When monitoring water quality in the power and industrial sectors, it is not always possible or practical to have an instrument for each parameter desired. Cost of acquisition may be an issue, or the complexity or labor required to make the measurement may make it formidable. Online analyzers can help from the labor aspect yet may require large capital expenditures. For example, while it may be desirable to analyze a sample stream for magnesium, calcium and sodium, it may be adequate to know the total ionic makeup of that sample.

In this discussion, alternatives will be considered for some of the more commonly performed measurements in a power or steam generation facility. The positives and negatives of each will be noted. In each case, while the surrogate may offer a less expensive or a less complicated method of analysis, the most significant advantage resides in the ability to obtain a real-time measurement.

The parameters to be considered in this discussion include the following surrogate methods.

IR (infrared)

The traditional method for monitoring total suspended solids (TSS) in a sample requires a gravimetric analysis. This is performed by pouring a known quantity of water (commonly one liter) through a filter of known weight, allowing it to dry, and then re-weighing the filter. The calculation will yield the TSS of the sample in milligrams per liter or ppm. However, one additional step is required for an accurate reading. Before setting the filter aside to dry, it should be rinsed with deionized water so that there are no additional solids deposited on the filter due to evaporation.

As this procedure illustrates, it is a time-consuming process that requires a dedicated technician to perform. The further shortcoming is that the data is only valid for that moment of time in the process. While for a relatively stable process this may not be an issue, one cannot always predict when an upset will occur or how long it will last. As a result, a false sense of security could result in serious repercussions.

Figure 1. TSS correlation to IR sensor. Source: HachFigure 1. TSS correlation to IR sensor. Source: HachWith these drawbacks in mind, the concept of monitoring TSS continuously using a surrogate method becomes interesting. One technology that has been well documented for this measurement utilizes an infrared beam of light at a wavelength of 860 nm aimed at the lens of a sensor at a 45° angle. The light is then reflected back at a 90° angle to a photo detector. Some versions may employ a second detector at 145°. By relating the obtained reading to an actual gravimetric analysis, the sensor can be calibrated to provide a continuous yet surrogate reading of TSS.

The correlation procedure involves obtaining a representative sample in a large bucket. The solids are suspended by stirring the sample. From this large sample, three sub-samples are taken for a laboratory determination of TSS in mg/L. The average TSS of the sample is determined. The solids are then re-suspended in the sample bucket. The IR probe is placed in the suspension and using the calibration, the average laboratory value of TSS is assigned to the probe.

One added advantage of this method is that it is not sensitive to color, so it can be used successfully even when color is present in a sample.

Conductivity

Conductivity can be utilized as a surrogate method in a couple of different manners. The first correlates total dissolved solids (TDS) in a sample to conductivity. Instead of performing a gravimetric laboratory test on a sample, conductivity can be used as a continuous surrogate. The gravimetric analysis involves evaporating water from a filter and measuring the residues left. If most of the dissolved solids are inorganic salts, these materials can be measured via conductivity and a correlation developed as a surrogate.

To determine the conversion factor for a specific solution of a known TDS value, the solution's conductivity is measured. The mg/L TDS value is then divided by the conductivity value reported. For example, a solution of a known TDS value of 64 g/L and the measured conductivity value of 100 mS/cm has a conversion factor of 64/100 or 0.64. It is important to know the conversion factor being used, especially when comparing user obtained TDS results with another lab's results, another test site or when comparing results with previously published or referenced data.

In ultrapure water, the concern is more related to monitoring dissolved ions than solids. Here hardness, as magnesium or calcium, sodium, chloride or sulfate ions can be measured online via analyzers designed specifically to detect each of these ionic species. While it may be expeditious to monitor one or more of these specific ions in some cases, for many applications it may be cost prohibitive or unnecessary to monitor each separately.

