WinNozl FAQ

Index:

1. What are geometry limits for WRC107 and WRC297?
2. Section VIII div I Code stress factors: I use the the following allowables recommended by Bednar: operating + local mechanical = 2Sa<Sy; operating + thermal = 1.25*(Sa + Satm). Where did the 1.5 primary combined and 3.0 secondary combined(default values) come from in WinNozl?
3. WinNozl uses a stress factor of 3 for gravity combined (secondary) loads. Isn't this excessive?
4. I use the allowable stress as the lesser of the shell or nozzle allowable. Does WinNozl use the shell material for the allowable stress?
5. Why do all forces and moments listed under allowable loads have the same value?
6. I keep getting the following warning message, "Pad width is smaller than 1.65*SQR(RT)= 989.38 mm". What shall I do?
7. What factors should I change on the FACTORS tab for my PD5500 model?
8. Why do my allowable design stresses appear incorrect?
9. WRC297 curve values can be negative shown in the detailed stress report?
10. Why are there two values of Fp in the API650 summary report?

Question 1: What are geometry limits for WRC107 and WRC297?

Answer: Winnozl Code Limits:

(1) - Subsequent revisions have conservatively extended d/D <=0.6 (but may be inaccuracies)

d - outside diameter of the nozzle
D - outside diameter of the shell.
T- shell thickness

Approximate limit of WRC107 for spherical shells is d/D = 0.33

Question 2:  Section VIII div I Code stress factors: I use the the following allowables recommended by Bednar: operating + local mechanical = 2Sa<Sy; operating + thermal = 1.25*(Sa + Satm). Where did the 1.5 primary combined and 3.0 secondary combined(default values) come from in WinNozl?

Answer: The 1.5 allowable membrane stress intensity and 3.0 primary +secondary stress limit come from ASME Section VIII div 2 4-130. The 3.0 S limit is not explicity stated in ASME VIII Div 1 but as stated in the PVRC nozzle design proceedings of 2/98, the 3S limit is a conservative criteria for div 1 stresses. Note: 3Sm is 3 x(Sm amb + Sm operating) as per ASME Section VIII div 2 Fig 4-130.1

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Question 3:  WinNozl uses a stress factor of 3 for gravity combined (secondary) loads. Isn't this excessive?

Answer: Gravity + pressure can be classified as a mechanical loading and since gravity contributes bending + membrane components of stress it can be classified in the PL+Pb+Q stress category to ASME Section VIII div 2 Fig 4-130.1 and therefore the allowable stress is Sm. bending stresses caused by external moment or pressure in the vesselwall near a nozzle or opening should be classified as Q i.e secondary stresses regardless whether caused by sustained or expansion loads. Secondary stresses, Q in PL+Pb+Q < 3Smav includes the following:

• Bending stresses due to pressure
• Bending stresses due to sustained forces and moments i.e gravity
• Bending stresses due to expansion forces and moments
• Membrane stresses due to expansion forces and moments

Question 4:  I use the allowable stress as the lesser of the shell or nozzle allowable. Does WinNozl use the shell material for the allowable stress?

Answer: Stress are always calculated in the shell not the nozzle to WRC107 and BS5500 and hence the allowable stress uses the shell material stress. Since stresses are calculated in the shell and the nozzle to WRC297, the shell and nozzle material stresses are both used respectively in the allowable stress calculations.

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Question 5:  Why do all forces and moments listed under allowable loads have the same value?

Answer: The allowable loads calculated are in accordance with the allowable load factors i.e "F" button under the Factors Tab which by default are set = 1.0 for all ratios. It is a good approach to enter ratios of in particular circumferential and longitudinal moments to radial load based on the existing pipe loads on the nozzle then the allowable loads will be in the same proportion.

Question 6:  I keep getting the following warning message, "Pad width is smaller than 1.65*SQR(RT)= 989.38 mm". What shall I do?

Answer: The pad width is the (pad radius - the nozzle radius ) and this WRC297 recommendation is not mandatory but is generally taken as the greater of than 1.65*SQR(RT) or d/2 whichever is greater which has been used successfully yielding generally conservative results. By following this recommendation then it is acceptable to consider the stress at the nozzle to shell junction as the thickness of the shell plus pad thickness and discontinuity stresses are neglible at the distance of than 1.65*SQR(RT) or d/2 whichever is greater.

Note: Winnozl will continue with local stress calculations even if the parameters are outside the WRC curves and provide a warning e.g "Calculations continued with T/t=5.1, for the nozzle" which is usually the limit of the WRC curves. The program does also block the user from continuing with a pad diameter < nozzle OD.

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Question 7:  What factors should I change on the FACTORS tab for my PD5500 model?

Answer: The design factors on the FACTORS tab are not relevant to hollow nozzles on spherical shells since PD5500 uses a different approach i.e shakedown to check these nozzles are within acceptable limits. Except the combined or total stress which has allowable = 2.25 Sm

The allowable stress for hollow nozzles on spherical shells is based on the ratio of shakedown factors P/Po as derived from PD5500 G.2.6. Refer to detail stress report where S1/S2 = 0.724 hence allowable pressure stress = actual stress/0.724 = 340.8/0.724 = 470.5

Similarly the shakedown combination shown in the summary report is compared to 1.0 in accordance with equation G.6 under PD5500 G.2.6.5.

Question 8:  Why do my allowable design stresses appear incorrect?

Answer: You have selected the Design code = PD5500 and Material library = PD5500.

However under the shell material is defined a user material "12CrMo910" which is not available in PD5500 library. When a user material name or user defined stresses are entered (shown as blue font) the program assumes the user is over-riding a selected library material. Hence the 199.3 read from the library is for a PD5500 code material and is meaningless for the user material "12CrMo910".

To select a library material for shell click on the "S" button on the SHELL tab or Edit/Shell material from the menu. When a material or stresses have been over-ridden (i.e user defined) the values are shown as blue in the program. To reset them back to library value, click on the RESET button in the material screen. Then selecting a new material from the drop down list will update all stresses - design, ambient yield stresses as recovered from the library.

Note: You can select a different material library for a design code e.g material library = ASME 2000 DIV I and design code = PD5500 but derivation of design stresses may be different.

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Question 9:  WRC297 curve values can be negative shown in the detailed stress report?

Answer: : For figures 18, 36 to 40 and 54 to 58 the membrane stress N? curve values can be +ve or -ve as indicated by (-) and (+) on the curves. Also refer to Table A-4 in WRC297 which shows an example N? = -0.07 obtained from fig 39.

Question 10:  Why are there two values of Fp in the API650 summary report?

Answer: Winnozl API650 Fp calculations (pressure end load on the opening) is the maximum from fluid force acting on the nozzle end (away from the tank) and design pressure whereas API650 only shows Fp = fluid force.

In ASME paper PVP-59 page 8 "Experimental investigation of stiffness coefficients and allowable loads for a nozzle in a flat bottom storage tank" co-authored by Tam and Billimoria (same authors as API650) they use this design pressure method instead of design height.

Where:

Fluid force acting on the nozzle end (away from the tank)
P = G (62.4 (H-L) and Fp1 = pi.a^2.P

Design pressure method
P = Sd t/R and Fp2 = pi.a^2.P

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