DCVG is short for Direct Current Voltage Gradient. In pipeline external corrosion control a voltage gradient is measured by two reference cells in contract with an electrolyte such as soil connected to a voltage meter. DCVG is also a cell to cell survey technique.
DCVG is also a technique for finding coating holidays on a buried well coated pipeline. The term DCVG was first used to describe this technique of finding and rating coating holidays on a buried pipeline. However, the use of cell to cell surveys on a buried pipeline was nothing new.
Cell to cell surveys before the development of DCVG was used to find corrosion hot spot areas on bare buried pipelines per the net protective criterion. This use of cell to cell survey is more controversial than DCVG and it tends to be called Hot Spot Survey. The DCVG procedure does copy this Hot Spot Survey method when taking side drains at coating holiday locations to determine if current flow in the soil is toward the pipe or away from the pipe. If current flow is away from the pipe then the coating holiday has created an active corrosion cell. If current flow is toward the pipe then the coating holiday is protected by the pipeline cathodic protection system. This is a matter of polarity of the voltage difference of a reference cell on top of the pipe and another reference cell located at a 90 degree angle away from the pipe. Since current flow is from positive to negative, if the voltage meter shows a positive number with the reference cell located on top of the pipe being connected to negative terminal of the voltage meter then current flow is toward the pipe and the pipe is protected per the net protective criterion.
This is from Wikipedia. The controversy generated by Wikipedia claiming that John Mulvany and Dr. John Leeds invented DCVG which has resulted in a silly discussion on Wikipedia would be best solved by saying the following:
It is true that the DCVG technique is universally accepted but to say that NACE International has a standard practice for the DCVG technique is not totally accurate. Yes, there are NACE International standard practices that mention the DCVG technique but there is no recommended practice as to how to do a DCVG survey. That is why there is so much controversy as to what is a real DCVG survey or a cheap cell to cell survey that is not really a true DCVG survey. Since lawyers tend to run NACE International and those lawyers are more concerned about lawsuits by those would would be upset if their DCVG equipment was suddenly made by a NACE International recommended practice only a cell to cell survey rather than a true DCVG survey; do not expect a DCVG recommended practice from NACE International anytime soon. So NACE International recommended practices speak of DCVG in a very general way making it possible for anyone to claim that a piece of equipment that does a cell to cell survey is capable of doing a DCVG survey. In later blog postings what NACE International recommended practices have to say about DCVG will be addressed but not now as this is an introduction to the topic of DCVG.
Frankly, this is one example of Wikipedia having a really poor article on a buried pipeline coating survey technique. I am running out of time so I will continue later with more blog postings on the DCVG technique of finding coating holidays on buried pipelines. What I will have to say on the topic will be controversial I am sure as this article as short as it is has a few controversial statements in it.
DCVG is also a technique for finding coating holidays on a buried well coated pipeline. The term DCVG was first used to describe this technique of finding and rating coating holidays on a buried pipeline. However, the use of cell to cell surveys on a buried pipeline was nothing new.
Cell to cell surveys before the development of DCVG was used to find corrosion hot spot areas on bare buried pipelines per the net protective criterion. This use of cell to cell survey is more controversial than DCVG and it tends to be called Hot Spot Survey. The DCVG procedure does copy this Hot Spot Survey method when taking side drains at coating holiday locations to determine if current flow in the soil is toward the pipe or away from the pipe. If current flow is away from the pipe then the coating holiday has created an active corrosion cell. If current flow is toward the pipe then the coating holiday is protected by the pipeline cathodic protection system. This is a matter of polarity of the voltage difference of a reference cell on top of the pipe and another reference cell located at a 90 degree angle away from the pipe. Since current flow is from positive to negative, if the voltage meter shows a positive number with the reference cell located on top of the pipe being connected to negative terminal of the voltage meter then current flow is toward the pipe and the pipe is protected per the net protective criterion.
The DCVG method was invented by the late Australian John Mulvany, an ex telecomms engineer, in the early 1980s. He developed and refined the technique in conjunction with Dr John Leeds, a professional corrosion engineer from Kent, England. Today, the DCVG technique is universally accepted and has its own method formally defined by NACE International.
