Dp19a1

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 1 of 64 Date December, 1999 Changes shown by ➧

CONTENTS Section

Page

SCOPE ............................................................................................................................................................ 4 REFERENCES ................................................................................................................................................ 4 DESIGN PRACTICES............................................................................................................................. 4 OTHER REFERENCES .......................................................................................................................... 4 DEFINITIONS .................................................................................................................................................. 5 BACKGROUND .............................................................................................................................................. 6 SEPARATOR TYPES AND APPLICATION.................................................................................................... 6 DECISION TREE FOR PRIMARY OIL / WATER SEPARATOR SELECTION........................................ 6 GENERAL APPLICATIONS .................................................................................................................... 6 API SEPARATORS................................................................................................................................. 7 PARALLEL PLATE SEPARATOR........................................................................................................... 7 SKIM PONDS.......................................................................................................................................... 7 DABURT SEPARATOR .......................................................................................................................... 8 PACKAGE OIL / WATER SEPARATOR ................................................................................................. 8 At Source Pretreatment........................................................................................................................ 8 Augment API Separator ....................................................................................................................... 8 Pressurized Wastewaters..................................................................................................................... 9 CYCLONE SEPARATOR........................................................................................................................ 9 CENTRIFUGES .................................................................................................................................... 10 QUICK, ROUGH SIZING BASIS ................................................................................................................... 10 API SEPARATOR ................................................................................................................................. 10 PARALLEL PLATE SEPARATOR......................................................................................................... 10 PACKAGE OIL / WATER SEPARATORS............................................................................................. 11 BASIC DESIGN CONSIDERATIONS ........................................................................................................... 11 OIL / WATER SEPARATION THEORY ................................................................................................ 11 CONTAMINANTS ................................................................................................................................. 12 VENDOR SPECIFIC INFORMATION ................................................................................................... 13 LAYOUT................................................................................................................................................ 12 MATERIALS OF CONSTRUCTION ...................................................................................................... 12 SEPARATOR COVERS ................................................................................................................................ 13 COVER TYPES..................................................................................................................................... 13 ADAPTABILITY OF COVERS TO EXISTING SEPARATORS.............................................................. 13 SEPARATOR COVER MATERIALS ..................................................................................................... 14 ALTERNATIVES TO COVERING SEPARATORS................................................................................ 14 OIL SKIMMERS ............................................................................................................................................ 14 SLOTTED-PIPE SKIMMER................................................................................................................... 14

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

Page

XIX-A1 2 of 64 Date December, 1999

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

CONTENTS (Cont) Section

Page

ROTARY DRUM SKIMMER ................................................................................................................. 14 HORSESHOE-TYPE FLOATING SKIMMER........................................................................................ 15 SELF-ADJUSTING, FLOATING OIL SKIMMER ................................................................................... 15 DISC SKIMMER.................................................................................................................................... 15 API SEPARATORS CONSTRUCTION DETAILS ........................................................................................ 16 INLET SECTION................................................................................................................................... 16 SEPARATION CHANNELS SECTION ................................................................................................. 17 DESIGN PARAMETERS............................................................................................................................... 21 DESIGN FLOW RATE .......................................................................................................................... 21 Existing Facilities ............................................................................................................................... 21 Grassroots Facilities .......................................................................................................................... 22 Contaminant Loadings ....................................................................................................................... 22 Load Growth / Contingency................................................................................................................ 22 INFLUENT OIL CONTENT ................................................................................................................... 22 OIL DROPLET SIZE DISTRIBUTION ................................................................................................... 22 INFLUENT TEMPERATURE ................................................................................................................ 22 SPECIFIC GRAVITY OF THE WATER PHASE ................................................................................... 23 SPECIFIC GRAVITY OF THE OIL PHASE........................................................................................... 23 VISCOSITY OF THE WATER PHASE.................................................................................................. 23 OPERATION AND MAINTENANCE ............................................................................................................. 23 IMPROVING PERFORMANCE OF EXISTING SEPARATORS ................................................................... 23 DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS................................................................ 24 API SEPARATOR ................................................................................................................................. 24 Channel Width and Depth.................................................................................................................. 25 PARALLEL PLATE SEPARATOR ........................................................................................................ 26 SAMPLE DESIGNS ...................................................................................................................................... 29 API SEPARATORS............................................................................................................................... 29 PARALLEL PLATE SEPARATOR ........................................................................................................ 31 COMPARISON ..................................................................................................................................... 32 NOMENCLATURE ........................................................................................................................................ 32 CALCULATION FORMS............................................................................................................................... 61 API SEPARATORS............................................................................................................................... 61 PARALLEL PLATE SEPARATORS ...................................................................................................... 63

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 3 of 64 Date December, 1999

CONTENTS (Cont) Section

Page

TABLES Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8

Summary of Primary Oil / Water Separator's Oil & Solids Removal .......................................33 Relative Cost Summary of Primary Oil / Water Separators ....................................................33 Advantages & Disadvantages of Primary Oil / Water Separators ...........................................34 Package Oil / Water Separator Vendors and Exxon Installations ...........................................35 Separator Covers Types Used In Exxon Affiliates ..................................................................36 Advantages & Disadvantages of Separator Covers................................................................36 Advantages and Disadvantages For Oil Skimmers ................................................................37 Potential Operation Problems and Solutions ..........................................................................38

FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23

Primary Oil / Water Separator Selection Guidelines ...............................................................40 API Separator .........................................................................................................................41 Cross Flow Parallel Plate Separator .......................................................................................42 Downflow Parallel Plate Separator .........................................................................................43 Daburt Separator ....................................................................................................................43 Hydrocyclones ........................................................................................................................44 Stokes Law Chart For Estimating Oil / Water Separator Area ................................................45 Gravity Displacement Type Oil / Water Separator ..................................................................46 Uncovered API Separators .....................................................................................................47 API Separator .........................................................................................................................48 Slotted Pipe Oil Skimmer........................................................................................................49 Rotary Drum Oil Skimmer.......................................................................................................50 Horseshoe Type Floating Oil Skimmer ...................................................................................51 Self-Adjusting, Floating Oil Skimmer ......................................................................................52 Disc Skimmer .........................................................................................................................53 Reaction Jet Inlets ..................................................................................................................54 Traveling Bridge Oil Skimmer and Sludge Collector...............................................................55 Four Shaft Collector Type Oil and Sludge Moving Device ......................................................55 Sludge Collection Hopper Arrangement .................................................................................56 Skimmed Oil Sump .................................................................................................................57 Typical Oil Droplet Size Distribution In Oily Wastewaters.......................................................58 Design Variables For Api Separators......................................................................................59 Recommended Values Of F For Various Values Of VH/Vt ......................................................60 Revision Memo 12/99

Made minor editorial changes. Revised oil/water separation theory equations and detailed oil/water separation equations for API and parallel plate separators to consistent units. Added section on specific vendor information required to assist in the selection of separation equipment. Included information on improving the performance of existing separators. Modified parallel plate total perpendicular cross sectional area to show it is not a function of the angle of inclination. Revised sample design equations to reflect above changes.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 4 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

SCOPE This section presents recommended process design considerations and procedures for primary oil / water separators. The primary oil / water separators covered in this section are:

•

API Separators

• •

Parallel Plate Separators / Interceptors

•

Daburt Separator

• •

Package Oil / Water Separator

Skim Ponds (Tanks)

Cyclone Separator

• ➧

Centrifuges A gravity type separator will separate other undissolved materials, e.g., solids, if their densities differ from that of water. However, the separator is usually sized for the separation of dispersed oil droplets from water / wastewater.

REFERENCES DESIGN PRACTICES 1. 2. 3.

DP XIX-A DP XIX-A2 DP XXIX-E

Guidelines for Selecting Wastewater Treatment Systems Flotation Units Civil Works

OTHER REFERENCES 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Design Practice 11-11-3, Exxon Company, U.S.A., Marketing Distribution, January, 1995. Black, B., Kilpert, R., Recent Developments in Oil / Water Separation Equipment, EE.66E.76, June, 1976. Bridgens, W.A.G., Covers for Antwerp API Separator, 92 EEEL 605, April, 1992. Bridle, M.K., Bayway Corrugated Plate Separator, 91 ECS2 163, November, 1991. Carlson, E.D., et al, MEFA: Minimum Emissions Facilities Assessment – Phase 2, EE.123E.92, February, 1993. Clesceri, L.S., Greenberg, A.E., Trussel, R.R., (ed.), Standard Methods for the Examination of Waste and Wastewater, American Public Health Association, Washington, 23rd Edition, 1993. Cozewith, C., Oil / Water Separation by Hydrocyclones, 59897, December, 1972. Eckenfelder Jr., W.W., Industrial Water Pollution Control, 2nd Edition, McGraw-Hill, New York, 1989. Feerick, C.P., Edwards, P., Boone, A., Evaluation of the Daburt Oil / Water Separator at the Fife Ethylene Plant, EE.20E.94, February, 1994. Foster, F.O., Whitegate Refinery Skim Pond Performance, 85 EEEL 363, February, 1985. Goodrich, Jr., R.R., Guidelines for Operating Oil and Water Separators, 88 ECS2 80, August 26, 1988. Green, G., Goodrich, Jr., R.R., Package Oil-Water Separators for Oily Wastewater Applications, EE.9E.87, February, 1987. Kaczmarek, S.A., API Separator Covers, A State-Of-The-Art Review – 1980, EE.101E.80, December, 1980. Kernkamp, A., Kilpert, R., API Separator Covers – A State-Of-The-Art Review, EE.71E.76, July, 1976. Konak, A.R., Bridle, M.K., BWN Vortoil Test Results, Oil Sands and Coal Facilities Engineering, July, 1988. Lipton, S., API-EPA Meeting Proposed Wastewater Regulations, 84ECS1 351, November, 1984. Meldrum, N., Conoco U.K. Ltd., Hydrocyclones: A Solution to Produced-Water Treatment, SPE Production Engineering, pp. 669 – 676, November, 1988. Monographs on Refinery Environmental Control – Management of Water Discharges – Design and Operation of Oil-Water Separators, API Publication 421, First Edition, February, 1990. Nelson, B.D., Vortoil Hydrocyclones Field Test Report for Exxon Company U.S.A. Baytown Refinery, December, 1991. Svarovsky, L., Hydrocyclones, Mining Magazine, pp. 99 – 105, August, 1988.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 5 of 64 Date December, 1999

DEFINITIONS Dissolved Oil Dissolved or Soluble oil (particle size of less than 0.45 micron or that passing through a standard 0.45 micron lab glass fiber filter) is the petroleum fraction that forms a true molecular solution with water. Dissolved oil cannot be removed by gravity separation; further wastewater treatment (for example, biological treatment) is necessary if removal of dissolved oil is required. Effective Area Effective area of a separator refers to the horizontal projection of the separator plates. Emulsified Oil Emulsified oil is defined as small oil droplets (in the range of 5–50 micron diameter) that form a stable suspension in the water as a result of the predominance of interparticle forces over buoyant forces. The presence of fine particulates also contributes to emulsion formation and stabilization. Oil and Grease Oil and grease is a key indicator test for effluent compliance in the petroleum industry and is reported in units of ppm or mg/L. It measures the total of dissolved and suspended "oil" fractions extracted from a wastewater sample by an organic solvent, typically freon (trichlorotrifluoroethane) or other similar solvent. Caution should be used in interpreting the results of this test since it can measure naturally occurring organic compounds, which may give artificially high readings in the oil test. Specific test procedures may vary, according to government regulations. It is important to specify the lab method used along with data. Parallel Plate Separators The following acronyms are interchangeably used for parallel plate separators. CPI Corrugated Plate Interceptor CPS Corrugated Plate Separator PPI Parallel Plate Interceptor PPS Parallel Plate Separator (Pressurized Plate Separator) TPI Tilted Plate Interceptor Rise Rate The velocity at which oil droplets move toward the separator surface. Settleable Solids Settleable solids is the term applied to the material settling out of suspension within a defined period of 1 hour, using a special settling tube called an Imhoff cone. Suspended Oil Suspended or "Free " oil is in the form of discrete oil globules of a size sufficient to rise as a result of buoyant forces, forming an oil layer on top of the water. Under proper quiescent flow conditions, free oil can be removed by gravity separation. For example, an API separator is designed to remove free oil droplet with diameters greater than 150 microns. Some vendors of packaged plate separators claim to remove particle sizes less than 50 microns. Total Suspended Solids Total suspended solids is the amount of suspended matter as measured by filtering a known volume of wastewater and drying solids at 217°F (103°C) and is reported in units of ppm or mg/L. Inorganic particles such as clay or grit as well as organic particles (biological solids including algae) contribute to a waste stream's suspended solids concentration.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 6 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

BACKGROUND Many refinery, petrochemical plant wastewater streams contain oil in three major forms: suspended (free), emulsified, and dissolved. A gravity type oil / water separator, e.g., API Separator, Parallel Plate Separator, is a specially designed container or device to provide flow conditions sufficiently quiescent so that particles of free oil rise to the water surface and coalesce into a separate oil phase. Primary oil / water separators are used to separate and recover for reuse the undissolved oil from water. They will not separate soluble substances, break down emulsions, or remove soluble compounds that contribute to the Biochemical Oxygen Demand (BOD). Separators also provide conditions that allow the settling of suspended solid particles. Gravity separation of free oil is most widely applied wastewater treatment process in Exxon. API separators have been the standard for free oil removal for many years. Over the past two decades, newly developed "packaged" parallel plate gravity separators have been applied to many of the services previously reserved for the API separator. Segregation of the wastewater streams can sometimes afford savings in separator construction, and often will result in an effluent containing less oil than it would contain if the streams were combined. For example, it may be feasible to segregate (1) oil-free wastewater (such stream as steam turbine condenser water, oil-free storm runoff water, once-through cooling water in C5 and lighter service, and cooling tower blowdown in C5 and lighter service); (2) oily cooling water (once-through cooling water in C6 and heavier service); and (3) oily storm water; and (4) process water (such streams as desalter waters, tank water drawoffs, stripper condensate, pump gland cooling water, etc.). Waste streams (1) and (2) (with the exception of storm runoff which can have a large accumulation of silt) contain little or no suspended matter, and whatever oil may be present is clean and readily separable. These streams can sometimes be treated in special-purpose, "clean water" separators, or sedimentation ponds. Mechanical sludge-removal equipment can usually be omitted from these special purpose separators. The use of auxiliary or unit separators at various specific units in the plant is a valid waste management practice, and the installation of such auxiliary separators should be considered. Unit separators can provide early leak detection and correction, and potential reuse of "like-kind" products at the source. Their use reduces the oil load on the plant separator and may improve the quality of the final effluent. However, there may exist debits such as increase in number of pumps and spacing requirements. Therefore, appropriate wastewater management, including source reduction, recycle and reuse options, should carefully be analyzed before starting a new design or revamp of an existing facility to optimize separator size and performance. Although the design procedure for oil / water separators and some general design considerations are given here, the design and construction of an oil / water separator installation or the modernization of an old one may present problems or require decisions based upon factors peculiar to a specific refinery's processing units, layout, and location.

SEPARATOR TYPES AND APPLICATION DECISION TREE FOR PRIMARY OIL / WATER SEPARATOR SELECTION A decision tree, Figure 1, has been provided to assist in the selection of the best primary oil / water separator according to the oil / water characteristics. Should the influent to the separator not meet the categories in the decision tree, refer to Section XIX-A, Guidelines for Selecting Wastewater Treatment Systems, for a broader selection.

