Cathodic protection for storage TanksFull description
Full description
Small write up about cathodic protectionFull description
Small write up about cathodic protectionFull description
Description complète
Hgyd
Deepwater offkhore structures am now quite corumom however a deep water development project still &uims significant capital investment on the part of the operator. Corrosion failure is not …Full description
Cathodic Protection Design
Full description
Description complète
Cathodic Protection DesignFull description
Cathodic Protection CalculationFull description
Chapter 5
Cathodic Protection Protec tion
Table of Content
Determining the required protective current needed.......................................... needed..............................................................1 ....................1 Selecting the type of protective system to use.............................................. use............................................................ ........................3 ..........3 Determining the number of anodes needed for various sacrificial materials.........................4 Choosing the type of anode material to use...........................................................................5
Table of figures
igure 5. 1 a. !mpressed " Current Cathodic Cathodic Protection System uses e#ternal Source to provide protective Current igure 5. 1 .b . Sacrificial "anode Cathodic Protection uses piece of metal more $lectronegative than the structure for Protection.................................3 igure 5. % &ypical Scrificial anode Consists of a flat slab of the consumable metal into 'hich fastening straps are cast.................................................. cast......................................................................... .............................................5 ......................5 igure 5. 3 (ayonet " &ype &ype !mpressed " Current anode is located for optimum Current thro' onto the condenser tube sheet............................................................... sheet.................................................................................. .......................) ....)
List of Ta Tables bles
&able 5. 1Current Densities *sed for various Condenser +aterials......................................% &able &able 5. % Current ,utput that can be $#pected from &yp &ypical ical Sacrificial -nodes +aterial. 4
Desiging Cathodic – Protection System For Power Plant Condenser Condenser..
Page 1 of )
Chapter 5
Cathodic Protection
Examle!"#
%$Design a cathodicprotection system for an uncoated 1/0///tube steam condenser having an e#posed 'aterbo#tubesheet surface area of 1/// ft% 2%. m%. 1 Determine the protective current needed for this condenser if the design current density is /.% amp ft% 2%.15 ampm % and 5 percent effective surface coverage 'ill be maintained. % o' many anodes of magnesium0 6inc0 and aluminum 'ould be needed in sea'ater to supply 5/ amp for protection7 3 Compare the number of anodes that 'ould be needed in fresh 'ater to supply 5/ amp for protection.
Determining the required protective current needed Cathodic protection of steam condensers is most often used to reduce galvanic corrosion of ferrous 'aterbo#es coupled to copperalloy tubesheets and tubes. Systems are also used to mitigate attac8 of both ironbased 'aterbo#es and copper alloy tubesheets in condensers tubed 'ith titanium or stainless steel. Cathodic protection is achieved by forcing an electrolytic direct current to flo' to the structure to be protected. &he name is derived from the fact that the protected structure is forced to be the cathode in a controlled electrolytic circuit. &here are t'o 'ays this current may be generated9 21 $ither an e#ternal direct current po'er source can be used0 as in an impressedcurrent system0 ig. : a0 or 2% a piece of a more eletronegative metal can be electrically coupled to the structure0 as in a sacrificial anode system0 ig. : b.&he first step in the design of a cathodicprotection system is to estimate the current requirement. &he usual procedure is to calculate the e#posed 'aterbo# and tubesheet area0 and then compute the total current needed by assuming a current density. !n practice0 current needs are often estimated by applying a test current to the structure and measuring the change in structure potential.
&able 5 lists actual current densities used by utilities to protect condensers made of several different combinations of metals. &he values given 'ere ta8en from a survey prepared for the $lectric Po'er ;esearch !nstitute <10 Pro?ect 1>:30 on 'hich this procedure and its source are based.
Page % of )
Chapter 5
Cathodic Protection
Table &. "Current Densities 'sed for (arious Condenser )aterials.
Figure &. " a. *mressed – Current Cathodic Protection System uses external Source to ro(ide rotecti(e Current Figure &. " .b . Sacrificial –anode Cathodic Protection uses iece of metal more Electronegati(e than the structure for Protection.
@ith a design current density of /.% amp ft % 2%.15 ampm %0 the total protective current need A /.% 21/// A %// amp. @ith the 5 percent effective surface coverage0 5 percent of the surface 'ill be e#posed through coating faults. ence0 the required protective current
Page 3 of )
Chapter 5
Cathodic Protection
'ill be /./52%// A 1/ amp. Clearly0 gross miscalculations are possible if the effectiveness of the coating is incorrectly estimated. &he value of /.% amp ft % 2%.15 ampm % is ta8en from the table mentioned above. -nother problem in estimating protectivecurrent requirements occurs 'hen condensers are tubed 'ith noble alloy tubing such as stainless steel or titanium. !n this case0 a significant length of tubing 2up to %/ ft B >.1 m may be involved in the galvanic action0 depending on the 'ater salinity0 temperature0 and the tube material. &his length dictates the anodecathode area ratio and0 thus0 the rate of galvanic corrosion. Protectivecurrent needs for this type of condenser can be unusually high.
