Document bV37DkVzJ165Zb8YEB237x3o

Control of Atmospheric Emissions From Petroleum Storage Tanks Informativu Report No. 2 TW Petroleum Comwltt-- Air PwUwtteH Control Anodotfon fcifeowotiv Report No. 2, prepared by Xw 71-3 Petroleum Committee of (tie Air . Pollution Control Awodotton. deob wHh R>e controi of hydrocarbon vapor minfoio from petroleum storage tanks. The report presents the theory of evaporation lose Informative Report No. 2 of the TI-3 Petroleum Committee was first submitted to APCA's Steering from/Rquids, describes emission control equipment and Hs maintenance, and out* fine*techniques used to estimate the magnitude of and la reduce emissions. " Committee and Technical Council on May 0, 1968. It was processed in accordance with the 14 step pro cedure outlined in the March 1963 Journal and was finally approved by APCA's Board of Directors on March 1,1970. In accordance with the objectives of the Association as they appear in Article XV, Section 4 of the By-Laws, each technical . This study by the Air Pollution Control Association's TI-3 Committee tells why THEORY Of EVAPORATION LO** FROM TANKS coordinating committee has the task of reviewing and amending its studies as often as necessary in tire end how hydrocarbon vapors escape from tanks in which petroleum and its products are stored and discusses means Evaporation is a natural proem whereby a volatile liquid is converted into a vapor. The driving force that light of technological changes. In aeoordanee with procedures adopted by tbs APCA Technical Council and the Board of PireeUm, Used to eetimate the magnitude of, and causes vaporisation is the vapor pressure ' the control of, such emissions. > of the liquid. - The report does not attempt to die- The liquid may be unconfined or en- it ia now published ae representing "the beet tHnHiq of the Associa tion." tmgpish between the hydrocarbon ends- ' doeed in a container, such as a tank. A sions that are relatively uqreactive, liquid confined in a tank may or may not suchas paraffins and naphthenes, and \ have a vapor space above the liquid. . are therefore not smog precursors, and ",. Where there ie such a vapor space, the the relatively reactive emiseions, such as vapor pressure will cause molecules in 6699 29 olefins and substituted aromatics, that ~ the liquid to vaporise and disperse may react with nitrogen oxides in the r; throughout the vapor space. At the presence of sunlight, to form osone snetv same time, some molecules in the vapor function of the rate oi emission to the other oxidation products which eon* \ space will return to the liquid. When atmosphere and the period of time in tribute to Los Angeles-type photo- . \moieeules leave and return to die liquid vohred. Primary factors affecting the chemical smog. ' it the same rate, equilibrium has been rate of emission are: true vapor pres- Facta presented herein were selected , established; and the vapor epaoe has sure of tbs liquid at storage tempera- frotn chemical texts, engineering reports, become saturated. At any given storage tures, temperature changes in the tank, and/oil industry publications. A list of pressure, the eqnilifarinm percent of the tank outage, tank diameter, schedule of fe&tanee aouroes concludes this report. volatile liquid vapor in the vapor spaee tank fillings and emptyings, tank eondi- It ii hoped that this report will aid- la directly proportional to the vapor tions, and type of tank. Saturation and control officers and others in making pressure of tin liquid. diffusion effects play relatively small objective assessments when judging the Evaporation km occurs, only when parts in the mechanism of the km and necAfor, or the establishment of, abate- vapors are emitted to the atmosphere. can be neglected for all practical pur- ineniprocedures in their oommunitiee. _ The amount of evaporation km is a poeea. . ' ' Journal of the Ah PoNutkKji Control Association STLCOPCB4066602 The true vapor pressure (TVP) at 1-020 storage temperature in a tank directly affects the rate of emissions to the at-- mospbere. Increasing TVP accelerates _ the rate of evaporation into any tank vapor space. Also, at saturation, a -0.30 -040 120 -- space contains proportionately more hydrocarbon vapors as the vapor pres* sunrises. Both of these factors increase the emission rate from a tank during any -090 -060 -070 $ 42 0 lio --= specific breathing or filling eycle. There is no standardised procedure for directly determining the TVP of a -0.80 -080 -1.00 ioo -i hydrocarbon mixture. The standard procedure (ASTM D 323) for deterrain- 90 H ing vapor pressure gives what is known as Reid vapor pressure (RVP) at a tem* a -1.90 perature of 100F. TVP at any given - a temperature can be estimated front cor- . ' | rZ00 , relation charts relating RVP, TVP, tern- .*= 80-| perature, and a fourth variable, ASTM distillation curve characteristics. - -S |-090 DefMtlora and Significance of Terms ' VaporPressure Vapor pressure is a measure of the 1-3.00 3.50 I j-4.00 force that tends to vaporise any volatile .. -S liquid. Molecular motion within the . .5 J-5.00 liquid is reponsiMe for this force and is | related to the composition of the liquid. Smaller molecules are more active; thus, * f-6.00 2. vapor pressures increase as the propertion of these low boiling (smaller mole- cule) components increase. Also, higher 1-7.00 J j^OOO temperatures increase molecular motion, whkh results in higher vapor pressure. Weld Taper Pressure (WF) =-9.00 -100 . ^11.0 ["Stops of Via ASTM distillation cures at 10 parcant wporCfad Dag F at 15 paicant minus das F at 5 pwcsnt ------------------------ re~----------------- - ; Reid vapor pressure is the absolute prateure in pounds per square inch de termined at 100F and a V/L ratio of 4. (Le., ratio of vapor volume to liquid volume as defined in ASTM designation. D.323) by using apparatus and prooedurea as standardised by the American Society for Testing