Chilled Water System Flushing Procedure
Fito Coulter
Although closed loop systems are designed to retain water, there are additional problems that may occur that cause water loss. These water loss problems are why it is necessary to clean and flush the system. Often people only consider open loop systems, which have water regularly cycle through them, as needing flushing and cleaning.
The major problem that closed loops have is water loss. There are many reasons why this may occur. One of the reasons is the packing seals either not being installed correctly or while they are being changed. This could also be caused by a full expansion tank or filters being backwashed with system water. Lastly, there may be leaks, these leaks could be undetectable.
The scale & corrosion inhibitor treatment level decreases in the loop as make-up water is added to replace leakage. Scale & 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.
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.
There are many places in a closed loop system where bacteria and rust can cause problems. Every component of the closed loop system needs to have water regularly flowing through it, along with biocide. There are certain areas that are particularly susceptible to stagnation and for bacteria and rust to build up. Secondary loops are especially susceptible to bacteria and rust as they may be closed during biocide treatment. It is important to open them whenever treatment is done as the untreated water in them could recirculate into the system. In expansion tanks there is no direct water flow, this can cause a cesspool of bacteria to colonize. This should be flushed and opened to let biocide through. Drip legs are another area of concern as the bacteria and other dirt can accumulate over time and make its way into the rest of the system. Dead legs can easily fill up over time, the water in them is always stagnant and is the perfect breeding ground for bacteria and other microorganisms. Drip legs and dead legs need to be drained regularly. They also need to be treated along with the rest of the system.
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.
During the installation of pipework, it is almost inevitable that debris will be introduced to the system. With carbon steel systems, there is the added risk of loose scale/corrosion products that could be left inside, to travel around, potentially damaging/blocking equipment. Most commonly, it is the chilled water and LPHW loops that are flushed, but it also good practice to flush other pipework, before they go into service. Pre-commissioning flushing is usually carried out in accordance with BSRIA BG-29.
Flushing is often a combination of various activities, used to ensure that all settled and suspended solids, dirt, installation debris and corrosion products are removed from the pipework before commissioning. This typically involves pumping water around the pipework at high-velocities, chemical cleaning, and (where necessary), the addition of corrosion inhibitors and biocides.
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The dynamic flushing stage is completed to remove the majority of the debris, dirt, and particles from inside the pipework system prior to moving on to the chemical cleaning stage, which will use chemicals to move the water quality to the next stage.
Also, removing debris from the system will assist to prevent blockages in terminal unit coils, sensitive equipment such as balancing and control valves, and the possibility of water quality degradation.
The commissioning consultant [CxA] should conduct their own review of the project documentation using the design specifications, layout drawings, and schematics, to understand if all requirements have been allowed.
Once the general/main contractor has been instructed on the project, and after they have had time to get their feet under the table, workshops and discussions should be organized to develop a flushing strategy for the project.
This is not environmentally friendly and also in some cases can re quire special licenses from a government body or water supplies company, to allow discharge of the chemicals and quantity of water needed.
So to combat this a temporary sidestream filtration system can be used installing cartridges or bags, that will filter a percentage of the water being flowed around the system, collecting the debris and particles, reducing the need for constant draining and filling of water.
As we know, the flushing and cleaning can be a bit of a freight train, once it is started it does not stop for anything, all hands to the pumps and something that really should take 12 weeks the Project Manager is trying to force through in 3 weeks.
So, the general purpose is to ensure that the works have been completed in line with the agreed method and that it is fully documented and if there are any issues later on, the documentation, witnessing and inspecting, should provide enough evidence that everything was completed in line with expectations.
Once pressure tested, leaving the clean mains water in the pipework for a period of time will start to corrode the pipework, cause issues with bacteria and degrade the water, this will result in a longer flush and clean.
The stages when the sampling will take place will be discussed and agreed upon with the commissioning manager/flushing and chemical cleaning specialist, prior to being detailed within the method statement.
After the completion of the cleaning and before handover there is usually a period of time where the system and commissioning tasks have moved on to water/hydrostatic balancing and equipment startup and functional testing.
As noted in the above table, this will be the responsibility of the flushing and chemical specialist and overseen by the commissioning manager, to ensure all documentation and records are kept and maintained.
Once the project and system has been handed over, the facilities company will take responsibility for managing and sampling the water quality to ensure it continues to be within the system parameters and requirements.
Within each step, there should be an area to take notes and sign/approve by the team prior to the next steps commencing. This will allow for the whole process to be documented and stored for any future reference.
All pre-requisite checks should be completed to ensure that the installation is ready for the flushing activities to commence, based upon an approved document that will usually form a section within the method statement.
The water supply should be dedicated, avoid using the temporary water feed that is used for the general site water. It needs to be fully managed and controlled by the contractor that has been employed to complete the water flushing.
Prior to using the mains water, it should be tested for quality against the project specification. This is to ensure that the water that is being used to fill the system is of the same / better quality than what the specification requires the pipework system to be.
If it is expected that the water will be sat in the system for some time, this will usually be the case for most large commercial and industrial projects, it should be dosed with biocide and inhibitors to slow the formation of biofilms and corrosion.
Depending upon the stage of flushing and cleaning, there will usually be a requirement to introduce a chemical to aid the process and will be specified by the specialist chemical engineer on the project, it should be checked that these are available, some common ones are listed below:
Generally, it is accepted that the permanent system pumps are used for flushing to help the process and make things easier, they are generally sized correctly and will be onsite connected to the pipework so no real need to cut in temporary supplies etc.
If they are used, it is good practice to, at the end of flushing and before the hydrostatic balancing takes place, that the manufacturer is instructed to inspect the pumps, externally and internally, to ensure there is no damage and to change any parts, prior to confirming the warranty is still in place.
To allow the water to circulate around the system, but not cause issues with the sensitive equipment and coils that are installed, flushing loops and bypasses should be installed to allow the water to bypass them and recirculate back to the pumps, allowing a complete circuit.
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