Nozzle Neck (Thickness and Reinforcement)
   
    ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,
    UG-37 Reinforcement Required for Openings in Shells and Formed Heads
                                           
                      (Enter values in yellow cells for calculations and in blue cells as information)
    ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,                       Page
UG-37 Reinforcement Required for Openings in Shells and Formed Heads
Introduction
      The following calculation for minimum internally corroded radial nozzle neck thickness and reinforcement is based on ASME VIII, Div 1, sections UG-37 and UG-45. Data Input shall be in Page 2.   
Pictures / Sketch
Nozzle size (NPS):
mm
Outside diameter of reinforcing element, Dp:
mm
Summary
Nozzle welded on 
Type of Head (If nozzle is welded on Head)
Maximum Limit of Reinforcement (Parallel to vessel wall)
mm
Maximum Limit of Reinforcement (Normal to vessel wall outside)
mm
Maximum Limit of Reinforcement (Normal to vessel wall inside)
mm
      Shell or Head Nominal Thickness, ts or th                       mm    
      The minimum nozzle reinforcing pad thickness, te:                     mm    
The minimum required nozzle neck thickness, as per UG-45: (For access openings and openings used only for inspection)
mm
The minimum required nozzle neck thickness, as per UG-45: (For other nozzles) 
mm
      The Nozzle Nominal thickness, tsn:                       mm    
      Nozzle for:                            
                                           
Check If tr ≤ t
Check If trn ≤ tn
Without reinforcing element
Is the available reinforcing area of At  greater than the required area of A:
With reinforcing element added
      Is the available reinforcing area of At  greater than the required area of A:              
ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,
Page
UG-37 Reinforcement Required for Openings in Shells and Formed Heads
Formula Data
Notes
CA = mm
Corrosion Allowance
U1 =
default value = 0
Shell Undertolerance Allowance (As per UG-16(c) & UG-16(d))
U2 = mm
Shell Undertolerance Thickness
D = mm
Shell Nominal inside Dia
Do = mm
Shell Outside Dia.
      DH = mm           Head Nominal inside Dia              
      DEH = mm           Ellipsoidal Head inside Dia. with corrosion removed        
      LTH = mm           Torispherical Head inner diameter of the skirt adjusted for corrosion      
d = mm (opening in Parent Material)
Finished diameter of circular opening
E =
Assuming E = 1.0
      E1 =                                
F =
1.00 shall be used for all conf. except that Fig. UG-37 may be used for integrally reinforced openings
      fr =             Sn/Sv                   
      fr1 =             Sn/Sv for nozzle wall inserted through the vessel wall         
      fr2 =             Sn/Sv                   
      fr3 =             (lesser of Sn or Sp) / Sv               
      fr4 =             Sp/Sv                   
h = mm default value = 0
Nozzle inside projection
      hH = mm If DH > 0 For Nozzle welded on Ellipsoidal Head 2:1. hH= DH/4   mm  
      K =             Factor in the equations for ellipsoidal heads, Table 1-4.1        
      K1 =             Spherical radius factor (see Table UG-37)          
      LEH = mm           Inside spherical or crown radius for ellipsoidal head, L = k1D      
M =
Assuming M = 1.0
L = 0
Applicable for LWNF only, Figure UG-40 sketch (e) 
P = bar
      Pe = bar default value = 0                          
      R = mm           Shell Inner radius                
      Rv = mm           Shell Inner radius with corrosion & undertolerance removed        
      Rn = mm           Nozzle Inner radius with corrosion & undertolerance removed      
      LHH = mm           Hemispherical Head inside spherical or crown radius adjusted for corrosion    
                                           
      Sn = MPa Material Specification as per ASME II-D Tables              
      Sp = MPa              
      Sv = MPa              
      T = °C                              
      ts = mm Shell Nominal Thk.     Nozzle welded on:
         
      t = mm Shell Nom. Thk. adjusted by Tolerances Head Type:  
If Nozzle is welded on Head    
      th = mm Head Nominal Thk. After forming   Nozzle for:  
         
      t = mm Head Nominal Thk. After forming adjusted by tolerances                
      t = mm Selected t                            
      ti = mm Nozzle Inside Proj. Thk.                        
      tsn = mm Nozzle Nominal Thk.                        
      tn = mm Nozzle Nominal Thk. adjusted by tolerances                    
      trs = mm                              
      tre = mm                              
      trh = mm                              
      trt = mm                              
      tr = mm Selected tr                           
trn = mm
Check If tr ≤ t
                                           