Conductivity is widely used to monitor the presence of ionic species in waters, especially in high pressure steam cycles where the water is required to approach ultrapure quality. Here, where it can often be narrowed down as to which species are present, conductivity can serve as an alternative or surrogate to monitoring the ion of interest.

Where there are particular limits for overall conductivity, or for certain species, this method could have some serious shortfalls. For example, the common value for degassed conductivity after cation exchange (CACE) is less than 0.2 uS/cm. At the same time, the chloride and sulfate limits are 2 ppb. It is possible that the degassed CACE could be within the required limits while the chloride or sulfate level would be out of specification if all of the conductivity was due to one of those contaminants.

The theoretical cation conductivity of a 2 ppb chloride (hydrochloric acid) solution is 0.063 µS/cm, barely above the theoretical minimum of 0.055 µS/cm. The steam’s chloride concentration would have to rise to nearly 20 ppb before the resultant cation conductivity would reach 0.2 µS/cm.

Nevertheless, conductivity is still a valuable tool as part of a facility’s overall water chemistry analysis.

Nephelometry

Recently, there have been several references to the use of laser nephelometry as a surrogate for monitoring iron (corrosion product transport) as a means of detecting flow accelerated corrosion (FAC) in the steam cycle of an industrial facility(1,2). This is in part due to the lack of a commercially available online analyzer with a detection level less than 5 ppb.

Figure 2. Concept of nephelometry. Source: HachFigure 2. Concept of nephelometry. Source: HachLaser nephelometers are capable of detecting a wide range of suspended particles down to the sub-micron range, well below the 2 micron limit of most particle counters. Assuming that all of the particulate matter is due to corrosion (all iron in an all ferrous system), a correlation can be made by digesting a grab sample and using the well proven FerroZine colorimetric method(3).

A digestion is required for the lab method since over 90% of iron transport occurs in the particulate form. This can be accomplished using the same FerroZine reagent, which contains thioglycolic acid (TGA). The advantage of this over using another acid, (e.g., hydrochloric acid) is that no additional iron impurities are introduced into the sample.

The result is a total iron concentration (ppb iron) that can be compared with the nephelometer reading (mNTU) to create an equation for the conversion. This equation can in turn be programmed into the controller (if it has that capability) or into a facility’s distributed control system (DCS) to read out directly in units of concentration (ppb). This surrogate method of measurement then offers a semi-quantitative online analysis of corrosion product transport.

Turbidity

While turbidity is related to nephelometry, the latter is much more suited to the process above. However, as the concept of silt density index (SDI) deals with somewhat larger particles (greater than 0.45 micron) and does not require the same trace level quantification as for corrosion particles, turbidity finds a potential fit as a surrogate for SDI. In concept, this technique is similar to that of monitoring corrosion transport, but the application and challenges are quite different.

The SDI is typically used as an indicator for potential fouling in a water purification system such as reverse osmosis (RO). Here there will be a combination of suspended solids contaminants, any of which could cause issues when they come in contact with membrane materials. It is important to first understand what the SDI is and how it is determined.

SDI is defined by ASTM Method 4189-07 as the time it takes to collect a 500 ml sample through a 0.45 micron, 47 mm diameter filter at the start of the test and comparing it with the time it takes to collect a 500 ml sample after water has flowed through the filter (at 30 psi) for 15 minutes. The resulting value indicates the plugging of the membrane as %/minute. The maximum SDI-15 value corresponds to 6.7 (100 ÷ 15). An SDI-15 value greater than 5 denotes a serious issue (75% plugging).Figure 3. SDI test kit. Source: Hach. Figure 3. SDI test kit. Source: Hach.

So, it is easy to see that a concentration value is not going to provide the information needed in order to use turbidity as a surrogate for SDI. If the determination is to be performed online, flow rate per minute will be required. An initial correlation would also be required, which has yet to be documented in the literature.

