Below is a detailed chilled water pipe flushing procedure that covers cleaning, flushing and chemical treatment requirements for an existing or new project.
Flushing of chilled water system shall be done as per the guidelines and requirements mentioned in the project specifications and relevant drawings etc.
Also instructions given from the chemical manufacturer and suppliers shall be followed for flushing chilled water system.
Overall project manager shall be responsible for the implementation of chilled water system flushing procedure at the project site.
Material Transport Handling and Storage
Special care shall be taken while chemicals are transported & offloaded from containers, rough handling can result in chemical spillage.
Use appropriate type of vehicles in transporting the chemicals and that they are protected against weather conditions.
As soon as the chemicals are received on the sites, these shall be inspected and approval from consultant to be taken.
Ensure that the storage location of the chemicals are ventilated & isolated from other materials & equipment at site.
Do not remove chemicals from original packaging containers in order to prevent spillage and leakages etc.
All packages for the chemicals shall be properly sealed, tagged, labelled & identified.
Health Safety and Environmental Requirements
Site specific HSE rules & regulations shall be complied all the times.
Supervisors will deliver tool box talks, relevant to chilled water flushing activities to all operatives involved in the activities.
All operatives will be equipped with minimum personnel protective equipment; hard hat, coveralls, safety boots, safety glasses, hand gloves, etc.
HSE officer will ensure that only qualified personnel shall perform the chemical flushing of chilled water system.
Warning sign boards shall be provided and access to working area shall be restricted.
Close monitoring by the site engineers during the chilled water flushing is highly recommended.
Extra care shall be taken while handling chemicals & workers should fully understand & aware of the hazards for mishandling the chemicals. MSDS shall be available and understood by the operatives.
Preliminary Checks
Before commencement of flushing, cleaning & chemical dosing activity, the supervisor/ engineer must ensure that:
The system has been successfully inspected by the consultant/client and approved.
Pressure testing of the chilled water piping is done and approved.
Necessary approvals i.e. check sheets are available.
No chemicals shall pass through the cooling coils of the HVAC equipment, flexible hoses shall be used to bypass the same to the available flushing loops.
All drain strainers are open for the initial flushing procedures, a temporary connection at the risers will be installed.
Air vents are provided as per the location mentioned in the approved drawings.
Arrangement for draining the flushed water are done and drain line is connected to the main drainage line.
General Chilled Water Pipe Flushing Procedure
The cleaning/ flushing & chemical dosing will be carried out in four processes as described below:
Flushing/ Cleaning with Clean Water
This process involves the use of clean water as a cleaning medium to remove inert suspended matter or unnecessary debris, jointing compounds, dust etc.
Procedure will be carried out with the cooling coils by passed.
This procedure shall be repeated until all deposits are removed from the chilled water piping system.
Flushing/ Cleaning with Chemicals
This process involves the use of flushing chemicals as per the approved material submittals and as per supplier instructions. It is recommended that this step should be done in the presence of supplier or manufacturer representative.
Flushing chemical dissolves accumulated oxide & scale deposits without attacking the parent metal and restores the system flow.
This process is also carried out with the cooling coils by passed, FCUs & AHUs are connected to the piping circuit after the system is emptied and cleaning chemical is not present in the system.
Chemical Treatment of the System
This process will ensure that the system is treated with inhibiting chemicals to prevent future corrosion and scale problems.
During this process, the chemicals are circulated through the cooling coils.
Chemical Treatment upon Commissioning
This process will ensure the system is checked for the quality of the chemicals in the circuit and respectively regulated upon commissioning of the system.
The process will also allow any adjustment of chemical dosages if necessary.
Flushing/ Cleaning Process with Clean Water
The first stage towards removal of the contaminants is to flush/ clean the system with clean water by dynamic flushing.
Main objective of the flushing/ cleaning the system with clean water is to remove as much dirt and debris from the pipework system as possible.
This step is important in order to reduce the likelihood of system blockage and to create the best possible circumstances for a successful chemical clean and subsequent water treatment regime.
Contaminants such as mill scale, jointing compound and building debris will inevitably be found in any newly fabricated pipework systems.
If allowed to remain in the system, these contaminants will make the system prone to blockage at strainer, control valves and small bore heat exchanger.
