Wall Thickness Calculation of Flat Head Under Internal Pressure  
   
  ASME Section VIII Div 1 - UG 34 - Unstayed Flat Heads and Covers  
  TEMA RCB 9.21 - Flat Channel Cover Deflection - Multipass Units  
     
     
     
    (Enter values in yellow cells for calculations and in blue cells as information)  
  DATA INPUT  
    Design Conditions     Material        
    Design Pressure, Pd =  bar g Flat Head Material Specification SA-516 Gr.70      
    Design Pressure, Pd =  N/mm2 Allowable Stress Flat Head @ Design Temp., S  =  N/mm2  
        Allowable Stress Flat Head @ Atmosf. Temp., S  =  N/mm2  
    Design Temperature, Td =  °C                                        
    Atmosferic Temperature, Td =  °C Bolting Material Specification SA-193 Gr.B7      
          Allowable Bolting Stress @ Design Temp., SB =  N/mm2  
    Modulus of Elasticity, E1 =  N/mm2 Allowable Bolting Stress @ Atmosf. Temp., SA =  N/mm2  
    Joint efficiency, E2 =   For Flange design bolt load = W        
Gasket Factor from Table 2-5.1 ASME VIII Div. 1, m =
O.D. surface gasket =  mm
    I.D. gasket =  mm Gasket or Joint-contact-surface unit seating load, y =  N/mm2  
N = (O.D. surface gasket – I.D. gasket)/2,  N =  mm
    (From Table 2-5.2 ASME VIII Div.1)   bo = N/2, bo =  mm Dimensions        
    When bo < 6 mm → b = bo         Nominal wall thickness, tn =
 mm    
    When bo > 6 mm → b = 2.5(bo)1/2         Minimum required thickness Including CA, tr =  mm    
                      If tr ≤ nt acceptable      
    (TEMA, Section 9, Table D-5)   Root areas of bolts, ab =  mm2           >                            
(Del ASME B16.5 or B16.47)   Number of Bolts,  =
                                                 
    Channel cover deflection at the center, Y =  mm 0.03" (0.8 mm) for nominal diameters thru 24" (610 mm)
0.125% of nominal diameter (nominal diameter/800) for larger sizes  		   
             
    Diameter of the bolt circle, C1 =  mm Del ASME B16.5 or B16.47    
    Factor depending upon the method of attachment, C2 =
in (d) 		  Del ASME VIII Div.1 UG-34(d)    
                                                 
    Corrosion Allowance, CA =  mm          
               
                                                 
  CALCULATIONS  
                                                 
    Gasket load reaction diameter, G =  mm   G = ODsurface gasket –  2 x b                      
    or Diameter, or short span, measured as indicated in Fig.UG-34, d                                            
                                                 
    Actual total cross-sectional root areas of bolts, AB =  mm2   AB = #bolts x aB                          
                                                 
    Radial distance from diameter G to bolt circle,hG =    mm   hG =   C1 - G                            
              2                            
                                                 
    Minimum required bolt load for operating cond., Wm1 =  N   Wm1 = 0.785G2P + (2b x 3.14GmP)                    
                                                 
    Minimum required bolt load for gasket seting, Wm2 =  N   Wm2 = 3,14 bGy                          
                                                 
    Total cross-sectional area of bolts for operating cond., Am1 =    mm2   Am1 =   Wm1                     (1)    
              SB                        
                                                 
    Total cross-sectional area of bolts for gasket seating, Am2 =    mm2   Am2 =   Wm2                     (2)      
              SA                          
                                                 
    [The greater between (1) and (2)], Am =  mm2                                        
                                                 
    Flange design bold load for operating cond., W1 =    N     W1  =  Wm1                          
    Flange design bold loadfor gasket seating, W2 =    N   W2 =   (Am+AB)SA                            
              2                            
    Flat Cover Design                                            
    Minimum required thickness [TEMA RCB 9.21]                                            
    Method for calculation of flat channel cover deflection                                            
    Required thickness without corrosion allowance, tr1 =  mm   t = ( G   (0.043G3P+0.5SBABhG))1/3       (3)    
            E1Y            
    Minimum required thickness [ASME UG-34 (c)]                                            
                                                 
