Nozzle Reinforcement Calculator — ASME Section VIII Div 1 UG-37 Area Replacement Method

By WeldFabWorld April 15, 2026 40 min read
Nozzle Reinforcement Calculator — ASME VIII Div 1 UG-37 | WeldFabWorld

Nozzle Reinforcement Calculator — ASME Section VIII Div 1 UG-37 Area Replacement Method

Calculator UG-37 Method Area Required A Area Available Reinforcement Zone Worked Example Pad Sizing Nozzle Types Practical Notes FAQ
Table of Contents
  1. Introduction — Why Nozzle Reinforcement Matters
  2. Nozzle Reinforcement Calculator
  3. The ASME UG-37 Area Replacement Method
  4. Area Required — Calculating A
  5. Area Available — A1, A2, A3, A41, A43
  6. The Reinforcement Zone — Limits and Boundaries
  7. Worked Example — Step by Step
  8. Reinforcing Pad Sizing
  9. F-Factor and Nozzle Orientation
  10. Nozzle Types and Weld Details
  11. Strength Ratios f r1, f r2, f r3, f r4
  12. Practical Engineering Notes
  13. Frequently Asked Questions

The nozzle reinforcement calculator on this page performs the area replacement calculation per ASME Section VIII Division 1, paragraph UG-37 — the standard method for determining whether a nozzle opening in a pressure vessel shell or head has adequate reinforcement. Every time a hole is cut in a pressure vessel shell to accommodate a nozzle, pipe connection, or manway, load-carrying metal is removed. ASME requires that an equal area of metal be provided within a defined reinforcement zone around the opening. This calculator computes the required area A, each component of available area (A1 through A43), the deficit that must be supplied by a reinforcing pad (A5), and the minimum pad outer diameter.

Nozzle reinforcement is one of the most frequently performed calculations in pressure vessel engineering, appearing on every vessel data sheet and design calculation package. Getting it right determines whether a vessel passes the Authorised Inspector’s review — and whether the connecting piping loads can be accepted at the nozzle. This article explains every term in the UG-37 calculation from first principles, provides a complete worked example with all intermediate steps, and covers pad sizing, weld area contributions, strength ratio adjustments for dissimilar materials, and the limits of the reinforcement zone.

Nozzle Reinforcement Calculator

ASME Section VIII Division 1 — UG-37 Area Replacement Method

Units:
Shell / Head Data
Nominal OD of the cylindrical shell or head
Actual ordered thickness (before corrosion)
From UG-27 shell thickness formula (no CA)
Per UG-37(a) — 1.0 for most radial nozzles
Nozzle Data
NPS pipe OD or custom nozzle neck OD
Ordered nozzle neck wall thickness
From B31.3 or UG-27 for nozzle at design pressure
Length nozzle projects inside vessel (0 if flush)
Weld Sizes
Outer fillet or groove weld leg at shell OD surface
Inner fillet weld leg at shell ID surface (if any)
Reinforcing Pad (if fitted)
Enter 0 if no pad is fitted (calculator will size required pad)
Enter 0 to calculate minimum required pad OD
Material Strength Ratios
Ratio: nozzle allowable / shell allowable stress
Ratio: pad allowable / shell allowable stress
Area Replacement Results — ASME UG-37
Area availability breakdown (vs required A):
A1 Shell excess A2 Nozzle excess A3 Inward projection Aw Welds A5 Pad required
Step-by-Step Formula Workings

The ASME UG-37 Area Replacement Method

When a nozzle is installed in a pressure vessel shell, a circular or elliptical hole is cut in the shell plate to pass the nozzle neck through. This hole removes a cross-sectional area of load-carrying metal from the shell wall. If not compensated, the opening would create a stress concentration and reduce the burst pressure of the vessel below the design value. ASME Section VIII Division 1, paragraph UG-37, requires that the removed area be replaced with an equivalent area of metal placed within a defined reinforcement zone centred on the opening.