      A41 leg = mm                              
      A43 leg = mm                              
                                           
                                         
      Nozzle Neck dimensions (ASME B36.10M & ASME B36.19M):                        
Pipe size (DN):
Nozzle Neck Nominal thickness, tsn:
mm
Schedule:
Nozzle Neck thickness undertolerance, U3: mm
Approximate ID:
mm
(undertolerance = 12.5%)
Approximate OD:
mm
Nominal thickness less 'c' & 'U3', tn:
mm
Check If trn ≤ tn
Nozzle Neck thickness trn for external pressure as per Section VIII Div 1 - UG 28(c) 
      trn for external pressure: mm default value = 0                      
Nozzle Pad details
      Dp =  mm                        
      te = mm                            
      A42 leg = mm default value = 0                          
ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,
Page
UG-37 Reinforcement Required for Openings in Shells and Formed Heads
                                           
      Calculations                     ASME VIII Reference        
                                           
      (a) Calculate limits of reinforcement                              
                                           
      Parallel to vessel wall, use larger of :                  UG-40            
                                           
      d =    mm               UG-40(b)(1)          
                                           
      Rn + tn + t =  mm               UG-40(b)(2)          
                                           
      Use   mm                            
                                           
      Normal to vessel wall outside use smaller of:                            
                                           
      2.5t =    mm               UG-40(c)(1)          
                                           
      2.5tn + te =  mm               UG-40(c)(2)          
                                           
      Use   mm                            
                                           
      Normal to vessel wall inside use smaller of:                            
                                           
      h =    mm               UG-40(c)(1)          
                                           
      2.5t =    mm               UG-40(c)(2)          
                                           
      2.5ti =    mm               UG-40(c)(2)          
                                           
      Use   mm                            
                                           
      (b) Shell, Head and nozzle required thickness:                            
                                           
      with                                    
      CA = mm                              
      P = bar                              
      Pe = bar                              
      DEH =  mm                              
      LTH =  mm                              
      RV =  mm                              
      LHH =  mm                              
      Rn =  mm                              
      Sv =  MPa                              
      Sn =  MPa                              
      E =                                 
      K =                                
      M =                                
                                           
      Required shell thickness:   (Internal Pressure)             Required shell thickness:   (External Pressure)
                                           
      trs = PRv + CA                   trs = PRv + CA        
      SE - 0.6P                   SE - 0.6P        
      trs =  mm                 trs =  mm      
                                           
                                           
      Required ellipsoidal head thickness: (Internal Pressure)             Required ellipsoidal head thickness: (External Pressure)
                                           
      tre = PDEHK + CA                   tre = PDEHK + CA        
      2SE - 0.2P                   2SE - 0.2P        
      tre =  mm                 tre =  mm      
                                           
                                           
      Required hemispherical head thickness: (Internal Pressure)             Required hemispherical head thickness: (External Pressure)
                                           
      trh = PLHH + CA                   trh = PLHH + CA        
      2SE - 0.2P                   2SE - 0.2P        
      trh =  mm                 trh =  mm      
                                           
                                           
      Required torispherical head thickness: (Internal Pressure)             Required torispherical head thickness: (External Pressure)
                                           
      trt = PLTHM + CA                   trt = PLTHM + CA        
      2SE - 0.2P                   2SE - 0.2P        
      trt =  mm                 trt =  mm      
                                           
                                           
Required nozzle neck thickness: (Internal Pressure)
Required nozzle neck thickness: (External Pressure)
As per Section VIII Div 1 - UG 28(c) - Thickness of Shells
trn = PRn + CA
and Tubes under External Pressure
      SE - 0.6P                              
trn =  mm
trn =  mm
                                           
    ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,                       Page
    UG-37 Reinforcement Required for Openings in Shells and Formed Heads                      
                                           
                                           
      (c) Determine required replacement area:                 Notes            
                                           
      A =  d tr F + 2tn tr F(1 - fr1)                   Ref: UG-37 (C)- Design for Internal Pressure.    
                                           
