Knowledge Base Answers Q:Why are nozzles not allowed to be exempt if they are located in Category A weld joint? UW-14(a) permits any opening that meets the reinforcing requirements of UG-37 or UG-39 to be located in a welded joint. Consequently, small nozzles located in a welded joint cannot be exempt from reinforcing area per UG-36(c)(3)(a). It is Codeware’ understanding that if a nozzle is located on a category A joint than the area reinforcement requirements must be satisfied. If a nozzle is located on a welded joint and failing the area replacement method than the designer may want to perform the analysis using the rules from Appendix 1-10. To activate this go to the first screen of the nozzle dialog and click on "Calculation Options". This button will be located in the bottom left hand corner of the dialog. Within this dialog check the option "Use Appendix 1-10 in lieu of UG-37".
Q:COMPRESS reports the formula "tr = P*K1*Do / (2*S*E + 0.8*P)" for "tr" in the UG37 calculation. How is it developed? This formula is applicable when the nozzle and its reinforcing area is located totally within the "dished" portion of the head, as defined in UG-37(a) for definition of "tr", subparagraph (c) for opening in an ellipsoidal head. For this case, the opening and reinforcement are located entirely within a circle of 80% of the head diameter. The required head thickness "tr" for purposes of area replacement is then based on that for a sphere of radius K1*D, where K1 is given by Table UG-37, and D is the shell diameter (diameter of the head). When the head has a specified outside diameter, COMPRESS applies the rule from Appendix 1-1 for thickness of spherical shells based on outside radius, subparagraph (2). Thus the term K1*D is substituted into equation (2) from Appendix 1-1 to determine the thickness of the equivalent spherical portion of the head. Step (1): From UG-37(a) tr (c), sphere radius = K1*D Step (2): Because the ellipsoidal head has specified outside diameter, COMPRESS uses Appendix 1-1 equation (2), with spherical radius = K1*OD. Consequently,
tr = P*Ro/(2SE+0.8P) = P/(2SE+0.8P) * K1*Do = P*K1*Do / (2*S*E + 0.8*P)
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Nozzle Design Questions
Knowledge Base Answers Q:How to model a socket welding coupling in COMPRESS. COMPRESS does not have the option to specifically model socket welding couplings. However, they can be modeled as standard nozzles based on their dimensions without the socket portion. The nozzle outer diameter should be specified as the coupling outer diameter and the nozzle’s bore diameter should be specified as the coupling’s inner core diameter. COMPRESS conservatively would not consider any additional reinforcing area in the “socket wall” of the coupling. See the examples below:
Example 1: Model Full Couplings as a type 3 nozzle – In the figure below, the coupling is modeled as a type 3 nozzle using the dimensions marked in red.
Example 2: Model Half Couplings as a type 1 nozzle – In the figure below, the half coupling is modeled as a type 1 nozzle using the dimensions marked in red.
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Nozzle Design Questions
Knowledge Base Answers Q:What is ASME Appendix 1-10? Why does COMPRESS apply Division 2 rules for nozzle design to my Division 1 vessel? For "large openings" analyzed per ASME Code Section VIII, Division 1, 2007 Edition and later, COMPRESS analyzes the opening by either of the following methods. Note the further discussion below about the weld path strength requirements of UG-41. Method A: Check by UG-37 and Appendix 1-7(a) area compensation rules. Perform Appendix 1-7(b) stress analysis if required by that paragraph. Method B:
Perform stress analysis using Appendix 1-10 stress analysis method (which is more comprehensive than the stress analysis of Appendix 1-7(b)). Note that the stress analysis method of Appendix 1-10 is used in lieu of the area compensation principles of UG-37 and Appendix 1-7(a). Note that both Appendix 1-7 and Appendix 1-10 have limitations on the range of sizes and types of construction to which they may be applied. If a nozzle falls outside of these restrictions then COMPRESS will use the rules of Division 2, Part 4.5 for the nozzle analysis, under the direction of Division 1, paragraph U-2(g). The designer can elect to use either the Appendix 1-7 method or the Appendix 1-10 method: Edit the nozzle and click the "Calculation Options" button; then select the desired method on the dialog:
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Nozzle Design Questions
Knowledge Base Answers
Appendix 1-7 has a number of limitations on the applicable geometry. COMPRESS automatically applies Appendix 1-10 if Appendix 1-7 is selected but the nozzle geometry falls outside the scope of that Appendix. If the nozzle geometry falls outside of the Code limitations on applicability of Appendix 1-10 then COMPRESS uses the rules of Division 2 Part 4.5 under the provision of Division 1 paragraph U-2(g). In this latter case there is no need for a Division 2 Code stamp, etc. See further discussion in the COMPRESS History document (select the COMPRESS Help menu, select "View History" command; search the document for the entry for build 6255): (14) Appendix 1-10: A new alternative method is now available to evaluate large openings in cylinders and cones. The designer can designate the analysis method globally on the Nozzles 1 tab of the Set Mode dialog or on an individual nozzle basis under the Areas option button in the Nozzle dialog. The new method provides a direct calculation of the maximum local primary membrane stress at the nozzle intersection (PL) by using a modified pressure area method to determine the magnitude of the discontinuity force resisted locally. Division 2 Part 4.5 is utilized to evaluate the strength of attachment welds as the UW-41 analysis is not applicable. It should be noted that the 2007 Edition of Division 2 uses this method for the design of all nozzles (all Rn /R ratios and nozzle configurations). For nozzle configurations that are not covered under Appendix 1-10 (eg. hillside nozzles in cylinders), Division 2 Part 4.5 is used as a U-2(g) analysis. Note that the title of Appendix 1-10 refers to "Alternative Method of Design of Reinforcement"; thus it is intended as an alternative method of analysis, NOT a new additional requirement
Q:How does COMPRESS determine which nozzles fall under the category of "large openings" subject to the rules of Appendix 1-7? For shells under 40" diameter, UG-36(b)(1) specifies that rules for reinforcing of nozzles per UG-36 through UG-43 apply to openings not exceeding one-half the vessel diameter. Appendix 1-7(b) applies to nozzles that exceed the limits of UG-36(b)(1). We interpret rules (a), (b), and (c) of 1-7(b)(1) to apply individually (ie: a nozzle meeting any of these size criteria must meet the rules of Appendix 1-7(b)(2),(3), and (4)). If the nozzle Rn/R also exceeds 0.7 as specified in 1-7(b)(1)(c), COMPRESS performs the stress analysis of Appendix 1-7(b)(2),(3), and (4) and then reports that the Rn/R ratio exceeds 0.7 and additional analysis is required per U-2(g).
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Nozzle Design Questions
Knowledge Base Answers Q:How does COMPRESS determine the maximum allowable working pressure (MAWP) and maximum allowable pressure (MAP) for nozzles? Several criteria must be met when determining the MAWP of a nozzle. These criteria include: 1. The nozzle wall required thickness(trn in UG-37 cannot exceed the nozzle thickness) 2. The vessel wall required thickness (tr in UG-37 cannot exceed the shell thickness) 3. The reinforcing area required cannot exceed the reinforcing area available 4. UG-45 minimum nozzle wall thickness 5. UG-41(b)(1) weld strength calculations 6. UW-16 minimum weld size requirements 7. For large openings, the rules of Appendix 1-7 are applied (or, alternatively, either Appendix 1-9 or 1-10) 8. Stresses due to pressure combined with external nozzle loads (WRC-107 & PVP Vol. 399) Because multiple criteria must be met there is no single equation that provides a direct calculation of the nozzle MAWP. COMPRESS solves for nozzle MAWP by iteration. COMPRESS performs the required calculations for all applicable criteria with an initial value for pressure. If none of the criteria are exceeded, COMPRESS increases the pressure and performs the calculations again. This process is repeated until one of the criteria is exceeded or until the MAWP of the component to which the nozzle is attached is reached. This limiting pressure as well as the governing criteria are listed in the nozzle report. The nozzle governing criteria is typically shown in the report before the reinforcement calculations. The MAWP is by definition calculated in the corroded condition and at the design temperature. Similar calculations are provided for the MAP which is by definition in the un-corroded (new) condition at ambient (test) temperature ("new and cold").