GENERAL APPLICATIONS Primary oil / water separators are used to remove gross amounts of free oil and suspended solids in process wastewaters. Typical inlet oil content ranges from 1,000 to 5,000 ppm (mg/L). The intent is to reduce the oil content to below about 50 to 100 ppm (mg/L) consistently and to recover the oil for recycle to the slop system. For some advanced primary oil / water separators, such as package separators and the Daburt separator, the effluent concentration has been determined to be as low as 10 to 20 ppm (mg/L), depending on the oil characteristics, such as oil droplet size. Typical effluent suspended solids content ranges from 20 to 200 ppm (mg/L). The oil / water separation efficiency is influenced by a number of factors, the most significant of which are inlet flow rate, oil concentration, oil droplet size, and the specific gravity of the oil to be separated. In addition, the extent of emulsion in the feed can have a great impact on the performance of the separator. Oil in oil / water separators, whether free, dissolved, or emulsified, is affected by the pH and energy induced upstream of the separator. Centrifugal pumps should be avoided and high pH should also be avoided. The specific applications for each major type of primary oil / water separator are briefly discussed below. Table 1, a summary of separators oil & solids removal, and Table 2, a relative cost summary, may also be helpful in deciding the primary oil / water separator type. The advantages and disadvantages for each separator are summarized in Table 3.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 7 of 64 Date December, 1999

SEPARATOR TYPES AND APPLICATION (Cont) API SEPARATORS The most common means of separating insoluble oil from water in refining and petrochemical wastewaters is the API (American Petroleum Institute) separator. The API separator consists of a rectangular concrete basin with multiple-channels (Figure 2) providing sufficient holdup time and quiescence to allow oil droplets to rise to the surface where they are skimmed away. Typically, this separator is set below grade, but depending on the topography of the site, above grade design may be considered for ease in sludge removal, and it should be covered to prevent volatile hydrocarbon emissions to the air. An API separator is sized to provide sufficient residence time to allow an oil droplet 150 microns in diameter to rise from the bottom of the separator to the surface, thereby providing complete removal of droplets 150 microns and larger. Smaller droplets have a slower rise rate (per Stokes' law, rise velocity is proportional to the square of the diameter), hence they are only partially removed. Influent water may contain an average of 1,000 to 5,000 ppm (mg/L) oil and a maximum of 10,000 to 50,000 ppm (mg/L). Effluent water oil content depends upon oil droplets size and size distribution, and upon the amount of emulsified and dissolved oil. With proper design, maintenance, and operation, the API separator effluent water oil content will usually be about 50 ppm (mg/L) or more, when measured by the oil and grease analysis.

PARALLEL PLATE SEPARATOR ➧

Parallel Plate Separator (PPS) is also commonly known as Parallel Plate Interceptor (PPI), Corrugated Plate Interceptor (CPI), Corrugated Plate Separator (CPS), or Tilted Plate Interceptor (TPI). These plate separators consist of a basin with numerous submerged parallel plates inclined about 45 – 60° in the direction transverse to flow (see Reference 7). Oil droplets rise beneath each plate, coalesce at the underside surface of each plate, and slide up the plate undersides to the edge of the basin, and from there rise to the water surface. Similarly, solids settle to each plate, agglomerate, and slide to the bottom of the basin. With PPS, oil drops need rise, at most, only the short distance between plates, rather than as far as the entire basin depth as with API separators. Therefore, much smaller residence time and hence greatly reduced basin dimensions are required as compared to API separators. Plate separators are generally designed to remove oil droplets larger than 60 microns compared to the 150 microns droplet size that can be removed by an API separator. The efficiency of an oil / water separator is inversely proportional to the ratio of its discharge rate to the unit's surface area. A separator's surface area can be increased by the installation of parallel plates in the separator chamber. The resulting parallel plate separator will have a surface area increased by the sum of the horizontal projections of the plates added. In cases where available space for a separator is limited, the extra surface area provided by a more compact parallel plate unit makes the PPS an attractive alternative to the API separator. Flow through a parallel unit can be two to three times that of an equivalent API separator of the same cross sectional area. According to vendors, the spatial requirements of oil / water separators can be reduced up to twofold on width and tenfold on length when a parallel plate unit is used in place of a conventional API separator. A variety of parallel plate equipment configurations are commercially available. Two major types of parallel plate separators marketed are: the cross flow parallel plate and the downflow parallel plate. In a cross-flow separator (Figure 3), flow enters the plate section from the side and flows horizontally between the plates. Oil and sludge accumulate on the plate surfaces above and below the wastewater flowing between the plates. As the oil and sludge build up, the oil globules rise to the separator surface and sludge gravitates toward the separator bottom. In a downflow separator (Figure 4), the wastewater flows down between the parallel plates, sludge deposited on the lower plates flows to the bottom of the separator, and oil accumulated beneath the upper plates flows countercurrent to the waste flow to the top of the separator.

SKIM PONDS A skim pond is a pond where wastewater is retained to allow oil, water, and sludge separation. They are generally located downstream of an oil / water separator, e.g., API separator, and are used as a polishing or secondary step to remove residual oil. Skim pond retention time can range from as low as 1 to 24 hours to a few months. After this period, oil skimmers and/or scraping devices are used to remove the surface oil or bottom sludges. Uses of skim ponds are strongly dependent on the cost of land and environmental regulations. Since the ponds are not covered, hydrocarbon emissions, odors and underground contamination are issues that must be considered before they are judged to be acceptable. Geometric shape of the pond is another design criteria for consideration, so that short-circuiting does not occur. That is, the wastewater should be evenly distributed over the entire pond area. Due to government regulations and land availability in most cases, skim ponds are undesirable for future applications, especially in the United States and in Europe. As an alternative, other oil / water separators configured as vessels that may be covered, such as, API separator, parallel plate separators, or secondary oil removal units such as flotation systems (see Section XIX-A2) should be installed.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

Page

XIX-A1 8 of 64 Date December, 1999

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

SEPARATOR TYPES AND APPLICATION (Cont) DABURT SEPARATOR The Daburt separator is a rectangular device with an inlet broad crested weir, a plate pack, an oil skimmer, and an outlet weir (Figure 5). A standard unit has a nameplate capacity of 330 gpm (75 m3/hr). In addition to using gravity, the Daburt design utilizes hydraulic energy forces to promote oil and water separation. By adjusting the levels of the inlet and outlet weirs, a stationary or standing wave is generated at the broad crested inlet weir. In hydraulic terms, these conditions correspond to the critical energy, depth, and velocity of the water flowing through the inlet channel. At these conditions there is a rapid transfer in hydraulic energy from potential (depth) to kinetic (velocity). While theory governing the separation of oil under these conditions is currently not well understood, it is possible that the acceleration across the wave increases the gravity effect, potentially enhancing the efficiency of separation. After flowing across the inlet weir, the water passes through a plate pack. The plates are spaced farther apart than in a traditional plate separator as their primary purpose is to provide a surface for microbial growth. The plate pack can be seeded with microorganisms, which may enhance oil removal by releasing or biodegrading oil that has remained attached to solids which shall be settled in the pack. Oil that floats to the surface of the settling chamber is then skimmed by either a floating disk skimmer or a slotted pipe, and the water exits via an underflow weir. Settled solids are removed via several sludge outlets. While there are currently no installations within Exxon, there are several installations throughout the world. Exxon, however, performed a pilot test at Fife Ethylene Plant. This test, which was not seeded with microorganisms, demonstrated that the Daburt Separator was highly efficient in removing oil droplets approximately 25 microns and larger for this specific wastewater. Testing is recommended before choosing this equipment to confirm performance and maintenance requirements. For more detailed information regarding Fife's test see Reference 12.

PACKAGE OIL / WATER SEPARATOR Package separators, or those separators manufactured to standard sizes typically ranging from 15 to 500 gpm (3.4 to 114 m3/hr), are presently being used in refining, chemical, marketing, production, and tanker applications. The package oil / water separators are a combination of coalescing and gravity separation plates. They are effective in high oil and solids service if careful attention is placed on providing adequate collection sumps and pumps to remove the oil and solids. The free oil removal for a given flow rate and separator is a function of the specific gravity of the oil and the water, the viscosity of the water (all of which are affected by temperature), the oil-droplet size distribution, and the overall oil concentration. Where possible, the units should be installed above grade so that the separator plate packs can be easily removed for cleaning and to minimize potential groundwater problems. Even under ideal conditions, gravity separators cannot be designed to remove oil droplets smaller than 10 microns. The capacity of a given separator is a function of the desired oil droplet size to be removed. There is generally no turn-down limitation on package separator; feed rates lower than the design rate result in better oil removals. Complete installation lists, including generally extensive refinery and chemical plant applications, are available from the vendors. Table 4 is a partial list of package oil / water gravity separators vendors and Exxon installations. At Source Pretreatment Package gravity separators can be used to pretreat oily wastewater from individual process units, tank draw-offs, and other high oil sources prior to sewer discharge. The oils are more concentrated at these points, resulting in more efficient removal. At source recovery is more likely to yield hydrocarbons in a purer, less contaminated form, thereby retaining a product value rather than merely slop oil value. Less oil to the sewer also reduces sludge production and improves wastewater treatment plant operation. Augment API Separator Package separators can be used in association with or in place of API separators to achieve lower effluent free oil levels. Unlike API separators, which are sized to remove 150 µm oil droplets and larger, package units are often sized for the removal of smaller diameter globules, thereby providing a higher quality effluent. Package separators may be used in place of API separators in both grass roots and revamp situations. These units may be an efficient means of meeting newer, more stringent regulations, and may obviate the need for more extensive secondary oil removal technologies, such as flotation, coalescing, and filtration.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 9 of 64 Date December, 1999

SEPARATOR TYPES AND APPLICATION (Cont) Package separators can also be used to treat skimmings from an API separator. In general, API separator skimmings tend to contain a significant amount of water. A package separator would serve to remove a significant amount of water from the skimmings, thereby reducing the load to (preventing overload) the plant slop system. Also, the use of a package separator to treat the skimmings would allow for more frequent or continuous skimming of the API separator. This results in improved API separator performance, especially during peak hydraulic flows where accumulated oil is often discharged with the API effluent. Frequent or continuous skimming serves to lower fugitive hydrocarbon emissions from API separators, and thus may obviate the need for expensive covers. The water phase separated from package-separator treatment of API skimmings is generally recycled back to the front of the API separator or sent to downstream treatment. Pressurized Wastewaters If wastewater is already pressurized, e.g., produced water, separators are available to perform oil / water separation. Package separators can be fabricated as closed vessels, operated at high pressures, and with no vapor space if desired. This capability makes them superior to API separators for treating oily wastewaters generated at high pressures, such as desalter water, produced water, and onsite process water. The pressure drop in going from high pressure to an atmospheric API separator will cause emulsification, poor quality effluent, and vapor release (emissions). This emulsification would not occur to an appreciable extent using a pressurized package separator. Also, the pressurized package unit would save pumping energy if subsequent treatment steps require elevated pressures. Pressurized systems can also be used to contain volatile emissions for recovery or treatment.

CYCLONE SEPARATOR Hydrocyclones have no moving parts, and separation is achieved by generating centrifugal forces of orders of magnitude higher than available in conventional gravity based separation equipment The oily water enters the hydrocyclone inlet (Figure 6) designed to impart a high velocity swirl to the fluid, which then flows through a concentric reducing chamber, a tapered tube, and, finally, a straight tube. The fluid accelerates through the concentric reduction tapered sections of the cyclone and the bulk of the separation occurs here. The smaller, slower moving oil droplets are recovered in the straight tube section. The lighter oil droplets migrate towards the lower pressure central core where an axial flow reversal occurs resulting in the lower density oil enriched phase being removed through a small diameter orifice in the center of the inlet head. The oil depleted water stream is flung to the outside of the vortex and exits continuously from the downstream end. Typical free oil removal efficiency is 60 to 99 percent. The oil and water separation efficiency is influenced by a number of factors, the most significant of which are inlet flow rate, reject rate, oil concentration, and differential pressure. A certain minimum flow rate is necessary to create the vortex motion and establish centrifugal separating forces which grow in intensity as the flow rate increases, thereby improving the separation efficiency. The efficiency increases to a point beyond which further increases in flow rate will cause performance deterioration. As flow rate increases, the pressure in the central core decreases, reducing the available pressure to drive the reject stream. The reject flow rate is inhibited resulting in a decline in the separation efficiency. The reject ratio is the ratio of the flow rate of the concentrated oil stream exiting the unit to the flow rate of the discharged clean water expressed as a percentage. As the reject ratio increases, the efficiency of separation increases until a plateau level is reached. Optimum separation efficiencies are achieved at about 1 percent reject ratio (20). The reject rate, and in turn, the reject ratio is affected by the level of inlet contamination. The higher the inlet oil concentration, the greater the required reject stream. High oil concentrations also result in high separation efficiencies. Large oil droplets move rapidly towards the central core, and at high oil concentrations, the probability of coalescence occurring to form larger oil droplets increases. The differential pressure is also an important operating parameter. Two distinct pressure drops exist across the hydrocyclone; the pressure differential between the inlet and the outlet and between the inlet and the reject stream. The latter is always the larger of the two and is more important since it determines the hydrocyclone capacity. The reject pressure can be maintained by automatically adjusting a pressure control valve in the reject line. The type of pump selected for handling oily water has a direct influence on system performance as do pump control and valve location. Centrifugal pumps are completely unsuitable for pumping oily water mixtures. Positive displacement pumps such as screw and gear pumps, while not entirely satisfactory, cause less droplet break-up than most. Although Exxon refineries and chemical plants have limited experience with hydrocyclones, e.g., field test at Baytown Refinery (22), this technology has been successfully installed and operated at some offshore production locations, and mining facilities where oil concentrations were very high (over 5%).

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 10 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

SEPARATOR TYPES AND APPLICATION (Cont) CENTRIFUGES As is the case with cyclones, centrifuges are based on the principles of centrifugal force. Relative to the wall of a container, any particle in a vortex will be accelerated outward. The vortex in the centrifuges is created by injecting the fluid tangentially into an approximately shaped chamber. Disc centrifuges are most often used for oil / water / solid separations into distinct phases where oil content is very high (over 5%). The concentric cones, or discs, serve much as the same purpose as the plates in a plate separator. Depending on certain design features, solids come out of disc centrifuges either as a slurry or as a thicker, more paste-like substance. Experience with this type of equipment is limited, but very good results were achieved at the Fawley Refinery where the stream to be treated was rich in solids and oil concentrations.

QUICK, ROUGH SIZING BASIS The following Quick, Rough Sizing Basis is for the purpose of quick oil / water separator size screening only. The rules of thumb should be used with caution. For design and detailed engineering, refer to Detailed Oil / Water Separator Size Calculations section of this Design Practice. A quick method to estimate gravity oil / water separator size (i.e., effective area) without data or details on the particular application at a given influent rate can be accomplished using Stokes chart (Figure 7). For example, given a temperature of 70°F and an oil / water specific gravity differential of 0.15, removal of oil droplet 60 microns and larger would require roughly 575 ft2 of effective area per every 200 gpm of influent. Note that effective area refers to the horizontal projection of the separator plates, and this Figure is for only one size separator.

API SEPARATOR The main critical parameter to size an API Separator is the flow rate. Knowing the design flow rate and using the procedure below, preliminary dimensions of an API Separator can be calculated. The minimum and the maximum flow rates for the following procedure are 1,500 gpm (340 m3/hr) and 7,000 gpm (1,600 m3/hr), respectively. For design flow rates less than 1,500 gpm (340 m3/hr), it is recommended to use the parallel plate or package oil / water separators. The rules of thumb below assume removal of oil droplet size greater than 150 microns and that there are 2 chambers. 1. Calculate the total cross sectional area for 2 chambers, in ft2, by dividing the flow, in gpm, by 22: Area = Flow / 22. 2. 3.

Assume the depth of each chamber to be 6 ft (1.8 m) for a design flow between 1,500 and 4,500 gpm (340 to 1,000 m3/hr) and 8 ft (2.4 m) for a design flow rate between 4,500 and 7,000 gpm (1,000 to 1,600 m3/hr). The width, in ft, for each chamber is the area from step 1 divided by 2 times the depth: Width = Area / (2 x depth)

4.