The philosophy is simple: the area of shell plate removed equals the area that must be restored. The metal used for replacement may come from several sources — excess shell wall thickness above the minimum required, excess nozzle neck wall thickness, the inward projection of the nozzle into the vessel bore, weld metal at the nozzle-to-shell junction, and a reinforcing pad if the other sources are insufficient. The calculation is a straightforward area balance: total available area must equal or exceed the required area.

Code Scope: UG-37 applies to single circular openings in cylindrical shells and formed heads where the opening diameter does not exceed the limits of UG-36. For openings exceeding those limits, or for multiple closely spaced openings where reinforcement zones overlap, additional rules in UG-42 and Appendix 1-7 apply. This calculator handles single isolated circular openings — the most common configuration in practice.

The Area Balance Equation

ASME UG-37 Area Balance Requirement: A_available ≥ A_required
A1 + A2 + A3 + A41 + A43 + A5 ≥ A

Where:
A = total cross-sectional area required to replace the opening
A1 = area available in the shell (excess shell wall within reinforcement zone)
A2 = area available in the nozzle wall (excess nozzle wall within reinforcement zone)
A3 = area available from inward nozzle projection (if any)
A41= area of outer nozzle-to-shell weld (triangular cross-section)
A43= area of inner nozzle-to-shell weld (if applicable)
A5 = area required from reinforcing pad (= A − A1 − A2 − A3 − A41 − A43, if positive)

Area Required — Calculating A

The required reinforcement area A is the product of the finished diameter of the opening in the shell and the minimum required thickness of the shell at that location, modified by the F-factor for nozzle orientation:

UG-37(c) — Required Reinforcement Area: A = d × t_r × F
Where: d = finished diameter of the opening = nozzle ID = nozzle OD − 2×t_n
t_r = minimum required shell thickness from UG-27 (without corrosion allowance)
F = correction factor for nozzle orientation (1.0 for radial nozzles; see UG-37 Fig.)

For a nozzle in a cylindrical shell, d equals: d = nozzle OD − 2 × t_n The result A is in mm² (metric) or in² (imperial)
Key Point on t_r: The t_r used in the A formula is the pressure design thickness of the shell at the nozzle location — calculated by the UG-27 formula without any corrosion allowance or mill tolerance added. It is not the ordered nominal shell thickness. Using t_nominal instead of t_r is a common error that gives an unconservatively large A value, which appears to require more reinforcement than is actually needed. Always derive t_r from the UG-27 formula at the design conditions.

Area Available — A1, A2, A3, A41, A43

A1 — Excess Area in the Shell

A1 is the area within the reinforcement zone contributed by the shell wall thickness in excess of the minimum required. It rewards the engineer for specifying a thicker-than-minimum shell — a shell ordered thicker for corrosion allowance or mill tolerance provides useful reinforcement credit at nozzle locations.

A1 — Shell Excess Area: A1 = (2×d_eff − d) × (t − F×t_r) Where d_eff = effective reinforcement width in the shell plane = larger of d or (R_n + t_n + t), where R_n = nozzle inside radius

Simplified for standard radial nozzles where d_eff = d: A1 = d × (t − F × t_r) This is the most common form used in practice for radial nozzles on cylindrical shells

A2 — Excess Area in the Nozzle Wall

A2 credits the nozzle neck wall thickness above the minimum required for pressure. A thicker nozzle neck — whether from pipe schedule selection or deliberate over-sizing — provides useful reinforcement within the limits of the reinforcement zone height.

A2 — Nozzle Wall Excess Area (outward projection): A2 = 2 × (t_n − t_rn) × min(2.5t, 2.5t_n) × f_r1
Where: t_n = nozzle nominal wall thickness, t_rn = nozzle minimum required thickness,
f_r1 = strength ratio = (nozzle allowable stress) / (shell allowable stress)
The factor 2 counts both sides of the nozzle, but only up to the height limit min(2.5t, 2.5t_n)

A3 — Inward Nozzle Projection

When the nozzle neck projects inward into the vessel bore (an intruding nozzle), the inward portion also provides reinforcement area. A3 is typically zero for flush or projecting-outward-only nozzles, which is the most common configuration in process vessels.

