Your plant's steam and water analysis systems (SWAS) instrumentation is incredibly sensitive, with detection limits that can be hard to comprehend. One of the most commonly used Units of measurement in a power plant SWAS is parts per billion (PPB). One PPB is the equivalent of one inch in 16,000 miles or one second in 32 years. Limits on specific parameters are less than 3 parts per billion and it is not uncommon to observe concentrations of less than .5 PPB on a steam sodium analyzer.
Limits that tight require accurate instrumentation. That's why it's essential to include regular maintenance and calibration for analyzers in your steam and water analysis systems (SWAS). And it's critically important to pay special attention to this technology during layups.
The importance of proper shutdown procedures
Each analyzer will have its own best practices for startup and shutdown. No matter what type or brand of analyzer you use, procedures should be defined/performed depending on the length of the shutdown and the type of instrument being laid up.
These analyzers measure chemical constituents like silica, phosphate, and chlorine. To measure silica in a water sample, the analyzers use acidic reagents that form blue molybdosilicic acid complex in a sample cell or reaction chamber. Analyzers must have a reagent delivery method. Peristaltic pumps or instrument air are commonly used to supply reagents to a colorimeter or reaction tube sample cell from their container.
When left stagnant, these reagents can crystalize or separate, causing a blockage in the line. This can be a challenging and time-consuming problem to diagnose. An analyzer might still pass calibrations but then be erratic following an outage. Even if you've checked that the reagent is getting to the cell, the volume of that reagent could be adversely affected by a partial blockage, which can cause reproducibility issues.
These challenges can be prevented by adequately laying up your silica analyzer before shutting it down. Depending on what brand of analyzer is in your plant, this can include a variety of steps, such as:
- Remove and store or dispose of all reagents
- Flush all reagent lines with demineralized water until you observe there are no reagents left in any tubing.
Ion Selective Electrodes (pH and sodium analyzers) Depending on the application, these electrodes can be fragile or robust, but none of them like to be dry. They’re shipped in protective caps full of an electrolytic solution to keep the measuring glass hydrated to prevent it from losing its sensitivity to H+ and Na+ ions. That’s where the importance of outage layup procedures comes in for pH and sodium analyzers.
Many sample cells are designed to hold a volume of water but inevitably end up dry from evaporation, leaks, vacuum on sample lines, or unknown causes. The result is often sluggish electrodes that might not pass calibration.
Typical pH analyzer layups can include a variety of steps, such as:
- Remove electrodes and store them in a pH storage solution (saturated KCL)
- Power down the instrument to protect electronics from surges
- Shortly prior to startup reinstall the probes in process water if possible
- Check the status of the reference electrode-electrolyte and refill if applicable. Some combination electrodes are not refillable and it may be difficult to judge the amount of life they have left in them. If a pH probe doesn’t seem trustworthy, it probably needs to be replaced
- Run probes in the sample until drifting stops. The longer this takes, the closer that electrode likely is to the end of its life. The conductivity of a sample stream is also a major contributor to the stabilization of the electrodes so if you’re measuring samples with very low conductance, expect this step to take several hours to a day
- Perform a regular buffer calibration
Typical sodium analyzer layups can include a variety of steps, for example:
- Power down the instrument and turn off sample flow
- Remove electrodes and store in electrode protective caps with DI water and a small amount of reference electrolyte (NH4Cl or KCl)
- Remove buffer, shut sample valve, and power down the instrument
- Upon startup, reinstall electrodes, new buffer, and let probes rinse down after establishing sample flow to the instrument until stability is observed.
- Perform a calibration
A site assessment can help you identify any issues with your SWAS analyzers before trouble strikes.
SysRep™ is your roadmap for extending the life of your aging steam and water analysis system – regardless of brand. Designed by Sentry Equipment, SysRep is a free, no-obligation, comprehensive health report on your SWAS.
Make sure you’re correctly maintaining and calibrating your SWAS analyzers. Schedule your SysRep™ assessment today. Contact us at +1-262-567-7256, or complete our online contact form for more information.