ASTM A240 304/304L 316/316L Stainless Steel Dished Head For Pressure Vessels

Product Specification

 Stainless Steel Head as Pressure Vessel Closure Component

A stainless steel head is a formed end closure for pressure vessels, heat exchangers, reactors, columns, and storage tanks. Stainless steel grades provide corrosion resistance across a wide range of process media, including acids, alkalis, chlorides (within grade limits), organic compounds, and high-purity fluids.

The head is formed from stainless steel plate or sheet by hot or cold pressing, spinning, or dishing. This product is designed and manufactured per ASME Section VIII Division 1 or Division 2, and GB/T 25198 (Pressure Vessel Heads). Heads can be supplied as formed blanks, with edge preparation for welding, or fully fabricated with welded flanges, nozzles, and inspection openings.

Head Types – Geometric Classification

2:1 Elliptical Head (Semi-Ellipsoidal)

• Most common pressure vessel head type

• Inside depth (h) = ID / 4

• Knuckle radius (r) ≈ 0.17 × ID

• Crown radius (R) ≈ 0.90 × ID

• Application: General pressure vessels, heat exchanger channels, reactor bottoms, storage tanks

• Advantages: Efficient pressure distribution; low forming cost for standard diameters

• Limitation: Higher forming strain at knuckle area; not suitable for very high pressure without increased thickness

Torispherical Head (Flanged and Dished)

• Crown radius (R): 0.80 × ID to 1.00 × ID (typically R = ID)

• Knuckle radius (r): 6% to 10% of ID (minimum 3% per ASME VIII-1)

• Application: Lower-pressure vessels, atmospheric tanks, manway covers

• Advantages: Shallow depth (reduced head height); lower forming cost than elliptical heads

• Limitation: Higher stress at knuckle; requires thicker material for same pressure compared to elliptical

Hemispherical Head

• Radius = ID / 2 (true sphere)

• Inside depth (h) = ID / 2

• Application: High-pressure vessels (above 10 MPa), cryogenic storage, nuclear vessels

• Advantages: Minimal thickness for given pressure and diameter; best pressure retention efficiency

• Limitation: Requires two-piece forming (two halves welded together) for large diameters > 1,500mm; higher manufacturing cost

Conical Head

• Cone apex angle: 30° to 120° (typically 60° for pressure vessels)

• Application: Hopper bottoms, slurry tanks, settling vessels, catalyst withdrawal zones

• Advantages: Provides smooth flow transition for solids or slurries

• Limitation: Higher stress at cone-to-cylinder junction; requires reinforcing ring or knuckle transition

Flat Head (Circular Plate)

• Thickness calculated per ASME VIII-1 UG-34

• Application: Low-pressure vessels, inspection covers, manway covers, heat exchanger channel covers

• Advantages: Simple fabrication; lowest cost for small diameters

• Limitation: Requires thicker material for pressure; high stress concentration at edge

Stainless Steel Material Grades – Selection per Service

Austenitic Stainless Steels

Duplex Stainless Steels

Precipitation-Hardening Stainless Steel

Forming Methods – Process Selection

Cold Forming

• Temperature: Ambient (20°C – 40°C)

• Equipment: Press brake, dishing press, or hydraulic press with matched dies

• Typical thickness: 2mm – 20mm (cold formable)

• Springback compensation: Die dimensions adjusted by 0.5% – 2.0% of crown radius per material yield strength

• Maximum forming strain: ≤ 15% for austenitic SS; ≤ 10% for duplex SS (to prevent cracking)

• Ferrite measurement (duplex after cold forming): Ferrite content must remain between 35% – 65% per ASTM E562. Cold forming may increase ferrite to > 65% – requires solution annealing.

• After cold forming: Solution anneal (heat to 1,040°C – 1,100°C, water quench) for austenitic SS if forming strain > 10% or if service requires maximum corrosion resistance (per ASME VIII-1 UG-79)

Hot Forming

• Temperature: 900°C – 1,100°C (austenitic SS); 1,000°C – 1,150°C (duplex SS)

• Heating rate: ≤ 150°C/hour (to prevent grain growth)

• Forming atmosphere: Inert gas purge or protective coating to prevent oxidation and scaling

• After hot forming:

  • Austenitic SS: Solution anneal at 1,040°C – 1,060°C, quench or rapid air cool

  • Duplex SS: Solution anneal at 1,040°C – 1,080°C, water quench (to restore ferrite/austenite balance)

