WATER TREATMENT
Solving common analyzer problems
Many plants have common problems with the same kinds of water sample panels and on-line analyzers. Although every site and sample panel is unique (Figure 3), there are some basic tips and tricks that can be used to address many of those problems. (For the larger context of this issue, see "Maintaining water sample panels improves plant availability.")
3. Where the action is. The back of a typical sample panel. Courtesy: Nalco
High-purity pH analyzer drift. High-purity water (condensate, boiler feedwater, demin water) has low ionic strength. On-line high-purity pH analyzers often use salt bridges or reservoirs to boost a sample's ionic strength. Figure 4 shows one popular high-purity pH analyzer configuration that includes a salt reservoir. It's important to replace salt bridges/reservoirs before they are exhausted. Most manufacturers recommend replacing them annually.
4. Worth its salt. A high-purity pH analyzer equipped with a salt reservoir. Courtesy: Nalco
It's also important to remember that most high-purity pH and conductivity analyzers provide more-accurate and -repeatable readings than wet tests of the same high-purity sample. But this sensitivity has a downside: Contact with air changes the pH and conductivity of high-purity samples.
In addition, bench-top pH meters must be calibrated specifically for high-purity water pH measurement. Ensure that your meters either are calibrated with low-ionic-strength buffers or that an ionic-strength booster is added to samples before analysis. It's best to use two bench-top pH meters: one for high-purity waters and one for low. The high-purity instrument should never be used to measure the pH of low-purity water.
High cation conductivity. Exhausted resin is the most common cause of high cation conductivity (Figure 5). Solving the problem is as simple as replacing the resin before it's depleted. Plants should maintain a full set of replacement resin on site that's ready for use. The best practice is to maintain the same number of replacement resin sets (or resin volume) in inventory as there are installed cation columns.
5. Shoot the moon. High cation conductivity caused by exhausted resin. Source: Nalco
Most of the cation conductivity resins in use are supposed to change color as they exhaust. Plant maintenance and chemistry personnel rely on this color change to indicate when the resin needs to be replaced. Unfortunately, sometimes the color change is subtle, or masked by the effect of a contaminant. For example, iron fouling can make a resin dark enough to obscure the change in hue. Other foulants can cause the resin to stop exchanging even if it's not exhausted, again masking the color change. If the color of a resin does change, it should be replaced while at least 10% of the resin remains unexhausted.
As a rule of thumb, replace the cation resin any time the cation conductivity consistently reads higher than 2 µS/cm (microSiemens/centimeter). Cation conductivity should never consistently read greater than this value. Finally, for accurate readings of degassed cation conductivity, ensure that the heaters are energized or that nitrogen is flowing any time that sample is flowing through the cation column. The degassed reading is supposed to eliminate interference caused by carbon dioxide, but that's only the case if the small reboiler heaters are energized or if the scrubbing nitrogen is being fed.
Unreliable or highly variable ORP. Oxidation-reduction potential (ORP) analyzers are some of the most difficult to calibrate and maintain, for several reasons. First, the probes themselves are subject to fouling and age rapidly. Most probe manufacturers recommend annual or biannual replacement even if the probe appears to be working correctly. Probe response tends to slow with age, and periodic replacement minimizes this problem.
Probe response can be verified by monitoring trends closely to ensure that the analyzer's readings change as expected. Does ORP increase when dissolved oxygen increases? Does ORP decrease when dissolved oxygen decreases or when a reducing agent (like a passivator) is added?
To calibrate an ORP analyzer, carefully follow the manufacturer's recommendations. ORP analyzers should not be offset to agree with dissolved oxygen data or other ORP readings. Offsetting ORP readings tends to throw off the calibration rather than improve accuracy. Instead, instrumentation and control personnel should perform a full calibration if an analyzer's accuracy is suspect. Again, using the proper calibration procedure is essential. Calibration reagents can actually destroy the probe if they're not properly applied and rinsed.
Finally, evaluate new technologies. New probe designs are in the pipeline and should be available within a year. The newer probes can actually monitor ORP without first cooling the sample. They promise significant improvements in responsiveness and accuracy.
Large deviations in low-range silica readings. This problem doesn't occur at all plants, but it has at several. Many sample panels use the Hach 5000 silica analyzer for continuous analysis. This model is generally reliable, but it is calibrated with a 500-ppb standard—the lowest-level standard that Hach can supply. The problem is that most high-purity streams have less than 10 ppb of silica, so calibrating the analyzer with a 500-ppb standard would lower its low-end resolution. Some plants see negative silica readings or poor agreement between the wet test results and the on-line analyzer.