and Materials. Thus, RVP is the vapor pressure of a -12.0 -13.0 -14.0 -15.0 -16.0 -17.0 -18.0 -19.0 r2ao In 8rt stMsnc* of distttstion date the Mkming: siersgi wtua of 8 may fcs used: Matargnollns - AdsMon gamins 2.0 tight naphths (9-14 pd RVP) 3.5 Naphtha (24 pel RVP) - 2.9 3.0- sample which has had its composition .' changed because of the vaporisation re- . quired tosaturate the vapor space of the bomb. : \ __ -: ' Tree Vapor Prassara (TVP) ^ PtSM L Vapor pressure* of gaaottna* and flnishad patrotaum products--1 lb. to 2S lb. RVP (APtBuSatln 2911). True'vapor pressure is the vapor pres sure of a liquid at a specified tempera, tun, when the composition of the liquid is not: changed by the vaporisation that occurs in most measuring procedures. With mixtures, such vaporisation results m lowering of the measured pressure. ' Vaporisation from a hydrpearboa mixture lowers the vapor preasunrof thy liquid because the higher vapor pressure components vaporise more readily, leav ing; tin liquid richer in the lower Vapor pressure components. Thus, for motor gasolines and similar wide-boding range mixtures:, (mixtures containing com ponentsWith a wide range of vapor prea- fentoali^TVP at 100F may be signifiWtt&fcjVMr than RVP because some vaporisation occurs as an inherent part c striking the confining walls. Air mole of making the Reid test. For a pirn cules, usually present in petroleum stor c compound, vaporisation will not ohange age tanka, similarly generate a partial the pressure; and TVP will equal RVP. pressure. The sum of all partial pree- TVP can be estimated from oorreta- sures equals the total pressure of the tioas relating it to RVP and ASTM system. The partial pressure of any -distillation curve (boiling range) dune* component ia proportionate to its volu- teristias. Charts in Figures 1 and 2 metrie fraction of the vapor space. show this relationship for gasolines and . In petroleum storage tanks, a space crude oils over a wide range of tempera . normally is in contact with the liquid tures. which emits the hydrocarbon vapor. Partial Bras--re Equilibrium exists when partial pressure of the hydrocarbon vapor squab the The partial pteewuc of hydrocarbon vapor presmre of the liquid and the in a vapor apace ia a measure of the force rates of vaporisation and condensation exerted fay the hydrooarbon molecules are equal. Vofcime 21, No. 2d <oT> to CO o> CM CO Q STLCOPCB4066603 --I a i-2 -1 -4 -5 I 1-10 1-15 --14 -IS ra>' -- 23 FleamL" Vapor pressures of erode^WBuSsttn ail). Sahostile af Viice >pw : : tank passes through the tank walls and .Vapor space in a tank, with respect to ' , to the liquid or vapor by conduction. a given component, is said to be satu , . Cemwetisn is the transfer of heat by rated when equilibrium exists between ' the movement of fluids induced either thecomponent and the vapor and liquid ' : ; mechanically or by temperature differ phases under given conditions of tem ences. . perature and pressure, and composition Ssfidun is the transfer of heat from of the vapor space is uniform through \ one body to another, not in contact with out;. , ' . .it, by means of wave motions The rate V, Degree of taiurotum is the percentage of abeorptkm of radiant energy by a of saturation with respect to a given ' body depends upon its area and surface component which prevails at any gbsur reflective propertiee, exposure, and the time or location under normal equilib temperature difference. Solar radiation rium conditions. " is the chief source of radiant heating of i- Diffusion is the' molecular motion., ^conventional petroleum storage tanks. which tends to uniformly distribute any component throughout a vapor space. Diffusion is one way by which newly vaporised hydrocarbons distribute Factsrm Necessary far Evaporation Laos Two'conditions must be present for evaporation to occur: ' themselves throughout* vaporspace m an effort to saturate it. \ - Vaporisation is the process whereby a liquid changes to a vapor, either with or without boiling. 1. Heat must be supplied, and 2. The vapor evolved must not bo : allowed to accumulate. ^ Therefore, to reduce atmospherio- Condensation is the reverse of vapor SMiimiom from a petroleum storage isation, when a vapor changes to a tank, wa must: liquid. : ^Conduction is a transfer of beat from : one part of a body to another port of the' 1. Boduoa the host input, and 1 Contain the hydrocarbon vapor. same body without involving motion : The total: amount of atmoapturio between parte of this body. Bast oom- ' wnhiinna from a tank depends upon the outside a petroleum storege rate of evaparatkm lorn and the time involved. Factors affecting the rate of evaporation lose are: true vapor pres sure, temperature changes in the tank, tank outage, tank diameter, schedule of the tank fillings and emptyings, me chanical condition of tank, and type of tank. A fixed roof tank without a vapor conservation system is the least efficient in controlling atmospheric emissions. Saturation and diffusion effects are only parts of the loes mechanism and are classed as dependent or secondary vari ables. Traa Vapsr Pressure True vapor pressure affects the rate of emission because it is the basic force causing vaporisation; it changes with liquid composition and temperature. TVP at storage temperatures is all-im portant. For hydrocarbon mixtures, this decreases with evaporation because of the change in liquid composition. TVP is usually determined from corre lations relating it to Reid vapor pressure (RVP). The effect of TVP on rate of breathing loss from a fixed roof tank involves two internal considerations--saturation con centration and diffusion and convection factors. The maximum concentration of hydrocarbons in the expelled vapor, known as the saturation concentration, increases in direct proportion to TVP. It follows that, if vented vapors were fully saturated, evaporation loss would increase rapidly as TVP approaches