The length, in ft, for each chamber is about 17 times the depth: Length = 17 x depth

PARALLEL PLATE SEPARATOR ➧

Two major parameters are essential to determine the size of a parallel plate separator: flow rate and Reynolds' Number. See Eq. (15) for derivation of the Reynolds' Number and assumed range of values for laminar flow. When these two quantities are known, the projected horizontal surface area, (AH), in ft2, can be quickly calculated by AH = 91.2 x Flow / NRe, where the flow is in gpm units. The length, in ft, of the separator can be calculated by Length = 0.01 NRe.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 11 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

QUICK, ROUGH SIZING BASIS (Cont) PACKAGE OIL / WATER SEPARATORS The following table lists a consolidation of package oil / water separator dimensions from various vendors. These dimensions are for rough size estimates only, the vendor should be consulted for final dimensions. Flow, gpm (m3/hr)

Length, ft (m)

Width, ft (m)

Height, ft (m)

50 (11.4)

9 (2.7)

3 (1.0)

7 (2.1)

100 (22.7)

9(2.7)

4 (1.2)

7 (2.1)

200 (45.4)

9 (2.7)

7 (2.1)

8 (2.4)

500 (113.5)

14 (4.3)

9 (2.7)

7 (2.1)

BASIC DESIGN CONSIDERATIONS OIL / WATER SEPARATION THEORY ➧

Oil / water separation theory is based on the rise rate of the oil globules (vertical velocity) and its relationship to the surface loading rate of the separator. The rise rate is the velocity at which oil particles move toward the separator surface as a result of the differential density of the oil and the aqueous phase of the wastewater. The surface loading rate is the water flow rate to the separator divided by the surface area of the separator. In an ideal separator, any oil globule with a rise rate greater than or equal to the surface loading rate will reach the separator surface and be removed. An ideal separator is assumed to have no short circuiting, turbulence, or eddies. The required surface loading rate for removal of a specified size of oil droplet can be determined from the equation for rise rate. The mathematical relationship for the rise rate is provided by a form of Stokes' Law:  S − So   Vt = 10720 δ2  w  µw  

(Customary)

Eq. (1)

 1000 g   ( ρ w − ρo ) δ2 Vt =    18 µw 

(Metric)

Eq. (1)M

where: Vt δ Sw So µw g ρw ρo

= = = = = = = =

Vertical velocity, or rise rate, of the design oil globule, ft/min (mm/sec) Diameter of the oil globule to be removed, cm (cm) Specific gravity of the wastewater at the design temperature, dimensionless Specific gravity of the oil present in the wastewater, dimensionless Absolute viscosity of wastewater at the design temperature, in cP (cP) Acceleration due to gravity = 981 cm/sec2 Density of water at the design temperature, (g/cm3) Density of oil at the design temperature, (g/cm3)

There are two fundamental principles in designing and operating oil / water separators. They are: 1. The performance of the separator will be highly dependent on the difference between the specific gravity of the water and that of the oil. The closer the specific gravity of the oil is to that of the water, the slower the oil globules will rise. 2. Since the oil globules' rise rate is inversely proportional to the viscosity of the wastewater, oil globules will rise more slowly at lower temperatures. In addition to the parameters mentioned above, changes in flow rate, salinity, oil globule size, and the method of measuring oil concentration can also affect the apparent efficiency of removal.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 12 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

BASIC DESIGN CONSIDERATIONS (Cont) CONTAMINANTS Caustic or other basic compounds tend to emulsify oil, adversely affecting the oil / water separator performance. Surfactants, such as non-biodegradable detergents and phosphates used in washing operations, can also cause problems in the oil / water separator. These contaminants may also add toxicity to the water discharge. For these reasons, the discharge of these materials to the separator should be avoided. An alternative is to use steam or high pressure water for washing operations. Additionally, a "mud box" or sediment trap upstream of the separator should be installed to knock out heavy sediments, thus minimizing sludge buildup in the separator. Also, a screen should be installed to catch any floating debris before it washes into the unit. ➧

VENDOR SPECIFIC INFORMATION Final sizing and economics are based on using the sizing methods described later in this section and vendor input. Information required by the vendor to assist in selection and sizing of separation equipment may include:

•

Type of process stream and influent flow rates

•

Raw water characteristics (influent oil content, oil droplet size distribution, specific gravity of water and oil phase, viscocity of the water phase, total suspended solids, dissolved solids, temperature, pH)

•

Location of installation (plot space, configuration)

• •

Expected operating hours / day Process flow sheet

Selection of standard vendor design sizes may reduce cost. Often the incremental cost of choosing a slightly larger size separation unit is not high and may be prudent to allow operating flexibility.

LAYOUT ➧

The gravity displacement type oil / water separator in Figure 8 shows some of the auxiliary equipment (pumps, valves, etc.) that will be required and should be taken into consideration when determining the layout of the separator. The layout of oil / water separators (Figures 9 and 10) strongly depends on the plot space and topographical features of the site. Ideally, the separator should be located in a place that is lower relative to the location of the areas to be served by the separator, but higher than the water discharge location from the separator. This will allow all the oily water collection lines and clean water discharge lines to gravity flow to / from the separator. When, this is not possible due to the topography of the site, the separator should be located so the oil water collection lines can gravity flow into the separator. The discharge lines from the separator can be pumped to their destination. Installation of Archimedes' screw pumps to raise the influent into the separator may be used. Although some emulsification may occur, these devices are believed to have less of an emulsifying effect than do centrifugal pumps, which are commonly used for other wastewater pumping applications. Although separators can be constructed above or below ground, above ground separators are preferred from a potential groundwater contamination viewpoint. If it is a below ground separator, it may require double-walled vessel with leak detection provided between the walls of the separator depending on local regulations. Techniques against freezing, for example, insulation and heat tracing and burying the separator below the frost line, may be needed at select locations for certain wastewater streams, such as dedicated separator for ballast water. However, in general, separators are not insulated or heat traced. When installed, the separator is typically surrounded by a layer of sand or gravel. The separator will likely experience a net positive upward buoyancy at low inventories, during a heavy rainstorm when the sand or gravel layer captures and retains a large amount of water, or if a high water table exists. Groundwater level during maintenance (cleaning) periods must also be considered. Hold down straps attached to appropriately sized "deadmen" anchors may be necessary to prevent the separator from rising out of the ground. Consult ER&E CIVIL WORKS SPECIALISTS for specific information.

MATERIALS OF CONSTRUCTION Reinforced concrete has generally been the construction material of choice for large in ground API separators. Steel separators have been commonly installed above grade. Such factors as corrosion, leakage, structural strength, and buoyancy should be considered in the selection of materials. For plate packed separators, carefully consider material selection to avoid collapse of plate separators due to elevated temperatures and aromatic content of oil. The designer should consult EXXON'S MATERIALS SPECIALISTS before specifying construction materials for Primary Oil / Water Separators.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 13 of 64 Date December, 1999

SEPARATOR COVERS Uncovered gravity-type oil / water separators in refineries produce hydrocarbon evaporation losses which vary with ambient temperature, proportion of light ends in the oil, influent oil temperature, wind effects, and design parameters particular to each location. The hydrocarbon evaporation rate from an uncovered separator in a large refinery can be tons per day and reports have indicated it can result in significant air pollution, odor problems, and loss of recoverable oil. In fact, estimates of these emissions made with various models indicate from 13 to 210 lb (6 to 95 kg) of hydrocarbon are lost per 1,000 bbl of wastewater; a covered separator, on the other hand, has been estimated to lose 2 to 8 lb. (1 to 4 kg) of hydrocarbon per 1,000 bbl of wastewater (17). While covers usually have the primary purpose of meeting hydrocarbon emission standards, they have the added dividend of odor reduction. Odor can be a problem since human threshold values are often at the parts per billion level. Although there are usually no specific odor standards to be met, an operation producing unpleasant odors can be classified a public nuisance, resulting in legal sanctions.

COVER TYPES

➧

As listed in Table 5, Exxon typically uses two main types of covers on API separators: fixed and floating. The fixed covers are a permanent concrete steel, or non-supporting foam / aluminum sandwich cover which partially covers the length of the separator. Blowoff hatches are sometimes provided to minimize damage in the event of an explosion in the vapor space. Explosion proof electrical equipment and a low-level of work activity in the area minimize the possibility of an explosion. Vapors may be collected and recovered by conventional vapor-recovery equipment or venting is provided via pressure-vacuum vents 10 to 15 ft (3 to 4.5 m) high. There are four types of floating covers: foamed slabs, polyurethane foam encapsulated with fiberglass skin, polyethylene balls, and steel pontoons. "Foamglas", a cellular glass insulating material, uses slabs that are laid directly on the water surface without being joined. Flight scrapers are low and sweep the oil under the slabs to a slotted pipe skimmer at the end of the cover. Storm surges, however, have caused the slabs to come out of position, thereby reducing the effectiveness of the cover. Conservaflote (or formally known as Hammondflote II) is a continuous laminar construction consisting of corrosion-resistant fiberglass skins applied over an impermeable closed cell rigid polyurethane core. A liquid and vapor type joint fastens the panels together and allows for slight differential movement. A gel coating on the top surface helps prevent deterioration by ultra-violet light. Hollow plastic spheres (polyethylene) have also been used to cover oil separators. They range in size from 1.5 to 4.0 in. (38 to 100 mm) in diameter and resemble heavy duty ping pong balls. The ball blanket adapts itself automatically with rising and falling liquid levels, thus maintaining a seal at all times. A second layer of balls results in a marginal further improvement in evaporation reduction and completely obscures the liquid surface. The steel pontoon cover is actually an entire automated slop oil recovery system. It features automatic pumpout of oil which has accumulated beneath the cover. The pontoon is designed to float in the oil layer; that is, above the oil / water interface. Sludge is removed semi-annually by draining one channel at a time. While the channel is empty, the pontoon rests on legs about 3 ft (1 m) above the bottom. Partial onstream cleaning is performed using hoses in the forebay to suck out some of the sludge. The explosion hazard associated with fixed covers is minimized in floating cover installation, since the vapor space is eliminated. The advantages and disadvantages for the two main types of separator covers are listed in Table 6.

ADAPTABILITY OF COVERS TO EXISTING SEPARATORS Many separators have mechanical systems which move the floating oil to a skimming device and scrape the settled sludges into bottom hoppers. These installations have either continuous flight scrapers or traveling bridges that move on rails mounted on the separator walls. A fixed roof can easily be installed over a continuous flight system. However, the traveling bridge would require construction of a low building or replacement of the bridge with a continuous flight system. Floating covers usually require modification of the conventional system. The flight scrapers may be lowered so that they function at the bottom of the floating sections, or oil may be allowed to accumulate under the cover and then be periodically pumped out. Unfortunately, oil / water separators do not have uniform dimensions or standardized equipment. They are, in general, designed on hydrodynamic principles set forth by the American Petroleum Institute, but there the resemblance ends. Hence, it is not possible to purchase a standard off-the-shelf cover; and tailor-made units need to be designed to fit specific feed situations.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 14 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

SEPARATOR COVERS (Cont) SEPARATOR COVER MATERIALS A variety of materials from metals to fabrics are available for covering API separators. Needs for structural strength, chemical and fire resistance, safety and convenience can influence the choice and design of a particular material. The two major factors in determining the fixed cover materials are corrosion resistance and hydrocarbon resistance. Typical fixed cover materials used are carbon steel with coal tar epoxy coating, fiberglass, aluminum, concrete, wood planks, or silicon-cloth fabric surrounded by layers of polytetrafluoroethylene (PTFE). Floating covers are constantly in direct contact with the oily surface, and high density polyethylene spheres, fiberglass encapsulated panels, and foamglass sheets have demonstrated good chemical resistance.

ALTERNATIVES TO COVERING SEPARATORS The amount of oil evaporated from a separator is a function of many factors including wind speed at the oil surface, the area of oil / air interface, the time of exposure, and the oil layer's temperature. Studies indicate that a decrease in influent temperature reduces the evaporation. Other recommendations for reducing evaporation include limiting the surface area of oil and time of exposure to air by recovering more oil in the forebay and initial regions of API separators.

OIL SKIMMERS Below are brief descriptions of oil skimmers used at Exxon affiliates. Also provided in Table 7 are the advantages and disadvantages for each of them.

SLOTTED-PIPE SKIMMER The slotted-pipe skimmer (Figure 11) is the oldest and most common type in separator channels, but may not be suitable for the preseparator flume if water level in the flume varies significantly. The slotted-pipe skimmer is recommended and is normally the preferred type for API separator channel sections. It has the capability of removing a large amount of oil that could be encountered in the event of a spill. As the name implies, this skimmer consists of a length of pipe with slots; a 60-degree slot is common. The pipe is mounted horizontally across the entire length of the channel and is supported at both ends. The pipe is rotatable and the skimmed oil and water flows into the pipe through the slots when they are rotated below the liquid surface. Pipe diameters from 8 to 20 in. (200 to 510 mm) are used; the size depends upon the capacity required, the variation in liquid level, and the distance the skimmings must travel within the pipe. With several settling bays in parallel, the slotted-pipe skimmers are usually connected in series. When used in a multiple, parallel-chamber separator installation, rotatable slottedpipe skimmers are connected end to end in a line that drains to a sump located at one side of the installation. The oil skimmed from the channel farthest from the sump must flow to it through each of the succeeding skimming pipes. As a result, each succeeding downstream skimmer pipe should be large enough to allow collected oil from other channels to flow by gravity to the sump. Normally, a 10 in. (250 mm) skim pipe should be used for runs up to 40 ft (12.2 m), and larger pipe for longer runs. The slotted pipe can be rotated by a simple lever arrangement, chain and sprocket by a rack and pinion linkage, by worm gear, or by motor. Normally, the worm gear arrangement is recommended, since it allows more precise skimming. If a slotted-pipe skimmer is used, it should be designed to permit backward rotation, in addition to forward, to allow recovery of oil that accumulates between the skimmer and the retention baffle. The slotted-pipe skimmer is operated manually and intermittently – as required to remove accumulated oil. Automation of the skimming with a slotted-pipe has not been satisfactory and is not recommended.

ROTARY DRUM SKIMMER The rotary drum oil skimmer (Figure 12) is also commonly used. It consists of a drum, mounted in a horizontal position, partially submerged below the surface of the settled oil layer. The drum is rotated by an external motor; as the drum rotates it picks up a film of oil which adheres to the drum surface. The oil film is removed by a doctor knife and directed to a trough. The optimum rotational speed for the drum achieves a peripheral speed on the order of 0.5 to 1.5 ft/s (0.15 to 0.5 m/s). The exact proper rotational speed will depend upon the amount of oil to be removed and its viscosity. Drum diameters of 12 in. (305 mm) and 24 in. (610 mm) are available from one vendor. Submergence is on the order of 0.5 to 2 in. (13 to 50 mm). Submergence is not critical as long as the drum is in contact with the oil layer; the drum selectively collects oil even if it is also in contact with water. It is preferable, however, to keep the skimming drum out of the water phase.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 15 of 64 Date December, 1999

OIL SKIMMERS (Cont) The rotary drum skimmer is not recommended if a large amount of oil is expected. When rotary drum skimmers are selected, specify that a variable speed drive and a polytetrafluoroethylene (PTFE) wiper blade are to be provided. The drum skimmer can be mounted in several ways. An "A"-frame mounted skimmer in which the frame rests directly on the separator bottom and requires no external supports is suitable for installation in existing separators. Pontoon-mounted rotary drum skimmers are applicable both where skimming is desired over many locations on the surface and where significant fluctuations in level are expected. These skimmers are available with special features; the skimmer drive, pump, and oil reservoir can be contained in the pontoon-mounted unit; ballasting provisions can be provided to adjust submergence; and two rotary drums can be provided for use where oil flows toward the unit from both directions.

HORSESHOE-TYPE FLOATING SKIMMER The horseshoe-type floating oil skimmer (Figure 13) consists of a floating collecting pan buoyed up by hollow chambers on three sides. The fourth side is open, contains an oil-skimming weir, and faces in the upstream flow direction to skim on-coming oil. This skimmed oil flows out a pipe or through a hose to a sump or other reservoir. Like the slotted-pipe skimmer, the floating skimmers can remove floatable solids in addition to oil. Floating oil skimmers can be a problem because of the difficulty in assuring that they will float at the proper level in a liquid whose density varies because of changes in waste stream composition. They are not recommended with a fixed liquid level, such as in the usual separator channel. Floating oil skimmers can be useful in the preseparator section if liquid level is expected to vary significantly in this section.

SELF-ADJUSTING, FLOATING OIL SKIMMER The self-adjusting floating oil skimmer, also known as the Baltimore Skimmer (Figure 14) consists of a floating tank with a portion of the front open near the top to form the oil-skimming weir. A hollow buoyancy chamber – in effect a tank within the tank – is included to float the skimmer at the proper level. The buoyancy chamber is mounted front of center, extends across the entire width of the skimmer, and is pierced by a number of pipes to allow the skimmed oil to flow to the reservoir behind the chamber. A vertical pump-suction pipe in this aft section allows the collected oil to be withdrawn. The device skims settled oil and collects it in the reservoir. When the collected oil is withdrawn through the pump suction line, the rear portion of the skimmer lightens, is buoyed to a higher elevation, and the skimming weir, consequently, dips lower, skimming more oil. Thus, the device is self-adjusting, skimming oil until full, then lowering the weir to skim more when the collecting pump is operated. As long as liquid is being withdrawn from the unit, it remains in the skimming position. The degree of tipping is proportional to the rate of liquid removal. Like the horseshoe-type floating skimmer, the self-adjusting, floating oil skimmer may present difficulty in assuring that it will float at the proper level in a liquid of variable density caused by changes in waste stream composition. In addition, floating scum, solids, etc., would tend to enter this skimmer but not be removed through the submerged pump suction line; so plugging – partial or complete – is likely. It may, however, be a useful oil skimmer with a varying liquid level, and in some instances might be appropriate in a preseparator section. It can handle large oil spills. Other floating oil skimmers have been devised for marine oil spills; some of these have been considered for preseparator channels, but are not discussed here.