    Required thickness without corrosion allowance, tr2 =  mm   t =   d ( C2P   + 1.9 W1hG ) 1/2       (4)    
    by operating condition       SE2   SE2d3          
                                                 
    Required thickness without corrosion allowance, tr2 =  mm   t =   d ( C2P   + 1.9 W2hG ) 1/2       (5)    
    by gasket seating       SE2   SE2d3          
                                                 
  RESULT  
                                                 
    Minimum required thickness, tr =  mm                                        
    [The greater between (3) (4) and (5)]                                            
                                                 
    Minimum required thickness Including CA, tr =  mm                                        
                                                 
                                                 
     
    Discussion and References  
    Information about Wall Thickness Calculation of Flat Head Under Internal Pressure    
    ASME Section VIII Div 1 - UG 34 - Unstayed Flat Heads and Covers    
    TEMA RCB 9.21 - Flat Channel Cover Deflection - Multipass Units    
    Standards, Paragraph & Appendix  
  - ASME Section VIII Div 1 - UG 34 - Unstayed Flat Heads and Covers    
  - TEMA RCB 9.21 - Flat Channel Cover Deflection - Multipass Units    
         
         
       
-
TEMA RCB 9.21 - Flat Channel Cover Deflection - Multipass Units
The effective thickness of a flat channel cover shall be the thickness at the bottom of the pass partition groove
(or the face if there is no groove) minus corrosion allowance in excess of groove depth. The thickness is to be
at least that required by the appropriate Code formula and thicker if required to meet proper deflection
criteria. The recommended limit for channel cover deflection is:
0.03" (0.8 mm) for nominal diameters thru 24" (610 mm) 
0.125% of nominal diameter (nominal diameter/800) for larger sizes
A method for calculation of channel cover deflection is:
Method for calculation of flat channel cover deflection Required thickness without corrosion allowance, t =
T = ( G (0.043G3P+0.5SBABhG))1/3
E1Y
Where:
Y = Channel cover deflection at the center, inches (mm)
G = Gasket load reaction diameter as defined by the Code, inches (mm)
E = Modulus of elasticity at design temperature, psi(kPa) 
T = Thickness under consideration, inches (mm)
P = Design pressure, psi (kPa)
SB = Allowable bolting stress at design temperature, psi (kPa)
AB = Actual total cross-sectional root area of bolts, square inches (mm2)
hg = Radial distance from diameter G to bolt circle, inches (mm)
If the calculated deflection is greater than the recommended limit, the deflection may be reduced by
acceptable methods such as: Increase channel cover thickness by the cube root of the ratio of calculated
deflection to the recommended limit. 
• Use of strong backs.
• Change type of construction.
Note: For single pass channels, or others in which there is no pass partition gasket seal against the channel
cover, no deflection criteria need be considered. 
-
ASME Section VIII Div 1 - UG 34 - Unstayed Flat Heads and Covers
When the circular head, cover, or circular blind flange is attached by bolts causing an edge moment [sketches
(j) and (k)] the thickness shall be calculated by:
t = d ( C2P
+ 1.9 W1hG ) 1/2
UG-34(c)(2) eq. 2
SE2
SE2d3
Where:
C =
a factor depending upon the method of attachment of head, shell dimensions, and other
increases the allowable stress for such constructions
to 1.5S.
items as listed in (d) below, dimensionless. The factors for welded covers also include a
factor of 0.667 that effectively increases the allowable stress for such constructions to 1.5S.
d =  diameter, or short span, measured as indicated in Figure UG-34
E =
joint efficiency, from Table UW-12, of any Category A weld as defined in UW-3(a)
hG =  gasket moment arm, equal to the radial distance from the centerline of the bolts to the line of
the gasket reaction, as shown in Table 2-5.3
P =
internal design pressure (see UG-21)
S =  maximum allowable stress value in tension from applicable table of stress values referenced
by UG-24
W = total bolt load given for circular heads for eqs. 2-5(d)(3) and 2-5(e)(4)
Sketches (j) & (k)