Technical Notes

Variable Definitions

• V_s, V_d: Total gas volumes (vessel gas space + connected piping) on suction and discharge sides.
• P_s, T_s, Z_s: Suction-side pressure, temperature, and compressibility factor at trip time.
• P_d, T_d, Z_d: Discharge-side pressure, temperature, and compressibility factor at trip time.
• μ_JT: Joule–Thomson coefficient used to approximate temperature change during pressure equalisation.
• P_so, T_so: Final settle-out (equalised) pressure and temperature after isolation and mixing.

Formulas / Logic

• Gas moles (real-gas corrected ideal form): n = (P·V)/(Z·R·T), with R = 8.314462618 J/mol·K.
• Settle-out EOS: Pso = (Zso·ntot·R·Tso)/Vtot.
• Energy balance (screening): assumes the same gas throughout and approximately constant Cv, so Tso is computed as a mole-weighted temperature.
• Joule–Thomson cooling (screening): discharge gas temperature adjusted as Td,eff = Td − μJT·ΔP, with ΔP taken from discharge pressure down to current Pso.
• Solution method: fixed-point iteration updates Tso and Pso until changes are small.

Assumptions / Notes

• This calculator is intended for engineering screening. For final design, use a validated EOS and system model (including real equipment volumes, elevation, and heat transfer).
• No heat transfer is assumed during equalisation (adiabatic/short-time settle-out). If long duration, ambient heat exchange can shift Tso.
• Single gas composition is assumed (no phase change and no significant composition change). If condensation is possible, a rigorous flash calculation is required.
• Piping database uses a simplified internal ID table (Sch 40). For accuracy, confirm against project piping class/spec.
• References (industry practice): API 617 (centrifugal compressors) and ASME PTC 10 (compressors and exhausters) provide context for compressor system practices; settle-out evaluation is commonly performed using mass + energy balance with appropriate property methods.