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Calculation of Flashing Losses/VOCEmissions from Hydrocarbon Storage TanksTHE BASICS . . .Q: What is VOC?

A: VOC is an acronym that stands forVolatile Organic Compounds. VOCare components of hydrocarbonliquids such as crude oil andcondensate. VOC means anycompound of carbon, ex-cluding carbon monoxide,carbon dioxide, carbonicacid, metallic carbides orcarbonates, and ammonium car-bonate, which participates in atmo-spheric photochemical reactions.

However, there is a list ofcompounds that are ex-

cluded from being VOCbecause they have negli-gible photochemical

reactivity such as methane,ethane, and fluorinated andchlorinated hydrocarbons.The list of chemicals excluded

from the definition of VOC isfound in 40 CFR 51.100 (s)(1).

Q: What are flashing losses/VOC emissions from hydrocarbonstorage tanks?

A: There are three types of emissionsfrom hydrocarbon storage tanks: workinglosses, breathing losses, and flashing losses.Flashing losses/VOC emissions occur when aliquid with entrained gases goes from ahigher-pressure to a lower-pressure. As thepressure on the liquid drops some of thelighter compounds dissolved in the liquid arereleased or “flashed” and some of the com-pounds that are liquids at the initial pressure/temperature transform from a liquid into agas/vapor and are also released or “flashed”from the liquid. As these gases are released,some of the heavier compounds in the liquidsmay become entrained in these gases and will

be emitted with them. Flashing losses/VOCemissions are greater as the pressure dropincreases and as the amount of lighter hydro-carbons in the liquid increases. The tem-perature of the liquids and the storage tankwill also influence the amount of flashinglosses/VOC emissions. These flashing losses/VOC emissions are then either vented to theatmosphere through the tank’s pressurerelief valve, hatch, or other opening, or theymay be vented to a capture and/or controlsystem. Flashing losses/VOC emissions fromhydrocarbon storage tanks include emissionsof VOC, hazardous air pollutants (HAP), andtoxic air contaminants (TAC).

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Q: Where do flashing losses/VOC emissions from hydrocarbonstorage tanks occur?

A: The main areas where tank-flashinglosses/VOC emissions occur are at:

· wellhead sites when produced liquidsare sent to an atmospheric storagevessels from the last pressurized vessel;

· tank batteries when produced liquidsare sent to an atmospheric storagevessels from the last pressurized vessel;

· compressors stations when producedliquids are sent to an atmospheric

storage vessels from the last pressurized vessel;

· gas plants when produced liquids aresent to an atmospheric storage vesselsfrom the last pressurized vessel; and/or

· when the liquids in the gas lines are“pigged” (physically purged of condensate) and then sent to an atmosphericstorage vessel.

Q: What are working and breathing losses/VOC emissions fromhydrocarbon storage tanks?

A: Working and breathing losses/VOCemissions from hydrocarbon storage tanksoccur in addition to flashing losses/VOCemissions. Working losses are due to dis-placement of the vapors within the storagetank as a tank is filled. Breathing losses aredue to displacement of vapors within the

storage tank due to changes in the tanktemperature and pressure throughout theday and throughout the year. Working andbreathing losses/VOC emissions can be esti-mated with the latest EPA TANKS programor its equivalent.

CALCULATION METHODS . . .Q: Do I need to estimate flashing losses/VOC emissions from my

hydrocarbon storage tanks?A: Yes, estimates of flashing losses/VOC

emissions from hydrocarbon storage tankswill be requested for every facility thathas potential flashing losses/VOCemissions from hydrocarbonstorage tanks (e.g., natural gascompressor stations, naturalgas processing plants, conden-sate tank batteries, crude petro-leum liquid storage facilities, etc.) How-ever, facilities that are determined to be deminimis or permit exempt facilities are notrequired to submit flashing losses/VOC emis-

sions for their hydrocarbon storage tanks.The graph on the following page has been

included to aid facility operators indetermining if flashing losses/VOC emissions from hydrocar-bon storage tanks need to be

calculated. The graph was devel-oped using the Vasquez-Beggs Equa-

tion (VBE) and the default parameterson Page 6 of this fact sheet (with the ex-

ception of the Stock Tank API gravity). Thisgraph cannot be used if the Stock Tank APIgravity of the hydrocarbon liquids in the

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storage vessel exceeds 60º. Operators maychoose to calculate their facility’s flashinglosses/VOC emissions from hydrocarbon

storage tanks using actual site data for amore accurate evaluation.

