(a)The following simple equations are offered for cal- culating approximate values oft, tI,tII, andAbbefore ap- plying the rules inY-4throughY-8. The equations are not intended to replace the rules; however, they should signif- icantly reduce the amount of work required to achieve a suitable design. Since the flanges are in metal‐to‐metal contact and interact, the stresses in one flange are influ- enced by the stiffness of the mating flange and theoreti- cally an unlimited number of designs can be found which satisfy the rules. In practice, however, economics, engineering judgment, and dimensional constraints will show which is the“best”design. It should be noted that the equations inTable Y-9.1assume that both flanges comprising an assembly have essentially the same modu- lus of elasticity and allowable stress.
(b)Equations for Trial Flange Thickness and Bolting ð39ị
ð40ị tc = greater ofta ortb
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ð41ị te = tawhentb <ta
te = tdwhentb >ta
ð42ị ð43ị
ð44ị
where
H1 = 0.785BII2P H2 = 0.785 (G2−BII2)P
l1 = (C −BII)/2
l2 = (C −G)/2 + (G −BII)/4 tg = smaller oftcortf
(c) Trial Values of t, tI,tII,and Ab. The simple equations given inTable Y-9.1should yield relatively good trial val- ues oft, tI,tII, andAbbut they do not assure that the“first trial design”will meet the requirements of Y-6through Y-7. As a result, it becomes necessary to select new trial values and reanalyze. In order to assist the designer in se- lecting the second trial values, the following comments concerning the behavior of different classes ofNonman- datory Appendix Yflanges are offered:
(1)The hub of a Category 1 or 2 flange of a Class 1 assembly reduces the radial stress at the bolt circle (due to a negative hub–flange interaction moment) and the longitudinal hub stress. As a result, a pair of Category 1 or Category 2 flanges will be thinner than a pair of iden- tical Category 3 flanges.
(2)Increasing the thickness of the reducing flange of a Class 3 assembly, when the nonreducing flange is Cate- gory 1 and 2, generally reduces the significant stresses in both flanges comprising the assembly. When the stress in Flange I (nonreducing) is excessive, increasingtIwill gen- erally be more effective in reducing the stresses; however, a nominal increase of the stresses in Flange II will occur due to the additional restraint provided by increasingtI. When the stress in Flange I is excessive and only margin- ally acceptable in Flange II, bothtIandtIIshould be in- creased with the emphasis placed ontI.
(3)A Category 3 reducing flange bolted to a Category 1 or 2 nonreducing flange produces a large overturning moment which tends to rotate Flange I in a negative di- rection. As a result, the radial stress at the bolt circle in Flange I will often be excessive due to a large, positive hub–flange interaction moment. As a result, it is usually necessary to increasetIso thattI =tII. The same problem does not occur when Flange I is Category 3 since there ex- ists no hub–flange interaction moment. When Flange I is an optional type treated as a loose‐type (Category 3), a hub–flange interaction moment actually exists but is dis- regarded in the analysis by assigning the flange to Catego- ry 3.
(4)When the longitudinal hub stress of a Category 1 or 2 flange is excessive, it can be reduced by increasing the size of the hub, org0wheng1 = g0; however, this will cause an increase in the radial stress at the flange–hub junction. WhenSHis excessive andSRis marginally ac- ceptable, an increase in the thickness of the flange is indi- cated in which case it may or may not be necessary to alter the size of the hub.
(5)When the longitudinal stress in the hub of the nonreducing flange of a Class 2 or Class 3 assembly is low compared to the allowable stress and the radial stress at the bolt circle is excessive, increasingSHby making the hub smaller (more flexible) will often reduce the radial stress at the bolt circle toSf. If it does not, an increase intIis indicated.
Table Y-9.1
Trial Flange Thickness and Area of Bolting for Various Classes of Assemblies and Flange Categories
Class (Assembly)
Category of Flanges
Suggested Trial Values
Nonreducing Reducing tortI tI I Ab
1 1 or 2 … 0.9ta …
3 … ta …
2 1 or 2 1 or 2 ta te
3 3 1.1ta 1.1tc
3 1 or 2 ta tc
1 or 2 3 1.1tg 1.1tg
3 1, 2, or 3 … 1.1ta 1.1tc
ASME BPVC.VIII.1-2019 Y-9
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Not for Resale, 07/02/2019 13:29:04 MDT No reproduction or networking permitted without license from IHS
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references:
Schneider, R. W., and Waters, E. O., The Background of ASME Code Case 1828: A Simplified Model of Analyzing Part B Flanges,Journal of Pressure Vessel Technology, ASME, Vol. 100, No. 2, May 1978, pp. 215–219
Appendix Ywere formerly contained in Code Case 1828, A Simplified Method for Analyzing Flat Face Flanges with Metal‐to‐Metal Contact Outside the Bolt Circle/Section VIII, Division 1.
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ð19ị
NONMANDATORY APPENDIX DD