DISC SKIMMER The disc skimmer (Figure 15) consists of one or more large discs which are set on a frame which may be mounted directly on a tank or on floats. The skimmer can be adjusted at varying angles to the horizontal so that a greater surface area of the disc is in contact with the surface oil. The skimmer normally is driven by a geared reversible air motor. This allows the disc to be run at low speeds where minimal water content is required and also it may be reversed to clear it from rubbish if necessary. Air power is favored for simplicity of operation, low wear and tear, easy maintenance and safety. Electrical motors may be a potential fire / explosion hazard. If an alternative to air is necessary, it is recommended to use a hydraulic motor. The disc may be made from a variety of materials, but experience with certain plastics appear to have oleophylic properties which have shown an increase in the take-off when compared with stainless steel. Burt Environmental Systems produces this type of skimmer and has been field tested at Fife Ethylene Plant and overall results were very good. It is, however, strongly recommended to check the metallurgy of the skimmer to make sure an appropriate material is used for the wastewater involved. Other examples of disc skimmers are DISCOIL and KEBAB which operate based on the adhesion of the floating oil to the appropriately treated surfaces of the discs. The discs are perpendicularly placed, partially immersed with respect to the surface of the liquid, and maintained in rotation with respect to their horizontal axis. The floating product extracted from the water is separated from the discs by means of special scraper blades, which convey it towards an accumulation tank from which the product is then transferred to a preselected place for storage. Trecate and Augusta refineries installed DISCOIL in their systems and it has performed well on light oils, as well as on the dirty rainwater retention basin. EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

Page

XIX-A1 16 of 64 Date December, 1999

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

API SEPARATORS CONSTRUCTION DETAILS The API separator consists of two sections: the inlet section and the separator channels. The dividing point is the gateways for channel shutoff. The inlet section generally includes a preseparator flume, trash rack, oil skimmer, retention baffle, and forebay. The separation section usually includes gateways, distribution devices, oil and sludge moving equipment, oil skimming equipment, oil retention baffle, sludge collection and removal facilities, and an effluent weir. The various components of both sections of API separator are shown in Figures 9 and 10.

INLET SECTION Preseparator Flume – The preseparator flume is the transition between the end of the inlet sewer and the separator forebay. It serves two functions: reduction of flow velocity and collection of floating oil. In flat terrain refineries, most of the oil flowing to a separator usually can – and should – be recovered in the preseparator flume. The transition between the sewer outlet and the preseparator section is designed to accomplish velocity reduction with a minimum of turbulence. The preseparator section is designed to reduce the horizontal flow velocity to about 10 to 20 ft/min. (3 to 6 m/min), with 1 to 2 minutes average retention time. The transition and separator sections of the preseparator flume may be required to be covered to reduce evaporation loss as mandated by government regulation. When this is the case, the vapor space should be enclosed by a barrier or wall at the downstream end of the cover. The preseparator section contains three pieces of equipment: a trash rack or bar screen, an oil skimmer, and an oil-retention baffle. Trash Rack – Trash racks or bar screens are provided to remove sticks, rags, stones and other debris. The trash rack consists of a series of bars or rods. Bars 3/8 in. by 2 in. (9.5 mm by 50 mm) with clear openings of 3/4 to 1 in. (19 to 25 mm) are typical. Trash racks can be cleaned manually or mechanically. Manually-cleaned racks are inclined 45 to 60° from the horizontal, depending upon the depth of the flume and space availability. Mechanically cleaned bar screens can be mounted vertically, but generally are sloped. A pan or trough is provided at the top of the trash rack to receive the refuse when the trash rack is cleaned. The refuse pan should be perforated to allow liquid drainage back into the flume. Oil Skimmer – The next piece of equipment downstream of the trash rack is usually an oil skimmer. This oil skimmer is included to collect that part of the influent oil that is already on top as the stream enters. An oil skimmer and oil retention baffle should normally be specified for the effluent end of the preseparator flume. Many types of oil skimmers can be used in oil separators. Slotted-pipe, rotary drum, horseshoe-type floating skimmer, selfadjusting floating skimmer, and disc skimmer have all been used. A sixth type, the rotary belt skimmer, is not commonly used in API separators. The types of skimmers that have been used include not only those that can be used in the preseparator flume, but also skimmers for the forebay and separator channels. See previous section for oil skimmer description and applications. Oil Retention Baffle – An oil retention baffle should be located not more than 12 in. (305 mm) downstream of the oil skimmer. The oil retention baffle should be high enough to prevent oil from flowing or splashing over it. Submergence of this baffle should not exceed 18 in. (460 mm). Separator Forebay – After flowing past the trash rack and under the oil skimmer and oil retention baffle, the water from the preseparator flume discharges into the separator forebay. This part of the separator acts as a header and distributes the influent to the separator channels. If the preseparator flume is oriented at a right angle to the separator channels, the separator forebay accomplishes the change in the direction of flow. The forebay part of an API separator can be seen in Figures 9 and 10. Sludge deposition is even more likely in the forebay than in the preseparator flume. Means for sludge removal should be provided, particularly if the flow diffusion device at the inlet to the separation section is of the reaction jet type. Water jets can be used to flush solids from the forebay into the separator zone. This practice can affect the quality of the effluent unless the separator channel is blocked off. Alternatively, the reaction-jet diffusion device can be put at the floor level of the forebay to allow solids to be scoured out of the forebay to the separator channels for collection. An oil skimmer may also be needed in the forebay, depending on whether or not oil is trapped by the flow distribution devices.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 17 of 64 Date December, 1999

API SEPARATORS CONSTRUCTION DETAILS (Cont) SEPARATION CHANNELS SECTION The separation channel is a simple flow through basin that presents no obstruction to the flow so that turbulence is minimized. Separators with long channels are sometimes subdivided into primary and secondary bays to reduce the travel of the sludge scrapers and oil skimmers. Gateways – Each channel is provided with one or more gateways at its inlet to allow shutting off the flow to the channel when desired. Permanently installed gates may be provided, but it is also acceptable to merely provide slots in the gateway piers such that metal sheets, wooden boards, or possibly plastic sheets of the proper size can be dropped into the slots when required to stop the flow. The gate frame and slots should be of suitable corrosion-resistant and erosion-resistant materials. Velocity Head Diffusion Devices – Immediately downstream of the inlet gateway is a diffusion device to distribute the flow equally over the cross-sectional area of the channel and to reduce flow turbulence. Two types of diffusion devices are available: vertical slot baffles and reaction jets. Only the reaction jet type should be used. 1. Vertical-Slot Baffle – A vertical slot baffle is formed from vertical posts. This type of distributor has suffered from severe fouling and is not recommended. 2. Reaction Jet Inlet – A reaction jet consists of a tube or orifice and a dished target baffle (Figure 16). The concave surface of the target baffle faces the orifice or tube. The flow of water from the orifice or tube is reversed by the baffle so that it impinges on the inlet wall of the separator. In this manner, the velocity head is dissipated, and the flow is effectively and uniformly distributed over the channel's cross-sectional area. Orifices are generally used if the separator forebay is large and flow direction is normal to the inlet wall. Orifices are flush with respect to the inlet wall. Tubes are indicated where approach velocities in directions at an angle to the inlet wall exceed 0.5 ft/s (0.15 m/s). The tubes project into the separator section from the inlet. Following Figure 16, the reaction-jet inlets should be installed based on the following criteria:

•

The baffle's radius of curvature (R), in inches, is equal to the tube or orifice diameter (D) in inches.

•

A diameter is selected such that it is sufficient to maintain a design flow velocity of 3 ft/s (0.9 m/s) in the tube or orifice.

• •

The baffle diameter (D + 1) is 1 in. (25 mm) larger than the tube or orifice diameter.

•

The baffle is located downstream of the tube or orifice at a distance equivalent to 0.25 to 0.6 of the tube diameter. A proper distance is required to avoid clogging.

•

A hole may be provided in the center of the baffle to improve distribution. The area of this hole should be 6% of the plane area of the baffle.

•

To facilitate removal of sludge from the forebay, the centerline of the reaction jet is located at the midpoint of flow depth in the main separator channel and at the bottom of the forebay (that is, the lowest point on the floor of the forebay should be at the midpoint of the separator channel).

Minimum tube diameter is 3 in. (75 mm).

•

Reaction jets are spaced uniformly across the width of the channel. The distance from the channel wall to the first jet is one-half the distance between adjacent jets. Reaction jets have a number of advantages:

•

They are less subject to clogging than vertical slot baffles.

• •

They provide good distribution over a wide range of flow rates.

•

They can be shut off easily, since they are relatively small orifices in solid barriers, thus obviating gates or dams for interrupting the flow for maintenance.

They are cheaper than vertical slot baffles.

• They may result in less oil in the channel effluent than with vertical slot baffles. Oil and Sludge-Moving Devices – The separator channels contain a mechanical device to move the separated oil and the sludge to the collecting area. The floating oil is moved to the downstream end of the separator; the settled sludge is moved to the upstream end. Two oil and sludge moving devices are available: the traveling bridge or span type and the flight scraper or chain type. Of the two, the traveling bridge (span) type, while the more expensive, has several advantages and is generally preferred. A description of each and certain important design considerations are given below.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 18 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

API SEPARATORS CONSTRUCTION DETAILS (Cont) 1.

Traveling Bridge or Span Type (Figure 17)– The traveling bridge or span oil / sludge moving device consists of one or two blades extending across the width of the channel hung from a beam or truss spanning the channel. The span rests on wheels in a carriage arrangement; the wheels rise on rails at the sides of the channel and travel the length of the basin. The wheels are chain-driven; the rails are located on top of the channel walls. The blades are adjusted to sequentially skim oil on the downstream travel and scrape sludge on the upstream flight. The one-blade arrangement accomplishes this by adjusting the height of the blade to either oil level or channel bottom using hoist or cam mechanism. The blade pivots on the underside of the bridge as height is changed. The two-blade arrangement achieves the sequential movement of material by either of two means. During oil movement, the sludge blade can be moved out of the channel or it can be feathered parallel to the channel bottom. At the end of the downstream run, the oil moving blade is raised out of the water and the sludge-moving blade is properly positioned to move sludge upstream on the return trip. Surface travel is typically on the order of 2 ft/min. (0.6 m/min.); bottom travel is on the order of 1 ft/min. (0.3 m/min.). This device can span and operate in one or several channels. Operation can be manual or continuous; but is, preferably, automatic and actuated intermittently by a cycle timer. The traveling bridge type oil / sludge moving device offers the following advantages over the chain type:

•

The parts requiring lubrication are located above the water.

•

It allows different travel speeds on the forward and reverse runs. Undesirable features of the traveling bridge device are:

• •

More expensive than other types.

•

Complicates covering the channels.

Requires a movable power cable; usually either a cable reel or a festoon system using a cable looped from a supporting wire.

A design specification for a traveling bridge type device should include the following features:

•

Provide full width skimming.

•

Provide totally enclosed, explosion-proof, weather-proof drivers.

• •

Provide overload protection.

•

Provide facilities for leveling the blades.

The current collector (third rail) type of power supply is not acceptable.

• 2.

Provide means to raise blades out of the water for maintenance. Flight Scraper or Chain Type (Figure 18) – The flight scraper type of oil and sludge moving device consists of two parallel, endless chains, one at each side of the channel, with flights connected to the chains across the channel width. The assembly is moved at a flight speed on the order of 1 to 2 ft/min (0.3 to 0.6 m/min) by motor-driven sprockets. As the flights at the oil layer travel downstream, separated oil is moved toward the oil skimmer; on the return trip the flights push the settled sludge on the bottom of the channel toward the sludge-collecting hoppers or trough at the inlet end. Flights can be spaced uniformly along the entire length of the chain, but flights on only one-half of the chain length are usually sufficient and are preferred because the smaller the number of flights, the less the turbulence. The flights should span the entire width of the channel. Typically, flights are 8 in. (205 mm) high and spaced on 10–ft (3 m) centers. Only one oil / sludge moving device per channel should be installed. Power requirements are low; a driver motor size on the order of 1/2 HP is normally provided. The motor and speed reducer assembly may be mounted directly on the concrete. Each oil / sludge moving device in a parallel channel installation should have its own drive unit operating independently of the others. Normally, these devices do not operate continuously, but only as accumulations warrant. Operation can be either automatic (timer) or manual. Advantages of the flight scraper type oil / sludge movers are:

• •

Low initial cost. Easier to accommodate a channel cover if desired for air pollution control or reduce evaporation losses.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 19 of 64 Date December, 1999

API SEPARATORS CONSTRUCTION DETAILS (Cont) The disadvantages of this device are:

• •

Requires underwater bearings.

•

Moves at the same speed on the top and the bottom of the separator.

Sludge can accumulate on chain and sprockets.

•

Chain sag can redistribute oil beneath the water surface. A design specification for flight scraper type oil / sludge moving devices should include the following features:

• •

Full width skimming.

•

Overload protection on driving sprocket.

• •

Provide shaft aligning facilities.

•

Provide chain guards for personnel protection on chains above grade or near stairs, ladders, platforms, accessways, etc.

•

Provide chain tighteners.

• •

Collection chain to have at least 30,000 pounds (133.4 kN) ultimate strength.

•

Two attachments in each chain strand to be of the pivoted type, for positive cleaning of lower channel corners.

• •

Provide angle tracks for supporting surface run of sludge collectors.

•

Use corrosion resistant anchor bolts.

• •

Flights to be spaced at 10–foot (3 m) intervals.

•

If wooden flights are used, they are to be 3 in. x 8 in. (75 mm x 205 mm) redwood or treated wood, and provided with wearing shoes to contact bottom rails.

Totally enclosed, explosion-proof, weather-proof drivers.

Provide flight leveling facilities.

At least 5 links of each chain strand to be pinned and cottered.

Provide rails flush with tank bottom for wear surfaces.

Flights to be 8 in. (205 mm) high.

•

Provide squeegees of spark-proof construction on two flights for positive cleaning of the tank wall at the water surface and at the tank bottom. Oil-Skimming Device – An oil-skimming device is provided at the end of each separator channel. Oil-skimming devices have been previously described under Oil Skimmers. The same devices can be used in the separator channels. A slotted-pipe oil skimmer is recommended for the separator channels. If minimum collection is required, a disc skimmer is recommended. Oil Retention Baffle – An oil retention baffle is provided just downstream of the oil skimming device, spaced not more than 12 in. (305 mm) downstream of the skimming device. The baffle is installed with a maximum submergence of 55% of water depth; it should extend to the top of the channel. Sludge Collection and Removal – The sludge moved by the pushing device to the inlet end must be collected and removed. The sludge may be considered hazardous depending on site specific environmental regulations. Consequently, the sludge should be disposed accordingly. Sludge can be collected in either of two types of containers: sludge hoppers or a sludge trough. Sludge can be removed by three means: underdraining using hydrostatic pressure, siphon pipe using hydrostatic pressure, and underdrain to a sludge pump. 1. Sludge Collection – The preferred method of collecting sludge is the hopper (Figure 19). The hoppers consist of inverted pyramids with sides sloped at least 45°. With an underdraw system, each hopper contains an exit pipe at the apex; the exits discharge into a sludge withdrawal pipe. With a siphon system, no bottom exit is required. The use of the V-bottom trough, an alternate sludge collecting device, requires a flowable sludge. The V-bottom trough is located on the downstream side of the inlet wall and is built in the channel across its total width. The bottom of the trough starts at a depth of 12 to 30 in. (305 to 760 mm) and slopes downward to a collecting sump at the other end of the channel. The sides of the "V" should slope at least 45°. Although not usually required, other sludge collecting devices are available. For example, with a hard-to-move sludge, screw conveyors or transverse chain flights can be used to move the sludge toward the collecting sump for pump-out.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 20 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

API SEPARATORS CONSTRUCTION DETAILS (Cont) Sludge Removal – Sludge can be removed by siphoning, gravity flow through an underdrain or by pumping. Siphoning and gravity flow through an underdrain rely upon the hydrostatic head of the water in the channel to pressure out the sludge, and can only be employed when the sludge does not require discharge to an elevation above water level. Topography may dictate otherwise. Sludge pumping can elevate the sludge above water level. Pump suction can be from a sump or through a hopper underdraw system. Underdraw systems, both gravity and pumped, are equipped with shut-off plug-type valves in the bottom outlet of every hopper. The valves are opened from a position at the top of the separators by lifting the plug. The plug lifting action can be manual, by a step-on pedal, or by a screw system. Automatic plug valves actuated by a timer can also be utilized. Sludge should be removed from one hopper at a time. The frequency of sludge removal will depend upon the amount collected. Pumping sludge from the collecting hoppers once per day is usually appropriate. Another alternative to the sludge removal mechanisms is manual sludge removal. With this method, separator chambers are taken out of service on an alternating basis as needed; the chambers are drained, and accumulated sludge and silt are removed either manually or by vacuum truck. For installations with excess separator capacity or where separator effluent oil levels are not critically affected by taking a separator channel out of service, thus increasing the channel surface-loading rate, such a procedure can be an alternative to an in-place mechanical sludge removal system. Effluent Weir – An effluent weir wall is located downstream of the oil retention baffle, spaced not more than 2 ft (0.6 m) downstream. The weir wall extends from the channel floor to a height equal to the water depth less the height of the water over the weir at normal flow rate. A sharp-crested or notched weir plate is attached to the downstream face of the weir wall along its top. Bolt holes in the plate should be elongated vertically so that the weir plate can be made absolutely level. Provision should be made to prevent leakage between the weir plate and the concrete weir wall. The head over an unnotched weir (the more common type) can be calculated from the following equation which is a rearrangement of the Francis formula with converted units. 2.

