DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION SYSTEMS

Transcription

1 DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION SYSTEMS Advanced Corrosion Course 2017 February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 1

2 OBJECTIVES To Provide Corrosion Control Personnel with the Basic Information to Properly Design an Impressed Current Cathodic Protection System for a Given Structure February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 2

3 PARAMETERS TO BE DISCUSSED Current Density Requirements for Cathodes Anode Requirements Groundbed Layout Circuit Resistance & Electrolyte Resistivity Rectifier Input & Output Specifications February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 3

4 A REVIEW OF IMPRESSED CURRENT FUNDAMENTALS February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 4

5 WHAT IS CORROSION? The Deterioration of a Material, Usually a Metal or Alloy, Because of a Reaction with its Environment February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 5

6 WHAT IS CORROSION? February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 6

7 WHAT IS CATHODIC PROTECTION? The reduction or elimination of corrosion by making a metallic structure cathodic with respect to the electrolyte in which it is installed February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 7

8 HOW DO WE DO THIS? (Where Does The Protective Current Come From?) Galvanic System February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 8

9 GALVANIC CURRENT SYSTEM Usually Magnesium Can Be Zinc Direct Connect Sometimes By Header Cable February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 9

10 WHERE DOES THE PROTECTIVE CURRENT COME FROM? External Source Impressed Current System February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 10

11 IMPRESSED CURRENT SYSTEM Transformer Power Supply + & - Cables Anode Cathode Dielectric Isolation February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 11

12 ANODE CONSUMPTION RATES (Impressed Current) HIGH Magnesium Zinc Aluminum Scrap Steel Cast Iron LOW High Silicon Chromium Bearing Cast Iron Treated Graphite Mixed Metal Oxides February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 12

13 APPLICATION for Impressed Current Protects Large Structures Protects Structures That Require Large Amounts Of Current Can Replace Depleted Galvanic Systems (Depleted Anodes Can Act As Holidays) February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 13

14 ADVANTAGES OF AN IMPRESSED CURRENT SYSTEM Variety of Applications with Flexibility Adjustable Voltage and Current Outputs Can Protect Large Areas or Structures Can Be Automatically Controlled February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 14

15 DISADVANTAGES OF AN IMPRESSED CURRENT SYSTEM Increased Maintenance Requirements Higher Operating Costs Stray Current Initial Cost of Installation Possibility of Cable Failure February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 15

16 RECTIFIER FAILURES 3% Power 6% Anodes 33% Cable 58% Rectifier February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 16

17 USEFUL DESIGN INFORMATION Pipeline Material Steel (grade), cast iron, wrought iron, or other? What is the known electrical resistance? Weight/Foot Wall Thickness Diameter February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 17

18 USEFUL DESIGN INFORMATION Coating What type of coating? What are the coating characteristics? February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 18

19 USEFUL DESIGN INFORMATION Leak History, CP History? Test Stations? Welded or Coupled Joints? Taps? Insulators? Are Maps or Drawings Available? Foreign Structures? Other Stray Current Sources? HVAC or HVDC? Operating Temperature? Is AC Power Available? February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 19

20 DESIGN OF AN IMPRESSED CURRENT GROUNDBED The Initial Steps Select Groundbed Site Select Groundbed Type February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 20

21 SELECTING A SITE Soil Resistivity Very important factor Affects the number of anodes and rectifier outputs Low resistance = Low output voltage Low resistance = Smaller rectifier February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 21

22 SELECTING A SITE Soil moisture Soil Moisture at Anode depth Usually Soil Resistivity Decreases as Moisture Increases Anodes Should Be Installed in Moist Areas Fluctuating Soil Moisture Can be Offset by a Constant Current Rectifier February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 22

23 SELECTING A SITE Interference With Buried Foreign Structures Locate groundbeds away from foreign structures whenever possible Contact local Corrosion Committees or foreign operators when installing impressed current systems February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 23

24 SELECTING A SITE AC Power Supply Availability Is AC power available? You may have to consider other options Solar units Generated power February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 24

25 SELECTING A SITE Accessibility For Work Crews The groundbed site selected must be accessible to construction vehicles for groundbed installation, testing, and repairs February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 25

26 SELECTING A SITE Purpose of Groundbed and Site Availability What is the intended purpose of the groundbed? Protect a long section of pipe or a short section? Will the location fit the intended purpose? Can easement be obtained? February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 26

27 SELECTING A GROUNBED TYPE BASED ON SITE SELECTION Site availability may dictate the type of groundbed that you use Expense of ROW Land limitations (roads, ROW s, other utilities, etc.) Shallow layers of high resistance soil Shallow layers of low resistance soil February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 27

28 CONVENTIONAL GROUNDBED DESIGN Conventional, Remote or Point-type Groundbed A type of groundbed that is usually installed perpendicular to the pipeline and the first anode is several hundred feet away from it February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 28

29 CONVENTIONAL GROUNDBED REMOTE The pipeline is outside of the anodic gradient of the groundbed caused by the discharge of current from the anodes to the surrounding soil February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 29

