{"operation":"document","citation":"PI-86-001","title":"Georgia Public Service Commission — Pipeline Safety Interpretation","source_type":"guidance","agency":"Pipeline and Hazardous Materials Safety Administration","status":"guidance","official":true,"published_on":"1986-01-23","effective_on":null,"summary":"PI-86-001 response to Georgia Public Service Commission concerning 193.2181.","machine_formats":{"json":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-86-001.json","markdown":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-86-001.md"},"app_url":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-86-001","source_url":"https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/legacy/interpretations/Interpretation%20Files/Pipeline/1986/PI86001.pdf","body":"<<<PAGE 1>>>\n\nGeorgia Public Service Commission\n244 Washington Street, S.W.\nAtlanta, Georgia 30334\nJanuary 23, 1986\nMr. Douglas Chisholm\nChief, Research Unit\nDPS 20\nOffice of Pipeline Safety\n400 - 7th Street, S.W.\nRoom 8409\nWashington DC 20590\nDear Mr. Chisholm:\nThe Georgia Public Service Commission is charged with auditing LNG facility construction in the\nState of Georgia to assure conformance to the Federal Regulation 49 CFR 193. In reviewing the\nregulation, we found that it is not clear what amount of vapor hold-up is allowed within the LNG storage\ntank dike. Because the amount of vapor hold-up allowed can be an important factor in determining the\ndownwind travel of a flammable gas cloud, we feel that this point needs to be clarified. We would like\nyour interpretation as to what fraction of the LNG storage tank dike free volume (dike volume less\nstorage tank volume up to the dike wall height less any liquid accumulation) is allowed for LNG vapor\naccumulation.\nNaturally, a reply at your earliest convenience would be greatly appreciated.\nSincerely,\nGlynn Blanton\nSection Chief\nUtilities Engineer\n193.2181\n86-01-23.doc\n\n<<<PAGE 2>>>\n\nMarch 3, 1986\nMr. Glynn Blanton\nSection Chief\nUtilities Engineer\nGeorgia Public Service Commission\n244 Washington Street, S.W.\nAtlanta, Georgia 30334\nDear Mr. Blanton:\nThis responds to your letter of January 23, 1986, to Dr. Douglas Chisholm requesting clarification of the\nfraction of storage tank impoundment space allocable to vapor detention under 49 CFR Part 193.\nPart 193 does not limit the space for “ vapor hold-up” in impounding system design. The minimum\ncapacity requirements of §193.2181 are designed to assure that some minimum space is available for\nvapor, but the operator may increase this capacity.\nI have enclosed a brief Technical Note by Mr. Walter Dennis providing some of his observations and\ncommentary on impounding system capacity and our current LNG research program.\nIn addition, I have enclosed copies of two Office of Pipeline Safety documents dealing with closely\nrelated issues.\nSincerely,\n(J.C. Thomas for)\nRobert L. Paullin\nDirector\nOffice of Pipeline Safety\n193.2181\n86-01-23.doc\n\n<<<PAGE 3>>>\n\nA TECHNICAL NOTE ON VAPOR DETENTION CAPACITY\nAND ITS EFFECTS ON VAPOR DISPERSION DISTANCE\nby\nMr. Walter Dennis\nAs you currently note, the vapor detention capacity can significantly affect the downwind travel of\nflammable LNG vapor (vapor dispersion distance). This is due to the potential for change in the\nproportionate detention capacity (the capacity relative to other spill conditions) to change the duration of\nLNG contact with heat transfer surfaces, the consequent degree of cooling, and the related rate of heat\ntransfer from those surfaces before initial vapor overflow occurs.\nUnder the prescribed model in §193.2059(c), a constant rate spill is presumed to continue at least until\nvapor overflows the diking (continuous spill). For this purpose, the term diking applies to either diking\nfor the impoundment of spilled LNG, or as appropriate, to an extended or additional barrier (if any)\ndesigned to increase holding volume for detention of evolving LNG vapor, as provided in\n§193.2059(d)(1)(iv). In applying the model, initial vapor overflow is assumed to occur when the\ncombined volume of evolved vapor, and the impounded volume of unvaporized spilled liquid equals all\nspace outside the component served that is provided for liquid impoundment and vapor detention. Thus,\noccurrence of initial vapor overflow is assumed at the moment of overflow due to overfill without\nconsideration of scooping by wind entrainment or ejection by vapor velocity.\nVaporization rate at the moment of initial overflow defines source strength in determining uniform unit\nsource strength under the model, the primary parameter that directly influences dispersion distance. The\nstrength at this moment is considered to be at its highest, thereby predicting the maximum dispersion\ndistance, since thereafter, vaporization rate is assumed to be diminishing as heat transfer surfaces in\ncontact with liquid will be cooling.