Squared-away sample panels
In these two cases involving either rare upsets in condensate quality or condenser tube leaks, a technician's mistrust of an instrument—due to prior reliability problems or poor O&M practices and training—caused additional damage to plant equipment especially during chemistry upsets during cycling operation. Such upsets are the result of overfeeding or underfeeding treatment chemicals just prior to unit shutdown or immediately after start-up—conditions that on-line monitoring would detect.
It's always more difficult to control the chemistry of a cycled plant, but ignoring or not maintaining your on-line analyzers just exacerbates the problem. Chemical feed systems may fail during the shutdown period. Pumps may become air-bound or otherwise cease to function. If on-line analyzers are missing, not working because repairs aren't a priority, or not trusted, the chemistry upsets caused by these failures may not be detected until the first grab samples of the cycle are taken. For this reason, many plants operate with no chemical feed (or too much chemical feed) for the first several hours following start-up.
Time is money
Many plants perform wet tests that do little to protect equipment. If a plant's sample panel is reliable, wet tests need to be performed only to verify the continued accuracy of the on-line analyzers. In the absence of reliable analyzers, wet tests become the first line of defense.
Because wet tests should be performed every four to six hours, operators who depend on them spend a lot of their time monitoring water chemistry, during which other problems go undetected (causing more upsets). Lowering chemistry's priority on the task list is no solution, because doing so produces the same result.
At most plants, sample panel maintenance is very time-intensive and entails weekly or biweekly calibration of on-line analyzers by the maintenance department. However, if more than one instrument shows some drift after it is returned to service, it's not uncommon for operators to believe that all of the analyzers need to be recalibrated or have their probes replaced. They then write work orders that maintenance fills, which starts the cycle again. In the end, maintenance spends as much time “fixing” working analyzers as operators do on chemistry.
Improved maintenance, operating, and design practices will solve many problems that are unjustly blamed on analyzers. Here are some suggestions that will keep your sample panel running in tip-top condition.
Delivering accuracy and reliability
Operators, because they are the primary users of the plant's sample panel (Figure 3), are best qualified to determine if it needs maintenance. Taking the following steps ensures that on-line analyzers read reliably and are repaired when they don't.
3. Water works. A typical sample panel with separate dry and wet sections. Courtesy: Nalco
Ensure analyzer accuracy and standardization. As with any sampling system, some assumptions must be made regarding the conditions that must be met to ensure accuracy and reliability. For pH instruments, the sample conditioning equipment (especially the temperature-controlling unit) must maintain a constant sample temperature that meets the manufacturer's specs. In addition, wet tests used to verify on-line analyzer accuracy must be performed using temperature-compensated probes. Because sample temperature has a large impact on pH readings, any deviation in temperature will produce a deviation in pH that is not due to calibration or instrument error.
The plant's chemistry trending software should be modified to add a set of calculations called “analyzer deviations.” These algorithms operate on the differences between the results of wet test samples and those of the on-line analyzers. Typical pH probe/analyzer combinations are accurate to about 0.1 pH unit. That being the case, some deviation between analyzers (on-line or bench-top) should be expected. An analyzer should be considered accurate as long as the deviation is within expected limits.
The analyzer deviation calculations also help determine the need for analyzer calibration or replacement. Each deviation calculation represents a ratio of a wet test reading divided by an on-line analyzer reading. For example, the deviation would be 1.00 if the wet test and sample panel readings were identical. For pH analyzers, the control limits are 0.95 to 1.05 (5% deviation). No standardization is required if the wet test result and on-line analyzer pH readings are within 0.2 pH unit of each other, or if the calculated deviation is between 0.95 and 1.05. For conductivity, the limits are 0.90 to 1.10 (10% deviation).
Operators should take responsibility for this important calibration check, which should be performed at least weekly. Operators should be trained to standardize any analyzer whose calculated deviation exceeds the limit specified for it.
Standardization is relatively quick and easy. Operators don't actually calibrate the analyzer—they offset its current reading by the amount of deviation determined by a wet test/on-line analyzer comparison. The procedure can be hard-coded into most chemistry monitoring software. Most chemistry trending programs can be configured to display an alarm with a link to the standardization procedure if a deviation is greater than the limit.
Flag out-of-service analyzers and equipment. Frequent high deviations or standardization failures may mean that an on-line analyzer needs to be replaced. If either is the case, operators should describe the problem on the work order so maintenance staff will examine the analyzer in detail.
If an operator generates a work order for an on-line analyzer, he or she should indicate having done so on a white board in the water chemistry lab (or use some other method) to flag that it is out of service. Operators typically mark the analyzer's reading on the shift log sheet as “OOC” (out of commission) or “OOS” (out of service) so operations managers can see at a glance what's not working. It's absolutely critical that operators be trained to believe an analyzer's readings unless it is flagged as OOC or OOS. There should be no second-guessing.
If a reading from a working analyzer is out of range, operators should take immediate action: a single retest, but no more. Two independent readings (from an on-line analyzer and a wet test) should be used to confirm that the parameter is out of range and that corrective actions should be taken.