Q: How do I estimate flashing losses/VOC emissions from myhydrocarbon storage tanks?

Calculations Required

No Calculations Required

A: The AQD will accept any of severalavailable methods of calculating flashinglosses/VOC emissions from hydrocarbonstorage tanks so long as the method selectedis used only for those site-specific situationsconsistent with its development and underly-ing assumptions. There are specific con-straints associated with each emission esti-

mation method. All supporting data used tocalculate the emissions, including identifica-tion of the calculation method and specificconstraints, description of sampling methodsand conditions, and copies of lab samplinganalyses must be provided to the AQD withthe emissions estimates.

The following are some of the methods of calculating flashing losses/VOC emissionsfrom hydrocarbon storage tanks:

· Vasquez-Beggs Equation (VBE);· Environmental Consultants and Research, Inc. (EC/R) Equation;· An equation of state (EOS) calculation program such as E&P Tank®;· Determination of the gas oil ratio (GOR) and throughput of the hydrocarbon liquids;· Process simulators (HYSIM®, HYSYS®, WINSIM®, PROSIM®, etc.); and· Direct measurement of emissions

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VBEThe VBE was developed in 1980 as part of

a research project at the University of Tulsa.More than 6,000 samples from oil fieldsworldwide were used in developing correla-tions to predict oil properties. The VBE canbe utilized as a default method to estimatepotential flashing losses/VOC emissions fromhydrocarbon storage tanks. This equationhas eight input variables: stock tank APIgravity, separator pressure (psig), tem-perature (°F), and gas specific gravity,volume of produced hydrocarbons(bbls/day), molecular weight of thestock tank gas, VOC fraction of thetank emissions and atmosphericpressure (psia). The VBE esti-mates the dissolved GOR of ahydrocarbon solution as afunction of the separatortemperature, pressure, gasspecific gravity, and liquidAPI gravity. Flashinglosses/emissions from the

EOS calculation programs (E&P TANK®)E&P TANK® is a software program de-

signed for use on personal computers. E&PTANK® was developed in an effort to esti-mate the working, breathing and flashingcomponents of hydrocarbon productiontanks. The E&P TANK program is based onthe Peng-Robinson (PR) EOS. An EOS is amathmatical equation relating to the relation-

ships between thermodynamic variables suchas pressure, temperature, and volume of aspecific material in theromdynamic equilib-rium. The minimum inputs needed for themodel are separator oil composition, separa-tor temperature and pressure, sales oil APIgravity and Reid Vapor Pressure (RVP), andsales oil production rate and ambient tem-

VOC storage tank are then estimated bymultiplying the GOR by the tank throughput,the stock tank gas molecular weight, and theweight fraction of VOC in the gases. Thismethod was designed for gases dissolved incrude oils. The equation is available in aspreadsheet format (EXCEL®) and can bedownloaded from the ODEQ website

http://www.deq.state.ok.us/AQDnew/resources/VBE_Aug2008.xls.

VBE calculations can alsobe done using the GRI-HAPCalc® model, whichruns in a Windows® format

and costs about $75 throughthe Gas Research Institute

(GRI). The AQD will acceptthis model or the VBE spread-

sheet calculations discussed above.The GRI-HAPCalc® program will also

speciate HAP emissions using site-specific data or default values.

EC/R EquationThe EC/R equation estimates

the ratio of each component in the liquidphase versus the vapor phase based on thetank pressure and the mole fraction of thevapor flashed. The flash emissions from thetank are then calculated using that informa-tion, the tank throughput, the density of the

hydrocarbon liquids, and the mass fraction ofeach component in the liquid. At pressuresbelow 1.6 atm (~8.8 psig), emissions areassumed to approach zero and at pressuresgreater than 5.1 atm (~60.3 psig), anotheremission estimation method is required.