tank relieving pleasure (a boiling condi tion). However, the diffusion and con vection of hydrocarbon vapor from the liquid surface to the vapor space is too slow to fully saturate it. Experience shows that vapors vented during normal tank breathing an usually only 80 to 00% saturated. Thus, the driving force can be looked upon as being the TVP of the liquid minus the partial pressure of the hydrocarbons in the vapor space. As TVP rises, this driving force will rise in direct proportion, if percentage satu ration in the vapor space remains con stant. Thus, both the saturation con- eideration and the diffusion and conven tion consideration suggest that actual emission rates are directly proportionate to rising TVP. Tank emissions while filling fixed roof 1 tanks are directly proportionate to in creased TVP because of the relationship between TVP and saturation concentra tion. ' Tswpsrstan Cksapsa la OM Tank Internal temperature changes brought about by atmospheric conditions can cause the tank vapor space to breathe. During the day, heat flosring through the roof and walls raises the vapor tem perature and expands the volume. The pure thermal effeet is augmented by vaporisation of hydrocarbons from the tank contents during the same period. Journal of tha Air rioNutlqnControl Association DSW 296691 STLCOPCB4066604 ?The best input abo may incnaae tha volume of vapor space available and awns, when filling, can be relatively liquid surface temperature and acceler pressure limitations of tin equipment. high. Emissions when filling variable ate vaporisation. At night, the reverse , If vapor space is allowed to change vapor space tanks are lees because these ^processes shrink the vapor and cause an ' volume at constant pressure, emissions tanks have added vapor storage capac intake of air. during the breathing cycle can be prac ity. The pressure tank also promotes [ Heat also causes natural convection in tically eliminated and emissions during condensation of hydrocarbon vapors .the vapor space which promotes evapo fillings can be reduced. Extent of emis during filling. ration from the liquid surface and aids in dispersing the hydrocarbon vapor. sions reduction is dependent upon the amount of variable vapor space pro Imptrfm Less - . ' - Tsn Giiias* vided. * Vapor emitted from the tank after liquid is removed is defined as emptying : The volume of vertical' tank vapor spaces- is directly proportionate to gam-css of emissions from Petroleum Tanks Iocs. Because vaporisation tags behind the expansion of the vapor spaoe during outage--the height of vapor space. For * fixed roof tank, higher outages mean greater emission rates because the larger vapor volume will breathe more. How ever, when outage is increased, heat input is not increased in direct propor tion. Heat enters the vapor space through the tank wall, the area of which increases in direct proportion, and through the tank roof, the area of which: remains unchanged. Furthermore, with added height of vapor space, resistance to transfer of hydrocarbon vapor from There are sue sources, or causes, of emissions from petroleum in storage: breathing lose, standing storage kies, filling loss, emptying lose, wetting loss, and boiling loss. reatfcbif Lees ' Vapor expelled from the tank because of the thermal expansion of existing vapors, expansion caused by barometric pressure changes, and/or increase in the amount of vapor from added vaporisa tion in the absence of a liquid level withdrawal, the partial pressure of the hydrocarbon vapor drops. Enough air enters during the withdrawal to main tain atmospheric pressure. When vaporisation into the new air reaches equilibrium, the vapor volume exceeds the capacity of the vapor space. This increase in vapor volume causes the emission. Emptying lose is common to all types of tanks except floating roof type and closed system pressure storage. Fixed roof tanks are most vulnerable. Pres the liquid surface to the vent increases. Therefore, the average concentration of hydrocarbon in vented vapor should falL Experience has confirmed that atmo spheric emission rate will increase lees change (except that which results from boiling) is defined as breathing lose. Breathing loss takes place in most types of tanks and occurs when limits of pressure or cofuste changer are exceeded. sure tanks and variable space tanks are lees subject to emission, but encounter. H if vapor-storage capacity is exceeded. Watttitf Lms than, in direct proportion to increase in Uninsulated fixed roof tanks, designed Vaporisation of liquid from a wetted tankoutage. for only a few inches of water pressure or tank wall, exposed when a floating roof Tank Olamstsr ' . vacuum, suffer relatively higher breath ing losses than tanks protected from a ie lowered by withdrawal of liquid, ia de fined aa wetting lose. This loss is small. Tank diameter influences volume of lorn or gain in heat by reflective coating, Vapor space and liquid surface oondi- burying, insulation, or shading. In Mh| Lne tion, assuming constant tank height. pressure tanks, which operate at 2j-f . Vapor expelled from the tank as a re Total atmospheric emissions from prig or higher, there is relatively little or sult of boiling of the liquid ie boiling breathing is lees than directly propor- no breathing loes. Variable vapor space lose. A fixed roof tank ie more subject innate to increase in vapor volume be tank systems reduce breathing losses. to this emission than a pressure tank. cause of tin less-than-proportionate in- . Floating roof tanks almost eliminate -crease in area for heat transfer into the vapor spaces and little or no breathing ATMOSPHERIC EMISSION CONTROL- vapor space. Furthermore, increasing loss occurs. . . PROCEDURES AND EQUIPMENT . tonk diameter should reduce the ton- perature. rise of the liquid surface be . Studlng Stores* Loss . - There are four basic tank designs: i cause the rising hot stock, Le., that