'Tis the Season: Cooling Tower/Condenser Shutdown
The warm weather is retreating and the days are getting shorter – fall is here. Here are a few things to consider at the end of the cooling season if your cooling system does not run throughout the year.
What needs protecting?
This article focuses on open recirculating systems. For these systems, the components under consideration are the cooling tower, system supply piping, distribution piping and the condenser itself. When the system is shut down and remains filled with water, corrosion and increased biological activity will compromise the metallurgy and health of your system. Increased corrosion rates and corrosion byproducts will form. Biological activity will increase and lead to slime growth and the potential for Legionella.
For these reasons, proper chemical treatment and physical cleaning is necessary to keep your system healthy during downtime. The challenge will be to balance system availably with asset preservation and reliability.
Wet vs. dry?
Choosing a Wet or Dry layup will be dependent on the expected length of time the system will be out of service. For our purposes, we will define an extended period as a system shutdown lasting 5 days or more.
A wet layup is defined as a storage practice that retains the circulating water within system equipment and piping. Besides the obvious ease of shutting a system down with a wet layup, the primary advantage is the ability to recommission the cooling system when needed.
Generally speaking, a system down for less than 5 days can make use of a wet layup. As a best practice, however, it is recommended that increased inhibitor/dispersant feed be utilized upon startup. Additionally the increased application of the biocide program should be utilized to inhibit biological activity while the system is stagnant.
For systems that are shut down for more than 5 days it is recommended that a dry layup be used. If operational necessity dictates the need for a quick startup, a wet layup can be used but requires specific activities to be effective.
The down side of a wet layup is that it requires routine attention for it to be effective if lasting for more than 5 days. The goal is to actively address corrosion, deposition, fouling while the system is in a fully wetted, stagnate environment. This requires chemistry adjustments and potential operational intervention in the form of circulation practices on a periodic or continuous low rate. Higher corrosion inhibitor levels are required for stagnant conditions and the chemistry must be maintained within acceptable limits. Microbiological control is generally problematic.
Freeze protection is a consideration.
A dry layup is defined as a storage practice that requires the complete draining of the circulating water from equipment and piping. Because there is no water, corrosion potential is reduced compared to a wet layup, but certainly not eliminated. Same goes for microbiological activity, with reduced potential but certainly not eliminated.
The disadvantages of a dry layup revolve around the time spent ensuring all the water is drained from the system, including low lying sections of piping and dead legs. When the warm weather returns, dry layup can require a lengthy startup time and procedure for filling, purging air and commissioning. Some of the best practices for a dry layup require fitting with appropriate low-point drains if not present, using dry air circulated through piping and equipment to help reduce the potential of corrosion, and the use of desiccant bags or vapor corrosion inhibitors for some equipment.
Many of the best protected systems during a dry layup still make use of chemical treatment to help further reduce the potential for corrosion and microbiological activity. Dispersible liquid products are used right before draining and are designed to form a corrosion inhibiting film on metal surfaces. An increase in non-oxidizing biocides are also introduced and circulated before draining.
“Shoulder season vs. extended shutdown?
The shoulder season is characterized by cool nights, when the system is shut down due to the reduced demand for cooling. During the day in these periods, warm days necessitate the startup of cooling systems or if the days are cool, the cooling system may not run at all for several days. In these transitional times, it is generally recommend carrying increased levels of corrosion inhibition actives and biocidal agents to help mitigate the effects of stagnant water.
Freeze protection is very important consideration for the areas that experience freezing temperatures. This is particularly true of systems under wet layup, but can also apply to dry layup systems that have not been adequately drained. The appropriate antifreeze protection is required in these applications.
What is your shutdown plan?
Doing nothing to protect your system when it is out of service due to seasonal operation, equipment repair or building shutdown is not an option. Increased corrosion and microbiological activity is sure to negatively impact the equipment reliability and integrity, and lead to health and safety concerns.
In addition to developing a plan for asset protection using a layup strategy, the cleaning of your cooling tower during this downtime is highly recommended. In fact, according to the OSHA Technical Manual – Section III: Chapter 7, V. Controls, B. Cooling Towers, Evaporative Condensers, and Fluid Coolers, 4. Frequency of Cleaning:
“Cooling towers should be cleaned and disinfected at least twice a year. Normally this maintenance will be performed before initial start-up at the beginning of the cooling season and after shut-down in the fall.” and “Any system that has been out of service for an extended period should be cleaned and disinfected.”
So despite whether or not your cooling tower is operational the entire year, it should be cleaned at least twice per year.
Now is the time to prepare and begin to execute an action plan for layup and tower cleaning. QualiChem has many products available to address these concerns. We have wet and dry layup formulations to meet the unique demands of cooling tower downtime. Application of these products and more detail on the products available will be covered in a future post.
Contact QualiChem to help with your shutdown strategy and chemical treatment needs.