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How do I determine maximum nozzle loads? (WRC-107)

How do I determine maximum nozzle loads? (WRC-107)

How do I determine maximum nozzle loads? (WRC-107)

(OP)
I have been asked a number of times "how do I determine the maximum loads that can be applied to a nozzle?" Even in my work pre-Codeware this question often came up.

It's a reasonable question, the vessel owner simply wants to know what maximum load they can apply to their new vessel. But there is no simple answer. This is because there is really an infinite number of combinations of loads that all produce the same stress in the shell. By definition, this stress equals the allowable stress.

Below is part of an FAQ to be posted to the Codeware website that addresses this issue.



Many engineering companies or vessel owners publish vessel design standards with specified loads that the vessel nozzles are to be designed for. But in some cases the vessel designer is asked to determine the maximum loads that the nozzle and vessel may withstand. For this there is no published standard by ASME, etc. Each problem must be approached separately because the answer will depend on the specific properties of the vessel and nozzle; ie: the answer will depend on the pressure, temperature, allowable stress, and the diameter and thickness of the shell and the nozzle. In fact, due to the nature of the engineering problem (the existence of combination of 3 forces and 3 moments to derive a single stress intensity) there is an infinite set of loads that will all produce the maximum stress in the shell as per the WRC-107 analysis.
 
Several issues relate to designing nozzles for the external loads: The nozzles must be designed so that the nozzle neck itself is adequate for the bending moments and axial and shear loads, as well as the welding being adequate. Generally the issues most relevant are the magnitudes of the stresses in the shell resulting from the external loads on nozzles. These stresses are often calculated using Welding Research Council bulletin 107. The calculated stresses must be within the corresponding allowable stresses. If nothing else governs, then by definition the maximum external load will be that which produces a calculated stress equal to the allowable stress.
 
The problem is that because the stresses in the shell result from a combination of 3 moments and 3 forces (2 bending moments, one torsional moment, one radial force, and two shearing forces) there is an infinite number of combinations of forces and moments that can all produce a calculated stress equal to the allowable stress. Thus there is an unlimited number of combinations of external loads that may be the "maximum external load" on the vessel; there is no single "magic combination".
 
The designer can manually determine a combination of loads and moments, or several combinations, that produce maximum stress. A problem can arise if the combination(s) provided to the customer are not satisfactory for their use and the customer comes back and wants a different analysis. The designer can sometimes be forced into an unproductive cycle of rework. Which may be okay if they are paid for the work...but this generally is not the case.

In general, the shear forces VL and Vc and the torsional moment Mt do not contribute to membrane stress; they contribute only to the shear stress which has only small effect on the resultant stress. Thus the major contributors to the total stress are the radial load P and the bending moment ML and Mc.
 
A procedure might be to determine a combination of forces and moments such that the shear forces are equal to each other and equal to some fractional multiple of the radial load (say, 10%), and assume that the bending moments are equal to the shear forces multiplied by some eccentricity (say, 10"). And assume that the torsional moment was same value. Thus the 3 moments and the 2 shears can be related to multiples of the single unknown variable, which is the radial force. Of course, these multiples are simply guesses and are up to the discretion of the designer.
 
COMPRESS provides an option that may help. The WRC-107 dialog contains several options to "solve" for the maximum value of one of the following: +Pr (radial load), -Pr, Mc, or ML. Note that because of the reversal of the load direction the radial load P may have different maximum values depending upon its direction. The maximum value found by COMPRESS is based on the simultaneous application of the values specified for the remaining forces and moments plus the design internal pressure (plus static head, if any). "Click" any of the four "Solve" option buttons and the corresponding input field on the WRC-107 dialog will immediately change to the maximum value for that load or moment. Another helpful tool is the fields at the bottom of the dialog that immediately show the calculated primary membrane stresses and the combined stress. Careful observation of the calculated stresses while varying the input loads and moments can allow the designer to home in on a reasonable set of "maximum loads" for the nozzle.


  Tom Barsh
  Codeware Technical Support

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