Fortunately, there's a way to address this problem. Plants can create a custom standard (50 ppb is common) by diluting the standard Hach 500-ppb standard with good-quality demin water. Once the custom standard has been created, its concentration must be verified using a laboratory spectrophotometer to perform an ultra-low-range silica test on it. Perform the test at least three times and verify that the results read within 5% of each other. If they do, then average the three readings and write this value on the standard bottle. The Hach 5000 accepts custom standards; refer to the manual for the procedure. Enter the value of the custom calibration standard and ensure that the instrument is set for automatic calibration. The unit will calibrate using the new, lower standard and will provide better low-end resolution.
If readings continue to show high deviation with wet test results, closely inspect the reagent tubes for plugs or cracks and check the “pinch” valves for proper operation. The Hach manual provides detailed troubleshooting procedures.
Sodium analyzer calibration drift. Calibrating sodium analyzers can be very difficult. Because many plants lack the understanding or knowledge to do so, most makers of sodium analyzers offer calibration training. The written calibration procedure provided with the instrument is sufficiently convoluted to stymie even the most-experienced technician. Nalco and Calpine advise plant managers to pay for annual OEM training of operators and techs on the proper calibration of sodium analyzers; it can significantly improve the reliability and accuracy of readings.
Also bear in mind that sodium analyzers are notorious for losing accuracy during cycling operation or whenever they lose sample flow. Using demin water to maintain sample flow significantly eases maintenance.
Finally, many plants do not calibrate sodium analyzers at the manufacturer's recommended frequency. Orion analyzers, for example, generate an error message after 30 days. Many plants continue to operate the analyzer even after receiving this alarm. Calibration drift is inevitable if monoethylamine is used as the buffering reagent. Drift may be minimized if diisopropylamine (DIP) is used instead. DIP is completely volatile, so there is no dilution of the reagent over time. Though this reagent change can minimize drift, sodium analyzers must still be calibrated at the frequency recommended by their manufacturer.
—Dan Sampson, (dcsampson@nalco.com) of Nalco Co.
PUMP MAINTENANCE
Qualifying rebuild shops
Routinely rebuilding old centrifugal pumps to their original specs makes no sense, given advances in pump rebuilding technology and inevitable changes in system performance over time. A qualified independent rebuild shop with modern design tools and experienced personnel can verifiably offer high-quality upgrades that improve both uptime and efficiency consistent with current system performance requirements.
Consolidation in the pump industry (Figure 6) is another reason to consider using a rebuild shop. Some pump makers now lack the same level of engineering competence they once had. There have been instances of vendors “downsizing” or “right-sizing” their inspection department into oblivion. In these cases, the company's customers pay the price in unexpected pump downtime and even unit outages.
6. Fewer options. Consolidation within and among major pump manufacturers continues. Courtesy: Heinz P. Bloch, PE
The qualified pump rebuild shop has both the tools and the experience needed to define a scope of work that goes beyond routine rebuilding or performance upgrading. It takes a lead role in defining the scope of work, and it begins by impressing on customers that a reasonably accurate definition will be possible only after a thorough incoming inspection. This task entails logging (on both a paper and a computer document) details such as the pump's type and model, the location of its plant and its type of service, its direction of rotation, and all of its O&M data.
Once a shop has inspected a pump and logged its salient details, the next steps are to describe its general condition and to propose in greater detail the work needed to rebuild or upgrade it. This process is called the condition review.
Condition reviews include taking photographs of as-received equipment and close-up shots of parts and components of special interest. The sizes of end floats and lifts and other detailed measurements are placed on a dimensional record form both before and after dismantling the pump. Components are marked or labeled, and hardware is counted and cataloged. Bearings, bushings, and impellers are removed. Blasting with beads or steam or another cleaning method is proposed and listed, along with an agreed-on completion date for this preliminary activity.
Nondestructive testing (NDT) is the next possible step, and it should be used whenever appropriate. A good pump rebuild shop will issue a form that identifies the chosen inspection method, perhaps using a liquid dye penetrant or magnetic particles. Although space limitations preclude a detailed discussion of NDT inspection here, competent pump repair shops recognize its importance and usually emphasize its necessity to pump owners.
Some pump condition reviews also include taking readings of electrical run-out at eddy current probe locations and measuring the shaft's balance and residual unbalance and the balance of individual impellers. The responsibility for performing these inspections, acceptance criteria, condemnation limits, and other items of interest are listed on a form. Ultimately, some inspection results also are documented on this form; others go on separate forms.
Recall that the term “form” was defined to include both hard-copy and computer documents. With this in mind, it should be clear that there will be a need to make a transition from documents that define the initial scope of work to documents that deal with material certification, documentation of as-achieved (or as-built) dimensions, the service fitness of auxiliary components, or repair quality. Nonetheless, it should suffice to say that defining the scope of work and the incoming inspection and condition review are important first steps in the pump repair process.
In future articles, we'll explore actual case histories of pump repairs, both good and bad, and explain how to work with a repair shop to ensure that it returns an overhauled pump with a new lease on life.
—Heinz P. Bloch, PE (hpbloch@mchsi.com) of Process Machinery Consulting.
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