Faster response time, less drift, less external influences and less maintenance. Optical oxygen sensors for the low ppb range offer it all.
Oxygen: the main source of corrosion
We all know the rusting of iron. It is the best known form of corrosion and is caused by contact with water in which oxygen is dissolved. Other forms of corrosion occur in power plants. Several of these are affected by the presence of oxygen in water, even in very low concentrations. Copper corrosion occurs at both low and high oxygen concentrations.
That is why it is all the more important to detect and monitor the dissolved oxygen (DO, Dissolved Oxygen) at various points in the process. Typical measurement locations are the condenser, the feed water, boiler water and the water treatment (MakeUp). The oxygen concentration is usually measured at these points in a branch of the process in which the water is diverted (bleeding line).
A separate location where permanent measurement of the oxygen content is extremely important is the stator cooling. In order to avoid copper corrosion, either very low (<15ppb) or very high (> 2ppm) oxygen values should be maintained. Values in between can cause enormous damage. In contrast to oxygen measurements in other locations, the measurement has to be done in-line at this point, without any contact with the environment.
Amperometric or Polarographic sensors
Electrochemical oxygen sensors have been used for decades to measure the concentration of dissolved oxygen in water. In most cases, it is essentially an anode and cathode electrode system, in an electrolyte solution that follows a redox reaction. The electrodes are separated from the water flow by means of a membrane. The measurement therefore depends, among other things, on the diffusion of oxygen through this membrane.
If you have any experience with this type of oxygen sensor, you’ll know all too well that they can be sensitive to the flow rate and that they require regular maintenance. Replacement of membrane and electrolyte is periodically necessary. The reaction in the electrochemical cell means that chloride is consumed. Because of this the entire anode/cathode system must also be cleaned or replaced from time to time. Perhaps the most important downside is the long polarisation time of several hours before the sensor is operational. Especially for low ppb values of dissolved oxygen.
The arrangement of the oxygen sensor: often a source of annoyance
The sample system itself with pipework and couplings is often the cause of alarms due to a high oxygen concentration. Couplings that are no longer completely fastened or diffusion of air oxygen through thin (flexible) plastic pipes can lead to an increased oxygen measurement. This is why it is important to keep the pipes as short as possible and to use as few couplings as possible. The material is preferably stainless steel. All plastic pipes are more or less permeable to air or oxygen. If plastic is unavoidable, definitely choose thick-walled pipes in PVDF, Nylon or polypropylene (PP).
Measuring oxygen at the speed of light
So-called optical sensors for measuring dissolved oxygen have been gaining momentum for some time. The measuring principle is not based on a chemical reaction, but on a physical principle. Light of a certain wavelength strikes the “chromophore” in the sensor and thus brings it into a higher energy state. The fall back to the ground state occurs by emitting light of a longer wavelength and with known intensity and delay. In the presence of oxygen, even in very small quantities, the delay, inter alia, changes. We then speak of a phase shift. By measuring this phase shift very accurately, the oxygen concentration can be calculated.
The advantages of optical sensors are clear. The response time is much faster than the classic electrochemical sensors, there is no electrolyte or membrane to replace and the sensor is insensitive to flux fluctuations (flow). The most obvious advantage is that an optical sensor does not require polarisation time. This means that the sensor is immediately ready for use.
In short: an optical sensor requires significantly less maintenance and is more stable, whereby the calibration interval can be drastically extended and is therefore much more reliable.
Use digitisation to your advantage
Optical oxygen sensors are digital by nature. The raw measurement signal is digitised and processed in the head of the sensor itself. Sophisticated algorithms determine when maintenance and calibration are required. The actual operating time, measured values and temperature are taken into account. Maintenance managers will certainly welcome this as it helps them in the predictive maintenance strategy. In addition to this information, the digital sensor offers extensive diagnostic options.
The evolution in oxygen sensors significantly helps you in reducing maintenance and lowering maintenance frequency. At the same time, the availability and reliability of the oxygen measurement increases. Operators can focus more on the water/steam process itself and respond better to actual corrosion problems. This optimises the overall cost of operating the power plant.
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