Q:Why is a different joint efficiency used for the Nozzle Required Thickness Per UG-27(c)(1) and the UG-45 Nozzle Thickness check? Paragraph UG-37(a) defines the nozzle required thickness "trn" as the required thickness for a seamless nozzle neck. Thus for the purpose of determining "trn" for use in UG-37, COMPRESS applies a joint efficiency E = 1.0. This is true even when another value has been entered for the nozzle's longitudinal seam joint efficiency. The value entered for the nozzle longitudinal seam joint efficiency (Calculations Options dialog) is used when determining the minimum required nozzle neck thickness per UG-45(a) in the equation for tr1.
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Nozzle Design Questions
Knowledge Base Answers Q:What happens when a nozzle’s reinforcement extends beyond the parent component and onto adjacent components? It is a problem when the Code limits of reinforcement extend beyond the nozzle's parent component and onto adjacent components. In such a case the excess thickness available in the adjacent component, to be utilized as reinforcing area, may not be at least equal to that available in the parent component, in which case the nozzle will be under-reinforced in that plane. Determining the thickness available in adjacent components is beyond what COMPRESS will address. However, in such a case, we suggest applying a user-defined limit of reinforcing so that reinforcing area beyond the parent component is not considered in the nozzle design. To specify such a limit edit the nozzle, click the "Areas" button, check the option to apply a user-specified limit of reinforcing, and enter a value. It is a particular problem when the nozzle is located on the knuckle region of the head. At one time, COMPRESS returned a warning when the Code limit of reinforcement for nozzles in heads extended beyond the head. At Codeware, we have discussed extending the warning to the nozzles in other components, but after some review and discussion it was decided to eliminate all such warnings. Please see comments below from Les Bildy, author of COMPRESS, that relate to this issue. In addition to changing the limit of reinforcing as above, you can also specify a "local shell thickness" for the purpose of nozzle calculations. Edit the nozzle, click the "Next" button, Click the "Auto" button adjacent to the "Shell Thickness" caption, and a dialog will appear. Remove the "Automatic" checkmark and then specify the local thickness to be used. Code Issues: The Code UG-37 rules actually make a number of assumptions including: 1.
The nozzle is attached to one type of geometric shape, i.e. it's either on a cylinder, head, or cone. It gives no guidance as to what to do if part of the assumed limit of reinforcement is shared between two different geometric shapes. 2. The nozzle is attached to a shell having a uniform thickness extending to the edge of the limit of reinforcement. 3. The nozzle is attached such that the limit of reinforcement doesn't enter into a local discontinuity stress zone such as a cone to shell juncture, saddle horn, body flange hub, not near an integral tubesheet etc. 4. Strictly speaking, in our opinion, when any of the above situations is encountered the UG37 rules don't apply and we are now into a U-2(g) (i.e. un-specified) analysis. The reasons that we removed the warning message are: Consistency. If we warn for the nozzle on head case why not all the other myriad cases? Reliability. We were getting the nozzle on head warning in cases where it wasn't required. It's actually quite difficult to produce a general purpose algorithm that can detect even this one situation and not "cry wolf". User confusion. What is the correct user response when the message is produced? Based on our investigations we are not at all convinced that a real issue exists anyway.
