Management of Cooling Towers

Guidance

The cooling system may consist of a cooling tower, evaporative condenser or other cooling element, the recirculating pipework, the heat exchanger, pumps and ancillary items such as supply tanks and pre-treatment equipment. All of these items should be subject to the management and control system. The ACOP says risks from legionella should be identified and managed. The following section on commissioning, operation and maintenance of cooling towers offers a guidance on some of the issues which need to be addressed in order to do this.

Commissioning

Systems should be properly commissioned so that they operate correctly and within the design parameters. It is essential that the commissioning process is carried out in a logical and defined manner.The responsibilities of the staff carrying out the commissioning process should be clearly defined with adequate time and resources allocated to allow the integrated parts of the installation to be commissioned correctly.The same precautions taken to prevent or control the risk of exposure to legionella during normal operation of cooling systems also supply to the commissioning process.
When commissioning (or recommissioning) a tower, the following points should be noted.
If a new system is not to be taken into immediate service, commissioning should not be carried out until the system is required for use and should not be filled until commissioning takes place (if filled for hydraulic testing, then the system should drained and not refilled until commissioning takes place).

If a new system is to be taken into use within a week, commissioning can be carried out and the system left charged with treated water which should include a biocide. This is equally important when recommissioning existing installations which have been substantially altered following a major design or modification.

The results of the commissioning process should be included as a section in the operation and maintenance manual.The availability of such baseline data enables periodic checks to be made to show that the installation continues to operate as intended.
Formal arrangements should be made to check that commissioning has been completed to the standard specified, e.g. an independent engineer witnesses the testing and countersigns the relevant documents.

Operation

Cooling systems and towers should be kept in regular use wherever possible. Where a system is used intermittently or is required at short notice, it should be run once a week and, at the same time, be dosed with water treatment chemicals and water quality monitored. The whole system should be run for long enough to thoroughly distribute treated water. If a system is out of use for a week or longer (up to a month), in addition to the above, the water should be treated with biocide immediately on re-use.
If it is out of use for longer than a month and there are continued management/ monitoring arrangements in place, the system should be kept full of treated water which should be checked (for biocide levels and water quality) and circulated once a week (see also paragraph 135). If it is not possible to ensure regular monitoring and circulation (for example, if a building falls out of use), the system should be drained and sealed, with a desiccant left in the system to reduce the effects of corrosion. Full recommissioning will be required before the system can be bought back into re-use. Cooling systems that do not operate continuously, such as cooling towers that cycle on and off automatically or those on regular standby duty, require particular attention with regard to the biocide programme to ensure that effective levels of biocide are maintained at all times.
Operation manuals should be available for each water system. These manuals should detail, in easily understood terms, operation and maintenance procedures which enable plant operators to carry out their duties safely and effectively.
The manuals should include equipment as fitted and represent the system as currently in operation, and include (also as fitted) system drawings and/or schematics, manufacturers' instructions for operation and system parameters such as capacities, throughputs and design temperatures. The total volume of the entire water circuit, i.e. tower pond, recirculation pipework and heat exchange equipment, should be known and recorded.
Specific information on the water treatment programme in use should be included. Where automatic dosing equipment is used, there should be a means of confirming that treatment is being applied. Irrespective of the dosing method, both the quantity and frequency of chemical application should be recorded.
Such records should be expanded to:
* include the results of system monitoring; and show any action required and confirmation that this has been carried out.

Manuals should include details of:
* normal control parameters;
* limits, with corrective actions, for out-of specification situations, or where plant operating conditions or make-up water quality have changed; and cleaning and disinfection procedures.
Where automatic controls are employed, either for chemical addition or to allow system bleed-off, they should be checked over their full operating ranges. In the specific case of conductivity controlled bleed-off, regular calibration of the conductivity cell should be carried out.
Standby equipment, such as towers and recirculating pumps, should operate on a rota basis e.g. daily on/off, or otherwise isolated and held dry. If there are standby cooling towers, specific procedures will need to be adopted to bring then into operation safely. When a biocide is added to a water system, all standby equipment or pipework should be brought into circulation so that the biocide is distributed throughout the entire system.

Maintenance

The operations manual should include a detailed maintenance schedule which lists the various intervals when the system plant and water should be checked, inspected, overhauled or cleaned. Provision should be made for the completion of every tasks to be recorded by the plant operatives.
Drift eliminators require particular attention with regard to maintenance so that aerosol release continues to be controlled. They should be inspected, cleaned and maintained to ensure that they are free from biofouling, corrosion, scale and other deposits and are well seated and undamaged.

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