Cleaning & Flushing Procedure for Closed Loops

If It’s a Closed Loop, Why Does it Need to be Flushed?

In theory, water should never leak out of a closed loop.  While there are closed loop systems that rarely lose water, most closed systems lose a small amount of water during the course of normal operation.  This typically happens when pump seals leak or molds are changed.  Unfortunately, most closed loop system designs do not account for this lost water by automatically adding closed loop treatment as make-up water is added to the loop.  The make-up water not only dilutes the corrosion inhibitor and reduces its effectiveness, but also adds oxygen which can promote corrosion on many surfaces.

What Happens to Corrosion By-Products in a Closed Loop?

The corrosion inhibitor treatment level decreases in the loop as make-up water is added to replace leakage.  Corrosion by-products tend to accumulate in closed loops since the water recirculates without blow down.  Iron is the most common corrosion by-product in closed loops.  Even though some iron is lost through leakage, the reduced treatment level rapidly exceeds the ability of the leak to control the amount of iron dissolved in the water.  As the concentration of iron increases, the solubility is exceeded, and precipitation occurs.  This results in iron deposits.

What is Microbiologically Influenced Corrosion?

Often referred to as MIC, Microbiologically Influenced Corrosion occurs when one or more species of bacteria colonize and begin feeding on the iron surfaces in a system.  Colonies can attack all types of ferrous metals including iron, mild steel, galvanized and stainless steel.  The metabolic process of Sulfate Reducing Bacteria (SRB) uses the conversion of iron to iron oxide to create energy.  Galionella also converts iron to iron oxide.  Clostridium excretes hydrogen ions which react with water to form strong organic acids.  The common result in pits in the surface of the metal that are hidden under tubercles of iron oxide.

How Do I Clean My Closed Loop System?

Removing the iron deposits involves a non-acid chemical cleaning with chilller flushing chemicals, a thorough flushing of the system to remove the iron, and a biocide to decontaminate the system.  Here are the steps:

  • If the water in the loop is already heavily fouled or dark in color, begin by flushing at a rate of 1-2 gallons per minute from a valve at a low point in the system. It is important to monitor the pressure on the circulating pump during this stage.  If the flushing rate exceeds the make-up rate, the system volume will drop and the pump will cavitate.  This can cause expensive damage to the pump bearings and impeller.  By making sure the pressure is maintained in the system, you ensure that the pump will not run dry.  Continue flushing until the water consistently runs clear.  Depending on the volume in the loop, this may take as long as 5-7 days.  Count on approximately 12 hours of flushing for every 1,000 gallons in the loop.
  • Once the water is clear, add the proper dilutions of the cleaning products to the system.
  • Circulate this cleaning solution for at least 48 hours. Circulate for 96 hours in heavily fouled systems.
  • After the cleaning period, flush the loop using the guidelines in the first step. Continue flushing until the conductivity of the water in the loop equals that of the make-up water.
  • Retreat with the appropriate closed loop corrosion inhibitor and bacteriostat.

If you have problems with this procedure or any other aspect of your closed loop system, contact your Chardon Master Service Technician.

matt welsh
Matt Welsh
Vice President, Water Consultant at | Website | + posts

Matt Welsh is the Vice President and Water Consultant at Chardon Labs.  He helps consult a wide range of customers utilizing various methods of water treatment, from chemical to chemical-free approaches, large and small applications, and across a wide range of geographical influences.  With 20 years of water treatment experience, including a wide range of troubleshooting and service in potable water and non-potable HVAC and industrial applications, he is an expert in water treatment chemistry for cooling towers, boilers, and closed-loop systems.