      A =    mm2                            
                                           
      (d) Calculate area contributions:                                
                                           
      Area available in the vessel wall, use larger value:                            
                                           
      A1.1 =  d(E1t - Ftr) - 2tn(E1t - Ftr)(1 - fr1)                            
                                           
      A1.1 =    mm2                            
                                         
      A1.2 =  2(t + tn)(E1t - Ftr) - 2tn(E1t - Ftr)(1 - Fr1)                            
                                         
      A1.2 =    mm2                            
                                 
      Use A1 =  mm2                            
                                     
                                           
      Area available in nozzle projecting outward, use smaller value:                        
                                           
      With reinforcing element added                   Without reinforcing element        
      A2.1 =  5(tn - trn)(fr2)(t)                   A2.1 =  5(tn - trn)(fr2)(t)        
                                           
      A2.1 =    mm2               A2.1 =    mm2    
                                           
      With reinforcing element added                   Without reinforcing element        
      A2.2 =  2(tn - trn)(2.5tn + te)(fr2)                 A2.2 =  5(tn - trn)(fr2)(tn)        
                                           
      A2.2 =    mm2               A2.2 =    mm2    
                                           
      Use A2 =  mm2               Use A2 =  mm2    
                                           
                                           
      Area available due to inward projection of nozzle, use smaller value:                        
                                           
      A3.1 =  5t ti fr2                                  
                                           
      A3.1 =    mm2                            
                                           
      A3.2 =  5ti ti fr2                                  
                                           
      A3.2 =    mm2                            
                                           
      A3.3 =  2h ti fr2                                  
                                           
      A3.3 =    mm2                            
                                           
      Use A3 =  mm2                            
                                           
                                           
                                           
                                           
                                           
    ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,                       Page
    UG-37 Reinforcement Required for Openings in Shells and Formed Heads                      
                                           
                                           
      Area available in welds:                                
                                           
      with reinforcing element added                   Without reinforcing element        
      A41 =  (leg)2 fr3   Outward Nozzle Weld             A41 =  (leg)2 fr2   Outward Nozzle Weld  
                                           
      A41 =    mm2               A41 =    mm2    
                                           
      A42 =  (leg)2 fr4   Outer Element/Pad Weld                          
                                           
      A42 =    mm2                          
                                           
      A43 =  (leg)2 fr2   Inward Nozzle Weld             A43 =  (leg)2 fr2   Inward Nozzle Weld  
                                           
      A43 =    mm2               A43 =    mm2    
                                           
      Area in reinforcing pad:                                
                                           
      A5 =  (Dp - d - 2tn)te fr4                                 
                                           
      A5 =    mm2                            
                                           
      (e) Total area available:                                
                                           
      With reinforcing element added                   Without reinforcing element        
      At =  A1 + A2 + A3 + A41 + A42 + A43 + A5               At =  A1 + A2 + A3 + A41 + A43      
                                           
      At =  mm2                 At =  mm2      
                                           
      At =  mm2 mm2     (A)       At =  mm2 mm2 (A')
                                           
      Is the available reinforcing area of At  is                 Is the available reinforcing area of At  is      
      greater than the required area of A:               greater than the required area of A:    
           
                             
                                           
                                           
                                           
                                           
                                           
    ASME BPVC VIII-1-2015,  UG-45 Nozzle Neck Thickness and,                       Page
    UG-37 Reinforcement Required for Openings in Shells and Formed Heads                      
                                           
                                           
      (g) Check nozzle neck thickness as specified in UG-45.                          
                                           
      For access openings and openings used only for inspection:                          
                                           
      tUG-45 = ta                                    
                                           
      For all nozzles other than access openings and openings used only for inspection, determine tb:                  
                                           
      tb = min[tb3, max(tb1, tb2)]                                
                                           
      tUG-45 = max(ta, tb)                                  
                                           
    where                                      
    ta =  minimum neck thickness required for internal and external pressure using UG-27 and UG-28 (plus corrosion allowance),  
      as applicable.                                  
    tb1 =  for vessels under internal pressure, the thickness (plus corrosion allowance) required for pressure (assuming              
      E = 1.0) for the shell or head at the location where the nozzle neck or other connection attaches to the vessel.              
    tb2 =  for vessels under external pressure, the thickness (plus corrosion allowance) obtained by using the external                
      design pressure as an equivalent internal design pressure (assuming E = 1.0) in the formula for the shell or                
      head at the location where the nozzle neck or other connection attaches to the vessel.                    
    tb3 =  the thickness given in Table UG-45 plus the thickness added for corrosion allowance.                    
    tUG-45 = minimum wall thickness of nozzle necks                              
                                           