➧

Q H = 0.351    I  Q H = 0.283    I 

2/3

(Customary)

Eq. (2)

2/3

where: H Q l

(Metric) = = =

Eq. (2)M

Head over the weir, in. (cm) Discharge rate, ft3/min. (m3/d) Length of weir, ft (m)

The effluent flows over the weir into a channel and flows by gravity or is pumped to the sewer. Turbulence created during the free fall over the weir into the outfall channel may create an oily aerosol and result in a pollution. This can be minimized by modifying the downstream face of the effluent weir wall to resemble a spillway. Skimmed Oil Sump – The skimmed oil from the main separator channels, as well as from the preseparator, generally flows by gravity to a common sump. The sump provides a reservoir for the pumpout pump, and in addition can be used to allow water to separate from the oil. The general arrangement for a skimmed oil sump designed to effect separation of the water from the skimmed oil, and separate pumpout of the two phases, is shown in Figure 20. The sump is equipped with two vertical centrifugal pumps, one to return separated water to the API separator inlet and the other to transfer the skimmed oil to slop-oil tankage. (An alternative arrangement, requiring more attention and manual operation is a single pump, alternate discharge routes with block valves, and a funnel at the discharge to the separator inlet to allow visual inspection for completion of separated water return). Level controls of the float, electric probe, or air differential pressure type actuate the pumps at high level and switch them off at low level (an alternate is manual actuation with automatic shutoff only). Frequently the sump is not designed to separate water from the skimmed oil, and the entire skimmed stream is pumped from the sump to a slop tank where the separation is done.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 21 of 64 Date December, 1999

API SEPARATORS CONSTRUCTION DETAILS (Cont) The sump and the pumps should be sized large enough to avoid continuous operation of the pumps, and should also be large enough to handle a large spill. If data are unavailable and cannot be determined to size the sump and sump pumps, the size of these facilities can be estimated on the following basis:

• • • •

Assume incoming oil can amount to 5,000 vppm on dry weather separator influent. Water withdrawn with the skimmed oil amount to about seven times the skimmed oil volume when a slotted or disc oil skimmer is used. Provide about 4 hours holdup time in the sump before pumpout. Provide for sump pumpout of oil in 20 to 30 minutes and sump pumpout of water in 1/2 to 1 1/2 hours.

DESIGN PARAMETERS DESIGN FLOW RATE One of the most important and complex parts of wastewater treatment facility design is to develop a design flow rate and the contaminant loading rate in terms of mass per time. The flow rate chosen for equipment sizing depends on location factors, equalization tankage, process unit operation and variability, stormwater management, load growth for expansion, preinvestment philosophy for infrastructure for future facilities, contingency, operating philosophy and penalties for not meeting wastewater quality requirements. Since the design flow rate is the key sizing factor and ultimately determines the cost and operability of treatment systems, it is recommended that careful analysis be made and experienced engineering assistance be consulted. Some key general principles should be used, unless site specified factors require adjustments: Clean stormwater segregation – All stormwater from clean land areas that are not potentially contaminated with process contaminants, such as oil, should be kept separate. The water can be allowed to runoff site via ditches facing and graded away from contaminated areas. Some locations decide to collect this water in ponds for firewater usage of reuse. Potentially contaminated stormwater – Stormwater that is in paved process areas or unpaved, potentially contaminated areas should be treated in a primary oil / water separation facility at the minimum, before discharge. In many locations, rainwater is held within dikes or diverted to holdup facilities (concrete basins, lined ponds, or tanks) to minimize the size of downstream treatment equipment when it is worked off following the storm. Estimating this stormwater flow is performed by mapping out the areas that will be affected, the type of area (paved or unpaved), and obtaining a history of rainfall intensity, duration and interval between storms. Hourly rainfall intensity may be used for sizing piping to prevent flooding in upstream areas, but design of rainwater retention for wastewater treatment units is usually the largest one in 25 year, 24 hour storm. Refer to Table 1 in Design Practice Section XXIX-E, Section on Civil Works, for specific guidance on how to estimate rain runoff rates. Paved areas have a runoff coefficient of 1.0 meaning that the entire amount (100%) of rain falling on these given areas translates to a volume in 24 hours. Unpaved areas use a runoff coefficient of 0.5 – 0.9 depending upon soil characteristics and slope. A coefficient of 0.7 can be used for estimating purposes. Hence, the contaminated stormwater retention basin would hold the sum of the flow rates from paved and unpaved areas for the 24 hour, one in 25 year storm. The design flow rate for oil / water separation units and downstream treatment units would be working off this volume in several days (usually 2 to 7 days), depending upon the frequency of storms (historical data). Three days can be used for a preliminary design flow basis. Again, site specific conditions may alter this workoff or design flow rate. For example, some locations may choose to reuse some of this water in the process areas, because it was shown to be very clean based on past experience. Other locations treat the first hour of stormwater (first flush) and divert the remaining water to the clean water discharge location. Hence, any reused water would not enter into the amount worked off to wastewater treatment. Existing Facilities The setting of the design flow rate for facility expansions is easier than for grassroots facilities, since experience with existing wastewater source generation and stormwater runoff will be helpful. Data can be collected and extrapolated for the additional facilities. When data are available, the flow rates should be plotted on probability paper or analyzed graphically. Usually, the 95% point is chosen for the design flow rate. It is important that this data set include flow rates that cover a minimum of 1 year, or at least during rainy seasons, to ensure seasonal variations are accounted for. In many existing facilities, stormwater runoff is commingled with other oil contaminated waters, so the measurement data on existing facilities is the total design flow rate. Special wastewaters such as ballast waters with a high salt content and those with potential for variable flow need to be considered. For additions to existing facilities, contaminated waters from process units or other sources must be added to the final figure used for the design flow rate. If some process units are expanded, data from existing units can be extrapolated for the facility addition.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 22 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

DESIGN PARAMETERS (Cont) Grassroots Facilities The estimation of design flow for grassroots facilities can be done by using the figures above for estimating the contaminated stormwater workoff from a retention basin or tank and all expected onsite process and utility / offsite wastewater sources that need treatment. For process sources and utility / offsites, the flow rate estimated from each unit should be its flow at design conditions. For example, the part of the overall design flow rate coming from a desalter should be its maximum expected rate or design water use rate. Usually the design flows are obtained from the heat and material balances that are performed by the process or offsite utility designers. If more comprehensive data is taken, it should be analyzed statistically, and the probability at the 95% level should be chosen. Contaminant Loadings In this section, and other Design Practices on wastewater treatment facility design, loading rates of contaminants, expressed as mass per time, are used for equipment sizing. The preferred way to obtain design contaminant loadings is to have paired data (flow X contaminant concentration) and this data should be analyzed statistically to produce probability figures. Usually, the 95% point would be used for design, but in selected other cases, higher probabilities may need to be chosen. The 95% point is usually chosen because it represents a realistic maximum, and excludes some outlying data. Load Growth / Contingency Different locations have different policies on investing in facilities. The new trend is to not build in load growth for future facilities into a capital investment. However, there are uncertainties in the development in the design flow rate. Also, technology improvements can occur with existing process equipment, that can improve throughput and generate more wastewater streams. A contingency for uncertainties of 20% is recommended, and a load growth factor, if any should be added to this contingency.

INFLUENT OIL CONTENT For some oil / water separators, especially the package oil / water separators, the quantity of oil concentration of influent is needed to perform design calculations. In an existing plant, oil content can be determined by sampling and analysis. There are several methods used to measure the amount of oil. They are: Gravimetric, Infrared Absorption (IR). For more detailed information regarding analysis methods, see Reference 10. For grassroots plant, the amount of oil can be estimated from design flow rate times 5,000 ppm (mg/L) or 0.5 wt%.

OIL DROPLET SIZE DISTRIBUTION In the past, most of the primary oil / water separators were the conventional API separators. For these, the oil droplet size was not an important factor, but with newer, packaged separators, and Daburt Separator, the oil droplet size is one of the most critical design parameters in determining the efficiency of an oil / water separator. Since the design of the oil / water separator also largely depends on the oil droplet size, it is very important to determine the size distribution for each application before screening the separators. Exxon has experiences with instruments, such as, Malvern 3200E, Sedi Graph, that produces an accurate oil droplet size distribution. If a measurement is not possible, Figure 21 provides oil droplet size distribution for typical oily wastewater streams. This figure illustrates the influence of oil droplet size distribution and concentration on separator design and performance. For an example, the stream shown in Figure 21A, 99% of all oil droplets will be 60 microns or larger and could be expected to be removed. Thus a properly designed separator with an influent free oil concentration of 1,000 ppm (mg/L) will yield an effluent with 10 ppm (mg/L) free oil (0.01 x 1,000 ppm = 10 ppm).

INFLUENT TEMPERATURE The composite separator influent temperature should be known to determine the appropriate values of specific gravity and viscosity. These physical properties may vary significantly with temperature and it is important that the proper values be used. If the influent temperature cannot be determined or estimated, use 95°F (35°C) for processing oily water. If the influent contains cooler water, such as rain, estimates should be made accordingly.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 23 of 64 Date December, 1999

DESIGN PARAMETERS (Cont) SPECIFIC GRAVITY OF THE WATER PHASE It is necessary to estimate the specific gravity of the aqueous phase in order to perform the separator calculations. Normally, this specific gravity is different from that of pure water because of dissolved solutes. The density may be less than that of pure water if it contains hydrocarbons or other organic metals, or it may be heavier than pure water if it contains dissolved salts. Where possible, the aqueous specific gravity of an actual sample should be measured. Where this is not possible the specific gravity of the water phase should be estimated based on knowledge of the other materials expected in the water. If no information is available use a specific gravity of 1.0.

SPECIFIC GRAVITY OF THE OIL PHASE It is also necessary to estimate the specific gravity of the oil phase. The oil phase is a mixed phase, where different density oils are all combined. Thus the density is generally different from that of pure oil, and a value determined by actual measurement should be used if available. Frequently, the oil droplets surround or are adsorbed upon one or more particles of suspended solid, forming a globule with a density that is some composite of that of the oil and the solid. The value of this composite specific gravity is not predictable and testing is required to determine it. If data is not available, the specific gravity should be determined or estimated. If the specific gravity of the oil phase cannot be determined or estimated, use the following typical specific gravities at 60°F (16°C):

•

Light Oils

0.85

•

Heavy Oils

0.95

VISCOSITY OF THE WATER PHASE The viscosity of the aqueous phase should be known at the temperature and solute concentration that will exist in the separator. This also should be determined by sampling where possible. If the water phase viscosity cannot be determined or estimated, use the viscosity of water.

OPERATION AND MAINTENANCE Since oil / water separators are relatively simple devices, there is sometimes a tendency to take them for granted and to devote all of one's attention to more complex wastewater treatment steps downstream of the separator. Doing so can be a mistake, because the primary oil / water separator is the first line of defense, protecting the more sensitive downstream treatment steps from high levels of oil that could seriously disrupt their operation. Past operational data provide one of the best indicators for controlling present operations. To ensure good control, it is advisable to routinely record the quantity and characteristics of both the oil and the sludge that are removed from the separator. Oil should be periodically checked for specific gravity, and sludge for percent solids. The oil concentration of the separator effluent should also be measured routinely. An occasional API test for determination of susceptibility to settling (API-STS Method (see Reference 21) or use of Imhoff cone) is also helpful in that it allows one to compare actual performance with the best that can reasonably be expected. Manual control by skilled operators is still preferable in most cases, particularly for API separators. If operations are automated, frequent and routine operator surveillance is recommended. The most frequently encountered separator operational problems and suggested solutions are listed in Table 8. It is apparent that many of the problems are associated with solids buildup and solids removal. Some early conventional API separators designs did not include sludge-removal devices. Instead, periodic shutdown, cleaning, and sludge removal were accomplished with vacuum or pumping devices or with clamshell-bucket equipment. The non-mechanical route to solids removal is simple and appears to work quite satisfactorily in a large number of refineries, where solids loads, flow variability, and separator size are factored into sludge removal operations. However, installation of covers and stricter effluent limits may make manual sludge removal less desirable. Parallel-plate separators can also be periodically drained as needed, and the plate chambers sprayed with a water jet to remove solids buildup; however, a means of removing sludge from the separator is also needed. ➧

IMPROVING PERFORMANCE OF EXISTING SEPARATORS Before any approach is chosen to improve separator performance, the reasons why the separator is not operating adequately should be identified. Poor separator performance often results from changes in the characteristics of the influent oil or flow and/or from inadequate oil / water separator design. By examining the design basis for the separator and comparing the current operating conditions such as flow rate, wastewater temperature, specific gravity of the oil, and channel dimensions, it can be compared to the suggested design criteria as a potential explanation for deteriorating performance.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 24 of 64 Date December, 1999

EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

IMPROVING PERFORMANCE OF EXISTING SEPARATORS (Cont) The presence of emulsified or dissolved oil may also contribute to poor separator performance. Individual process streams, specifically desalter brines, caustic streams, sour waters, and delayed coker cutting water should be investigated for oil load and potential for affecting separator performance. Dissolved oils can often be present in refinery caustic streams, and an increase in pH from fresh caustic entering the wastewater prior to the separator can increase the solubility of the oil. In addition, surfactants encourage the formation of emulsions. Free oil can also become emulsified through excessive agitation or turbulence. Another approach to improving separator performance is upgrading the existing oil skimming equipment. Equipment upgrades make the separator more modern using the latest skimming technology as flight scrapers have been shown to break and/or corrode over time resulting in high replacement costs. Two types of skimming improvements have been recommended within Exxon recently. First, floating disk-type skimmers are recommended for "drop-in" improvements which do not require taking the separator out of service. These types of skimmers have been applied in calm water oil spill applications, lagoons, separators, and independent tests have shown them to be quite efficient and versatile for different oils. Floating type skimmers are especially useful when flow rates can vary greatly, especially under storm conditions. Secondly, a company called Oil Skimmers, Inc. have developed a long, oleophilic tubing skimmer and scraper device to handle solids and low volumes of oil pickup for approximately 7K USD per 100 x 20 ft (30 x 6 m) separator bay with easy installation. This system is applicable if low VOC emissions are a regulatory requirement. Other factors that may have adverse impacts on the oil removal and contribute to some residual oil showing up in the separator effluent includes (1) the presence of significant amounts of coke fines will impede wastewater oil droplets settling, and (2) insufficient hydraulic distribution of the water to the separator bays, especially during stormwater events.

DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS All the calculations below are normally done for summer, winter, and average conditions. The case resulting in the largest separator will govern.

API SEPARATOR ➧

Rising Droplet Velocity – The rate of rise is calculated by an expression of Stokes' Law [Eq. (1)] for the terminal velocity of spheres in liquid medium. For an oil droplet with a diameter of 150 microns (0.015 cm). Eq. 1 can be written as:  S − So   Vt = 2.41  w  µw   

(Customary)

Eq. (3)

 ρ − ρo   Vt = 12.263  w  µw   

(Metric)

Eq. (3)M

where: Vt = Sw = So = (15°C) µw = ρw = ρo =

Rate of rise of an oil droplet in wastewater, ft/min (mm/sec) Specific gravity of wastewater, at temperature of flow, relative to water at 60°F (15°C) Specific gravity of oil in wastewater, at temperature of flow, relative to water at 60°F Absolute viscosity of the wastewater at temperature of flow, cP (cP) Density of water at temperature of flow, (g/cm3) Density of oil at temperature of flow, (g/cm3)

Horizontal Velocity – The design mean horizontal velocity is as: VH = 15 Vt

Eq. (4)

where: VH Vt

= =

Superficial horizontal flow velocity, ft/min (mm/sec) Rate of rise of an oil droplet in wastewater, ft/min (mm/sec)

and is limited by a maximum velocity of 3 ft/min (15 mm/sec).

