Understanding Valve Pressure Ratings: PN vs. Class Explained
Valve Pressure Ratings are a fundamental consideration in the design of any fluid‑handling system because they define the maximum allowable working pressure a valve can sustain at a given temperature. Engineers frequently encounter two parallel systems—PN (Nominal Pressure) used in European/ASIAN standards and Class ratings defined by ASME B16.34—and the similarity in nomenclature often leads to confusion about which value applies at operating temperature. Misinterpreting these ratings can result in under‑specified valves, excessive stress on the pressure boundary, and unexpected leaks or failures.
This article clarifies the origins, definitions, and temperature‑dependent behavior of PN and Class ratings, explains how standards such as EN 1092‑1 and ASME B16.34 derive their values, and provides practical guidance on converting between the two systems. It also examines the influence of body material, design factors, and test pressures on the allowable working envelope, and outlines selection criteria that help purchasers match valve capability to process conditions while avoiding common pitfalls.
By establishing a clear, engineering‑focused framework for comparing PN and Class ratings, the discussion supports safer, more reliable valve selection and reduces the risk of costly downtime caused by pressure‑rating misunderstandings.
Fundamentals of Pressure Rating Systems: PN vs. Class
Pressure rating defines the maximum sustained internal pressure a valve may experience in normal service at a reference temperature. Two dominant systems are used worldwide: the PN (Nominal Pressure) series, rooted in European DIN/EN and Chinese GB standards, and the ASME/ANSI Class series, prevalent in American‑based projects.
| Aspect | PN (DIN/EN/GB) | Class (ASME/ANSI) |
|---|---|---|
| Unit | bar (metric) | psi (imperial) |
| Definition | Fixed reference value at ~20 °C (e.g., PN10 = 10 bar) | Pressure‑temperature relationship; the number does not equal a pressure value |
| Typical flange standard | EN 1092‑1 / GB/T 9119 | ASME B16.5 |
| Common applications | Water supply, HVAC, municipal piping, general industry | Oil & gas, power generation, petrochemical, high‑energy processes |
Because Class ratings vary with temperature, a Class 150 valve allows about 285 psi (≈19–20 bar) at 100 °F, but the allowable pressure drops as temperature rises. PN ratings, by contrast, are specified at ambient temperature and must be derated manually for higher service temperatures using material‑specific curves.
The two systems are not interchangeable; differing flange dimensions, bolt hole patterns, and design standards mean a PN‑rated valve cannot be directly substituted for a Class‑rated counterpart without requalification. Vahid Valves reflects this split in its catalogue: products such as the Metal Wedge Gate Valve are offered in Class 150, while the majority of gate, knife, check, globe and hydrant lines are rated PN 10/16 (or PN 6/10 for knife valves). Engineers should select the rating system that matches the project’s piping standards and then apply the appropriate temperature derating to ensure the valve’s maximum allowable working pressure stays within the nameplate limit.
Understanding PN Ratings: Metric Basis and European Standards
The PN designation comes from European EN/ISO standards and indicates a nominal pressure in bar that a valve can withstand at a reference temperature, usually 20 °C for water services. It is not a measured limit but a rounded value used for flange compatibility; the actual MAWP is derived from the material’s allowable stress at that temperature, applying the safety factors in EN 1092‑1 and ISO 7005‑1.
Understanding these concepts is essential when evaluating Valve Pressure Ratings for system design.
Reference temperature and temperature derating
PN ratings are quoted at the reference temperature because material strength varies with heat. As temperature rises, the allowable stress of ductile iron, carbon steel or stainless steel decreases, so the MAWP must be reduced according to the material‑specific derating curves found in EN 1092‑1 annexes. For example, a valve marked PN 16 may be limited to 12 bar at 120 °C, while at 20 °C it can sustain the full 16 bar.
Relation to bar and test pressures
By definition, 1 PN equals 1 bar of pressure at the reference temperature. Consequently, PN 10 corresponds to 10 bar, PN 16 to 16 bar, and PN 25 to 25 bar. During manufacture, the valve body is hydrostatically tested at 1.5 × PN (e.g., 24 bar for PN 16) and the seat is checked at 1.1 × PN (≈ 17.6 bar for PN 16) to verify integrity, as outlined in ISO 5208.
These principles apply across Vahid’s product range, where items such as the Spring Check Valve PN 10/16 and the Globe Valve PN 10/16 are rated using the same EN‑based PN system, ensuring compatibility with EN 1092‑2 flanges and EN 558 face‑to‑face dimensions.