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Determination of the GORDetermination of the hydrocarbon liquid

GOR can be obtained by collecting a pressur-ized sample upstream of the storage tank (i.e.separator dump line). The flashing losses/emissions can then be determined by multi-plying the GOR by the throughput of the

perature and pressure. The separator oilcomposition can be determined using ananalysis of the low-pressure separator oil,high-pressure separator oil, or low-pressureseparator gas, or using an analysis from thegeographical database provided with theprogram. The database is sorted by geo-graphic region, sales oil physical properties,and separator pressure and separator tem-

perature. The selected case should be se-lected such that the parameters are similar toyour particular facility. Using these inputs,the program estimates flashing losses/emis-sions as well as working and breathing losses.The program costs about $450 (for non-members in 2004) from the American Petro-leum Institute (API).

tank. An extended hydrocarbon analysis ofthe flash gas from the sample should also beconducted to identify the concentrations ofthe individual components of the tank’s flashemissions.

Process simulators (HYSIM®, HYSIS®, WINSIM®, PROSIM®, etc.)other parameters (e.g.temperature, pressure,and flow) for theprocess being simu-lated. Process simula-tors are not con-strained by API gravity. Thismethod of estimating potential flash emis-sions can be very expensive and complicatedand is expected to be more accurate whenestimating flashing losses/VOC emissions fromhydrocarbon storage tanks than other emis-sions estimation methods.

Process simulators are computer modelsthat use EOS and mass and energy balances tosimulate petroleum processes for a variety ofengineering purposes. There are severaldifferent manufactures of process simulators(HYSIM®, HYSIS®, WINSIM®, PROSIM®, etc.)each utilizing similar basic principles. Processsimulators are mainly used in process designand have been demonstrated to accuratelypredict natural gas and petroleum processes.Process simulators can be used to estimateflash emissions and speciate these emissions.Required inputs may include an extendedpressurized condensate analysis as well as

Direct measurement of emissionsActual testing of emissions fromtanks can also be performed todetermine flash emissions. Sincethere are no currently approved

U.S. EPA reference methodsthat are developed specifi-cally for measuring emis-

sions from storage tanks,

approved reference methods and modifiedreference methods or other approved meth-ods may be used to characterize and deter-mine these emissions, with prior approval ofthe AQD. However, it should be noted thatsuch testing is just a snapshot of the emissionsand should be used in conjunction with asafety factor to establish emission limits.

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Q: Are there some default values that may be used when calculatingflashing losses/VOC emissions from hydrocarbon storage tanks atmy facility?

A: Yes, while some of the variables of theVBE equation are facility-dependent, such asseparator pressure (psig) and throughput(BOPD), other variables are not as easily

obtainable or may require default values thatmay be used. The values shown below havebeen accepted as default values for VBEcalculations:

Q: Are there restrictions or limits to take into consideration whenusing any of the calculation methods?

A: Yes, the VBE and the EC/R equationare relatively simple calculations and can beused for a general estimate of flashing losses/VOC emissions from hydrocarbon storagetanks. However, if these simpler estimationmethods result in flashing losses/VOC emis-sions that are greater than 50 TPY, anothermore accurate method shall be used. If usingthe more accurate emission estimation meth-ods shows that the simpler emission estima-tion methods are conservative (calculatemore emissions), then the simpler emissionestimation methods can be used for permit-ting and reporting annual emissions.

Also, when using a process simulator, careshould be exercised to ensure that theamount of lighter hydrocarbons (propane,ethane, and methane) remaining in the flash

oil (hydrocarbon liquids remaining in the tankafter flashing occurs) is not significant (>4%).If a process simulator and the EPA TANKSprogram are used together to calculate flash-ing and working and standing losses/emis-sions from a tank, respectively, it should benoted that the EPA TANKS program doesnot calculate emissions of lighter hydrocar-bons that evolve from the liquids due to theirlow boiling points. The EPA TANKS pro-gram does not even allow entering com-pounds lighter than pentane. As stated inAP-42 (7/97), Section 7.1, “the equations arenot intended to be used in estimating lossesfrom unstable or boiling stocks or frommixtures of hydrocarbons or petrochemicalsfor which the vapor pressure is not knownor cannot be readily predicted.” If the

Inputs Units Default ValueStock Tank API Gravity API 78.0Separator Temperature °F 60.0Separator Gas Specific Gravity —- 0.9Stock Tank Gas Molecular Weight lb/lb-mole 49.0VOC Fraction of the Stock Tank Gas Fraction 0.8Atmospheric Pressure psia 14.7

Other values may be used as long as they have been obtained from site-specific data collectionor are based on a “representative sample.”

