Pressure vessel heads play a vital role in the safety and functionality of pressure vessels. These sealed containers are designed to hold toxic or non-toxic gases or fluids at pressures significantly different from the environment outside the vessel.
Pressure vessel heads help distribute the internal pressure throughout the container. This specialized component also seals the ends of the vessel to prevent internal gas or fluid leaks. If a pressure vessel head is unable to handle the pressure, it may leak or cause damage to nearby people and property.
The design and construction of pressure vessels and the various heads that keep their contents securely contained are highly regulated by the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code (BPVC), specifically Code Section III in the U.S. and Canada.
Along with these mandatory design requirements, choosing the right pressure vessel head impacts the overall design, performance and suitability of the containment vessel for specific applications. Therefore, it’s essential to consider the various types of pressure vessel heads designed today and their advantages.
Semi-Elliptical (SE) Heads
SE heads are one of the most common head designs used in pressure vessels. The head is shaped like half of an ellipse in a specific ratio of 2:1 — meaning the head depth is four times the head width.
The unique design of SE heads offers many advantages to pressure vessel applications:
- Pressure handling: This design is perfect for high pressures owing to its height-to-weight ratio.
- Cost-effective: These heads are made of thinner materials, such as a flat plate, which helps reduce their fabrication costs.
- ASME code compliance: The ASME code recommends two approximations for SE heads to withstand pressure effectively — one with a sphere of the radius equal to 90% of the head’s diameter (0.90D) and the other with a knuckle radius measuring 17% of the head’s diameter (0.17D).
ASME Flanged and Dished (ASME F&D) Heads
As the name suggests, these heads are ideal for pressure vessels designed for moderate pressures. The flange, or outer rim, has a flat, extended portion. While the shape of the head is dished, meaning it curves inward. These features contribute to the overall strength and structural integrity of the pressure vessel head.
Based on design pressure and vessel size, ASME F&D heads offer many advantages:
- Height restriction compatibility: These heads have a flatter or lower profile, making them ideal for pressure vessels with height restrictions.
- Thicker construction: Based on design pressure and vessel size, ASME F&D heads are thicker than semi-elliptical heads for enhanced strength and durability in moderate-pressure applications.
- Offset Seam: The head possesses an offset in the head-to-shell circumferential seam, which is usually managed at a 3:1 ratio. This configuration helps to manage stress concentrations and ensure structural integrity.
- Unique applications: ASME F&D heads can be used in various industries and moderate-pressure applications. Their flatter heads make them ideal for mixing tanks, as the mixer can be mounted at a lower position in the tank.
Flat Heads
Flat heads have completely flat surfaces, making them suitable for applications that require flat inside surfaces. These pressure vessel heads are more common in no-pressure applications where stress concentration is not a concern, such as holding tanks or storage tanks that store materials at no pressure. When used for pressure applications, flat heads can be more costly than other options.
Flat heads are also recommended when the design of a pressure vessel calls for a head with a lower profile or height.
Hemispherical Heads
Hemispherical heads have a radial shape that resembles a half-sphere, where the head depth is half the diameter. The head’s design features two pressure heads placed back to back that create a storage sphere that efficiently stores materials at high pressure.
Hemispherical heads have a thinner profile than cylindrical shells. They follow a standard code that specifies a 3:1 taper during the transition. Hemispherical heads are the thinnest head type and one of the most expensive options. They are typically formed by welding sheets rather than the flat sheets used to make other head types.
These heads are ideal for large-diameter vessels and high-pressure applications.
Explore ASME Pressure Vessel Heads From BEPeterson
Pressure vessel heads come in various shapes and designs to accommodate different pressure requirements, structural considerations, and operational needs. If you are unable to choose a head type for your application, turn to a trusted manufacturer like BEPeterson.
We have been providing pressure vessels in different head types for decades, including high-pressure tanks or ASME code and non-code pressure vessels with F&D heads and SE heads. Reach out for a free quote based on your application requirements.
