Water Chemistry Important to Avoid Equipment Damage
Technology advances for heat recovery steam generators (HRSGs) at power plants continue to provide higher efficiency, with major gains recognized in natural gas–fired and cogeneration systems in recent years. A critical part of improved operation for facilities utilizing high-pressure, high-temperature steam is the monitoring of steam and water chemistry.
Today’s HRSG systems often require high-purity water, and strict water and steam chemistry control and monitoring. The wrong chemistry, and introduction of impurities into a system, can damage equipment, including boilers and turbines—damage that can be made worse due to the high temperatures and high pressures of today’s systems.
The problem also may be exacerbated by frequent cycling of power generation units, a situation more common today as power loads vary with the integration of renewable energy. Unit shutdowns, and then startups, put more stress on systems, a notable factor as more combined cycle gas-fired units replace coal-fired boilers. Monitoring for contaminants in a system always has been important during normal operations; the risk of damage becomes greater in today’s systems as they cycle on and off more regularly, particularly when they are not shut down or started up properly.
Following best practices for layup procedures, to avoid system leaks or other problems after maintenance periods, often requires preparation and practice. There are several important considerations to address to ensure there are no surprise failures when returning a plant to service after a maintenance or other outage.
Operators can begin by considering the length of the layup, which could be just a few hours, but also could last a few days or even weeks. Plant water treatment equipment needs to be protected, particularly from corrosion, during any idle period to help minimize startup time.
Knowing the status of water in the system is important during any idle period. Is the water fresh or brackish? Does your plant use seawater? If pumps are continuing to operate (which may depend on operating costs), is the water chemistry being monitored and controlled?
Knowing the corrosion resistance of the system is another factor. The goal is to prevent fouling of equipment, and do it at the lowest cost. A good layup strategy may include keeping circulating water pumps in operation, with sufficient velocity to prevent suspended solids from settling, and to create enough head loss to help balance the water box flow. Industry experts say a standard practice should be to keep water boxes full, and to continue use of a biocide. If the system uses ball cleaners, ensure that they are properly sized and replaced before they wear out. Use an antiscalant and always monitor the pH of the system’s water.
Experts note other important practices for long-term outages. In these cases, it could be necessary to drain the tubing. If the condenser unit is lower than the cooling tower basin, ensure the system has the proper pumping capabilities and solid valves to keep water from re-entering the system.
If fouling or scaling is found in the tubing, it will likely be easier to clean while still wet. Depending on the alloy, potable water may be used to push out water with high levels of TDS (total dissolved solids). Some alloys, such as stainless steel and copper, may be better handled with a condensate rinse. The faster tubes are dried after cleaning, the less time for corrosion to occur, so consider blow drying or using dehumidified air during this process.
Industry experts have said that investing in a high-performance tubing alloy is likely the best solution to protect the water system and avoid leaks. The use of lower-performance tubing may mean pumps must run continuously to maintain biocides, and it’s important not to assume biocides will provide long-term protection in the absence of water flow.
Never shut down a pumping system and leave large amounts of water in the tubes. Stagnant water can cause damage. And avoid low flows, which can allow for more sedimentation and microbial growth in the system.
—POWER staff.