Flange design IN PRESSURE VESSEL

Bolt design (conti…)
Actual bolt area required (Ab) > (Am)
Total number of bolts will be multiple of four.
Number of combinations are possible in providing the required bolting area.
In general, large number of smaller size will provide same area as of small number of large bolts.
Minimum bolt spacing is based on wrench clearing that limits maximum number of bolts.
The maximum bolt spacing is limited by the permissible deflection that would exist between flanges.
Bs(max) = 2db+ 6 +0.5
Where, Bs(max) is maximum bolt spacing, db diameter of bolt, t is flange thickness, m is gasket factor

Minimum bolting area required (Am) and actual bolting area provided(Ab)
Am = Am1 or Am2, which ever is greater

Am1 = minimum bolting area required at operating condition
= Wm1/fb
fb = Maximum allowable bolt stress at operating condition

Am2 = minimum bolting area required at atmospheric condition
= Wm2/fa
fa = Maximum allowable bolt stress at atmospheric condition

Bolt design
Two different types of loads are acting bolts.
At bolting up condition (At atmospheric condition), Wm2
At operating condition, Wm1
Wm2 = πbGy
Wm1 = H+ Hp = 4 2P + 2bπGmP
Where, G = diameter at the location of gasket load reaction
G = Mean diameter of gasket = + 2, for b0 <= 0.25 inch
G = OD of gasket – 2b, b0 > 0.25 inch
H = Total hydrostatic end force = 4 2P
Hp = Force required to keep the gasket without leaking or in seating condition = 2bπGmP

b = b0, If b0 <= 0.25 inch
b = sqrt(b0)/2, b0 > 0.25 inch (Keep value of b0 in inch and answer will be in inch then convert it to mm)
Width of gasket
= 0 2
Where, N = Actual width of gasket, d0 and di are outside and inside diameter of gasket respectively
Since because of different types of flange facings and gaskets, total width of gasket is not utilized or total width of gasket is never affected by any force. Also bolt load is applied on the side which creates deflection in flange and hence total width of gasket does not bear the load but some effective part of gasket is bearing the load. Hence two more widths are defined.
b0 = Basic gasket seating width for different type of flange facing and different types of gasket.
b = Effective gasket seating width

How to select gasket seating stress 'y' and gasket factor 'm'?

Gasket Design
d0 = OD of gasket
di = ID of gasket
y = gasket seating stress
m = gasket factor
Design of integral flange
Design of integral flange is almost similar to loose flange.
Equations for finding forces will remain same (Hd, Ht and Hg remain same)
But lever arm for the forces will vary.
hd = R + (g1/2) , where g1 = thickness of hub at larger end
g1 = thickness of hub at larger end
= 2*g0 for values of g0 upto 1.5 inch
= 1.5*g0 for values of g0 greater than 1.5 inch
R = Radial distance from bolt circle to point of intersection of hub and back of flange
hg = (C-G)/2
ht = (R+g1+hg)/2
Thickness of flange, t = 0.72 + CA + MA

Thickness of loose flange
t = + CA + MA
Where, Mmax = Maximum moment, Mo or Ma*ffo/ffa
ffo = Maximum allowable stress of flange material at operating condition
ffa = Maximum allowable stress of flange material at atmospheric condition
fallow = ffo
CF = Bolt pitch correction factor = 2 +
B = ID of flange
t = thickness of flange
Bs = Actual bolt spacing
d = bolt diameter
Y = factor involving K = 1 1 [0.66845 + 5.7169 2 10 2 1 ]
K = ratio of outside diameter to inside diameter of flange = A/B

Total bending moment acting on a flange at bolting up condition or atmospheric condition, Ma = W * hg
Where, W = Design bolt load at bolting up or atmospheric condition = [Am+Ab]*fa/2.
hg = lever arm for design bolt load at atmospheric condition = (C - G)/2

Design of loose flange (cont…)
Flange Outside diameter
= 2
Nmin = minimum gasket width
Ab = actual bolt area provided
fb = Maximum allowable bolt stress at atmospheric condition
G = diameter at the location of gasket load reaction
y = gasket seating stress

Check for gasket width
A = C +2E
A = OD of flange
C = bolt circle diameter
E = Edge clearance = (dbh/2) +24
dbh = bolt hole diameter

Design as a loose flange or integral flange?
Integral Flange : Integral flanges are those in which the construction is such that the flange obtains support from its hub and the connecting nozzle (or pipe). The flange assembly and nozzle neck form an "integral" structure. Ex. welding-neck flange.

Loose flange : Loose flanges are attached to the nozzle (or pipe) in such a way that they obtain no significant support from the nozzle neck and cannot be classified as an integral attachment. Ex. Screwed or lap joint flange.
Check for the constraints
g0 <= 5/8 inch, g0 is thickness of hub at small end. For ring flange g0 is thickness of shell or pipe.
B/g0 <= 300, B is ID of flange = OD of shell
P = Internal design pressure <= 300 psi
Design temperature <= 700 oF

If above all four constrained are satisfied then flange is designed as loose flange, otherwise it is designed as an integral flange.
Design of loose flange
Total bending moment acting on a flange at operating condition,
MO = Md + Mt +Mg
Where, Mo is total bending moment at operating condition.
Md is bending moment created by hydrostatic end force acting on inside area of flange (Hd) and lever arm for Hydrostatic end force (hd)
Md = Hd*hd
Where, Hd = hydrostatic end force acting on inside area of flange = 4 2P
hd = lever arm for Hydrostatic end force = Radial distance between bolt circle diameter and ID of flange = (C-B)/2

Mg is bending moment created by gasket load Hg (difference between flange design load and total hydrostatic end force and lever arm for force Hg (hg)
Mg = Hg*hg
Hg = Difference between flange design load and total hydrostatic end force = W – H = Wm1- 4 2P
hg = lever arm for force Hg = (C - G)/2

Mt is bending moment created by force Ht (difference between total hydrostatic end force and the hydrostatic end force on area inside of flange and lever arm for force Ht (ht)
Mt = Ht*ht
Ht = Difference between total hydrostatic end force and hydrostatic end force acting on inside area of flange = 4 2P - 4 2P
ht = lever arm for force Ht = (hd + hg)/2

Design of non-standanrd flange

Bolt Circle diameter,
C = B'+2(g1+R)
Where, C is bolt circle diameter and B' is ID of shell
g1 = thickness of shell + plus thickness of weld = approximately, 1.5*thickness of sheel
= 0 02+10+ h2
Where, d0 is OD of gasket, D0 is OD of shell and dbh is bolt hole diameter.
Design bolt load at bolting up or atmospheric condition, W = [Am+Ab]*fa/2.
Design bolt load for operating condition, W = Wm1

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Flange design IN PRESSURE VESSEL

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