In the power industry, every minute is critical. A smooth plant startup is vital to operations, and directly linked to lay-up procedures. A thorough understanding of critical lay-up and startup strategies and procedures, paired with a proper corrosion control protection program can prevent unscheduled downtime, reduce operating costs, and increase plant reliability.
In the gas fired combined cycle market, whether you are a baseload facility or cycle regularly, asset preservation is at the top of the list of objectives. It is ever so important in the operational mode, but also critical in a standby/shutdown situation. A delay of just twenty minutes can result in tens of thousands of dollars of lost revenue depending on the size of your operation and power prices. How you protect your equipment during idle times has a direct impact on startup time, plant efficiency, and unscheduled downtime.
IDENTIFYING THE ROOT CAUSE OF INEFFECTIVE LAY-UP PROCEDURES
Although many common failures typically occur during operation, the root cause can often be traced to ineffective lay-up procedures. Common failures, which usually have a root cause of ineffective lay-up include condenser leaks; turbine deposits; boiler tube failures caused by corrosion fatigue or hydrogen transport; and feed water heater corrosion product transport. A well-documented strategy can help operations run smoothly.
The specific lay-up strategy will align with the duration of time the plant will be idle, how much time is available for shutdown, how quickly the plant needs to be operational, and the expected speed to operation. Although lay-up strategies will differ from plant to plant, depending on varying shutdown and startup factors, corrosion protection is paramount whether the idle time is just hours or as long as several months. The plant’s water treatment equipment inclusive of zero liquid discharge systems, open and closed loop cooling systems, and evaporative air coolers must be protected in order to minimize startup time.
CHOOSING THE RIGHT PROCEDURES FOR YOUR PLANT
Lay-up strategies can vary depending on how long the plant will be idled. Idle times usually fall into one of the following categories: based on the power purchasing agreement (PPA), hourly, daily, weekly, or beyond weekly. For hot lay-up where need is typically hourly to daily, slight pressure and proper cycle chemistry should be maintained. There should also be a vacuum on the condenser to ensure the proper steam seal is maintained.
For wet lay-up, where there is typically a daily to weekly need, drum levels and turbine vacuum should be maintained while ensuring a nitrogen or steam blanket is sustained. With all wet chemistry, regular testing procedures should apply. For long-term lay-up, the HRSGs should be drained “hot,” meaning the hot well should be drained dry, and the turbine should be maintained at a specific relative humidity, as many turbine issues happen due to improper lay-up. Sample panels and analyzers should also be protected.
THE IMPORTANCE OF CORROSION PROTECTION
To protect water treatment equipment from corrosion, recirculate from filter effluent to clarifier influent to keep the system moving and the pumps active. The flow will be determined by system volume, typically around 20 to 50 percent should be recirculated. Shut down and clean day tanks, and determine proper blowdown for the clarifier. Backwash filters and leave them full of water with bleach for longer term shutdown. Remove cartridge filters and clean out housings. Reverse osmosis (RO) units should be ran every day with biocide to control biofouling.
For seasonal operation, remove and bag membranes in 1 percent bisulfite solution. Clean out chemical tanks and pumps. Mixed bed systems can be idle for a few days without running, but should be ran in conjunction with the RO unit to keep water fresh for shorter terms. For seasonal shutdown, they should be regenerated at end of season and cation/anion beads separated in mixed beds. Mixed beds should be aired prior to startup.
In cooling towers, it is important to add biocide daily and monitor results in the open loop. While you can reduce the scale and corrosion inhibitor, you must continue to monitor corrosion during idle time. For longer term outages, drain and clean the cooling tower basin while inspecting spray nozzles and repairing or replacing as needed. Open water boxes need to be inspected and cleaned. Clean the condenser tubes and check sacrificial anodes and replace if necessary. With the evaporative air-coolers, you should drain and clean out the drain sump while inspecting the media for fouling slump, shifting, or any damage. If damage is significant, the media should be replaced.
SPECIAL CONCERNS FOR CHEMICAL STORAGE
In regards to chemical storage during lay-up, bleach will continue to be fed to systems in most cases, but all day-tanks that are used for other applications should be cleaned out for long-term storage. Bleach should be purchased in smaller quantities and kept out of the sun as time, temperature, and UV rays can cause bleach to decrease in activity. Ensure that the vent and vent dryer on the acid tanks are operable to keep moisture out. The caustic tank heaters should be checked, and nitrogen blankets should be considered to keep the CO2 out, as moisture can reduce the activity of the caustic. Chemical pumps from other products which will not be fed should be removed and cleaned or rebuilt as necessary.
Film forming products (FFP) and film forming amines (FFA) are alternatives for online protection as well as off line lay-up protection. FFPs and FFAs place a thin hydrophobic barrier between metal surface and liquid/steam environments. The protective film provides a corrosion inhibiting barrier to oxygen, carbon dioxide, and moisture that can supplement lay-up practices as well as alternative protection for troublesome environments in some operating units such air-cooled condensers (ACC). These products have proven to be effective with changing modes of operation units are now facing such as frequent shutdowns of unknown duration, load changes, and faster startups.
ACHIEVING STARTUP SUCCESS
Every moment is critical in the power industry, and a smooth plant startup is vital to operations. Startup success and plant operations are directly linked to lay-up procedures. Written procedures and necessary equipment modifications can help ensure a smooth startup and lay-up. Make sure to have the right expertise for proper lay-up and startup. Take the time to do it right, as a delay of just twenty minutes can result in tens of thousands of dollars in lost revenue depending on size of your operation and power prices.
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