in,., Vapor emission resulting from causes 1. Fixed-roof tanks contact with the tank wall, must spread other than breathing or change in liquid 2. Floating roof tanka In ai thinner film over tin surface area. _. level, is defined as standing storage loss. In floating roof tanks, the largest poten- ' . i Tank CmmUUms \ ' r tial standing storage loss is attributed to : A. Conventional open type (most common) - B. Covered float tanks .' Mechanical condition is another fee- improper fit of the seal and shoe to the (a) Internal floating covers for tor affecting emission rgtee'of tanka. shdL This condition exposes some fixed-roof tanks Openvents result in high emiesionawhect liquid surface to the atmosphere. Wind * (b) Covered floating roof gusty or turbulent winds cause fqpid . affects this source of lose. Also, a small - 3. Variable vapor spaoe tanka . pressure changes in tanks in which vobt- ' amount of vapor may permeate through 4. Pressure tanks tile liquids are stored. Rapid emissions occur as short puffs. Any hols in the\ tank roof, diaphragm, seal, or aeceaaory > results in the same type of emission. ; Where there are two or more openings the flexible membrane seal between shoes and roof. Other sources may be vapor escape from gaging hatches (proa- sure taatum) vents, fittings, or other openings. - v Each is designed for special storage re quirements and has special requirements for accessories, maintenance, and effi cient operation. in tin tank, emissions are further in creased. Pressure difference^ which re sult from wind or thermal effects, cause a constant flow of air through soma openings into the vapor space and tin; outflow .-of vapor through other openihcfc types! Tank . - RMuf Lets Vapor expelled from a tank as result of filling, irrespective of the exact mechanism by which the vapors are produced, is common to all types of tanks except the floating roof tank and dosed system pressure storage (such as for liquefied petroleum gas). It ocean Find Roof Tanks The minimum accepted standard for storage of volatile hydrocarbons ie the fixed roof tank. Since it is usually the least ooetiy to construct, it sews as a base in justifying expenditures necessary to construct conservation type tanka. It is designed to operate at only slight ThSitype of tank or storage system when the pressure inside the tank ex internal pressure or vacuum and is Willeffecttbe emission rate experienced. ceeds the relief pressure. For fixed roof susoeptible to emissions from breathing, . Th^UHwpt of emission depends on tin tanks, the relief pressure is low and emis- filling, and emptying. ` Me* 1971** Volume 21, No. S . 20 DSW 296692 STLCOPCB4066605 Accessary Equipment Floating Roof Tanks Maintenance ef Tank The fixed roof tank has several open A floating'roof tank is an effective ings in the roof for venting, gaging, and ... conservation device for stocks of motor sampling. To maintain a gas-tight roof, fuel volatility. The basic design vir accessory equipment must be provided. . tually eliminates vapor space. This re The accessory for the vent opening is sults in low emissions due to breathing, called a breather valve (pressure vacuum) filling, and emptying. relief or conservation vent.' This device prevents the inflow of air or the escape of vapor until some pre-set vacuum or pressure is reached. Most breather valves, especially the metal type, allow . some leakage below the pressure vacuum rotting. A tight valve is important in (educing atmospheric emissions, Until recent years the most commonly used floating roof tank was the conven tional open type floating roof tank. There are three basic designs of the open type floating roof tank used by the petroleum industry: pan, pontoon, and double-deck. Efficient and safe operation of any mechanical device that moves inter mittently requires inspection and main tenance at regular intervals. 1 Shoes must fit well, seals must be in good con dition, the roof should be level at all times. Breather valve and bleeder vent also must operate satisfactorily. Seals should be inspected periodically for tightness and general condition. Sections of the seal that deteriorate should be replaced. Design el Accessory Cqelgment r The pressure vacuum settings of a breather valve are dictated by the struc . - Fsa-Tygs Fleeter ^ tural characteristics of the tank and In this design a single deck covers must be within safe operating limits. A most of the liquid surface. A seal is at certain amount of pressure or vacuum tached to rim of the deck. The deck beyond these settings is necessary to slopes to the center for drainage. This overcome pressure drop in order, to ob-' type roof has found disfavor within the tain required flow. The pressure set industry due to its instability. It is not tings for vent valves to be installed on recommended except for very special large tanks constructed in accordance with API Standard 650, "Welded Steel ! Tanks for Oil Storage" (1964), usually are limited to ounce because the roof plates will start to lift when the pressure . . : rises much above 1 os. For small tanks, and all tanks having,special structural. features, the pressure range can be in- creased in accordance with the manufacturers'recommendations. ' This design was developed in 1928 to provide greater stability and roof buoyancy. The pontoons are arranged to provide floating stability under heavy loads of water and snow, and are aoceptable designs for most purposes.' Two accessories are necessary to the operation of the floating roof tank: a breather valve for the rim space and a bleeder vent for the roof. One breather valve (sometimes two) is provided at the outer edge of all types. It is similar in design to that used on fixed roof tanks. The bleeder vent of the pontoon and double-deck tanks allows air trapped under the roof to escape before the roof floats and prevents a vacuum as the roof comes to rest on its supports. The following