1、Item No. 24206 NACE International Publication 7H100 (2011 Edition) This Technical Committee Report has been prepared by NACE International Specific Technology Group (STG) 11,* “Water Treatment.” Evaluation of Boiler Tube Deposit Mass Loading (Deposit Weight Density) Methodology March 2011, NACE Inte
2、rnational This NACE International (NACE) 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, proc
3、esses, or procedures not 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
4、 anyone against liability 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
5、 negate the usefulness 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 t
6、heir applicability in 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
7、NACE report are also responsible 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. CAUT
8、IONARY NOTICE: The user 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 NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old.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
9、current information on 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 purpose of this technical committee report is to provide information on the current methodology for m
10、easuring the mass per unit area of deposits in boiler tubes (commonly called “deposit weight density” or DWD). The report describes the common methods of removing and measuring the deposits in boiler tubes. It also contains data collected by several laboratories during “round robin” testing of actua
11、l boiler tube samples using the common methods. All those involved in the recommendation, evaluation, and performance of DWD should find this report useful. This report compares and evaluates different methods of determining deposit weight density. This report does not address the use of such data t
12、o determine boiler cleaning need. This report was originally prepared by Work Group T-7H-6f, “Deposit Weight Density Methodology,” a component of Task Group T-7H-6, “Failure Analysis Boiler Waterside”, under Unit Committee T-7H, “Corrosion and Its Control in Steam-Generating Systems.” It was reaffir
13、med in 2011 by Specific Technology Group (STG) 11, “Water Treatment.” It is issued by NACE International under the auspices STG 11. _ *Chair Eunice Murtagh, GE Power (b) chemical solvent cleaning; and (c) glass bead blasting cleaning. The first two methods are described in ASTM(2) D3483.2 The third
14、procedure is a more recently developed method that is similar in approach to mechanical scraping but utilizes a different medium.3 DWD Methodologies Sample Selection and Preparation Deposit accumulation is often heaviest on tube surfaces that experience the highest heat transfer, but representative
15、areas depend on boiler design and operating conditions. Samples are often selected from areas near the top burner elevation in the center of a division wall, side wall locations near the top burner elevation, near the center of the burner walls, in the first pass of once-though boilers with multipas
16、s circuitry, areas where the boiler circulation is sluggish, or other high heat-transfer sites designated by the boiler manufacturer or having a troublesome operational history, such as hydrogen damage. Each representative boiler tube sample is typically identified by showing the location of the tub
17、e, the direction of coolant flow, and the hot and cold sides in accordance with ASTM D887.4 All three deposit removal methods assume that the boiler tube sample has been cut without the use of any lubricating fluids. If a cutting torch is employed to remove a tube from the boiler, a sample at least
18、3 ft (1 m) long minimizes deposit contamination from torch spatter. If the boiler tube sample has been removed via dry sawing or grinding wheel, a sample with a minimum length of 2 ft (0.7 m) is in accordance with ASTM D3483, although shorter (6 to 12 in 150 to 300 mm) test specimens are often used.
19、 The tube ends of the test specimen are sealed to avoid the loss of internal deposits, if the DWD is not determined at the original boiler site. Handling of the test specimen is important, because rough handling can dislodge deposits and introduce errors. The removed deposit mass over a given tube a
20、rea can be measured directly, as in the mechanical scraping method, or determined indirectly by subtracting the weight of the test specimen after complete deposit removal from the test specimen weight before deposit removal, as used in the solvent and bead blasting methods. Because boiler deposit de
21、nsities can vary considerably from neighboring tubes or even within the same test specimen, very small deposit removal areas (2 to 3 in2 13 to 19 cm2) tend to skew DWD values higher or lower. Very large test specimen areas (1) Electric Power Research Institute (EPRI), P.O. Box 10412, Palo Alto, CA 9
22、4303. (2) ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959. NACE International 3 (e.g., 20 in2 130 cm2) tend to require longer times for DWD determinations and to lower deposit densities through averaging. Typically, minimum and maximum areas for DWD determination are
23、 specified for each of the removal methods. The specific deposit mass loading (DWD) is expressed as the deposit mass divided by the area from which the deposit was removed. The specific deposit weight or deposit density results are usually expressed as (1) grams per square foot; (2) milligrams per s
24、quare millimeter; or (3) milligrams per square centimeter. Conversion factors for the units of deposit density are as follows: mg/mm2 to mg/cm2multiply by 100 mg/mm2 to g/ft2multiply by 92.9 mg/cm2 to g/ft2multiply by 0.929 None of the DWD results alone provides a measure of the deposit thickness. T
25、he deposit or scale thickness can provide additional information to determine boiler cleaning requirements. However, the degree of deposit porosity, thermal conductivity, adherence to the tube substrate, bulk deposit composition, distribution of individual deposit constituents, water chemistry, and
26、boiler operating conditions and pressure are also important parameters in the decision-making process for possible chemical cleaning. Deposit thickness can be estimated by a scale thickness indicator during on-site testing in the boiler. Alternatively, a point micrometer can provide readings before
27、and after deposit removal procedures to obtain a good estimate of deposit thickness. A more complete method involves mounting selected test specimens in epoxy and measuring the thickness and morphology by optical microscopy. A waterwall deposit can vary in its complexity and material components. The
