1 – Why should you have a deaerator?
Raising steam for your facility requires water, and depending on the steam plant’s size and design, the volume of feedwater needed can be small or quite large. Some applications allow for steam condensate to be recycled back to the boiler which is reused as part of the feedwater, with the rest coming in as fresh water, called makeup. The makeup water comes from your local utility or well water source. It is often cold and contains many dissolved gases, with oxygen being the most prevalent. Regardless of the volume of condensate returns you have, the makeup water needs to be conditioned before it can be used as boiler feedwater. The conditioning requirement for the boiler's feedwater quality must achieve 4 goals, as outlined in both the 2015 ASHRAE Handbook chapter on Water Treatment , and the ASME Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers (CRTD-81). These goals are:
- Dissolved oxygen removal through mechanical means, in order to reduce the need for water treatment chemical oxygen scavengers (Sulfite, hydrazine, DEHA, etc)…. reduce your water treatment chemicals
- Dissolved gas removal through mechanical means, in order to protect the steam distribution network and condensate network from oxygen pitting and carbonic acid corrosion.... prevent corrosion and failure
- Dissolved non-condensable gas removal, in order to increase steam system efficiency…. increased steam potential
- Heat the cold water to avoid thermo-shock to the boiler system (economizer, boiler)…. longer steam boiler life
A deaerator is a specifically designed piece of equipment to achieve all 3 of these goals. Often the deaerator is a combination of two vessels; one vessel for removal of oxygen and non-condensable gases (scrubbing section), and a second vessel for storing hot feedwater (storage section). It is this 2 vessel design that provides superior results and efficiency as compared to simply having one steam heated “feedwater tank”. The deaerator’s scrubbing section mechanically breaks up the water into droplets so that oxygen and dissolved gases are removed from the water and vented out of the vessel before entering the storage section. The resulting high quality feedwater has far lower concentrations of dissolved oxygen and non-condensable gases, as compared to a conventional feedwater tank, leading to minimized operating costs and improved steam quality.
2 – How do I know my deaerator is working well?
As described above, the deaerator is far more effective than a steam heated feedwater tank because of its mechanical design and ability to use steam pressure and temperature to condition the makeup water into feedwater. The three main parameters indicative of a properly operating deaerator are temperature, pressure and dissolved oxygen removal. Ensuring these three parameters are within design limits is crucial for maximum performance and return on investment of the deaerator. The temperature and pressure should be checked at least daily, and logged in an electronic database for trending analysis. If fluctuations in temperature and/or pressure are observed, maintenance should be scheduled for the controls on the deaerator. Dissolved oxygen removal can be assessed by regularly scheduling a Dissolved Oxygen Study (DO2 study), which is a controlled test on the system that measures the dissolved oxygen level at the outlet of the deaerator. DO2 studies are important because they can identify when something has failed mechanically inside the deaerator, limiting its ability to remove oxygen or other non-condensable gases. A physical inspection of the deaerator will not often reveal if something has failed internally. DO2 studies should be scheduled at least on an annual basis, with the results logged and stored electronically for trend analysis. The DO2 study itself is relatively simple, but does require some planning and setup:
- A representative sample must be obtained from the storage section of the deaerator, and cooled to 70-80°F before testing can be performed. The recommended setup for proper sampling is a stainless steel sample cooler, installed in such a way that a sample can be continuously collected during the DO2 study.
- The boiler chemical (Sulphite) program must be disconnected from the deaerator for the duration of the test, and ideally temporarily connected to the feedwater line(s) feeding the steam boiler(s), downstream of any re-circulating lines, such as those found on feedwater pumps and feedwater economizers.
- Sampling and testing should occur on a flowing sample, while testing the deaerator during all of its modes of operation. Ideally the unit should be tested when it is taking on makeup, when it is satisfied and over a period of normal operation so as to measure the effectiveness of the temperature and pressure controls on the deaerator.
- Testing equipment should be used for the range of dissolved oxygen to be tested. This can range from 10-15ppm in the cold makeup water, down to 7-50ppb for the storage section of a properly operating deaerator.
3 – What is my water treatment provider’s role?
Your water treatment provider should be monitoring the temperature, pressure, and demand for chemical oxygen scavenger at least monthly, and logging this data into an electronic database for trend analysis. The easiest way to spot a failing deaerator is by observing changes in the “normal” levels of chemical consumed, or the relationship between pressure and temperature. Your water treatment provider should also be reviewing your daily logs of temperature, pressure and chemical consumption to spot potential issues. If you are doing your own DO2 studies, the water treatment provider should be using your data as well to help determine the effectiveness of the deaerator. At the very least, your water treatment provider should be performing a DO2 study themselves, to ensure the deaerator is functioning properly.
4 – What is my boiler mechanical contractor’s role?
Your boiler mechanical contractor’s role is important as well, as they need to coordinate inspections and possibly repairs, in order to act on any deviations observed during the regular monitoring of deaerator performance indicators. A DO2 study will reveal incorrect deaerator operation, and often the fix is to repair/replace the spray head or a tray inside the “scrubbing” section of the deaerator. Bringing your mechanical contractor and water treatment provider together as a team is a great first step in ensuring your deaerator continues to operate at maximum efficiency.