Monitoring of corrosive products
Lukas Staub, Michael Rziha and Marco Lendi. VGB PowerTech 3|2019
See here the original article: “Turbidity measurement as trend monitor for particulate corrosion products”.
The monitoring of corrosive products is essential to determine the effectiveness of the chemical treatment of the water-steam cycle. Today, trend determination for corrosion products in thermal power plants is even more important. This is due to the number of thermal power plants that exist as a result of the increased use of renewable energy sources in the grids.
The exact and complete determination of corrosion products, which are almost always present as undissolved particles, can only be carried out by complex and time-consuming analytical methods. For modern thermal power plants, these manual and analytical methods are quite inadequate due to the short time and strong oscillation, which makes it impossible to follow (peaks) in a complete and satisfactory way. Although these processes cannot be completely replaced by in-line measurement systems.
However, some parameters can be monitored online, trending and being useful. The technical possibilities and limits of turbidity measurement are discussed as a trend for corrosive products.
Introduction to turbidity measurement.
Light scattering is a physical phenomenon, of utmost importance for the understanding of turbidity. The theory of light scattering is quite complicated, since scattering depends on different physical parameters:
- Particle size, shape and dielectric properties (absorption, refraction…),
- Wavelength spectrum and polarization of the illuminating light beam,
- Illumination direction and detection.
Particles much smaller than the wavelength scatter the light symmetrically around the beam mainly in the forward and backward direction. Particles of size comparable to the wavelength and larger particles scatter predominantly in the forward direction, the larger the particle, the more intense it is.
The particle size dependence is less pronounced at an angle of 90° to the incident beam. In addition, the intensity of the scattered light depends on wavelength and particle size. The smaller the particle, the more efficiently it scatters the shorter wavelength light. The dielectric properties of the particles, i.e. the refraction and absorption of the incident light beam, also influence the intensity of the scattered light.
In general, the greater the difference in the refractive index of the particle with respect to the refractive index of the water, the more intense the dispersion will be. If the particles are colored and also absorb light in the wavelength range of the beam, the intensity of the scattered light is attenuated.
Normative methods
A consequence of the dependence of turbidity on different parameters is that turbidity can only be used as a characteristic property of a sample if the measurement method is standardized. For reporting purposes, as required in drinking water production, EPA and ISO established EPA Standard Methods 180.1 and ISO 7027 respectively. Both standard methods define in detail how the turbidity meter should be designed, as well as the turbidity units (NTU FNU and FAU respectively).
In addition to these two standard design configurations, there are approved alternative methods, such as GLI-Method-2 or SWAN’s Turbiwell LED white-Method-1. Common to all these methods is the nephelometric measurement principle: the scattered light is detected at an angle of 90° to the incident light beam. (Figure 1 )
Fig. 1. Unregulated design (left); ISO and USEPA compliant design (right) .
For applications where it is not required to follow a standardized measurement method, the design with measurement at various angles is widely used. The 90-degree scattered light signal is split into forward and/or backward transmitted light, respectively. The advantage of this design is its ability to cancel unwanted effects due to fouling of the optics or colored samples.
Turbidimeter sensitivity
The sensitivity of a turbidimeter depends on its specific design and measurement method. The sensitivity curve is determined by measuring a series of Formazin standard solutions of different concentrations.
Each type of analyzer has its own sensitivity curve. The graph in Figure 2 shows the average normalized sensitivity curves for two turbidity meters with different design.
Fig. 2. Sensitivity curves of two turbidimeters with different designs. Non-regulated (red) and ISO or USEPA regulated design (blue)
The relationship between turbidity and signal is not linear; it is determined by a polynomial.
All manufactured instruments of a given type should be constructed and fitted as identically as possible. Then, the individual sensitivity curves have the same shape and can only differ by a proportionality factor: the calibration factor.
Application in drinking water.
Turbidity measurement is an important parameter in the drinking water industry. It affects both the acceptability of water to consumers and the selection and efficiency of treatment processes. In particular the efficiency of chlorine disinfection as it influences chlorine demand and protects microorganisms. It can also stimulate bacterial growth when free chlorine decreases.
The two most important guidelines regarding the measurement of nephelometric turbidity of drinking water are listed in Table 1.
Table 1. Comparison of two different turbidity measurement settings.
Both regulations define Formazin as a primary standard. Formazin is a white, water-insoluble polymer. Its dispersion remains stable for a long time. The main difference between the USEPA and the ISO standard is the light source.
