Many engineers assume that if a valve is rated PN16, it’s safe for any system running below 16 bar—but that’s a costly mistake. In reality, PN rating affects valve selection far beyond static pressure numbers, especially when real-world conditions like high temperature, fluid surges, or corrosive media come into play. Misunderstanding PN ratings can lead to leaks, damaged equipment, or complete system failure. This article breaks down what PN ratings truly mean, why they matter, and how to choose valves that perform safely under actual working conditions.

 

What PN Rating REALLY Means (Industry Truths)

The 20°C Benchmark: 

A PN (Pressure Nominal) rating, such as PN16, indicates the maximum allowable working pressure of a valve at a reference temperature of 20°C. However, as operating temperatures rise, the material strength of the valve components can decrease, leading to a lower maximum allowable pressure. For instance, a PN16 valve made from certain materials may only be rated for approximately 11.2 bar at 200°C.

This temperature-dependent derating is crucial in applications involving elevated temperatures, such as steam systems or high-temperature chemical processes. Failing to account for temperature effects can result in selecting valves that are not adequately rated for the actual operating conditions, leading to potential system failures.

Material Matters: 

The material composition of a valve significantly influences its pressure-temperature performance. For example, cast iron valves generally have lower pressure and temperature ratings compared to those made from stainless steel. At 200°C, a PN16 cast iron valve may be rated for a lower pressure than a PN16 stainless steel valve, which can maintain its full 16 bar rating up to 200°C.

Understanding the material-specific pressure-temperature ratings is essential for selecting the appropriate valve for a given application. Using a valve made from a material not suited for the operating conditions can compromise system integrity and safety.

PN vs. MAWP: 

While PN ratings provide a standardized pressure classification at a reference temperature, the Maximum Allowable Working Pressure (MAWP) is the highest pressure at which a valve is rated to operate at a specific temperature, as defined by the ASME Pressure Vessel Code.

MAWP accounts for the material strength, design, and operating temperature, offering a more precise measure of a valve’s pressure-handling capability under actual service conditions. Relying solely on PN ratings without considering MAWP can lead to selecting valves that are not adequately rated for the intended application, especially in systems with varying temperatures and pressures.

 

The 2 Silent Killers of Valve Pressure Capacity

Temperature Trap: How Heat Drains Pressure Tolerance

Temperature significantly impacts a valve’s pressure-handling capacity. As temperatures rise, the strength of valve materials diminishes, reducing their ability to withstand pressure.

Material Derating Examples:

• Cast Iron:

  • At 100°C (212°F), the pressure rating can decrease to approximately 70% of its value at 20°C (68°F).

• Steel Pipe:

  • Design pressure must be adjusted using a temperature derating factor (T) as specified in regulations.

Real-World Example:

A PN16 valve, rated for 16 bar at 20°C, may only be safely operated at approximately 11.2 bar when the temperature reaches 100°C. This reduction underscores the importance of considering temperature effects in valve selection.

 

Hammer Effect: When Water Shock Destroys Systems

Sudden changes in fluid flow can cause pressure surges known as water hammer or hydraulic shock. These surges can be several times greater than the system’s normal operating pressure, leading to catastrophic failures.

Understanding Water Hammer:

• Occurs when a fluid in motion is forced to stop or change direction suddenly, such as when a valve closes quickly.

• The abrupt change creates a pressure wave that travels through the pipeline, potentially causing damage.

Data Highlight:

Pressure spikes from water hammer can reach 3 to 5 times the system’s normal pressure. For instance, in a system operating at 15 bar, surges could spike to 45–75 bar, exceeding the design limits of standard PN16 valves.

Case Study:

In a paper mill, unmanaged water hammer caused by the sudden closure of a swing check valve led to pressure waves that damaged equipment, including pumps and boilers. The pressure surges also stressed pipes and valves, resulting in cracks and failures. This incident highlights the destructive potential of water hammer when not properly mitigated.

 

5 Critical Factors for Safe PN Selection

Selecting the appropriate PN (Pressure Nominal) rating for valves is crucial for system safety and longevity. Beyond matching the PN rating to the system’s operating pressure, several factors must be considered to ensure optimal performance and prevent failures.

1. Operating Temperature vs. Material Limits

Valve materials have specific temperature thresholds beyond which their pressure-handling capabilities diminish. For instance:

• Cast Iron: At 100°C, cast iron valves may experience a reduction in pressure capacity to approximately 70% of their rating at 20°C.

• Steel: Steel pipes also exhibit decreased strength at elevated temperatures, necessitating derating factors to adjust their pressure ratings accordingly. 