Understanding Class Ratings: ASME B16.34 and Imperial Basis
The ASME Class system, defined in ASME B16.34, assigns pressure‑rating designations such as Class 150, Class 300, Class 600, etc., to valves, flanges and fittings. Unlike the PN system that works in bar, the Class numbers are index values that reference a pressure‑temperature table; the table gives the maximum allowable working pressure (MAWP) in psi for a given material at a specific temperature. The rating is therefore not a direct pressure unit but a pointer to a derating curve that accounts for loss of material strength as temperature rises.
Temperature Derating in the Class System
As service temperature increases, the allowable pressure decreases according to the material’s allowable stress values from ASME BPVC Section II. For example, a Class 150 valve made of carbon steel may be rated for 285 psi at 100 °F, but only about 185 psi at 500 °F. The designer must consult the B16.34 table for the selected body and trim material to determine the correct MAWP at the actual operating temperature; applying the room‑temperature rating without derating can lead to over‑pressurization and potential failure.
Vahid Valves lists a Metal Wedge Gate Valve with a Class 150 designation in its catalogue, indicating that the valve’s pressure‑temperature rating follows the ASME B16.34 table for the chosen material. When selecting this valve for high‑temperature water or steam service, the engineer should verify the temperature‑dependent MAWP from the table rather than assuming the nominal 150 psi‑equivalent rating applies at all temperatures.
Cross‑Reference Tables: PN to Class Equivalents and Limitations
Pressure‑nominal (PN) ratings and ASME/ANSI class designations both express a valve’s maximum allowable working pressure, but they are rooted in different standard systems and reference conditions. PN values are fixed numbers expressed in bar and are defined at approximately 20 °C ambient temperature; for example, PN10 corresponds to 10 bar and PN16 to 16 bar. In contrast, an ANSI class such as Class 150 does not denote a single pressure value; it represents a pressure‑temperature relationship taken from ASME B16.34 tables. At the reference temperature of about 38 °C (100 °F) a Class 150 valve is rated for roughly 285–290 psi, which is equivalent to 19–20 bar—higher than the PN16 rating at the same temperature.
Because the two systems use different reference temperatures, material allowances, and flange standards (EN 1092‑1 versus ASME B16.5), a direct one‑to‑one conversion does not exist. The table below shows the approximate pressure ranges for common PN ratings and the corresponding Class 150 limits at the ANSI reference temperature, illustrating where the values converge and where they diverge.
| PN rating | Pressure (bar) @ 20 °C | Pressure (psi) @ 20 °C | Class 150 pressure (bar) @ 38 °C | Class 150 pressure (psi) @ 38 °F |
|---|---|---|---|---|
| PN10 | 10 | 145 | 19–20 | 285–290 |
| PN16 | 16 | 232 | 19–20 | 285–290 |
| PN25 | 25 | 363 | 19–20 | 285–290 |
As the table shows, even the highest common PN rating (PN25) exceeds the Class 150 limit only when temperature is low; at elevated temperatures the Class 150 rating drops significantly, while PN ratings remain fixed unless corrected for temperature using material‑specific derating factors. Consequently, engineers must consult the appropriate pressure‑temperature curves for the selected material and standard rather than relying on a simple numeric conversion. Vahid’s catalog lists many valves—such as gate, globe, check, and knife valves—rated PN10 or PN16 for water, wastewater, irrigation, and industrial fluid applications, underscoring the practical use of PN ratings in the regions where the company’s products are specified.
Material Influence on Pressure‑Temperature Ratings
For a valve stamped with a given PN or Class rating, the allowable working pressure varies with service temperature and the material of the pressure‑containing parts. Standards EN 1092‑1 (PN) and ASME B16.34 (Class) give base ratings at a reference temperature (≈20 °C) and then apply temperature‑dependent derating factors that reflect the loss of allowable stress in the selected material.
Vahid’s catalog shows that many PN 10/16 valves (Spring Check, Combination Air, Ball Check) have ductile‑iron bodies (GJS400) to EN 1563, while higher‑class items like Metal Wedge Gate Valve Class 150 use metallic bodies. Spindles are stainless steel (1.4021).
Because ductile iron loses allowable stress faster with temperature than stainless or carbon steel, a PN 16 valve of GJS400 reaches its reduced pressure limit at a lower temperature than an equivalent Class 150 valve whose body is a steel alloy. The stainless‑steel trim does not change the body’s pressure‑temperature capability, but it helps maintain seat integrity and operation at higher temperatures where the body would otherwise be the limiting factor. This consideration is especially important in steam service where temperatures exceed 200 °C.
