Pipes, Valves, and Pipe fittings often utilize a pressure rating, but many times individuals confuse the Class associated with these products to represent the actual pressure rating. Classes, associated with fittings and valves, are dimensionless numbers used to provide guidance when discussing how strong certain pipe fittings are.
You may ask, what is a pressure rating? The pressure rating of a fitting/valve, or its Maximum Allowable Working Pressure (MAWP), is the maximum pressure a fitting/valve can safely operate at for prolonged periods of time. Manufacturers use mathematical calculations, performance testing, and a Factor-of-Safety (FOS) to determine what a fitting/valve's MAWP will be.
Additionally, various Standards have taken into account pipe fittings/valves shapes, dimensions, materials, connection ends, and temperatures to calculate appropriate pressure ratings. The American Society of Mechanical Engineers (ASME) B16 series of Standards has many of the Classes that would be relevant in the PVF (pipe, valve & fitting) industry.
Common ASME B16 Series Standards for the PVF Industry
| ASME Standard | Type of Products | Classes |
|---|---|---|
| ASME B16.1 | Gray Iron Pipe Flanges and Flanged Fittings | Classes 25, 125, and 250 |
| ASME B16.3 | Malleable Iron Threaded Fittings | Classes 150 and 300 |
| ASME B16.4 | Gray Iron Threaded Fittings | Classes 125 and 250 |
| ASME B16.5 | Pipe Flanges and Flanged Fittings, NPS ½ through NPS 24 | Classes 125, 150, 250, 300 |
| ASME B16.11 | Forged Fittings, Socket-Welding and Threaded | Classes 2000, 3000, 6000 |
| ASME B16.15 | Cast Copper Alloy Threaded Fittings | Classes 125 and 250 |
| ASME B16.24 | Cast Copper Alloy Pipe Flanges, Flanged Fittings, and Valves | Classes 150, 300, 600, 900, 1500, and 2500 |
| ASME B16.42 | Ductile Iron Pipe Flanges and Flanged Fittings | Classes 150 and 300 |
All fittings (regardless of type/material) that are identified by a "Class" have a Pressure Rating associated with them. This Pressure is determined by the operating temperature of the piping system, and takes into account how the fitting material reacts to the operating temperature. Often times, we will hear from individuals that "this fitting is Class 300, it must have a 300psi pressure rating." Comments along these lines are not correct, and could potentially lead to serious injuries.
Various standards will have a pressure-temperature table that calls out what the pressure rating of a specific pipe fitting is, at a specific operating temperature. The below example is from ASME B16.15 and governs how Cast Copper Alloy Fittings are rated by class for a given operating temperature.
ASME B16.15 – Table I-1: Cast Copper Alloy Threaded Fittings
| Temperature (°F) | Class 125 (psi) | Class 250 (psi) |
|---|---|---|
| -20 to 150 | 200 | 400 |
| 200 | 190 | 385 |
| 250 | 180 | 365 |
| 300 | 165 | 335 |
| 350 | 150 | 300 |
| 400 | 125 | 250 |
At a system operating temperature of 200° F, a class 125 fitting would have a pressure rating of 190PSI, while a class 250 fitting would have a pressure rating of 385PSI.
As stated above, material and temperature play a major part in determining a fitting/valve's pressure rating. ASME B16.5 Table 1.1-1 (and the subsequent Pressure-Temperature Ratings Tables for other materials) not only calls out the material but can reference how the material is produced (Forging, Casting, or Plate). The material designations for Group 2.2 are shown below.
ASME B16.5 – Table 2-2.2C: Group 2.2 Material Designations
| Nominal Designation | Forgings | Castings | Plates |
|---|---|---|---|
| 16Cr-12Ni-2Mo | A182 Gr. F316 (1) | A351 Gr. CF3M (2) | A240 Gr. 316 (1) |
| 16Cr-12Ni-2Mo | A182 Gr. F316H | A351 Gr. CF8M (1) | A240 Gr. 316H |
| 18Cr-13Ni-3Mo | A182 Gr. F317 (1) | ... | A240 Gr. 317 (1) |
| 19Cr-10Ni-3Mo | ... | A351 Gr. CG8M (3) | ... |
ASME B16.5 – Table 2-2.2C: Group 2.2 Working Pressures
| Temp (°F) | Working Pressures by Classes, psig | ||||||
|---|---|---|---|---|---|---|---|
| Class 150 | Class 300 | Class 400 | Class 600 | Class 900 | Class 1500 | Class 2500 | |
| -20 to 100 | 275 | 720 | 960 | 1,440 | 2,160 | 3,600 | 6,000 |
| 200 | 235 | 620 | 825 | 1,240 | 1,860 | 3,095 | 5,160 |
| 300 | 215 | 560 | 745 | 1,120 | 1,680 | 2,795 | 4,660 |
Once the material and manufacturing process has been determined, the next step is to follow the chart to determine the working pressure at the operating temperature for the class of fitting/valve in question. The below example will walk you through determining a working pressure for a ductile iron (A536 65-45-12) fitting.