This is from Wikipedia. The controversy generated by Wikipedia claiming that John Mulvany and Dr. John Leeds invented DCVG which has resulted in a silly discussion on Wikipedia would be best solved by saying the following:
The DCVG method as a technique for finding and rating coating holidays on buried dielectrically coated pipelines was developed by the late Australian John Mulvany, an ex telecomms engineer, in the early 1980s. He developed and refined the technique in conjunction with Dr John Leeds, a professional corrosion engineer from Kent, England. Today, the DCVG technique is universally accepted and has its own method formally defined by NACE International.To say that John Mulvany and Dr. John Leeds invented DCVG would be like saying that both invented cell to cell survey. Therefore I can understand the discussion that has resulted in Wikipedia on this matter.
It is true that the DCVG technique is universally accepted but to say that NACE International has a standard practice for the DCVG technique is not totally accurate. Yes, there are NACE International standard practices that mention the DCVG technique but there is no recommended practice as to how to do a DCVG survey. That is why there is so much controversy as to what is a real DCVG survey or a cheap cell to cell survey that is not really a true DCVG survey. Since lawyers tend to run NACE International and those lawyers are more concerned about lawsuits by those would would be upset if their DCVG equipment was suddenly made by a NACE International recommended practice only a cell to cell survey rather than a true DCVG survey; do not expect a DCVG recommended practice from NACE International anytime soon. So NACE International recommended practices speak of DCVG in a very general way making it possible for anyone to claim that a piece of equipment that does a cell to cell survey is capable of doing a DCVG survey. In later blog postings what NACE International recommended practices have to say about DCVG will be addressed but not now as this is an introduction to the topic of DCVG.
Assuming that the buried pipeline is protected using Impressed Current Cathodic Protection (ICCP - as most are), then any defects in the coating will result in electrical current flowing from the surrounding soil and into the pipe. These currents cause voltage gradients to be set up in the soil, which can be measured using a voltmeter. By looking at the direction of these gradients, the location of coating faults may be identified. By plotting the direction of voltage gradients around a fault, the type and nature of faults may be deduced. By measuring the localised soil potentials with respect to remote earth, a measure of the effectiveness of the Cathodic protection may be calculated.It appears a Brit wrote this because of the way some English words are spelled the British way rather than the USA way; but it could have been a Canadian too. Wikipedia gets it half right.
Assuming that the buried pipeline is protected using Impressed Current Cathodic Protection (ICCP - as most are), then any defects in the coating will result in electrical current flowing from the surrounding soil and into the pipe. These currents cause voltage gradients to be set up in the soil, which can be measured using a voltmeter. By looking at the direction of these gradients, the location of coating faults may be identified. By plotting the direction of voltage gradients around a fault, the size of the coating holiday and the position of the coating faults may be deduced. By measuring the potential of a reference cell on top of the pipeline with respect to a remote earth reference cell, an active corrosion cell can be found per the net protective current criterion. [There is not time to deal with the IR Drop issue here and I will attempt to address that matter in a later post.]The person who wrote the original Wikipedia article is using vague language it appears in an attempt to make it sound as if the procedure is more complex than reality. It really is nothing all that complex. Since Dr. Leeds typically attempts to make the simple sound extremely complex; maybe the person who wrote this article was trained in the DCVG technique per Dr. Leeds. I have been condemned as a Corrosion Control Heretic for making the topic too easy to understand for the common masses making it appear that Corrosion Control Experts are overpaid.
Frankly, this is one example of Wikipedia having a really poor article on a buried pipeline coating survey technique. I am running out of time so I will continue later with more blog postings on the DCVG technique of finding coating holidays on buried pipelines. What I will have to say on the topic will be controversial I am sure as this article as short as it is has a few controversial statements in it.
I devised a method of using two half-cells and a sensitive ammeter in 1975 in Nigeria, before accurate digital volt meters were available for field use. I have copies of documentation relating to a contract I signed for the first of these surveys to be carried out at Bomu Manifold on the Shell-BP trans Niger pipeline by Mark Derefaka who later became senior corrosion consultant to the Nigerian National Pipeline Corporation. You will find all of this stuff on http://www.cathodic-protection-network.com I am rogeralexander2@o2.co.uk Roger Alexander and I also worked for John Leeds for a short while. I have been trying to find out about this Malvany guy and why he did not become a consultant on this matter.
ReplyDeleteIf the half-cell is a reference potential then how do we get a voltage between two of them?