Engineers selecting a Vahid valve for hot‑water, steam or thermally cycled systems must consult PT‑rating tables for body material. The same PN or Class marking therefore does not guarantee identical pressure capability across product line; the material choice shifts the derating curve and must be matched to process temperature to avoid over‑pressuring the pressure boundary.
Selection Criteria for Engineers and Purchasers
Selection starts with the system’s design pressure and temperature. Understanding Valve Pressure Ratings helps engineers match the valve to the system’s design conditions. The valve’s pressure rating (PN or Class) gives the maximum allowable working pressure at a reference temperature—20 °C for PN, 38 °C for Class. As temperature increases, the allowable pressure drops according to the material’s derating curve, so both values must be checked together. Designers should also consider possible pressure spikes or transient conditions that may exceed the steady‑state rating.
Design Pressure and Temperature
Engineers compare the design pressure to the valve’s rated pressure at the expected temperature. If the temperature exceeds the reference, the effective rating is reduced using the pressure‑temperature tables from EN 1092‑1 or ASME B16.34. This prevents over‑pressuring the body or seat.
Code and Flange Compatibility
PN‑rated valves follow EN 1092‑2 (metric bolt pattern), while Class‑rated valves use ASME B16.5 (inch dimensions). Vahid’s catalog includes PN‑rated items such as the Spring Check Valve PN 10/16 and Class‑rated items like the Metal Wedge Gate Valve Class 150, each with the matching flange drilling. Mixing PN and Class flanges without adapters can misalign bolt holes and compromise sealing.
Material and Cost Considerations
Material affects rating and price. Ductile iron (GJS400) is typical for Vahid’s PN 10/16 valves, suitable for water and non‑corrosive liquids up to 80 °C. Higher Class ratings often require carbon or stainless steel for added strength or corrosion resistance. Although PN valves usually have lower material cost, total cost of ownership—flange compatibility, installation effort, and long‑term maintenance—should be weighed against the purchase price.
Common Misconceptions and Pitfalls
Valve pressure ratings, such as PN10, PN16, and Class 150, are often misunderstood or used interchangeably, leading to incorrect valve selection and potential system failures. One common misconception is that PN16 is equivalent to Class 150. However, these ratings belong to different standard systems and have distinct pressure-temperature relationships.
PN ratings, commonly used in European and Chinese standards, are defined at ambient temperature (around 20°C) and represent a fixed pressure value in bar. For example, PN10 corresponds to 10 bar, while PN16 corresponds to 16 bar. In contrast, ANSI Class ratings, used in American standards, are based on a pressure-temperature relationship and do not directly represent a specific pressure value in psi or bar.
Another pitfall is ignoring temperature derating. As temperature increases, the allowable working pressure of a valve decreases. For instance, a Class 150 valve can handle 285 psi at 100°F but only 185 psi at 500°F. Similarly, PN ratings are affected by temperature, and their pressure capacity decreases as temperature rises.
Mixing metric and imperial flange standards can also lead to compatibility
Engineering Takeaways
When selecting valves, understanding the pressure rating system is crucial for ensuring compatibility and safety. The two primary pressure rating systems are PN (Nominal Pressure) and Class (e.g., Class 150, Class 300). PN ratings are commonly used in Europe and other parts of the world, while Class ratings are predominantly used in North America.
Engineers should consider the system’s operating conditions, including temperature and fluid type, when choosing between PN and Class ratings. It’s essential to consult the manufacturer’s documentation and relevant industry standards, such as API or ASME, to ensure accurate comparisons and proper selection.
Practical cautions include being aware of the differences in pressure ratings between the two systems and avoiding direct conversions, as they are not always equivalent. Additionally, engineers should verify the valve’s material, design, and testing conditions to ensure they meet the system’s requirements.
Frequently Asked Questions
What is the difference between PN and Class pressure ratings?
PN (Nominal Pressure) and Class (e.g., Class 150) are two different pressure rating systems used for valves. PN is commonly used in Europe and other parts of the world, while Class is widely used in North America. Both systems indicate the maximum pressure a valve can withstand, but they are based on different standards and testing procedures. PN ratings are typically defined by European standards such as EN 1092, while Class ratings are defined by American standards like ASME B16.34.
How do I determine the correct pressure rating for my application?
To determine the correct pressure rating for your application, consider factors such as the maximum operating pressure, temperature, and fluid type. Consult with relevant industry standards, such as EN 1092 or ASME B16.34, and ensure the valve’s pressure rating matches or exceeds your application’s requirements. Additionally, consider factors like back pressure, differential pressure, and any potential pressure spikes or surges.
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