Pressure vessels are comprised of various components with different function, material characteristics and fabrication requirements.
The shell is the primary component that contains the pressure. Pressure vessel shells are welded together to form a structure that has a common rotational axis. Most pressure vessel shells are cylindrical, spherical and conical in shape.
Photo 1: Pressure Vessel Shell
All pressure vessel shells must be closed at the ends by heads (or another shell section). Heads are typically curved rather than flat. Curved configurations are stronger and allow the heads to be thinner, lighter, and less expensive than flat heads. Heads can also be used inside a vessel. Heads are usually categorized by their shapes. Ellipsoidal, hemispherical, torispherical, conical, toriconical and flat are the common types of heads.
Photo 2: Pressure Vessel Head
Figure 1: Types of Heads
A nozzle is a cylindrical component that penetrates the shell or heads of a pressure vessel. The nozzle ends are usually flanged to allow for the necessary connections and to permit easy disassembly for maintenance or access. Nozzles are used for the following applications:
- Attach piping components for flow into or out of the vessel
- Attach instrument connections, (e.g., level gauges, thermowells, or pressure gauges).
- Provide access to the vessel interior at manholes.
- Provide for direct attachment of other equipment items, (e.g., a heat exchanger or mixer).
Nozzles are also sometimes extended into the vessel interior for some applications, such as for inlet flow distribution or to permit the entry of thermo wells.
Photo 3 : Pressure Vessel Nozzle
The type of support that is used depends primarily on the size and orientation of the pressure vessel. In all cases, the pressure vessel support must be adequate for the applied weight, wind, and earthquake loads.
Photo 4 : Pressure Vessel Saddle Support
Tall, vertical, cylindrical pressure vessels are typically supported by skirts. A support skirt is a cylindrical shell section that is welded either to the lower portion of the vessel shell or to the bottom head (for cylindrical vessels). Skirts for spherical vessels are welded to the vessel near the mid-plane of the shell.
The skirt is normally long enough to provide enough flexibility so that radial thermal expansion of the shell does not cause high thermal stresses at its junction with the skirt
Small vertical drums are typically supported on legs that are welded to the lower portion of the shell. The maximum ratio of support leg length to drum diameter is typically 2:1. The number of legs needed depends on the drum size and the loads to be carried. Support legs are also typically used for spherical pressurized storage vessels. The support legs for small vertical drums and spherical pressurized storage vessels may be made from structural steel columns or pipe sections, whichever provides a more efficient design.
Horizontal drums are typically supported at two locations by saddle supports. A saddle support spreads the weight load over a large area of the shell to prevent an excessive local stress in the shell at the support points. The width of the saddle, among other design details, is determined by the specific size and design conditions of the pressure vessel. One saddle support is normally fixed or anchored to its foundation.
The other support is normally free to permit unrestrained longitudinal thermal expansion of the drum. A typical scheme of saddle support is shown on below figure.
Figure 2: A saddle Support
Lugs that are welded to the pressure vessel shell, may also be used to support vertical pressure vessels. The use of lugs is typically limited to vessels of small to medium diameter (1 to 10 ft.) and moderate height-to-diameter ratios in the range of 2:1 to 5:1. Lug supports are often used for vessels of this size that are located above grade within structural steel. The lugs are typically bolted to horizontal structural members to provide stability against overturning loads; however, the bolt holes are often slotted to permit free radial thermal expansion of the drum.
Figure 3: Typical Scheme of Lugs
5. Insulation and Linings
Pressure vessels fabrication might require an insulation material / lining to manage the temperature and to protect against corrosion for special applications. These materials can be insulation blankets, refractory linings, or corrosion-resistant coatings.
Photo 5 : Pressure Vessel Insulation
6. Instrumentation and Controls
Pressure vessels are equipped with instrumentation and control systems to monitor and regulate pressure, temperature, and other process parameters for a safe and efficient running of the operation. The major controls are;
- pressure management
- temperature management
- level measurement
- flow measurement
- safety and alarm systems