How to Calculate Vapor Dispersion

Vapor dispersion calculations are required for liquified natural gas terminals to delineate safe boundaries in light of potential spills. Federal regulations stipulate that LNG spills must be collected in impounded areas sited such that any resulting vapor clouds dissipate to below half the lower flammable limit before reaching property boundaries. EPA-approved models are required to properly calculate vapor dispersion. However, unique parameters must be entered for LNG calculations.

Instructions

• 1

  Download the Dense Gas Dispersion Model (DEGADIS) simulation softward from the Goddard Space Flight Center website. De-archive the code onto a PC. Transfer source code files to a VAX computer system. Rename several files as directed in the "AAREADME.TXT" file. Before compilation, print this file, along with the "BUILD.COM" batch file.

• 2

  Convert DEGADIS model into executable form. Evaluate block parameters, keeping in mind that unique parameter values will be entered for LNG vapor dispersion simulations. Determine field attributes, name, data type, numeric type and dimensions.

• 3

  Enter federally required parameters into the model. The average gas concentration in air is 2.5 percent. Using data available from the National Weather Service, enter dispersion conditions longer than 90 percent of known distances. Atmospheric stability (Pasquill Class) is equivalent to F. Wind speed is 2.01 meters per second (4.5 miles per hour) at 10 meters height. Relative humidity is 50 percent. Use the average atmospheric temperature for the region. Contour elevation is .5 meters. The surface roughness factor is .03 meters. However, higher site-determined values are necessary if the upwind and downwind vapor cloud will encounter dense vegetation and the height of the vapor cloud is more than 10 times the height of any obstacles.

• 4

  Run the model. Interpret the results to determine the distance from the center of the spill impoundment area that vapors will disperse to 1/2 lower flammable limit, considered a safe level.