Taħt Liema Kombinazzjoni Speċifika ta 'Konċentrazzjoni Residwu ta' Chloramine u pH 316 Stainless Steel Heater Sheath Jiżviluppa Pitting f'Ilma sħun Muniċipali f'55 grad

Mechanism of pitting corrosion of disinfectant in potable water systems Monochloramine (NH2Cl) is being increasingly used as a secondary disinfectant in municipal hot water systems (domestic hot water tanks, recirculation loops in hotels and hospitals, point of use heaters in commercial buildings) to replace free chlorine and minimise trihalomethane formation for 316 stainless steel sheathed electric heating tubes. Chloramine is less aggressive to stainless steel than free chlorine but can produce pitting at higher temperatures especially when the pH is not in the proper range for chloramine stability. 316 stainless steel is passive at chloramine levels below 2-3 ppm at a common residential hot water temperature of 55 °C and a pH of 7.5-8.5. However, below pH 7.0 chloramine breaks down into free chlorine and ammonia which leads to a more aggressive oxidising environment. Chloramine is unstable at pH > 9.0, and decomposes to dichloramine and nitrogen trichloride. Pitting begins at chloramine levels >4-5 ppm f'pH 7.0-8.0, jew taħt eskursjonijiet tal-pH. Din ir-riċerka tbassar il-limiti tal-pH u l-konċentrazzjonijiet ta 'chloramine li jibdew pitting fuq 316 għant f'55 ⁰C. Il-mekkaniżmu ta' pitting indott mill-kloramina-u d-dipendenza fuq il-pH Il-monokloramina (NH2Cl) hija aġent ossidanti aktar dgħajjef mill-kloramina ħielsa (HOCl/OCl-) Il-kloramina, madankollu, hija degradata minn bosta rotot f'temperaturi ogħla minn 50 grad. F'pH newtrali għal kemmxejn alkalin (7.0-8.5) id-dekompożizzjoni tagħti ammonja u nitroġenu b'ammonti żgħar biss ta 'kloru ħieles rilaxxat. F'pH < 7.0, id-dekompożizzjoni tinbidel għal kloru ħieles (HOCl) u dikloramina (NHCl 2 ), it-tnejn li huma ossidanti aktar qawwija li jistgħu jiddekomponu l-film passiv fuq 316. F'pH > 9.0 kloramina tiddekomponi malajr għal trichloride tan-nitroġenu (NCl3), li huwa speċi volatili ħafna u ossidanti ħafna. Il-mekkaniżmu tal-pitting huwa dak tal-pitting tal-kloru ħieles. Tkissir lokalizzat tal-film passiv Cr 2 O 3 u l-propagazzjoni sussegwenti tal-fossa indotta mill-klorur. Iżda l-kloramina teħtieġ konċentrazzjoni aktar b'saħħitha (madwar 2-5x dik tal-kloru ħieles) biex tikkawża pitting fl-istess temperatura. Limiti kwantifikati ta' pitting f'55 grad Testijiet ta' immersjoni kkontrollati ta' 316 stainless steel (finitura elettrollustrata, Ra=0.2 μm) f'ilma muniċipali sintetiku (100 ppm Cl−, 80 ppm SO42−, 120 ppm CaCO3) f'diversi konċentrazzjonijiet ta' monokloramina u pH għal sigħat ta' boutting ta' 3500 gradi determinati għal 500 ppm ta' pitting. bidu. pH (55 grad ) Monochloramine (ppm bħala ekwivalenti Cl2) Ħin tal-Inizjazzjoni tal-Pitting għall-Mogħdija tad-Dekompożizzjoni tal-Kloramina (Sigħat) Profondità Massima tal-Ħofra Wara 3000 Siegħa (mikron) Ħin għall-Perforazzjoni (ħajt ta' 1.5 mm) Servizz Rakkomandat ta' 55 grad<1.0 7.0-8.5 Stable >10,000 <10 >30 sena Iva 1.0-2.0 7.5-8.5 Stabbli 5,000-10,000 10-20 15-30 snin Iva 2.0-3.0 7.5-8.5 Stabbli 20-40 sena L-aktar 8-15-il sena 3,000-5,000 Aċċettabbli 3.0-4.0 7.5-8.5 Madwar 1,500-3,000 40-80 4-8 snin 000 40-80 4-8 snin Deterjorament {15} Marġinali} Moderata Deterjorament 800-1,500 80-120 2 sa 4 sninMhux rakkomandat 5.0-7.0 7.5-8.5 Diżintegrazzjoni sinifikanti 400-800 120-200 1-2 snin le 7.0 7.5-8.5 Tħassir rapidu<400 >200 <1 year Unacceptable 2.0-3.0 6.5-7.0 (pH excursion)Free chlorine release 1,000-2,000 60-100 3-5 years Not recommended 2.0-3.0 <6.5 Free chlorine + dichloramine 300-800 100-180 1.5-2.5 years Not acceptable NCl3 formation 2.0-3.0 >9.0 500-1,200 80-150 2 - 3 yeras Unacceptable 3.0 < 7.0 Severe degradation < 500 > 150 < 1.5 years Temperature effect on chloramine aggressiveness