Managing Cooling Water: Chemistry Control
This article, which explores the reasons why you should manage your industrial cooling water, is the second in an occasional series on water management basics and technologies.
Despite the importance of properly functioning cooling tower systems, operational control of cooling water treatment programs frequently is neglected, and thus is the single most common cause of program failure. The best possible combination of corrosion, scale and deposition control chemicals, with effective biocides, is completely worthless if not consistently and correctly applied to the cooling water.
Cooling water chemistry control begins with cooling water cycles, or the number of times that the dissolved salts in the fresh makeup water are concentrated by evaporation from the cooling system. This parameter commonly is obtained by measuring the conductivity of the cooling water and dividing it by the measured conductivity of the makeup water. Cycles also can be calculated using other parameters such as chlorides and dissolved solids. They are common to both the makeup and cooling water, and are not expected to be affected in any great degree by chemical additions or precipitations, or by makeup water and blowdown water volumes.
Control of cycles is critical in systems using nonsoftened makeup water. No chemical treatment program can cope with the excessive levels of hardness salts in cooling water resulting from very high cycles of concentration. Excessive levels of hardness salts in cooling water result in increased potential for scale formation. Cycles are controlled by discharging, or blowing down, concentrated water from the system and replacing it with fresh water. Blowdown, as this discharge is called, lowers the system concentration of dissolved solids.
One may ask: “Why use chemical treatments to operate at additional cycles when often no scale will form when operating at lower cycles?” The best answer is that operation at increased cycles substantially lowers both the makeup and blowdown requirements, cutting the cost of fresh water and sewage disposal. With corrosive waters, increasing the cycles so that the water is rendered less corrosive is an inexpensive means to improve corrosion control. Another point is that operation at increased cycles permits use of effective corrosion inhibitors that may be too costly to employ at the higher blowdown rates resulting from low-cycle operation.
Occasionally, the quality of the makeup water is so bad that a scale inhibitor must be employed just to use the water. In such cases, cycling reduces the cost of the scale inhibitor to an economic level.
Environmental requirements and insufficient amounts of fresh makeup water are new forces driving cooling tower operation to higher cycles. Increasing the cycles provides a simple, economic solution. In one example, sewer-tap fees at a new plant were about $100,000, but were reduced to $10,000 simply by increasing cycles from three to six. Environmental agencies also are taking a much closer look at facility water use. One recent National Pollution Discharge and Elimination Systems (NPDES) permit in Arizona contained a condition that the user operate at 17 cycles in order to reduce its fresh water consumption.
Maximum economic cycles are the cycles' value at which the total operating cost for the entire program is lowest. This value is based on the costs for water, sewerage, and chemical treatment for each specific application. One unique set of parameters consisting of cycles, inhibitor chemistry, inhibitor dosage, makeup and blowdown can be found for each makeup water/facility combination that gives the lowest total operating cost.
The first step in selection of maximum economic cycles -- and many times, the basic treatment program -- is to calculate the calcium-carbonate saturation index (SI) of the makeup water. Prior to cycling, the SI is utilized to determine if the makeup water is scaling or non-scaling. The makeup water is then cycled over a typical range of two to 10 cycles, and the resulting SI, makeup rate, blowdown rate, inhibitor chemistry needed, and chemical dosages are calculated to determine the maximum economic cycles. These calculations can be accomplished using a computer program or with a hand calculator.
The calculation of maximum economic cycles is complex and usually is justified only for larger facilities. In the majority of cases, using a chemistry that permits three to six cycles operation results in a total operating program cost close to the absolute minimum cost. Specific product selection for control of scaling is then based upon the SI at the selected cycles.
Two methods have been found to yield the best control of cycles. The first, which is required when makeup-water chemistry varies substantially or the facility has uncontrolled water losses, is to use an automatic control system. The system measures the conductivity (which is proportional to the level of dissolved salts) of the cooling water. When the conductivity reaches the determined control level, an automatic valve activates and high dissolved-salt-content water drains from the cooling water system. Replacing this blowdown with new makeup water lowers the conductivity of the cooling water and deactivates the automatic valve.
The second means is the makeup proportional method, which measures the amount of makeup water added to the cooling system, then drains a proportional amount of blowdown from the system via an automatic valve controlled by a timer activated by a makeup-water meter. To operate at 10 cycles, approximately 100 gal of blowdown is drained for every 1,000 gal of makeup water added to the cooling water system.
While this method is only applicable when the makeup water is of fairly consistent quality and the system leakage is low, it yields the same degree of control and is less costly to install and maintain than a conductivity-based system. An additional advantage is elimination of the conductivity sensing electrode, which requires routine maintenance.
Many methods have been used over the years to control the addition of chemical inhibitor products to cooling systems. Common practices have included manual batch feed, timer-controlled feed, constant feed, simultaneous blowdown/chemical injection (feed/bleed), proportional feed and active-control feed (where a relevant parameter is measured and inhibitor feed used to maintain a setpoint).
For many plants, the best chemical-inhibitor control method is the one using feed proportional to either makeup or blowdown flow. All chemical products, with the exception of biocides, are dosed according to the amount of blowdown from the cooling system, which is directly proportional to the amount of makeup added.
Proportional chemical-feed systems are quite simple. They are based upon metering the amount of makeup or blowdown water flow. They activate a chemical metering pump via a timer to add an amount of inhibitor proportional to the amount of makeup or blowdown. With makeup water generally being higher quality than blowdown, it is best to meter the makeup to reduce meter-maintenance problems.
Computerized and online control systems also are marketed. In my opinion, they have no technical advantage over the basic control methods discussed above.
Biocide feed demands that users remember that the dose makes the poison. In other words, there is a dosage, or toxic threshold, below which a biocide will not work. The critical dosage point and time required for effective microbiological control varies substantially depending on the biocide. In addition, microorganisms adjust very quickly to toxins in their environment. Constant use of a single biocide (except oxidizers) merely results in establishing a resistant flora in the cooling system being treated.
The best method for adding biocides is on a slug (intermittent) dosage basis using the system volume to establish initial dosage. Slug dosage can be accomplished either manually or automatically using timers and pumps. Excellent results can be obtained by either method, although health and safety considerations in the handling of toxic biocides have increased the use of automatic feed systems and on-site oxidizing biocide generation. PCE
Sidebar: Operator Attention
While automatic systems for control of cycles, chemical feed and biocide addition will improve the probability for success of any cooling water treatment program, a certain level of attention by plant operations personnel is required. Replacing empty chemical drums, finding water leaks and repairing leaking areas only can be done by an operator. A daily check of the conductivity and makeup meter readings is necessary, as well as a visual and operational check of the chemical feed pumps and drums.
Chemical tests of the cooling water should be made and logged at least once a week by the operator to ensure that proper levels of treatment chemicals are being maintained in the cooling water. Larger plants may benefit from an increased amount of testing due to the potential costs of upsets.
A technical representative of the treatment-chemical supplier should visit the plant on a routine basis to check operation of the automatic systems, undertake his own chemical tests of the cooling water using reagents, and assist the plant operator with any problems.
Even if telemetry or computer-based controls are used, someone still must prime the chemical pumps, change out the empty drum, or track down the leak. Trained operators and experienced service personnel are vital to obtaining good cooling water system operation.
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