707/19/04

amount of the lighter hydrocarbons pre-dicted to remain in the flash oil by the pro-cess simulator or other program is significant,another method should be used to calculatethe working and standing losses/emissions. Inthese cases, the working and standing losses/

VOC emissions can be calculated using asimple distillation calculation in the processsimulator or the temperature of the tank canbe increased to reduce the amount of lighterhydrocarbons in the flash oil.

Q: Do I need to perform individual gas or liquid sampling toestimate flashing losses/VOC emissions for each hydrocarbonstorage tank or pigging operation?

A: No, the AQD is aware there may besome cost and potential problems associatedwith taking samples from each crudeoil or condensate-gas systemor during each pigging opera-tion and that there is somevariability in the compositionof the hydrocarbon liquidscollected. In order to avoidtaking samples in each case,facilities can use default valuesprovided in some models (e.g.E&P TANK®, GRI-HAPCalc®, etc.)or “representative analysis” for commonlylocated oil/gas facilities. If a database or “rep-resentative sample” of condensate/oil is used

to represent the actual composition of thefacility’s condensate/oil, the company should

select one that has similar properties andcomposition to thefacility’s condensate/oil.Sampling is typically justi-fied when there is a po-tential to violate an appli-cable requirement (e.g.,high throughput facilities,

facilities where the pressure drop isvery high, when the variability of the

hydrocarbon liquid constituents changesfrequently, or when the amount of emissionsare calculated to exceed major source per-mitting thresholds).

PERMITTING . . .

Q: If a facility is required to estimate flashing losses/VOC emissionsfrom hydrocarbon storage tanks, does it need an Air Quality permit?

A: If a facility is a de minimis facility as defined under OAC 252:100-7 or satisfies the fol-lowing permit exempt requirements, a permit is not required:

· The facility has actual emissions every calendar year that are 40 TPY or less of each regu-lated air pollutant;

· The facility is not subject to an emission standard, equipment standard, or work practicestandard in 40 CFR Part 60 (NSPS) or 40 CFR Parts 61 and 63 (NESHAP);

· The facility is not a potential “major source” as defined in OAC 252:100-8; and· The facility is not located in a non-attainment area;

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Q: Is a facility required to include flashing losses/VOC emissionsfrom hydrocarbon storage tanks located at their facility that areowned by another company?

A: For sources with tanks adjacent to orco-located at their facility, whether the flash-ing losses/VOC emissions from those hydro-carbon storage tanks should be included inthe facility’s potential to emit should bebased on current guidance for the definitionof a stationary source. For example, to make

this determination, consideration should begiven to the 2 digit SIC code of the tank andthe source, the location of the tanks in rela-tion to the source, the ownership or controlof those tanks and the rest of the source, andwhether or not the tanks are required as asupport facility.

Q: What is the schedule for reporting hydrocarbon storage tankflashing losses/VOC emission on my annual emission inventories?

A: For those facilities requiredto submit emission inventories asspecified in OAC 252:100-5, flashinglosses/VOC emissions from hydro-carbon storage tanks are requiredto be reported on their 2003 and

2004 calendar year emission invento-ries and every year thereafter. Revi-sion of emission inventories for calen-dar years prior to 2003 will not gener-ally be required.

EMISSION REDUCTION OPTIONS . . .Q: What are some options to reduce flashing losses/VOC emissions

from hydrocarbon storage tanks?

A: There are several options available to reduce the amount of flashing losses/VOC emis-sions from hydrocarbon storage tanks and they include:

· Implementing pollution prevention techniques that reduce the amount of emissionssuch as installation of an intermediate or low-pressure separator to reduce the pressuredrop from the line separators to the hydrocarbon storage tank and allow intermediatevapor recovery.

