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A P I P U B L X 4 2 1 90     m 0732290 0087364    5                                                                       ...
Monographs on                     Refinery Environmental Control-                     Management of Water Discharges      ...
SPECIAL NOTES                       1. API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL                       NA...
A P I PUBL*L12L 70 W 0 7 3 2 2 7 0 0087367                                                              FOREWORD          ...
CONTENTS                                                                                                                  ...
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A P I PUBL*w1l2L 90               m     0732290 087370                                                                    ...
A P I PUBLm423 90              m     0732290 087373                                                                      0...
A P I PUBL*:421 90            m     0 7 3 2 2 9 0 0087372 4            m                                                  ...
A P I PUBL*1l2L 90               m     0732270 0087373 b               m        4                                         ...
A P I PUBL*42L 90            m 0732290 0087374 8 m                                                        DESIGN          ...
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A P I PUBL*q2L.S0 W 0 7 3 2 2 7 0 0 8 7 3 7 b                                                                         0   ...
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APL PUBL*92L 90 U 0732270 O O B 7 3 B L 5 U       12                                                                API PU...
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  1. 1. A P I P U B L X 4 2 1 90 m 0732290 0087364 5 $-"-5"5 Monographs on Refinery Environmental Control- Management of Water Discharges Design and Operationof Oil-Water Separators API PUBLICATION 421 FIRST EDITION, FEBRUARY 1990 American Petroleum Institute 1220 L Street, Northwest Washington, D.C. 20005 41 . - "-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services ~~
  2. 2. Monographs on Refinery Environmental Control- Management of Water Discharges Design and Operationof Oil-Water Separators Refining Department API PUBLICATION 421 FIRST EDITION, FEBRUARY 1990 American Petroleum InstituteCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  3. 3. SPECIAL NOTES 1. API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE, WITH RESPECT TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE, AND FEDERAL LAWS AND REGULATIONSSHOULD BE REVIEWED. 2. API IS NOT UNDERTAKINGTO MEET THE DUTIES OF EMPLOYERS, MANU- FACTURERS, OR SUPPLIERS TOWARN AND PROPERLY TRAIN AND EQUIP THEIR EMPLOYEES, AND OTHERS EXPOSED, CONCERNING HEALTH AND SAFETY RISKS AND PRECAUTIONS, NORUNDERTAKTNG THEIR OBLIGATIONS UNDER LOCAL, STATE, ORFEDERAL LAWS. 3. INFORMATION CONCERNING SAFETYAND HEALTH RISKS AND PROPER PRECAUTIONS WITH RESPECT TO PARTICULAR MATERIALS AND CONDI- TIONS SHOULD BE OBTAINED FROMTHE EMPLOYER, THE MANUFACTURER OR SUPPLIER OF THAT MATERIAL, OR THE MATERIAL SAFETY DATA SHEET. 4. NOTHING CONTAINED ANY API PUBLICATIONIS TO BECONSTRUED AS IN GRANTING ANY RIGHT, BY IMPLICATION OR OTHERWISE, FOR THE MANU- FACTURE, SALE, OR USE OF ANY METHOD, APPAFUIVS, OR PRODUCT COV- ERED BY LETTERSPATENT. NEITHER SHOULD ANYTHINGCONTAINED IN THE PUBLICATION BE CONSTRUED AS INSURING ANYONE AGAINST LIABIL- ITY FOR INFRINGEMENT OF LETTERS PATENT. 5. GENERALLY, API STANDARDS ARE REVIEWED AND REVISED, REAF- FIRMED, ORWlTHDRAWN AT LEAST EVERY FIVE YEARS. SOMETIMES ONE- A TIME EXTENSION OF UP TO TWO YEARS WILL BE ADDED TO THIS REVIEW CYCLE. THIS PUBLICATION WILL NO LONGER BE IN EFFECT FIVE YEARS AF- TER ITS PUBLICATION DATE AS AN OPERATIVE API STANDARDOR, WHERE AN EXTENSION HAS BEEN GRANTED, UPON REPUBLICATION. STATUS THE OF PUBLICATION CAN BE ASCERTAINED FROM THE API AUTHORING DEPART- MENT [TELEPHONE (202) 682-8000]. A CATALOG OF API PUBLICATIONS AND MATERIALS IS PUBLISHED ANNUALLY AND UPDATED QUARTERLY BY API, 1220 L STREET, N.W., WASHINGTON, D.C. 20005. Copyright O 1989 American Petroleum Institute ~ __~ .COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  4. 4. A P I PUBL*L12L 70 W 0 7 3 2 2 7 0 0087367 FOREWORD The MonographsonRefinery Environmental Control consist of four series- Management of Emissions to Air, Management of WaterDischarges, Management o Solid f The Wastes, and Management o the Subsurface Environment. monograph series are ongo- f ing projects of the Committee on Refinery Environmental Control of the API Refining De- partment and are intended to be used byengineers responsible for the design, construction, operation, and maintenance of disposal systems for wastes generated in refineries. This monograph provides design guidance for gravity-type oil-water separators for use in petroleum refmeries, provides practical advice on solving operating problems and im- proving separator performance, and presents information on performance of existing r e f ï - ery separators. A I publications may be used by anyone desiring to do so. Every effort has been made P by theInstitute to assurethe accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this pub- lication and hereby expressly disclaims any liability or responsibilityfor loss or damage re- sulting from its use or for theviolation of any federal, state, or municipal regulation with which this publication may conflict. Suggested revisions are invited and should be submitted to the director of the Refining Department, American Petroleum Institute, 1220L Street, N.W., Washington, D.C. 20005.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  5. 5. CONTENTS Page SECTION 1"GENEFUL .............................................................................. 1 1.1 Introduction ..................................................................................................... 1 1.2 Background ..................................................................................................... 1 1.3 Basic Theory ................................................................................................... 1 1.4 Application ...................................................................................................... 2 1.5 Referenced Publications .................................................................................. 3 SECTION 2-CONVENTIONAL OIL-WATER SEPARATORS .... 3 2.1 Design Procedures ........................................................................................... 3 2.1.1 General ..................................................................................................... 3 2.1.2 Wastewater Characterization .................................................................... 