Anba ki konbinezon espesifik nan konsantrasyon rezidyèl kloramin ak pH 316 Nerjaveèi aparèy chofaj djenn devlope pitting nan dlo cho minisipal nan 55 degre.

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 nan pH 7.0-8.0, oswa anba randone pH. Rechèch sa a predi papòt pH yo ak konsantrasyon kloramin ki kòmanse pitting sou 316 djenn nan 55 ⁰C. Kloramin-pwovoke mekanis pitting ak depandans pH Monochloramine (NH2Cl) se yon ajan oksidan ki pi fèb pase klò gratis (HOCl/OCl-) Kloramin, sepandan, se degrade pa anpil wout nan tanperati ki pi wo a 50 degre. Nan pH net ak yon ti kras alkalin (7.0-8.5) dekonpozisyon bay amonyak ak nitwojèn ak sèlman ti kantite klò gratis lage. Nan pH < 7.0, dekonpozisyon an chanje nan klò gratis (HOCl) ak dichloramine (NHCl 2 ), tou de nan yo ki pi pwisan oksidan ki ka dekonpoze fim nan pasif sou 316. Nan pH > 9.0 kloramin rapidman dekonpoze nan trichloride nitwojèn (NCl3), ki se yon espès temèt trè oksid. Mekanis nan pitting se sa yo ki nan pitting klò gratis. Dekonpozisyon lokalize fim pasif Cr 2 O 3 ak pwopagasyon twou san fon an ki fèt aprè klori. Men, kloramin bezwen yon konsantrasyon pi fò (apeprè 2-5 fwa klò gratis) pou lakòz pitting nan menm tanperati a. Limit quantifye nan pitting nan 55 degre Tès imèsyon kontwole nan 316 asye pur (fini elektwopoli, Ra=0.2 μm) nan dlo minisipal sentetik (100 ppm Cl−, 80 ppm SO42−, 120 ppm CaCO3) nan divès kalite konsantrasyon monokloramin ak pH yo detèmine pou 3500 èdtan degre pi yo detèmine pou 3500 èdtan pi. inisyasyon. pH (55 degre) Monochloramine (ppm kòm ekivalan Cl2) Tan Inisyasyon Pitting pou Chloramine Dekonpozisyon Pathway (èdtan) Max Pwofondè twou apre 3000 èdtan (mikwon) Tan pou pèforasyon (1.5 mm miray) 55 degre Sèvis Rekòmande<1.0 7.0-8.5 Stable >10,000 <10 >30 ane Wi 1.0-2.0 7.5-8.5 Estab 5,000-10,000 10-20 15-30 ane Wi 2.0-3.0 7.5-8.5 Estab 20-40 ane Sitou 8-15 ane 3,000-5,000 Akseptab 3.0-4.0 7.5-8.5 Anviwon 1,500-3,000 40-80 4-8 ane Degradasyon majinal {15} Moded 800-1,500 80-120 2 rive 4 anePa rekòmande 5.0-7.0 7.5-8.5 Dezentegrasyon enpòtan 400-800 120-200 1-2 ane pa gen 7.0 7.5-8.5 Dezentegrasyon rapid<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 Upgrade a 316L electropolish Nenpòt ak randone 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 Duplex Upgrade Idantifikasyon jaden pitting ki te pwodwi pa kloramin Pou yon aparèy chofaj 316 nan dlo cho minisipal ak ti kras klò gratis men li te ye tretman kloramin, twou yo anjeneral piti (0.1-0.5 mm dyamèt), souvan (5-20 twou/cm2), epi regilyèman espas. Sifas metal la nan twou san fon an endike yon mòfoloji klori-kondwi pitting. Diferans kle a se ke pa gen okenn klò gratis nan dlo a esansyèl (yon endikatè ke kloramin la ap kraze). Jounal chimi dlo a pral gen valè kloramin ki pi wo a 3-4 ppm ak pH pi ba pase 7.0 oswa pi wo a 9.0 nan diferan moman. Tès dlo esansyèl la pou klò gratis ak kloramin pral konfime kalite dezenfeksyon an. Solisyon echèk sa yo se diminye konsantrasyon kloramin nan mwens pase 2.5-3.0 ppm oswa kenbe pH la byen nan 7.5-8.5 oswa ajou nan duplex 2205 Nerjaveèi. Solisyon tretman pou sistèm kloramin ki wo Si li pa posib pou diminye konsantrasyon kloramin la a 3 ppm, pwosedi sa yo ap diminye risk pou pitting: Electropolishing: Electropolished sifas (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) oswa 316L ak kontwòl enklizyon nitrure Titàn (TiN). Konklizyon: 316 aparèy chofaj lavi @ 55 degre pH ak Chloramine kontwòl 316 Nerjaveèi kouvri aparèy chofaj nan minisipal dlo cho nan 55 degre montre pitting kòmanse pi wo a alantou 3-4 ppm monochloramine nan pH 7.5-8.5 nan 1,000-3,000 èdtan ak pèforasyon nan 2-5 ane. Le pli vit ke ph desann pi ba pase 7.0 kloramin kraze nan klò gratis e menm 2-3 ppm kloramin se byen agresif epi li pral twou nan 1-2 ane. 316 djenn ak done sou klò ak pH ak yon seri pH nan 7.5-8.5 ak monokloramin.<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.