Special Flanges

Rules for special flanges with different geometry and/or loading are given in Appendix 2 of VIII-1. Included are: 2–9 for split loose flanges, 2–10 for noncircular shaped flanges with a circular bore, 2–11 for flanges subject to external pressure, 2–12 for flanges with nut-stops, and 2–13 for reverse flanges. Flanges with other geometry and loading shall follow U-2(g).

4.6.2.1 Reverse Flanges. Reverse flanges are described in Appendix 2-13 of VIII-1. They are similar to standard flanges, except some of the loads on the flange ring cross section may be applied at different loca- tions and in a reverse direction, possibly causing a reverse moment. VIII-1 has chosen to use the term αBto

convert a standard flange to a reverse flange. Example 4.7 gives an example problem and a filled-in sheet for a reverse welding neck flange with a ring-type gasket. See Appendix D for a blank fill-in Sheet D.4 (Reverse Welding Neck Flange with Ring-Type Gasket).

The method of analysis for a reverse flange is similar to that used for an integral flat head with a large, sin- gle, circular, centrally-located opening, as given in Appendix 14 of VIII-1. For both analyses, a special lim- itation of the geometry is given. When K≤ 2, calculated stresses are acceptable; however, when K > 2, calculated stresses become increasingly conservative. For this reason, use of the analysis procedure should be limited to K≤ 2.

Example 4.7 Problem

Using the rules of Appendix 2 of VIII-1, determine the minimum required thickness of a reverse welding neck flange, shown in Fig. E4.7, with the following design data:

Design pressure = 2000 psi; Design temperature = 650°F; Flange material is SA-105; Bolting material is SA-325 Gr. 1;

Gasket is spiral-wound, fiber-filled, stainless steel, 13.75 in. I.D. × 1.0 in. wide; No corrosion allowance.

Note: This flange has facing details, gasket size, and bolting that are the same as those given in Example 4.5; however, this is a reverse welding neck flange with different flange dimensions.

Solution

(1) The allowable tensile stress of the bolts from II-D at the gasket seating and operating conditions (design temperature) is Sa= Sb= 20.2 ksi.

(2) The allowable tensile stress of the flange from II-D at the gasket seating is Sfa= 20.0 ksi and operating

conditions is Sfo= 17.8 ksi.

(3) The diameter of the gasket line-of-action, bolt loadings, bolt number and diameter, and the crushout width are the same as in Steps 3–5 of Example 4.5.

(4) The total flange moment for the gasket seating condition is the same as in Step 6 of Example 4.5.

MGS= 2,098,000 in.-lb

(5) The total flange moment for operating condition is:

Flange Loads

HD= (π/4)B2p

FIG. E4.7

HG= Wm1 – H = 556,000 – 355,500 = 200,500 lb HT= H – HD = 355,500 – 821,900 = –466,400 lb Lever Arms hD= 0.5(C + g1– 2go– B) = 0.5(22.5 + 1.8125 – 2 × 1.8125 – 22.875) = – 1.094 in. hG= 0.5(C – G) = 0.5(22.5 – 15.043) = 3.729 in. hT= 0.5[C – 0.5(B + G)] = 0.5[22.5 – 0.5(22.875 + 15.043)] = 1.771 in. Flange Moments MD= HD× hD = (821,900)(– 1.094) = – 899,200 in.-lb MG= HG× hG = (200,500)(3.729) = 747,700 in.-lb MT= HT× HT = (–466,400)(1.771) = – 826,000 in.-lb Mo= MD+ MG+ MT = – 977,500 in.-lb

Use the absolute value in the calculations.

(6) Shape factors from Appendix 2 of VIII-1 for K are

K = A/B′ = 26.5/13.25 = 2.0

T = 1.51 Z = 1.67 Y = 2.96 U = 3.26 αR= (1/K2) {1 + [3(K + 1)(1 – µ)]/πY} = 0.419 TR= [(Z + µ)(Z – µ)] αRT = 0.857 YR= αRY = 1.241 UR= αRU = 1.366 g1/go= 1.0 ho= (Ago)1/2 = [(26.5)(1.8125)]1/2_{= 6.930} h/ho= ∞

From Appendix 2 of VIII-1:

F = 0.909 V = 0.550 f = 1.0

e = F/ho

= 0.909/6.930 = 0.131

d = (UR/V)hog2o

= 56.543

(7) MGS= 2,098,000 in.-lb and Sfa= 20.0 ksi and Mo= 977,500 in.-lb and Sfo= 17.8 ksi. Since the moment

at operating condition is less than 0.5 times the moment at gasket seating condition with a slightly less allow- able stress, only the gasket seating condition is calculated. Assume a flange thickness of t = 4.0 in.

