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Utilities Don’t Get SteamedAvoid plant downtime and potential injuries by addressing steam generation chemistry issues Brad BueckerAbstractHigh purity water and the steam produced from it constitute the lifeblood of most process plants. In this articlethe author examines several of the most important issues related to proper water treatment and chemistrycontrol in steam generators.High purity water and the steam produced lems. These become more se-vere as boiler pressures from it constitute the lifeblood of most pro- and temperatures increase. Fortunately, the power in- cess plants. Equipment failures and curtailed dustry has learned some lessons that directly apply toproduction due to water/steam issues can cost a site chemical plants, particularly ones that generate high-hundreds of thousands of dollars or more annually. pressure steam for process needs or electrical genera-Much worse, some failures can cause injury or death. tion. For example, Tables 1 and 2 summarize guidelinesSo, here, we’ll examine several of the most important developed by the Electric Power Research Instituteissues related to proper water treatment and chemistry (EPRI) for makeup water effluent and condensatecontrol in steam generators. pump discharge (CPD) for heat recovery steam gener- ators (HRSGs) [2]. Let’s begin with a case history. A number of yearsago, a colleague and I visited an organic chemicals An examination of the effects of some of these im-plant in the Midwest that every two years or so had to purities reveals why the limits are so low. Considerreplace the steam superheater bundles in four 550-psig chlorides. Even small amounts that enter the steampackage boilers due to internal scaling. We first were generator, say, from a condenser tube leak or contam-shown a recently removed bundle; roughly ¼-inches inated condensate return, if chronic and not neutral-thick deposits covered the internal tube surfaces. We ized by the boiler-water treatment program, will con-then inspected the boilers and immediately noticed centrate under deposits on boiler internals. Chloridefoam issuing from the saturated steam sample lines.Subsequent investigation revealed that total organ- Table 1. Conductivity and sodium measurements canic carbon (TOC) levels in the condensate return to the warn of contaminant ingress to the steam generator.boilers sometimes reached 200 ppm — ASME guide-lines [1] call for a maximum TOC concentration of 0.5 Normal Chemistry Limits for HRSG Makeup Water*ppm in boilers of this pressure. So, it was easy to seewhy much foam existed in the boiler water and why Parameter Normal Limitimpurities carried over to the superheaters on a con- Silica (C) <10 ppbtinual basis. Specific conductivity (C) <0.1 µS/cm Total organic carbon (P) <100 ppbThe Impact of Impurities Sodium (C) <2 ppb Chloride (P) <2 ppb Impurities cause corrosion, scaling and other prob- Sulfate (P) <2 ppbBrad Buecker is a process specialist for Kiewit Engineering & Design, Lenexa, Kan. * Adapted from Reference 2. C = continuous sampling and p = peri-E-mail: [email protected]. odic samplingChemical Industry Digest. March 2017 65

Utilities with either polishing mixed-bed ion exchange units or electrodeionization for final conditioning. Because RO membranes are very susceptible to particulate fouling, upstream filtration is required with micro- or ultrafil- tration increasingly popular for this purpose [4].Figure 1. Hydrogen gas molecules penetrate into the metal Chemical Treatment Issues wall — notice the thick-lipped failure here. Source: ChemTreat. A couple of decades ago, the common belief was that all oxygen should be eliminated from boiler feed-salts in the high temperature boiler environment can water, as otherwise it would cause severe corrosion.react with water per: This often is true when a unit is offline and air can en- ter the system. However, during normal operation, thisMgCl2 + 2H2O + heat k Mg(OH)2 m + 2HCl .......(1) idea has been proven false unless the condensate/feed- water system contains copper alloys. Regardless, this The hydrochloric acid produced may cause general belief led to a chemical program for feedwater condi-corrosion by itself — worse yet, the acid will accumu- tioning that became known as all-volatile treatment re-late under deposits, where it can react with iron to gen- ducing, AVT(R), with ammonia or amine feed to es-erate hydrogen. The hydrogen gas molecules penetrate tablish a mildly basic pH and reducing agent (oxygeninto the metal wall where they then combine with car- scavenger) injection to eliminate any oxygen that es-bon atoms in the steel to generate methane (CH4): caped a mechanical deaerator. For high-pressure units, the common reducing agent once was hydrazine but2H2 + Fe3 C k 3Fe + CH4 q .......