When selecting the right valve for industrial applications, understanding pressure and temperature ratings is crucial.
Many professionals in the field struggle with terms like Class, Rating, and PN, which can lead to costly mistakes if misunderstood.
This guide will help you understand these concepts so you can select valves with confidence.
Let’s start with the basics and understand the common pressure units you’ll encounter when working with industrial valves.
PSI stands for Pounds per Square Inch, the most widely used pressure measurement unit in English-speaking countries. It’s a fundamental unit that measures force applied over a square inch of area.
For reference, 3000 PSI equals approximately 206 bar, while 6000 PSI is around 413 bar. These conversions are essential when working with international specifications.
PSIG refers to Pound per Square Inch Gauge, representing pressure relative to atmospheric pressure as read on a pressure gauge. The “G” indicates we’re measuring the difference between internal pressure and ambient atmospheric pressure.
When you measure a car tire’s pressure, you’re reading PSIG – the difference between air inside the tire and outside air. At sea level, absolute air pressure is approximately 14.7 psi, so a tire pressure of 30 PSIG means the absolute pressure inside is 44.7 psia (pounds per square inch, absolute).
While the International System (SI) uses pascal as the official pressure unit, bar is more commonly used in European industrial settings. One bar equals 100,000 Pascals or 100 Kilopascals.
The conversion between bar and psi is straightforward: 1 bar = 14.5 psi. This conversion is crucial when dealing with equipment from different regions.
The pressure a valve can withstand depends on multiple factors, including construction material and operating temperature. Various standards and nomenclatures exist to clarify these specifications.
ANSI Class is primarily used for flanged valves, defining the maximum pressure the valve can withstand at various temperatures. The primary pressure classes include 125, 150, 250, 300, 400, 600, 900, 1500, and 2500.
Many professionals use abbreviations like LBS, LB, CL, or # interchangeably when referring to these classes. Historically, the 1973 nomenclature used terms like 150LBS, but modern standards prefer CL (for example, CL 150).
It’s vital to distinguish between PSI (pressure unit) and ANSI Class. A working pressure of 150 PSI (10 bar) is not the same as CL 150 (which can handle approximately 20 bar at ambient temperature).
Nominal Pressure (PN) is primarily used in European standards including DIN, EN, BS, and ISO. This classification directly indicates the valve’s operating pressure in bar.
For example, a PN250 valve works at 250 bar pressure (approximately 3,625 psi) and doesn’t correspond to ANSI Class 250. This distinction is crucial when sourcing valves internationally.
WOG stands for Water, Oil, Gas and represents a pressure rating for valves at ambient temperature (typically -29°C to 38°C). According to the MSS-SP-25 standard, WOG is synonymous with CWP (Cold Working Pressure).
The measurement is straightforward: 600 WOG equals 600 psi at room temperature. Remember not to confuse this with Class ratings. For example, 2000 WOG equals 2000 PSI, which is approximately equivalent to CL 800 or PN 140.
MAWP refers to the highest pressure at which a valve is rated to operate at a specific temperature. Derived from the ASME Pressure Vessel Code, this is not the burst pressure but rather the safe operational maximum.
MAWP must always be equal to or greater than the design pressure, and units are always indicated (e.g., MAWP 1500 PSI).
Understanding how European and American standards correlate is essential when working with international equipment. The API 6D standard provides equivalences between PN and Class ratings.
| PN (EN 1092-1/ISO-7268) | Class (ASME B16.34) |
| PN 20 | CL 150 |
| PN 50 | CL 300 |
| PN 64 | CL 400 |
| PN 100 | CL 600 |
| PN 150 | CL 900 |
| PN 250 | CL 1500 |
| PN 420 | CL 2500 |
Outside the API 6D standard, several commercially used equivalences exist:
| PN | Class |
| PN 140 | CL 800 |
| PN 690 | CL 4000 |
| PN 750 | CL 4500 |
Class 800 is listed in API 602 and is equivalent to approximately 132.4 bar or 1920 psi pressure. It serves as an intermediate class between 600 and 900, often rounded to PN140 or 2000 PSI.
A crucial point many overlook is that valve pressure ratings change with temperature. The same valve can withstand different pressures depending on its operating temperature and construction material.
For example, a Class 1500 valve made from 316 stainless steel can withstand 248 bar at 25°C, but only 166 bar at 250°C. The same class valve made from Carbon Steel A105 can handle 255 bar at 25°C and 209 bar at 250°C.
This is why you must consult material-specific pressure-temperature tables to determine the appropriate valve class for your application conditions.
Several terms describe different aspects of pressure in valve applications, and understanding these distinctions is important for proper selection.
This is the pressure at which equipment works under normal conditions. It represents the everyday pressure the valve will experience in your system.
Design pressure equals operating pressure plus any excess pressure that may occur during operation. This accounts for pressure fluctuations and provides a safety margin.
As mentioned earlier, this is the maximum pressure at which the manufacturer indicates the equipment can safely operate. It should never be exceeded during normal operations.
This is the pressure at which manufacturers test equipment without leakage (or with an agreed leakage coefficient). These tests typically run for short durations.
The pressure at which equipment suffers irreversible damage. This is rarely tested to actual rupture and represents a catastrophic failure point that should never be approached in practice.
The difference between PSI, Class ratings, and PN values might seem like engineering minutiae,but these distinctions determine whether your industrial systems operate safely or fail catastrophically.
At Lianke Valve, we’ve seen firsthand how proper valve selection based on accurate pressure-temperature understanding prevents downtime and extends equipment life. Since 1982, our journey from a small machinery plant to specialists in pinch valves and fluorine-lined products has taught us one truth: in industrial valve applications, precision matters.
Dictionary of pressure valves. Let us clarify the difference between class, rating, and PN at once.
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