Introduction to Spherical & Cylindrical Pressure Vessels
What are Pressure Vessels?
According to the ASME Boiler and Pressure Vessel Code (BPVC), Code Section VIII :
- A pressure vessels is a container designed to hold gases and liquids at a pressure substantially different from the ambient pressure.
- pressure vessels are containers for the containment of pressure, either internal or external.
- This pressure may be obtained from an external source or by the application of heat from a direct or indirect source as a result of a process, or any combination thereof.
Note : The ASME Boiler & Pressure Vessel Code (BPVC) is an American Society of Mechanical Engineers (ASME) standard that regulates the design and construction of boilers and pressure vessels
The ASME Code is the construction code for pressure vessels and contains mandatory requirements, specific prohibitions, and non-mandatory guidance for pressure vessel materials, design, fabrication, examination, inspection, testing and certification.
Shape of a Pressure Vessel
Pressure vessels can theoretically be almost any shape, but shapes made of sections of spheres, cylinders, and cones are usually employed. A common design is a cylinder with end caps called heads. Head shapes are frequently either hemispherical or dished (torispherical). More complicated shapes have historically been much harder to analyze for safe operation and are usually far more difficult to construct.
Theoretically, a sphere would be the best shape of a pressure vessel. Unhappily, a spherical shape is tough to manufacture, therefore more expensive, so most pressure vessels are cylindrical with 2:1 semi-elliptical heads or end caps on each end. Smaller pressure vessels are assembled from a pipe and two covers. A disadvantage of these vessels is that greater breadths are more expensive.
Uses of Pressure Vessel
Pressure vessels are used in a variety of applications in both industry and the private sector. They appear in these sectors as industrial compressed air receivers and domestic hot water storage tanks. Other examples of pressure vessels are diving cylinders, recompression chambers, distillation towers, autoclaves, and many other vessels in mining operations, oil refineries and petrochemical plants, nuclear reactor vessels, submarine and space ship habitats, pneumatic reservoirs, hydraulic reservoirs under pressure, rail vehicle airbrake reservoirs, road vehicle airbrake reservoirs, and storage vessels for liquified gases such as ammonia, chlorine, propane, butane and LPG.
Spherical Pressure Vessel (Sphere)
This type of vessel is preferred for storage of high pressure fluids. A sphere is a very strong structure. The even distribution of stresses on the sphere’s surfaces, both internally and externally, generally means that there are no weak points. Spheres however, are much more costly to manufacture than cylindrical vessels.
Storage Spheres need ancillary equipment similar to tank storage – e.g. Access manholes, Pressure / Vacuum vent that is set to prevent venting loss from boiling and breathing loss from daily temperature or barometric pressure changes, Access ladders, Earthing points, etc.
Spherical Pressure Vessel
An advantage of spherical storage vessels is, that they have a smaller surface area per unit volume than any other shape of vessel. This means, that the quantity of heat transferred from warmer surroundings to the liquid in the sphere, will be less than that for cylindrical or rectangular storage vessels.
Spherical Tanks
Miro refinery Karlsruhe, Germany
Cylindrical Pressure Vessel
Cylinders are widely used for storage due to their being less expensive to produce than spheres. However, cylinders are not as strong as spheres due to the weak point at each end.
This weakness is reduced by hemispherical or rounded ends being fitted. If the whole cylinder is manufactured from thicker material than a comparable spherical vessel of similar capacity, storage pressure can be similar to that of a sphere.
Lifting and handling of a Pressure vessel
Larger image lifting and handling of a Pressure vessel
Pressure Vessel Heads
Ellipsoidal Head, Hemispherical Head and Torispherical Head are three types of ASME Pressure Vessel Dished Heads.
ELLIPSOIDAL HEAD
This is also called a 2:1 elliptical head. The shape of this head is more economical, because the height of the head is just a quarter of the diameter. Its radius varies between the major and minor axis.