3 2.1.3 Establishing Design Flow ...................................................................... a 4 2.1.4 Step-by-step Design Calculations ............................................................ 6 2.1.5 Design Example ......................................................................................... 9 2.2 Construction Details ........................................................................................ 11 2.2.1 General ..................................................................................................... 11 Section .............................................................................................. 2.2.2 Inlet 11 Section .................................................................................... 2.2.3 Separation 13 2.2.4 Other Design Features .............................................................................. 19 2.3 Performance .................................................................................................... 21 SECTION 3"FROVING THE PERFORMANCEOF EXISTINGSEPARATORS ............................................... 22 3.1 General Approach ........................................................................................... 22 3.2 Process Troubleshooting ................................................................................. 22 3.2.1 Design Review ......................................................................................... 22 3.2.2 Emulsified Oil .......................................................................................... 22 3.2.3 Flow Surges .............................................................................................. 24 3.3 Dealing With Emulsified Oil .......................................................................... 24 3.4 Retrofitting Existing Separators With ParallelPlates ..................................... 24 SECTION LPAEtALLEL-PLATE SEPARATORS................................ 25 4.1 General ............................................................................................................ 25 4.1.1 Introduction .............................................................................................. 25 4.1.2 Design ....................................................................................................... 25 4.1.3 Wastewater Characteristics Required for Separator Sizing ...................... 25 4.1.4Parallel-Plate Surface Area ...................................................................... 25 4.1.5 Maintenance ............................................................................................. 26 4.2 Construction Details ........................................................................................ 26 4.3 Performance .................................................................................................... 26 SECTION 5"OPERATION AND MAINTENANCE OF OIL-WATER SEPARATORS ........................................... 28 SECTION &REFERENCES ...................................................................... 30 APPENDIX A-DEWATION OF BASIC EQUATIONSFOR DESIGN OF OIL-WA-R SEPARATORS ...................................................... 31 APPENDIX B-DETERMINATION OF SUSCEPTLBILITY OF SEPARATION .............................................................................. 33 APPENDIX C-SELECTED RESULTS: 1985 API OIL-WATER SEPARATOR SURVEY ............................................................... 35 APPENDIX D-TEST FOR SPECIFIC GRAVITY OF OIL INWASTEWATER 39 VCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  6. 6. -~ APUBL*421 I 7007322700087367 4 Page Figures l-Specific Gravity of Clear Water (Fresh and Sea) for Temperatures Between 40°F and 120F .............................................................................................. 5 2-Absolute Viscosity of Clear Water (Fresh and Sea) forTemperatures Between 40°F and 120°F ............................................................................... 6 3-Design Variables for Oil-Water Separators ................................................... 8 &Recommended Values o f F for Various Values of ................................. 10 5-Conventional Oil-Water Separator (Uncovered) ........................................... 12 &"-Reaction-Jet Inlets .......................................................................................... 14 7-Vertical-Slot Baffle ........................................................................................ 15 8-Four-Shaft Collector-Type Oil- and Sludge-Moving Device......................... 15 9-Traveling-Bridge Oil Skimmer and Sludge Collector ................................... 16 IO-General Arrangement of Rotatable Slotted-Pipe Skimmer .......................... 17 11-Rotary-Drum Oil Skimmer .......................................................................... 18 12-Horseshoe-Type Floating Skimmer ............................................................. 18 13-Self-Adjusting Floating Skimmer ................................................................ 19 14-Covers for Oil-Water Separators ................................................................. 20 15-Performance of ConventionalOil-Water Separators ................................... 21 16"Correlation Between Influent and Effluent Oil Levels for Existing Oil-Water Separators ................................................................................... 23 17-Cross-Flow Parallel-Plate Separator ............................................................ 27 18-Downflow Parallel-Plate Separator .............................................................. 28 19-Performance of Parallel-Plate Separators .................................................... 29 Tables 1-Applicability of EPA Methods 413.1 and 413.2 ............................................ 4 2-Typical Rangas for the Basic Design Variables ofParallel-Plate Separation 25 C-1Celected Results of 1985 API Oil-Water Separator Survey: Conventional Separators ............................................................................ 36 C-2"Selected Results of 1985 API Oil-Water Separator Survey: Parallel-Plate Separators ............................................................................ 37 D-1-Oil-Density Variation with Temperature (15API-35"API) ...................... 40COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  7. 