exhaust etc. What they have given is the resloved values as per CASE 2.

I cannot recall the example given in NEMA SM23.However, resolution of Forces and Moments from one point to another in space follows simple laws of vector . Forces when moved carries a couple wth it, moments go as is as they are free vectors. That is all and nothing else. It does not matter what NM23 or ano other document says as ultimalely it is vector algebra which governs. What SM 23 tells ( and may be having multiple interpretations) is about the resolution point.This may be at the intersection of shaft and exhaust nozzle axis or at the center of the exhaust nozzle.Beyond this, it is vector algebra .

Regards

I'm satisfied. Also, people should be aware of the multiple math errors in the examples when sizing the equivalent diamters of the nozzles.