accessories should be inspected and replaced or repaired . regularly: a. breather valves {v'Mtfcrtsnancs of Accessary Egulpmsiit . Double Deck West ' b. bleeder vents v - To maintain accessories in a gas-tight :. The roof in this case is a series of port condition, they should be inspected and . . toons. This design is preferred because restored periodically. Pallets of the of its greater stability and insulating metaJ-to-metal breather valves which qualities. become warped in service should be' . e. gage hatches ! d. other openings from which vapor may escape Chaim el Mwt . machined to restore a gas-tight fit. De Seal Design : The value of a highly reflective paint fective diaphragms of valves should be . in reducing atmospheric emissions from replaced. . - The most critical feature in a floating floating roof tanks is lees than for other Liquid-seal breather, valves may be roof tank is the fit of the seal. The usual types. Reflective paint on the pan-type affected by dilution or loss of liquid and seal consists of a relatively thin-gage roof may be justified because it will re may have to be inspected, cleaned, and shoe or ring supported against the tank duce the chance for boiling. It is not so maintained at frequent intervals. A shell around the edge of the roof. The important on the pontoon and double loose-fitting gage hatch lid can be made . bottom of the ring is below the liquid deck roofs because those are designed to nearly gas-tight by replacing the gasket surface. The top is a few inches above provide insulating barriers to heat trans qt machining the seat. Flame arrestors . the top rim of the roof.. A piece of fer. Reflective paint on tire shell may and flash screens can become clogged^ flame-retardant rubberised cloth cloees be useful because it may reduce boiling with dust, rust, and ice. Such obstruo-\ the space between the ring and roof. in the seal area and may help maintain tion in the venting system can cause severe damage to the tank from exoesjrive internal pressure or vacuum. ?'. Choice ef Mat \ Another type of seal consists of a flexi' 'Me tube, fastened to the roof and oo- cupying the annular space between roof . and shell. The tube is held on the liquid surface and completely eliminates a tower liquid temperature throughout the tank. Iirtsrwal Flsethig Covers ler FlaeJ WseTTeaks > Painting is important in reducing the vapor space. Installations of internal floating atrdqspherio emissions as well as pre In tanks with riveted shells a some covers date from 1965 in France, and in serving the tank. Use ofreflective paints ' what higher emission rate for the mors - recent yean there has been a strong will minimise heat input by reducing volatile stocks may occur because surge of interest in the United States in i)metal temperature. White paint is a riveted heads and overlapping steel the internal floating eover as an eco empls and effective means of reducing plates hold the ring away from the tank nomical and practical conservation de jamisslqns from fixed roof tanks.: . heiL . vice for fixed roof tanks. `JM: ' Journal of the Ak PeKutiofl Control Association DSW 296693 STLCOPCB4066606 Some typee of internal floating coven" " " Cevmed Floating Reef Tank ' holder capacity is exceeded. In cooling, currently available are listed below: 1. Aluminum cover. 2. Plastio coated-fabrio . cover sup- - .( ported on an inflatable oircular tubular float 3. Nylon fabric cover supported on a . ; peripheral ring of aluminum floats. The covered floating roof tank is not new. First installations were made in the late 1930's but has only gained wide' acceptance in the 1960's. Aa the name implies, it is a fixed roof tank with a floating roof inside. Basic design of the covered floater is a pan float in a fixed roof tank; the seal the vapors then in the holder are drawn back into the tank. Variable vapor space capacity usually is sufficient so that one holder may be used in conjunc tion with interconnecting fixed roof tanks operating at substantially the same pressure. Emissions due to filling are not materially reduced. Rapid installation is characteristic of all types of internal floating covers. No welding is required nor do door sheets have to be cut in the tank to make the installation. The tanks do have to be taken out of service and gas freed for installation of the semi-rigid types. If tank has been in leaded gasoline servioe, special cleaning and inspection will be required. . The inflatable ring design can be in . stalled without taking the tank out of service." Its use is restricted- to tanks which do not have roof support columns. These inflatable type covers are not normally available for tank diameters greater than 25 feet. : Each of the principal types is . equipped with a resilient peripheral seal between the floating cover and the tank wall. Seal designs include a wiper type, tubular type, and resilient materials encased in chemical-resistant and wear- - resistant envelopes. The effects of tank contents on seals must be considered, and it is important that any manufac turer be advised of the intended service. . As .internal floating covers are very light, only antirotation cables aw .. ' necessary to preserve alignment. Static; charge is not a problem as in the cone- roof tank. Anti-static cablds equalise any difference in potential between the floating cover and the tank Shell. ' . Ample vapor space ventilation is es sential. Vent areas equivalent to 0.20 . square foot per foot of tank diameter is . usually sufficient for all normal pumping rates. , As a rule, a minimum of four vents is'provided for aerodynamic effi ciency. Maximum spacing of not ove^\ 32 feet- between vents around the circumference is considered desirable. Damage to the roof by overfilling can be -\ \ is usually of the less costly nonmetallio construction. The rolling stairway, roof drain, and wind girder of conven tional floating roof design are