    DN =      mm                
    trn =        mm                          
    ta =        mm             trn with corrosion and undertolerance removed    
    tb1 =        mm             tr with corrosion and undertolerance removed    
    tb2 =        mm                          
    tb3 =        mm                    
    tb =        mm                          
                                           
    For access openings and openings used only for inspection:                          
    Minimum Nozzle Neck Thickness required, tUG-45 = ta         mm                
                                           
    For other nozzles:                                   
    Minimum Nozzle Neck Thickness required, tUG-45 = max(ta, tb)       mm                
                                           
    Nozzle Neck Nominal thickness, tsn =             mm                
                                           
                                     
    Discussion and References
   
    Information about Reinforcement Required for Openings in Shells and Formed Heads
    ASME Section VIII Div 1 paragraph UG-37
    Estandares, Paragrafos y Apendices
  - ASME BPVC.VIII.1  Rules for Construction of Pressure Vessels
  - ASME BPVC SEC II-D Material Properties
  - ASME Section VIII Div 1 paragraph UG-37  Reinforcement Required for Openings in Shells and Formed Heads
     
Introduction
      The rules in this paragraph UG-37 apply for the calculation of the reinforcement of the opening where the nozzle neck will be welded.  
Figure UG-37.1, Nomenclature and Formulas for Reinforced Openings
Nomenclature
    A =  total cross‐sectional area of reinforcement required in the plane under consideration (see Figure UG-37.1)
      (includes consideration of nozzle area through shell if Sn/Sv < 1.0)
    A1 =  area in excess thickness in the vessel wall available for reinforcement (see Figure UG-37.1) 
(includes consideration of nozzle area through shell if Sn/Sv < 1.0)
    A2 =  area in excess thickness in the nozzle wall available for reinforcement (see Figure UG-37.1)
    A3 =  area available for reinforcement when the nozzle extends inside the vessel wall (see Figure UG-37.1)
    A5 =  cross‐sectional area of material added as reinforcement (see Figure UG-37.1)  
    A41, A42, A43 =  cross‐sectional area of various welds available for reinforcement (see Figure UG-37.1)
c =  corrosion allowance
U1 = shell undertolerance allowance (as per UG-16(c) & UG-16(d))
U2 = shell undertolerance thickness
U3 = nozzle undertolerance thickness
D =  inside shell diameter
    Dp =  outside diameter of reinforcing element (actual size of reinforcing element may exceed the limits of 
reinforcement established by UG-40; however, credit cannot be taken for any material outside these limits)
d = finished diameter of circular opening or finished dimension (chord length at midsurface of thickness 
excluding excess thickness available for reinforcement) of nonradial opening in the plane under 
consideration, in. [see Figures UG-37.1 and UG-40]
    E =  1 (see definitions for tr and trn)
    E1 =  1 when an opening is in the solid plate or in a Category B butt joint; or
    E1 =  0.85 when an opening is located in an ERW or autogenously welded pipe or tube. If the ERW or autogenously 
welded joint is clearly identifiable and it can be shown that the opening does not pass through this weld joint, 
then E1 may be determined using the other rules of this paragraph; or
    E1 =  joint efficiency obtained from Table UW-12 when any part of the opening passes through any other welded 
      joint
F =  correction factor that compensates for the variation in internal pressure stresses on different planes with
respect to the axis of a vessel. A value of 1.00 shall be used for all configurations except that Figure UG-37 
may be used for integrally reinforced openings in cylindrical shells and cones. [See UW-16(c)(1).]
    fr =  strength reduction factor, not greater than 1.0 [see UG-41(a)]
    fr1 =  Sn/Sv for nozzle wall inserted through the vessel wall
    fr1 =  1.0 for nozzle wall abutting the vessel wall and for nozzles shown in Figure UG-40, sketch (j), (k), (n) and (o).
    fr2 =  Sn/Sv 
    fr3 =  (lesser of Sn or Sp)/Sv
    fr4 =  Sp/Sv 
