WRC-107 - Tips
- Former user (Deleted)
- Tiradej Bunyarattaphantu
- Mike Clark
What is the WRC-107 Analysis?
The WRC-107 Analysis calculates the combined local stress intensity from external loads at the junction of an attachment and a shell or head. The method may be employed for structural supports or nozzles.
What are the limitations?
The analysis is based on empirical data. Certain geometries falling outside this data have no experimental basis to support them and it is up to the designer to determine if the method is a valid approximation outside the range covered by the empirical data. In addition, the method does not cover stress from internal pressure; however, DesignCalcs does allow for internal pressure loading.
The second major limitation is the stress combination. This method combines the membrane and bending stresses. Membrane only stress will have a set of allowable stress criteria if you look to Section VIII, Division II, and the combined membrane and bending stress will have its own allowable stress criteria. In addition, if you need to consider peak stress, the failure modes you need to check per Division II get more complicated.
Finally, the WRC-107 reference does not clearly indicate the attachment details for the attachment to the host. When utilizing this method, be careful when considering tilted or hillside nozzles or attachments where full penetration groove welds are not used.
How does the WRC-107 coordinate system work?
See the figures below for reference. For both heads and cylinders as hosts, the radial load P is positive if it is inward. Choose arbitrary 1-1 and 2-2 axes that are normal to each other for heads. A shear load V2 acts in the 1-1 direction and causes the M1 moment. A shear load V1 acts in the 2-2 direction and causes the M2 moment.
For cylindrical hosts, the axes are the longitudinal direction and the circumferential direction. A positive shear load VC acts in the positive circumferential direction and creates the positive moment MC. The positive shear load VL acts in the positive longitudinal direction and creates the positive moment ML.
How does the Solve For radio button work?
When stresses are selected, the analysis will calculate the combined local stress intensity from all of the loads entered. When one of the other options is selected, the analysis will set the combined stress to its maximum and solve for the item selected with the other items set.
What should I enter for the stress multipliers?
These multipliers are used to determine the allowable combined stress. Cs represents the stress multiplier and Cy represents the yield multiplier. You may choose which combination of stress, yield, and multiplier to use and whether to consider the attachment properties. When the attachment properties are considered, the minimum of the selected yield, the stress of the host, or the stress of the attachment is used to determine the allowable stress.
The ideal values for the stress multipliers depend on several factors, including the duration and geography of the load. If the load is to be applied one time and then released (such as for lifting lugs), a higher allowable may be justified. A typical operating nozzle load would be limited to a lower allowable. See the stress classifications in Section VIII-II for further guidance. Pay special attention to the primary local and secondary stresses and the slight differences between them.
An example of a typical allowable stress for primary local membrane stress would be 1.5*S. An example of a typical allowable stress for primary local membrane plus secondary bending stress would be Sps where Sps may be either 3*S or 2*Sy. Be careful when specifying 3*S if the allowable stress criteria is based on 90% yield instead of 66-2/3% yield.
When do I use the Stress Concentration Factor?
This is only used for cyclic type loadings or for brittle materials. See appendix B in WRC-107 and proceed with caution.
How does the analysis handle reinforcing pads for nozzles?
The analysis will calculate the stresses at the periphery of the nozzle to host/pad combined thickness and it will calculate the stresses at the periphery of the pad diameter to host junction. This is probably a conservative leaning check unless the repad thickness is very close to external projection of the nozzle or the width of the pad is very narrow; in either of those cases, the check is more accurate.
For a large diameter pad (e.g. a pad with 2x the diameter of the nozzle neck), it may be suitable to increase the host thickness to represent the combined thickness of the pad and the host - assuming the allowable stress of both are the same and the nozzle has a quality attachment detail to both the pad and the host wall.
Can I bump up the host thickness to represent the combined host and pad thickness for a structural attachment?
Yes, you can. However, this approximation is the most viable when the attachment is attached through the pad to the host wall and the details include full penetration groove welds. In addition, the pad should be fairly large compared to the attachment size (e.g. 8” x 8” pad for a 4”x 4” attachment) and the pad should have an allowable stress similar to the host.
How can I import a nozzle or a host that I have already designed?
On the Vessel/Attachment tab, select the Nozzle Browser to import the information for the nozzle and its host. To add a structural attachment, select the host browser and bring the host information into the WRC-107 form. Then you can manually enter the attachment information.
Why do I need to enter the diameter when the host is an elliptical head?
The analysis treats the head as a sphere and uses the diameter entered to determine the spherical radius.
Are the wind loads, elevations, and diameters from the Vessel Information and Attachments/Loadings windows used in the WRC-107 Analysis?
In order for wind loads to be considered in the WRC-107 Analysis, they must be directly entered on the Loads tab of the WRC-107 window. The analysis does not draw this data from any other location.
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