7. A P I PUBL*w1l2L 90 m 0732290 087370 0 O m Design and Operation Oil-Water Separators of SECTION I-GENERAL 1.I Introduction Free oil is in the form of discrete oil globules of a size suf- ficient so that the globules can rise as a result of buoyant The purpose of thismonograph is to provide design guid- forces and form an oil layer on top of the water. Under ance on gravity-type oil-water separators for use in petro- proper quiescent flow conditions, free oil can be removed by leum refineries, to provide practical advice on solving gravity separation. This removal is a function of residence operating problems and improving separator performance, time, differences inspecific gravity and temperature, and the and to present information on the performance of existing stability of the emulsion. Some coarse oily solids have a spe- separators. cific gravity greater than 1.0and will settle to the bottom of This monograph deals solely with gravity-type oil-water a separator. Most of the oiland associated fiie solids in re- separators, that is, those that rely on differences in specific f i e r y wastewater have a specific gravity of less than 1.0 and gravity to separate oil globules from a wastewater stream. will rise to the waters surface. Two types of oil-water separators are covered in this mono- Emulsified oil is in the form of much smaller oil droplets graph: or globules with a diameter of less than 20 microns (mostly a. The conventional, rectangular-channel unit. in the 1-10-micron range). These globules form a stable sus- b. The parallel-plate separator. pension in the water as a result of the predominance of inter- particleforcesoverbuoyantforces.Thepresence of Note: Throughout this monograph, the term conventional oil-water separa- tor is used in place of the term A P Z separator and refers to rectangular-chan- particulates also contributes to emulsion formation. Regard- ne1 units designedin accordance with the criteria published earlierAFT. by less of how long a true oil-water emulsion stands under qui- The term APZseparator has become almost a generic term, sometimes used escent conditions, a separate oil phase will not form. incorrectly to refer to any gravity-- oil-water separator. Emulsified oil may be removed bychemical addition and co- Historically, the design of conventional oil-water separa- alescing or by flotation but not by gravity separation alone. tors was based on criteria developed from a 3-year, API- Dissolved oil is the petroleum fraction that forms a true funded research study initiated in 1948 at the Engineering molecular solution with water. Dissolved oil cannot be re- Experiment Station at the University of Wisconsin Cl]. Since moved by gravity separation; further wastewater treatment then, numerousoil-water separatorsbased on the API-devel- (for example, biological treatment) is necessary if removal of oped design criteria have been designed and put into opera- dissolved oil is required. tion throughout the petroleum industry. The criteria were The fraction of oil that is removable from a wastewater developed as voluntary guidelines for designing conven- stream by gravity separation is affected by the type of oily tional oil-water separators. Many other separators based in material present. The method of measuring oil concentration part on the API-developed design criteria have been adapted can also affect the apparent efficiency of removal. Conven- for a variety of other industrial wastewater treatment appli- tional oil-water separatorsremove only free oil; stable emul- cations. sions and dissolved oil require additional treatment. , This monograph retains pertinent information from Chap- ters 5 and 6 of the 1969 edition of the API Manual on Dis- 1.3 Basic Theory posal of Refinery Wastes, Volume on Liquid Wastes,and In essence, an oil-water separator is a chamber designed incorporates the recent findings of a literature review and to provide flow conditions sufficiently quiescent so that 1985 survey of refiery separators. Data from this survey of globules of free oil rise to the water surface and coalesce into the design and operation of oil-water separators have been a separate oil phase, to be removed by mechanical means. incorporated in this monograph. Oil-water separation theory is based on the rise rate of the Although the theory of oil-water separators is briefly dis- oil globules (vertical velocity) and its relationship to the sur- cussed, the emphasis of this monograph is on practical guid- face-loadingrate of the separator. The rise rate is the velocity ance for those involved in the design or operation of oil- at which oil particles move toward the separator surface as a water separators. result of the differential density of the oil and the aqueous phase of the wastewater. The surface-loading rate is the flow 1.2 Background rate to the separator divided by the surface area of the sepa- A refinery wastewater stream may contain oil in three ma- rator. In an ideal separator, any oil globule with a rise rate jor forms: free, emulsified, and dissolved. Some pertinent greater than or equal to the surface-loading rate willreach features of each of these are briefly discussed below: the separator surface and be removed. An ideal separator is 1COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  8. 8. A P I PUBLm423 90 m 0732290 087373 0 2 U 2 P BI AI N U LC TO AI P 421 assumed to have no short circuiting, turbulence, or eddies. a. The performance of the separator will be highly de- The required surface-loading rate for removal of a specified pendent on the difference between the specific gravity ofthe size of oil droplet can be determined from the equation for water and that the oil. the closer the specific gravity of the of rise rate. The derivations ofthe basic equations for oil-water oil is to that of the water, the slower oil globules will rise. the separator design are given in Appendix A.The mathematical b. Since the oil globules rise rate is inversely proportional relationship for the rise rate is provided by a form of Stokes to the viscosity of the wastewater, oil globules will rise more Law: slowly at lower temperatures. y = (g/ 18Y)(PWPO)DZ (1) Both of these factors play an important role in selecting de- Where: sign conditions in the design procedure for oil-water separa- tors presented in 2.1. V, = vertical velocity, or rise rate, of the design oil glob- For those further interested in the theoretical aspects of ule, in centimeters per second. particle settling, a derivation of Equations 1 and 2 is pre- g = acceleration due to gravity sented in Appendix A. = 981 centimeters per second squared. p = absolute viscosity of wastewater at the design tem- 1.4 Application perature, in poise. pw = density of water at the design temperature,in grams As stated above, oil-water separators are designed to re- per cubic centimeter. move free oil only. If emulsified or dissolved oil is present, p,, = density of oil at the design temperature, in grams one cannot expect an oil-water separator to remove it, and per cubic centimeter. additional downstream treatment may be