A3 — Inward Projection Area: A3 = 2 × min(h, 2.5t_n) × (t_n − t_rn) × f_r1
Where h = actual inward projection length of nozzle inside the vessel
h is limited to min(h, 2.5t_n) for reinforcement credit purposes
A3 = 0 when nozzle does not project into the vessel interior (most common case)

A41 and A43 — Weld Areas

The triangular cross-sectional area of the fillet welds at the nozzle junction contributes to the total available area. A41 is the outer fillet weld area (nozzle-to-shell outside surface weld), and A43 is the inner fillet weld area at the shell inside surface, if any. For full-penetration groove welds, the weld area credit is taken differently and the effective area depends on the groove geometry.

A41 — Outer Nozzle-to-Shell Weld Area: A41 = (w1)² × f_r1 Where w1 = leg size of outer fillet weld (triangular area = 0.5×w², but ASME uses w² for conservative credit)

A43 — Inner Fillet Weld Area (if nozzle projects inward): A43 = (w3)² × f_r1 Where w3 = inner fillet weld leg size; A43 = 0 if no inner weld
t_r zone A1 A1 A2 A3 A41 A41 d (bore) t t_n d (zone each side) 2.5t or 2.5t_n REINFORCEMENT ZONE AREA COMPONENTS A1: Shell excess A2: Nozzle excess A3: Inward proj. A41/A43: Welds A5: Reinforcing pad required area Zone width: ±d from CL | Height: min(2.5t, 2.5t_n)
Figure 1 — Cross-section of a radial nozzle in a cylindrical shell showing the reinforcement zone boundary (dashed orange rectangle) and the five area components: A1 (blue, shell excess), A2 (green, nozzle wall excess), A3 (purple, inward nozzle projection), A41 (teal, outer fillet weld), and A5 (the reinforcing pad area if required). The zone extends d on each side of the nozzle centreline and up to min(2.5t, 2.5t_n) above the shell surface.

The Reinforcement Zone — Limits and Boundaries

Only metal within the reinforcement zone counts toward the available area. The zone is a rectangle in cross-section centred on the nozzle axis, with the following boundaries:

Direction Limit Formula Governs When
Radial (width in shell plane), each side Larger of d or (R_n + t_n + t) d_eff = max(d, R_n + t_n + t) d_eff > d when nozzle wall is thick relative to shell
Height above shell OD surface Smaller of 2.5t or 2.5t_n h_limit = min(2.5t, 2.5t_n) 2.5t_n governs when nozzle is thinner than shell
Depth below shell ID surface (inward) Smaller of 2.5t_n or the actual inward projection h h3_limit = min(h, 2.5t_n) Only applies when nozzle protrudes into vessel
Pad outer diameter Must remain within radial zone (d_eff each side) D_p/2 ≤ nozzle CL + d_eff Pad cannot extend beyond reinforcement zone
Practical Tip: For most standard radial nozzles on cylindrical shells, the reinforcement zone width on each side equals d (the nozzle bore diameter), giving a total zone width of 2d. This is the simplified form used in the majority of hand calculations. The extended width d_eff = max(d, R_n + t_n + t) only becomes relevant for nozzles with unusually thick walls relative to the shell, which is uncommon in standard process vessel designs.

Worked Example — Step by Step

Design Data: Carbon steel separator vessel, SA-516 Grade 70. Shell OD = 1,600 mm, shell nominal wall t = 20 mm. Shell minimum required thickness t_r = 14.5 mm (from UG-27 at design conditions). Nozzle: NPS 8 (OD = 219.1 mm), Schedule 40 (t_n = 8.18 mm), nozzle minimum required thickness t_rn = 5.2 mm (from B31.3). No inward projection (h = 0). Outer fillet weld leg w1 = 12 mm. No inner fillet weld. No reinforcing pad initially. F = 1.0 (radial nozzle). All same material, f_r1 = f_r2 = 1.0.
Step 1 — Nozzle bore diameter d: d = nozzle OD − 2×t_n = 219.1 − 2×8.18 = 219.1 − 16.36 = 202.74 mm

