1、 1 Item No. 24229 NACE International Publication 11106 This Technical Committee Report has been prepared by NACE International Task Group 152* on Cooling Water Systems: Monitoring and Control Monitoring and Adjustment of Cooling Water Treatment Operating Parameters May 2006, NACE International This
2、NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes, or procedures n
3、ot included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liabil
4、ity for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness
5、 of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in
6、relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Users of this NACE report are also r
7、esponsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this report. CAUTIONARY NOTICE: The use
8、r is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may receive current information on
9、 all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200). Foreword The efficient and safe operation of a cooling tower system typically involves a substantial amount of routine monitoring of che
10、mical, physical, and microbiological phenomena. This technical committee report is intended for personnel directly responsible for daily operation and control of cooling tower systems, facility engineering and maintenance personnel, and water treatment company sales and technical staff personnel. Th
11、e purpose of this report is to provide a concise compilation of what are considered common practices in this area. Monitoring and control of cooling systems generally occurs in three phases: Initial system surveys, conducted after assuming responsibility for the management or operation of a new or u
12、nfamiliar cooling system; Monitoring and adjustment of cooling tower operating parameters during a campaign of operation; and Inspections and measurements of the condition of a cooling tower system during off-line periods such as outages or turnarounds. This report is specifically concerned with the
13、 second topicmonitoring and adjustment of cooling tower operation on a day-to-day basis during periods of routine operation. This technical committee report was prepared by Task Group (TG) 152 on Cooling Water Systems: Monitoring and Control. TG 152 is administered by Specific Technology Group (STG)
14、 11 on Water Treatment, and sponsored by STG 46 on Building Systems and STG 62 on Corrosion Monitoring and MeasurementScience and Engineering Applications. It is issued by NACE International under the auspices of STG 11. _ *Chair Michael E. Rogers, Alberta Technology Make-up water flow; Analytical t
15、ests of the circulating water; and Monitored performance parameters. The pump delivery rates are adjusted as needed to maintain the desired chemical levels in the water. Alternative methods for accomplishing this control are discussed below. Chemical testing is discussed in a later section. Feed Bas
16、ed on Blowdown This method, commonly called the “bleed and feed” method, typically involves a conductivity controller with outputs to drive one or more chemical pumps. The conductivity set point is defined by the desired cycles of concentration in the circulating water. When the conductivity rises a
17、bove this point, the blowdown solenoid valve opens, and the chemical pumps operate. When the conductivity drops below the set point, the solenoid valve closes, and the chemical pumps stop. Chemical levels in the water are determined analytically by methods described later, and the pumping rates are
18、adjusted to raise, lower, or maintain the desired levels. “Bleed and feed,” as this method is often called, is the simplest, least expensive, and most commonly used way to control chemical feed to cooling water systems. It is also the least reliable method. Anything that interferes with operation of
19、 the conductivity controller or the blowdown solenoid valve also interferes with proper chemical feed. When blowdown begins, make-up water typically does not flow until the water level in the basin drops sufficiently to activate the level control device (either a simple float or an electronic contro
20、l). Because the water level is controlled by evaporation as well as blowdown, make-up water flows without blowdown, causing chemical levels to vary. With careful testing and incremental adjustment of pumping rates, these variations can usually be kept within the established control parameters. In ma
21、ny cases, this is all that is used. Feed based on blowdown is applicable only in systems with consistent make-up water quality. Feed Based on Make-up A more precise way to control chemical levels in a cooling tower system is to feed based on make-up water flow. This method ensures that all make-up w
22、ater entering the tower is treated, regardless of blowdown, and that no treatment is fed when make-up water is not flowing. Make-up control of chemical feed helps to eliminate the overfeed and underfeed problems that are inherent in the “bleed and feed” method based on conductivity control. Control
23、of chemical feed based on make-up water flow is often accomplished using either a flow meter or a totalizing water meter. The simplest method is to include a sensing flow meter that sends an electrical signal whenever water is flowing. This signal is used by the tower controller to activate one or m
24、ore chemical pumps. As in the “bleed and feed” method, chemical levels are measured analytically and the pump stroke is adjusted to maintain the chemical dosage within range. This method, although it is about as simple as the “bleed and feed” method, is not often used. The cost is higher, the flow m
25、eter needs maintenance, and results are not as favorable as those achieved with a totalizing water meter. A totalizing make-up water meter is usually set to send an electrical impulse to a controller every time a preset volume of water passes through the meter. This impulse is then used by the contr
26、oller to trigger one or more chemical pumps that operate for a preset time. With a totalizing water meter, there are three methods available for adjusting the chemical feed rate: 1. The amount of make-up water flow used to trigger the chemical pumps; 2. The pump strokethat is, the amount of chemical
27、 delivered per stroke; and 3. The time that the pump operates during each delivery sequence. The water meter is sized for the system, and the pulse frequency is not normally adjustable. Properly sizing the meter and incrementally adjusting the pump stroke and timing normally keeps inhibitor dosage l
28、evels well within control ranges. Barring sudden changes in operating load or timing, frequent adjustments in pump parameters are not typically made. As a further refinement of totalizing make-up water meter control, special chemical pumps that deliver an accurate NACE International 8 preset volume
