Monographs on

Refinery EnvironmentalControl--- Management of Water Discharges

Design and Operation of Oil-Water Separators

API PUBLICATION 421

FIRST EDITION, FEBRUARY 1990

Petroleum

iI _ AmericanInstitute

HelpingYou Get TheJob

DoneRightt"

Monographs on

Refinery EnvironmentalControl--

Management of Water Discharges

Design and Operation of Oil-Water Separators

Refining Department API PUBLICATION 421

FIRST EDITION, FEBRUARY 1990

Petroleum

II AmericanInstitute

Helping You Get The Job

Done Right._

P

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Copyrigh© 1t989Am_c_nPctzolcIns__m tum

FOREWORD

The Monographs on Refinery Environmental Control consist of four series--- Management of Emissions to Air, Management of Water Discharges, Management of Solid Wastes, arid Management of the Subsurface Environment. The monograph series Rre ongo- ing projectsoftheCommitteeon RefineryEnvironmentaClontroloftheAPI RefiningDe-

pattrnentandareintendedtobe usedby engineersresponsiblefortbedesign,consmacfion,

opemlion, and maintenance of disposal systems for wastes generated in refineries.

This monograph provides design guidance for gravity-type oil-water separators for use in pelzoleum refineries, provides practical advice on solving operating problems and im- proving separator performance, and presents information on performance of existing refin- ery separators.

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the d_rz contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this pub- lication and hereby expressly disclaims any liability or responsibility for loss or damage re- suiting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict.

Suggested revisions are invited and should be submitted to the director of the Refining Department, American Pe_oleurn Institute, 1220 L Street, N.W., Washington, D.C. 20005.

iii

CONTENTS

Page

SECTION I---G_ .............................................................................. I

1.1 Introduction ..................................................................................................... I

1.2 Background ..................................................................................................... 1

1.3 Basic Theory ................................................................................................... I

1.4 Application ...................................................................................................... 2

1.5 Referenced Publications .....................................................................:.;.......... 3

SECTION 2 CONVENTIONAL OIL-WATER SEPARATORS .... 3

2.1 Design Proceflmes ........................................................................................... 3

2.1.1 General ..................................................................................................... 3

2.1.2 Wastewater Chara_rization 3

2.1.3 Establishing a Design Flow 4

2.1.4 Step-by-Step Design Calculations ............................................................ 6

2.1.5 Design,Example ................................................................................_........ 9

2.2 ConstructionDetails 11

2.2.1 General 11

2.2.2 Met Section 11

2.2.3 Separation Section 13

2.2.4 Other Design Features 19

2.3 Performance 21

SECTION 3---IMPROVING THE PERFORMANCE OF

EXISTING SEPARATORS 22

33.2.1 GeneralProcess TrouApprobleshooach ...........................................................................................ting ................................................................................. 2222

3.2.1 Design Review 22

3.2.2 Emulsified Oil 22

3.2.3 How Surges ..........................................................................; 24

3.3 Dealing With Emulsified Oil 24

3.4 RetrOfittingExisting Separators With ParallelPlates 24

SECTION 4 PARALLEL-PLATE SEPARATORS 25

4.1 General 25

4.1.1 Introduction 25

4.1.2 Design 25

4.1.3 Wastewater Characteristics Required for Separator Sizing 25

4.1.4 Parallel-Plate Surface Area .................................................................; 25

4.1.5 Maintenance 26

4.2 Consm_ctionDetails 26

4.3 performance 26

SECTION 5 OPERATION AND MAINTENANCE OF

OIL-WATER SEPARATORS 28

SECTION 6---REFER.ENCE_ 30

APPENDIX A--DERIVATION OF BASIC EQUATIONSFOR DESIGN OF

OIL-WATERSEPARATORS 31

APPE_4DIXB--DErER.MINATION OF SUSCEPTIBILITYOF

SEPARATION 33

APPENDIX C----SSEPARATORELEt:It:u RESURVEYSULTS: t985............................................................API OK,-WATER ;., 35

APPENDIX D-TEST FOR SPE_II_ICGRAVITY OF OIL IN WASTEWA2F-_ 39

V

Figures

1---Specific Gravity of Clear Water (Fresh and Sea) for Temperama'es Between

40°i:: and 1200F .............................................................................................. 5

2--Absolute Viscosity of Clear Water (Fresh and Sea) for Temperatures

Between 4B°F and 120°F ............................................................................... 6

3---Design Variables for Oil-Water Separators ................................................... 8

4---Recornmended Values of F for Verioes Values of vH/V_ ................................. 10

5--Conventional Oil-Water Separator (Uncovered) ........................................... 12

6---Reaction-let Inlets .......................................................................................... 14

7--Vertical-Slot Baffle ........................................................................................ 15

8--Four-Shaft Collector-Type Oil- and Sludge-Moving Device ......................... 15

9---Traveling-Bridge Oil Skimmer and Sludge Collector ................................... 16

10----General Arrangement of Rotautble Slotted-Pipe Skimmer .......................... 17

11---Rotary-Drum Oil Skimmer .......................................................................... 18

12 Horseshoe-Type Floating Skimmer ............................................................. 18

13---Self-Adjusting Floating Skimmer ................................................................ 19

lel Covers for Oil-Water Separators ................................................................. 20

15----Performance of Conventional Oil-Water Separators ................................... 21

