In valve and piping systems, understanding what DN (Diameter Nominal) and PN (Pressure Nominal) mean is essential for choosing the right valve size and pressure rating. These two specifications define how a valve connects and performs within a pipeline — DN refers to the nominal diameter that determines the valve’s physical fit, while PN indicates the pressure class that defines its strength and safe operating limits. Selecting valves with the correct DN and PN ensures proper compatibility, leak-free connections, and long-term system reliability, making them key factors in industrial, water, and HVAC applications.

 

What Does DN Mean in Valves?

In valve and piping parlance, DN stands for Diameter Nominal (sometimes called Nominal Diameter). It is a simplified size designation, widely adopted in metric systems, that provides a conventional reference for the “size” of a valve or pipe. According to ISO 6708, DN is an alphanumeric label—not a precise measurement—that indirectly relates to the bore or connection diameter.

Nominal Diameter vs Actual Diameter

One crucial point to understand is that DN does not equal the true internal diameter (ID) or external diameter (OD). Rather, it is a rounded or standardised number used for compatibility and reference. In practice, the actual internal diameter may vary depending on wall thickness, manufacturing tolerances, and pressure class.

For example, a valve marked DN 50 might have an internal bore slightly above or sometimes even below 50 mm. The actual bore could be 48–52 mm, depending on design and pressure requirements. In thin-walled pipe situations, the DN value often more closely approximates the internal diameter, but deviations still exist.

Because DN is a “nominal” figure, you’ll sometimes see it described as a “designation size” or “reference size,” rather than a precise dimension.

 

Common DN Sizes and Units

DN values are expressed in millimetres, but the “DN” label itself is dimensionless (you don’t say “DN 50 mm,” only “DN 50”). Common DN sizes range from very small (e.g. DN 10, DN 15) up to very large valves in industrial systems (e.g. DN 1000 or more) depending on the standard set.

In many standards, the series of DN values follows a logical progression or preferred-number series to avoid an excessive number of intermediate sizes.

To give you an idea of typical correspondences:

• DN 50 often maps roughly to a pipe with outer diameter around 60.3 mm (for certain pipe standards).

• DN 100 might correspond to roughly 114.3 mm OD in some systems.

Valves, flanges and fittings are usually manufactured to match these standard DN values to facilitate interchangeability.

 

Role of DN in Valve and Pipe Compatibility

The most immediate role of DN is to ensure compatibility: valves, flanges, pipes and fittings with the same DN are (in principle) designed to mate without mismatch. In other words, a valve rated DN 80 is intended to be interchangeable with other system components labelled DN 80, regardless of manufacturer.

However, compatibility is not solely about DN. Even if two items share the same DN, they might differ in wall thickness, pressure rating, flange drilling pattern, or end connection type. That is why DN must be considered together with other specifications (such as PN) when selecting valves.

By using DN as a standardised reference, engineers and specifiers can quickly filter options, cross-reference components, and verify that a proposed valve will fit within the intended piping layout. In short, DN gives a shared “language” of size within metric systems—even though the actual internal dimensions will always depend on more detailed design considerations.

 

What Does PN Mean in Valves?

In valve parlance, PN stands for Pressure Nominal (sometimes also called Nominal Pressure). It is a standardised label used across metric and European systems to indicate the pressure rating of a valve, flange, pipe or fitting under reference conditions. The “PN” designation gives engineers an easy shorthand for the pressure class a component is manufactured to withstand.

Understanding Nominal Pressure Rating

When a valve is marked with, say, PN 16, that means it is designed to safely withstand a nominal internal pressure equivalent to 16 bar under standard conditions (typically at or near 20 °C). That said, PN is not necessarily the actual working pressure at all temperatures or for all media. The “nominal” in PN means it is a reference (or classification) value used for design, compatibility checks, and comparison.

Because materials behave differently under heat, the actual allowable working pressure at elevated temperatures is often lower than the nominal pressure. Hence, while PN refers to a maximum reference pressure, engineers must also consider derating when temperatures exceed the standard reference point. For many valves, data sheets provide charts showing allowable pressure versus temperature. For instance, a PN16 valve might only be rated for around 11 bar at 200 °C, depending on its material construction.