required. A princi- D = diameter of the oil globule to be removed, in cen- pal function of the oil-water separator is to remove gross timeters. quantities of free oilbefore further treatment. In this capac- ity, the oil-water separator protects more sensitive down- The vertical velocity ofan oil globule in water depends on stream treatment processes from excessive amounts of oil. the density and diameter of the oil globule, the density and Since separator skimmings are typically recycled, and not oil viscosity of the water, andthe temperature. The oil globules recovered can end up as sludge, efficient recovery results in vertical velocity is highly dependent on the globules diam- minimization of waste. eter, with small oil globules rising much more slowly than The performance of gravity oil-water separators varies larger ones. It has been determined from the API research with changes in the characteristics of the oil and wastewater, mentioned above that, using the example design procedures including flow rate, specific gravity, salinity, temperature, described in this monograph, oil globules with a diameter viscosity, and oil-globule Performanceis also a function size. greater than or equal to 0.015 centimeter (150 microns) can of design and operational constraints and of the analytical be expectedto be removed effectively a gravity separation in methods used to measure performance. The API survey data chamber without plates. included in this monograph reflect the large amount of vari- With the oil globule diameter fixed at 0.015 centimeter, ability that is a result of these many factors. However, the Equation 1 can be simplified (see Appendix A) to yield, in survey data indicate that increasing separator size, as mea- English units, sured by surface-loading rate, results in improved perfor- v = 0.0241"- S, P- so (2) mance, as measured by effluent oil and grease. Unit design needs to take into account the impact on downstream oil re- Where: moval processes (for example, dissolved-air flotation) to de- termine whether incremental improvements in performance V, = vertical velocity of the oil globule, in feet per can be justified. minute. Parallel-plate separators are based on a newer technology. S," = specific gravityof the wastewater at the design tem- They require less space than do conventional oil-water sep- perature (dimensionless). arators and are theoretically capable of achieving lower con- S = specific gravity of the oil present in the wastewater , centrations of effluent oil. Petroleum industry data are (dimensionless, not degrees API). insufficient to conclude that parallel-plate units offer overall = absolute viscosity of the wastewater at the design superior performance. temperature, in poise. There are petroleum industry applications in which Procedures for determining specific gravities for oil and oil-water separators are the only end-of-pipe treatment pro- wastewater and for wastewater viscosity are given in 2.1.1. vided. These are usually cases in which the only effluent re- Equation 2 embodies two fundamental principles that strictions specified are for oil or suspended solids and the should always be in mind when designing and operating kept wastewater in question consistently contains sufficiently low oil-water separators: amounts of emulsified and dissolved oils. In some applica-COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  9. 9. A P I PUBL*:421 90 m 0 7 3 2 2 9 0 0087372 4 m DESIGN OPERATION OF OIL-WATER AND SEPARATORS 3 tions, the oil-water-separator is provided as aprotective de- 1.5 Referenced Publications vice for containment of spills and leaks (for example, on once-through cooling water). Another example instance is an The following publications are cited in this monograph: in which a stream is discharged a publicly owned treat- to ment works and theoil-water separator is used to ensure AST” compliance with requirements for pretreatment oil andof D 445 Test Methodfor Kinematic Viscosity of Trans- grease. parent and Opaque Liquids (and the Calcu- It should be stressed that whenever an oil-water separator lation of Dynamic Viscosity) is considered for an application where it must standalone, D 1429 Test Methods Specific Gravity o Water and for f the amount of emulsified and dissolved oils in the wastewa- Brine ter stream must be properly quantified, because these oils D 1888 Test Methods for Particulate and Dissolved will not beremoved by the separators. More important, the Matter in Water . design procedures in this monograph relate to refinery pre- D 3921 Test Methodfor Oil and Grease for Petroleum treatment of contaminated wastewater. Other applications re- Hydrocarbons in Water quire special consideration. One aspect of oil-water separator design that is some- EPA2 times overlooked is that whether intended to or not, an “Standards ofPerformance for VOC Emissions from Pe- oil-water separator also functions as a sedimentationbasin. troleum Refinery Wastewater Systems” (40 Solid particles more dense than water (for example, soil and Code of Federal RegulationsPart 60, Subpart coke particles) will tend tosettle out in the separator. Proui- QQQ) sion musttherefore be made deal with the removal of set- to “Hazardous Wastes from Specific Sources” (40 Code of tleable solids that accumulate in the separator. Federal RegulationsPart 26 1.32) SECTION 2-CONVENTIONAL OIL-WATER SEPARATORS 2.1 Design Procedures jointly by the American Public Health Association(APHA), the AmericanWater Works Association (AWWA), and the 21 .I GENERAL . Water Pollution Control Federation(WPCF). The presence API has establishedcertain design criteria for determining of compounds suchas organic sulfur compounds can cause the various critical dimensions andphysical features of the artificially elevated results in the oil-and-grease determina- separator. These are presented in a series of step-by-step de- tion. Since oil is a complex mixture different compounds, of sign calculations. The design calculations require that certain different test methods can give different results. differ- These characteristics of the wastewater be known. The morethor- ences may not beconsistent between waste streams. A sum- oughly the wastewater is characterized, the more predictable mary of the applicability of these tests is given in Table 1. the performance of the separator will be. An analogous ASTM method for determining oil and grease and petroleum hydrocarbons in water is D 3921. This 2 1 2 WASTEWATER .. CHARACTERIZATION method is similar to EPA Method 413.2, except that thevalid concentration range is 0.5-100 milligrams perliter. Again, 2 1 2 1 General ... none of these methods distinguishes among emulsified, free, When selecting test methods for oil in wastewater, one or dissolved oils. must considerthe testobjective. A number of methods are Over thirty .years ago, API developed an operational available for measuring oil, and the solids fraction includes method for determining the emulsified and dissolved oil total, filterable, settleable, and dissolved fractions. Depend- fraction. This method,entitled “Determinationof Suscepti- ing on the objective, any of the methods maybe applicable. bility to Oil Separation,” is presented in Appendix B. The method employsthe use of a separatory funnel to roughly 2 1 2 2 Free, Emulsified, and Dissolved Oil ... The total oil and grease content of a wastewater can de- be termined by the U.S. Environmental Protection Agency’s lAmerican Society for Testing and Materials, 1916 RaceStreet,Philadel- (EPA’s) Methods 41 3.1, “Gravimetric Separation,” and f ~ ~ ~ ~ ~ ~ ~ Agency.