16--Correlation Between Influent and Effluent Oil Levels for Existing

Oil-Water Separators ................................................................................... 23

17--Cross-Flow Parallel-Plate Separator ............................................................ 27

18---Down/Iow Paralle1-Plate Separator .............................................................. 28

19---Performanoe of Pm'allel-Plate Separators .................................................... 29

Tables

1--Applicability of EPA Methods 413.1 and 413.2 ............................................ 4

2--Typical Ranges for the Basic Design Variables of Paranel-Plate Separation 25

C-l---Selected Results of 1985 API Oil-Water Separator Stu'vey:

Conventional Separators ............................................................................ 36

C-2---Seleeted Results of 1985 API Oil-Water Separator Survey:

Pzrallel-Plate Separators ............................................................................ 37

D-l---Oil-Density Variation with Temperature (15°API-35°API) ...................... 40

Design and Operation of Oil-Water Separators

SECTION1--GENERAL

1.1 Introduction Free oil is in the form of diserete oil globules of a sizesuf-

ficient so that the globules can rise as a result of buoyant The purpose of this monograph is to provide design guid- forces and'form an oil layer on top of the water. Under ance on gravity-type oil-water separators for use in petro- proper quiescent flow conditions, free oil can be removed by leum refineries, to provide practical advice on solving gravity separation. This removal is a function of residence operating problems and improving separator performance, time, differences in specific gravity and temperature, and the and to present information on the performance of existing stability of the emulsion. Some _arse oily solids have a spe- separators, cific gravity greater than 1.0 and will settle to the bottom of This monograph deals solely with gravity-type oil-water a separator. Most of the oil and associated fine solids in re- separators, that is, those that rely on differences in specific finery wastewater have a specific gravity of less tb_n 1.0 and

gravity to separate oil globules from a wastewater s_eam, will rise to the water's surface.

Two types of oil-water separators are covered in tiffs mono- Emuls_ied oil is in the form of much smaller oil droplets graph: or globules with a diameter of less than 20 microns (mostly a.' The conventional, rectangular-channel unit. in the 1-10-micron range). These globules form a stable sus-

b. The parallel-plate separator, pension in the water as a result of the predominance of inter-

Note:Throu_aoutd_smono_h, thctcnnconven_onaloil-watersepara- particle forces over buoyant forces. The presence of torisu_c-.dinpLaceoftheummAPIseparator_drefcrstorcctaugular-ehan- particulates also contributes to emulsion formation. Regard- n¢lunitsdesignedmaccordancewithth_¢duniapublishedearlierby API. less of how long a tree oil-water emulsion stands under qui-

The_APlseparatorhasbecom_almostagene_ctenn, tomerJmesused escent conditions, a separate oil phase will not form.

incom:ctlyto referto anygravity.typeoi/-wat_sc0arator. Emulsified oil may be removed by chemical addition and co- Historically, the design of conventional oil-water eepma- aleseing or by flotation but net bygravity separation alone. tors was bused on criteria developed from a 3-year, API- Dissolved oil is the petroleum fraction that forms a true

funded research study initiated in 1948 at the Engineering molecular solution with water. Dissolved oil cannot be re- Experiment Station at the University of Wzseonsin [ 1]. Since moved by gravity separation; further wastewater tzeetment

then, numerous oil-water separators based on the API--devel- (for example, biologlcaltmatmen0 is necessary if removal of oped design criteria have been designed and put into opera- dissolved oil is required.

tion throughout the petroleum industry. The criteria were The fraction of oil that is removable from a wastewater developed as voluntary guidelines for desi_nlng conven- stream by gravity separation is affected by the type of oily tional oil-water separators. Many other separators based in material present. The method of measuring ell concentration part on the API-developed design criteria have been zd_pted can also affect the apparent efficiency of removal. Conven- for a variety of other industrial wastewater treatment appli- tional oil-water separators remove ouly free oil; stable emul-

cations, sions and dissolved oil require additinnal treatment.

Thismonographsetaln_pertinent inferm_on fromChap

tersSand6 ofthe1969editionoftheAPIManualonDis- 1.3 BasicTheory

posal of Refinery Wastes, Volume on Liquid Wastes, and

incorporates the recent findings of a literature review and In essence, an oil-water separator is a chamber designed 1985 survey of refinery separators. Data from this survey of to provide flow conditions sufficiently quiescent so that the design and operation of oil-water separators have been globules of free oil rise to the water smffaee and coalesce into incorporated in this monograph, a separate oil phase, to be removed by mechanical means.

Although the theory of oil-water separators is briefly dis- Oil-water separation theory is based off the rise rate of the

cussed, the emphasis of this monograph is on practical gnid- oil globules (vertical velocity) and its relationship to the sur- anee for those involved in the design or operation of oil- face-loading rate of the separator. The rise rate is the velocity

at which oil particles move toward the separator surface as a

water separators, result of the differential density of the oil and the aqueous phase of the wastewater. The surface-loading rate is the flow

1.2 Background rate to the separatordivided by the surfacearea of the sepa-

A _fmerywastewamrsin;ammay contsinoilinttn'eema- rater. In an ideal separator, any oil globule with a rise rate jor forms: free, emulsified, and dissolved. Some pertinent greater than or equal to the surface-loading rate will reach

feetures of each of these are briefly discussed below: the separator surface and be removed. An ideal separg,tot is