elimi nated. Possibility of a flammable mix ture in the space between the fixed and floating roofs was probably a factor in delaying user acceptance. Modern de signs provide ventilation by way of hooded openings in the fixed roof or in the upper portion of the tank shell. Ventilation area usually falls in the range of 0.06 to 0.20 square feet per foot of circumference. Jamming of the floating roof against the fixed roof by overfilling can be prevented by provid ing an overflow hole to release liquid above a certain level. This overflow is usually an indicator only and not de signed to handle maximum filling rates. Important advantages over the con ventional open type floating roof are re duced maintenance and operating eoeta and improved product protection. There is no snow to remove and the freeaing of seals to the shell is prevented. The'floating roof drain is eliminated, and pollution problems which could occur due to its faulty operation are' avoided. Conversion of a cone roof of. any aiae to a covered floater is usually leas costly than conversion to an open top floater. Practical designs which permit support columns to penetrate the float have been developed. Deck and seal repairs are more diffi cult, and roof support column seals may cause problems. Column seal design varies with tank vendors and a number of each are in service. From the stand point of losses, it is desirable to minimise the number of roof support columns. Costs may be adversely affected when this ia done. ' - Designer Tank Two typee of variable space tanks are used: the lifter roof and the flexible diaphragm. There are two types of lifter roof tanks: wet seal and dry seal. In each type, the roof is telescopic. Not being rigidly attached to the shed, the roof can move up or down as the vapor above the liquid expands or con tracts. - With the wet seal lifter roof the upper part of the tank is surrounded by an annular space, called a "trough" or "launder," that contains a sealing liquid. A dip skirt extends from the roof into the liquid to provide a seal, while the roof is free to move up and down a distance of four to ten feet or more. The seal liquid may be water, light oil, or an anti-freese solution. The dry teal lifter roof lank operates similarly to the liquid seal lifter. In stead of a liquid seal, rubberised cloth or dry seal is used to retain the vapors as the roof moves. For both, the operating pressure ranges from ounces to 4 ounces per square inch depending upon the type, tank diameter and height of lift. Because this pressure controls the operation of the entire system, intercon nected fixed roof tanks must be designed and constructed to withstand operating pressure, plus pressure drop' in the system, plus a reserve to keep the vent from leaking. Flexible diaphragm tanke serve the same purpose as lifter roof tanks by pro viding expansion capacity through movement of a diaphragm. There are two types: the integral unit and sepa rate unit. Makrtaaaacc ef Tank Keeping a variable vapor space tank prevented by equipping the tank with an overflow? ' Variable Vapor Specs Tanks ' or system gas tight requires careful maintenance. The level of sealing : Properly installed and fitted internal A variable vapor space tank is par- liquid in the wet seal lifter tank should floating covers provide roort effective tieularly suited to reducing emissions be checked twice a year. Water, which vapor1 conservation than standard, open from breathing. Because of the effec may have condensed in thei bottom of pontoon-type floating roofs. This is due tiveness in reducing breathing loss, this the dip ring of the roof. Tne launder to added protection from rain and wind. tank has been widely accepted, par should be filled, with seal liquid to the Installation of an internal floating raver ticularly where tank throughput is low. design level. istheteast expensive method for ran- . Expanding vapors are stored, tem Snow should not be permitted to vertingen existing fixed roof to vapor porarily in a gas holder device and accumulate on lifter roof tanks. The conservation. vented to the atmosphere only when the added pressure sensed by snow at an Volume 21, No. 5 Id DSW 296694 STLCOPCB4066607 average depth of 4 to 8 inches (depend no emissions from breathing. Emis ing upon the pressure setting of the sions, when filling, may vary widely, vents) will cause the pressure vent por depending upon product characteristics, tion of the volume control valve to open. vent valve setting, and flow pipe ar Also, this increased pressure will cause rangement. vents in interconnected fixed roof tanks to leak or open. The entire system will Ossltn of Tsnk become inoperative; Two classes of pressure tanks are in . Dsslfn of Accessory Equipment Breather valves are standard acces- eories for each tank in the variable vapor apace system. Relief settings for breather valves on fixed roof tanks are slightly higher than those on conserva tion tanks. Because the variable vapor' space capacity is not adequate to retain all vapor expelled during filling, the 'breather valve must accommodate . vapor flow at maximum fill rate. . Each type of lifter roof tank generally has a volume control valve; some oper ate on vent valve only. This permits vapor to escape from the conservation .tank when the roof has reached the ex treme upper limit of travel. It also allows air to enter the tank to avoid .drawing a vacuum beyond one inch of water. . ;y Other accessories that aid in reducing emissions are (1) automatic gaging de vices and temperature recorders, (2) mechanical control of vapor-line shutoff valve connected to regular gage hatch, and (3) gage wells. general use: low-pressure tanks operat ing between 2.5 and 15 peig; and highpressure tanks operating up to 250 peig or higher. Low-pressure tanks can tolerate only a slight vacuum, ordi narily 1 to 2 oa per square inch. Higher . pressure tanks can be designed to with