~Thecode ofFederalRegrc,ations Y ~ ~ ~ ~ ; c t i o n 413-23 “Infrared Spectrophotometry” 121*Corresponding Printing- Office, Washington, D.C. is available from the U.S. Government - tests areStandardMethods503A and 503B [3], published 20402. /COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  10. 10. A P I PUBL*1l2L 90 m 0732270 0087373 b m 4 API PUBLICATION 421 Table 1-Applicability of EPA Methods ing for low velocities in the channel preceding the main sep- 413.1 and 413.2 arator basin. This channel can serve as a type of grit chamber and can be beneficial in reducing solids problems within the Concentration Range for Which the separator itself. Method Valid" Representativeness Is of Settleable solids (those that settle by gravity over a given Method (milligrams per liter) Results Analytical period of time) are usually measured on a volumetric basis EPA 413.1 5-1000 Measures only nonvolatile hydrocar- (see Standard Method 209E.3.a [3] or EPA Method 160.5 bons (volatilizingat >70°C), exclud- [2]). The results of this procedure indicate the approximate in ing residue materials insoluble volume of settled materials that will need to be removed Fluorocarbon 113 from the bottom of an oil-water separator. The characteris- EPA 413.2 0.2-1000 Measuresvolatile most hydrocabons in addition to heavier compounds,ex- tics of the settleable solids should be noted to determine how cluding residue materials insoluble in much grit is present. Based on the tradeoff between the Fluorocarbon 113 quantity of grit in the wastewater and the amount of separa- aWastewaters withoil concentrations greaterthan the maximum can be tor down time required for grit and sludge cleanout, a grit readily analyzed by sample dilution. collector may be advisable. simulate the quiescent settling conditions in an oil-water 2.1.2.4 OtherWastewaterCharacteristics separator. Wastewater allowed to separate in the funnel for is No less important than the two parameters discussed in 30 minutes. Settleable solids are then withdrawn from the 2.1.2.2 and 2.1.2.3 are the specific gravities of the oil and water phase, and the water phase is analyzed oil andfor water phases and the absolute viscosity of the wastewater, grease (the more current EPA Methods 413.1 or 413.2 or the which are both evaluated at the minimum design tempera- equivalent APHA/AWWA/WPCF Standard Methods 503A ture. Assuming all other conditions are equal, the greater the and 503B should be used in lieu of older API methods.) The difference in specific gravity between the water and oil amount of oil and grease remaining in the water phase after phases, the better the oil-water separation will be. The spe- the separation period represents theoil that is not susceptible cific gravity of the oily phase can be determined using the to gravity separation. procedure given in Appendix B.The specific gravity and ab- The amount of free oilcan be determined by analyzing a solute viscosity of the wastewater should be determined ex- comparable wastewater sample for total oil and grease using perimentally where possible. In the absence of such data, the EPA methods or the APHA/AWWA/WPCF standard values for the specific gravity and absolute viscosity of fresh methods discussed above and subtracting the amount of oil and saline water can be estimated from Figures and 2. 1 and grease remaining in the water phase, as found using Ap- ASTM D 1429 can be used to determine the specific gravity pendix B. D of wastewater. ASTM Method 4-45can be used tomeasure It is recommended that Appendix B or a comparable the viscosity of wastewater. The temperature of the wastewa- method be used in designing an oil-water separator, partic- ter can have a pronounced effect on the efficiency of sep- the ularly in applications where the separator must stand alone. arator. In general, for the temperature range experienced in refinery wastewaters, the lower the temperature, the poorer 2.1.2.3 Solids Content the oil-water separation will be. Although the primary function of an oil-water separator is 2.1.3 ESTABLISHING A DESIGNFLOW to remove free oil from aqueous waste streams, the solids content of the wastewater is important in selecting separator The design flow for a conventional oil-water separator is appurtenances. Total suspended solids can be measured us- primarily determined by two factors: the expected wastewa- ing Standard Method 209C [3], EPA Method 160.2 [2], or ter flow rate, and the safety factor desired to accommodate ASTM D 1888. Thesuspended-solids content of an oily flow variations. Considerationshould also be given topossi- wastewater may not be accurately measured by these meth- ble future refinery expansion. Unless flow equalization is ods if compounds that volatilize at temperatures at or below provided upstream of the separator, the design flow should 103°C are present. These compounds may adsorb to partic- be based on the maximum flow rate attributable to current ulate matter in the sample and cause difficulties in determin- and future oil-contaminatedprocess wastewaters and storm- ing when the sample has been dried to constant weight. water runoff. Regulatory requirements for spill control can Ignition of the dried sample from Standard Method 209C also be a factor in identifying stormwater treatment require- or an equivalent (following Standard Method 209D [3] or ments. EPA Method 160.4 [2]) indicates the quantity of organic and Uncontrolled surface runoff or storm drainage can greatly inorganic materials present in the wastewaters solid compo- increase flow to a separator for a short period of time. In nent. Sand and similar materials may be removed by design- such a case, flow equalizationupstream ofthe oil-water sep-/ .COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  11. 11. A P I PUBL*42L 90 m 0732290 0087374 8 m DESIGN OPERATION AND OF OIL-WATER SEPARATORS 5 1 .O30 1.020 .? z g 1.010 .