 

Units for PN: Bar, MPa and More

The PN number is dimensionless in itself—it acts as a label (e.g. PN 10, PN 25, PN 100)—but the underlying pressure is most often expressed in bar. So PN 10 implies a nominal pressure of 10 bar under reference conditions. In technical documentation it is sometimes converted to MPa (megapascal), where 1 bar = 0.1 MPa. For example, a PN16 valve corresponds roughly to 1.6 MPa under standard conditions.

In European or DIN/EN standards, PN is the usual designation. In American or ANSI systems, pressure classes (e.g. Class 150, Class 300) are more common. These systems are not directly identical, but there are commonly accepted equivalences (for example, Class 150 is often roughly comparable to PN 20) depending on temperature and material conditions.

 

How PN Reflects Maximum Operational Pressure at Standard Conditions

By definition, PN indicates the maximum internal pressure a valve can handle safely under standard (reference) conditions—typically around 20 °C. It assumes ideal material strength and standard fluid behaviour. However, as operational conditions change—especially temperature—the material’s strength and allowable pressure change too, so the valve’s safe working pressure can decrease relative to the nominal rating.

Because of this, valve manufacturers often publish pressure-temperature derating charts, which show how a valve’s allowable pressure reduces at higher temperatures. Engineers must consult these charts rather than relying solely on the PN label when specifying valves for hot or otherwise demanding services.

 

Typical PN Values and Pressure Classifications

In practice, valves and piping components are manufactured in discrete PN classes. Some commonly encountered classes are PN 6, PN 10, PN 16, PN 25, PN 40, PN 64, PN 100 and beyond. These reflect standardised steps in pressure capacity to simplify selection and compatibility.

These PN steps often follow a series (for example, a Renard-type series) to limit the proliferation of intermediate ratings.

It is also noteworthy that PN is a common denominator for matching components: typically only elements (valves, flanges, pipes) with the same DN and the same PN are considered reliably compatible in standard designs.

Finally, when comparing with ANSI/ASME classes, there are approximate equivalences between PN and Class ratings (e.g. PN 20 ≈ Class 150, PN 50 ≈ Class 300) — but these must be used cautiously, always checking temperature and material constraints.

 

Difference Between DN and PN

Although DN and PN often appear side by side on valves and pipes, they represent two very different physical concepts. DN is all about size; it is a nominal diameter designation. PN, on the other hand, is about strength—it’s a nominal pressure rating. When you pair the two correctly, you ensure that the valve both fits and performs under expected conditions.

DN establishes what bore or “size class” the valve belongs to, providing compatibility with pipes, flanges and fittings in the same nominal size series. But a valve that fits in diameter may still fail if its pressure rating is too low. That’s where PN comes in: it sets a benchmark for how much internal pressure the valve can reliably withstand under standard conditions (typically at ~ 20 °C). So while DN ensures it “connects,” PN ensures it “holds.”

They are both essential. If you pick a valve with the correct DN but an insufficient PN, under high pressure the valve may leak, deform or burst. Conversely, choosing a valve with a very high PN but mismatched DN does nothing if you cannot physically connect it to your pipeline. Thus DN and PN must be considered together during valve specification.

In practice, DN and PN work hand in hand. When you design or procure a valve, you first identify the required bore size (DN) to match the piping system. Then, knowing your operating conditions (pressure, temperature, medium), you choose a valve whose PN rating meets or exceeds that requirement (after applying any derating factors). Only valves sharing both the correct DN and sufficient PN are deemed reliably compatible and safe for use.

 

How DN and PN Affect Valve Performance and Safety

The DN and PN specifications are not just labels — they play a direct role in how a valve behaves in a system and how safe that system remains under stress. When DN is undersized relative to the required flow, the fluid velocity rises, which increases frictional losses, turbulence, erosion and pressure drop. Such conditions can degrade efficiency, increase wear and even lead to flow instability or cavitation. Conversely, if DN is oversized, the valve may struggle to regulate flow and may become unnecessarily expensive or bulky.

