In industrial applications, selecting the appropriate high-pressure valves is critical, yet often confusing due to terminology like pressure ratings, ANSI Class, and nominal pressure (PN). These concepts not only determine whether valves can operate safely and reliably but also significantly impact system efficiency and longevity. This article provides an in-depth analysis of these key technical parameters to assist engineers and procurement professionals in making informed decisions.

First, we must clarify several common pressure units and their meanings:

• PSI (Pound per Square Inch): The most common imperial unit for pressure measurement. For example, 3000 PSI equals approximately 206 bar, while 6000 PSI equals about 413 bar.
• PSIG (Pound per Square Inch Gauge): Indicates pressure relative to atmospheric pressure. When measuring car tire pressure, the gauge displays PSIG. Absolute pressure (PSIA) adds atmospheric pressure (approximately 14.7 psi at sea level). Using PSIG and PSIA helps avoid confusion about pressure types.
• Bar: While the International System of Units (SI) uses Pascals (Pa), bar remains widely used in European industrial applications. One bar equals 100,000 Pascals or 100 kilopascals.

Conversion Between Bar and PSI

One bar approximately equals 14.5 psi. Understanding these units and their conversions is fundamental for proper high-pressure valve selection.

The pressure capacity of valves, piping, and pressure equipment depends not only on units but also on materials and operating temperatures. Industrial standards have established several classification systems:

• Rating: Refers to the total pressure a valve body and stem can withstand without leakage. Different manufacturers, standards, and valve types may use varying expressions.
• ANSI Class: Primarily used for flanged valves, indicating the maximum pressure flanges or valves can withstand at different temperatures. Common classes include 125, 150, 250, 300, 400, 600, 900, 1500, and 2500. Note that LBS, LB, CL, and # symbols are often interchangeable for ANSI Class. However, PSI (pressure unit) differs from ANSI Class. For example, 150 PSI working pressure (about 10 bar) differs from CL 150 working pressure (about 20 bar).
• Nominal Pressure (PN): A European standard (DIN, EN, BS, ISO) classification method based on working pressure values, typically expressed in bar. For instance, PN250 valves operate at 250 bar (approximately 3,625 psi), which doesn't correspond to ANSI Class 250.

PN to ANSI Class Conversion

The API 6D standard establishes correspondence between PN and ANSI Class:

Additional commercial classifications include:

Pressure class numbers don't equal actual working pressure values. ANSI Class includes temperature-pressure charts to determine allowable working pressures at specific temperatures. For example:

• A 316 stainless steel CL 1500 valve withstands 248 bar at 25°C but only 166 bar at 250°C.
• An A105 carbon steel valve of the same class handles 255 bar at 25°C but 209 bar at 250°C.

Therefore, valve selection must reference standard temperature-pressure charts and consider actual operating conditions.

• WOG (Water, Oil, Gas): Indicates a valve's pressure rating at ambient temperature. Note that "Gas" refers to vaporized liquids (like steam), not flammable gases. MSS-SP-25 standard considers WOG, WO, GLP, WWP, and W as synonyms for CWP (Cold Working Pressure). For example, 600 WOG equals 600 psi at room temperature.
• MAWP (Maximum Allowable Working Pressure): The highest pressure at which a valve can operate at specified temperatures. Derived from ASME pressure vessel codes, MAWP must equal or exceed design pressure.

• Operating Pressure: Normal working pressure.
• Design Pressure: Operating pressure plus potential overpressure during operation.
• MAWP: Manufacturer-specified maximum working pressure.
• Test Pressure: Pressure applied during manufacturer testing to verify leakage standards.
• Burst Pressure: Pressure causing irreversible damage (typically not tested).

After evaluating all pressure and temperature factors, select a commercial pressure rating slightly exceeding actual requirements. Normal operating conditions should always remain within the valve's rated specifications to ensure safe, reliable performance.

Proper understanding of high-pressure valve technical parameters—including pressure ratings, ANSI Class, and PN—is essential for system safety and efficiency. Valve selection must account for working pressures, temperatures, materials, and relevant standards, ensuring MAWP meets or exceeds design pressure. This comprehensive analysis aims to clarify these technical specifications for informed decision-making.