Flanges used in plant piping for refineries, petrochemical installations, oil and gas installations are manufactured in accordance with ASME B16.5 or ASME B16.47. ASME B16.5 covers pressure-temperature ratings for pipe flanges and flanged fittings from NPS ½ inches to 24 inches. ASME B16.47 covers pressure-temperature ratings for large diameter steel flanges from NPS 26 inches to NPS 60 inches.
Unlike ASME B16.5 which covers flanges and flanged fittings, the scope of ASME B16.47 is limited to flanges. Whereas ASME B16.5 covers pressure rating designations up to class 2500, the scope of ASME B16.47 is limited to class 900 rating. As per section 303 of ASME B31.3, flanges manufactured to ASME B16.5 and ASME B16.47 standard are deemed to be suitable for the pressure-temperature limits specified in the standards.
Pressure Rating Designation
The available pressure rating designations under ASME B16.5 are: 150, 300, 400, 600, 900, 1500 and 2500. The terminology for pressure rating designation is “Class” followed by the dimensionless numbers stated above. Thus, the designation for pressure-temperature ratings as per ASME B16.5 are as follows:
- Class 150
- Class 300
- Class 400
- Class 600
- Class 900
- Class 1500
- Class 2500
The pressure-temperature ratings as per ASME B16.47 are as follows:
- Class 75
- Class 150
- Class 300
- Class 400
- Class 600
- Class 900
Some engineers may still refer to the legacy terminology 150 pound, 300 pound i.e. the dimensionless number followed by pound. These designations do not refer to the pressure capacity of the flange. The pressure rating designation is related to the working pressure of the flange by equations which are further clarified in article below.
List of Material Specifications Table-1A
The list of material specifications for flanges and flanged fittings are covered in Table-1A of ASME B16.5. Table-1A of ASME B16.5, 2017 edition covers the following three material groups:
Material Group 1.1 to 1.18 (Carbon and Low Alloy Steels)
Material Group 2.1 to 2.12 (Chrome Nickel Steels)
Material Group 3.1 to 3.19 (Non-ferrous Alloys)
Extract of Table-1A from ASME B16.5 is shown below. Note that the first column refers to the material group. Within each material group are several materials listed with their nominal designation and applicable ASTM specification for forgings, castings and plates. The pressure-rating table for the applicable material group is listed in column three of this table.
Pressure Temperature Ratings Table
Extract of Table 2-1.1 applicable for material group 1.1 is provided below. There are forty-nine such tables listed in ASME B16.5 each applicable for the corresponding material group. The units of working gage pressure are in bar and temperatures in degrees Celsius. Appendix II of ASME B16.5 also lists the pressure-temperature ratings using psi units for pressure and degrees Fahrenheit units for temperature. Similar tables are available in ASME B16.47. From table below we can interpret the following:
Class 150 rating flange in ASTM A105 material can withstand 19.6 bar pressure at 38°C
Class 150 rating flange in ASTM A105 material can withstand 13.8 bar pressure at 200°C
Example 1: ASTM A105 flange is required to be used for design pressure of 50 bar and design temperature of 150°C, the following steps should be followed to determine the required pressure rating of the flange.
Step 1: Check Table-1A and find which material group refers to ASTM A105 material. In this case, ASTM A105 can be found in material group 1.1.
Step 2: Check in column-3 the applicable table for pressure temperature rating for this material group. In this case, the applicable table is 2-1.1. Go to this table.
Step 3: The nominal designation and ASTM specification for the applicable materials are listed in top section of Table 2-1.1. The bottom section of the table refers to the temperature and the working pressure for each class. First follow the design temperature in the left column and mark the point where the temperature corresponds to 150°C.
Next move to the right from this point and locate the 50 bar pressure. For class 300 the working pressure is 45.1 bar, hence class 300 rating is not suitable for the design conditions. The next column for class 400 has a working pressure of 60.1 bar, hence this would be a suitable pressure rating for the flange. However, class 400 is not a very commonly used pressure class. The next pressure rating is class 600 which will be the appropriate rating for the design conditions. For temperatures which are between the stated values the pressure values can be interpolated from the tables. Interpolation between class designations is not permitted.
Establishing the Pressure-Temperature Ratings
Appendix A of ASME B16.5 provides the procedures on how the pressure-ratings for ASME B16.5 are derived. The pressure-temperature ratings for only class 300 and higher flanges are established by the following equation:
Pt = (C1S1Pr/8750)
Pr = 8750*[Pt/(C1S1)]
where
C1 = 10 when S1 is expressed in MPa units and the resultant Pt will be in bar units (C1 = 1 when S1 is expressed in psi units and the resultant Pt will be in psi units)
Pr = pressure rating class index. For all designations Class 300 and above, Pr is equal to the class designation (e.g., for Class 300, Pr p 300).
Pt = rated working pressure, bar (psi), for the specified material at temperature T.
S1 = selected stress, MPa (psi) for the specified material at temperature T. The value of S1 is the lesser of 60% of the specified minimum yield strength at 38°C or 60% of the yield strength at temperature T.
Example 1: ASTM A105 flange is required to be used for design pressure of 50 bar and design temperature of 150°C, the following steps should be followed to determine the required pressure rating of the flange.
In this section we will verify the conclusions made in Example 1 by using the above equation.
Pt = 50 bar
C1 = 10
S1 = 133 MPa
Pr = 8750*[50/10*133] = 328 Class
Since the calculated value is above 300 Class, we would have to choose Class 400 or higher flange rating for these design conditions.