- o c o a l c% 1.o00 0.990 0.980 40 50 60 70 80 90 1 O0 110 120 Temperature, degrees Fahrenheit Figure i-Specific Gravity of Clear Water (Fresh and Sea) for Temperatures Between 40°F and 120°F arator is beneficial. In the event that such equalization cannot runoff that never contacts oil should be excluded from the be provided, the separator must handle the design storm flow process wastewater separator system. of direct runoff from the drainage area. The size of the storm When flow exceeds the design capacity of the separator, flow load on a separator can be estimated based on fre- the separator continues to function, but it does so with re- quency, intensity, duration of rainfall. Data on these fac- and duced efficiency. The frequency and duration of hydraulic tors for various localities can be obtained from state and overloads can be reduced by providing additional separator federal agencies. The amount of runoffis a function of rain- capacity. Alternatives are to divert flow that exceeds the de- fall intensity, soil porosity, the size of the drainage area, and sign flow allowance to an impounding basin for temporary the percentage of the area that is paved. If practical, surface storage or to separate extraneous flows from the system (forCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  12. 12. A P I PUBL*:L(21 90 m 0732270 0087375 T m 6 API PUBLICATION 421 0.04 0.03 0.02 0.01 0.009 0.008 0.007 0.006 0.005 Figure 2-Absolute Viscosity of Clear Water (Fresh and Sea) Temperatures Between 40°Fand 120°F for example, uncontaminated runoff, utility blowdowns, and 2.1.4 STEP-BY-STEPDESIGNCALCULATIONS non-oily process wastewater) for separate handling or tie-in to wastewater the treatmentsystemdownstream of the 2.1.4.1 General oil-water separator. In certain cases, diversion of low-oil, high-pH waste streams from the separator can enhance its As discussed above, the following parameters are required performance. for the design of an oil-water separator:COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  13. 13. A P I PUBL*q2L.S0 W 0 7 3 2 2 7 0 0 8 7 3 7 b 0 L W DESIGN AND OPERATION OF OIL-WATER SEPARATORS 7 a. Design flow (Q,), the maximum wastewater flow. The Where: design flow should include allowance for plant expansion V, = vertical velocity, or rise rate, of the design oil glob- and stormwater runoff, if applicable. ule, in centimeters per second. b. Wastewater temperature. Lower temperatures are used for g = acceleration due to gravity conservative design. = 981 centimeters per second squared. c. Wastewater specific gravity (S,). y = absolute viscosity of wastewater at the design tem- d. Wastewater absolute (dynamic) viscosity Q. perature, in poise. e. Wastewater oil-fraction specific gravity (S,). Higher val- p, = density of waterat the design temperature,in grams ues are used for conservative design. per cubic centimeter. f. Globule size to be removed. The nominal size is 0.015 p = density of oil at the design temperature, in grams , centimeter, although other values can be used inconjunction per cubic centimeter. with Equation 1if indicated by specific data. D = diameter of the oil globule to be removed, in cen- The design of conventional separators is subject to the fol- timeters. lowing constraints: S, = specific gravity of the wastewater at the design tem- perature (dimensionless). a. Horizontal velocity (vH)through the separator should be S, = specific gravity of the oilpresent in thewastewater less than or equal to 3 feet per minute or equal to 15 times (dimensionless, not degrees MI). the rise rate of the oil globules (V,),whichever is smaller. b. Separator water depth (d) should not be less than 3 feet, to After an oil-globule rise rate (V,) has been obtained from minimize turbulence caused by oil/sludge flight scrapers and Equation 1or 2, the remaining design calculations may be high flows. Additional depth may be necessary for installa- carried out as described i 2.1.4.2 through 2.1.4.7. n tions equipped with flight scrapers. It is usually not common practice to exceed a water depth of 8 feet. c. The ratio of separator depth to separator width typically 2.1.4.2 HorizontalVelocity (vH) ranges from 0.3to 0.5 in refinery service. The design mean horizontal velocity is defined by the d. Separator width is typically between 6 and 20 feet and smaller of the values for vH,in feet per minute, obtained from conforms to standard dimensions for flight scraper shaft the following two constraints: lengths specified for sludge removal. e. By providing a minimum of two separator channels, one V , = 1 y I3 5 (3) channel is available for use when it becomes necessary re-to move the other from service for repair or cleaning. These constraints have been established based on operating experience with oil-water separators. Although some sepa- The following design suggestions are also made: rators may be able to operate at higher velocities, 3 feet per minute has been selected as a recommended upper limit for a. The amount of freeboard specified shouldbe based on conventional refinery oil-water separators. Most refinery consideration of the type of cover to installed and the be process-water separators operate at horizontal velocities maximum hydraulic surge used for design. much less than 3 feet per minute at average flow. All separa- b. A length-to-width ratio (LIB) of at least 5 is suggested to tors surveyed had average horizontal velocities of less than provide more uniform flow distribution and to minimize the 2 feet per minute, and more than half had average velocities effects of inlet and outlet turbulence on the main separator less than 1foot per minute, based on typical or average flow channel. rates (see Appendix C). Maximum flow rates were not re- c. When the target diameterof the oil globules to be re- ported in the survey; however, design flow rates were typi- moved is known to be other than 0.015 centimeter, Equation cally 1.5-3 times the typical average flow rates. 1should be used to compute oil-globule rise rates. Equation 2 assumes an oil-globule size of 0.015 centimeter and repre- sents a typical design approach. 2.1.4.3MinimumVerticalCross-SectionalArea (A,) (See Figure3) Figure 3 shows an oil-water separator and depicts the de- sign variables listed above. Repeating Equations l and 2, Using the design flow to theseparator (&,J and the se- lected value for horizontal velocity (vH),the mínimum total r: = (8118Y)(PWPO)PZ (1) cross-sectional area of the separator (A,) can bedetermined from the following equation: r: = 0.0241- S, - P so (2) Ac = QmIVH (4)COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  14. 