(2) safer chemicals now have supplanted it. Formation of the gaseous methane and hydrogen It now is known that AVT(R) chemistry will inducemolecules causes cracking in the steel, greatly weaken- flow-accelerated corrosion (FAC) in feedwater systems;ing its strength (Figure 1). this can cause wall thinning (Figure 2) and ultimately lead to catastrophic failures. In the last 30 years, sev- Hydrogen damage is very troublesome because it’s eral FAC-induced failures in the U.S. have resulted innot easily detected. After such damage has occurred, fatalities. In brief, when steam generators go into ser-the plant staff may replace tubes only to find that oth-er tubes continue to rupture. I once was part of a team Fig. 2. Single-phase FAC caused the substantialthat had to deal with hydrogen damage in a 1,250-psig wall thinning shown. Source: ChemTreat.utility boiler. Operations personnel insisted on run-ning the unit for several weeks with a known condens-er leak. Even though the team did its best to maintainadequate boiler water chemistry, the ultimate outcomewas extensive hydrogen damage that required a com-plete boiler retubing. Measurements of conductivity and sodium are verystraightforward and are excellent for detecting contam-inant ingress to the steam generator. Of course, suchmonitoring only has real value if it leads to promptremedial actions by chemists or operators[3]. Organiccompounds, as already noted, can cause problems inthe steam generator and can break down at high tem-peratures to form small-chain organic acids and car-bon dioxide, which may significantly influence steamand condensate return chemistry. Meeting the makeup water guidelines requires a re-liable high-purity water treatment system. A very com-mon approach employs two-pass reverse osmosis (RO)66 Chemical Industry Digest. March 2017

UtilitiesTable 2. Organics can break down into small-chain organic acids that AVT(O). impact fluid chemistry. Regarding feedwater chemistry monitor-Normal Chemistry Limits for HRSG Condensate Pump Discharge* ing, the normal limits cited in Table 2 for cat-Parameter Normal Limit or range ion conductivity, pH and sodium for CPD apply. This is understandable because ma-pH (C) 9.2–9.8 ny modern industrial steam generators andSpecific conductivity (C) Consistent with pH established by virtually all HRSGs don’t have feedwa- ammonia or amine feed ter heaters; thus, the condensate chemistryCation conductivity (C) ≤0.1 µS/cm should change very little in its passage toDissolved oxygen (C) ≤20 ppb the steam generator. However, the suggest- ed dissolved oxygen range for HRSG feed-Sodium (C) ≤2 ppb water is 5–10 ppb. Also recommended is to-Total organic carbon (P) ≤200 ppb tal iron monitoring, preferably with a cor-* Adapted from Reference 2. C = continuous sampling and P = periodic sampling. rosion product sampler, to ensure the pro- gram, whether AVT(O) or an alternative, isvice, the carbon steel develops a thin layer of magne- adequately protecting the condensate andtite, Fe3O4. The combination of a flow disturbance and feedwater piping. With proper chemistry, the totalthe reducing environment causes iron ions to leach out iron content in the feedwater should remain less thanof the steel/magnetite matrix, resulting in the wall thin- 2 ppb. If, for some reason, AVT(R) is necessary, a corro-ning. Temperature and pH affect the extent of the dis- sion product sampler also will collect copper corrosionsolution; it usually reaches a peak at a temperature of products, which provide critical data on copper corro-about 150°C and increases with lower pH (e.g., Table 3. Monitoring of these parameters is essential for9 and below). protecting steam purity. So, the areas most prone to this attack are Recommended Hrsg Boiler-Water Monitoring Points*the feedwater/economizer system of conven-tional steam generators and the low-pressure Parameter Guidelinesand, sometimes, intermediate-pressure econo- pH (C)mizers and evaporators in HRSGs. See Reference 2 for a control chart. Dosage is based on TSP feed only, with a maxi- For units that contain no copper alloys in mum free caustic concentration of 1 ppm.the feedwater system — which almost always Immediate unit shutdown is required if theis the case for HRSGs — the recommend- boiler pH starts dropping and reaches 8.0.ed feedwater treatment has become all-vol- Phosphate (C) See note above.atile treatment oxidizing, AVT(O). This pro- Specific conduc-gram allows the small amount of oxygen that tivity (C) Measures the overall impurity level in the(normally) leaks in through a condenser to re- boiler water. Control range is unit- andmain, with perhaps even a bit of supplemental Silica (C) pressure-dependent.oxygen injected, such that the feedwater dis-solved oxygen levels stay in a 5–10 ppb range. Sodium (C) Can be coordinated with steam silica analy-Addition of ammonia or an amine maintains ses to prevent carryover to turbines. Controlthe pH in a mid- to upper-9 range. Under these range is unit- and pressure-dependent.conditions, the magnetite layer becomes inter- Important to determine the free caustic concentration.spersed and overlaid with a layer of ferric ox- Cation conduc- Provides a