Annual and periodic testing for cathodic protection
Testing for cathodic protection must be done often. By conducting this testing, you can make sure that cathodic protection is being achieved, find out what needs to be adjusted or fixed, and stay by the law. Regular, periodic testing of cathodic protection systems allows for the necessary preventative maintenance, which:
- Verifies the cathodic protection system is in good working order.
- protects against structural deterioration from corrosion
- reduces the price of future CP system repairs
- Ensure adherence to regulations
- Testing for Tank Cathodic Protection
Tanks should be straightforward to cathodically guard, at first glance. The efficiency of the tank bottom cathodic protection system should therefore be easy to test. That is not the situation.
Why is it challenging to test cathodic protection systems for above-ground storage tanks?
On a grand scale, we have a sizable, rotund construction that rests on an engineering base. However, the truth is that there are electrolyte and structural problems. Tank bottom CP system testing is much more challenging than you might initially believe when testing problems are added to the equation.
What problems impede tank CP system testing and performance?
One significant problem with tank bottoms is the volume of inventory inside the tank. To ensure more thorough contact of the tank bottom with the sand cushion beneath the tank, the weight of the tank’s product presses down on the tank bottom. the base of a bare tank, however, has the option to flex. It makes less direct touch with the sand cushion as a result.
As a result, measurements of potential on a full tank are usually less negative than equivalent readings on the same tank when it is empty. Readings on tanks that are no longer in service are avoided. However, it’s still a good idea to note the tank level when taking possible readings for tanks that are already in use.
The resistance is substantially higher when a tank is empty. It is difficult to establish whether we genuinely have a higher current density in the parts that remain in touch with the sand because the current output is much lower at the same applied voltage.
The isolation of the structure is another problem. When assessing tank bottoms, it’s crucial to look into how isolated the tank is from the piping and earthing systems. To make sure that the cathodic protection current is focused on the tank’s bottom and not picked up from any surrounding structures, the tank frequently has isolation devices in place. This must be verified as part of the testing procedure for isolated tanks.
What electrolyte problems can impact the testing and operation of CP systems?
For new tank construction and tank retrofits, the AMPP (previously NACE) and API requirements both suggest using a high-quality, high-resistance sand cushion. The enormous amount of sand needed for merely 12″of the tank bed might have a big impact. This can amount to around 900 tonnes of material for a 150-foot-diameter tank. It will depend on the density of the sand. Using substantial 30,000 lb capacity dump trucks, this might be up to 60 truckloads.
It is not guaranteed that the sand will be uniformly distributed and contain the same amount of moisture, even if it comes from the same source. We have witnessed completely different sand being used in various parts of the same tank in extreme situations.
It is simply impossible to verify that the electrolyte in the tank is homogeneous once it has been constructed.
The moisture content of the sand may fluctuate over time. Additionally, it is normal to see floodwater and rains seep through the sand foundation. This depends on the seal chime’s quality and the tank’s secondary containment system’s design (release prevention barrier and dikes).
Sand resistivity is significantly influenced by moisture content, which can also affect cathodic protection effectiveness. Over time, the electrolyte could undergo substantial alteration. Any native or depolarized potential measurements made during setup and commissioning cannot be utilized to evaluate polarisation in the years that follow.
It is crucial to continuously take precise and reliable measurements of potential. Installing fixed reference electrodes underneath the tank during construction has traditionally been the norm.
Under tanks, copper-copper sulfate reference electrodes are most frequently utilized. Reliability over time is a major issue with this type of reference electrode. Within the first 10 to 15 years of service, faulty prospective data are frequently encountered.
Compared to the installed reference electrodes used to check the efficiency of the CP system, tanks typically have a substantially longer useful life. On older tanks, it’s common to find both “good” reference cells, which certify the functional operation of the CP system, and “bad” reference cells, which give false readings. Therefore, it is challenging to verify that the tank
Continuously taking accurate and trustworthy measurements of potential is essential. Traditionally, fixed reference electrodes have been installed beneath the tank during construction.
The most popular reference electrodes used under tanks are copper-copper sulfate electrodes. A significant problem with this kind of reference electrode is its durability. Faulty prospective data are typically seen in the first 10 to 15 years of service.
Tanks usually have a much longer usable life compared to the installed reference electrodes used to assess the effectiveness of the CP system. Both “good” reference cells, which attest to the CP system’s proper working, and “bad” reference cells, which produce erroneous readings, are frequently found on older tanks. Consequently, it is difficult to confirmthat the tank satisfies requirements.