14. - - A P I PUBLm42L 70 M 0732270 0087377 3 m 8 API PUBLICATION 421 rulcuay - TOP VIEW I -L- I END SIDE VIEW A, = totalcross-sectionalarea. AH = total surface area of separator. B = widthofchannel. d = depth of water. L = lengthof channel. n = total number of separator chambers. Q, = influent flow. v, = horizontal velocity. V, = rise rate of oil globules. Figure 3”Design Variables for Oil-Water Separators Where: Where: A, = minimum vertical cross-sectional area, in square A, = minimum vertical cross-sectional area, in square feet. feet. Q,= design flow to the separator, in cubic feet per Fractional numbers of channels are rounded up to the next minute. whole number, based on engineering judgment. vH= horizontal velocity, in feet per minute. 2.1.4.4ChannelWidthandDepth 2.1.4.4Number of Separator Channels Given the total cross-sectional area of the channels (A,) Required (n) and the number of channels desired (n),the width anddepth In general, the most economical installation is one that of each channel can be determined. A channel width (B) of minimizes the number of channels required. However, a between 6 and 20 feet should be substituted into the follow- minimum of two channels is recommended to allow for sep- ing equation, solving for depth (d): arator maintenance without bypassing the entire separator. d = A, / Bn To minimize the number of channels, the cross-sectional area should be maximized in conformance with the design con- Where: straints set above. Typically, the maximum cross-sectional d = depth of channel, in feet, dimensions recommended for a singlechannel are 20 feet A, = minimum vertical cross-sectional area, in square wide and 8 feet deep (160 square feet), On this basis, the feet. number of channels (n)required is calculated as follows: B = width of channel, in feet. n = A,/160 (5) II = number of channels (dimensionless).COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  15. 15. ? API PUBLm42L 70 0732270 0087378 5 DESIGN OPERATION OF OIL-WATER AND SEPARATORS 9 The channel depth obtained should conform to the ac- is greater than or equal to the water depth divided by the re- cepted ranges for depth (3-8 feet) and for the depth-to-width tention time will reach the surface, even if it starts from the ratio (0.3-0.5). If the depth obtained fails to meet either of bottom of the chamber. When the rise rateis equal to the these criteria, different separator widths should be tried until overflow rate, this relationship is expressed as follows: a depth that meets these criteria is obtained. 2.1.4.5 Separator Length Qm Once the separator depth and width have been determined, the final dimension, the channel length (L),is found using Where: - the following equation: di = depth of wasfewater in an ideal separator, in feet. Ti = retention time in an idealseparator, in minutes. L = F(vH/ v ) d (7) Li = length of an ideal separator, in feet. Where: Bi = width of an ideal separator, in feet. v, = ovefflow rate, in feet per minute. L = length of channel, in feet. F = turbulence and short-circuiting factor (dimension- Equation 8 establishes that the surface area required for an less). ideal separator is equal to the flow of wastewater divided by vH = horizontal velocity, in feet per minute. the rise rate of the oil globules, regardless ofany given or as- V, = vertical velocify of the design oil globule, in feet signed depth. per minute. By takinginto account the design factor (F), the minimum d = depth of channel, in feet. horizontal area (AH), is obtained as follows: If necessary,the separators length should be adjusted to be A, = F(?) at least five times its width, to minimize the disturbing ef- fects of the inlet and outlet zones. Where: Equation 7 is derived from several basic separator relations: F = turbulence and short-circuiting factor (dimension- less). a. The equation for horizontal velocity (vK = Ac/Qm),where Q, = wastewater flow, in cubic feet per minute. A, is the minimum total cross-sectional area of the separator. b. The equation for surface-loading rate (V,= Qm/AH),where A is the minimum total surface area of the separator. H 2.1.5 DESIGN EXAMPLE c. Two geometricalrelations for separator surface and cross- 2.1 5 1 WastewaterCharacteristics section area (AH = LBn and A, = dBn),where n is the number of separator channels. In this example, the wastewater has the following charac- teristics: A derivation of this equation is given in Appendix A. a. A design flow rate (Q,) of 4490 gallons per minute. The turbulence and short-circuiting factor ( F ) is a com- b. A minimum temperature of 105°F. posite of an experimentally determined short-circuiting fac- c. A specific gravity () of 0.992. S, tor of 1.21and a turbulence factor whose value depends on d. An absolute (dynamic) viscosity () of 0.0065 poise. J the ratio of mean horizontal velocity (vH) the rise rate of to e. A maximum oil specific gravity ( , of 0.92. S) the oil globules (V,). A graph of F versus the ratio v& is given in Figure 4; the dataused to generate the graph are also given. 2.1 S.2 DesignCalculations 2.1 S.2.1 The rise rate for the oil globules is calculated us- 2.1.4.6MinimumHorizontalArea ing Equation 2. For a globule with a diameter (D) 0.015 of centimeter, In an ideal separator-ane in which there is no short-cir- cuiting, turbulence, or eddies-the removal of a given sus- pension is a function of the overflow rate, that is, the flow = 0.0241- sw - so rate divided by the surface area. The overflow rate has the P dimensions of velocity. In an ideal separator, any oil globule 0.992 - 0.92 = 0.0241 whose rise rate is greater than or equal to the overflow rate O. 0065 will be removed. This means thatany particle whose rise rate = 0.267COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  16. 16. ____ "" API PUBL*L121 90 m 0732270 0087377 7 W 10 API PUBLICATION 421 1.8 1.7 1.6 1.5 1.4 1.3 1.2 2 6 1610 14 20 %/I: VHW, Turbulence Factor (F,) F = 1.2(FJ ~ ~ 20 1.45 1.74 15 1.37 1 .a 10 1.27 1S 2 6 1.14 1.31 3 1 .O7 1.28 Figure 4"Recommended Values of F for Various Valuesof v , / & 2.1 5 2 . 2 The maximum allowable mean horizontal veloc- Since n must be greater than or equal to 2, this value is ac- ity is calculated using Equation 3: ceptable. , V = 15y I3 2.1 5 2 . 5 The width and depth of the channels are calcu- = 15(0.267) = 4 lated using Equation 6: 4 > 3 d = A,/Bn A velocity of 3 feet per minuteis used because this is the Assuming a channelwidth (B) of 20 feet, maximum recommended mean horizontal velocity. 200 2.1 5 2 . 3 The minimum vertical cross-sectional area is d = = 5 (20)(2 channels) calculated using Equation 4: The value of 5 obtained for d meets the requirement that d be Ac = Qm/VH greater than or equal to 3 but less than or equal to 8. How- Q, = 4490 / 7.48 = 600 ever, withd = 5, dlB = 0.25, which fails to meet the require- A, = 60013 = 200 ment that dlB be greater than or equal to 0.3 but than or less equal to 0.5. Therefore, a smaller value for B must be tried. 