Horizontal Shell & Tube Heat Exchanger on Two saddles

[DK44]

When analysing Two saddles of Horizontal Shell & Tube Heat Exchangers, how saddle reaction at each saddle is calculated. PV Elite qives vertical saddle loads at each saddle, but does not provide the calculations. I presume the complete Heat Exchanger to be treated as a horizontal beam with length equal to the total length of Heat exchanger and total weight of Exchanger is considered as uniformly distributed load over total length. Also, how to judge the best location of the saddles. The shells are some times cylindrical and some times Kettle type. I am looking for detailed saddle reaction load calculation method.

 

[SnTMan]

DWARAKAKRISHNA, are you concerned only with the reaction at each saddle? These are simply found from the dimensional relation of the CG to the saddles.

If you are interested in the stresses imposed in the shell, this is normally done by the so-called Zick calculations, which (I believe) PVE does perform. However the method is not especially accurate for shell & tube exchangers, as it assumes a vessel that is symmetrical about the supports. You can find details of the calculations in standard handbooks such as Moss, Bednar, etc. or the Zick paper itself.

About the only really accurate way is to run one calculation for each end of the exchanger, adjusting the weights such that the reactions at the saddle correspond to reality. Note the vessel is treated by the software as being symmetrical for each case. Even this is troublesome. I should say that I run these calculations in Code Calc, not PVE, which may handle the asymmetry, I don't know.

My experience has taught me to not especially trust these calculations. If they tell me something is overstressed, well I don't exactly believe it. If they tell me the opposite, I don't exactly believe that either

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DK44]

Thank you Mike for your comments.
I am looking at present for calculation method for saddle reactions for a Horizontal Shell & Tube Heat Exchanger (with cylindrical shell)as stated in my previous question.
And also how the Heat Exchanger is considered on two saddles, is it as a beam with Total weight converted to UDL throughout the length of the beam?
The problem:
Total weight of Exchanger = 102 T (it included two End channels + Middle shell with Tube bundle inside it).
Total length of the Exchanger (extreme ends)=10.7 M (can this be considered as the total length of the Beam)
CG of the Exchanger = 5.23 M from left end.
Left saddle placed at 2.128 M from left end.
Right saddle placed at a distance of 5.500 M from left saddle (or at a distance of 3.072 M from right end).
If I convert the total weight as UDL it works out to 102 / 10.7 = 9.53 T/M (is this method OK ?). If not, how to consider loading of the beam?
Request a suitable guidance in assessing saddle reactions for above.

 

[SnTMan]

DWARAKAKRISHNA, as stated the reactions at the saddles (loads imposed on the foundation, or loads imposed on the shell BY the saddles, makes no difference) are found from simple statics:

Load on right saddle: 102 T * (5.23-2.128)/5.5 = 57.5 T
Load on left saddle = 102 - 57.5 = 44.5 T

Again, if you are interested in stresses in the shell imposed by these reactions you will need to use Zick methods.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Wind and seismic must be considered, operating and empty conditions. See TEMA 8th edit. (no 9th edit.)

Regards
r6155

 

[Trestala]

The reaction at each saddle depends on the location of COG of the equipment (excluding the saddle weight) from the saddle location. If the COG is in center in between saddles, the weight is supported equally on each saddle. If the COG is 25% of the saddle span from the left saddle, and 75% of the saddle span from the right saddle, then 75% of the weight is supported on the left saddle.

In PVElite, the Saddle Force Q from the weight is calculated that way. The saddle reaction from seismic forces can also be considered distributed in relation to the COG. For wind loads, it depends on where the total wind pressures can be considered as concentrated relative to the saddle location. And yes, PVElite doesn't show the calculation for the values needed to get the saddle force, Q.

 

[DK44]

RaymondN. Thank you for your remarks.
1. How to assess from PV Elite calculations, whether the saddle locations are optimum.
2. PV Elite also provides M1 and M2 at both saddles. What is the significance, especially M2 for both saddles.
3. Is maximum negative bending moment vs maximum positive bending moment criteria applicable. How to take it from PV Elite Calculations.
4. Can they provide manual calculations for Saddle loads considering wind and seismic loads also.

 

[Trestala]

DWARAKAKRISHNA,

1. How to assess from PV Elite calculations, whether the saddle locations are optimum.

The optimum saddle location is where the loads are distributed evenly and in which M1 and M2 should be around the same amplitudes, except when you can use a horn stiffener. That is my opinion.

2. PV Elite also provides M1 and M2 at both saddles. What is the significance, especially M2 for both saddles.

In PVElite notation (as well as in general), M1 is the longitudinal bending moment at saddle location, and M2 is longitudinal moment at midspan between saddles.

3. Is maximum negative bending moment vs maximum positive bending moment criteria applicable. How to take it from PV Elite Calculations.

The signs of the value just refers to the direction of the load.

4. Can they provide manual calculations for Saddle loads considering wind and seismic loads also.

The calculation by PVElite are pretty much the same and they consider wind and seismic loading. You can refer to ASME VIII, Div.2 Part 4.15.3. or the paper by LP Zick, or a Pressure Vessel Design Book where they discuss the calculation method for saddle supported vessel.

 

[r6155]

Best practice for HE is calculations with 3 COG: the left side of saddle 1, between saddles and the right side of saddle 2.

Regards
r6155

 

[DK44]

Thank you Raymond for your comments.

r6155.
3 COG method is new to me. What loading is to be considered for each COG. and how the total load to be distributed.
How do you calculate Saddle loads and moments with this. Any software uses this method?

 

[r6155]

DWARAKAKRISHNA
See any handbook with beam formulas: Roark’s, AISC, ......

Regards
r6155