In the power industry, every minute is crucial. That’s why there are few things more devastating than returning from a maintenance outage and learning that an on-time startup will be delayed due to condenser or key BOP exchanger leaks. These surprise failures are not uncommon, and yet many of the problems are easily preventable with the right layup preparation and practice. Here are some easy to follow guidelines that will ensure your layup goes smoothly, and not a minute too late.
Layup Considerations
One of the first factors to consider is the length of the layup. 3 hours, 3 days, 3 weeks? Each plant operation is unique and the length of time is very specific to the plant. The specific layup strategy will support the amount 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 layup procedures will vary from plant to plant, corrosion protection is of utmost importance. The plant water treatment equipment must be protected to minimize startup time.
Other factors that need to be considered for making layup decisions include:
- Is the water fresh, mildly brackish (many tower water may be considered this), seawater?
- If the CW pumps continue operating, how well is the chemistry monitored and controlled?
- What is the pump operating cost?
- How corrosion resistant is your alloy?
The goal when analyzing these factors is fouling prevention at the lowest cost and ease of maintenance. Ready for a deeper dive? Let’s look into these considerations a bit further.
Do’s – Short-Term Layup Practices
Depending on your tube material and financial review, this time frame may be a few hours to a few days. Below are suggestions that may be added to your checklist and incorporated into your layup strategy.
- Keep the circulation water pumps operational. At a minimum, keep velocity at 5 ft/second minimum. This velocity needs to be sufficient to prevent the settling of suspended solids and should create enough head loss to help balance the waterbox flow.
- Institute a standard practice that ensures that the water boxes are full.
- If using a biocide practice, continue to do so. Make sure monitoring of your standard parameters is continued and notifications to proper personnel are in place.
- If your system is has a ball cleaning system, continue to run it. Make sure that the properly sized balls are used and they are replaced as they wear.
- Continue the use of your antiscalant. Alternatively, drop the pH to prevent scaling. However, continue pH monitoring as acid valves may not be 100% reliable and the results can be catastrophic.
- On the shift after the shut-down, have someone double-check the chemistry and shutdown lineup.
- Formalize the procedure and train to ensure it is followed in case of an emergency.
Do’s – Long-Term Layup Practices
After your team agrees on what time frame is considered “long term”, prepare a practice based on the following suggestions:
- Drain the tubing. If the condenser is lower than the cooling tower basin, the system will need a capable pumping system and reliable valving to prevent water re-entry.
- If the tubing has some scaling or fouling, consider cleaning tubes while they are still wet.
- When higher performance alloys are in use, particularly with mildly brackish and seawater CW systems, rinse the tubing with potable water to remove high TDS water.
- With the low-performance stainless steels and copper alloys, rinse the tube ID with condensate.
- After rinsing, blow-dry or dehumidified air through the man ways. Faster drying reduces the time for tube corrosion to occur. Some plants have fabricated portable manifolds to fit the openings and to increase airflow. If the condenser is drained and rinsed while hot, drying may be minimized.
- Formalize the procedure and train to ensure it is followed in case of an emergency.
In such a case where resources are limited and all steps cannot be accounted for, the most cost-effective long-term solution may be to invest in a high-performance tubing alloy. Although the cost may be twice that of the low-cost alloys, that difference can be easily made up from the cost of a couple of forced outages to plug leaking tubes. The most popular alloys include titanium grade 2, super-ferritic stainless steels like SEA-CURE® (UNS S44660), and 6% molybdenum-containing austenitic stainless steels like AL6XN® (UNSN08367).
Consider running pumps continuously while maintaining biocides when lower performance tubing is used. The biocide impact needs to be monitored to ensure a residual of 0.3 to 0.5% free chloride or equivalent at the outlet.
When the higher performance alloys are installed but you are unable to drain the condenser, you may want to still consider running the pumps for a few hours daily to keep the tube surfaces cleaner. This should be paid back by preventing the loss of thermal performance when this is not done.
Don’ts – Both Long Term and Short Term Layup Practices
Regardless of the length of the layup, there are common mistakes that, while seemingly innocuous, can wreak havoc on equipment.
- Don’t shut down the CW pumps and leave the tubes full of water! Even a few hours of stagnant water can damage some of the more sensitive tube alloys. This may be difficult in plants designed with the condenser lower than the cooling tower basin. If building a new plant, or considering adding a tower do to 316B concerns, it will make life much easier if the tower basin is lower than the condenser.
- Don’t assume that the biocides will provide long term protection without flow! A shock biocide charge will rarely remain after a few hours.
- Do not expect positive tubing cleanliness by continuing flow with low flow pumps! This question has come up numerous times in new construction.
- The condenser design relies on head loss to ensure relatively even flow through all of the tubing. With low flow, the head loss is minimal and the flow will take the easiest route through the bundle, which is usually the shortest. Tubing nearer the top will very likely remain stagnant. In some bundles, the lower head may not completely fill the entire condenser, creating alternating wet/dry conditions. Typical head loss across a bundle with 6 to 9 ft/s can be 12 to 22 ft. The low flow will allow sediments to settle and microbial growth will be cemented to the bottom of the tube. Many bacteria produce polysaccharides that act as the glue for the silt and sand particles.
- Unless certain provisions are maintained in the condenser system to keep the entire condenser full, many of the tubes may be only partially full, producing more aggressive wet/dry conditions in those tubes.
With the significant increase in cycling operations, tube layup practice has become important. Lack of an effective layup strategy can lead to start-up delays, when minutes, let alone days, are critical. For long tube life and predictable thermal performance, short term shutdown practices and long term layup procedures need to be carefully developed and followed. Practices may vary based on tube material and water source, and each location’s procedures should be customized based on its unique factors keeping the goal of asset protection and corrosion prevention in the forefront.
To discuss your power plant’s upcoming layup strategy, contact us today.