Selecting the type of protective system to use Protectivecurrent needs generally determine 'hether an impressedcurrent or sacrificial anode system should be used. or a surface condenser0 the sacrificialanode system generally become impractical at current levels over 5/ amp. or a sacrificialtype system0 the current output can be estimated by determining the effective voltage and the resistance bet'een anode and structure. &he effective voltage bet'een anode and structure is defined as the anodetostructure opencircuit voltage minus the bac8emf associated 'ith polari6ation at both anode and structure. &his voltage depends primarily on the choice of materials0 as sho'n in &able >.
Table &. + Current ,utut that can be Exected from Tyical Sacrificial -nodes )aterial.
;esistance of the metallic path is usually negligible for an uncoated structure and the electrolytic resistance is dominant. or a coated structure0 this resistance may become significant. &he ma#imum achievable current output can be estimated by considering the case of an uncoated structure.
Page 4 of )
Chapter 5
Cathodic Protection
Determining the number of anodes needed for various sacricial materials &able > gives a range of current outputs estimated for different sacrificial materials 'ith an anode of the dimensions sho'n in ig. . &hus0 for any sacrificial material0 number of anodes needed A 2required protectivecurrent output0 amp 2current output for the specific sacrificial material0 amp. Since the condenser being considered here is cooled by sea'ater0 'e 'ill use the values in the first column in &able >. or magnesium0 number of anodes required A 5/%.3 A %1.)3 say %% anodes. or 6inc0 number of anodes required A 5//.: A>%.5 say >3 anodes. or aluminum0 number of anodes required A 5//.: A >%.5 say >3. rom a practical standpoint0 >3 sacrificial anodes is an e#cessive number to install in most condenser 'aterbo#es. &he respective service of these anodes at 5/ amp are about three months for magnesium0 si# months for 6inc and aluminum. &his short service further reduces the practicality of sacrificial anodes at high protective current levels. o'ever0 in fresh 'ater0 the current output is lo'er and is limited by the higher resistance of the 'ater. Corresponding service lives are 5 to 1/ years for magnesium0 and 4/ to >/ years for 6inc and aluminum. Protective coating further reduces the effective 'etted surface area and lo'ers the required protective current at the same time as it limits the current output of the anodes.
Figure &. + Tyical Scrificial anode Consists of a flat slab of the consumable metal into which fastening stras are cast
Page 5 of )
Chapter 5
Cathodic Protection
Choosing the type of anode material to use Several different factors affect the choice of anode material in both sacrificial and impressedcurrent systems. Choice of a sacrificialanode material is largely determined by the current density needed0 but the efficiency of the material is also important. !n an anode material that is 5/ percent efficient0 half the material deteriorates 'ithout providing any useful current. &ypical electrochemical efficiencies are9 magnesium0 4/ "5/ percent 6inc0 / percent aluminum0 :/ percent. ere are brief features of several important anode materials9 Magnesium anodes provide a high driving voltage0 but are not as efficient as zinc or aluminum . Zinc anodes are excellent as sacrificial material; at temperatures above 140 F (60 C), zinc ma passivate 0
providing almost no protective current. !luminum anodes are not "idel used to protect surface condensers because of performance problems# $teel anodes are used in a fe" po"er plants to protect copper%allo tubes&eets, but t&e are less efficient t&an traditional materials# !mpressedcurrent systems0 ig. : a0 use anodes of platini6ed alloy0 lead alloy0 or iron alloy. Platini6ed and leadalloy anodes are favored in sea'ater0 'hile ironalloy anodes are favored in lo'salinity 'ater. 'latinized% and lead%allo anodes can be operated at higher current density than those of iron alloy0 so fe'er anodes are needed in the 'aterbo#. Platini6edtitanium anodes can be operated at current densities up to 1/// ampft% 21/0)>4 ampm% and voltages up to about : in sea'ater. Such anodes should have a service life of 1/ to %/ years0 dependin on t&e current densit and t&e platinum platin t&icness# *ead%allo anodes are "idel used in sea"ater applications. &hese anodes can be operated at current densities as high as 1/ to %/ amp ft % 21/).> to %15.3 amp m % 'ith a life e#pectancy of more than 1/ years.
Related Calculations. &his procedure outlines the essentials of si6ing anodes for protecting
steam surface condensers. or more detailed information0 refer to the report mentioned in step 1 of this procedure. Data for this procedure 'ere compiled by Eohn ;eason and reported in Po'er maga6ine0 using the report mentioned earlier. by Eohn ;eason and reported in Po'er maga6ine0 using the report mentioned earlier.
Page > of )
Chapter 5
Cathodic Protection
Figure &. /ayonet – Tye *mressed – Current anode is located for otimum Current throw onto the condenser tube sheet