• For thickness > 20mm: Quench required to prevent carbide precipitation and sigma phase formation (duplex)

Spinning (Flow Forming)

• Process: Plate is rotated and pressed over a mandrel to form the head contour

• Temperature: Cold or hot (warm spinning at 200°C – 400°C for duplex)

• Typical thickness: 2mm – 12mm

• Application: Thin-walled heads for storage tanks, low-pressure vessels, large diameters

• Advantages: Low die cost; suitable for diameters > 2,000mm

• Limitation: Thickness reduction at crown area (typically 5% – 10% wall thinning); thickness must be measured and compensated in blank design

Thickness Design – Minimum Required Thickness

The minimum required thickness for each head type is calculated per ASME VIII-1:

2:1 Elliptical Head (per UG-32(d))

• Formula: t = (P × D) / (2 × S × E – 0.2 × P) × correction factor

• Where: P = design pressure, D = inside diameter, S = allowable stress, E = joint efficiency

• Correction factor for 2:1 elliptical: 1.0 (standard factor applied for D/2h = 2.0)

• For D/2h ≠ 2.0: Apply stress index calculation per Appendix 1-4

Torispherical Head (per UG-32(e))

• Formula: t = (0.885 × P × L) / (S × E – 0.1 × P)

• Where L = crown radius (typically L = D for standard torispherical heads)

• Knuckle radius must be ≥ 6% of D (minimum 3% per ASME VIII-1)

Hemispherical Head (per UG-32(f))

• Formula: t = (P × R) / (2 × S × E – 0.2 × P)

• Where R = inside radius (D/2)

Flat Head (per UG-34)

• Formula: t = d × sqrt(C × P / S × E)

• Where: d = diameter, C = edge constraint factor (0.33 – 0.50)

• C value depends on edge attachment method (welded, bolted, or integral)

Thickness Tolerance (Performed Head)

• Minimum formed thickness after forming: ≥ 90% of nominal thickness per ASME VIII-1 UG-79

• Thinning allowance must be included in blank plate thickness selection

• Thickness measurement: 10% of surface area (minimum 5 points) – ultrasonic thickness gauge

Dimension Tolerances (Per ASME VIII-1 UG-81 / GB/T 25198)

Welding and Fabrication – Weld Preparation

Head-to-Shell Weld Joint

• Bevel angle: 30° – 37.5° (single V or double V)

• Root gap: 2mm – 4mm

• Root face: 1mm – 2mm

• Filler metal:

  • 304L – ER308L

  • 316L – ER316L

  • 321 – ER321

  • 347H – ER347

  • Duplex 2205 – ER2209

• Weld process: GTAW (TIG) or SMAW (stick) or SAW (submerged arc) – per ASME Section IX

• Backing gas: Argon purge for root pass on austenitic and duplex SS

Nozzle Attachments and Reinforcing Pads

• Nozzles: Welded to head crown or knuckle area – reinforcement pad required per ASME VIII-1 UG-37 if opening exceeds allowable size

• Minimum distance between nozzle center and head tangent line: ≥ 0.5 × nozzle OD (to prevent weld overlay of knuckle zone)

• All attachments: 100% PT on final weld surface

PWHT for Stainless Steel Heads

• Austenitic SS (304L/316L): PWHT not typically required; however, solution annealing at 1,040°C – 1,100°C with water quench is recommended after hot forming or after welding of thick sections (> 20mm) to restore corrosion resistance

• Duplex SS: Solution anneal at 1,040°C – 1,080°C with water quench after any hot forming or welding of thick sections to restore ferrite/austenite balance and prevent sigma phase

• 321/347H: Solution anneal at 1,040°C – 1,060°C after hot forming or welding if sensitization is a concern

• Stress relief (400°C – 450°C) for 17-4PH after welding – per manufacturer's recommendation

Surface Finish and Passivation

Internal and External Surface

• Mill finish (as-formed): Standard forming surface, Ra 3.2μm – 6.3μm

• Pickled finish: Acid bath (HF/HNO₃) to remove scale and oxide layer – restores corrosion resistance

• Passivated finish: Nitric acid or citric acid treatment to remove iron contamination and re-form passive layer – required for pharmaceutical, food, and corrosive service

• Electropolished finish: Ra ≤ 0.4μm – for pharmaceutical, biotech, ultra-pure water service

• Mechanical polished finish: Ra 0.8μm – 1.6μm (grit 180 – 320) – for food, dairy, cosmetic service

