That is not a bad reference guide. One obvious admission would be where you have anodes for cathodic (if you are doing this). Order a copy of Peabody's Control of Pipeline Corrosion. Every 1000' seems a bit overkill, but I guess it depends on what kind of coating/how many anodes you are throwing on the line.

Thank you for replying. Will order the Peabody book.

If you talked to some CP engineers you'd have one every 100m....

Usually dictated by location and access, i.e. you can't have it in the middle of a field.

Each CP engineer is law unto themselves in my experience so you could go from 100m between them to 1500m depending on who you choose.

But that set of criteria look's Ok to me

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I never heard someone say that they installed too many. Often as time goes on due to construction and such some must be abandoned, and having some redundancy comes in very handy.

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Not to high jack the forum, but LittleInch is right - ask a CP Engineer what to do on this, and you will get a different response from each one. For the record, I have designed many miles of pipeline without even putting a reference electrode in (just test stations with a test wire and current carrying wires). It all depends on how you are accounting for IR drop and the sophistication of the operator's corrosion monitoring. A large company does not mean that they have sophisticated corrosion procedures. I know several such large operators who I have worked with over the years that will do "head scratchers" on the CP side. My standard is to show them (test stations, reference electrodes, and/or anodes) how I want to and then send the client's Corrosion Department the plans to have them nitpick unless they have explicitly told you what they want otherwise.

As a CP engineer there are some documents that can give you guidance for test post distance I.E. BS EN 12954. However, as a general rule your list is quite comprehensive with the exception of 1000 feet, I would suggest 1000 meters is more normal.

It would be of great interest to know what type Cathodic Protection suystem is being proposed for pipline. Is it going to be a sacrificial system or immpressed current system? A sacrificial system would certainly have more test posts as sacrificial anode do not protect well over long distance, thus requiring anodes at fairly short distances, each anode or anode bank should be terminated in a post that is suitable for taking pipeline potentials. However on a 5000 feet long pipe a single transformer rectifier and ground bed would be norm, situated close to pipeline center with suitable ground bed connection post and drain point test post.

The pre-construction CP survey carried out by a competant CP Senior Technician will give you the most guidance on system requirement, this survey should include soil resistivity and soil pH data. Without a survey being carried out it is quite possible the wrong type of CP system may be installed. It would also highlight and document the requirement of any additional test facilities.

I would however suggest the installation of DC coupons and permanent reference elecrodes at each test facility, this is a requiremnet of the UK's biggest pipeline operator and is documented in their design procedure.

Thank you for the responses. I asked my project manager if we could retain a CP engineer to review our plans and specifications but he indicated that it wouldn't be worth the cost, even though we have a healthy budget with the design at 90%.

Garry, the client has indicated that they would determine if a sacrificial or impressed system would be desirable sometime in the future, when deemed necessary to protect the pipe; they just want test stations with copper copper sulfate half cells for now. BTW, the geotech report shows that the minimum resistivity of the soil ranges between 1,100 and 2,300 ohm-cm.

If you have good exterior coating, you should not need many MG anodes for this. A single multi-anode groundbed should be sufficient. If it is bare exterior - you will need A LOT - and it would be best to space MG anodes along the pipeline at regular intervals. Since you don't know what type of future CP system/the location/etc - I would let them add those themselves at a future date. They will need to expose the pipeline anyhow to connect the wires. Space test stations and electrodes per your above guidelines and increase the spacing (over 1000'). The various points about access and damage protection are good/valid. If there are fences/trees/power poles, place the test stations close to them if you can.

You can add coupons as a few have suggested if you want also. But if your client doesn't current use them/understand them - they are honestly a waste of money (unless they consult out their corrosion monitoring).

Exterior coating is cement mortar. Geotech reports a low exposure category for water-soluble sulfate content per ACI.

Makes sense. This is not a tradition dielectric (insulating coating) like I am used to working with. I believe though that your test station question is answered sufficiently.

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1000 ft spacing is a "bit" of an overkill? More like a mile, unless there is some REAL reason you need one closer.

Having seen the above information with regards to soil resistivity and cement coating it puts a whole new complextion on the issue. 1100 Ohm.cm or 11 Ohm.m is higher than is normally accepted as suitable for sacrificial anodes, consult NACE and ICORR standards as both recommend sacrificial anode to only be used where resistivity is less than 1000 Ohm.cm (10 Ohm.m). Further a cement coating is also a highly resistive electrolyte and will add significant overall circuit resistance between the anodes and the pipe structure. The reason for this is (again as per NACE and ICORR) is sacrificial anodes have a very limited driving voltage between the anode and the cathodic structure for example a standard Magnesium alloy anode has a theoretical natural potential of -1500mVDC whilst the steel cathode has a theoretical natural potential of -550mVDC, therefore the driving voltage is only 950mVDC. This results a very limited distance the current will be efective for corrosion control. Therefore when calculating the number of anodes required for a protective potential with high resistivity, the cost of anodes far outweighs the benefit. Lots of anodes in groups close together means lots of test posts for farmer Giles and council hedge cutters to destroy.

With regards to the chemical makeup of the electrolyte low sulphate is good, but the real enemy, not only for reference electrode selection but for protection against corrosion of the pipe is chlorides. The reason for this is the pipe is going to be coated in cement motar, assuming this is a high pH cement it would make the steel passive (consult the porbaix diagram for corrosion of steel versus pH value). Chlorides have 2 major influences on this type of installation. Firstly if cholrides are absorbed into the cement motar, it will change the steel pipe from being passive to being active. This means the pipe will start to corrode and the corrosion procuct has a volume 3 time larger than the steel that has been consumed causing the cement motar to fracture and break up. Additionally cholrides attack Cu/CuSO4 reference electrodes causing them to fail. Therefore, if the pipe route is under a tidal water way or there is standing brackish water in the vicinity of the pipeline chloride is a real threat to this pipeline and the installed Cu/CuSO4 reference electrodes.

With these considerations my advice would be to recommend an impressed current system installed at the midpoint of the pipe section. As this monitoring point would also be a drain point for the Cathodic Protection system and is well within recommended distance from either end of the pipe for future surveys such as Close Interval Potential Surveys. Then you can consider other test points to fill the requirements you listed above.

Try the book Peabody's Control of Pipeline Corrosion, this has helped me a lot to understand when and where to put reference electrodes. And also, don't forget to put them at sensitive locations. Like Train junctions, any electrical facility closeby (powerplant etc).

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