Frequently it is difficult to decide if a unit being incorporated into a piping system should be classed as piping or as a pressure vessel. No precise differentiation exists; hence the classification must be left to engineering judgment.
The unit should be called piping, however, if:
1. As a part of the piping system, its primary function is to transport fluid from one location to another within the system. A header or manifold for the distribution of fluid would fall into this category. Special design features or accessories added to permit secondary functions would not change its classification. For example, enlargement of any part or all of a header to provide a degree of pulsation dampening, or to accumulate and remove liquid from a gas in connection with its primary function, would not, in itself, classify the piping system or any part of it as a pressure vessel.
2. The element under consideration is available from, and is classified by recognized piping equipment suppliers as a piping component or accessory. Examples would include certain types of strainers, filters, steam traps, steam separators, expansion joints, and metering devices. Units which are normally constructed in accordance with the Code, however, should not be included in this category, but classified as pressure vessels.
Even if fabricated exclusively from pipe and fittings, a unit other than a commercial piping accessory should be classified as a pressure vessel, if:
1. Its primary purpose is not to transport fluid, but to process fluids by distillation, heat exchange, separation of fluids, or removal of solids.
2. Its primary function is to store fluids under pressure.
Pressure vessels normally contain various internal components that are attached directly to a vessel’s shell, such as distributor trays, catalyst support grids, baffles, and demister pads, etc. These internal components apply loads to the shell, and thereby develop stresses that must be added to those resulting from the internal pressure. The weight of the internal components plus the weight of liquid or catalyst supported by the component must be considered. In addition, the pressure drop across the component will apply an additional load to the shell that must be considered separately from the influence of the pressure drop on the design pressure.
The internal loads in vertical pressure vessels are usually downward, developing a compressive stress in the vessel’s shell that counteracts the longitudinal tensile stress developed by the internal pressure. Therefore, it is rare that internal loads affect the design of a vertical vessel. An exception could be encountered with an upflow vertical vessel, if a high pressure drop occurs across an internal component. This would develop a tensile stress in the shell that would add to the longitudinal stress developed by the internal pressure. Note that the weight of the internal contents of a vessel (i.e., internal components, catalyst, and fluids, etc.) will affect the design of the vessel’s support, both by directly increasing the compressive stress and indirectly by amplifying the overturning moment in the event of an earthquake.
Lifting lugs must be provided for moving and erecting a pressure vessel. The location of these lugs and the loads that will be applied to them depend on how the vessel will be moved and erected. The details are usually worked out between the fabricator of the vessel and the construction contractor who will be responsible for erection. A rigging diagram should be provided to the fabricator by the construction contractor. The static loads on the lugs resulting from the weight of the vessel are usually multiplied by a factor, depending on the construction contractor’s lifting procedure and experience, to accommodate dynamic loads that can be developed during the lifting.
There are no Code criteria for maximum allowable stresses that can be used for design of the lifting lugs. They are designed to prevent damage to the vessel during installation only, and they have no effect on the integrity and reliability of the vessel during operation.
Other equipment, such as reboilers or valves, occasionally are connected directly to pressure vessel nozzles without providing separate support. The weight of this equipment results in forces and moments on the nozzles that develop local stresses in the vessel’s shell similar to the stresses caused by piping connections. If these stresses are great enough to affect the design of the nozzle and reinforcement of the opening, when added to those attributable to internal pressure, additional support for the attached equipment should be considered.
Platforms and ladders are frequently supported by direct attachment to a pressure vessel. The normal practice is to provide a sufficient number of clips welded to the vessel’s shell such that the local load transmitted to the shell by any individual clip is not great enough to affect the design of the vessel. This may not always be practical for vessels with a low design pressure and a relatively thin shell. For these vessels, the external load can usually be distributed over a larger area to reduce the stresses developed in the shell, by providing a reinforcing pad on the shell for attachment of the clip. However, it is not advisable to use a reinforcing pad if the operating temperature of the vessel will exceed 450°F, and other design approaches should be investigated.
Stresses can be developed in a pressure vessel shell due to forces and moments that result from piping connections to nozzles. The magnitude of these forces and moments applied to a vessel are relatively insignificant and need not be considered for the design of a vessel. This is especially true for vessels designed according to Section VIII, Division 1, where extensive experience has shown that the required Safety Factor of 4 is sufficient to compensate for these relatively small loads without detailed analyses. Heavy equipment attached to nozzles (such as valves and bridles) should be supported to minimize the external loads acting on the nozzle.