USEPA defines a tungsten lamp (white light) and ISO defines an infrared light source at 860 nm. Both light sources have their advantages. Sensitivity to whitish residues in water is better with the tungsten lamp but suffers from biases due to solution color. The discussion of the different light sources used in turbidity meters becomes very important when the application changes. In the drinking water industry, the impurities expected to be measured are white as particles.
Therefore, it makes sense to calibrate and verify such instruments with a white colored polymer of similar appearance. But what happens if these instruments are used in the water-steam cycle to detect corrosive products?
These particles obviously differ in color, shape and size distribution from the impurities in drinking water. In the next chapter, a series of tests is recorded to show the potential of different turbidity meter designs with iron oxide particles.
Turbidity measurement of iron oxide powder
Corrosive products in the water-steam cycle can take many forms. The main chemical crystals formed are magnetite and hematite. In the following experiments, only magnetite, iron (II, III) oxide powder in different particle sizes, was used.
Influence of the light source.
In the first experiment, iron (II, III) oxide powder with an average size of 1 μm was inserted into the sample. The response was measured with two turbidity meters of the same design. The two instruments differ only in their light source. In Figure 3, the black line was the response of an analyzer with a tungsten lamp type light source. The red line was the measurement of a light source at 860 nm.
Fig. 3. Injection of iron (II, III) oxide powder (average particle size: 1 μm) with a concentration of 50 ppb as the total iron particles. The response was measured with similar turbidity meters but with a different light source.
The amount of iron (II, III) oxide dust injected was 50 ppb of the total particulate iron.
The use of a light source with a wavelength of 860 nm is clearly an enormous advantage for the detection of black particles. For the same sample, the response for the two light sources was 0.181 FNU (860 nm) and 0.054 FNU (tungsten lamp) respectively.
Due to different drinking water regulations, the analyzers can also be equipped with different light sources. This fact may lead to a false conclusion:
“[…] Since metal oxide particles are generally dark, they absorb rather than reflect light, so nephelometry is not a preferred method for this application […]” [2].
The above statement is true if an analyzer is used with a tungsten lamp. But with an infrared light source, the color of the particle does not have the same influence. Experimental data support this theory.
For the measurement of black iron oxide powder (II, III), only one wavelength according to ISO 7027 (860 nm) is suitable.
Single- or multi-detector design (angle of measurement)
Two turbidity analyzers with 860 nm light source but different designs were compared.
The response of the single-detector design (at 90°) was greater, but the signal was noisier than with the multi-detector analyzer (at different angles).
The ability of the multi-detector design model to cancel unwanted effects due to colored samples is an advantage for the measurement of iron (II, III) oxide powders. But the benefit compared to the influence of the light source is minimal. (Figure 4 )
Fig. 4. Injection of iron (II, III) oxide powder (average particle size: 1 μm) with a concentration of 50 ppb as total particulate iron. The response was measured with two turbidity meters using an 860 nm light source but with different designs (red= 1 detector, green= multiple detectors).
Influence of particle size.
Iron (II; III) oxide powder is commercially available in various grades and forms. Two products were dispersed in separate bottles with similar iron concentrations. In one bottle, the powder had an average particle size distribution of 0.95 μm. For the second bottle, the powder had particles no larger than 50 nm.
Figure 5 shows the two standard solutions – the difference in turbidity of the solutions are clearly distinguishable.
Correlation of turbidity and particulate corrosive product measurement.
In water treatment plants, the quality of drinking water is defined with respect to nephelometric formazan units. It is the standard unit of turbidity with respect to the calibration with formazin. Despite showing a trend on turbidity, this unit has no use for the water-steam cycle. That is why there is great interest in transferring the abstract term turbidity to a concrete concentration. Most of the time, the turbidity term correlates with the concentration of iron in the sample.
Detection limit and particle size distribution.
Figure 5 showed the different turbidities that can be obtained for standard solutions with the same iron concentration. Therefore, particle size distribution plays an important role for the correlation of turbidity with iron concentration.
Or in other words: only if the particle size distribution of a sample remains constant over time, a correlation can be established. Another consequence of this is the fact that the correlation must be on a real sample, in the field. A correlation of turbidity with iron in the field of sample A is not required to match the correlation of sample B.
A good example of how particle size influences this correlation is the detection limit measured with two types of iron oxide powders (II, III) of different particle size. Based on the signal-to-noise ratio, the detection limit was obtained [6]. For iron (II, III) oxide powder with a particle size of 1 μm, a detection limit of 0.5 ppb Fe could be achieved. If the experiment is repeated with the same substance, but with a particle size in the nanometer range, a detection limit of 15 ppb Fe can be achieved (Table 2).