We have measured stationary electrodes that, after only a few years, give false values. Additionally, after a year, operators perceive stationary electrodes to be inaccurate. This is not a result of the electrode’s effectiveness, but rather of the dry surroundings of the cell.
Combining the copper-copper sulfate reference electrode with a reference electrode of the zinc type is one solution. Reference electrodes made of zinc become steadier over time. For the duration of the tank’s life, they can deliver efficient service. However, the base potential of different zinc reference electrodes can be different. The zinc reference electrode should therefore frequently be buried alongside a copper-copper sulfate reference electrode.
In this manner, the copper-copper sulfate reference cell can be used to “calibrate” the zinc reference electrode.
Micro-slotted PVC pipe is becoming more and more popular for use as a pull tube. To take continuous “profile” readings from one edge of the tank to the other, a calibrated reference electrode can now be pulled through the tube. A single draw tube may be used in some circumstances, but two pull tubes may also be placed to enable the taking of additional measurements.
It is crucial to guarantee that the electrode inside the draw tube has electrolytic contact with the sand surrounding it while measuring potential using one. To facilitate this interaction, there must be sufficient water in the tube. A voltmeter with an input impedance larger than the default Fluke meter’s 10 M-ohm resistance should also be used. Various meters have input impedances of at least 100 M-ohm.
What should the proper requirements for tank bottom cathodic protection be?
The -850mV Instant-Off potential and the -100mV polarization requirements are the two most relevant criteria, and both are relevant when properly applied.
On a sizable naked structure in a well-ventilated area, this condition may be difficult to meet. As a result, we frequently consider the 100 mV of polarisation requirement, which is another suitable criterion.
The 100 mV requirement can be applied in one of two ways. The first is a formation criterion that is based on contrasting the polarised potential with a recognized initial potential, also known as the native potential. As was previously mentioned, the tank’s baseline can lose its validity over time.
The second method is known as polarisation decay, and it compares a polarised potential to a depolarized potential.By removing the current sources and letting the tank depolarize for a few days to a few weeks, the depolarized potential is obtained. Again, as the electrolyte changes, the depolarized potential may alter over time. Therefore, it is advised that each annual structure-to-electrolyte potential study include the collection of a fresh depolarized potential.
It is significant to note that heated tanks operating at temperatures higher than 30 C (86 oF) do not meet the 100 mV shift criteria. According to studies, hot structures need polarisation of up to 300 mV. Studies have also shown that polarisation levels are similarly higher in regions where sulfate-reducing bacteria (SRB) are present.
For systems with mixed metals, the 100 mV polarisation criteria are likewise invalid. A mixed-metal system may develop on steel tank bottoms where specific mill scales are present. As a result, the 100 mV criterion’s applicability can be questioned. The problem of mill scale is still being researched.
Finally, as mentioned earlier, it is best to measure potentials under tanks using a multimeter with a greater input impedance. There is a large resistance through the tube for pull tube readings. A level of inaccuracy is added by the potential for substantial resistance from the surrounding dry sand for stationary electrodes. Although it won’t eliminate the inaccuracy, a higher input impedance meter helps to lessen it.
Tanks can be challenging to test, and without the right instruction, knowledge, and tools, it is all too simple to obtain a false impression of the CP system’s true performance. Consider getting a second opinion from a professional if your tank’s CP system doesn’t seem to be functioning before taking more harsh action.
Cathodic protection testing for oil, gas, and water pipelines, above-ground storage tanks, power plants, energy facilities, deep wells, and other infrastructure assets can be scheduled and carried out by qualified Consultech technical staff on an annual or recurring basis. A written report outlining test results and suggestions for cathodic protection maintenance is issued after results have been examined by NACE-certified specialists.
Benefits of Using Consultech
- Experience with various kinds of cathodic protection systems spanning over 58 years
- Cathodic protection system testing, installation, and upkeep experience for all kinds of structures globally
- NACE corrosion engineers and professionals with high levels of training
- team members with prior client-side CP project expertise
- having experience leading corrosion divisions of big utilities and pipeline industries and multimillion-dollar CP projects
- Compared to certain regularly used CP systems, Consultech’s several unique cathodic protection technologies are more affordable and have longer lifetimes.
Your corrosion issues are CONSULTECH’s primary concern. In addition to project management and comprehensive turnkey solutions, we are prepared to help with your requirements for corrosion engineering and field service, including design, production, installation, commissioning, and continuing maintenance. Additionally, we provide a huge selection of unique corrosion protection materials.
Contact our team of corrosion specialists for additional information, to ask a question, or to request a price. Within 24 hours, we’ll give you a call or reply to your email. Call +919824077820 for assistance and email consultech_cp@yahoo.com right away.