· Installing vapor recovery systems to reduce the amount of emissions from the hydro-carbon storage tanks. For example, the COMM Engineering, Environmental Vapor Recov-ery Unit™ has been verified by the EPA as a viable cost-effective method for control ofemissions from hydrocarbon storage tanks.

· Installing pollution control devices such as flares to convert the VOC emissions tocombustion products that consist primarily of CO2 and water.

· Establishing federally enforceable permit conditions on operational parameters suchas throughput and the maximum pressure drop allowed before venting to the atmosphere.

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ASSISTANCE . . .ASSISTANCE . . .ASSISTANCE . . .ASSISTANCE . . .ASSISTANCE . . .Q: Are there any programs or ways that a facility can get assistance

when trying to calculate flashing losses/VOC emissions fromhydrocarbon storage tanks and when applying for a permit?

A: Yes, the DEQ provides three methods of assistance for facilities:

· Pre-Application Conference - Prior to filing an application, you may request a confer-ence with staff to answer questions and identify important issues. This meeting is typicallyhandled by AQD staff who will be reviewing your application and drafting your permit.

· Small Business Assistance Program (SBAP) - This program provides guidance andassistance to small businesses (100 or fewer employees) regarding state and federal air regu-lations. This program is administered through the DEQ’s Customer Services Division (CSD).

· Permit Assistance Teams - A more formalized method of educating your facility aboutregulatory requirements is the Permit Assistance Team. Staff from the AQD and CSDwork as a team to answer permitting questions about your facility. The goal is to provideup-front information to facilitate the permitting process for our customers. This programis also administered through DEQ’s CSD.

Also, please work closely with your permitdrafter to ensure that the draft permit pre-pared for your facility represents actualconditions at your facility and is appro-

For general assistance contactour Customer Service Division,toll free at 1-800-869-1400, orfor specific assistance contactthe AQD at (405) 702-4100.

The information provided hererepresents our best understanding ofhow flashing losses/VOC emissionsfrom hydrocarbon storage tanks willbe addressed by the AQD. However, it issubject to change at a later time, and is not

priate for issuance. This will ensure that yourpermit is issued in a timely manner without

unnecessary delays.

Forms and other information areavailable on our web site(www.deq.state.ok.us.) or uponrequest.

final until established by rule or asa final determination in issuanceof a permit. Any reference to

software or equipment within this factsheet is not an endorsement by the state ofOklahoma.

This publication is issued by the Department of Environmental Quality as authorized by Steven A. Thompson, Executive Director.200 copies have been produced at a cost of $.1435 each. Copies have been deposited with the Publications Clearinghouse of theOklahoma Department of Libraries. 07/19/04 \\Filemaker1\web\CMullins\factsheets\Air\CalculationLosses.pmd.

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References

E&P TANK Version 2.0 for Windows Software. Copyright (C) 1996-1999 by The AmericanPetroleum Institute and The Gas Research Institute.

GRI-HAPCalc™ 3.0 Software. Copyright (C) 1996-1999 by the Gas Research Institute. http://www.gastechnology.org/webroot/app/xn/xd.aspx?xd=10AbstractPage\11984.xml

Emission Inventory and Improvement Program. Volume I: Chapter 10: Preferred and AlternativeMethods for Estimating Emissions from Oil and Gas Filed Production and Processing operations.U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. Re-search Triangle Park, North Carolina. 1999http://www.epa.gov/ttn/chief/eiip/techreport/volume02/ii10.pdf

Akin, Tom and William H. Battye, 1994. Memorandum to Martha E. Smith, U.S. Environ-mental Protection Agency. Environmental Consultants and Research, Incorporated. Octo-ber 18, 1994. Durham, North Carolina.

Test and Quality Assurance Plan: COMM Engineering, USA, Environmental Vapor Recov-ery Unit, Greenhouse Gas Technology Center, Southern Research Institute, ResearchTriangle Park, NC, U.S. EPA, Environmental Technology Verification Program, April 2002.http://www.epa.gov/etv/pdfs/testplan/03_tp_evru.pdf