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Nozzle Design Questions
Knowledge Base Answers
Engineering Issues: During the course of validating Codeware's nozzle design proposal to the Div 2 re-write committee, we ran several hundred FEA models to try and get a better understanding of the behavior of nozzles in and around the formed head knuckle region. It turns out that the Code area replacement rule assumptions don't work very well for nozzles in the knuckle region of formed heads. Some observations:
FEA shows that for many high D/t ratios that meet the existing Code rules the PL stress present in the knuckle region of the head is considerably higher than the 1.5S allowed by the Code. Except in extreme cases this is usually o.k. because in practice the head deflects a little and takes on a better shape (geometric strengthening). Generally, FEA shows that as the nozzle is moved from the center of the head into the knuckle region the stresses increase as expected. In contrast, UG-37 area requirements stay the same until you reach the 80% radius at which point they suddenly increase to a higher level where they stay (even as you move the nozzle further from the head center). According to FEA, the result of adding a nozzle in the knuckle region is not always the same. There are cases where adding a nozzle in the knuckle region actually reduces the stress in the head. I believe this may be due to the nature of the compressive buckling present in the knuckle; the nozzle tends to act like a local "stiffener" for thinner heads. Contrast this to UG-37, where adding a nozzle is assumed to always increase the stress in the head. According to FEA, not all methods of providing additional reinforcement are equivalent. Thickening a nozzle in the knuckle region may lead to an increase in stress in the head instead of the expected decrease. One possible explanation may be the nozzle as a local "stiffener" metaphor. If the nozzle gets too "stiff" it restrains the knuckle region, causing a higher local bending. UG-37 assumes that adding extra material in the nozzle is just as good as adding a pad. If FEA is correct, then this is not the case; there are situations where adding a pad is the ONLY way to correct the problem.
Q:Why does COMPRESS determine nozzle weld sizes per UW-16 using corroded thicknesses? COMPRESS determines nozzle fillet weld sizes based on the corroded thicknesses of the shell wall and nozzle neck per the following ASME Code paragraphs:
UG-16(e) states that the dimensional symbols used in all design formulas throughout ASME VIII-1 represent dimensions in the corroded condition.
UG-37 defines "t" and "tn" to be the nominal thickness of the shell and nozzle wall respectively. Figure UG-37.1 illustrates that the nominal thicknesses do not include the corrosion allowance "c".
UW-16 repeats the same definitions for "t" and "t n" found in UG-37. ASME VIII-1, Appendix L uses the corroded thickness when calculating the required minimum fillet weld size in subparagraph L-7.5, Example 5. (see L-7.5.3(a) Inner reinforcing element).
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Nozzle Design Questions
Knowledge Base Answers
How does COMPRESS determine the chord opening for offset nozzles in cylinders? COMPRESS determines the chord opening for offset nozzles in cylinders using the method described in ASME Section VIII, Division 1, Appendix L; see Figure L-7.7.1. Following this method, COMPRESS uses the chord on the radius of the mid-plane of the cylinder required thickness in the corroded conditions. Note that the cylinder design pressure is used when determining the required thickness of the cylinder. If the nozzle's design mode is "Design P, Find MAWP & MAP", then this results in a slightly larger chord than if the nozzle's MAWP was used: at MAWP the required thickness would be greater and the chord will be smaller. The use of the required thickness at design pressure is thus slightly conservative. Numerical Example:
cylinder ID = 60 int pressure = 180
cylinder thk = 1.625 allowable stress = 18800
cylinder corrosion = 0.12 E =1
corroded IR = cyl ID / 2 + corr = 60 / 2 + 0.12 = 30.12
t reqd = P * IR / (S * E - 0.6 * P) = 180 * 30.12 / (18800 * 1 - 0.6 * 180) = 0.290049
radius to mid-plane of reqd thk, Rm = cyl ID / 2 + corr + reqd thk / 2 = 60 / 2 + 0.12 + 0.29 / 2 = 30.2650246
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Nozzle Design Questions
Knowledge Base Answers
nozzle OD = 24 nozzle corrosion = 0.12
nozzle thk = 0.625 nozzle offset = 14
nozzle IR = nozzle OD / 2 - nozzle thk + nozzle corr = 24 / 2 - 0.625 + 0.12 = 11.495
alpha 1 = arc cos((offset + nozl IR) / Rm) = arc cos((14 + 11.495) / 30.265)) = 32.606473
alpha 2 = arc cos((offset - nozl IR) / Rm) = arc cos((14 - 11.495) / 30.265)) = 85.252265
delta = alpha 2 - alpha 1 = 85.2523 - 32.6065 = 52.6458
chord = 2 * Rm * sqrt(1 - cos ^ 2 (delta / 2) = 2 * 30.265 * sqrt(1 - (cos(52.6458 / 2)) ^ 2) = 26.8408
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Nozzle Design Questions