On the pressure side, PN sets the benchmark for how much internal pressure the valve can tolerate under reference conditions. Operating a valve above or close to its PN rating—especially at elevated temperature—can overstress the material, leading to deformation, fatigue cracking or outright failure. That’s why derating curves are essential: they show how much you must reduce allowable pressure at higher temperatures or under aggressive media.

Together, DN and PN define a “safe envelope” for valve performance. If you mismatch them—for example, pairing a low PN valve with a high-pressure system or a small DN valve with a high-flow demand—you risk leaks, ruptures or catastrophic failure. In high-risk industries like oil, gas or chemical plants, such mistakes can lead to safety incidents, downtime or environmental harm.

 

Examples of DN and PN on Valve Labels

When you inspect valves in the field or in a catalogue, you’ll often see combinations such as DN100 PN25 stamped or cast on the body. These labels are not decorative — they immediately communicate two essential parameters: nominal diameter and nominal pressure. Let’s break down what these labels mean in practice, and how to interpret them correctly.

Sample Label: “DN50 PN16”

Imagine a valve body engraved or cast with “DN50 PN16”. The DN50 means the valve belongs to the nominal diameter class 50 mm. That is to say, the internal bore is intended to match other DN50 piping components, although the true internal diameter may be slightly different (depending on wall thickness or design).

The PN16 part of the label means the valve is rated for 16 bar under standard reference conditions (typically at or near 20 °C). In many engineering systems, a PN16 rating indicates a safe working pressure of 16 bar under ambient conditions, though as temperature or media change, the limit may be derated.

Thus, the full label “DN50 PN16” conveys that this valve is sized for a 50 mm nominal pipeline and rated to handle 16 bar at reference conditions. This is one of the more common combinations in water, heating, and general industrial systems.

 

Standards and Industry Norms for DN and PN

When you look into valve specifications across the globe, you’ll find that DN and PN don’t exist in a vacuum — they are defined and constrained by international and regional standards. This ensures compatibility, safety and predictability when valves, pipes and flanges are designed, manufactured and installed.

International & European Standards Governing DN and PN

The ISO 6708 standard is key in defining nominal diameters in piping systems. It specifies how the “DN” designation is to be used for pipework components, treating it as a dimensionless nominal size—not a precise measurement.

On the pressure side, European norms under the EN series (such as EN 1333) define discrete steps for PN values (PN 2.5, PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100, PN 160, etc.). These steps align with pressure-temperature designations and ensure a shared scale across manufacturers.

For valves themselves (especially flanged metal valves), standards like EN 558 (which sets face-to-face / end-to-end dimensions) and EN 1092-1 (on flange dimensions and matching) are widely used to guarantee that a valve labelled as DN X, PN Y will physically align with piping systems built to EN flange standards.

In the realm of steel ball valves, DIN EN 1983 prescribes DN ranges (e.g. DN 4, 6, 8 … up to DN 900) and supported PN / Class ranges (e.g. PN 6, 10, 16, 25, 40, 63, 100, and ANSI Class 150, 300 etc.).

Thus, across much of Europe and markets that adopt ISO/EN standards, DN and PN are not arbitrary values — they are tightly bounded by these normative frameworks.

 

Regional & Industry Variations in Standards

While ISO/EN dominates in Europe, many other regions or industries adopt or mix other standard systems, leading to variations and sometimes confusion.

In North America, ANSI / ASME standards prevail. Here, pressure ratings are often expressed in Class (e.g. Class 150, Class 300), which do not correspond one-to-one with PN values. Many specifiers use equivalence tables (e.g. Class 150 ≈ PN 20 under certain materials/temperature conditions), but those equivalences must be used cautiously and always validated against pressure-temperature charts.

Valve standards under ANSI / ASME include B16.34 (design, materials, pressure-temperature ratings), B16.10 (face-to-face dimensions), and B16.5 (flanges) — so valves built to these standards will follow different dimensional and pressure rating rules than EN valves.

In some industries — for example oil & gas, petrochemicals or power plants — API standards or sector-specific norms may impose additional constraints (e.g. higher temperature classes, special materials, more stringent test regimes). In those cases, valve suppliers often publish both PN/ISO and API/ANSI rating versions to satisfy cross-market demand.