Step 2 — Required Reinforcement Area A: A = d × t_r × F = 202.74 × 14.5 × 1.0 A = 2,939.7 mm²

Step 3 — Reinforcement Zone Limits: Zone width each side: d_eff = max(d, R_n + t_n + t) R_n = d/2 = 101.37 mm; R_n + t_n + t = 101.37 + 8.18 + 20 = 129.55 mm d_eff = max(202.74, 129.55) = 202.74 mm → zone width = d Zone height above shell: min(2.5×20, 2.5×8.18) = min(50, 20.45) = 20.45 mm

Step 4 — Area A1 (Shell excess): A1 = d × (t − F×t_r) = 202.74 × (20 − 1.0×14.5) A1 = 202.74 × 5.5 A1 = 1,115.1 mm²

Step 5 — Area A2 (Nozzle wall excess): A2 = 2×(t_n − t_rn) × min(2.5t, 2.5t_n) × f_r1 A2 = 2×(8.18 − 5.20) × 20.45 × 1.0 A2 = 2 × 2.98 × 20.45 A2 = 121.9 mm²

Step 6 — Area A3 (Inward projection): A3 = 0 (no inward projection)

Step 7 — Weld Areas: A41 = (w1)² × f_r1 = (12)² × 1.0 = 144 mm² A43 = 0 (no inner weld)

Step 8 — Total Available (without pad): A_avail = A1 + A2 + A3 + A41 + A43 = 1115.1 + 121.9 + 0 + 144 + 0 A_avail = 1,381.0 mm²

Step 9 — Area deficit requiring pad: A5 = A − A_avail = 2,939.7 − 1,381.0 A5 required = 1,558.7 mm² → Reinforcing pad REQUIRED

Step 10 — Minimum Pad OD (set pad thickness t_p = shell t = 20 mm, f_r2 = 1.0): A5 = (D_p − d − 2t_n) × t_p × f_r2 1558.7 = (D_p − 202.74 − 2×8.18) × 20 × 1.0 D_p = (1558.7 / 20) + 202.74 + 16.36 = 77.94 + 219.1 D_p minimum = 297.0 mm → specify 300 mm OD pad (round up to nearest 25 mm: 325 mm)
Area Accounting — NPS 8 Nozzle on OD 1600 Shell (20mm wall) A1 = 1,115 1,381 available A5 = 1,559 (pad needed) 0 1,381 A = 2,940 mm² A1=1,115 (shell) A2=122 (nozzle) A41=144 (weld) A5=1,559 (pad reqd) Required Reinforcing Pad Minimum OD = 297 mm → Specify 325 mm (rounded up) Pad thickness = 20 mm (matching shell), same material (f_r2 = 1.0)
Figure 2 — Area accounting for the worked example: NPS 8 nozzle on a 1,600 mm OD shell with 20 mm nominal wall. The available area from shell excess (A1), nozzle wall (A2), and weld (A41) totals 1,381 mm² against a required area of 2,940 mm². A reinforcing pad supplying 1,559 mm² minimum is required, giving a minimum pad OD of 297 mm (specify 325 mm).

Reinforcing Pad Sizing

When the available area from the shell, nozzle, and welds is insufficient, a reinforcing pad is welded around the nozzle neck on the outside surface of the shell. The pad is a flat ring of plate, centred on the nozzle, with an inner hole matching the nozzle OD and an outer diameter sized to provide the required area A5.