Not acceptable The rate of decomposition of monochloramine rises with temperature, hence chloramine is more aggressive at higher temperatures. Water Temperature (oC) Maximum Monochloramine for 5-Year Life pH 7.5-8.0 (mg/L)Maximum Monochloramine for 10 Year Life at pH 7.5-8.0 (mg/L)Recommended Action 40 5.0 3.0 Standard practice 50 3.5 2.5 OK 55 3.0 2.0 Marginal above 2.5 ppm 60 2.2 1.5 Chloramine reduction if >2 ppm 70 1.5 1.0 316 not recommended, upgrade material 80 1.0 0.5 316 not appropriate Chloramine Treatment Practical Water Chemistry Specifications The following specifications will ensure long term pitting free service for 316 sheathed heaters in municipal hot water systems treated with monochloramine at 55oC. System Type Target Monochloramine (ppm) pH Range Max. Temp. (°C) Heater Life Expectancy (years)Hot water heating ideas for households1.0-2.0 7.5-8.5 55 10-15 Commercial (hotel/hospital) OK 1.5-2.5 7.5-8.5 55 8-12 Test for chloramine regularly. big circulation system 2.0-3.0 7.8-8.2 55 5-8Active pH regulation as required Any with chloramine >3.0 >3.0 7.5-8.5 55 2-4 Aġġorna għal 316L electropolish Kwalunkwe b'eskursjonijiet tal-pH<7.0 <3.0 <7.0 56 2-5 pH control necessary Any at temperatures above 60°C 1.5 7.5-8.5 >60 2-5 2205 Aġġornament Duplex Identifikazzjoni tal-qasam tal-pitting prodott mill-kloramina Għal heater 316 f'ilma sħun muniċipali bi ftit klorin ħieles iżda trattament magħruf tal-kloramina, il-fosos huma tipikament żgħar (0.1-0.5 mm dijametru), frekwenti (5-20 fosos/cm2), u spazjati regolarment. Il-wiċċ tal-metall fil-fossa huwa indikattiv ta 'morfoloġija ta' pitting mmexxija mill-klorur. Id-differenza ewlenija hija li m'hemm l-ebda kloru ħieles fl-ilma bl-ingrossa (indikatur li l-kloramina qed tkisser). Ir-reġistru tal-kimika tal-ilma se jkollu valuri tal-kloramina 'l fuq minn 3-4 ppm u pH taħt 7.0 jew ogħla minn 9.0 f'ħinijiet differenti. L-ittestjar tal-ilma bl-ingrossa għal kloru ħieles u kloramina jikkonferma t-tip ta 'diżinfezzjoni. Is-soluzzjoni għal dawn in-nuqqasijiet hija li titbaxxa l-konċentrazzjoni tal-kloramina għal inqas minn 2.5-3.0 ppm jew iżżomm il-pH mill-qrib f'7.5-8.5 jew taġġorna għal duplex 2205 stainless steel. Soluzzjonijiet ta' trattament għal sistemi ta' kloramina għolja Jekk ma jkunx possibbli li titnaqqas il-konċentrazzjoni ta' kloramina għal 3 ppm, il-proċeduri li ġejjin inaqqsu r-riskju ta' pitting: Elettropolixxjar: Wiċċ elettropolished (Ra<0.2 µm) increases chloramine threshold by ~1 ppm. pH stability The pH should be maintained in the range 7.8 – 8.2 by adding a pH controller. Decomposes to free chlorine at low pH. Decomposes to NCl3 at high pH. Lower heater setpoint from 55C to 50C (40-50% reduction in chloramine decomposition rate) Alloy upgrade: Duplex stainless steel 2205 (higher pitting resistance equivalent number, PREN >35) jew 316L b'kontroll tal-inklużjoni tan-nitrur tat-titanju (TiN). Konklużjoni: 316 heater life @ 55 grad pH u Chloramine kontroll 316 stainless steel sheathed heaters f'ilma sħun muniċipali f'55 grad juru pitting bidu fuq madwar 3-4 ppm monochloramine f'pH 7.5-8.5 f'1,000-3,000 siegħa u perforazzjoni fi 2-5 snin. Hekk kif il-ph jinżel taħt is-7.0 kloramina jkisser għal kloru ħieles u anke 2-3 ppm kloramina hija pjuttost aggressiva u se tinżel fi żmien 1-2 snin. 316 għantijiet b'dejta dwar kloru u pH u firxa ta 'pH ta' 7.5-8.5 u monokloramina<2.5 ppm for 10 year life shall be specified by engineers for chloramine treated water. Electropolished 316L or duplex 2205 is suggested for chloramine levels over 3 ppm or where pH control is not reliable. The present method, connecting chloramine concentration and pH to measured pitting initiation times at 55°C, allows users to define water treatment programmes which prevent disinfectant-driven pitting in municipal hot water systems.