Limit for CP "ON" potentials
Limit for CP "ON" potentials
(OP)
Under CP,Our burried gas pipelines are protected through 850 off criteria. This is referred to variously as “850 off”, “polarized potential” or “instant off potential”.
This criterion is applicable to impressed current and galvanic systems where the
protective current can be interrupted. Further the "off" potetials have a limited value of -1100 m V maintained in our pipeline to avoid hydrogen evolution which can damage the coating. As our pipeline is old and have coating issues we find dificulty of protecting some sections of the pipe line under"850 off" criteria. Having said that we experiance that we can achive the protection levels by incrasing the "ON potentials" by driving to much higher limits than usual(-1800 m V). Our consultant opnion is that as well as for "off potentials" there should be limitation to "ON potentias" and they are also indicative of the voltage stress applied to the coating and has limited to the "on potentials" of our pipeline to -1400 m V. Whats your thought on this subject. Any papers or past experiance that help me to clarify this issue? Appreciate your feed back.
This criterion is applicable to impressed current and galvanic systems where the
protective current can be interrupted. Further the "off" potetials have a limited value of -1100 m V maintained in our pipeline to avoid hydrogen evolution which can damage the coating. As our pipeline is old and have coating issues we find dificulty of protecting some sections of the pipe line under"850 off" criteria. Having said that we experiance that we can achive the protection levels by incrasing the "ON potentials" by driving to much higher limits than usual(-1800 m V). Our consultant opnion is that as well as for "off potentials" there should be limitation to "ON potentias" and they are also indicative of the voltage stress applied to the coating and has limited to the "on potentials" of our pipeline to -1400 m V. Whats your thought on this subject. Any papers or past experiance that help me to clarify this issue? Appreciate your feed back.





RE: Limit for CP "ON" potentials
ISO 15589 and NACE RP 0169 might give you more guidance, but for old pipelines it is very common for low protection sections or spots to be provided with additional sacrificial anodes or additional transformers and ground beds to "boost" the protection from remote impressed current systems without turning the system up so much that other sections are being "fried".
Hope someone with a bit more in depth knowledge than me answers your question.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Limit for CP "ON" potentials
Some options if you want to limit on potentials to -1400mV:
Install additional rectifiers at lower potential areas, this could be costly and the hardest part is usually getting AC power hook up to those locations.
Complete CIS, find larger areas of poor coating, recoat these areas. Pipe won’t require as much current for protection and can keep rectifier outputs down. Usually costly, but could be worth it in the long run or if there is no other solution.
Move to a 100mV criteria, would need native potentials .
I would only recommend localized anode installation as an absolute last resort. If you go this route, I would bring the cables up in a bond box, so you can disconnect or interrupt the anodes, otherwise you will never know what the true polarized potential is for those areas.
RE: Limit for CP "ON" potentials
RE: Limit for CP "ON" potentials
"brimmer" talks about limiting to -2500mV,mainly for safety reasons. What are the safety conserns under high on potentials for a pipeline? What is the condition of your coating and do you carryout DCVG surveys to your pipeline? Due to poor coating conditions, our pipeline is unable to manage under DCVG as the number of repairs are very high. Therefore we manage under a srtict ILI program. When did you change your on potential criteria to -2500mV? Have you increased it gradualy within many years to arrive at -2500mV ?
RE: Limit for CP "ON" potentials
RE: Limit for CP "ON" potentials
RE: Limit for CP "ON" potentials
In few of our compressor stations which separates the main pipeline from the station the insulation joints are buried. Therefore there is a section, which immediately goes under ground to the buried IJ is not protected (Approx 12 Meters). As per vendors recommendations we had followed a localized ground bed design to protect the 12 meter section of the pipeline. Due to various reasons we are unable to polarize the plant side and therefore cannot carry out the On/Off criteria as specified in the standard. Hence the vendors recommendation is to carryout spot On potentials for every meter for 12 Meter section as part of the full line survey for main pipeline. With comparison to CP standards I find it difficult to justify the On potential criteria which we have been using under the given circumstances. Appreciate your thoughts on this issue.
RE: Limit for CP "ON" potentials
If this section is plant side of the IJ and no other isolation then the pipe will be earthed inside the plant no? It doesn't sound very surprising to me that you have no potential as all the current is just leaking away or have I misunderstood your situation? A small sketch might be useful here to define your issue.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Limit for CP "ON" potentials
RE: Limit for CP "ON" potentials
I've seen this a number of times although it is not the best solution, partly because the buried section of pipe on the plant side is unable to be protected by CP. This is because there is no electrical isolation (this is what the IJ is doing) between any CP input ( n this case your sacrificial input) and the plant pipework which is either earthed properly or effectively earthed by all the support connections, instruments etc. Therefore any potential applied to the pipe promptly dissipates back to earth - hence no potential.
Any time I've seen it and it comes up in the HAZOP, the mitigation is to thoroughly inspect the buried section of coating and then apply extra coating or wrapping to provide further protection in lieu of the lack of CP.
Unless you can electrically isolate this section from the plant ( in which case why didn't they put the IJ there?) your sacrificial anode is doing nothing and is a waste of space.
Without seeing it or any drawings it's difficult to say why this solution was put forward.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Limit for CP "ON" potentials
RE: Limit for CP "ON" potentials
This section of the pipe was excavated and repainted 8 years ago. Under the most recent DCVG carried out in this year shows that overall coating condition of the pipeline is good. Having said that vendor couldn't carryout DCVG for this section as he was unable to put a signal due to isolation issues.