UV absorption

Certain organic compounds absorb ultraviolet (UV) light. While often this claim is made in general, only those containing aromatic rings or conjugate double bonds can be effectively detected. As not all organic compounds absorb light at the UV level, the measurement of UV absorption — the spectral absorption coefficient (SAC) measurement —represents an independent “total” parameter for these dissolved organic substances in water. The advantage of this technology is the ability to detect the change in organic load in a process stream almost instantly. SAC measures UV absorption at a wavelength of 254 nm.

Figure 4. Organics diagram. Source: HachFigure 4. Organics diagram. Source: HachStudies have shown the compatibility between SAC and chemical oxygen demand (COD) or total organic carbon (TOC). Data shows that SAC can be used as a correlative method to predict other oxygen demand parameters, but SAC is not an exact measurement of the total parameters(4,5). It is important to note that any SAC correlation depends on the water source and can change with changing water quality conditions.

While this approach works well with water samples containing a high concentration of these organic substances, it cannot identify them by type.

UV fluorescence

The method of (UV) fluorescence is not an all-encompassing solution, but can be especially useful in determining the presence of certain organics dissolved in water. This technique is much better suited for monitoring small concentrations of the organic groups noted in the previous section. Both monocyclic aromatic hydrocarbons (BTEX) and polycyclic aromatic hydrocarbons (PAH) are regularly present in crude oil and refined oil products. As with other surrogate methods, there is the task of correlating the oils present with the instrument reading.Figure 5. UV fluorescence sensor. Source: HachFigure 5. UV fluorescence sensor. Source: Hach

It has been demonstrated that the system of conjugate bonds present in these compounds can be detected by spectrometry as well as fluorimetry. Field testing has demonstrated that signals produced by a UV fluorescence sensor are proportional to the oil-in-water concentration(6). The difference between UV absorption (discussed above) and UV fluorescence is that in the latter a sensor measures light at a wavelength of 360 nm that fluoresces when a sample absorbs light at a 254 nm wavelength.

The sensor response can be calibrated by using either commercially available standards or based on a grab sample analysis. Degree of quantification of the results will depend on the consistency of the oil content.

Conclusion

In summary, the methods discussed may provide alternatives that would not normally be considered. While not being direct replacements for the methods for which they are surrogates, they offer an option of continuous monitoring, often with a lower price point and degree of complexity. As long as the limitations are recognized and accepted, the advantages may be worth considering.

About the author

Ken holds a Bachelor of science degree in chemistry from John Carroll University in Cleveland, and has been active in the power industry for over 26 years. In his current role at Hach, Ken provides technical support on all aspects of water quality monitoring for power, steam and industrial applications. He has authored and co-authored articles appearing in several industrial publications, presented at numerous utility and water chemistry conferences and given webinars on monitoring of industrial waters that have reached over 60 countries.

References

1) Kuruc, K., Johnson, L. “Further Advances in Monitoring Low-Level Iron in the Steam Cycle.” PowerPlant Chemistry. 2015, 17(2), pp. 86-89.

2) Johnson, L. “Monitoring Iron Transport in the Steam Cycle via Grab Sample and On-Line Methods.” PowerPlant Chemistry. 2015, 17(4), pp. 218-222.

3) Stookey, L. L., Analytical Chemistry 1970, 42(7), 779.

4) Baumann, P., Krauth, K.: Vergleichende Untersuchung zur Bestimmung der organischen Belastung im Abwasser (Comparative study to determine the organic pollution of wastewater); Report by the Institute for Residential Waterworks, Water Quality and the Waste Industry, Stuttgart University (1994).

5) Matsché, N., Sturmwöhrer, K.: CSB-Bestimmung durch UV-Absorption (Determination of COD by UV Absorption) Special I 137 (1996), pages 25-31.

6) Malkov, Vadim and Sievert, Dietmar, PowerPlant Chemistry 2010, 12 (3), Oil-in Water Fluorescence Sensor in Wastewater and Other Industrial Applications

To contact the author of this article, email GlobalSpeceditors@globalspec.com