2.1 5 2 . 4 The number of separator channels required is Assuming a channelwidth (B) of 18 feet, calculated using Equation 5: 200 d = = 6 n = Ac/160 (18)(2 channels) = 2001160 The value of 6 obtained for d meets the requirement that bed = 125 greater than or equal to 3 but less than orequal to 8. In addi- :n =2 . tion, withd = 6, d/B = 0.3, which meetsthe requirement thatCOPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  17. 17. A P I PUBL*42L 90 M 0732290 0087380 3 m DESIGN OPERATION OF OIL-WATER AND SEPARATORS 11 d/B be greater than or equal to 0.3 but less than orequal to 2.2.2.2 Preseparator Flume 0.5. Furthermore, B meets the requirement that it begreater The preseparator flume is located between the end of the than or equal to 6 but less than or equalto 20. Therefore, a inlet sewer and the separator forebay and serves to reduce channel width of 18 feet and achannel depth of 6 feet are ac- flow velocity and separate light oil. Velocity reduction is es- ceptable dimensions. sential for evendistribution of flow to the separator chan- 2.1 5.2.6 The length of the separator is calculated using nels. Grit deposition and oil separation will occur as a result Equation 7: of the velocity reduction. Velocity reduction with minimum L = F(vH/ v ) d turbulence is accomplished by a transition section located between the inlet sewer andthe preseparator section. For From2.1.5.2.1and2.1.5.2.2,v,/lr=3/0.267=11.23.There- good hydraulics, the sides of the transition section are flared fore, from Figure 4,F = 1.55. laterally and the floor is sloped from the sewer invert to the separator-floor elevation. L = (1,55)(11.23)(6) = 105 The effluent end of the preseparator section usually con- A check should performed to determine that LIB is greater be tains three pieces of equipment: a bar screen or trash rack, an than or equal to5. In this case, LIB is equal to 5.8, so the oil skimmer, and an oil-retention baffle, in that order. To check is satisfied. minimize evaporative losses, the preseparator flume and transition section may be required to be covered by govern- 2.1 5 2 . 7 The separator configuration resulting from this ment regulations. example consists of two channels, each 105 feet long and 18 Note: Refer to “Standards Performance forVOC Emissions íÏom Petro- of feet wide, with a water of 6 feet plus desired freeboard. depth leum Refinery Wastewater Systems” (40 Code o Federal Reg~tlations f Part The series of calculations above illustrates the fundamental 60, Subpart QQQ, which addresses air emissions fiom oil-water separa- tors. separator chamber dimensions needed for detailed unit de- sign, The construction details given in 2.2 provide informa- Floating oil that has been trapped in this section of the flume tion on oil- and sludge-removal equipment and other factors should be skimmed and removed at periodic intervals. pertinent to the implementation of separator design. Velocities in the preseparator section on the order of 10-20 feet per minute, with a 1-2-minutedetention time, are considered satisfactory. Experience has shown that most of 2.2 Construction Details the oil flowing to a separator can be recovered in the presep- 2.2.1 GENERAL arator flume or preseparation section. If needed, means should be provided for removal of settleable solids deposited A conventional oil-water separator installation (see Fig- in this section. These means can include grit rakes or provi- ure 5) consists of two basic sections-the inlet section and sion of multiple channels in the preseparator flume to allow the oil-water separator channels. The components of these one channel to betaken out of service for cleaning without sections are discussed in 2.2.2 through 2.2.4, and informa- having to bypass the entire oil-water separator. tion on the types of appurtenances currently used is pro- vided. 2.2.2.3Trash Rack Trash racks or bar screens can be placed upstream of the 2.2.2 INLET SECTION separator to remove sticks, rags, stones, and otherdebris that 2.2.2.1 General would interfere withthe operation of downstream equip- ment. The trash rack consists of a series of inclined bars The inlet area serves to distribute flow to the separator spaced on 1-2-inch centers, at an angleof 45-75 degrees channels. Some separation of oil from the wastewater occurs from the horizontal, depending on the depth of the flume. A here in addition to removal of some grit and floating debris. perforated pan draining into the flume is provided to receive If the separator is to receive highly variable flows, such as the debris removed from the bars. Both manually cleaned de- those from uncontrolled stormwater runoff, the overall de- vices and mechanically cleaned trash racks are available. sign may need to incorporate criteria for flow equalization. The maincomponents of a typical inlet section are illus- 2.2.2.4 Oil Skimmer and Retention Baffle trated in Figure 5. These may include a preseparator flume, a trash rack or automatic bar screen, an oil skimmer, and a An oil skimmer is generally installed downstream of the forebay. The gateways for channel shutoff are the division trash rack for removal ofoil from the inlet section. Rotatable between the inlet section and the separator channels. I total f slotted-pipe skimmers are normally preferred for this loca- separator system bypass is desired, additional gates should tion. A floating skimmer can be employed where the flow be installed at the head of the inlet section. level is expected to vary significantly.COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services
  18. 18. APL PUBL*92L 90 U 0732270 O O B 7 3 B L 5 U 12 API PUBLICATION 421 4 a Preseparator flume ___) diffusion device . section c .- c O o t"-llIl- 8 .- C O a o o Skimmed- -+ 1 oil line Rotatable slotted- Oil-retention baffle N / Flow d Effluent weir and wall / Incoming sewer Effluent flume B J section Channel gateways SECTION A-A Velocity-head Flight scraper chain sprocket Oil-retention baffle Gateway pier diffusion device T Flights Flight scraper chain Rotatable slotted- pipe skimmer 1 .Effluent weir and wall Effluent sewer Separat f ,Effluent flume collection hopper Slot for channel gate Sludge pump suction pipe Discharge from sludge collection hopper (with plug) SECTION B-B are are Note: Separator pumps allocated for specific service as follows but manifolded to permit sparing and to provide flexibility of operations. Pump used transfer of the oil-water mixture from the skimmed-oil sump A is to to the water-removal vessel. Pumpis used to transfer of sludge from the separator to the sludge-handling facil- B ities. Figure 5-Conventional Oil-Water Separator (Uncovered)COPYRIGHT American Petroleum InstituteLicensed by Information Handling Services

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