2. Fluid Type: Water vs. Steam vs. Corrosive Media

The nature of the fluid flowing through the valve significantly impacts material selection and PN rating:

• Water: Generally compatible with a wide range of materials, but considerations for temperature and potential contaminants are necessary.

• Steam: Requires materials that can withstand high temperatures and pressures; improper selection can lead to rapid degradation.

• Corrosive Media: Fluids like acids or alkalis necessitate corrosion-resistant materials such as stainless steel or specialized alloys.

3. System Dynamics: Pump Controls & Valve Actuation Speed

Rapid changes in flow, such as sudden pump starts or quick valve closures, can cause pressure surges known as water hammer. These surges can exceed the valve’s PN rating, leading to potential damage.

• Mitigation Strategies:

  • Implementing slow-opening and slow-closing valves.

  • Using surge anticipation valves to absorb pressure spikes.

  • Designing control systems that manage pump start-up and shutdown sequences effectively.

4. Elevation Changes: Why Gravity Amplifies Pressure

In systems with significant elevation differences, gravity can influence pressure:

• Ascending Flow: Fluid moving uphill experiences pressure losses due to gravitational forces.

• Descending Flow: Fluid moving downhill can gain pressure, potentially exceeding the valve’s PN rating.

5. Safety Margins: Why 1.5x Isn’t Always Enough

While a common practice is to apply a 1.5x safety factor to the system’s operating pressure, this may not suffice in all scenarios:

• Dynamic Systems: Systems subject to frequent pressure fluctuations or surges may require higher safety margins.

• Critical Applications: Industries where valve failure can lead to significant hazards or downtime might necessitate more conservative safety factors.

 

Practical Solutions for Risk Mitigation

Upgrade Tactics: When to Use PN25 Instead of PN16

Selecting a higher PN-rated valve, such as PN25, over a standard PN16, can provide an added safety margin in systems prone to pressure fluctuations, high temperatures, or corrosive media. This upgrade is particularly advisable when:

• Operating Conditions Exceed Standard Ratings: If the system experiences temperatures or pressures that approach or exceed the limits of PN16 valves, upgrading to PN25 can prevent premature failure.

• Presence of Water Hammer: In systems where rapid valve closures or pump startups are common, the resultant pressure surges can exceed the capacity of PN16 valves. PN25 valves offer a higher threshold to withstand these surges.

• Critical Applications: For systems where valve failure can lead to significant safety hazards or operational downtime, the additional robustness of PN25 valves provides a necessary safeguard.

System Design Fixes

Slow-Close Valves vs. Hammer Risks

Implementing slow-closing valves can significantly reduce the risk of water hammer by allowing gradual deceleration of fluid flow, thereby minimizing pressure surges. This approach is particularly effective in systems with frequent valve operations. 

Air Vents and Check Valves for Surge Protection

Installing air release valves and check valves can mitigate pressure surges by:

• Air Release Valves: These valves expel trapped air from pipelines, preventing air pockets that can cause pressure fluctuations.

• Check Valves: They prevent backflow, which can lead to sudden pressure changes and potential damage.

Full-Weld vs. Bolted Connections at Critical Nodes

Choosing between full-weld and bolted connections depends on the application’s criticality and maintenance requirements:

• Full-Weld Connections: Offer superior strength and leak resistance, making them suitable for high-pressure or hazardous systems.

• Bolted Connections: Provide ease of maintenance and flexibility but may be more susceptible to leaks under high-pressure conditions. 

The 3-Step Safety Checklist for Valve Specifiers

1. Assess Operating Conditions: Evaluate the system’s pressure, temperature, and fluid characteristics to determine the appropriate valve specifications.

2. Consider System Dynamics: Analyze factors such as flow velocity, potential for water hammer, and elevation changes that may impact valve performance.

3. Select Appropriate Valve Features: Choose valves with suitable PN ratings, materials, and design features (e.g., slow-closing mechanisms, corrosion resistance) to ensure reliability and safety.

 

Conclusion

PN rating affects valve selection far more than many engineers realize—assuming a PN16 valve can safely handle 16 bar in all conditions is a costly mistake. From temperature-driven derating and material limitations to surge pressures and system dynamics, real-world conditions can push a valve well beyond its nominal limits. Safe valve selection requires more than just matching system pressure to a PN number—it demands a full understanding of fluid type, thermal impact, installation environment, and proper safety margins. Need help selecting the right valve? Contact our engineering team for a custom consultation.