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Sizing of Vents

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  1. Aniket Shende 61 Rahul Sheth 62 Kunal Shinde 63 Advait Soni 64 Chaitanya Sudrik 65 Sizing of Vents

  2. Introduction Venting systems include • Venting devices, piping or vent header systems, disposal systems Vent systems required for • Purging, Cleaning • Safety Considerations

  3. Venting systems: • Venting devices Valves-PRVs, PSVs -conventional , balanced Rupture disks, Breaking Pins Pressure Vacuum Vents Sizing of Relief Valves

  4. Critical flow

  5. Relief headers Worst case scenarios Standard pressure drop calculations • Disposal Systems Quenching, Chilling Separation of vapors and liquids Flares- ground, elevated

  6. Methods for vent sizing • No universal method of vent sizing for all situations • If vent size is too large then the venting system becomes uneconomical • If it is inadequate then it can cause accidents • Codes: • API 2000 standards • NFPA 30 • OSHA

  7. Purging Process: • Complex fluid dynamic process • Discharge flow, pressure decay depend on gas properties, vent line configuration, gas flow

  8. Safety considerations: • Process considerations 1)Valve malfunction, equipment failure 2) Sudden rise in feed inlet pressure 3) Runaway exothermic reactions 4) Process gas exceeding explosion limits 5) Dust explosion • Fire considerations 1) External fire adjacent to pressurized vessels 2) External Ignition

  9. Methods for vent sizing • K- factor Method • RUST Method • Vent Ratio Method • Nomograph or Cubic law Method

  10. 1. K factor method • Factor K Method by Simmonds and Cubbage • For compact rectangular vessels • Max. explosion pressure in vented vessel related to vent area

  11. Av is area of smallest side • For St 2 gas mixtures

  12. Vessel volume between 1 to 1000 m3 • L/D ratio less than 5:1 • Does not account for vent ducts

  13. RUST Method • Spherical flame front propogation • Used for equipments in soap and detergent industries • Rust equation • The shape factor C is defined as

  14. Limited to maximum rate of pressure rise of 345 bar /s • L/D ratio should be less than 2.5:1 • Does not account for vent ducts

  15. Vent Ratio Method • Put forth by Palmer (1973) • Vent Ratio = Area of Vent • Simplest method for sizing of vents Volume of vessel

  16. Vent Ratio Method • Vent ratios for various size ranges provided • Vent ratios on the basis of experiments in Hartmann apparatus • The Hartmann apparatus measures the maximum rate of pressure rise in the vessel for a given size range

  17. Recommended Vent ratios

  18. Drawbacks of Vent Ratio Method • Can be used for vessels of up to 30 cu.m only • For larger vessels, predicted vent size much greater than actually required and hence uneconomical • Assumes homogenous conditions and rapid propagation of flame throughout the reactor – conditions true in case of smaller vessels

  19. Modifications of Vent Ratio for Larger Vessels

  20. Nomograph or Cubic law Method • Method developed by Heinrich and Bartknetcht • Based on a cubic relation between maximum rate of pressure rise and inverse of the volume • (dP/dt)max α V ^ (-1/3)

  21. Nomograph or Cubic law Method • Nomographs are used to determine vent size

  22. Nomograph or Cubic law Method • P stat - the vent opening pressure • P red – reduced pressure from P max • First determine the P red • From the volume of the reactor and P red draw a horizontal line onto the second graph and intersect it with the appropriate K st value • The horizontal axis gives the vent area

  23. Drawbacks of Nomograph or Cubic law Method • Lowest pressure is 0.2 barg • Vessel volume must be between 1 and 1000 cubic m • The vessel Length: Diameter ratio should be lesser than 5:1

  24. Selection of Vent sizing method • Weak vessels- Vent ratio method • Low maximum rate of pressure size-RUST method • Turbulent dust clouds- Vent ratio method

  25. Selection of Vent sizing method • Vessel Shape- for conical vessels Nomograph method more suitable • For cylindrical vessels K- factor method more useful

  26. Thank You