Introduction to Different Pressure Vessel Head TypesMay 16, 2019
Pressure vessels are designed to contain toxic or non-toxic gases or fluids at different pressures. This pressure may be induced by an external application of heat, from an internal source, or a combination of both. If a pressure vessel is unable to handle the pressure, it may leak or cause damages to people and property where it is placed. Thus, the design and construction of these pressure vessels are regulated by the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code (BPVC), Code Section III in the US and Canada. This code recommends mandatory design requirements as well as non-mandatory guidance for materials to be used for pressure vessel design, manufacturing, examination, testing, and so on. Shaped head and body are basic considerations of any pressure vessel design. SE heads and ASME F&D heads are two important types of pressure vessel heads designed today. This post discusses their specific advantages, as well as a few other head types.
Comparison between SE Heads and ASME F&D Heads
The following comparison of various pressure vessel head types will help you understand their importance better.
- Semi-Elliptical (SE) Heads: This elliptical shape head is designed in a 2:1 ratio, and it is also known as 2:1 head. In this head, the width to the depth ratio is 2:1. It means, the head depth is four times the head width. This SE head features a half ellipse, so its head depth is usually quarter of its diameter.
This head shape is economical, and is perfect for high pressures owing to its height to weight ratio. Semi-elliptical head has radius varying between the minor and major axis, in the ratio of 2:1. This varying ratio allows it to hold gases or fluids at high pressure. These heads are made of thinner materials such as a flat plate, which help reduce their fabrication costs.
Typically, pressure vessel fabricators design these heads with three radii, which is approximated to an ellipse. The ellipse usually has a smallest outer diameter, a large crown, and an intermediate radius. The code recommends the approximation of SE head, one with a sphere of the radius 0.90D and other with a knuckle radius 0.17D.
The ASME code design formula for 2:1 SE is similar to that of a cylinder, where the design thickness for a cylinder is defined as 0.500″, and that for the semi-elliptical head is 0.4947″.
- ASME Flanged and Dished (ASME F&D) Heads: As the name suggests, these heads are ideal for pressure vessels designed for moderate pressures. These heads possess a flatter or lower profile, which makes them ideal for pressure vessels that have a height restriction. The F&D heads are shallower than 2:1 heads, which means they need not be dished as much and as deep as a 2:1 head.
Based on design pressure and vessel size, ASME F&D heads are thicker than 2:1 elliptical heads. This is best explained through the example below:
- ASME F&D 60″ head would need a 69″ circle blank to form the head. You need to add 14% to OD of the requested head for a blank size.
- ASME 2:1 60″ elliptical head would need a 72” circle blank to form the head. You need to add 20% to OD of the requested head for blank size.
In the forming phase, the material of the head flows to the straight flange or outer flange, thereby making the heads thicker. The head possesses an offset in the head-to-shell circumferential seam, which is usually managed at 3:1, as per the Code.
As mentioned before, ASME F&D heads have flatter heads than 2:1 elliptical heads, which makes them an ideal choice for mixing tanks as the mixer can be mounted at a lower position in the tank.
A Few More Important Types of Pressure Vessel Heads
Pressure vessels may also be designed with any of the following head types:
- Flat Heads: These heads have flat surfaces, which makes them ideal for applications that demand flat inside surfaces. Flat heads are considered ideal for no pressure applications, when used for pressure applications, these pressure vessel heads may be quite expensive than other heads discussed here. This is why flat heads are mainly used for holding tanks or storage tanks that store materials at no pressure. These heads are also recommended in applications where lower head heights than ASME F&D heads are needed.
- Hemispherical Heads: These heads have a radial geometry, where the head depth is half the diameter. These storage heads feature two pressure heads that are placed back to back, thereby creating a storage sphere, which efficiently stores materials at high pressure. Hemispherical heads feature thinner heads than shell and use a standard code 3:1 that taper during the transition. Normally, hemispherical heads are formed using welded sheets rather than flat sheets used in other head types mentioned here. This makes it the thinnest head, and also one of the most expensive types. These heads are recommended for applications that demand high-pressure storage or large diameter vessels.
The above information will help you understand the specific advantages of different head types, and their applications. If you are unable to make a decision, it is always better to approach a manufacturer like BEPeterson, who has been providing pressure vessels in different head types over the years. The company regularly provides high pressure tanks or ASME code and non code pressure vessels with F&D heads and SE heads.