stand full vacuum. Low-pressure tanks are constructed in many shapes and sixes depending upon the operating pressure range. One low-pressure tank, the noded hemispheroid, is designed for a pressure range of 2.5 to 6 peig. It consists of a cylindrical shell with curved pintea in the top and bottom which may be either smooth or noded. In the plain hemispheroid, there are ring gilders at the intersections of roof, shell, and bottom to withstand compression at points when the vessel is subjected to internal pressure. The noded spheroid tank has been widely accepted for operating pressure* ranging up to 15 peig. Storage capaci ties range from 40,000 to 120,000 barrels. These tanks have curved shells with in 3 Maintenance ef Accessary Equipment ternal ties and trusses and one or more Maintenance of breather valves .is even more important on a variable vapor . space system than on a fixed roof tank nodes in the roof and bottom. Besides s ring girder and brackets around the external base of the shell, internal : alone because of the elevated pressures . needed in the connected fixed roof tank. -> ' .When a variable vapor-s|Seoe unit is part of a multiple tank system, the leakage of one breather valve will tend to nullify ; the conservation effect of the entire system.... _- ' . ! Choice of faint . . / 1.00 \. .. The same factors apply to this type ofV _ ' bmk as were discussed under fixed roof \ tanks. . ' Pressure Tanks - Pressure tanks are storage tanks which withstand relatively luge pres sure .yariationswithout emitting hydro- ; carbon vapor. Some tanks withstand . ' , only the pressure variations caused by . daily; temperature changes; others pre- yyent boiling of volatile stocks. There " 3prcMurs tanks experience little or name *.. Adjustment factor for amal dfanwtar tanka (API BuSatin 2911). trusses support the curved portion of the shell end nodes in the roof. A center column serves as a roof support. The spheroid is available for storage capacities ranging up to 40,000 barrels and operating pressure* ranging up to 30 paig. Spheroids are smooth in appear ance and have no internal framing. Design ef Accessories Special gas-tight accessories are re quired because of high-pressure opera tions. Effectiveness and safety of the equipment depend upon careful selec tion of property designed accessories. Maintenance af Tank and Accessory Equipment Pressure tanks should be inspected frequently to insure that tank and accessories are in gas-tight condition. Rupture of either pilot or main di aphragm of a pilot-operated relief valve will cause a valve to open below the set pressure. Valve performance should be cheeked. Diaphragms should be in spected for cracking or other signs of deterioration and replaced as necessary. Still-type breather valves should be in spected frequently to detect and correct fading and sticking of pallets. Flame arrestors should be inspected and cleaned periodically to prevent clogging with dirt or ice. . Ckalca of Paint To minimise pressure variations, tanka should be painted with a heatreflective paint. Although color is not as critical as on fixed roof and variable vapor space tanks, white is commonly used. Taper Recovery Systems These systems collect vapor from storage tanks and send it to a gaa re covery plant. They have sensitive con trols and -remove vapor as pressure builds up during pumping or breathing. Vapor is compressed and recovered by absorption or condensation. Refinery or natural gas is sometimes . used for repressuring the vapor space in tanka when air or a conceive atmo sphere is undesirable in the vapor re covery system. Vapor* are withdrawn as the internal pressure increases, and the repressuring gee is admitted to the tank when air normally would be drawn in. Some provision'must be made to prevent oollapee of structure when there is insufficient gas to maintain pressure. Journal of the Air Pollution ^Control Association 0 S\N 296695 STLCOPCB4066608 Maintenance at Iptmii Vapor recovery systems require that ail tanks be kept gas tight and instru mentation and fittings be adequately maintained. MEASUREMENT OF ATMOSPHERIC EMISSIONS FROM PETROLEUM STORAGE TANKS Measurement of atmospheric emis sion can be accomplished in three basio ways:. . , . 1. decrease in stock volume : 2. change in stock properties 3t measurement of vapors vented Tahle I Paint factors. Roof Tank color Shall Paint (actor, F, Paint In Paint in condition poor condition* Whits Aluminum (specular) White Aluminum (specular) White Aluminum (diffusa) White Light gray Medium gray White White Aluminum (specular) Aluminum (specular) Aluminum (diffuse) Aluminum (diffuse) Gray Light gray Medium gray 1.00 1.04 1.16 1.20 1.30 1.39 1.30 1,33 1.46 1.16 1.13 1.24 1.29 1.35 1.46 1.36 The vafciaa for palnta In "poor condition" ara prevfdad only aa a guide; It la not within the scope of thla report to daflna poor conditions. Special techniques can increase the effectiveness of each approach. ; API Bulletin 2512 (see Reference ' Sources) defines these test methods in Ffatad IM Tanks detail. Part I of this bulletin discusses Breathing loss of gasoline from fixed die. banc approaches to measuring roof tanks may be calculated from the evaporation losses and offers guidance following equation. for selecting methods for use under specific conditions. Part II describes eight methods. ',f These methods are not simple or in expensive. The petroleum industry has, D,uH*mT,mF,C (1) therefore, correlated test data and de veloped equations for calculating evap where oration loss rates. `' Lf ** breathing loss, in barrels per Calculation of Hydrocarbon Vapor Emissions from Tanks ' . . year ' ?' "; F true vapor pressure at bulk .. " liquid temperature in psia, Many methods for calculating hydro \ from Figure 1. Average carbon emissions from petroleum storage : ' . liquid body temperature usu- tanks have been developed frqm expert- : - . ally is available from gaging