HEMISPHERICAL HEAD
A sphere is the ideal shape for a head, because the pressure in the vessel is divided equally across the surface of the head. The radius (R) of the head equals the radius of the cylindrical part of the vessel.
TORISPHERICAL HEAD
These heads have a dish with a fixed radius (CR), the size of which depends on the type of torispherical head. The transition between the cylinder and the dish is called the knuckle. The knuckle has a toroidal shape.
KLÖPPER HEAD
This is a torispherical head. The dish has a radius that equals the diameter of the cylinder it is attached to. The knuckle has a radius that equals a tenth of the diameter of the cylinder.
KORBBOGEN HEAD
This is a torispherical head also named Semi ellipsoidal head (according to DIN 28013). The radius of the dish is 80% of the diameter of the cylinder (CR = 0.8 x D0.). The radius of the knuckle is (KR = 0.154 x D0).
Flat head
This is a head consisting of a toroidal knuckle connecting to a flat plate. This type of head is typically used for the bottom of cookware.
Pans typically have a bottom in the shape of a flat head
Diffuser head
Typical application of a diffuser head: the bottom of an aerosol spray can
This type of head is often found on the bottom of aerosol spray cans. It is an inverted torispherical head.
Conical head
This is a cone-shaped punkhead.
Note : What is the Difference Between the Design Formula of a Cylindrical Shell and a Spherical Shell?
In the same design condition with the same design pressure, design temperature and material, the thickness you obtain from the internal pressure formula for spherical pressure vessel will be half that of the cylindrical pressure vessel.
The design formula for the cylindrical shell is t = PR/ (SE-0.6P) and for the spherical shell is t = PR/ (2SE-0.2P)
When the “t” is represent Thickness, “R” the Inside Radius,” S “the Allowable Stress, “P” the Design Pressure and “E” the Joint Efficiency;
For example, if your design pressure is 250 psi, inside radius 20 inch. , allowable stress 20,000 psi and joint efficiency 1.
Your thickness for cylindrical shell will be 0.24 inch. or 6.10 mm, and for the spherical shell, it will be 0.125 inch. or 3.175 mm.
What is the Application and Advantage of a Spherical Pressure Vessel?
When you need to make storage for a great amount of pressurized liquid or gas, meaning you need a big volume, then you need a big pressure vessel, so a spherical shell pressure vessel would be more economical.
As you see above, the thickness is half that for a similar design condition. Also placing a long cylindrical pressure vessel might not be suitable in regards to the regarding plant layout.
What is the Disadvantage of a Spherical Pressure Vessel?
Spherical shell pressure vessels are more expensive than cylindrical pressure vessels to fabricate, and this higher price is only justifiable for large vessels.
What is Difference in Fabrication Method Between Cylindrical and Spherical Shell Pressure Vessel?
Cylindrical shell pressure vessels generally are built in the shop and then transferred to the plant field except for long process towers, which might be built in two pieces and completed in the field by one circumferential weld.
This is only because of the facilitating shipping process.
But spherical shell pressure vessels are assembled in the field. The plates generally are formed in the rolling shop and then carefully transferred to the field for assembly.
What is the Difference in the Post Weld Heat Treatment (PWHT) Process?
If PWHT is a code or process requirement, the PWHT for cylindrical shell pressure vessels generally are done in the furnace, and if the vessel is too long, it is done in two heating process with a 5 ft. overlap.
But PWHT for spherical shell pressure vessels are done by one or more high velocity burners that are fired into the vessel using the top or bottom manways (or both) as burner entry and exhaust positions.
This is done because using a PWHT furnace is not possible.
What Kinds of Materials are Stored in a Spherical Pressure Vessel?
Most of the LNG (Liquid Natural Gas) and LPG (Liquid Petroleum Gas) tanks are Spherical Vessels.
Butane, Propane, Ammonia, oxygen, hydrogen and nitrogen also are stored in spherical vessels.