Welcome, Guest
Username: Password: Remember me

TOPIC: Cathodic protection

12 years 4 months ago #8665

  • nvmani2005
  • nvmani2005's Avatar
  • Offline
  • New Member
  • Posts: 3
  • Karma: 0
1) Sacrificial Anode and 2) Impressed current systems are available for Cathodic protection.
Are there guidelines where such systems are selected? Which is better system?

Cathodic protection 12 years 4 months ago #4042

  • Jop
  • Jop's Avatar
  • Offline
  • Moderator
  • Posts: 1052
  • Thank you received: 14
  • Karma: 11
I have always considered this to be an electrical related issue. You may try contacting an Electrical Engineer who has experience with Cathodic Protection. However I offer the following:

Here are some web sites that you might try for information:
http://www.cathodicprotection.com/

http://www.delweg.com/

http://www.state.sd.us/denr/des/ground/ ... ection.htm

http://www.epa.gov/oust/ustsystm/cathodic.htm

http://www.deq.state.mi.us/documents/de ... memo18.pdf

http://www.npl.co.uk/lmm/docs/cathodic_protection.pdf

And From Wicapedia:

Galvanic CP

Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminium. The electrochemical potential, current capacity, and consumption rate of these alloys are superior for CP than iron.

Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode.

Impressed Current CP

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and others.
A cathodic protection rectifier connected to a pipeline
A cathodic protection rectifier connected to a pipeline

A typical ICCP system for a pipeline would include an AC powered rectifier with a maximum rated DC output of between 10 and 50 amperes and 50 volts. The positive DC output terminal is connected via cables to the array of anodes buried in the ground (the anode groundbed). For many applications the anodes are installed in a 60 m (200 foot) deep, 25 cm (10-inch) diameter vertical hole and backfilled with conductive coke (a material that improves the performance and life of the anodes). A cable rated for the expected current output connects the negative terminal of the rectifier to the pipeline. The operating output of the rectifier is adjusted to the optimum level by a CP expert after conducting various tests including measurements of electrochemical potential.

Telephone wiring uses a form of cathodic protection. A circuit consists of a pair of wires, with forty-eight volts across them when the line is idle. The more positive wire is grounded, so that the wires are at 0 V and -48 V with respect to earth ground. The 0 V wire is at the same potential as the surrounding earth, so it corrodes no faster or slower than if it were not connected electrically. The -48 V wire is cathodically protected. This means that in the event of minor damage to the insulation on a buried cable, both copper conductors will be unaffected, and unless the two wires short together, service will not be interrupted.

If instead the polarity were switched, so that the wires were at 0 V and +48 V with respect to the surrounding earth, then the 0 V wire would be unaffected as before, but the +48 V wire would quickly be destroyed if it came into contact with wet earth. The electrochemical action would plate metal off the +48 V wire, reducing its thickness to the point that it would eventually break, interrupting telephone service. This choice of polarity was not accidental, corrosion problems in some of the earliest telegraphy systems pointed the way.

This should give you a starting place
Do it once and Do it Right

When we repaired the bottom 11 years 7 months ago #4572

  • juan1977
  • juan1977's Avatar
  • Offline
  • Senior Member
  • Posts: 63
  • Karma: 0
When we repaired the bottom floor, annular and the bottomsection of the wall of our AST, part of the work we did is also to install a CP. What we have used as the basis for the design is the ANSI/API RP 651. The actual installation was supervised by an electrical engineer, however its need as been specified by a mechanical engineer in-charge of the whole project (who happen to be me) - it involved several big AST's.

Re: Cathodic protection 11 years 7 months ago #4579

  • gpsvn
  • gpsvn's Avatar
  • Offline
  • Premium Member
  • Posts: 94
  • Karma: 0
1) Sacrificial Anode and 2) Impressed current systems are available for Cathodic protection.
Are there guidelines where such systems are selected? Which is better system?

It's all about cost and practicality, I guess. If you do a jacket for an offshore platform, then it's almost certain that you'll have sacrificial anodes. These are designed for the life of the jacket or will be replaced after certain time. If you do pipelines then there are choices, I think you need to evaluate the costs associated with each choice.

For atmospheric storage tanks and underground pressure vessel, I think sacrificial anode is a good choice.

im not assure about the 11 years 7 months ago #4580

  • juan1977
  • juan1977's Avatar
  • Offline
  • Senior Member
  • Posts: 63
  • Karma: 0
im not assure about the other plant, but in chemical, refineries and petrochem - impressed current is the method of choice. we're talking here a pretty big piece of equipment that needed protection from rust - and as neil young says, never sleep.
Moderators: Jop
Time to create page: 0.486 seconds

 

 

Latest Jobs Feed

Feed not found.

Company News Feeds

Company News Feeds

Login Form

You are not logged in.

Search

}());