Reinforcing Pad Area Supplied: A_pad = (D_p − d − 2×t_n) × t_p × f_r2
Where: D_p = pad OD, d = nozzle bore, t_n = nozzle wall, t_p = pad thickness, f_r2 = pad/shell stress ratio

Solving for minimum pad OD D_p: D_p = d + 2×t_n + A5 / (t_p × f_r2)

Constraint — pad must stay within reinforcement zone: D_p ≤ d + 2×d_eff = 3×d (approximately, for radial nozzles where d_eff = d) If the required D_p would exceed the zone limit, increase pad thickness t_p instead
Pad Thickness Practice: In most fabrication shops, the reinforcing pad is made from the same plate as the shell (same specification, same thickness) to simplify the material procurement and welding procedure qualification. Setting t_p = t (shell nominal thickness) and f_r2 = 1.0 (same material) is the standard starting assumption. Increasing pad thickness beyond the shell thickness is permitted and reduces the required pad OD, but adds weight and may complicate PWHT. Decreasing pad thickness requires a larger OD.

F-Factor and Nozzle Orientation

The F-factor in the UG-37 area formula modifies the required area based on the direction of the nozzle axis relative to the shell principal stress directions. For a radial nozzle in a cylindrical shell (the normal case), the nozzle axis is perpendicular to the shell axis and the opening is perpendicular to the maximum hoop stress direction — F = 1.0. For a hillside nozzle where the nozzle axis is tilted, or for nozzles whose axis lies along the cylinder axis, the opening is oriented more favourably with respect to the hoop stress and F may be less than 1.0, reducing the required area.

Nozzle ConfigurationShell TypeF-FactorNotes
Radial nozzle, axis perpendicular to shell axisCylindrical1.0Most common case — no reduction
Nozzle axis parallel to shell axis (axial)Cylindrical0.5Opening axis aligned with lower longitudinal stress
Hillside nozzle at angle θ to radialCylindricalPer UG-37 Fig.F between 0.5 and 1.0 depending on angle
Nozzle in spherical zone of headEllipsoidal/Spherical1.0Biaxial stress state; F = 1.0 per UG-37(a)
Nozzle in conical sectionCone/Transition1.0Hoop stress governs; conservative to use 1.0

Nozzle Types and Weld Details

The weld configuration at the nozzle-to-shell junction affects both the weld area credit in the reinforcement calculation and the weld procedure requirements. ASME VIII Div 1 UW-16 specifies minimum weld sizes for nozzle attachments. The most common nozzle weld types are:

Nozzle TypeWeld DetailA41 CreditTypical Use
Set-in (Insert) nozzleFull penetration butt weld through shell; optional outside filletw1² × f_r1High-pressure, code-critical nozzles
Set-on (Pad-type)Fillet weld at shell OD; nozzle does not penetrate shellw1² × f_r1Lower-pressure attachments, instrument nozzles
Insert with internal projectionNozzle passes through shell and protrudes inward; inner fillet weldA41 + A43Heavy-wall vessels; maximises total weld area
Self-reinforcing (integrally reinforced)Forged nozzle with integral reinforcement; no separate padPer UG-37 integrally reinforced nozzle rulesHigh-pressure, small-bore, eliminates pad

Strength Ratios f r1, f r2, f r3, f r4

When the nozzle, reinforcing pad, or shell materials have different allowable stresses, the area contributions of each element are scaled by the strength ratio — the ratio of the element’s allowable stress to the shell allowable stress. This ensures that weaker materials provide proportionally less credit toward the reinforcement area.

SymbolDefinitionApplied ToValue When Same Material
f_r1S_nozzle / S_shellA2, A3, A41, A431.0
f_r2S_pad / S_shellA5 (pad area)1.0
f_r3min(S_nozzle, S_pad) / S_shellWeld areas connecting both nozzle and pad1.0
f_r4S_pad / S_shellOuter fillet weld between pad and shell1.0
Common Application: In most oil and gas process vessels, the shell, nozzle, and pad are all the same material specification (e.g., SA-516 Grade 70 for all three). In this case all f_r values are 1.0 and can be ignored. Strength ratios become important when the nozzle is stainless steel (SA-312 TP316L) set into a carbon steel shell (SA-516 Grade 70), or when the pad material is a lower-grade carbon steel than the shell. Since stainless steels often have a lower allowable stress than carbon steel at the same temperature, f_r1 < 1.0 in these cases, reducing the nozzle wall credit.

