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
| Grade | UNS | Temp Range | Chloride Limit | Application |
|---|---|---|---|---|
| 304L | S30403 | -196°C to +425°C | ≤ 200ppm | Mildly corrosive, clean process fluids, food and beverage, pharmaceutical |
| 304 | S30400 | -196°C to +600°C (derated above 425°C) | ≤ 200ppm | Similar to 304L but higher carbon (not recommended for welded service without PWHT) |
| 316L | S31603 | -196°C to +425°C | ≤ 200ppm (service) / ≤ 250ppm (slightly higher with higher temp derating) | Organic acids, salt solutions, chloride-containing hydrocarbons (moderate), pharmaceutical |
| 316 | S31600 | -196°C to +600°C (derated above 425°C) | ≤ 200ppm | Similar to 316L; not recommended for welded thickness > 16mm without low-carbon grade |
| 321 | S32100 | -196°C to +540°C | ≤ 200ppm | Elevated temperature service (425°C – 540°C); titanium-stabilized to prevent carbide precipitation |
| 347H | S34709 | -196°C to +600°C | ≤ 200ppm | High-temperature service (up to 600°C); niobium-stabilized for creep resistance |
| 310S | S31008 | -196°C to +700°C | ≤ 200ppm | High-temperature furnace service; oxidation resistance > 600°C |
Duplex Stainless Steels
| Grade | UNS | Temp Range | Chloride Limit | Application |
|---|---|---|---|---|
| 2205 (S32205) | S32205 | -40°C to +280°C | ≤ 300ppm at 80°C | Offshore, seawater, chlorides, CO₂/H₂S service |
| 2507 (Super Duplex) | S32750 | -40°C to +250°C | ≤ 500ppm at 60°C | Severe chloride service, high-strength requirement |
Precipitation-Hardening Stainless Steel
| Grade | UNS | Temp Range | Application |
|---|---|---|---|
| 17-4PH | S17400 | -40°C to +315°C | High-strength covers, flanges, and closures requiring high yield strength |
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)
| Parameter | 2:1 Elliptical | Torispherical | Hemispherical | Measurement Method |
|---|---|---|---|---|
| Crown radius (R) | ±1.5% | ±1.5% | ±1.0% | Template / 3D scan |
| Knuckle radius (r) | ±5% | ±5% | N/A | Template / 3D scan |
| Inside depth (h) | ±1.0% | ±1.0% | ±1.0% | Depth gauge |
| Out-of-roundness at tangent line | ≤ 1.0% of ID | ≤ 1.0% of ID | ≤ 0.5% of ID | Diameter tape |
| Straight flange length | ±2mm | ±2mm | ±2mm (if applicable) | Caliper / tape |
| Straight flange out-of-roundness | ≤ 0.5% of ID | ≤ 0.5% of ID | ≤ 0.5% of ID | Diameter tape |
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)
| Examination | Method | Scope | Acceptance (Per ASME VIII-1) |
|---|---|---|---|
| Surface cracks | PT (liquid penetrant) | 100% of formed surface + weld areas | No cracks, no porosity > 0.8mm, no clustered porosity |
| Laminations | UT (straight beam) | 100% of plate blank | Laminations ≤ 20mm² per 100cm² |
| Weld seams (if welded) | RT or UT (shear wave) | 100% (per UW-51) or spot (per UW-52) | Per ASME VIII-1 UW-51 or UW-52 |
| Thickness verification | UT (ultrasonic) | 10% of surface | ≥ 90% nominal |
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
Head type – 2:1 Elliptical / Torispherical / Hemispherical / Conical / Flat
Shell ID (inside diameter) – determines head size and depth
Design pressure – determines minimum thickness per ASME VIII-1 UG-32
Design temperature – determines allowable stress (S) per ASME II-D and material grade
Process fluid – determines stainless steel grade (304L / 316L / 321 / 347H / Duplex)
Chloride concentration – if > 200ppm, select 316L (≤250ppm), Duplex 2205 (≤300ppm), or Super Duplex 2507 (>300ppm)
Forming method preference – Hot forming (thick > 20mm) or Cold forming (thin ≤ 20mm)
Surface finish requirement – Mill / Pickled / Passivated / Electropolished / Mechanical polished
Nozzle attachments required – provide nozzle schedule (size, rating, orientation, projection)
PWHT requirement – Based on material, thickness, and service (usually solution anneal for SS)
NDE requirement – Per ASME VIII-1 or per project specification
Weld preparation – Edge bevel type (single V / double V) and bevel angle (if supplied with weld prep)
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