\nFrom the foregoing, it is seen that with an increase in proportionate detention capacity, the time needed\nto fill the impoundment-detention space and reach maximum source strength will increase. With a\nreduction in the proportionate capacity, this time delay will diminish. It is evident, thereby, that both\ncontact duration of the liquid and consequent cooling of heat transfer surface in contact will vary directly\nwith proportionate detention capacity.\nConversely, the rate of heat transfer along with the vaporization rate, and related source strength will\nvary inversely with proportionate detention capacity. It follows that, under the prescribed model, the\npredicted dispersion distance will vary inversely with the proportionate detention capacity— the former\ndiminishing as the latter increases and vice versa.\nThus, in §193.2059, there is no specified fraction of impoundment-detention space arbitrarily dedicated to\nvapor detention. Rather, as defined in §193.2059(d)(1)(iv), the space dedicative to vapor detention is the\ntotal space available for liquid impoundment and vapor detention minus the volumetric space occupied by\nimpounded liquid at the moment of initial overflow due to overfill.\nThis definition is necessary, since for a given total impoundment-detention capacity, the latter fraction\nwill vary with spill volume, spill rate, differential enthalpy in spillage, and similar design specific variables.\nFor example, under a given design, an increase in overall detention-impoundment volume provided by\nincreasing the height or perimeter of vapor detention fencing would be allocable only in part to vapor\ndetention, since the increase in time for initial overflow from overfill would result in an additional liquid\nspill volume which must be accommodated.193.2181\n86-01-23.doc\n\n<<<PAGE 4>>>\n\n2\nOf course, in spite of this increase in spill volume, time duration to initial overflow would still be\nincreased with a consequent reduction in source strength and predicted dispersion distance. Solution of\nrespective liquid-vapor volumes would be relatively simple, once cumulative vaporization and liquid\naccumulation is established either as a volume-time function or simply by iterative convergence.\nAlthough you refer only to storage tank impoundment, the foregoing applies to all impounding-detention\nsystems.\nThe prescribed model under §193.2059 was developed only with conventional low remote diking in mind.\nTherefore, it may be important also for you to be aware of certain limitations. For conventional designs\nand clear field dispersion, predictions are generally thought to be overly conservative. But this has never\nbeen conclusively evaluated, and some comparisons give rise to uncertainties. Certain design conditions,\nhowever, could result in hazardous nonconservatism.\nOne (which the model cannot address) is channeling or diversion of the vapor by large downwind\nstructures or other topography. Very large detention capacity, where source strength based on initial\noverflow could be significantly less than actual source strength due to wind entrainment, is another. A\nthird problem is envisaged with multiple diking.\nHigh close-in diking, a more recently proposed design, presents a fourth and potentially more serious\nproblem. This problem results from the potential for actual source strength to continue increasing (if\nactual LNG spillage continues) after initial vapor overflow, thereby exceeding the theoretical maximum\nsource strength. It is seen that this would occur where the heat transfer rate continues to increase,\ndespite cooling, as the contact area continues to increase with the rising level of LNG from continuing\nspillage into the narrow impoundment annulous [sic].\nBecause of limitations in predictive capability of the current model, costs for protection distance at new\nplants could be economically burdensome. Preclusion of expansion at most existing plants would be\nlikely. Yet unsafe conditions could prevail with certain designs. OPS recognized this problem even at\nthe writing of current standards, but available options were limited.\nAccordingly, in 1983, OPS initiated a six phase research program, and subsequently was joined in co-\nsponsorship by the Gas Research Institute, to resolve this problem. The program is intended to develop\ndefinitive and verified methodologies and procedures for regulatory application of wind tunnel simulation\nindependently or conjunctively with a select mathematical model to predict dispersion distance where\ndiffusion is influenced by: (a) eddy entrainment from excess capacity LNG vapor detention systems, (b)\nwake turbulence from on site structures and natural obstacles, and (c) topographically induced diversion\nor meander. Independent physical simulation will be dependent on scale. With such methodologies,\nprotective distance for dispersion may be safety reduced by as much as one order of magnitude with tank\ntop transfer and designs to provide the conditions described in (a), (b), and (c) above. Although results\nof this effort will not be in place until after 1988, it may be useful for you and operators under your\njurisdiction to be aware of this potential development in planning for expansion.\n193.2181\n86-01-23.doc","truncated":false,"body_characters":9323}