Blog circa ASME Standards Guide for Pressure Vessel Head Selection

Imagine a massive pressure vessel containing forces powerful enough to destroy everything within it. The head—the end closure of this vessel—must be precisely designed and perfectly fitted to ensure absolute safety. Choosing the wrong head can range from compromising efficiency to causing catastrophic failure. Struggling with head selection and calculations? This ASME-compliant guide will provide the knowledge needed to navigate even the most complex operational conditions.

According to ASME Sec. VIII, UG-32 standards, three primary head types dominate industrial applications: torispherical, ellipsoidal, and hemispherical. Each possesses unique geometric characteristics and mechanical properties suited for specific scenarios. Proper selection is paramount for safe pressure vessel operation.

Comprising a spherical crown and knuckle transition zone, torispherical heads offer manufacturing simplicity and economic advantages, making them widely adopted. Key specifications include:

• Crown radius (R): Typically equals head diameter (D)
• Knuckle radius (r): Conventionally 6% of diameter (0.06D)
• Height (H): Approximately 0.1935D (H ≈ D/5.17)
• Volume (V): Without straight flange: V ≈ 0.088 * [D(in)/2]³

Selection Note: While economical, these heads exhibit significant stress concentration at the crown-knuckle junction. Thorough pressure capacity evaluation against safety standards is essential.

With superior stress distribution and enhanced pressure resistance, 2:1 ellipsoidal heads represent an optimal middle ground. Their geometry features:

• Crown radius (R): 0.9D
• Knuckle radius (r): 0.17D
• Height (H): Approximately 0.25D (H ≈ D/4)
• Volume (V): Without straight flange: V ≈ 0.1309 * [D(in)/2]³

Selection Note: Their complex manufacturing process increases costs, but they outperform torispherical heads in high-pressure applications.

Exhibiting optimal stress distribution, hemispherical heads deliver unparalleled pressure resistance. Critical dimensions include:

• Inside radius (IDR): Exactly 0.5D
• Height (H): Equals radius (H = D/2)
• Volume (V): Without straight flange: V ≈ 0.2618 * [D(in)/2]³

Selection Note: Their manufacturing complexity and premium cost reserve them for critical applications like nuclear reactor vessels.

ASME standards mandate additional thickness beyond calculated minimums to account for:

• Corrosion allowance: Compensates for material degradation
• Manufacturing tolerance: Accommodates production variances
• Erosion allowance: Addresses material wear from fluid dynamics

The standard thickness increment rules specify:

• For thickness ≤ 0.5 inches: Add 0.04 inches
• For thickness > 0.5 inches: Add 0.04 inches or 4% (whichever is greater)

Optimal head selection requires multidimensional analysis of:

• Operating pressure: Primary determinant of head type and thickness
• Temperature range: Affects material performance characteristics
• Media properties: Dictates material compatibility and wear allowances
• Manufacturing feasibility: Considers production capabilities and costs
• Economic factors: Balances performance requirements with budget constraints