h =  distance nozzle projects beyond the inner surface of the vessel wall. (Extension of the nozzle beyond the 
inside surface of the vessel wall is not limited; however, for reinforcement calculations, credit shall not be
taken for material outside the limits of reinforcement established by UG-40.)
    K1 =  spherical radius factor (see definition of tr and Table UG-37)
L = length of projection defining the thickened portion of integral reinforcement of a nozzle neck beyond the 
outside surface of the vessel wall [see Figure UG-40 sketch (e)]
P =  internal design pressure (see UG-21), bar
    Pe =  external design pressure, bar
    R = inside radius of the shell course under consideration
    RV = inside radius of the shell course under consideration with corrosion & undertolerance removed              
    Rn =  inside radius of the nozzle under consideration with corrosion & undertolerance removed                
    S =  allowable stress value in tension (see UG-23), MPa                        
    Sn =  allowable stress in nozzle, MPa (see S above)                        
    Sp =  allowable stress in reinforcing element (plate), MPa                        
    Sv =  allowable stress in vessel, MPa (see S above)                        
    ts = specified vessel nominal wall thickness including corrosion allowance and undertolerance.              
    tsn = specified nozzle nominal wall thickness                          
    te = thickness or height of reinforcing element (see Figure UG-40)
    ti = nominal thickness of internal projection of nozzle wall
    t = specified vessel wall thickness, in the corroded condition, see UG-16(e), (not including forming allowances).            
      For pipe it is the nominal thickness less manufacturing undertolerance allowed in the pipe specification.            
    tn =  nozzle wall thickness. Except for pipe, this is the wall thickness not including forming allowances. For pipe, 
use the nominal thickness [see UG-16(d)].
    tr =  required thickness of a seamless shell based on the circumferential stress, or of a formed head, computed by
the rules of this Division for the designated pressure, using E = 1, except that:
(a) when the opening and its reinforcement are entirely within the spherical portion of a torispherical head,
      tr is the thickness required by 1-4(d), using M = 1;
      (b) when the opening is in a cone, tr is the thickness required for a seamless cone of diameter measured D
      where the nozzle axis pierces the inside wall of the cone;
      (c) when the opening and its reinforcement are in an ellipsoidal head and are located entirely within a
      circle the center of which coincides with the center of the head and the diameter of which is equal to 80%
      of the shell diameter, tr is the thickness required for a seamless sphere of radius K1D, where D is the shell 
diameter and K1 is given by Table UG-37.
    trn =  required thickness of a seamless nozzle wall
    T =  design temperature                              
W =  total load to be carried by attachment welds (see UG-41)
    DH/2hH = ratio of the major to the minor axis of ellipsoidal heads, which equals the inside diameter of the skirt of the
head divided by twice the inside height of the head.
Formulas:
        Without Reinforcing Element        
                                       
A =  d tr F + 2tn tr F(1 - fr1)
Area required
A1 =  d(E1t - Ftr) - 2tn(E1t - Ftr)(1 - fr1)
Area available in shell, use larger value
2(t + tn)(E1t Ftr) - 2tn(E1t - Ftr)(1 - Fr1)
A2 =  5(tn - trn)fr2 t
Area available in nozzle projecting outward;
          5(tn - trn)fr2 tn     use smaller value      
        A3 =  5t ti fr2     Area available in inward nozzle; use smaller value    
          5ti ti fr2                    
          2h ti fr2                    
        A41 =  outward nozzle weld = (leg)2 fr2     Area available in outward weld      
        A43 =  inward nozzle weld = (leg)2 fr2     Area available in inward weld      
          If A1 + A2 + A3 + A41 + A43 ≥ A     Opening is adequately reinforced      
          If A1 + A2 + A3 + A41 + A43 ˂ A     Opening is not adequately reinforced so       
                        reinforcing elements must be added and/      
                        or thickness must be increased      
                                       
