Interpretation Response #PI-10-0017 ([Fulbright & Jaworsky L.L.P.] [Ms. Lisa Tonery])
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date:
Company Name: Fulbright & Jaworsky L.L.P.
Individual Name: Ms. Lisa Tonery
Location State: NY Country: US
View the Interpretation Document
Response text:
PHMSA Interpretation #PI-10-0017
PHMSA Response Letter
December 22, 2010
Ms. Lisa Tonery
Fulbright & Jaworsky L.L.P.
666 Fifth Avenue, 31st Floor
New York, NY 10103-3198
RE: Interpretation of the Flammable Vapor-Gas Dispersion Protection Requirements for Liquefied Natural Gas Facilities in 49 CFR § 193.2059
Dear Ms. Tonery:
By letter dated June 25, 2010, you requested an interpretation of 49 CFR § 193.2059, the regulation that prescribes the flammable vapor-gas dispersion protection requirements for liquefied natural gas (LNG) facilities. Specifically, you asked whether Freeport LNG Development, L.P. (Freeport LNG or the Company) can use a particular source term model (FLNG Source Term Model) to determine the vapor-gas-dispersion (VGD) exclusion zone for the sump in a natural gas liquids (NGL) extraction system that the Company plans to build at its LNG Import Terminal in Quintana Island, Texas.1[1]
In our opinion, the FLNG Source Term Model uses generally-accepted assumptions for heat transfer and conservative assumptions for pool spreading and vapor retention. Accordingly, Freeport LNG can use that source term model to determine the VGD exclusion zone for its NGL extraction system, provided the conditions described in this letter are satisfied.
Background
The Pipeline and Hazardous Materials Safety Administration (PHMSA) issues Federal safety standards for determining the location of new LNG facilities. [2] Those standards require that an operator or governmental authority exercise control over the activities that can occur within an "exclusion zone," the area around an LNG facility that could be exposed to unsafe levels of thermal radiation or flammable vapor gas in the event of a release. [3] They also require that certain mathematical models be used to calculate the size of these exclusion zones. [4]
In the case of VGD, two computational models may be used: (1) the DEGADIS Dense Gas Dispersion Model (DEGADIS), an integral model that simulates the downwind dispersion of dense gases in the atmosphere, and (2) FEM3A, another dispersion model designed "to account for additional cloud dilution which may be caused by the complex flow patterns induced by tank and dike structure." [5]
If DEGADIS is used to calculate the VGD exclusion zone for an LNG facility, the user must first specify the vaporization rate and radius as a function of time that will serve as the input for that dispersion model. That input, or source term, is a representation of the physical phenomena that occur immediately after an LNG release, but prior to atmospheric dispersion. [6]
PHMSA has stated that the source term used with DEGADIS must have a suitable basis to comply with 49 CFR § 193.2059. [7] "Otherwise, a user could select whatever source term is likely to produce the most favorable outcome (e.g., the smallest or largest possible exclusion zone, or even at random.)" [8] That result would not be consistent with the limitations of DEGADIS or our obligation to ensure that the public is protected from the hazards associated with an LNG plant.
PHMSA has also determined that a particular source term model, SOURCE5, can no longer be used with DEGADIS to calculate the VGD exclusion zone for an LNG facility. [9] That determination was supported by the critical findings in a recent technical report (i.e., that SOURCE5 does not accurately represent the likely effects of pool spreading and the resulting flammable vapor flashing or vapor accumulation within impoundments, and that these deficiencies could compromise the integrity of an operator's VGD exclusion zone for an LNG facility.) [10]
PHMSA has further stated that a source term model should account for the effects of jetting and flashing in appropriate cases, including where a design-spill scenario involves a failure of pressurized piping or equipment. [11] In our view, using a source term model which ignores such an effect (or any other phenomena that has a similar influence on the discharge, vaporization, or conveyance of LNG) could also compromise the integrity of an operator's exclusion zone analysis (e.g., by distorting the downwind dispersion of flammable vapor gas, a result that would not be consistent with public safety.)
Analysis
Your letter states that Freeport LNG plans to use DEGADIS to calculate the VGD exclusion for the NGL extraction system sump. It further states that the Company hired a third-party expert, Exponent, Inc. (Exponent), to determine the source term that should be used as the input for that dispersion model, and that Exponent developed the FLNG Source Term Model for that purpose.
In our opinion, the FLNG Source Term Model uses generally-accepted assumptions for heat transfer and conservative assumptions for pool spreading and vapor retention. Specifically, it assumes instantaneous spreading of the LNG pool across the floors of the NGL extraction system sump, an assumption that the Company has shown, through a parametric analysis, will produce the longest flammable vapor-gas cloud. It also uses the generally-accepted, one-dimensional transient Fourier heat transfer method to determine the amount of vapor generated by the sump's concrete floors and walls. The Company has further shown that the passive mitigation measure that will be installed in that sump, a Foamglass block system, will have a negligible effect on the VGD exclusion zone for that portion of the NGL extraction system. Finally, the FLNG Source Term Model does not assume that any of the vapors produced during an LNG release are retained by the sump's walls.
For these reasons, we conclude that the FLNG Source Term Model has a suitable basis and can be used as the input for DEGADIS under these circumstances. [12] We note, however, that Freeport LNG has not yet accounted for other phenomena that could influence the discharge, vaporization, or conveyance of LNG to that sump, including the effects of jetting and flashing for any design-spill scenarios that involve the failure of pressurized piping or equipment. Such a showing must be made to properly calculate the VGD exclusion zone for that portion of the NGL extraction system.
Sincerely,
John A. Gale
Director, Office of Standards and Rulemaking