The forces and moments attributable to piping connections can be calculated using the computer program CAESAR (see the Piping Manual), if it is suspected that they are high enough to affect the design of the vessel.
External loads applied to a pressure vessel are usually of a local nature. WRC Bulletin 107 provides the almost universally accepted methodology for calculating the stresses that these loads develop in the shell of a vessel, and for determining the effect that they will have upon the design (i.e., the minimum required thicknesses for the affected components of the vessel).
The stresses developed in the vessel’s shell by local external loads must be added to the stresses attributable to the internal pressure. It is important to recognize that they are normally either local primary membrane stresses, or bending stresses. Therefore, as explained in Section 100, the total stress obtained by adding the stresses developed by local external loads to those attributable to the internal pressure is permitted to reach 1.5 times the maximum allowable design stress given for the material of construction in the Code. Consequently, only relatively high external loads are likely to affect the design of a vessel.
The internal process environment that a pressure vessel is exposed to during operation can frequently cause the material to corrode. Therefore, a corrosion allowance must be added to the calculated minimum thicknesses required for each component of the vessel. The Code assigns to the owner/user of the vessel the responsibility for specifying the corrosion allowance. This is necessary to prevent corrosion from reducing the thicknesses below the required minimums during operation. Recommendations for determining corrosion allowances, based on operating experience, are discussed in Section 500.
An alternative to a corrosion allowance, especially when the process environment would result in a very high corrosion rate, is to employ a corrosion-resistant cladding or to apply a corrosion-resistant coating.
External corrosion is rarely significant. Pressure vessels usually either operate at high temperatures that prevent the condensation of moisture, or weather shielding is provided. Therefore, an external corrosion allowance is not usually necessary.
The potential wind and earthquake loadings on a pressure vessel are specific to the geographic location where the vessel is installed. Design parameters for the United States can be obtained from maps published by API. Refer also, to the Civil and Structural Manual. Section 440 of this manual describes how these design parameters are used to determine the maximum potential loads for design of the vessel.
Both wind and earthquake loads create overturning moments that develop longitudinal stresses in the shell of vertical vessels. The weight of the internal contents of a vessel will amplify the overturning moment resulting from earthquake loading, and must be taken into consideration when calculating the longitudinal stresses. It is not necessary to design a pressure vessel for the simultaneous occurrence of maximum wind and earthquake loads.
The longitudinal stresses developed in the vessel’s shell by the wind and earthquake loads must be added to the longitudinal stresses attributable to the internal pressure. However, the longitudinal stress attributable to the internal pressure is nominally one-half of the hoop stress, which is the maximum principal stress that governs the design of the vessel for internal pressure. Furthermore, the combined stresses for wind or earthquake and internal pressure are allowed to exceed by 50% the maximum allowable design stress for the material of construction given in the Code.
Higher stresses are permitted because a vessel is only intermittently subjected to severe wind and earthquake loads. Consequently, wind and earthquake loads will not usually affect the design of a pressure vessel shell. The major exceptions would be vessels designed for low pressures with relatively thin shells and a high weight of internal contents. Wind and earthquake loads can have a very significant effect on the design of the support and anchoring for a vertical vessel.
A minimum permissible temperature is required to be displayed on the nameplate of a Division 2 vessel. Unlike the nameplate for a Division 1 vessel, a coincident pressure is not displayed for the minimum permissible temperature. However, the pressure during startup or shutdown is explicitly not permitted to exceed 25% of the design pressure (defined in the Code as 20% of hydrotest pressure) at temperatures below the minimum permissible temperature.
The minimum permissible temperature can be determined for a Division 2 vessel in the same way the minimum design temperature is determined for a Division 1 vessel, as discussed above. However, the curves in Division 2 concerning CVimpact test requirements for the various materials of construction differ somewhat from those in Division 1. The curves in Division 2 tend to require CV-impact testing at slightly higher temperatures for the same material of construction and component thickness, which is probably related to the greater toughness required to resist brittle fracture at the higher design stresses in Division 2.