mental studies over the years. The \ l " records; however, if this in American Petroleum Institute took an . formation is not readily avail important step to coordinate knowledge able, it may be estimated by in this field when it sponsored the `'8ym- adding 6F to the average poeium on Evaporation Loss" , at the . ; . ambient temperature from Institute's 32nd Annual Meeting in . ' meteorological records 1952. This led to establishment of the\ D -- tank diameter, in feet APT Evaporation Loss Committee which, since that time, has studied and ' . ^ H correlated experimental results to obtain useful control techniques and equations \ average outage, in feet, includ ing a correction for root voL ume for predicting evaporation loan rates. T -- average daily ambient temper- Currently, this work has not been > \ ature change, in degrees F adopted as an API Standard; ,it is pro-.. (difference between U. 8. sented for information purposes only. Weather Bureau average - The correlations, formulae, and fac daily raoorded maximum and tors for' calculating hydrocarbon vapor minimum temperatures) emissions published by the API Evapo Fr paint factor from Table I (from ration Loss Committee are used as the API Bulletin 2518). basis', for, this section of the report C adjustment factor fbr tank di Recommended methods of calculation ameter aa indicated in Figure follow-' 3 (from API Bulletin 2518). Working Loss (filling and emptying loss) of gasoline from fixed roof tanks may be calculated from the following equation: _ F - 3PF 10,000`Kr (2) when F -- working loss, in barrels P -- same as defined above ' V -- volume of liquid pumped into tanks in barrels Kr -- turnover factor aa indicated in ' Figure 4 (from API Bulletin 2518). Breathing loss of crude oil from fixed roof tanks may be calculated frbm the following equation: L--K-(m>)(uT^pTx " D' ilH*aT*"FjC (3) ' where K, is factor to adjust gasoline breathing loss equation to breathing loss of crude oil - 0.58. The remainder of the nomenclature is the seme as defined for the gasoline equation except that the true vapor pressure, P, is determined from Figure 2. Working loos of crude oil from fixed roof tanks may be calculated from the following equation: ( _ 2.25PVF-~maoKr (4) Nomenclature is the same as defined for Equation (2) except that the true vapor pressure (P) is from Figure 2. May 1971 " Volume 21, No. 5 257 DSW 296696 STLCOPCB4066609 X \1.0 A*0.8 \|o.6 Nets: For 36tumovtrs ptr ye*r or less K T-1.0 l04 \ S D - tank diameter in feet ^for tanks 150 ft or less in diameter, use D11; for tanks larger than m150 ft. in diameter, use 160M P " true vapor pressure of the stock at its average storage tem perature, in psia. (This may be determined from the Reid vapor pressure using appro 0 100 200 300 400 . Tunwwers per year Annul throughput ' Tank capacity {/'*> .v- > FlfuraA. Effect of turnover on wording loss (API Bugatln ISIS). . priate conversion curves for finished products and crude oils in Figures 1 and 2) F " average wind velocity, in miles per hour K, TM a recommended seal factor: K. " 1.00 for tight-fitting seals (typical of modern metallic and nonmetallic seals); K. -- 1.33 for loose-fitting seals (typical of seals built prior to 1942) ! Fleatins Hoof Tanka Kt * a recommended factor dis tinguishing between gasoline Standing storage evaporation lose may be calculated from the following .'equation: , and crude oil storage: K. -- 1.00 for gasoline; K, -- 0.75 for crude oil K,, -- a recommended paint factor for color of shell and roof: K, -- 100 for light gray or alumi 'Mrhsm;; JUt, (5) num; K, -- 0.90 for white Variable Vapor Space Systems Filling losses from variable vapor I'ilbrJr standing storage" evaporation . space systems can be estimated from the ' loss, in barrels per year following formula: A tank-type factor' which ' changes as follows: ' op p - Wv'- -251w . Kt 0.045 for welded tank with pan or pontoaomn where " s' ` roof, single or doubbkta , . F -- filling loss, barrels K, seal > 0.11 for riveted ta--n1k_ V\ P w true vapor pressure at bulk liquid temperature in psia with pontoon roof, \ Ft ~ volume of liquid pumped in, double seal . ' barrels - K, 0.13 for riveted tank Ft -- volume of expansion capacity, with pontoon roof,' barrels single seal N -- number of transfers into the Kt 0.13 for riveted tank . system with pan roof,' - double seal ' Caassrslaas ta Taas/Pays ' K, 0.14 for riveted tank To place'these values interma more with pan roof, single conventional in atmospheric quality seal oontrol usage, the following formula is recommended: B LW 730,000 (7) where B -- hydrocarbon emissions (tons/ day) L -- total evaporation loss from tank (barrels/year) W " density of hydrocarbon vapors Qb/42-gallon barrel) The loss equations presented here do not have API approval as standard procedures and are submitted for infor mation only as recommended methods for calculating emissions to the atmo sphere from petroleum storage tanks. These loss equations are based on a cor relation of industry-wide test data. REFERENCE SOURCES ON EVAPORATION LOSSES API Bulletin 2512 Tentative Methods of Measur ing Evaporation Loss from Petroleum Tanks and Trans portation Equipment 2513 2514 Evaporation Loss in the Petro leum Industry--Causes and Control Evaporation Loss from Tank Cars, Tank Trucks, and 2515 ,2516 2517 ' 2518 Marine Vessels ' Use of Plastic Foam to Reduce Evaporation Loss Evaporation Loss from Low Pressure Tanks Evaporation Loss from Float ing Roof Tanks Evaporation Loss from Fixed 2519 2520 Roof Tanks Use of Internal Covers for Fixed Roof Tanks to Reduce Evaporation Loss Use of Variable Vapor Space 2521 Systems to Reduce Evapora tion Loss ,- Use of Pressure Vacuum Vent Valves for Atmospheric Pres 2522 . sure Tanks to Reduce Evapo ration Loss Comparative. Methods for Evaluating - Conservation Mechanisms for Evaporation Loss ttt Journal of the Air PoHutioq Control Association DSW 296697 STLCOPCB4066610