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 25 of 64 Date December, 1999

DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS (Cont) Minimum Vertical Cross-Sectional Area – This is the total cross-sectional area for all channels (see Figure 22). Ac =

Qm VH

A c = 0.277

Qm VH

where: Ac = Qm = VH =

(Customary)

Eq. (5)

(Metric)

Eq. (5)M

Minimum total vertical cross-sectional area, ft2 (m2) Wastewater design flow rate, ft3/min (m3/hr) Superficial horizontal flow velocity, ft/min (mm/sec)

Number of Separator Channels Required – Typically, the maximum cross-sectional dimensions recommended for a single channel are 20 ft wide x 8 ft deep (160 ft2) (6 m x 2.5 m = 15 m2). On this basis, the number of channels, n, required is calculated as follows: n =

Ac 160

(Customary)

Eq. (6)

n =

Ac 15

(Metric)

Eq. (6)M

where: n Ac

= Number of separator channels required, fractional number of channels are rounded up to the next whole number =

Minimum total vertical cross-sectional area, ft2 (m2)

Regardless of whether or not Ac exceeds 160 ft2 (15 m2), provide at least two channels to allow the separator to maintain operation when one channel is down for maintenance. Although at least two channels are provided, the separator is sized for the design flow rate with all channels in service. When more than one parallel channel is installed, a common separating wall is recommended. Channel Width and Depth Choose a width of channel, B, between 6 to 20 ft (1.8 to 6 m) and determine the depth of channel as follows: d =

Ac Bn

Eq. (7)

where: d Ac B n

= = = =

Channel depth, ft (m) Minimum total vertical cross-sectional area, ft2 (m2) Channel width, ft (m) Number of channels

Check for the following 2 design constraints: 3 ≤ d ≤ 8 ft

(Customary)

Eq. (8a)

0.9 ≤ d ≤ 2.4 m

(Metric)

Eq. (8a)M

0 .3 ≤

d ≤ 0. 5 B

Eq. (8b)

If the depth obtained fails to meet either of these criteria, different separator widths should be tried until a depth that meets these criteria is obtained. Several combination of dimensions will result in the desired cross-section for a particular system. The distributing influence of the inlet and outlet zones is reduced with long channels. Usually, the most economical installation is the one with the smallest number of channels (maximum cross-sectional area per channel) greater than one.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 26 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS (Cont) ➧

Flow Correction Factors – Determine the flow correction factors for turbulence and short-circuiting. Turbulence Correction – From the values of Vt and VH determined previously (Eqs. 3 and 4), calculate VH/Vt. From this value, find the correction factor for turbulence, Ft, from the table below the plot in Figure 23. Short-Circuiting Factor: Fs = 1.21 Total Design Correction Factor – Read F directly from Figure 25 or use the following equation: F = Fs Ft = 1.21 Ft

Eq. (9)

Separator Length – Determine the separator length between oil retention baffles. L =F

VH d Vt

Eq. (10)

where: L F VH Vt d

= Length of the separator channel between inlet distributor and oil retention, ft (m). Round up to the next whole number in ft or up to a half meter. = Flow correction factor = Superficial horizontal flow velocity, ft/min (mm/sec) = Rate of use of an oil droplet in wastewater, ft/min. (mm/sec) = Channel depth, ft (m)

Minimum L/B Ratio – To provide more uniform flow distribution and to minimize the effects of inlet and outlet turbulence on the main separator channel, the suggested length-to-width ratio (L/B) is at least 5. From Eqs. 7 and 10, check if L/B is 5 or greater, otherwise choose a different B and try again. ➧

PARALLEL PLATE SEPARATOR The parallel plate separators design is based upon separating oil droplets greater than 60 microns. In addition, the following parameters are required:

•

Plate inclination measured from the horizontal: 45° ≤ β ≤ 60°

•

Perpendicular plate spacing: 0.75 ≤ Sp ≤ 1.5 in. (19 ≤ Sp ≤ 38 mm)

•

Assume a Reynolds' number: 500 ≤ NRe ≤ 2,000 Rising Droplet Velocity – The rate of rise is calculated by an expression of Stokes' Law for the terminal velocity of a 60 micron (0.006 cm) diameter oil droplet [see Eq. (1)].  S − So   Vt = 0.386  w   µw 

(Customary)

Eq. (11)

 ρ − ρo   Vt = 1.962  w   µw 

(Metric)

Eq. (11)M

where:

Vt Sw So µw ρw ρo

= Vertical rate of rise of a 60 micron diameter oil droplet between plates due to density gradient only, ft/min (mm/sec) = Specific gravity of wastewater, at temperature of flow, relative to water at 60°F (15°C) = Specific gravity of oil in wastewater, at temperature of flow, relative to water at 60°F (15°C) = Absolute viscosity of the wastewater at temperature of flow, cP = Density of water at temperature of flow, (g/cm3) = Density of oil at temperature of flow, (g/cm3)

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 27 of 64 Date December, 1999

DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS (Cont) The rise rate can also be expressed as the following: Vt =

Sp vertical distance between plate X = = retention time between plate tr tr cos β where: β Sp X tr

= = = =

Eq. (12)

Plate inclination measured from the horizontal Perpendicular spacing between plates Vertical spacing between plates Retention time between plates

Retention Time – Eq. (12) can be rearranged to calculate the retention time between the plates. tr =

Sp

Eq. (13)

Vt cos β

The retention time can also be expressed as the following: tr =

A p Lp holdup volume in plate package = flowrate Qm where: tr Ap Lp Qm

➧

= = = =

Eq. (14)

Retention time Perpendicular cross-sectional area Length of parallel plate package Design flow rate

Reynolds' Number – Reynolds' number is a dimensionless number interpreted as the ratio of inertial forces to viscous forces in the fluid. Since Stokes' Law is applicable, the flow is laminar. Therefore, for most applications, the Reynolds' number can be assumed to be between 500 and 2,000. If necessary, the Reynolds' number can be calculated by using the following expression: NRe =

NRe =

Sp V ρ w gc µ w Sp V ρ w µw

=

=

Sp V

ν Sp V

ν

where: NRe = Sp = V = = µw gc = ρw = ν =

(Customary)

Eq. (15)

(Metric)

Eq. (15)M

Reynolds' Number, dimensionless equivalent diameter of velocity of interest (plate spacing), ft (m) Average velocity of wastewater, ft/s (m/s) Absolute viscosity of the wastewater at temperature of flow, lbf-s/ft2 (Pa-s) Gravitational conversion constant = 32.2 lbm-ft/lbf-s2 Density of water at temperature of flow, lbm/ft3 (g/cm3) Kinematic viscosity of the wastewater at temperature of flow, ft2/s (m2/s)

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 28 of 64 Date December, 1999

EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DETAILED OIL / WATER SEPARATOR SIZE CALCULATIONS (Cont) ➧

Total Perpendicular Cross-Sectional Area – Calculate the total cross-sectional area perpendicular to the parallel plates from the Reynolds' number, Eq. (15). A new factor, Sw is introduced to convert density to specific gravity in Eq. (16). Note that viscosity units are changed from (Pa-s) to (cP) in Eq. (16)M. (Conversion factors: 10 Pa-s = cP). Ap =

Ap =

17.25 S p Q m S w µ w NRe 0.28 S p Q m ρ w µ w NRe where: Ap Sp Qm Sw µw ρw

➧

= =

Eq. (16)

(Metric)

Eq. (16)M

Total perpendicular cross-sectional area, ft2 (m2) Perpendicular spacing between plates, ft (mm) Design flow rate, ft3/s (m3/hr) Specific gravity of wastewater, at temperature of flow, relative to water at 60°F (15°C) Absolute viscosity of the wastewater at temperature of flow, cP Density of water at temperature of flow, lbm/ft3 (g/cm3)

Parallel Plate Package Length – Determine the length of the parallel plate package by combining Eqs. 13, 14, and 15. Note that viscocity is represented as cP in Eqs. 17 and 17M. Lp =

Lp =

3.2 x 10 −4 µ w NRe S w Vt cos β 4.96 x 10 −6 µ w NRe ρw Vt cos β where: Lp NRe Sw Vt µw ρw

➧

= = = =

(Customary)

(Customary)

Eq. (17)

(Metric)

Eq. (17)M

= Length of parallel plate package, ft (m) = Reynolds' Number, dimensionless = Specific gravity of wastewater, at temperature of flow, relative to water at 60°F (15°C) = Rate of rise of a 60 micron diameter oil droplet in wastewater, ft/min (mm/sec) = Absolute viscosity of the wastewater at temperature of flow, cP = Density of water at temperature of flow, (g/cm3)

Horizontal Cross-Sectional Area – Determine the horizontal surface area projected by the inclined plates. This area includes both the frontal area and the base area represented by Lp. This area is only somewhat useful, since it does not include other factors that influence the unit footprint such as inlet and outlet chambers, inlet ennergy dissipation etc. A H = A p sin β + WL p cos β where: AH Ap β W

= = = =

Eq. (18)

Horizontal surface area, ft2 (m2) Total perpendicular cross sectional area, ft2 (m2) Plate inclination measured from the horizontal, degree Width of plate pack, ft (m)

The number and area configuration of plates required, in conjunction with the open (not plate-filled) surface area of the separator (if significant), comprise the total required surface area, AH. Owing to the great variability among manufacturers with respect to plate size, spacing, and inclination, it is strongly recommended that a vendor be consulted for specification of these parameters.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

SAMPLE DESIGNS The following sample calculations are based on the characteristics below:

•

A design flow rate (Qm) of 4,490 gpm (1,019 m3/hr)

•

A minimum temperature of 105°F (41°C)

•

A specific gravity (Sw) or density (ρw, g/cm3) of 0.992

•

An absolute (dynamic) viscosity (µw) of 0.65 cP

•

A maximum oil specific gravity (So) or density (ρo, g/cm3) of 0.92

API SEPARATORS 1.

The rise rate for the oil globules is calculated using Eq. (3):  S − So   Vt = 2.41  w  µw    0.992 − 0.92  = 2.41   0.65   = 0.267 ft / min [1.358 mm / sec ]

2.

The maximum allowable mean horizontal velocity is calculated using Eq. (4): VH = 15 Vt = 15 (0.267 ft / min) = 4 ft / min [20.4 mm / sec ] Since VH calculated exceeds the limiting 3 ft/min (15 mm/sec), use the limiting mean horizontal velocity.

3.

The minimum vertical cross-sectional area is calculated using Eq. (5): Ac =

Qm =

Ac =

Qm VH 4,490 gpm 7.48 gal / ft

3

= 600

ft 3 min

600 ft 3 / min 3 ft / min

[

= 200 ft 2 18.8 m2

Page

XIX-A1 29 of 64 Date December, 1999

]

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 30 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

SAMPLE DESIGNS (Cont) 4.

The number of separator channels required is calculated using Eq. (6): n = Ac / 160 = 200 ft2/160 ft2 = 1.25 Therefore, n = 2

5.

Since n must be greater than or equal to 2, this value is acceptable. The width and depth of the channels are calculated using Eq. (7): d =

Ac Bn

Assuming a channel width, B, of 20 ft (6 m), d =

200 ft 2 (20 ft ) (2 channels)

= 5 ft [1.57 m] The value of 5 ft (1.57 m) obtained for d meets the requirements that d be greater than or equal to 3 (0.9) but less than or equal to 8 ft (2.4 m). However, with d = 5, d/B = 0.25, which fails to meets the requirements that d/B be greater than or equal to 0.3 but less than or equal to 0.5. Therefore, a smaller value for B must be tried. Assuming a channel width, B, of 18 ft (5.4 m), d =

200 ft 2 (18 ft ) (2 channels )

= 6 ft [1.8 m]

6.

The value of 6 ft obtained for d meets the requirement that d be greater than or equal to 3 (0.9) but less than or equal to 8 ft (2.4 m). In addition, with d = 6, d/B = 0.3 which is ≥ 0.3 ≤ 0.5. Furthermore, B meets the requirement that it be greater than or equal to 6 (1.8) but less than or equal to 20 ft (6 m). Therefore, a channel width of 18 ft (5.4 m) and a channel depth of 6 ft (1.8 m) are acceptable dimensions. From Steps 1 and 2 above, VH 3 ft/min = = 11.24 Vt 0.267 ft/min

7.

Therefore, from Figure 23, F = 1.55. The length of the separator is calculated using Eq. (10): L = F

VH d Vt

= 1.55 (11.24 ) (6 ft ) = 105 ft [32 m] A check should be performed to determine that L/B is greater than or equal to 5. In this case, L/B is equal to 5.8, so the check is satisfied.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 31 of 64 Date December, 1999

SAMPLE DESIGNS (Cont) PARALLEL PLATE SEPARATOR 1.

The rise rate for the oil globules is calculated using Eq. (11) as follows:  S − So   Vt = 0.386  w  µw    0.992 − 0.92  = 0.386   0.65   = 0.0428 ft / min [0.217 mm / sec ]

➧

2.

NRe = 2,000 Sp = 1.5 in (38 mm)

Assuming in addition that: then from Eq. (16), Ap = =

17.25 Q m ρ w µw 34.5 (1.5 in.) (4490 gpm) (0.992)) 0.65 cP (2000)

[

= 88.7 ft 2 8.2 m 2 ➧

3.

]

The length of the parallel plate package is calculated from Eq. (17): Lp =

=

3.2 x 10 −4 µ w NRe S w Vt cos β 3.2 x 10 − 4 (0.65 cP ) (2000 )

(0.992) (0.0428 ft/min) cos 60o

= 19.6 ft [6 m] ➧

4.

Assuming in addition that: W = 10 ft (3.1 m) The horizontal surface area (AH) projected by the inclined plates can be calculated from Eq. (18): A H = A p sin β + WL p cos β = (88.7 ft 2 ) (sin 60) + (10 ft ) (19.6 ft ) (cos 60)

[

= 174.8 ft 2 16.2 m 2 ➧

5.

]

Assuming that the separator will be similar to a conventional, rectangular channel API separator, each one with a package of tilted plates; then for each channel the parallel package should be: AH = 87.4 ft2 [ 8.1 m2] and Lp = 19.6 ft [6 m]

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 32 of 64 Date December, 1999

EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

SAMPLE DESIGNS (Cont) COMPARISON The reader should note that the given wastewater characteristics in the sample problems are the same for the conventional API separator and parallel plate separators. However, the parallel plate separator is smaller and removes smaller droplets of oil. The results for both cases can be compared as follows: SEPARATOR DESCRIPTION

CONVENTIONAL API

Number of channels Channel cross sectional area Length of channel required Minimum oil drop diameter removed Wastewater flow rate

PARALLEL PLATE

2

2

160 ft2 (14.9 m2)

87.4 ft2 (8.1 m2)

105 ft (32 m)

19.6 ft (6 m)

150 microns (0.15 mm)

60 microns (0.06 mm)

4490 gpm (1019.2 m3/h)

4490 gpm (1019.2 m3/h)

NOMENCLATURE Ac AH Ap B d F Fs Ft g H L l Lp NRe n Q Qm So Sp Sw

Minimum vertical cross-sectional area, ft2 (m2) Horizontal cross-sectional area, ft2(m2) Perpendicular cross-sectional area, ft2 (m2) Channel width, ft (m) Channel depth, ft (m) Total design correction factor Short-circuiting correction factor Turbulence correction factor Acceleration due to gravity, (cm/s2) Head over the weir, in Channel length, ft (m) Length of weir, ft Length of parallel plate pack, ft (m) Reynolds' Number, dimensionless Required number of channels Discharge rate, ft3/min. Design flow rate, the maximum wastewater flow including allowance for plant expansion and stormwater runoff, if applicable, ft3/min, gpm (m3/hr) Wastewater oil-fraction specific gravity Perpendicular plate spacing, in (mm) Wastewater specific gravity

T

Wastewater temperature, °F (°C)

tr V VH Vt X

Oil droplet retention time between plate, min. Average velocity of wastewater, ft/s (m/s) Horizontal flow velocity, ft/min. (mm/sec) Rate of rise of oil globule through water phase for API separators, ft/min. (cm/sec), or rate of rise a 60 micron diameter oil droplet in wastewater ft/min (mm/sec) for parallel plate separators Vertical spacing between plates

β

Parallel plate inclination measured from horizontal, degrees

δ

Diameter of oil droplet, cm

µw

Absolute (dynamic) viscosity of the water phase, cP, lb/ft-s (Pa-s)

