Controlled-volume diaphragm metering pumps offer a practical and efficient way to inject precise amounts of chemicals for cooling tower water treatment.



All closed-loop cooling towers entrain a flow of water that must be treated continuously to maintain the maximum operating efficiency.1 Water is the bioreactor of life and is both a catalyst for the growth of organic material, especially at the various temperature gradients found in cooling towers, and a medium for inorganic particle circulation. A buildup of excessive suspended matter in the circulating water of the cooling tower eventually will find flaws in the tower piping and cause blockages and other problems in the piping and pumping apparatus of the tower. This situation will reduce the overall ability of the tower to exchange heat and lower the overall efficiency of the heat transfer.

Additionally, in any tower, makeup water must be added periodically to offset general system losses. This makeup water affects the pH of the water in the system. If the pH is too acidic or alkaline, it can cause corrosion on galvanized cooling towers. As a result, any cooling tower in which the water is left untreated will have high maintenance and energy requirements.

Successful water treatment seeks to mitigate both organic and inorganic circulation through biological remediation and settling of inorganic particulates, while also controlling the pH of the water. For most closed-loop cooling towers, unless otherwise specified by the tower manufacturer, the most practical and efficient method is to inject biological control chemicals using a controlled-volume diaphragm metering pump (generally referred to simply as a metering pump). A metering pump can provide precise dosing of specific volumes of chemicals automatically on a timed or as-needed basis for both natural and mechanical draft systems.

The compact nature and simplicity of the metering pump makes it an attractive method for introducing water treatment chemicals to a cooling tower.

Most pumps that are included in a cooling tower system are within the piping network and make up part of the overall system. The controlled-volume diaphragm metering pump is the only pump that is not directly part of the system, but rather adds to it. For this reason, the pumps should be placed against the system for maximum efficiency and ease of maintenance.

The metering pump uses a diaphragm composed of a chemically compatible material to draw the chemical from a supply on an intake stroke and inject it into the process on the discharge stroke (figure 1). Check valves keep the volume true. Backpressure from the system, along with the changing pressure from the diaphragm movement, keeps the check valves in place. Most manufacturers use this type of arrangement as it has proven to be the most reliable and consistent in the metering pump industry.

The stroke or pulse of a metering pump provides an exact volume of chemical each time. Summing the number of strokes or pulses provides an overall volume of chemical over time, know as flow rate. The rate at which chemical is introduced can be controlled by the number of pulses or strokes, or the length of the stroke itself. Most manufacturers design their pumps to be proportional for the stroke length to the volume introduced to the process stream.2 This characteristic makes the metering pump a successful device for chemical addition, allowing for accurate dosing and simple fine tuning of dosing quantities.

Cooling Tower Chemical Treatment

The main determination for using chemicals to treat the water in a cooling tower is water entrainment. The fact that the water volume is considered fixed, or entrained at a volume, within a closed-loop system indicates that certain physical factors are present:
  • There is a pressure in the system that must be overcome to get the chemical into the process.
  • The proper amount of chemical required to remediate any undesirable elements in the water must be added to maximize tower operation.

Metering pumps can meet both of these requirements.

Although the pressure in cooling towers is not as high as in boilers, there is considerable resistance from the flow of water as it moves through the piping system. In small tower operations, it is generally sufficient to add chemicals through an injection point as long as the pump pressure is at least 5 psi greater than the transport pressure of the water being circulated. In larger towers, spring-loaded injection quills can provide a sufficient injection point and disperse the chemical, so it spreads through the flow profile on a more even basis, providing maximum distribution within the process flow.

The volume of chemical also is an important component when determining water treatment requirements. For biological growth control, chemical manufacturers will provide a recommended dose of chemical proportional to the anticipated quantity of biology that requires remediation. This recommendation is translated into a dosing rate that the metering pump can deliver automatically.

For most closed-loop cooling towers, the most practical and efficient water treatment method is to inject biological-control chemicals using a controlled-volume diaphragm metering pump.

In the case of pH adjustment, the dose is determined based on the water quality, with the goal of achieving a pH as close as possible to 7 (neutral). Water quality tests can be performed either with batch detection using litmus paper test kits or with online instrumentation directly attached to the metering pump. The latter method can provide an instant alert when a problem with water pH is detected. The metering pump then can be used to meter the correct chemical dosage into the system. This method is referred to as flow-pacing and is the most accurate way to add a volume of chemical treatment to the cooling tower process system.

In conclusion, the compact nature and simplicity of the metering pump makes it an attractive method for introducing water treatment chemicals to a cooling tower. The stroke of the chemically compatible diaphragm overcomes the relatively small system pressure to provide an accurate volume of chemical treatment to a process. The precise chemical additions are made automatically as needed, rather than randomly adding or injecting an unknown or inexact quantity of chemical to perform the desired task. Additionally, instrumentation can be connected directly to the pumps to provide flow-paced dosing based on actual changes in the water chemistry. By using a metering pump, a plant can achieve the maximum benefit from its cooling tower water treatment program with minimum effort.

References

1. Herro, H. and Port, R., Nalco Guide to Cooling Water Systems Failure Analysis, McGraw-Hill Inc., New York, 1993, p. 83.2. McCabe, R., Lanckton, P., and Dwyer, W., Metering Pump Handbook, Industrial Press, 1984, p. 139.3. Harrison, J., Water Treatment Fundamentals, Water Quality Association, 2004, p.60.

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