                                       
        With Reinforcing Element Added        
                                       
        A =  same as A, above     Area required      
        A1 =  same as A1, above     Area available      
        A2 =  5(tn - trn)fr2 t     Area available in nozzle projecting       
2(tn - trn)(2.5tn + te)fr2
outward; use smaller area
A3 =  same as A3, above
Area available in inward nozzle
        A41 =  outward nozzle weld = (leg)2 fr3     Area available in outward nozzle      
        A42 =  outer element weld = (leg)2 fr4     Area available in outer weld      
        A43 =  inward nozzle weld = (leg)2 fr4       Area available in inward weld      
        A5 =  (Dp - d - 2tn)te fr4     Area available in element      
          If A1 + A2 + A3 + A41 + A42 + A43 ≥ A     Opening is adequately reinforced      
                                       
                                       
    Discussion and References  
    Information about Nozzle Neck Thickness    
    ASME Section VIII Div 1 paragraph UG-45    
    Standards, Paragraph & Appendix  
  - ASME BPVC.VIII.1  Rules for Construction of Pressure Vessels    
  - ASME BPVC SEC II-D Material Properties    
  - ASME Section VIII Div 1 paragraph UG-45  Nozzle Neck Thickness    
       
    Wall thickness of the nozzles necks (tube)    
      aaaaaaaaa                                  
                                         
                                         
                                         
                                         
                                         
                                         
    For access openings and openings used only for inspection    
    Minimum required thickness [ASME UG-45]    
    tUG45 = ta                              
    ta = minimum neck thickness required for internal and external pressure using UG-27 and UG-28 (plus corrosion and threading allowance), as applicable. The effects of external forces and moments from supplemental loads (see UG-22) shall be considered. Shear stresses caused by UG-22 loadings shall not exceed 70% of the allowable tensile stress for the nozzle material.     
    Longitudinal Joint                      
      ta1 = P R     (1)                
      S E - 0.6 P                    
    Circunferential Joint                      
      ta2 = P R   (2)                
      2 S E + 0.4 P                  
                                         
    ta = max(ta1,ta2)                            
    Internal design pressure [ASME UG-27 (c)]    
    Longitudinal Joint                        
      P1 = S E ta1       (1)                
      R + 0.6 ta1                      
    Circunferential Joint                        
      P2 = 2 S E ta2       (2)                
      R - 0.4 ta2                      
                                         
    Where:    
    P: Design Pressure, in bar    
    R: Inner Radius, in mm    
    S: Allowable Stress, in Mpa    
    E: Welding Efficiency    
    ta: Wall Thickness, in mm [The greater between (1) and (2)]    
    ta = specified vessel wall thickness in the corroded condition, (not including forming allowances). For pipe it is the nominal thickness less manufacturing undertolerance allowed in the pipe specification.     
    (1) Limitations: UG-27 and Appendix 1: t < (R/2), P < 0.385 SE, If one isn’t met, review by Appendix 1-2. Being R the inner radius.    
    (2) Limitations: UG-27 and Appendix 1: t < (R/2), P < 1.25 SE, If one isn’t met, review by Appendix 1-2. Being R the inner radius.    
                                         
    Notes:    
    S: is tabulated in the ASME Code Section II, Part D (Table 1A).    
    E: is tabulated in the ASME Code Section VIII Div 1 (Table UW-12).    
                                         
    For other nozzles:                       
    tb = min[tb3, max(tb1,tb2)]                      
    tUG45 = max(ta,tb)                            
    ta = minimum neck thickness required for internal and external pressure using UG-27 and UG-28 (plus corrosion and threading allowance), as applicable. The effects of external forces and moments from supplemental loads (see UG-22) shall be considered. Shear stresses caused by UG-22 loadings shall not exceed 70% of the allowable tensile stress for the nozzle material.    
    tb1 = for vessels under internal pressure, the thickness (plus corrosion allowance) required for pressure (assuming E = 1.0) for the shell or head at the location where the nozzle neck or other connection attaches to the vessel but in no case less than the minimum thickness specified for the material in UG-16(b).    
    tb2 = for vessels under external pressure, the thickness (plus corrosion allowance) obtained by using the external design pressure as an equivalent internal design pressure (assuming E = 1.0) in the formula for the shell or head at the location where the nozzle neck or other connection attaches to the vessel but in no case less than the minimum thickness specified for the material in UG-16(b).    
    tb3 = the thickness given in Table UG-45 plus the thickness added for corrosion allowance.