Table. 2. Detection limit of turbidity measurement with respect to particle size distribution based on experimental data with iron oxide powder (II, III).
Comparing the accuracy of turbidity meters, a detection limit set for an “iron concentration” is totally useless if the particle size is not being indicated.
Comparative methods.
According to the IAPWS technical guide “Corrosion Product Sampling and Analysis for Combined Cycle and Fossil Plants,” three terms are defined with respect to “corrosion products” (Figure 6): [1]
Fig. 6. Definition of different terms according to [1].
- Dissolved corrosion products: ionized form. This fraction is not detected by turbidity measurement.
- Particulate corrosion product: corrosion products in suspension. This fraction is detected by turbidity measurement.
- Total corrosion products: the sum of dissolved and particulate corrosion products.
It depends on the sample chosen and the analytical method, whether you are measuring the total corrosion products or only a fraction of the particulate corrosion product.
With the usual corrosion product sampling, suspended solids are collected – retained – captured with a screen. This fraction is defined as “particulate corrosion products”. Everything that passes through the screen is defined as the “dissolved” fraction. It follows that the “dissolved” corrosion products include all particles passing through the filter. But with turbidity measurement, even solids in the nanometer range are detected. If the fraction of dissolved corrosion product is negligible compared to the fraction of particulate corrosion product, the turbidity can be correlated to the total corrosion products.
Useful tips for a turbidity-iron/copper correlation.
If a turbidity measurement correlates with an iron/copper concentration, the following tips should be considered:
- The hand sample or the corrosive product sample must be obtained from the same point where the sample is taken by the turbidity analyzer.
- Stable conditions are required.
- The comparative method for iron/copper analysis must have an appropriate detection limit.
- Only if the fraction of “dissolved iron/copper” is negligible, the turbidity can be correlated with the “total corrosion product”.
- The relationship between turbidity and iron/copper concentration is only linear over a small part of the concentration.
Plants with cyclic operation and total iron.
In the so-called “good old days”, where most of the plants were managed with a base load, or at least with very few load variations, total iron analyses were only requested at most once a day. Or at most 3 times a week due to the fact that strong fluctuations were not expected and did not occur. However, the situation changed completely with the disappearance of base-loaded plants, hence the importance of having a detailed knowledge of the total iron trend.
The stable conditions recommended in all current guidelines are not achieved in plants with flexible operation.
Taking samples manually for analysis and evaluation is practically impossible, as plants have few chemical personnel with the time and equipment necessary to perform laboratory analysis. On the other hand, without these trends and data, developing a maintenance strategy is impossible. This can lead to significant and costly damage such as wear on control valves, particle impact and erosion, and tank problems, etc.
Considering also the experienced concentrations of total iron during these events (reaching a maximum of up to x mg/kg!), precise analytical methods in the lower μg / kg range are certainly unnecessary and such high precision is not required.
Need for precise measures
Proxy methods such as on-line turbidity monitoring can help. However, it should be clearly stated that these methods can by no means replace a proper analysis. The conversion from turbidity to concentration will always have a significant error. Therefore, this measure should not be used to replace an adequate analysis. However, as a trend measure and to have an estimate of approximate concentration levels (few μg / kg, or some 100 μg / kg, etc.), turbidity measurement can be a helpful tool. It will serve to indicate where corrosive products are released and how they are distributed.
Case studies
The 2 graphs in Figure 7 show the use of online turbidity in different locations and situations.
Fig. 7. Fluctuation of corrosive products in feed water caused by large load variation.
The values achieved and shown in Figure 8 were obtained thanks to a parallel measurement program, with optimal samples collected. As mentioned above, individual values may differ slightly, but overall the trend was always consistent.
Fig. 8. Turbidity (as Fe) during start-up
These trends also clearly demonstrate that relying solely on samples obtained under stable loading conditions can lead to false conclusions. This can influence the operator, giving him a false sense of security.
Conclusion.
Turbidity measurement is a good method for monitoring trends of particulate corrosive products in the water-steam cycle.
The analyzer must be equipped with a light source in accordance with ISO 7027 (860 nm).
The correlation of turbidity with iron/copper depends on several particle properties. Such as particle size and particle size distribution, and these properties are particular to each plant and may change with time. Therefore, turbidity measurement cannot replace an accurate analysis. Since the conversion from turbidity to concentration will always have a significant error.
However, as a trend analysis and for an estimate of approximate concentration levels (few μg/kg, or some 100 μg/kg, etc.), on-line turbidity measurement can be a useful tool. It tells us where corrosive products are found and how they are distributed. This will ultimately support the operation and maintenance strategies for each plant.
