Steam Safety Valves

Overview

Engineering reference data for Steam Safety Valves in steam condensate.

Key Formulas

Steam Quality

$x = \frac{m_v}{m_{total}}$

Mass fraction of vapor in two-phase mixture.

Enthalpy of Wet Steam

$h = h_f + x \cdot h_{fg}$

Specific enthalpy of wet steam.

Flash Steam

$m_{flash} = m_{liquid} \frac{h_f - h_{f2}}{h_{fg2}}$

Steam generated when condensate flashes to lower pressure.

Condensate Load

$m_c = \frac{Q}{h_{fg}}$

Condensate generated by heat transfer.

Variables

Discharge Area Calculation

The minimum required discharge area for a steam system relief valve can be determined using the following formula:

$A = \frac{\dot{m}}{51.5 \cdot k_d \cdot k_{bp} \cdot P^{1/2}}$

Where:

• A is the minimum discharge area (in²)
• $\dot{m}$ is the required relieving mass flow capacity (lbs/hr)
• $k_d$ is the discharge coefficient, typically *0.975
• $k_{bp}$ is the back pressure coefficient, *1.0 for systems discharging to atmosphere
• P is the relieving pressure (psia), calculated as: Set Pressure (psig) + Over Pressure (psig) + Atmospheric Pressure (14.7 psia)

Note: This formula is derived for steam service. Always verify application-specific coefficients and standards.

References

• Original Source