For an out-of-service analyzer, operators must increase the frequency of wet tests for any reading that it supplies. For example, if the pH analyzer for an HRSG's HP drum is out of service, operators should increase the frequency of wet testing that drum's pH to once every four hours. Similarly, if a silica analyzer is out of service, operators must wet-test all of the systems sampled by it once every four hours. This approach accomplishes two goals. First, it provides increased protection if an analyzer is out of service. Second, it creates some urgency on the part of the operators to get the analyzer back up and running (since their workload increases when the analyzer is out of service).
At most plants, shifting primary responsibility for sample panel maintenance to the operator decreases overall maintenance costs. Under such a regime, analyzers are calibrated on an as-needed basis rather than every week, typically saving about four I&C man-hours per week. The frequency with which pH and ORP (oxidation-reduction potential) probes are replaced decreases from about once every six months to about once a year. Another benefit of this approach is that conductivity probes no longer need to be replaced as a step in the preventive maintenance program. Replacing the probes only if they fail will generate net savings averaging about $18,000 per year per site. Sites that do not maintain their sample panel will obviously see their overall maintenance costs rise after implementing this philosophy. But usually the increase is more than offset by the higher unit availability made possible by a reliable and accurate steam sample panel.
4. Settle down. On-line analyzer deviations before and after a change in sample panel maintenance strategy. Source: Nalco
Figure 4 confirms the tangible positive effects of changing a sample panel maintenance strategy. The graph shows how one plant's average analyzer deviation ratio, discussed earlier, fell precipitously with a shift in maintenance philosophy. The correlation between wet test results and sample panel readings was extremely low before the change but much higher after it. The improvement in the accuracy and reliability of on-line analyzers allowed operators to decrease their wet testing frequency to once per day. Figure 5 shows typical, much lower analyzer deviations several weeks after the change.
5. Great expectations. Typical analyzer performance after the maintenance strategy change shows only minor deviations in readings. Source: Nalco
Make alarms active in the DCS. Critical parameters should be alarmed on the plant's DCS, and these alarms should be received in the control room. EPRI provides specific recommendations for DCS alarms in the publication, “Cycle Chemistry Guidelines for Fossil Plants.” Nalco's recommendations for the critical alarm parameters include:
- All pH readings
- All specific conductivities
- All cation conductivities
- All sodium analyzers
- All silica analyzers
- All dissolved oxygen analyzers
In addition to DCS alarms, most plants have access to chemistry-trending software, the plant data historian, or both. These data should be examined often to verify that steam cycle chemistry is within required limits. Operators should review the last 24 hours of sample panel trend data (at a minimum), just as they review DCS data for the turbine. In many cases, the trend data can be used to detect changes or upsets even if the analyzer providing the reading is in need of repair or maintenance.
Clean and preserve during downtime. As mentioned earlier, cycling operation makes it extremely difficult for on-line analyzers to operate accurately and reliably. The intermittent sample flow allows analyzer probes to dry out and rewet, increasing wear on them and shortening their life. These problems can be minimized if probes are cleaned with demineralized water and if demin water or condensate is routed through idle sample points.
If its probes are coated with corrosion products, an on-line analyzer will be sluggish and inaccurate. Taking advantage of a cycled plant's downtime to clean and preserve probes can increase analyzers' accuracy and extend probe life after operation resumes. Idle probes should be removed at least monthly and cleaned with demin water and a lint-free cloth. After flushing the sample “T” with demin water, refill the probe header, and re-insert the probe. Remember to restore the flow of demin water to idle sample points after the probe has been cleaned.
Most sample panels have several different locations where the demin water tie-in could be made. One good location is on the downstream (cool) side of the sample coolers. There's usually an existing union on all of these sample points that could be used to introduce demin water. This retrofit would require the installation of a block valve on the cooler effluent, to prevent both the backflow of demineralized water through the sample lines and the simultaneous flow of normal sample and demin water through the sample panel. A quick-disconnect and “T” can be installed downstream of this new block valve. Demin water would then enter the idle sample point through the quick-disconnect and flow through the sample point via the “T.” Figure 6 shows one possible arrangement for each sample point.
6. Keep it clean. A simplified schematic of a demin water flushing system for a sample panel. Source: Nalco
This arrangement provides several advantages. First, it preserves analyzers by maintaining flow through them even when the plant (or an HRSG) is shut down. Second, it minimizes sample line fouling because the continuous demin water flow during shutdown will tend to flush corrosion products that accumulate in the sample lines during operation. Third, it allows sample panel maintenance to be performed regardless of plant status.
Plants that have made this retrofit have reaped substantial benefits. But before you follow suit, here are two caveats. First, you must devise a way to distinguish whether a sample point is receiving demin water or a normal sample. One plant did so by creating engraved nameplates with “Demin water” on one side and “Sample” on the other. The plaques hang on a chain on the front of the panel around the block valve for each sample point. Operators simply turn the plaque around to read the sample point's status.
The second caveat is to take care to ensure that demin water and normal samples are not fed to the same sample point at the same time. Mixing the two creates several potential problems and should be avoided. Sample point shutdown and start-up procedures can be modified (or created) to address this potential problem.
—Dan Sampson (dcsampson@nalco.com) is a power industry technical consultant for Nalco Co. He authored “Fleetwide standardization of steam cycle chemistry” in the March 2006 issue of POWER.
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