L = {[(te + 1)/TR] + (t3/d)}

= 1.778 + 1.131 = 2.909

Longitudinal hub stress:

SH= fMGS/Lg21B′

= [(1)(2,098,000)]/[2.909)(1.8125)2_(13.25)] = 16,570 psi

Radial flange stress:

SR= {[(4/3)te + 1]MGS}/Lt2B′

= [(1.699)(2,098,000)]/[(2.909)(4)2_(13.25)] = 5780 psi

Tangential flange stress:

ST= [(YRMGS/t2B′) – ZSR(0.67te + 1)]/β = {[(1.241)(2,098,000)/(4)2_{(13.25)] – [(1.67)(5,780)(1.351)]}/(1.699)} = 4610 psi Combined stresses: 0.5(SH+ SR) = 0.5(16,570 + 5,780) = 11,180 psi 0.5(SH+ ST) = 0.5(16,570 + 4610) = 10,590 psi

Tangential flange stress at B′:

S′T= (MGS/t2B′){Y – [2K2(0.67te + 1)/(K2– 1)L]} = [(2,098,000)/(4)2_{(13.25)]{2.96 – [2(2)}2_{(1.351)/(3)(2.909)]}} = 17,040 psi (8) Allowable stresses: SH≤ 1.5Sf: 16,570 psi < 26,700 psi SR≤ Sf: 5780 psi < 17,800 psi ST≤ Sf: 4610 psi < 17,800 psi 0.5(SH+ SR) ≤ Sf: 11,180 psi < 17,800 psi 0.5(SH+ ST) ≤ Sf: 10,590 psi < 17,800 psi S′T≤ Sf: 17,040 psi < 17,800 psi

4.6.2.2 Full-Face Gasket Flanges. Although Fig. 4.1 (p), Shows a flange with a full-face gasket which permits part of the gasket to lie outside the bolt circle, no design procedure exists for such a flange. This type of gasket may be used with either a loose or an integral flange. See Appendix D for blank fillin Sheets D.5 (Slip-on Flange with Full-Face Gasket) and D.6 (Welding Neck Flange with Full-Face Gasket). One of the basic differences with a full-face gasket is that a reverse moment is generated from that part of the gasket loading outside the bolt circle.

Most often, a full-face gasket is used where the m and y factors are relatively low, so that the bolt loading is kept within acceptable limits. A full-face gasket design generally results in the total moments from gasket seating and from operation to be fairly low, and consequently, only a nominal flange thickness is required. However, bolt loads are usually higher.

4.6.2.3 Flat-Face Flange with Metal-to-Metal Contact Across the Face or at the Outer Edge. Appendix Y of VIII-1 contains rules for the design of a flat-face flange with metal-to-metal contact across the whole face or with a metal spacer added to the outer edge between pairs of flanges. Gasket loadings usu- ally are small, as most gaskets are of the self-sealing type. In order to make an analysis easier, assemblies are classified and individual flanges are categorized. Once this is established, the rules for analysis are given in VIII-1.

Classification of Assemblies

Class 1: A pair of flanges which are identical except for the gasket groove

Class 2: A pair of nonidentical flanges in which the inside diameter of the reducing flange exceeds half the bolt circle diameter

Class 3: A flange combined with a flat head or a reducing flange with an inside diameter that is small and does not exceed half the bolt circle diameter

Categories of Flanges

Category 1: An integral flange or an optional flange calculated as an integral flange Category 2: A loose-type flange with a hub that is considered to add strength

Category 3: A loose-type flange with or without a hub—or an optional type calculated as a loose type— where no credit is taken for the hub

The analysis of an Appendix Y flange is similar to that made for an Appendix 2 flange, except for the addi- tional load and moment caused by the contact or prying effect. The contact force, HC, and its moment arm,

hcinvolve an interaction between the bolt elongation and the flange deflection and the moments from the bolt

loading and pressure loading.

The bolt loading for the operating condition is

Wm1 = H + HG+ HC (4.17)