30 CONVENTIONAL GROUNDBED February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 30

31 CONVENTIONAL GROUNDBED The main difference between a perpendicular and parallel groundbed has to do with the gradient effect (Assuming the line of parallel anodes is at a distance from the pipeline equal to that of the first anode in a perpendicular groundbed) February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 31

32 CONVENTIONAL GROUNDBED Another way of putting it! How far from the pipeline do I start my groundbed? February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 32

33 CONVENTIONAL GROUNDBED R. Rudenberg s formula (Used to calculate voltage gradients in uniform soil due to an energized groundbed) February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 33

34 Example: I = 5 amperes ρ = 10,000 ohm-cm y = 150 feet x = 100 feet Vx = x = volts February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 34

35 CONVENTIONAL GROUNDBED If x (The distance to point x from the groundbed in ft) is more than 10y (The length of the groundbed in soil in ft), a simplified formula can be used. February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 35

36 CONVENTIONAL GROUNDBED Simplified Formula February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 36

37 TESTS REQUIRED FOR GROUNDBED DESIGN Soil Resistivity Tests Current Requirement Tests February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 37

38 SOIL RESISTIVITY TESTS Conducted along the route of the pipeline Conducted at groundbed sites February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 38

39 CURRENT REQUIREMENT TESTS Set-up a Temporary Groundbed Driven ground rods Guardrails Metal fence posts Tin foil in a stream Portable rectifiers, DC welders, batteries February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 39

40 CURRENT REQUIREMENT TESTS Energize the temporary GB Obtain On and Off readings along the pipeline Use a high input resistance voltmeter and coppercopper sulfate reference electrode Increase the current output until protection is achieved February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 40

41 CURRENT REQUIREMENT TESTS If the output of the temporary groundbed is not enough to achieve full protection, the following process can be used to determine the correct current requirements February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 41

42 CURRENT REQUIREMENT TESTS Output of temporary GB = 1.25 amps Potential Shifts V Polarization = V g OFF After V g OFF Initial Location V g ON Initial V g OFF Initial V g ON After V g OFF After V Polarization Test Station # Test Station # Test Station # February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 42

43 CURRENT REQUIREMENT TESTS Required Shift = = 180 mv Lowest Shift = 140 mv Total Current Required to Meet -.850V Polarized Lowest Voltage Shift Required Voltage Shift = Current Used During Test Current Required (X) 0.140v = 1.25A 0.180v X X = 1.607A February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 43

44 Design Step #1 Calculate Current Requirement Current Response Testing Theoretical Current Calculations February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 44

45 Theoretical Current Calculation I T = Area * Current Density * % Bare For Pipelines: Area = π*d*l Current Density = 2 ma/ft2 (bare steel) February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 45

46 Design Step #2 - Determine Number of Anodes W = Consumption Rate*Desired Life*Required Current Utilization Factor February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 46

47 Design Step #3 Calculate Groundbed Resistance Use Formulae on Pages 24 and 25 for Different Groundbed Types/Configurations Single Vertical (Deep Anode) Multiple Vertical Single Horizontal Multiple Horizontal February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 47

48 Calculating Anode Resistance to Earth - Single Vertical Anode RV Where = Single Vertical Anode (H.B. Dwight Formula) ρ 8L = ln 1 L d R v = Resistance-to-earth of single vertical anode-to-earth in ohms ρ = Layer soil resistivity in ohm-cm L = Anode length in feet d = Anode diameter in feet February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 48

49 Design Step #3 Multiple Vertical Anodes Multiple Vertical Anodes in Parallel (Erling D. Sunde Formula ) ρ 8L 2L R V = ln 1+ ln N NL d S Where = R v = Resistance-to-earth, in ohms, of the vertical anodes connected in parallel ρ = Layer soil resistivity in ohm-cm L = Anode length in feet N = Number of vertical anodes in parallel d = Anode diameter in feet S = Spacing between anodes in feet February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 49

50 Design Step #3 Single Horizontal RH Single Horizontal Anode (H.B. Dwight Formula) ρ 4L + 4L S + L S S + L = ln + 1 L ds L L Where: R H = Resistance-to-earth, in ohms, of the vertical anodes ρ = Layer soil resistivity in ohm-cm L = Horizontal Anode length in feet d = Anode diameter in feet S = Twice anode depth in feet February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 50

51 Design Step #4 Determine Rectifier Voltage Size Determine Total Circuit Resistance R Total = R Groundbed + R Structure + R cable R Structure = R coating /Area (Table II, pg. 17) Cable Resistance is usually negligible unless traversing really long distances February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 51

52 Structure-to-Earth Resistance February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 52

53 Structure-to-Earth Resistance February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 53

54 Cable Resistances (pg 33) R C = R cable x L cable Where: R cable = Standard resistance of cable (pg 34) L cable = Length of cable used (figure 4-1) L cable = L negative + L pos(1) + 1/2L pos(2)** ** = Average current in L pos(2) is ½ total current because of parallel anodes February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 54

55 Design Step #4 Determine Rectifier Voltage Size Use Ohms Law V = IR Where: I = Current Required R = Total Circuit Resistance R = groundbed + cable + structure-to-earth February 21-23, 2017 Keith Boswell, National Pipeline Services, LLC. 55