Practical Engineering Notes

Connection to Shell Thickness

The t_r value used in the reinforcement area calculation comes directly from the pressure vessel shell thickness calculator (UG-27). The larger the t_r (higher design pressure, larger vessel, lower allowable stress), the larger the required area A — meaning high-pressure vessels have more demanding nozzle reinforcement requirements. A thicker ordered shell (larger t) reduces the pad requirement by increasing A1, the most efficient source of reinforcement area.

Multiple Nozzles and Overlapping Zones

When two nozzle reinforcement zones overlap — when the outer edge of one zone falls within the zone of an adjacent nozzle — the UG-42 rules for multiple openings apply. In this case, the reinforcement for each opening must account for the fact that some shell material is shared between the two zones and cannot be double-counted. Close nozzle spacing is common on vessels with multiple instrumentation connections in a short spool, and requires careful engineering review to confirm that sufficient reinforcement area remains for each opening.

Nozzle Loads and Pad Size

The reinforcing pad calculated here is sized for pressure design per UG-37 only. Where the connecting piping imposes significant nozzle loads (forces and moments from thermal expansion, dead weight, seismic, or wind), the pad may need to be larger and thicker to distribute these loads into the shell without overstressing the nozzle-to-shell junction. For critical nozzles on large vessels in oil and gas facilities, a WRC-107/WRC-537 local stress analysis is typically performed to check the shell stress at the nozzle junction under combined pressure and piping loads. This analysis is beyond the scope of UG-37 but is required by most pressure vessel design codes for nozzles with significant piping loads.

Welding and Inspection: All nozzle-to-shell welds are Category D joints under ASME VIII Div 1 and must be executed under ASME Section IX qualified welding procedures. Minimum weld sizes per UW-16 must be confirmed. For vessels with full radiography (E = 1.0), nozzle welds in the Category A and B radiography sequence must also be examined. For sour service vessels, the nozzle weld HAZ hardness must meet NACE MR0175 limits — typically requiring PWHT of the entire vessel, which affects the nozzle-to-shell weld in the same heat treatment cycle as the main seam welds.