ν

Kinematic viscosity of the wastewater at temperature of flow, ft2/s (m2/s)

ρo

Density of oil at the design temperature, lb/ft3 (g/cm3)

ρw

Density of water at the design temperature, lb/ft3 (g/cm3) EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 33 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

TABLE 1 SUMMARY OF PRIMARY OIL / WATER SEPARATOR'S OIL & SOLIDS REMOVAL

INSOLUBLE ORGANIC (FREE OIL) REMOVAL EFFICIENCY, %

FINE SOLIDS PARTICLE REMOVAL EFFICIENCY

HIGH SOLIDS LOADING REMOVAL

OIL DROPLET REMOVAL EFFECTIVENESS (OIL DROPLET SIZE)

1,000 to 5,000

60 to 90

Low

High

> 150 microns

Parallel Plate Separator

1,000 to 5,000

80 to 90

Low

Medium

> 60 microns

Skim Ponds

1,000 to 5,000

60 to 90

Low

High

> 60 microns

Daburt Separator

1,000 to 5,000

90 to 99

Medium

Low

> 20 microns

Package Oil/Water Separator

1,000 to 5,000

90 to 99 (per vendors)

Low

Low

> 20 microns (per vendors)

Cyclone Separator

1,000 to 5,000

60 to 99

High in oil phase, low in water phase

Low

> 20 microns

> 50,000

N/A

N/A

N/A

N/A

TYPICAL INLET INSOLUBLE ORGANIC CONCENTRATION, ppm (mg/L)

API Separator

SEPARATOR TYPE

Centrifuges

TABLE 2 RELATIVE COST SUMMARY OF PRIMARY OIL / WATER SEPARATORS SEPARATOR TYPE

COST

API Separator

Medium

Parallel Plate Separator

Medium

Skim Ponds Daburt Separator

Low (Regulation Dependent) High

Package Oil/Water Separator

Low

Cyclone Separator

High

Centrifuge

Very High

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 34 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

TABLE 3 ADVANTAGES & DISADVANTAGES OF PRIMARY OIL / WATER SEPARATORS SEPARATOR TYPE API Separators

ADVANTAGES • •

Parallel Plate Separators

Skim Ponds

Daburt Separator

Package Oil/Water Separator

Cyclone Separator

Centrifuge

Used for large, over 500 gpm (114 m3/hr), variable oil loading and flows

DISADVANTAGES •

• Recommended for gross oil and suspended oil • removal

Use large plot space Need higher residence time Higher mechanical maintenance requirements

•

Available as shop fabricated, skid mounted packages

•

Potential for solids to plug passages between plates

•

Low effluent free oil concentration

•

•

Ease of operation

Higher frequency and more tedious cleaning requirements, especially for high solids streams

•

Easily covered

•

•

Availability for pressurized applications

Lower tolerance to shock oil loading due to smaller oil holdup capacity

•

Used to remove incremental amount of free oil •

Use large plot space

•

Not recommended for gross oil and suspended solids removal

•

May cause underground contamination if not properly fired

•

Cannot be covered; to contain vapors

•

Can outperform plate separators

•

•

Comparable to most flotation units at a significantly lower capital and operating cost

Finely suspended solids particles are not removed

•

There are no economics of scale. Multiple standard units required for large flow rate

•

Shop fabrication/skid mounting

•

•

Less plot space

Potential solids plugging of passages between plates

•

High oil removal

•

Frequent cleaning in high solids services

•

Ease of operation

•

Low tolerance to shock oil loadings

•

Ease of covering

•

Pressurization option

•

Small in size

•

Inflexible once installed and operated

•

Ability to operate in any physical orientation

•

•

Capable of operating at high pressures

Sensitive to instabilities in feed flow rate and solids concentration

•

Volume of reject stream is very low

•

Limitation on the separation performance in terms of sharpness of cut, range of operating cut size, dewatering

•

Requires significant ∆ P

•

Rarely used for normal refinery wastewater except for high oil content (over 5%)

•

Greatly reduce space requirements

•

High power and maintenance requirements

•

Very high loading capacity

•

Susceptible to shock loads

•

Problems with abrasion

•

Rarely used for normal refinery wastewater except for high oil content (over 5%)

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

➧

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section

Page

XIX-A1 35 of 64 Date December, 1999

TABLE 4 PACKAGE OIL / WATER SEPARATOR VENDORS AND EXXON INSTALLATIONS * CATEGORY Corrugated Plate Separators

MANUFACTURER Pielkenrood-Vinitex B.V.

KNOWN EXXON INSTALLATIONS •

Rotterdam Oxo Plant

•

Billings Refinery

•

Pelican Island Terminal, LA

•

Ethylene Plant, Mossmorran, Fife, Scotland

•

2 at unspecified refineries

•

6 at unspecified terminals

The Netherlands Monarch Separators Inc. CJB Developments Ltd. England Lancy International, Inc. ESI

•

Smith & Loveless Novel Plate-Type Separators

Santa Ynez Unit

None

Quantek, Inc. (formal Fram Industrial Filter Corp.)

•

Mont Belvieu Plastic Plant

•

Production - Blackjack Creek, Jay, Florida

Wemco

•

Benicia Refinery

•

Production in Gulf Coast

•

Rotterdam Refinery

•

Exxon tankers

Skimovex B.V. The Netherlands Other Plate-Type Separators

Butterworth Inc. Facet Enterprises, Inc.

Enhanced Coalescing Gravity Separators

AFL Industries, Inc.

ENQUIP

None •

Exxon Company, U.S.A., Norwalk, CT

•

Exxon Pipeline Co., Arkansas Pass, TX

•

Exxon Co., Baltimore, MD

•

Exxon Company, U.S.A., Teterboro, NJ

•

Marketing Distribution Terminal, Charlotte, NC

Hyde Products Inc. McTighe Industries, Inc.

None •

Esso Petroleum Canada, Norman Wells Refinery

•

Marketing Terminals at Joliet, IL

•

Marketing Terminals at Baltimore, MD

Remedial Systems, Inc.

None

Pollution Control, Inc.

None

Great Lakes Environmental Inc.

•

Exxon Company, U.S.A., unspecified

* Note that these are past Exxon installations. For current operating data consult individual sites.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 36 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

TABLE 5 SEPARATOR COVER TYPES USED IN EXXON AFFILIATES AFFILIATE

TYPE OF SEPARATOR COVER

UNIT / SECTION

Altona

Fixed

N/A

Antwerp

N/A

Forebay

Augusta

Fixed

N/A

Baton Rouge

Fixed

CPS

Baytown

Fixed

Main channel, forebay

Benicia

Fixed (Dismantled)

Inlet channel, forebay

Fawley

None

None

Fos

Fixed

Forebay

Ingolstadt

Floating

Forebay

Port Jerome

N/A

Inlet channel

Rotterdam

Floating

Forebay

Fixed

Mainbay

Slagen

N/A

Inlet channel, forebay

Strathcona

Fixed

Mainbay

Trecate

Fixed

Forebay

N/A: Not Available

TABLE 6 ADVANTAGES AND DISADVANTAGES OF SEPARATOR COVERS COVER TYPES Fixed

Floating

MATERIALS

ADVANTAGES

Rigid Steel, Aluminum Concrete, light weight aluminum foam sandwich

•

Foamglass Slabs (cellular insulating material)

•

Inexpensive

•

Easy to move

Adaptable to flight skimmers

DISADVANTAGES •

Vapor space requires special safety measurements

•

Vapor recovery system sometimes required

•

Flight scrapers must be removed or submerged

•

Problem with breakage when dropped

•

Slabs may move out of position with storm surges

Conservaflote (polyurethane foam encapsulated with fiberglass skin)

•

Can remove sludge without removing entire cover

•

Flight scrapers must be removed or submerged

•

Easy to move

•

Polyethylene balls tend to wash away

Polyethylene Balls (hollow, high density)

•

Inexpensive

•

•

Easy to move

Flight scrapers must be removed or submerged

Steel Pontoon

•

Fully automated

•

Expensive

•

Decreased operator time

•

Many moving parts

•

Little water in slop oil

•

Flight scrapers must be removed

•

Little emulsification

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page

XIX-A1 37 of 64 Date December, 1999

TABLE 7 ADVANTAGES AND DISADVANTAGES FOR OIL SKIMMERS SKIMMER TYPE Slotted-Pipe Skimmer

Rotary Drum Skimmer

Horseshoe-Type Floating Skimmer

Self-Adjusting Floating Oil Skimmer

Disc Skimmer

ADVANTAGES •

High capacity

•

Simple

•

Economical

•

Low maintenance

•

No utilities required

•

Can remove floatable solids in addition to oil

•

Minimizes water skimmed off with the oil

•

Continuous and automatic

DISADVANTAGES •

Removes relatively large amounts of water with the oil (80% water or more)

•

Requires manual operation

•

Usually not continuous

•

Operable with only a limited variation in liquid level

•

Limited oil removed, cannot handle massive spills (roughly 1 gpm/ft of length)

•

Has problems with heavy greases and heavy objects, which slide off

•

Floating debris can interfere with oil pickup

•

Requires electric power

•

Requires maintenance for rotating machinery

•

Does not skim floatable solids

•

Submergence varies with the density of the liquid

•

Removes water with the oil

•

Subject to fouling, which can upset buoyancy and trim and cause undesirable performance

•

Adjust automatically to changes in liquid level

•

High capacity

•

Operates continuously and automatically

•

Adjusts automatically to changes in liquid level

•

•

Avoids skimming water when no settled oil layer is present

Submergence varies with the density of the liquid

•

Requires usual inspection operator intervention to operate the pump

•

Ability to lift low percentage of water

•

•

Can be adjusted for different types of oil

Limited capacity. Capacity depends on type of oil

•

Continuous and automatic

•

Requires electric power

•

Higher capacity than Rotary Drum Skimmer per running length

•

Requires maintenance for rotating machinery

•

Does not skim floatable solids

•

Comes as a self contained unit for remote locations

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 38 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

TABLE 8 POTENTIAL OPERATION PROBLEMS AND SOLUTIONS POTENTIAL OPERATION PROBLEMS •

Mechanical failures of flight skimmers or scrapers caused by foreign object, such as, tools, gloves, rags, and the like falling into the separator basin and getting caught in the flight mechanism.

•

For separators with sludge removal pumps, plugging of pump suction lines with foreign matter and silt, as well as failures of pump seals.

•

Mechanical failure of flight scrapers caused by buildup of grit or silty sludge deposits on the bottom and edges of the separator chambers.

•

Mechanical failure of: -

Flow distributor

-

Oil retention baffle (corroded or not sealed at ends)

-

Oil skimmer

SUGGESTED SOLUTIONS •

The area around the separator should be kept free from debris and other objects that could fall or blow into the separator.

•

All mechanical moving parts should be kept clean and properly lubricated. It is particularly important to keep exposed gear mechanisms heavily greased.

•

If the scrapers are out of service, consider using portable disk or surface skimmer to remove oil to slop system.

•

Solids-removal problems at the upstream bar screen.

•

Check operation of forebay surface skimmer.

•

Excessive sludge build-up, reducing effective retention time.

•

Sludges should be removed 1 to 4 times per year to prevent buildup in API separators.

•

Limit build up of sludge level in main channel to < 1.5 ft (0.5 m).

•

Adjust main channel slotted pipe skimmer to insure oil removal.

•

Minimize oil spills

•

Buildup and discharge of oil as result of infrequent skimming operations.

•

Excessive depth of surface oil layer, which can cause oil near the interface to be "pulled" under the oil retention baffle.

•

Long holdup of surface oil. This can cause two problems. The oil can weather, causing density to increase and ultimately settle to the bottom where anaerobic action can occur. Bacteria/fungi can attach themselves to the under side of the oil layer.

•

Unintentional leakage of heavy oil into the sewer.

•

Intentional draining of heavy oil into the sewer from units or from • cleaning operations.

•

High oil loads to wastewater sewer.

•

•

Excessive flow, reducing retention time.

Poor flow distribution, reducing effective retention time

Reduce high oil loads to wastewater sewer from few isolated sources, e.g., tank drawoffs, desalters, pump leaks.

•

If some high oil loads to wastewater cannot be prevented, consider installing a small CPS on this oily waste stream.

•

Maintain the correct forebay level control in API separator by adjusting inlet weir level.

•

Divert stormwater to stormwater diversion basin.

•

Remove "dead spots", pockets or oil accumulation locations in the sewer system to prevent large oil loads during storm events.

•

Keep average wastewater velocity in API separator less than 3 ft/min (1 m/min).

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page

XIX-A1 39 of 64 Date December, 1999

TABLE 8 (Cont) POTENTIAL OPERATION PROBLEMS AND SOLUTIONS POTENTIAL OPERATION PROBLEMS

SUGGESTED SOLUTIONS

•

Too many fine oil droplets or emulsions in feed.

•

Use low dosages (1 to 10 ppm) of high molecular weight polymer/polyelectrolyte to flocculate oily solids in the separator.

•

Unusually high soluble oils.

•

Check to pinpoint the major sources of soluble oils, and segregate, if possible

•

High pH, causing stabilization of oil-in-water emulsions.

•

Keep pH below 9 - caustic or other basic compounds tend to emulsify oil, reducing oil settling performance.

•

Control surfactant use especially during turnarounds, and foam producing chemicals used in fire training.

•

Separators equipped with covers can experience a variety of problems, including cover degradation, pontoon submergence, and shifting of the cover out of position during very high flows.

•

These types of problems can usually be avoided by means of proper design and selection of appropriate construction materials.

•

For parallel-plate separators, plugging of the plate chambers with buildup of solids and foreign matter.

•

Clear the plates by removing the accumulated solids, flushing the plate pack with water or air, or mechanical cleaning.

•

If significant solids levels are detected, the plate slope should be 60°, and periodical blowdown of accumulated solids should help most of plate separators' plugging problems.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 40 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

FIGURE 1 PRIMARY OIL / WATER SEPARATOR SELECTION GUIDELINES Start

Does Facility Oil Content Exceed Effluent Limit?

No

Oil / Water Separator Not Required

Yes Is Free Oil Content To Be Removed >1000 PPM?

No

Secondary Oil/Water Separator Required (See DP XIX-A2)

Yes Primary Oil/Water Separator Required

Use Centrifuge Separator Yes

Is Oil Droplet Size > 60 Microns?

No

Need Fine Solids Particles Removal?

Use Cyclone Separator

Yes

Yes

Use API Separator

Use Parallel Plate Separator

Is Oil Droplet Size > 150 Microns?

No Use Daburt Separator

No

Use Package Oil/Water Separator

Notes: (1) Yes/No decision that are provided are not absolute, they are meant to provide a rough guideline and exceptions are expected. (2) For situations that do not fit into this general decision tree, contact ER&E environmental specialists for assistance. (3) Selection and design of units are based on free oil removal. However, each treatment unit will remove some degree of suspended solids. (4) Most Regulatory oil limits are based on total oil, since the treatment units remove free oil, it is suggested the wastewater samples be analyzed by the method specified by the regulators and for both free and dissolved oil. (a) If dissolved oil is less than the oil limit, physical separation of the oil will be sufficient. (b) If dissolved oil is greater than the oil limit, additional treatment such as biological treatment or activated carbon may be necessary. DP19A1F01

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 41 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 2 API SEPARATOR

Adjustable Inlet Weir Oil Skimmer Oil Layer

Adjustable Outlet Weir

Clean Water Outlet Channel Outlet

Inlet Sediment Trap

Sludge Concrete Basin DP19A1F02

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

Page

XIX-A1 42 of 64 Date December, 1999

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

FIGURE 3 CROSS FLOW PARALLEL PLATE SEPARATOR

Effluent Weir Oil Baffle Oil and Scum Overflow Influent

Oils Influent Effluent

Flow Corrugated Plates

Distribution Oil Weir

Settled Solids

Settled Solids

DP19A1F03

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 43 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 4 DOWNFLOW PARALLEL PLATE SEPARATOR

Adjustable Outlet Weir

Oil Skimmer

Oil Layer

Adjustable Inlet Weir

Oil Globules

Outlet

Inlet

Concrete

Clean-Water Outlet Channel

Sediment Trap Corrugated-Plate Pack

Concrete

DP19A1F04

Sludge Pit

FIGURE 5 DABURT SEPARATOR Tilted Disk Skimmer

Inlet Weir

Oil Coalescence And Separation

Deflector Outlet Weir

Settling Chamber

Bio Pack

Primary Sludge Separation

Aerator (If Required)

Sludge Outlets

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DP19A1F05

DP19A1F06

Involute Inlet

Oil Core

Oil Core

Reject Stream (Typically 2% Of Main Flow)

Vortex

Involute Inlet

Outer Pressure Shell

Inner Liner

Clean Water Surrounding Inner Liner

Clean Water Outlet

Parallel Section

Page

Vortex

Oily Water Inlet

XIX-A1 44 of 64 Date December, 1999

The spin imparted to the fluid as it enters the cyclone generates very high centrifugal forces of well over 1000 g, and this greatly enhanced gravitational effect causes practically instantaneous separation of oil from water.

