Cooling towers - the name itself gives the air of the opposition to fire. Their existence is for the soul purpose of removing heat. However, cooling towers contain very real fire hazards: Polyvinyl Chloride (PVC) fill; Fiberglass Reinforced Plastic (FRP) walls (FRP is often mistaken as an acronym for fire resistant plastic), fan stacks, fan decks, fan blades, louvers, partitions, and catch basins; Acrylonitrile Butadiene Styrene (ABS) and Polypropylene nozzles; and wood. Cooling towers are susceptible to fire when they are online or off. When a cooling tower is online, there are several dry areas within the building with fire hazards available to fuel a blaze. When cooling towers are offline, they are at greater risk for fire with a larger volume of dry space to invade. Don't wait until a fire erupts in a cooling tower to find out the fire sprinkler pipes have failed.
Causes: Cooling Tower Pipe Deterioration
Pipe corrosion and scale formation are part of the lifecycle of pipes within a cooling tower. The humid living conditions of these pipes are the perfect catalyst for the deterioration of fire protection pipes. In fact, without pipe and general metal component up-keep, it is estimated that a cooling tower will need to be completely torn down and rebuilt approximately every twenty years. There are three main triggers of pipe break-down in a cooling tower.
1. Contaminated Water
Many cooling towers are open and exposed to the outside environment. Air pollution particulates and dissolved gases contaminate the water. When evaporation occurs, the pure vapor water enters the atmosphere, while the contaminated water remains in the cooling tower in a concentrated state. This accelerates the corrosion process.
The corrosion process is dictated by the velocity of the water. High velocity water increases the volume of corrosion by transporting oxygen and particulates to the metal pipes at a quick rate, providing the opportunity to contaminate a larger volume of piping. When the water velocity is low, it localizes the particulate distribution, causing greater damage to a smaller area. The deteriorating matter is sitting in one spot, giving it the opportunity to eat away at the pipe in a concentrated manner.
Contaminated water affects the pH balance of the water, providing new opportunities for deterioration and corrosion.
2. pH Balance
Water in cooling towers comes from a variety of sources, and the source will dictate the pH balance of the water. The damage caused by particulates becomes exasperated when the water loses its pH balance and becomes either too acidic or too alkaline. The imbalance of pH can cause either scale formation or corrosion, depending on the number it reaches on the pH scale.
Water that is a pH balance of six or lower is acidic and increases corrosion. Acidic water mixed with oxygen and contaminates like chloride or sulfate will cause corrosion. Corrosion is the deterioration of metal through chemical or electrochemical reaction with its environment.
Water that has a pH balance of 8 or higher is too alkaline, causing scales to form. Alkaline water has high concentrations of calcium and/or magnesium. The calcium or magnesium carbonate becomes deposited on pipes, creating scales. When scales build up, it can trap oxygen, causing corrosion. Without annual inspection, testing, and maintenance, a facility would not learn of the pipe deterioration until a fire event happens, leaving the facility without water supply for their fire protection sprinklers.
Scale buildup may cause microbial growth between the scales and pipes. This can cause pipe corrosion called microbial induced corrosion (MIC) in addition to the already present scale problem.
There are three different types of attacks on the metal:
• General Attack: Corrosion is evenly distributed over the metal surface.
• Pitting Attack: Small areas of the metal are corroded, like in the case of MIC.
• Galvanic Attack: Corrosion when two different metals are in contact.
3. Warm Environment
The warm environment is the perfect atmosphere for microbes to thrive. The microbial growth causes differential aeration. Differential aeration, also known as oxygen concentration cells, occurs when oxygen is trapped between scaling and the metal pipe. This entrapment will not allow the oxygen to diffuse naturally and uniformly. Without the ability to diffuse evenly, oxygen develops into a concentration of cells, causing the cell concentration to eat away at the pipe where it is trapped. Every 25-30 degree increase in temperature doubles the corrosion rate.
The contaminated water, pH balance, and warm atmosphere all act in combination to corrode or scale fire protection pipes.
Ensuring Safety in the Event of a Fire
Because of the nature of a cooling tower, it is difficult to find a convenient time to take it offline for inspection, testing, or maintenance. However, annual inspections could lead to a longer lifespan for cooling tower fire protection. The environment of a cooling tower is the ideal formula for pipe decay. It cannot be stopped, but it can be delayed.
To delay pipe decay, annual inspection and testing needs to be completed by a qualified fire protection solution provider. If corrosion or scale build up is found early, it could be rectified when it is only a small problem and delay the need for a completely new system. If corrosion or scale formation is found and maintenance is needed, the piping system can be replaced with stainless steel, which decays at a much slower rate than standard steel.
Do not delay in replacing a fire protection system when it has corroded beyond the point of repair. Cooling Tower fire protection systems, in general, need to be replaced every 7-10 years due to corrosion. Without annual inspection, the only way for a facility to learn that the fire protection pipe system is corroded or has significant scale formation is if there is a fire and the fire protection solution fails.
When a fire protection sprinkler system replacement is needed, choose a fire protection solution provider that can meet the accelerated timeline that cooling towers require. Many aspects of the installation can be done beforehand or offsite to minimize the outage: design, survey, material drop off, safety training, and custom pipe fabrication. For more information on cooling tower hazards, FM Global has compiled an in-depth data sheet available here or click here for FEMA's cooling tower fire hazard training.