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Sizing of Vents

Aniket Shende 61 Rahul Sheth 62 Kunal Shinde 63 Advait Soni 64 Chaitanya Sudrik 65. Sizing of Vents. Introduction. Venting systems include Venting devices, piping or vent header systems, disposal systems

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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

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