general measurement of an-ide hydrate (FeOOH) that, with elimination tivity (C) ions in the boiler water, including the veryof the reducing environment, is very protec- harmful chloride ion. Control range is unit-tive. However, this program only is effective and pressure-dependent.in high purity water with a cation conductivity Chloride (P) Important for corrosion prevention. Controlof less than 0.2 µS/cm. Otherwise, oxygen cor- range is unit- and pressure-dependent.rosion would result. Sulfate (P) Important for corrosion prevention. ControlSo, plants where condensate return could range is unit- and pressure-dependent.produce elevated conductivity shouldn’t use * Adapted from Reference 2. C = continuous sampling and P = periodic sampling Chemical Industry Digest. March 2017 67

UtilitiesTable 4. Monitoring of these parameters is especially important if is particularly true for plants that gener- the steam drives a turbine. ate electricity via steam turbines. Table 3 summarizes the most important mea- Recommended Steam Sample Parameters* surements.Parameter Normal Limit Notes To a large extent, the chemistry guide- lines for feedwater and boiler waterCation con- ≤0.2 µS/cm A general indicator of total impurities chemistry aim to prevent excess impuri-ductivity (C) in the steam. ty carryover to the steam — this is espe- cially critical if the steam drives a turbineSodium (C) ≤2 ppb An excellent analysis to determine or turbines. Steam turbines are precision mechanical carryover from the machines that require careful installa- boiler drum. If sodium is entering tion, balancing and operation. (For more the turbine, so, too, may other harm- on steam turbines, see: “Count on Steam ful impurities including chloride and Turbines,” http://goo.gl/fXS2OD.) Table 4 sulfate. These salts can cause pitting, details the most crucial guidelines. stress corrosion cracking and corro- sion fatigue in a low-pressure turbine. Prevent ProblemsSilica (C) ≤10 ppb Silica in water vaporizes, with such Proper steam generation chemis- vaporous carry-over into steam very try is critically important, as is the need pressure-dependent. Silica will coat to monitor and control chemistry at all turbine blades and reduce efficiency. On-line monitoring is a must.* Adapted from Reference 5. C = continuous sampling and P = periodic sampling. times. Neglect of condensate return, boil- er feedwater, boiler water or steam chem-sion control. istry can prove quite costly both from a monetary and a safety standpoint.Boiler Water Treatment In addition, proper steam generator shutdown, la-For eighty years, steam-generation chemists have yup and startup procedures are critical issues, particu-utilized sodium phosphate compounds for corrosion larly to prevent offline oxygen corrosion .[6,7]control and prevention of solids deposition in the wa- Referencesterwall circuits of drum-type boilers. Today, for high-pressure units, tri-sodium phosphate (TSP — Na3PO4), 1. “Consensus on Operating Practices for the Control ofis the only recommended species, perhaps with a small Feedwater and Boiler Water Chemistry in Modern Boilers,”amount of supplemental caustic (NaOH) for pH ele- ASME, New York City (1994).vation at startups. TSP generates mild alkalinity in theboiler via : 2. “Comprehensive Cycle Chemistry Guidelines for Com- bined-Cycle/Heat Recovery Steam Generators (HRSGs),” Publ. No. 3002001381, EPRI, Palo Alto, Calif. (2013).Na3PO4 + H2O ok Na2HPO4 + NaOH .......(3) 3. Buecker, B., and D. McGee, “An Intelligent System for Improved Water/Steam Chemistry Control and PlantThe alkalinity to some extent will mitigate the ef- Reliability,” Power Engineering (May 2014).fects outlined in Eq. 2. TSP also provides benefit by re- 4. Buecker, B., “Micro- or Ultrafiltration and Reverse Osmosis: A Popular Combination for Industrial Water Treatment,”acting with hardness ions (calcium and magnesium) to Industrial WaterWorld (January/February 2014).form a soft sludge that can be blown down. A drawback to TSP is that its solubility greatly de- 5. “Technical Guidance Document: Steam Purity for Tur-creases as temperatures exceed 300°F. Thus, in high- bine Operation,” Intl. Assn. for the Properties of Water andpressure units at full load, most of the phosphate pre- Steam, London (2013).cipitates on waterwall tubes and other internals. Thisphenomenon commonly is known as “hideout.” Many 6. Mathews, J., “Layup Practices for Fossil Plants,” Powerplant chemists now operate units with a bulk water (February 2013).phosphate concentration of ≈1–2 ppm, knowing thatmost of the original phosphate dose has hidden out 7. Buecker, B. and D. Dixon, “Combined-Cycle HRSGand will redissolve at reduced boiler load or shutdown. Shutdown, Layup, and Startup Chemistry Control,” Power Engineering (August 2012). Boiler-water chemical treatment and monitoring in Reprinted with permission from Chemical Processing.large measure is designed to protect steam purity. This All rights reserved.68 Chemical Industry Digest. March 2017