What is Pressure-temperature rating?
According to ASME B16.5 para 2.1 Pressure–temperature ratings are maximum allowable working gage pressures in bar (Psig) units at the temperatures in degrees Celsius (F) shown in Tables 2-1.1 through 2-3.17 for the applicable material and class designation. We can say that the pressure rating (or some time called as pressure class) is the maximum pressure ( MAWP) a piping system should be capable of withstanding at given temperature. The pressure rating related to the temperature of piping system; the higher the process temperature, the less pressure can be handled by the piping system. The detail of pressure rating can be seen in Tables II-2-1.1 through II-2-3.17 of ASME B16.5. For intermediate temperatures, linear interpolation is permitted.
The temperature shown for a corresponding pressure rating is the temperature of the pressure-containing shell of the component. In general, this temperature is the same as that of the contained fluid. Use of a pressure rating corresponding to a temperature other than that of the contained fluid is the responsibility of the user, subject to the requirements of applicable codes and regulations.
what is the hydrostatic test pressure?
According to para 2.6 Flanged joints and flanged fittings may be subjected to system hydrostatic tests at a pressure of 1.5 times the 38°C (100°F) rating rounded off to the next higher 1 bar (25 psi) increment. The detail of testing condition, please refer to para 8.2.3 ( test fluid ) and para 8.2.4 ( test duration ).
Group of material
Materials for flanges and flanged fittings are listed in Table 1A with the restriction that plate materials shall be used only for blind flanges and reducing flanges without hubs. The most common material is Flange material A 105 for “normal temperature” which is carbon steel material. For high temperature operation, we can use “Cr-Mo alloy” material such as A182 gr F11 or A182 gr F22.
what is working pressure?
the working pressure is limited by material group, pressure class and working temperature. For example, a welding neck flange A 105 class 150 can be used up to working pressure of 285 Psig when the service fluid temperature is 100 F. If the service fluid temperature increase up to 400 F, then the working pressure should be limited to 200 Psig. The higher the service fluid temperature, the less pressure can be handled by the pipe, fitting and valve body. Please refer to Table II-2-1.1 for Group 1.1. materials.
Here the best practice to find the maximum working pressure:
- Check the material specification. e.g flange material specification is A 105
- Check the material group in table 1A. Flange A 105 is carbon steel material that listed in Group 1.1
- Find pressure-temperature Table II-2 for related material group. for example, if the material is listed under group 1.1 then we must use Table II-2-1.1
- Check the rating class ( 150, 300 to 2500 )
- Check the service fluid temperature. For intermediate temperatures, linear interpolation is permitted.
- Go the right side to find the maximum working pressure.