The suggestion that any refrigerant-containing component is a vessel is too broad a definition, because ANSI/ASHRAE Standard 15–94 excludes tube-type condensers and evaporators from the category of vessels. However, Section 9.11.1, which does apply to condensers, requires that the component shall have an ultimate strength sufficient to withstand three times the design pressure for which it is rated. Furthermore, auxiliaries such as gauges and bodies of valves are not considered vessels. The usual interpretation of Standard 15–94 is that a vessel is a component containing liquid refrigerant (except for the tube-type components described above). Thus, an oil separator is not considered to be a vessel. Standard 15–94 groups some of the components into different categories, for example:
• Components smaller than 152 mm (6 in) diameter with a design pressure less than 103.4 kPa gauge (15 psig) are exempt from the requirements listed below.
• Pressure-containing components smaller than 152 mm (6 in) diameter with a design pressure greater than 103.4 kPa gauge (15 psig) must be listed with an approved testing laboratory or shall meet the requirements of the ANSI/ASME Boiler and Pressure Vessel Code. For a component to be listed, it must have been tested and identified as acceptable by an approved, nationally recognized testing laboratory.
• Vessels larger than 152 mm (6 in) diameter with a design pressure greater than 103.4 kPa gauge (15 psig) must conform to Section VIII of the ANSI/ASME Boiler and Pressure Vessel Code.
ANSI/ASHRAE Standard 15–94 provides specifications for pressure-relief devices on vessels and what their settings should be. A pressure vessel must be equipped with a pressure-relief device if there is any possibility that the vessel could be valved off from another part of the system where a pressure-relief device exists.
A key characteristic of a vessel is its design pressure. The design pressure depends on the refrigerant and whether the vessel is on the high- or low-pressure side of the system. As Table 13.1 shows, the design pressure (as specified in ANSI/ASHRAE Standard 15–94) is the saturation pressure at various maximum temperatures of refrigerant liquid.
The pressures listed in Table 13.1 are the minimum, and designers often request a rounded-off pressure slightly higher than those listed. The fabricator, in turn, must work with standard material thicknesses which, in combination with the vessel diameter, determine the pressure capability of the vessel. Therefore, the vessel from the fabricator may be rated at a pressure slightly higher than requested, or the fabricator may contact the designer to ask whether a slightly lower design pressure than requested could be permitted. The vessel is hydrostatically tested by the fabricator to 1.5 times the design pressure, which provides a simultaneous test for leaks. The test pressure does not mean that the safety factor on the vessel is only 1.5, because the calculated shell thickness is such that the safety factor is usually about 4 or 5.
The welding operations on the vessel are tightly prescribed in order to qualify the vessel, which refers to both the welder and the procedures. For the procedures to be qualified, specific materials must be used for the weld, and the welder must be qualified by passing tests on whatever type of weld is in question (pipe, couplings, shell, heads, etc.).
The buyer of the vessel expects a certification, often called the stamp, which is a stainless steel nameplate with the symbol U (for unfired) within the ASME emblem, as shown in Fig. 13.1a.
The owner and operator of a plant usually want the stamp to be visible, even if the vessel is ultimately insulated. The fabricator can be requested to mount an extension bracket, as in Fig. 13.1b, so that the nameplate is not concealed by the insulation.
Table 13.1 shows that a lower design pressure would be permitted for a lowside vessel. Many designers, nevertheless, specify a high-side design pressure even for low-pressure receivers and constant-pressure receivers, so that if ever there is a need to store liquid during shutdown, the vessels will be pressurerated for this purpose.