Sizing of Vents

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Sizing of Vents

• Aniket Shende
  61
• Rahul Sheth 62
• Kunal Shinde 63
• Advait Soni 64
• Chaitanya Sudrik 65

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Introduction

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

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Venting systems

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

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• Critical flow

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

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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

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Purging Process

• Complex fluid dynamic process
• Discharge flow, pressure decay depend on gas
  properties, vent line configuration, gas flow

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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

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Methods for vent sizing

• K- factor Method
• RUST Method
• Vent Ratio Method
• Nomograph or Cubic law Method

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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

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

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• Vessel volume between 1 to 1000 m3
• L/D ratio less than 51
• Does not account for vent ducts

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RUST Method

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

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• Limited to maximum rate of pressure rise of 345
  bar /s
• L/D ratio should be less than 2.51
• Does not account for vent ducts

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Vent Ratio Method

• Put forth by Palmer (1973)
• Vent Ratio Area of Vent
• Simplest method for sizing of vents

Volume of vessel
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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

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Recommended Vent ratios
Maximum Rate of Pressure Rise (bar/s) Vent Ratio (1/m)
lt345 1/6.1
lt690 1/4.6
gt690 1/3.1
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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

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Modifications of Vent Ratio for Larger Vessels
Volume Range (cubic m) Modification
30-300 Vent ratio reduced linearly from 1/6 to 1/25
300-600 1/9.2
gt700 1/3.1
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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 a V (-1/3)

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Nomograph or Cubic law Method

• Nomographs are used to determine vent size

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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

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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 51

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Selection of Vent sizing method

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

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Selection of Vent sizing method

• Vessel Shape- for conical vessels Nomograph
  method more suitable
• For cylindrical vessels K- factor method more
  useful

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• Thank You