The microorganisms may also initiate corrosion and may encourage the growth of microorganisms.
System is filled with clean water and circulated for at least 1 hour with the system pump, the pump is also used to drain & refill the system whenever necessary.
The system pumps are secured with strainers. If necessary, add temporary strainers on each floor which will have a balanced flushed result.
During the balance flush, the system pressure is monitored to ensure that the filling and draining are balanced and to ensure that no water shall be lost from the system.
While maintaining a balance flush the flushing velocities are measured & recorded accordingly.
The flushing velocity will be determined by taking the system schematic volume.
During the flushing, the strainers are removed & cleaned and refitted periodically.
The water velocity required for flushing must be sufficient to pick up and carry the majority of the dirt and debris in the system.
It is much feasible for much larger particles to find their way into the systems, most of the debris will be below 5mm in diameter.
Application of Chilled Water Flushing Chemicals
The main purpose for chemical cleaning is to remove oxide deposits from the metal surfaces.
Chilled water piping system will be dosed with specified amount of chemical i.e. Clean Guard 70F02 cleaning agent at a level of 0.5-1 Kgs per cubic meter of system volume.
Clean Guard 70F02 (Cleaning chemical) disperses accumulated oxide & scale deposits without attacking parent metal, thus restoring full system flow and ensuring ideal heat transfer.
Because of its non-aggressive nature, Clean Guard 70F02 is ideal for use in multi-metal systems and can be used whilst system is on line and operating under normal load condition which is particularly important during commissioning.
Metal ions such as iron, copper, calcium & magnesium form relatively stable & soluble organic complexes with Clean Guard 70F02.
The dispersed deposits shall be taken out during flushing of the system with water.
Water samples are taken regularly until analysis indicates that the system has been cleaned and restored to full operational capacity.
The rising levels of iron can be regularly monitored to determine exactly when the cleaning has been completed.
Cleaning chemical should be circulated in the system with circuit functioning normally.
Maximum you can circulate for 72 hours or minimum 24 hours prior to draining and flushing.
Clean Guard 70F02 will be circulated in the system until the sample indicate that no further increase in iron level is occurring prior to start to flush, a reference sample should be collected from the system in the sample bottle.
At this point, draining and flushing will be commenced and will continue until iron level is less than 2 ppm and all traces of the chemical and sediments are removed.
Back flushing of FCUs, AHUs, Chillers/ Heat Exchangers are to completed and connected with the main header one by one.
Dynamic flushing to be continued and then it is passivated with Clean Guard 70F03 @ 1kg/m³.
It should be circulated for 24 hours.
The system should be flushed until it is running clear, colorless, and odorless water and the iron level is less than 2 ppm.
Once the cleaning operation is completed with water in the system, the same should be treated with a corrosion & scale inhibiting treatment to prevent any such future problem.
Remove, clean & reinstall the strainers upon completion of chilled water system flushing.
Inhibition
When the system Clean Guard 70F02 has been satisfactory cleaned, it will be inhibited using the suitable chemical i.e. Corro Guard 33L01 at the rate of 2.5 kgs/ m³ of the system volume.
During this process, Corro Guard 33L01 is added at a dosage level of 2-3 kgs/m³ of water, a thin mono molecular layer of the inhibitor is formed on the metal surface.
This passivation layer prevents the dissolved oxygen from coming into the direct contact with the metal surface thus preventing the corrosion reaction from taking place.
After it has circulated for 24 hours, a sample will be drawn and tested to confirm whether an adequate inhibitor reserve has been established or whether an additional inhibitor is required.
The uniform film formed after addition of corrosion inhibitor is a make/ break film which exist on the metal surface till the residuals are present.
If the residuals fall below control limits (800-900 ppm) then the film ceases to exist and corrosion can start.
In order to maintain the control limits regular dosing is required using automatic or manual dosing techniques as applicable.
Biological Control
Chilled water systems are ideal breeding ground for bacterial and other microorganisms and it is necessary to control biological growth in the system.
Manufacturer recommendations entail shock dosing of biocide Bio Guard 40H06 @ 0.05 kgs/m³.
The product is dosed into the system using manual dosing pot and is dosed once every three months in order to control the growth of microorganisms.
Therefore, it is very important that residual levels are constantly monitored and maintained.