Theory Of Pressure Relief,1

Theory of Pressure Relief Overview of presentation - Tube rupture - Modified 2/3 rule - Control valves - Pressurization path - required relief rate - Ideal vent area - Actual relief rates - inlet / discharge piping - critical / sonic flow

Tube Rupture Causes of failure; thermal shock, vibration, corrosion with potential of high pressure side overpressuring equipment on low pressure side

1. For non-flashing (sub-cooled) liquids, the relief rate through a tube rupture (two orifices) is given by

Wtube = 3782d Where:

2 tube

∆Pρ f K tot

Wtube = Required relief rate dtube = Internal diameter of tube ∆ P = Differential pressure ρ Fluid density f = Ktot = Total resistance for velocity head loss

(lbm/h) (in.) 2 (lbf/in ) 3 (lbm/ft )

2. For gas, vapor, or flashing liquid, the relief rate through a tube rupture (two orifices) is given by

2 Wtube = 3782Ydtube

Where:

∆P K tot vv

Wtube = Required relief rate dtube = Internal diameter of tube ∆ P = Differential pressure vv = Fluid specific volume (Pseudo mixture specific volume used for 2-phase flow) Y= Net expansion factor (Y = 0.8 is conservative) Ktot = Total resistance for velocity head loss

(lbm/h) (in.) 2 (lbf/in ) 3 (ft /lbm)

Control Valves ❚ Use maximum upstream pressure for calculating max flow through valve. ❚ consider flow through bypass valve if one exists plus through control valve. This allows for start up condition.S t e a m FC

Pressurization Path - consider common relief valve between vessels and the path that connects them - “check valves” not considered acceptable to prevent over pressure from reverse flow unless leakage < 30% - RO not means for limiting capacity since may not be installed - control valves with limit stops not acceptable to limit flow as these could be removed. - wide open control valve can be used as credit to limit flow

Required relief rate (cont.) 6. Fire Relief - assume properties of most volatile pure component - environmental factor to 1 unless fire rated insulation is installed (not commonly done). No credit for jackets or sprinkler systems - max liquid level 80% process vessels or 90% storage vessels Use API 520 to determine heat input (for pressure vessel) OR API 2000 for low pressure vessel - upto 25 ft above pool fire source for fire API guide