Working pressure 0.8 MPa, temperature 200°C — what pressure rating should be selected?

Key conclusion: Prioritize selecting Class 150 (PN16) Level

At a work stress level of0.8 MPa, the medium temperature is200 ℃Under these operating conditions, conventional carbon steel or stainless steel piping components can generally be selected.Class 150(Corresponding to Chinese national standard PN16 or European standard PN20) pressure rating. At ambient temperature, the rated pressure for this class is approximately 1. 6~2.0 MPa Even in a high-temperature environment of 200°C, although the material strength experiences some degradation, its allowable working pressure still fully meets the design requirement of 0.8 MPa.

Why does temperature affect pressure ratings?

The pressure rating of pipe flanges, valves, and fittings is not fixed; rather, it strictly adheres to“Pressure-Temperature Rating”Curves. As temperature increases, the yield strength and tensile strength of metallic materials gradually decrease. Both international standards and domestic chemical industry standards provide detailed derating charts. Taking commonly used 20# steel or WCB carbon steel as an example, when the temperature rises from ambient to 200°C, the allowable pressure for Class 150 typically decreases by approximately 10%~ 15%, but the remaining load-bearing capacity is still far greater than 0. 8 MPa.

Key Material Selection and Standard Alignment Recommendations

• Carbon Steel Pipe Fittings and ValvesIt is recommended to select Class 150, with materials such as 20#, A105, or WCB. These materials exhibit stable microstructures at 200°C, are well-established in manufacturing, and offer the best overall cost-effectiveness.
• Stainless Steel Piping SystemIf the medium is corrosive or requires a high degree of cleanliness, 304 or 316L material may be used instead. These materials exhibit excellent high-temperature strength, fully meet Class 150 requirements, and offer longer corrosion-resistant service life.
• Special Alloys or Nonferrous Metals: For gray cast iron, copper alloys, or duplex stainless steel, refer to the temperature-pressure curves specified in the relevant standards. Some low-melting-point or brittle materials experience a significant reduction in pressure-bearing capacity at 200°C; in such cases, upgrading to Class 300 may be necessary to ensure structural safety.

Practical Considerations for Use

• Design Margin CalculationEngineering codes typically require the design pressure to be no less than 1.1 times the working pressure, with due consideration for transient impacts such as water hammer, pump start-up and shut-down, and pressure fluctuations. With a working pressure of 0.8 MPa and a standard safety factor applied, Class 150 still provides ample margin.
• Selection of Seal Face TypeUnder high-temperature service conditions, it is recommended to prioritize raised-face (RF) or ring-type joint (RTJ) flanges and avoid flat-face (FF) flanges whenever possible. Thermal expansion and contraction can easily cause a loss of bolt preload in flat-face flanges, leading to failure of the static seal.
• Compliance Confirmation: The final selection must be verified item by item against the mandatory standards applicable to the project’s industry (such as GB, SH, HG, ASME, etc.), and stress analysis and strength verification must be performed by qualified professional engineers.