Pickling and Passivation Procedure

• Pickling solution (for austenitic SS): 10% – 20% HNO₃ + 1% – 3% HF, temperature 20°C – 50°C, immersion time 10 – 60 minutes

• Passivation solution (for austenitic SS): 20% – 40% HNO₃, temperature 40°C – 50°C, immersion time 30 – 60 minutes

• After treatment: Rinse with demineralized water, dry with oil-free air

• Passivation verification: Ferroxyl test per ASTM A380 – no blue coloration (indicates free iron removal)

Inspection and Testing per Head

Dimensional Inspection

• Crown radius: Template or 3D scanning – tolerance per ASME VIII-1 UG-81

• Inside depth: Depth gauge – tolerance ±1.0%

• Thickness: Ultrasonic thickness gauge at 10% of surface area – min. thickness ≥ 90% nominal

• Out-of-roundness: Diameter tape at tangent line – ≤ 1.0% of ID

• Straight flange length and roundness: Caliper and diameter tape – tolerances per drawing

Non-Destructive Examination (NDE)

Pressure Testing (when welded to vessel or tested as standalone closure)

• Hydrostatic test per ASME VIII-1 UG-99: Test pressure = 1.3 × design pressure × (S at test temp / S at design temp)

• Test medium: Clean water or demineralized water (to avoid chloride stress corrosion on SS)

• Chloride limit in test water: ≤ 50ppm (for austenitic SS) to prevent SCC; ≤ 200ppm for duplex

• Hold time: 30 minutes minimum

• Acceptance: Zero pressure drop; no visible leakage at weld joints

Documentation per Shipment

• Material test certificates (EN 10204 3.1 or 3.2) – plate material and filler materials

• ASME U-stamp data report (if applicable)

• GB/T 25198 data sheet (if domestic standard)

• Dimensional inspection report (crown radius, depth, thickness, out-of-roundness, straight flange)

• NDE reports – PT surface scan, UT lamination scan, RT/UT weld scan (if welded)

• Forming procedure specification (FPS) and forming records (temperature, heating rate, cooling rate)

• Thickness measurement report (ultrasonic thickness gauge records)

• Pickling/passivation certificate (if performed) – including bath concentration, temperature, and duration

• Passivation verification test (Ferroxyl test) – if required

• Heat treatment chart (solution annealing time-temperature record) – if performed

Selection Checklist – Stainless Steel Head

1. Head type – 2:1 Elliptical / Torispherical / Hemispherical / Conical / Flat

2. Shell ID (inside diameter) – determines head size and depth

3. Design pressure – determines minimum thickness per ASME VIII-1 UG-32

4. Design temperature – determines allowable stress (S) per ASME II-D and material grade

5. Process fluid – determines stainless steel grade (304L / 316L / 321 / 347H / Duplex)

6. Chloride concentration – if > 200ppm, select 316L (≤250ppm), Duplex 2205 (≤300ppm), or Super Duplex 2507 (>300ppm)

7. Forming method preference – Hot forming (thick > 20mm) or Cold forming (thin ≤ 20mm)

8. Surface finish requirement – Mill / Pickled / Passivated / Electropolished / Mechanical polished

9. Nozzle attachments required – provide nozzle schedule (size, rating, orientation, projection)

10. PWHT requirement – Based on material, thickness, and service (usually solution anneal for SS)

11. NDE requirement – Per ASME VIII-1 or per project specification

12. Weld preparation – Edge bevel type (single V / double V) and bevel angle (if supplied with weld prep)

13. Test pressure – For hydrotest per UG-99 (if supplied as pre-tested closure)

Design Limitation Statement – Stainless Steel Head

The stainless steel head as a standalone product is subject to the following limitations:

• It is not rated for internal pressure until welded to a shell or flange; pressure rating is derived from the complete assembled vessel

• Not suitable for chloride-containing service > 200ppm without selecting appropriate grade (316L for ≤250ppm, Duplex for ≤300ppm, Super Duplex for higher concentrations)

• Not suitable for hydrotest with water containing chloride > 50ppm (for austenitic SS) – risk of chloride stress corrosion cracking (SCC)

• Not suitable for service temperatures above 600°C (austenitic SS) or above 280°C (duplex SS) – material phase transformation or creep will occur above these limits

• Not suitable for severe thermal cycling (> 1,000 cycles/year) without fatigue analysis per ASME VIII-2 – residual stresses from cold forming may accelerate fatigue failure

• For heads requiring solution annealing after forming, the part must be re-quenched; cooling rate affects final mechanical properties