Per Dr. John Leeds, John Malvany was a consultant and a close friend of Dr. Leeds up to the day he died. Dr. Leeds credits John Malvany for helping him develop the DCVG survey technique. The quote in my published article above mentioning John Malvany indicates that he is the late John Malvany which means he has died.
ReplyDeleteThe saturated copper-copper sulfate half-cell is a reference cell [reference electrode] not a reference potential. Voltage is measured by the difference in potential between two half-cells.
One thing a bit strange about Dr. Leeds per my memory is he prefers the use of a 50% solution copper-copper sulfate reference cell for his DCVG survey equipment rather than the traditional saturated solution copper-copper sulfate reference cell. By accident I mixed a traditional saturated copper-copper sulfate reference cell for use with his DCVG equipment because I failed to read the directions that came with the equipment. I was later corrected on this error by Dr. Leeds during the training course.
Greetings as you are the first reader to post a response on this blog.
I am a bit confused because you signed in as Rodger Alexander but you made the following statement: "Roger Alexander and I also worked for John Leeds for a short while."
ReplyDeleteOK, who is the "I" in this sentence?
I very much note the Roger Alexander internet business marketing effort to sell a radically new way of monitoring cathodic protection in an age of smart pigs. I can tell you are not familiar with the fact that now days in the USA a pipeline integrity program is based upon metal loss inline inspection tools (smart pigs) and not the old school copper-copper sulfate reference cell.
Naturally the pipelines that cannot be smart pigged are still monitored the old fashioned way using a reference cell and a multimeter. However this is the real old tech way of doing pipeline integrity.
If I signed in as 'RODGER' it was a typo..... but I am old, so I am allowed typos.
ReplyDeleteI am Roger Alexander the founder of Cathodic Protection Network and owner of the websites connected and linked to it.
I have been in close contact over the years with John Tiratsoo who organises pigging conferences world wide and keeps me up to date in that respect.
The sole reason for the development of the Alexander Cell was that I could not find a satisfactory way of showing that corrosion has stopped using the techniques that are used in science laboratories. These require that the reference is a reaction potential that is conducted to the electrolyte at the anodic interface in such a way that it does not disturb the reaction to be measured. I was closely involved with the development of the 'off potential' theory and worked closely with the scientist that proposed it. I was the first field engineer to attempt to gather data using the off potential theory.
I reasoned that we needed a corrosion cell to test in the same way that the medical profession use a sample of a disease on which to test the cure. The only method available was weight loss coupons and they have the disadvantage of being long term measurements requiring scientific discipline to apply in the field.
I first thought that simply measuring the current going onto a bright steel coupon would be sufficient in combination with measurements of the potential of the soil at intervals approaching the metal and the groundbed.
This does not satisfy the requirements of the Pourbaix diagrams as the voltage measurements are made in 'open circuit'.
It also measures the electrolyte potential at a combination of both anode and cathode of the corrosion cell under inspection.
I stepped back and asked if we really needed to know the potentials at all as Faraday had proved that the corrosion current was proportional to the weight of metal going into solution.
The Alexander Cell is a corrosion cell in which the current can be measured in closed circuit condition and from that measurement, absolute values can be obtained to establish the exact point of equilibrium at which corrosion stops.
It is simple to use, understand and interpret.
At each location it is subject to the pH, temperature and pressure ofits environment and can even be made from the same metal as the pipeline or structure under test.
If left in situe for 24 hours, corrosion products are visible to the naked eye confirming that the measurements relate to correct observation.
As for the big marketing drive, I have no money and no financial backer. All of the Alexander Cells that are available at present have be made by hand and the desgn is readily available. If you can't work it out from the mass of information and data on the internet I will help you make them yourself. If you want my help you will have to pay my expenses as I am on state pension and top up welfare benefits. If anyone wants to hire me for a fee I will lose my benefits so we would have to work out a deal with the tax and benefits people so that I can feel secure about my future. All of this is relevant to stopping corrosion as it is part of my very existance.
Perhaps I do not understand your system but doesn't it suffer at least some of the same limitations as a copper-copper sulfate reference cell. Isn't it simply a point reading that doesn't apply to the complete pipeline. I understand how your method works in a laboratory but I don't see how it is an improvement over a standard copper-copper sulfate reference cell.
ReplyDelete