Design considerations: Atmospheric Relief Systems vs. Atmospheric Vent Systems

Asked by lonstruthers 5 years, 7 months ago | 2 Answers

2 Answers

judyp 5 years, 7 months ago

Atmospheric Vent Systems

The design of atmospheric vent system discharge piping should be in accordance with API STD 2000.

Not all vents need to be separated individually or connected to one common discharge point.  Sub-systems can be formed based on location, venting rates, and maintenance requirements.  The effect of back flow or backpressure from one source on another should be considered.  For single source vents such as tanks, 2 in pipe should be the minimum size and 3 in the minimum size for system headers.  All piping should be sloped back to the source.

At onshore facilities lighter-than-air (LTA) vent lines should be terminated at high points in such a way as to minimize normal rainwater from entering the lines.  This can be accomplished by terminating lines with 180° bends pointing down and wire mesh screens to prevent water and foreign matter from entering.  Preferred venting is at least 10 ft (3 m) above normal personnel access areas and one foot above building eaves.

On offshore platforms, gas blanketed and atmospheric tanks, and other small LTA vents should be piped past the edge of the platform for discharging at high, outboard points.  Diesel fuel tanks or other small heavier-than-air (HTA) vents should be piped below the cellar deck.

Low pressure atmospheric tanks and vessels containing non-volatile liquids such as diesel fuel, lube oil, treating chemicals, etc., should be provided with open vent lines.

Flashback protection should be considered for vent lines if the following factors are present:

  • Systems contain volatile liquids (those with flash points below 100°F (37°C) or which have operating temperature above their flash points), which could emit hydrocarbon vapors or other combustibles.
  • Extensive and large vent header systems.
  • Systems which may contain air or flammable vapors, with the possibility of explosive mixtures existing for significant periods.
  • Potential for occurrence of pyrophoric deposits within the system.

Continuous purging of vent lines is the generally recommended method of providing flashback protection.  Flame arresters are less desirable alternatives, being subject to corrosion and blockage.  For many small vents, purging is usually not practical.  For these, flame arresters may be installed on the discharge ends of the vent lines.  If flame arresters are used, they should be arranged so that their internals can be inspected without disassembling piping.  If a flame arrester is used in vent piping (i.e., not in a pressure vent system), bypass overpressure protection should be provided (e.g., water seal leg, rupture disk, or rupture pin valve) in the event of plugging or maintenance of the flame arrester.

Pressure-vacuum relief devices (PV) or secondary vent lines should be considered to protect tanks or vessels in case the primary vent control devices foul or otherwise obstruct flow.  Flame arresters are not permissible on PV discharges.

Atmospheric vent discharge points should be outside of buildings and away from hot exhaust or other ignition sources and away from ventilation intakes.

judyp 5 years, 7 months ago

Atmospheric Relief Systems

The outlet piping on relief valves discharging directly to the atmosphere should comply with the following:

  • Diameter should be at least equal to the diameter of the relief valve outlet flange.
  • Diameter should limit the exit velocity at design rate to 75% of sonic to limit noise and to avoid choked flow.
  • Exit velocity should be at least 100 ft/sec (30 m/s) for the design rate to ensure effective dispersion.
  • Design rating should be ANSI 150 minimum.

Piping should terminate at a location that will have acceptable radiant heat densities if accidentally ignited.

  • Piping should terminate away from any lines, equipment, or vessels to prevent impingement and for offshore platforms to at least 5 ft (1.5 m) beyond the platform edge.
  • Piping should terminate a minimum of 10 ft (3 m) above equipment, deck/grade, or normal personnel access areas or 1 ft (30 cm) above adjoining building eaves, whichever is higher (see Section 6.1).
  • Discharge direction should be outward or upward.
  • Piping should discharge away from potential ignition or hot exhaust sources and away from ventilation intakes.

For relief valves vented to the atmosphere and not equipped with body drain connections, drain holes should be provided in the outlet piping to ensure that no liquids collect downstream of the valves.  The drains should be piped to safe locations where they will not impinge on personnel, equipment, etc., or be ignited upon relieving.

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