Section

The de-oiling hydrocyclone is a liquid/liquid hydrocyclone where the main, or bulk flow is water and where the oil concentration is in the range of 0-10,000 ppm.

DESIGN PRACTICES WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 6 HYDROCYCLONES

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page

XIX-A1 45 of 64 Date December, 1999

FIGURE 7 STOKES LAW CHART FOR ESTIMATING OIL / WATER SEPARATOR AREA Oil Droplet Micron Size

0.0004

Specific Gravity Differential

500

0.0006 0.0008

400

0.001 300 0.002 200

0.004

150

0.006 0.008 0.01

100

0.015

80

0.02 60

0.04

50 0.06

40

0.08 0.1 0.15

30

0.2 20

0.4 0.6

15

0.8 1.0

10 40

60

80 100 140

2 180 15

32

20

30

40

50

60

80

100

200

300

400

600

800

1000

212 Water Temperature °F

Capacity (gpm) per 575 ft 2 of Effective Area **

How to Use Chart:* 1. Locate water temperature. 2. Draw vertical line up at water temperature to oil droplet particle size. 3. Draw horizontal line from oil droplet size to specific gravity differential. 4. Draw vertical line down to flow capacity for typical 575 ft

2

unit.

* See "Quick, Rough Sizing Basis" of this Design Practices for more detailed explanation of the chart. ** Effective area of a separator refers to the horizontal projection of the separator plates.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DP19A1F07

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 46 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

FIGURE 8 GRAVITY DISPLACEMENT TYPE OIL / WATER SEPARATOR

Hydrocarbon Side Pumpout Connection

Sampling Hatch Vent

Water Discharge (Pumped) (Note 1) Hydrocarbon Sensor

Carbon Canister

Vent

Sludge Pumpout Connection (Note 2)

NC Hydrocarbon Discharge

Sampling Hatch

N.O.

Flush Drain Inlet

Leak Detection

Hydrocarbon Water Inlet Hydrocarbon Level

N.O . Water Discharge (Gravity Flow) (Note 1)

Water Level Hydrocarbon Stop Ck Valve

Water Sump

Separation Chamber

8

Water Transfer Pipe

NC-Normally Closed NO-Normally Open

Hydrocarbon Sump

Notes: 1. Water discharge is either gravity flow or pumped. 2. Pumpout inlets shall be 3 in. above bottom of separator.

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DP19A1F08

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 47 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 9 UNCOVERED API SEPARATORS Inlet Sewer

Trash Pan

Trash Rack

Rotary Drum Oil Skimmer

Covered Preseparator Flume Flow Transition Section

Preseparator Section

Separator Pumps

Working Platform

Skimmed Oil Line

Notes: (1)

Sludge Removal from Forebay by Clam Shell or Vacuum Truck

(2)

Separator Pumps are allocated for specific services as follows but are manifolded to permit sparing and to provide flexibility of operations. Pump A –Transfer of settled water from covered Skimmed Oil Sump to Inlet of Preseparator Flume

A B C

Oil Retention Baffle Forebay(1) Gateway Pier Gate Slots Gateways Flow

Covered Skimmed Oil Sump

A

B

Flight Scraper Chain Sprocket Flight Scraper Chain

Flow

Rotatable Oil-Skimming Pipe

Effluent Weir and Wall

Water Level Flow Transition Section

Oil-Retention Baffle

Effluent Flume Flow

Effluent Sewer

B

Trash Pan

Covered Preseparator Flume

Sludge Pump Suction

Wood Flight

Skimmed Oil Line

Inlet Sewer

Diffusion Device

SludgeCollecting Hoppers

Pump B – Transfer of Separated Oil from covered Skimmed Oil Sump to Slop Handling Facilities Pump C – Transfer of Sludge from Separator to Sludge Handling Facilities

A

Separator Channel

Curtain Trash Rack Preseparator Section

Rotary Drum Oil Skimmer

Channel Gateways

Gateway Piers

Section A-A Diffusion Device (Reaction Jet)

Flight Scraper Chain Sprocket

Separator Channel

Flight Scraper Chain

Gateway Pier Forebay

Water Level

Effluent Sewer

Flow

Slot For Channel Gate Sludge-Collecting Hopper Discharge With Plug DP19A1F09

Effluent Weir and Wall

Oil-Retention Baffle

Sludge-Collecting Hopper Sludge Pump Suction Pipe

Effluent Flume

Rotatable Oil Skimming Pipe

Section B-B

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 48 of 64 Date December, 1999

EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 10 API SEPARATOR

Overflow to Retention Pond Preseparator 40' 10'

75'

5'

10'

Ditch

10' 1" 1"

12'

Acid Gravity Flow Caustic Gravity Flow

4'

4' 10' Oily Water Sewer

12'

Paved Overflow Ditch

Skimmed Oil Water From Sump

Slop Oil Slump 8'

From Retention Pond 8" Vent to Atmosphere 25'

8'

Flow Control Valve 2500 gpm Max. 10" Vent to Atmosphere 25' Skimmed Oil to Sump 8" 4" Water From Sump Top of 10" Shear Gate Separator Traveling

Mechanical Bar Floating Screen Cleaner Oil 10" Removable Skimmer Cover Approx. NLL

Top of Weir

Approx. NLL Oily Water Sewer 2 Bar Screens 72" 4" Plug Valves

Sluice Gate

6"

3 Reaction Jets Cleanout 6"

Sludge, Pump 50 gpm, 15 psi

Rotatable, Oil-Skimmer Pipe

Bridge

Max. Level 8' 2'-6" 5'-1"

4'-11"

Oil-Retention Baffle Adjustable Effluent Weir

6'-0"

Cleanout 6" Plug Valves 3"

DP19A1F10

Oil to Slop-Oil Tank

To Sludge Thickener

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES Section

Page

XIX-A1 49 of 64 Date December, 1999

FIGURE 11 SLOTTED PIPE OIL SKIMMER

Blanked End

30° Chamfer Inside

DP19A1F11

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

Page

XIX-A1 50 of 64 Date December, 1999

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

FIGURE 12 ROTARY DRUM OIL SKIMMER

Recovered Oil Outlet Pipe

Flow

Oil Trough

Doctor Knife

Drive Motor

Drum

Top

Oil Retention Baffle

Oil Trough Oil Retention Baffle Adhering Oil Layer

Drum

Oil

Oil

Water

DP19A1F12

Drum Water

Front

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

Side

Doctor Knife

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 51 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 13 HORSESHOE TYPE FLOATING OIL SKIMMER

Flow

Retention Cable A Flume Wall

Oil Skimming Weir Skimmed Oil Outlet

Oil Collecting Pan

Removable Plug For Adding Liquid Ballast

A PLAN

Oil Skimming Weir Flow Buoyancy Chamber

Water Level

Skimmed Oil Outlet DP19A1F13

SECTION A-A

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 52 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 14 SELF-ADJUSTING, FLOATING OIL SKIMMER

Brace As Required

Pump Suction

Skimmed Oil Pipes

Oil Skimming Weir TOP VIEW A

FRONT VIEW

A Pump Suction Lifting Ring Skimming Weir

Skimmed Oil

DP19A1F14

SECTION A-A

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page

XIX-A1 53 of 64 Date December, 1999

FIGURE 15 DISC SKIMMER Product Data A Skimming Modules B Hydraulic Power Pack C Frame and Walkway D Disc and Scraper E Existing Oil Discharge Pipe

D E

A A C

B

Fixed

E C B

A

D

F

G

DP19A1F15

Floating

Product Data A Skimmer Head B Skimmer Hydraulic Drive C Suction and Delivery Pump D Buoyancy Tanks (Removable) E Hinged Covers F Manifold and Flow Control Valves G Hydraulic Power Pack

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 54 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 16 REACTION JET INLETS

Target Baffle Support

Orifice

R=D d=

0.06 (D + 1)2

D D+1

Target Baffle (0.25 - 0.6) D Orifice Type

Inlet Wall

d=

0.06 (D + 1)2

R=D

D D+1

(0.25 - 0.6) D Tube Type

DP19A1F16

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 55 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 17 TRAVELING BRIDGE OIL SKIMMER AND SLUDGE COLLECTOR Control Panel

Bridge Drive Unit Drive Line Shaft

Disk Brake

Toe Plate

Towing Trolley Cable Support

Idler Wheel

Retractable Skimmer Blade

Maximum Water Level

Retractable Sludge-collector Blade DP19A1F17

FIGURE 18 FOUR SHAFT COLLECTOR TYPE OIL AND SLUDGE MOVING DEVICE Drive Unit Chain Tightener

Chain Support Track

Freeboard

Flow

Flight

Chain Sprocket

Oil-collection Device

Water Surface

Main Separator Channel

Velocity-head Diffusion Device

DP19A1F18

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 56 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON ENGINEERING

FIGURE 19 SLUDGE COLLECTION HOPPER ARRANGEMENT

Sludge Hoppers

A

A

B

PLAN

Plug

Hopper Outlet From Adjacent Bay Sludge Outlet Pipe

B SECTION A-A

SECTION B-B

DP19A1F19

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

To Sludge Pump

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 57 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 20 SKIMMED OIL SUMP Manual SW

P1

Manual SW

Slop Oil to Slop Tanks

To Atm. 10' LH (Cl) LL (CO)

LH (Cl) LL (CO)

Level Switch ( ∆ P Type) M

M

6" Min. Vent (150 mm)

LHA

P1

Water to Separator Inlet

18" Typ. (455 mm) Pump Start Position

Pump Shut-off Position

From Oil Skimmer 18" min. (455 mm min.) min

Stop Oil Pump*

Removable Trash Screens 1" x 1" Mesh (25 mm X 25 mm)

12 Oil Level LIC (Float Type)

1/4

min

Water Pump*

Oil/Water Interface LIC (Float Or Electric Probe)

* Vertical submerged suction type, centrifugal pump capable of handling solids up to 1/4" (6 mm) diameter

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DP19A1F20

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 58 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 21 TYPICAL OIL DROPLET SIZE DISTRIBUTION IN OILY WASTEWATERS A. TYPICAL PROCESS STREAM

A: Typical Process Stream 500

Oil Droplet Size - microns

300 200 150 100 80 60 40

20 Cumulative Percent of Oil

0.01 0.1

1

10

30

60

90

99

99.9 99.99

B: TYPICAL STREAM FROM STORAGE AREAS

B: Typical Stream From Storage Areas (a) At beginning of storm (b) After-10 minutes of storm

500

Oil Droplet Size - microns

300

(a)

(b)

200 150 100 80 60 40

20 Cumulative Percent of Oil

0.01 0.1

1

10

30

60

90

99

99.9 99.99

DP19A1F21

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page

XIX-A1 59 of 64 Date December, 1999

FIGURE 22 DESIGN VARIABLES FOR API SEPARATORS

L

B Separator Channel A Qw

n=2

AH Separator Channel B B

Forebay TOP VIEW

L

B

B

QM VH

d

d

Ac

Vt SIDE VIEW

AC

=

END VIEW

Total Cross-Sectional Area.

AH

=

Total Surface Area of Separator.

B

=

Width of Channel.

d

=

Depth of Water.

L

=

Length of Channel.

n

=

Total Number of Separator Chambers.

QM =

Influent Flow.

VH

=

Horizontal Velocity.

Vt

=

Rise Rate of Oil Globules

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DP19A1F22

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 60 of 64 Date December, 1999

EXXON ENGINEERING

PROPRIETARY INFORMATION - For Authorized Company Use Only

FIGURE 23 RECOMMENDED VALUES OF F FOR VARIOUS VALUES OF VH / VT 1.8

Combined Turbulance And Short-Circuiting Factor, F

1.7

1.6

1.5

1.4

1.3

1.2 2

6

10

14

16

20

VH / Vt

Recommended Values of Turbulence Factors VH / Vt 20 15 10 6 3

Ft 1.45 1.37 1.27 1.14 1.07

Recommended Value of Short-Circuit Factors VH / Vt

Fs

ALL

1.2 DP19A1F23

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 61 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

CALCULATION FORMS API SEPARATORS Job Name:

Job No.:

Location:

Date:

Service:

Revision No.:

Design Parameters

Summer

Winter

Average

2

3

4

Influent Oil Concentration, ppm (mg/L) Effluent Oil Concentration, ppm (mg/L) Influent Temperature, T, °F (°C) Specific Gravity of Water, Sw, (g/cm3) Specific Gravity of Oil, So, (g/cm3) Absolute Viscosity of Water, µw, cP Flow Rate of Waste Stream, Qm, gpm Concentration of Emulsified Oil, ppm (mg/L) pH of Flow Stream Suspended Solids Concentration, ppm (mg/L) Inlet Oil Size Distribution, % by wt.

1

Oil Particle Size, Microns % Concentration by Weight Calculations

Summer

Winter

Average

Eq. #

Equation (Customary Units)

3

 S − So Vt = 2.41  W  µw

Rising Droplet Velocity, ft/min

Horizontal Velocity, ft/min

4

VH = 15 Vt ≤ 3 ft, whichever is smaller

5

Ac =

6

n =

-

6 ≤ B ≤ 20 ft

7

d =

-

0.3 ≤

9

F = Ft Fs = 1.21 Ft

10

V L =F H d Vt

-

L/B≥5

Minimum Vertical Area, ft2

Number of Channels Channel Width, ft Channel Depth, ft

d/B Ratio Flow Correction Factor Separator Length, ft

L/B Ratio

  

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

Qm VH

Ac 160

Ac , 3 ≤ b ≤ 8 B n d B

≤ 0 .5

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 62 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

CALCULATION FORMS (Cont) API Separator Duty Specifications Number of Channels: Width, ft (m)

Per Channel:

Total:

Depth, ft: (m) Length, ft (m) Skimmer?

Yes:

Automated Control

Yes:

Insulation?

Yes:

Covers?

Yes:

Is installation above grade?

Yes:

Type:

No: No: No:

Type:

No: No:

Materials:

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING

DESIGN PRACTICES

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS EXXON ENGINEERING

Section

Page

XIX-A1 63 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

CALCULATION FORMS (Cont) PARALLEL PLATE SEPARATORS Job Name:

Job No.:

Location:

Date:

Service:

Revision No.:

Design Parameters

Summer

Winter

Average

2

3

4

Influent Oil Concentration, ppm (mg/L) Effluent Oil Concentration, ppm (mg/L) Influent Temperature, T, °F (°C) Specific Gravity of Water, Sw, (g/cm3) Specific Gravity of Oil, So, (g/cm3)

Absolute Viscosity of Water, µ w, cP Flow Rate of Waste Stream, Qm, gpm Concentration of Emulsified Oil, ppm (mg/L) pH of Flow Stream Suspended Solids Concentration, ppm (mg/L) Inlet Oil Size Distribution, % by wt.

1

Oil Particle Size, Microns % Concentration by Weight Additional Design Parameters Assumed Reynolds' Number (500 ≤ NRe ≤ 2000) Plate Inclination (45° ≤ β ≤ 60°): Plate Spacing (0.75 ≤ Sp ≤ 1.5 in.) Calculations

Summer

Winter

Average

Eq. #

Equation (Customary Units)

11

 S − So Vt = 0.386  w  µ w 

Rising Droplet Velocity, ft/min.

Retention Time, min.

13,

Sp

14

tr =

16

Ap =

17

SH =

18

Lp =

Perpendicular Cross-Sectional Area, ft2

Vt cos β 34.5 Sp Qm S w µ w NRe

Horizontal Cross-Sectional Area, ft2

Separator Length, ft

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

   

Ap sin β

3.2 x 10 −4 µ w NRe S w Vt cos β

DESIGN PRACTICES Section

WATER POLLUTION CONTROL

PRIMARY OIL / WATER SEPARATORS

Page

XIX-A1 64 of 64 Date December, 1999

PROPRIETARY INFORMATION - For Authorized Company Use Only

CALCULATION FORMS (Cont) Parallel Plate Separator Duty Specifications Number of Separators: Length, ft (m)

Width, ft (m)

Height, ft (m)

Number of Packs Automated Controls?

Yes:

No:

Insulation?

Yes:

No:

Covers?

Yes:

Type:

No:

Materials:

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON ENGINEERING