Furthermore, in certain regions (especially where legacy systems are common), designers might encounter a mixture of metric (DN/PN) and imperial (NPS/Class) standards in the same plant, which makes understanding equivalents and tolerances critical.

 

How to Choose the Right DN and PN for Your Application

Choosing the correct DN and PN is not a matter of guesswork — it requires a holistic view of your system’s demands. Here’s how to proceed, with key factors and tips to guide your valve selection.

Factors Influencing Size and Pressure Rating Choice

First, the fluid or gas (“media”) that your system carries plays a central role. If the medium is corrosive, high in solids or abrasive, or prone to foaming, that may require thicker walls or special lining, which in turn affects what DN or PN is feasible.

Temperature is another critical factor. As the operating temperature rises, material strength often falls, so you may need a valve with a higher nominal pressure (PN) even for the same nominal size. For example, a valve rated at PN16 at 20 °C might only be safely usable at a lower pressure at 150 °C, depending on material.

Pressure dynamics in the system — not just the steady-state pressure — must also be accounted for. Transients, water hammer, vibration or pulsation can push pressures beyond the normal working point. Hence you often apply a safety margin or choose a valve with a PN rating above the nominal operating pressure.

Additionally, the flow rate and pressure drop constraints matter. If you choose a DN that is too small relative to desired flow, it increases the velocity and pressure loss across the valve. In contrast, an oversized DN may degrade control precision or inflate costs unnecessarily.

Material compatibility is also key. If the material selected for valve body, seals or lining must resist corrosion, erosion or chemical attack, this might force a design with thicker walls or different geometries, which impacts the nominal diameter or pressure class you can reliably achieve.

 

Considerations Based on Media, Temperature, and Pressure Conditions

When dealing with hot or high-temperature media, you must consult derating curves provided by manufacturers. These charts show the maximum allowable pressure at various temperatures. A valve that is safe at its nominal PN in ambient conditions might need to be de-rated for hotter environments.

If the medium is abrasive or contains suspended solids (e.g. slurries, particulates), wear on internal surfaces can reduce wall thickness over time, so choosing a more conservative PN class or more robust materials can extend service life.

For corrosive or chemically aggressive media, select valve materials (body, seats, coatings) with sufficient resistance. This sometimes means thicker sections or special alloys, which can influence what DN or PN classes are practical.

In pressure pulsating systems (like pumps or reciprocating compressors), account for cyclic stress. A valve operating near its maximum pressure repeatedly might fatigue more rapidly. Using a higher PN rating and proper materials helps mitigate that risk.

 

Tips for Selecting Valves with Proper DN and PN Ratings

• Start from system requirements — determine the operating flow, pressure, and temperature, then infer a conservative design pressure (with safety margin) and required flow path size.

• Refer to manufacturer data — review pressure-temperature derating charts for the specific valve materials and construction. The nominal PN is a starting point, not a guarantee under all conditions.

• Allow margin for transients — don’t spec the valve exactly at expected pressure; choose a PN class above that to tolerate surges or occasional deviations.

• Verify material compatibility — ensure the valve’s body, trim, seats and seals are suited for the media and temperatures you expect.

• Ensure matching DN across components — the valve’s DN must match or appropriately adapt to the adjoining piping, flanges or fittings to avoid mismatches or flow disturbances.

• Consult with suppliers — when your application is unusual (very high temperature, aggressive chemistries, pulsating pressure) bring your system parameters to the valve manufacturer and ask for bespoke solutions or recommendations.

 

 

Conclusion

In summary, DN  and PN  are two of the most important parameters in valve specification, defining a valve’s size and pressure rating. Correctly matching DN ensures proper fit within a piping system, while selecting the right PN guarantees safe performance under operating pressures and temperatures. Choosing the wrong combination can lead to leaks, reduced efficiency, or even system failure. By understanding what DN and PN mean and applying them correctly during valve selection, engineers and plant operators can ensure long-term system reliability, safety, and performance across all industrial and fluid control applications.