Density and molecular weight of common materials

💡 Plant Engineering Rules of Thumb & Safety Limits

• Standard vs. Normal Conditions: Always check your reference state! Standard conditions are typically defined at 15.6°C (60°F) and 1 atm (101.325 kPa), whereas Normal conditions are often 0°C (32°F) or 20°C (68°F). This distinction can alter gas volume estimations by over 7%!
• Ideal Gas Law Deviation (Compressibility): The ideal gas approximation holds well for low pressures (< 5 bar) and high temperatures. For high pressures or near-critical conditions, always use real gas equations of state (e.g., Peng-Robinson) or include the compressibility factor \(Z\) via: \[ \rho = \frac{P \cdot M}{Z \cdot R \cdot T} \]
• Liquid Compressibility: For most industrial designs, liquids are considered incompressible. Under extreme pressures (> 100 bar), density changes slightly but remains negligible for standard pump and piping hydraulics.
• Safety Margins: In relief valve (PSV) and gas piping sizing, design for the worst-case density (minimum temperature and maximum pressure yields highest density and peak mass flow).