Monitoring Dissolved Oxygen in High Pressure Boiler Systems

When it comes to controlling and monitoring dissolved oxygen levels in boiler feedwater, it pays to get it right. Many power plants use fossil fuels or nuclear power to heat purified water into pressurized steam that drives turbines to produce electricity. Since steam is so important to power generation, water quality is critical. The quality of boiler feed water must be carefully controlled throughout a plant’s operations if you want to optimize your system performance and extend the lifespan of your equipment. If that’s your goal, you want to remove virtually every bit of dissolved oxygen coursing through your system. Dissolved oxygen in water is so corrosive that if levels go unchecked, your boiler system will be destroyed. Even low concentrations can be highly destructive, causing localized corrosion and pitting of metal system components with damage that extends far below the surface. Improper water quality management results in plant inefficiency, costly repairs, downtime, and eventually the need to purchase replacement equipment. It’s pay now or pay later. Do the maintenance for relatively low cost on the front end, or skip it and pay a lot more for repairs in the long run.

There are two primary ways or methods that boiler operators use to reduce DO: through mechanical controls and chemical controls. Mechanical deaeration, commonly the first and most economical technique, can reduce DO levels down to around 10 parts-per-billion. Plant operators can also employ chemical treatment using oxygen scavengers like hydrazine, DEHA and carbohydrazide to consume the remaining DO, and CHEMetrics offers test kits for these analytes. The problem here is that competing chemical reactions between oxygen and boiler surfaces are likely to occur, resulting in system corrosion. That means routine monitoring is crucial to confirm you’ve been removing enough DO.

There are a few other common monitoring methods and while these online analyzers do offer continuous DO measurement, they require routine calibrationm and those calibrations can drift. The analyst also has to ask: Are the sensors beginning to corrode? Is the validity of online results being compromised by something else? We are in fact the only manufacturer of portable colorimetric test kits that detect trace levels of dissolved oxygen in boiler applications down to the parts-per-billion range, and that’s without the need for equipment maintenance or calibration. In addition, CHEMetrics is the sole-source supplier of test kits specified in the A-S-T-M Standard Test Method for Low-Level Dissolved Oxygen in Water.

For testing both online and with a kit, this is a good place to point out that it’s imperative sample water not be exposed to the air because atmospheric oxygen introduces contamination, and that leads to high dissolved oxygen readings and error. Therefore, the analyst can’t rely on ‘grab’ samples. That’s why our DO kits come with a special sampling tube that’s mounted vertically and connected to a continuous flow from the boiler system’s sampling port. After a samples is cooled to ambient temperature, the operator submerges a vacuum-sealed ampoule, then snaps the tip. The sample is drawn into the ampoule where the dissolved oxygen reacts instantaneously with the highly sensitive colorimetric Rhodazine D reagent. The color changes almost immediately, then you compare the developed color in the test ampoule to the ampoule standards in our round comparator.

In conclusion – Dissolved oxygen must be controlled and monitored, not only to avoid catastrophic failure of steam-generating boiler systems, but also to maximize plant performance and prolong the life of boiler equipment.

CHEMetrics kits are used as a primary means to measure DO in boiler systems; as a secondary means to verify online equipment readings; and as a back-up when online equipment simply isn’t working. There are a whole host of advantages that come with using CHEMetrics Dissolved Oxygen testing products:

They’re portable – small enough that you can carry a kit in one hand and hold an ampoule between two fingers.
They’re affordable.
They’re easy to use – with instructions an unskilled operator can read in less time than it takes to run the whole test that delivers results in one minute.
These kits provide everything needed to complete your analysis with no maintenance and no calibration required, and...
They are an ASTM Standard Test Method.

We’ve earned a reputation for manufacturing world-class, easy-to-use DO test kits with sensitivity down to 2 parts-per-billion that have been making life easier for plant operators for more than half a century.

CHEMetrics – For simplicity in water analysis.
www.chemetrics.com