ASME B16.42 – Table 3.1-1C: Ductile Iron Pipe Flanges & Flanged Fittings
| Temperature (°F) | Working Pressure, psi | |
|---|---|---|
| Class 150 | Class 300 | |
| -20 to 100 | 250 | 640 |
| 200 | 235 | 600 |
| 300 | 215 | 565 |
| 400 | 200 | 525 |
| 500 (Note 1) | 170 | 495 |
| 600 | 140 | 465 |
| 650 | 125 | 450 |
Note 1: The maximum temperature for ASTM A536 Grade 65-45-12 is 500°F.
ASME B16.42 Table 3.1-1C.
Class: 300
Operating Temperature: 300° F
Following the Table, it shows that in this configuration, a 300 Class Fitting operating at 300° F will have a Working Pressure of 565 psi.
What should become apparent is that almost always the class of a fitting/valve does not signify the working pressure. Multiple factors need to be taken into consideration when determining what pressure a given fitting/valve will perform under for optimal performance. Differences in base material and operating temperature have the largest influence on the working pressure of a fitting, but other factors such as connection method, the standard being specified, what is flowing in the pipe (application) will also play a role.
The below tables from ASME B16.5, and MSS-SP-114 will show the difference between class 150 stainless steel (316) fittings threaded vs socket weld. ASME B16.5 will govern the Socket Weld Flange, while MSS-SP-114 will govern the Threaded Fitting. Even though both fittings are made from Stainless Steel (316), and are designated "Class 150", they have different Pressure Ratings when the two Standards are applied.
The B16.5 socket weld ratings come from the Group 2.2 material table shown earlier, while the threaded ratings come from MSS-SP-114 below. You can browse Merit's stainless steel 150# fittings in the ordering catalog.
MSS SP-114 – Table 1: Class 150 Castings & Class 1000 Wrought Fittings
| Temperature (°F) | Class 150 – Castings (psig) | Class 1000 – Wrought (psig) |
|---|---|---|
| -20 to 150 | 300 | 1000 |
| 200 | 265 | 910 |
| 250 | 225 | 825 |
| 300 | 185 | 735 |
| 350 | 150 (a) | 650 |
| 400 | – | 560 |
| 450 | – | 475 |
| 500 | – | 385 |
| 550 | – | 300 |
Note (a): Permissible for service temperature up to 366°F, reflecting the temperature of saturated steam at 150 psig.
Class 150 Stainless Steel (316) ASME B16.5 Socket Weld Flange at 200° F = 235psi pressure rating
Class 150 Stainless Steel (316) MSS-SP-114 Threaded Fitting at 200° F = 265psi pressure rating
The application for the piping system (what is flowing in the pipes and its intended purpose) will also play a factor in the pressure rating. ASME B31.3 (Process Piping) has guidelines which will limit the established class's pressure-temperature rating depending on what the pipe system is constructed from and what the application parameters call for. This is generally reflected as a firm maximum pressure allowed for a specific fitting or application, or some form a limiting factor (Factor-of-Safety) applied to the results of a Pressure-Temperature Table.
As you can see from the above, a fitting with a class does not guarantee that is the pressure rating, nor is it a "one-size-fits-all" statement to establish a pressure rating. The engineers designing the system must take all of the above factors, and many more that were not discussed, into consideration when choosing the piping components for their design. Having a baseline understanding of what class is and how it needs to be applied to the components of a piping system is a benefit to anyone involved in the PVF community.
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Explore Resource CenterAuthor: Marcus (Mark) Estrella, Director of Product Line Management, Merit Brass Company
This article references the following industry standards and educational resources on pressure class and pressure-temperature ratings.
8/24/2026 10:06:42 AM