Frequently Asked Questions

What is the area replacement method for nozzle reinforcement per ASME UG-37?
The area replacement method requires that the cross-sectional area removed by a nozzle opening in a pressure vessel shell be compensated by an equal area of metal within a defined reinforcement zone. The required area A = d × t_r × F, where d is the nozzle bore, t_r is the minimum required shell thickness, and F is the orientation factor. The available area is the sum of A1 (shell excess), A2 (nozzle wall excess), A3 (inward projection), A41 and A43 (weld metal), and A5 (reinforcing pad if needed). Total available area must equal or exceed A for code compliance.
How is the reinforcement zone defined in ASME UG-37?
The reinforcement zone is a rectangle centred on the nozzle centreline. Its width in the shell plane extends a distance equal to the larger of d (nozzle bore) or (R_n + t_n + t) on each side. Its height above the outer shell surface extends to the smaller of 2.5t or 2.5t_n. For inward projecting nozzles, depth below the inner shell surface is limited to the smaller of 2.5t_n or the actual projection length h. Only metal within this zone counts toward the available reinforcement area. For most radial nozzles on cylindrical shells, the zone width on each side equals d, giving the simplified form A1 = d × (t − t_r).
What is area A1 in the ASME UG-37 nozzle reinforcement calculation?
A1 is the excess area in the shell wall within the reinforcement zone. It represents the shell thickness above the minimum required t_r, multiplied by the reinforcement zone width. For a standard radial nozzle: A1 = d × (t − F × t_r). A larger A1 reduces or eliminates the need for a reinforcing pad. Engineers sometimes specify a slightly thicker shell than strictly required by pressure to provide built-in nozzle reinforcement, avoiding the fabrication cost of pads on all nozzles.
When is a reinforcing pad required for a nozzle?
A reinforcing pad is required when the total area from the shell (A1), nozzle wall (A2), inward projection (A3), and welds (A41 + A43) is less than the required area A. Smaller nozzles on thick-walled shells often need no pad. Larger nozzles on thin-walled or high-pressure vessels almost always need one. A practical indicator: when the nozzle bore d approaches or exceeds approximately 30% of the shell inside diameter, a pad is typically required. The calculator on this page determines the needed pad area and minimum pad OD automatically.
What is the F-factor in the ASME UG-37 nozzle reinforcement formula?
The F-factor accounts for nozzle orientation relative to the principal stress directions in the shell. For the most common case — a radial nozzle in a cylindrical shell where the nozzle axis is perpendicular to the shell axis — F = 1.0. For nozzles whose axis lies along (or parallel to) the shell axis, the opening is oriented more favourably and F = 0.5, halving the required area. For hillside nozzles at intermediate angles, F varies between 0.5 and 1.0 per ASME UG-37 Figure. For nozzles in spherical zones of formed heads, F = 1.0.
Can the excess nozzle wall thickness contribute to reinforcement area?
Yes. Area A2 credits the nozzle wall thickness above the minimum required for pressure (t_n − t_rn), scaled by the strength ratio f_r1 and limited to the reinforcement zone height of min(2.5t, 2.5t_n). A2 = 2 × (t_n − t_rn) × min(2.5t, 2.5t_n) × f_r1. A thicker nozzle schedule therefore reduces the required pad area. For a nozzle with significant excess wall (for example, a Schedule 160 nozzle where the pipe schedule was selected for other reasons), A2 can be substantial and may eliminate the need for a pad entirely on smaller-diameter openings.
What size should a nozzle reinforcing pad be?
The minimum pad OD is: D_p = d + 2t_n + A5/(t_p × f_r2), where A5 is the remaining required area, t_p is pad thickness, and f_r2 is the pad-to-shell stress ratio. In practice, pad thickness is set equal to the shell nominal thickness and the same material is used (f_r2 = 1.0). The resulting D_p is rounded up to the nearest 25 mm. The pad must remain within the reinforcement zone, so D_p cannot exceed the nozzle OD plus 2 × d_eff. If the required D_p would exceed this limit, the pad thickness must be increased.
What is the difference between a set-in and a set-on nozzle for reinforcement?
A set-in nozzle has the nozzle pipe passing through a hole in the shell, attached by a full penetration weld at the shell junction. Its excess wall contributes on both sides of the shell, maximising A2. A set-on nozzle sits on the outside surface and is attached by fillet welds only; it does not project through the shell, so A2 is based only on the outward projection height. Set-in nozzles provide better reinforcement efficiency. Set-on nozzles are simpler to fit and weld but rely more heavily on the reinforcing pad. ASME UW-16 specifies minimum weld dimensions for both configurations.
Does UG-37 apply to nozzles in dished ends as well as cylindrical shells?
Yes. ASME UG-37 applies to openings in both cylindrical shells and formed heads. For nozzles in ellipsoidal or torispherical heads, the t_r used in the area calculation is the head thickness required by UG-32 at the nozzle location. For openings near the centre of a 2:1 ellipsoidal head, this equals the shell thickness per UG-32(c). For nozzles in the knuckle zone of a torispherical head, the local required thickness may be higher. The calculator on this page includes a “Formed Head” tab where the head t_r can be entered directly from the UG-32 calculation.

Recommended Reference Books

📚
Pressure Vessel Design Manual — Dennis Moss
The go-to practical reference for nozzle reinforcement, pad sizing, and UG-37 calculations with detailed worked examples and design charts.
View on Amazon
📚
ASME Section VIII Division 1 Code
The authoritative code covering UG-37 area replacement, UW-16 weld requirements, and UG-42 multiple openings for pressure vessel nozzles.
View on Amazon
📚
Pressure Vessel Technology — Bednar
In-depth engineering analysis of nozzle stress concentrations, reinforcement theory, and WRC local stress evaluation for pressure vessel connections.
View on Amazon
📚
Companion Guide to ASME Boiler & Pressure Vessel Code
Expert commentary on ASME code intent and application, including detailed discussion of UG-37 area replacement rationale and common calculation mistakes.
View on Amazon

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