Page 53 - Pressure Vessel Design Manual

General Design 39

h=y+6 bolts required is:
1 [
q=- 1+ 3(K + 1)(1 - u)] n = A,dR;,
K2 XY
To find the size of bolt for a given quantity:
For factors, F, U, FI,, and U12, see Table 2-7.1 of the
ASME Code [l]. R,= Am/n

With these two equations a variety of coinbinations can be
determined.
Special Flanges
Step 2: Determine the bolt circle diameter for the selected
Special flanges that are required to be designed should bolt size.
only be used as a last resort. Whenever possible, standard
flanges should be utilized. In general, special designs as out- C = B + 2,1+2H
lined in this procedure are done for large or high-pressure
designs. Flanges in this category will be governed by one of The flange O.D. may now be established.
tu70 conditions:
A=C+2E

1. Gasket seating force, Wm7 Step 3: Check the minimum bolt spacing (not an ASME
2. Hydrostatic end force, H
requirement). Coinpare with the value of B, in Table 2-5a.
For high-pressure flanges, typically the hydrostatic end B, = C/n
force, H, will govern. For low-pressure flanges, the gasket
seating force will govern. Therefore the strategy for Note: Dimensions R,, R, E, and B, are from Table 2-5a.
approaching the design of these flanges will vary. The strategy
is as follows: Step 4: After all of the preliminary dimensions and details
are selected, proceed with the detailed analysis of the
e For low-pressure flanger flange by calculating the balance of forces, moments,
<I. Minimize the gasket width to reduce the force neces- and stresses in the appropriate design form.
sary to seat the gasket.
b LJse a larger number of smaller diameter bolts to mini-
mize the bolt circle diameter and thus reduce the Gasket Facing and Selection
moment drm which governs the flange thickness.
c Utilize liubless flanges (either lap joint or plate flanges) The gasket facing and type correspond to the service con-
to minimize the cost of forgings. ditions, fluid or gas handled, pressure, temperature, thermal
shock, cyclic operation, and the gasket selection. The greater
e For high-pressure jlanges the hazard, the more care that should be invested in the
High-pressure flanges require a large bolt area to counter- decisions regarding gasket selection and facing details.
act the large hydrostatic end force. Large bolts, in turn, Facings which confine the gasket, male and female,
increase the bolt circle with a corresponding increase in tongue and groove and ring joint offer greater security
the moment arm. Thicker flanges and large hubs are against blowouts. Male and fernale and tongue and groove
necessary to distribute the bolt loads. Seek a balance have the disadvantage that mating flanges are not alike.
between the quantity and size of bolts, bolt spacing, and These facings, which confine the gasket, are known as en-
bolt circle diameter. closed gaskets and are required for certain services, such as
TEMA Class “R.”
For tongue and groove flanges, the tongue is more likely to
be damaged than the groove; therefore, from a maintenance
Design Strategy standpoint, there is an advantage in placing the tongue on
the part which can be transported for servicing, i.e., blind
Step 1: Determine the number and size of bolts required. As flanges, manway heads, etc. If the assembly of these joints is
a rule of thumb, start with a number of bolts equal to the horizontal then there will be less difficulty if the groove is
nominal size of the bore in inches, rounded to the nearest placed in the lower side of the joint. The gasket width should
multiple of four. First, calculate Wlnl or Wm2, A,, is equal be made equal to the width of the tongue. Gaskets for these
to the larger of W,,,, or Wrn2 divided by S,. The quantity of joints are typically metal or metal jacketed.