29、of chemical with each pump cycle are available. This method eliminates the need for pump stroke and timer adjustments and typically results in a constant rate of chemical delivery based on make-up water flow. As with other methods, analytical tests of the circulating water are used to set the chemic
30、al delivery rate. This is the most precise method for feeding chemicals based on make-up water flow. It is readily adaptable to automated control, and it is also the most expensive method. Accurate feeding by this method depends on accurate control of concentration cycles. Feed Based on Performance
31、Indicators Another method of on-line control utilizes fouling simulators, corrosion detectors, and performance parameters such as oxidation reduction potential (ORP) in combination with make-up flow(s) and other on-line nonperformance monitors such as pH and conductivity, plus product detection anal
32、yzers. These analyzers are configured to incorporate algorithms that adjust the feed of corrosion inhibitors, pH adjustment chemicals, deposit inhibitors, and microbiocides. Feed forward and feed back trims are also incorporated to achieve the desired cycles of concentration, as well as fouling and
33、corrosion set points. A difficulty with the use of this method is that effects of dosage changes on corrosion and scaling are typically quite slow, introducing significant lag into the control. The algorithms contain effective compensation for this lag time, as well as a verification function of the
34、 nonperformance-based monitors data acquisition. Test devices used in connection with these systems typically measure the corrosion or fouling tendency of water on a simulated surface or probe. The assumption is that the simulator is configured and operated to reflect the behavior of the water in th
35、e production equipment. This assumption is not always valid. There are currently no accepted industry standards in this area. Chemical Methods of Inhibitor Dosage Control Monitoring and control of corrosion and fouling inhibitor levels in cooling water systems typically involve some analytical testi
36、ng of the circulating water. The method of analysis has traditionally been based on manual laboratory techniques. However, much progress has been made in the development of automated, on-line chemical analysis techniques. The results of these tests are usedeither directly or indirectlyto control the
37、 feed of chemicals to the water. It is therefore very valuable to know exactly what is being tested, how the results are expressed, and what the data mean in terms of levels of total product and active components in the water. These topics are discussed in this section of the report. Tracers and Act
38、ive Ingredients Tracer chemicals, intended only for dosage rate measurement and control, are sometimes added to water treatment products. Ideally, tracer chemicals are soluble over the range of cooling water conditions, and are inert and unreactive in the system. That is, they do not precipitate or
39、in other ways react with other water treatment chemicals or components of the water. They do not take part in any corrosion- or scale-inhibiting reactions, and are not attacked by oxygen, chlorine, or other aggressive chemicals. Obviously, simple and reliable analytical methods are available for any
40、 tracer chemical. A chemical that meets these conditions is sometimes used to control the dosage rate of scale or corrosion inhibitor formulations. The water treatment vendor often establishes a control range for this chemical. Examples of such tracers are low levels of molybdate, vanadate, and fluo
41、rescent molecules used in on-line control. Molybdate, usually in the form of sodium molybdate, is also used as a corrosion inhibitor. There is often a distinction between the measurements of tracers and active ingredients. That is, the tracer measures the amount of total product fed into the water.
42、It does not measure the remaining amount of any specific active component of the product. Active ingredients in a formulation can precipitate, will be absorbed on surfaces, chemically degraded, or otherwise consumed in the cooling system. To the extent that these processes occur, there may be differ
43、ences between the “theoretical” active ingredient concentration determined by the tracer and the actual levels seen by chemical analysis. If the differences are excessive, it can indicate a performance problem. If tracer chemicals are used for routine monitoring and control of treatment chemical lev
44、els in cooling water, these analyses are supported by periodic analyses for active components. This activity is normally performed by water treatment vendor personnel during regular service visits. On-Site Water Analyses To be useful in practical field situations, analytical tests are typically fast
45、, simple, and usable by personnel who are not trained chemists. These tests are reliable over a range of concentrations normally found in circulating cooling water, and in the presence of common dissolved and suspended impurities. These tests are used to control the concentrations of both chemical s
46、pecies in the water and active materials that are often added for corrosion and mineral scale control. Reagents and procedures for field water analysis are typically provided by water treatment vendors. They are also available from other sources such as those outlined by L.S. Clescerl, et. al.3 Unit
47、s of Expression When interpreting the results of water analysis, it is important to remain aware of the units used to express concentrations. It is common in the water treatment industry to represent concentrations in units of milligrams/liter (mg/L) or parts per million (ppm). Mg/L is a NACE Intern
48、ational 9 unit of mass per unit of volume, while ppm is a unit of mass per unit of mass. These two units are equivalent only in dilute water solutions where the density of the solution is very close to 1, as in most cooling water systems. It has become common practice to represent all forms of hardn
49、ess and alkalinity (Ca+2, Mg+2, HCO3-, CO3-2) in units of mg/L or ppm of CaCO3. This practice dates from the early days of water treatment, when a common unit of measure was needed to express these concentrations for water softening calculations. For example: The concentration of calcium in cooling tower water is usually represented as mg/L of Ca+2or as mg/L of CaCO3. The equivalent weight of the Ca+2ion is 20 (atomic weight at. wt. divided by charge 40 2). The equivalent weight of CaCO3is 50.05 (molecular weight divided by charge of component
