dished head

What is a Dished Head?

A dished head, also called a torispherical head, is a formed closure welded to the end of a pressure vessel or tank. It consists of a shallow spherical crown with a knuckle radius that transitions smoothly to a straight flange for welding. Dished heads are widely used as closures for storage tanks, heat exchangers, filters, columns, and process vessels where moderate pressure and cost-effectiveness are priorities.

Standards and Specifications

Dished heads are manufactured to ASME F&D (flanged and dished) standard dimensions, and conform to ASME BPVC Section VIII, EN 10028, and DIN 28013. The standard crown radius is equal to the vessel diameter, with a knuckle radius of 6 percent of the diameter. Sizes range from 150 mm to 6000 mm and larger, with wall thicknesses from 3 mm to 50 mm. They are supplied with or without a straight flange depending on welding requirements.

Materials and Grades

Carbon steel Q235B, Q345R, and ASTM A516 Gr 70 are standard for vessel heads. Stainless steel 304/304L and 316/316L provide corrosion resistance for chemical and food equipment. Alloy steel and clad materials are available for high-temperature or corrosive service. Plates are sourced from certified pressure-vessel-grade mills with full mechanical and chemical test reports.

Pressure and Design Considerations

Dished heads are designed per ASME BPVC Section VIII using the formulas for torispherical heads. The knuckle radius is critical: a smaller knuckle increases stress concentration but reduces overall head depth. Heads are rated for internal pressure, with vacuum and external pressure checks as needed. Forming methods include hot pressing, cold spinning, and spinning for large diameters.

Applications

Dished heads close the ends of storage tanks, pressure vessels, heat exchangers, steam accumulators, filters, separators, and reaction vessels in chemical, petrochemical, pharmaceutical, food, and water treatment industries. They are also used as end caps for large-diameter pipe spools and piling.

Advantages

Dished heads are shallower than hemispherical heads, reducing overall vessel length and manufacturing cost. The torispherical shape provides good pressure distribution and is easier to form in large diameters. Standard dimensions simplify engineering and procurement. The straight flange allows circumferential welding to the vessel shell with proper joint alignment.

Manufacturing and Installation

Heads are hot-pressed or cold-spun from plate, then normalized or stress-relieved as required. The head is welded to the vessel shell using a full-penetration butt weld. Post-weld heat treatment may be required for thick-wall or high-strength materials. NDT including radiographic or ultrasonic examination verifies weld integrity, and the completed vessel undergoes hydrostatic testing.

Quality Assurance

Each dished head is identified with material grade, heat number, dimensions, and manufacturer's stamp. Inspection includes dimensional verification, plate UT, hardness testing, and visual examination. Material test certificates and forming records are provided. Heads are delivered with the weld end beveled and coated with rust-preventive oil, packed for shipment.