Relief Valve Vent Line Length

Overview

Engineering reference data for Relief Valve Vent Line Length in sanitary drainage.

Key Formulas

Manning's Equation

$Q = \frac{1}{n} A R_h^{2/3} S^{1/2}$

Open channel flow.

Slope

$S = \frac{\Delta h}{L}$

Hydraulic gradient.

Variables

Formula Derivation & Engineering Basis

The maximum vent line length formula is derived from compressible flow equations considering adiabatic expansion of the relief gas through the pipe. It assumes:

• Gas behaves as an ideal air-like fluid
• Flow is isentropic (friction losses are approximated)
• Backpressure at the vent exit is atmospheric (14.7 psia)
• Relief pressure pr is in psig (gauge pressure)

Key Physical Relationship:
$l_m \propto \frac{(1.1p_r + 14.7)^2 \cdot d_i^5}{q_a^2}$

This shows that allowable length increases sharply with higher pressure and larger pipe diameter, but decreases quadratically with increased mass flow rate.

Practical Installation Considerations

1. Safety Margin: Always apply a safety factor (typically 0.75–0.85) to the calculated maximum length to account for:

   • Fitting losses (elbows, tees)
   • Corrosion/erosion over service life
   • Uncertainty in relief pressure during actual events

2. Material Effects: The formula uses general air properties. For specific gases (steam, hydrocarbons, corrosives), consult ASME PTC 25 or NFPA 68 for adjusted calculations.

3. Exit Considerations: The formula assumes open discharge to atmosphere. If venting into a manifold or closed header, backpressure effects require additional analysis.

4. Minimum Diameter Check: Ensure calculated d_i provides sufficient flow area—cross-reference with valve manufacturer’s minimum outlet size requirements.

References

• Original Source