The term ‘pressure vessel’ represents a broad range of different systems, all designed with the express purpose of confining a large amount of fluid to a small space. This definition includes impressive feats of engineering such as nuclear reactors and aeroplanes, to humbler everyday items such as a deodorant canister.
Technologies that rely on the dependable containment of pressurised liquids and gases are nothing new. Nonetheless, the design and manufacturing of pressure vessels presents an interesting engineering challenge: Persuading a fluid to occupy smaller volumes than it ‘naturally’ would often requires using a colossal amount of energy, and as a result, compressed liquids and gases exert a lot of strain on their containers.
Containing the Strain from Pressurised Fluids
The challenge of containing this strain must be met in two ways: firstly, by the application of strong materials (steel’s notorious toughness and relative abundance make it the material of choice for the majority of applications), and secondly by designing the container so that the strain exerted on it by its pressurised contents is minimised.1
Mathematically, the solution to the problem of minimising strain has a simple geometric solution: a sphere. Of course, the engineering solution is not so simple. Spherical pressure vessels are incredibly difficult to make. While NASA may opt to painstakingly manufacture perfectly spherical, carbon fibre cryogenic tanks,2 the majority of applications call for a cheaper, more pragmatic solution. Something that distributes strain reasonably well, and can be manufactured reliably and economically.
The resulting vessel shape is one that anyone who has ever visited a petrol station or used a camping stove will be well accustomed to: a predominantly cylindrical vessel with a convex “head” at each end.
The steel cylindrical pressure vessel meets the demands of the vast majority of pressure vessel applications. But don’t let the familiar appearance fool you – despite being commonplace, these vessels are meticulously designed to favour ease-of-production, while retaining a resilient and robust geometry.
The cylindrical middle section can be easily formed from a rectangular piece of steel, while a lack of abrupt edges ensures strain is distributed well. Although hemispherical heads provide better strain distribution, shallower heads are frequently used instead.3 Known in the industry as “dished” heads, they are an effective compromise between strain minimisation and manufacturability.4 Far easier to produce, these heads can attain the same resistance to pressure by being made slightly thicker.
The Different Geometries of Dished Heads
Dished heads generally have one of two geometries: Torispherical or semi-ellipsoidal. Torispherical heads, (also known as Klopper or Decimal heads) consist of a fixed-radius dish, joined to the cylinder by a toroidal “knuckle”. The relative ease of manufacturing has made torispherical heads the most popular pressure vessel head shape, finding use in petrochemical plants, recompression chambers, distillation towers, and a variety of storage applications.
Semi-elliptical heads are another frequently used option – deeper and more spherical than a torispherical head, they are more difficult and therefore more expensive to produce, but can handle more demanding applications. Semi-elliptical heads are better suited to slightly higher-pressure applications where overall cylinder length is still important
Manufacturing Steel Dished Heads
Manufacturing steel dished heads has two main stages. Firstly, the steel is produced to the correct thickness and cut to shape, usually using numerically controlled plasma cutting machines or industrial circular shears. Once cut to shape, the steel is shaped into a head using either “spinning” or “flanging”. In the spinning approach, the steel is spun on a hydraulic lathe and pressed to a tool.
The tool shapes the steel to the desired head shape and can control both the knuckle and dish radius, allowing the entire head to be produced in one go. Flanging is a two-step process designed to expedite the final assembly of the cylinder: The steel is cold pressed into a shaped cap, then formed with a pressure roller so that it exhibits a straight flange at the point of connection with the cylinder.
Gases and liquids at higher-than-ambient pressures are uncooperative even at the best of times, and extreme caution must be observed in the manufacture of cylindrical vessel heads to ensure resistance to corrosion, and sufficient tensile strength to withstand the pressures of the intended application. The history of pressure vessel development and operation is fraught with accidents– and as a result, pressure vessel manufacture is tightly regulated and adherence to strict guidelines is essential.5,6
The reliable and efficient production of dished heads for pressure vessels requires both an intimate knowledge of the materials involved and the infrastructure to precisely manufacture and distribute them.
Steel Dished Heads Provided by Masteel
Masteel UK is aiming to eliminate the difficulties involved in dished head production by drawing on expertise as a global steel manufacturer. Working with a number of worldwide manufacturers, Masteel oversees all elements of the production of dished heads, from steel production to shipping of the end product. Masteel stocks pressure vessel grade steel that has been stringently tested to meet the requirements of the industry, including testing for resilience to the effects of hydrogen induced cracking (HIC). Masteel is able to produce steel and supply it to a manufacturer faster than normal dished head producers can procure it, enabling the rapid production of dished heads that adheres to exact specifications.
References and Further Reading
- Lei Zhu & J T Boyle 2000. Optimal Shapes for Axisymmetric Pressure Vessels: A Brief Overview. Journal of Pressure Vessel Technology. 122 (443)
- Sourabh Lawate & B. B. Deshmukh. 2015. Analysis of Heads of Pressure Vessel. International Journal of Innovative Research in Science, Engineering and Technology. Vol. 4, Issue 2.