28、 deposit can be analyzed quantitatively by wet chemistry methods. Scanning electron microscopy/X-ray energy dispersive spectroscopy can provide additional information by mapping the in situ elemental distribution and observing the morphology of the boiler deposits. Optical spectrographic techniques
29、can be used to identify many species semi-quantitatively. X-ray secondary fluorescence is commonly used for the identification of elements that might be present in the deposit. X-ray diffraction can identify crystalline species in the deposit. ASTM D3483 specifies drying the deposit in an oven at 10
30、5 C (221 F) for one hour, but usually such drying is avoided unless noticeable moisture is observed in the deposit. Deposits can range from primarily monolithic scales to highly banded structures having multiple alternating (e.g., alternating iron-rich and copper-rich) deposit layers with a total th
31、ickness of 0.050 in (1.3 mm) or more. Atwood and Hale5 identified five major types of deposit based on their characteristics. The representative deposit areas and unusual features are often photographed before and after deposit removal. Mechanical Method for Deposit Removal The following procedure i
32、s based on ASTM D3483, Test Method AMechanical Removal.2 A suitable representative tube sample (6 to 24 in 150 to 610 mm) is selected. The fireside (hot) half of the tube is cut from the casing (cold) side by cutting longitudinally without any lubrication. The external fireside surface scales are gr
33、ound off or covered with masking tape to prevent contamination of the waterside deposits. In addition, any metal chips introduced by cutting are removed with a magnet or other suitable device without disturbing the tube deposit. The boundaries are marked for deposit removal on the respective interna
34、l hot and cold side surfaces. The selected hot waterside surface is typically the most heavily fouled area. ASTM D3483 recommends a minimum tube length of 6 in (150 mm) be selected for DWD, but often the deposits are removed from sections as small as 1 to 3 in (25 to 76 mm), depending on conditions
35、such as the test specimen diameter and the variability of deposit loading. Although many DWD values are based on 1 to 3 in2 (6 to 19 cm2) areas, some laboratories have selected 9 to 12 in2 (58 to 77 cm2) as a routine area to improve DWD results due to possible deposit variations that can exist along
36、 tube surfaces. The deposit is first removed by brushing. Often, the bulk of the deposit is removed by careful scraping with a screwdriver, curved scalpel, or spatula followed by removal of harder and more tenacious oxides or scales with a vibratory (hand engraver) tool. After the deposit is removed
37、 fully within the designated tube area, the collected deposit is weighed to the nearest 0.1 mg. The area of deposit removal can be measured directly or by using a pattern made NACE International 4 from a clean piece of graph paper. The DWD is determined by dividing the weight of the removed deposit
38、by the corresponding removal area. While complete removal of deposits from straight-bored tubing has not proved difficult for experienced personnel, a residue of deposit can remain on metal surfaces when less experienced operators attempt to remove deposits, leading to undervaluing of the true depos
39、it loading. It is difficult to remove all deposits from rifled tubing due to inaccessible areas where deposits cannot be adequately removed by a spatula or vibratory tool. One boiler manufacturer has utilized an apprentice system that has allowed only three persons to remove deposits mechanically ov
40、er the last 30 years in order to maintain quality assurance and reliability of the DWD results. In summary, the advantages of the mechanical (scraping) removal method are: Minimized tube metal loss and deposit contamination are easily obtained by the scraping procedure. Removal personnel can obtain
41、a “feel” of the deposit structure and the deposit adherence to the tube substrate. The weight of the deposit removed from the representative area is measured directly. Deposit removed by the mechanical method can be retained for subsequent analysis. Proper deposit removal can be obtained with a mini
42、mum of capital expense. Disadvantages of the mechanical method are: It is the most labor-intensive and the most time-consuming (particularly with large sample batches) of the DWD removal methods. The DWD values are dependent on operator experience; inconsistent (generally low) DWD values are sometim
43、es obtained by inexperienced personnel when certain scales are dense and tenaciously bonded to the test specimen tube surface. Deposits from rifled tubing are difficult to remove completely. Solvent Method for Deposit Removal The following procedure is based on ASTM D3483, Test Method BSolvent Remov
44、al2 and a description by Atwood and Hale.5 In practice, a 2 in (50 mm) ring with the highest deposit accumulation is selected and cut from an initial tube section 6 to 24 in (150 to 610 mm) long. After the center of the fireside (hot) portion of the boiler tube is identified, a lathe is used to turn
45、 down the outer tube diameter to bare metal, thereby removing all outer deposits and allowing for greater sensitivity in test specimen weighing. The tube ring is cut or milled to separate the fireside half from the casing (cold) half; any metal chips are carefully removed. Each tube half is weighed
46、to an accuracy of 0.1 mg before solvent cleaning commences. The test specimen is cleaned by immersing it in a beaker of inhibited 10% hydrochloric acid cleaning solution held at 71 C (160 F). ASTM D3483 does not indicate a specific time necessary for complete removal by test specimen immersion. Atwo
47、od and Hale state that one to two hours are usually needed to remove most deposits completely, although heavier deposits sometime require longer exposure times. Esmacher3 suggests that by wire brushing the deposit from the tube surface after only 5 to 10 minutes of immersion and then observing the t
48、est specimen surface, total immersion time can be reduced. NACE Work Group T-7H-6f identified the need for guidelines for determining the end-point when the deposit is completely removed by the solvent method during CORROSION/92; the end point can be a significant variable with the potential to affe
49、ct DWD results. If test specimens show the presence of copper plating, they are immersed in an ammonium persulfate solution (10 g/L) until all indications of plating disappear. After cleaning is complete, the test specimens are washed in warm water, neutralized in sodium carbonate solution (10 g/L), rinsed in warm (50 to 70 C 122 to 158 F) water, and dried with acetone. Because the solvent method involves an acid that, if poorly inhibited, can dissolve and corrode all NACE International 5 exposed tube steel surfaces, the
