Taħt Liema Kombinazzjoni Speċifika ta' Konċentrazzjoni ta' Ossiġenu Maħlul ta' Ilma ta' l-Għalf u pH 316 Sheath ta' l-Azzar li ma jsaddadx Jsofri Fluss-Korrużjoni Aċċellerata (FAC) f'200 grad f'Ilma ta' l-Għalf tal-Boiler ta' Veloċità Għolja-

Xoljiment tal-Film Passiv f'Ilma ta 'Temperatura Għolja ta' Purità Għolja

The high velocity boiler feedwater systems operating at 200°C (typical for high-pressure boiler economisers or feedwater heaters) can be a problem for 316 stainless steel sheathed electric heating tubes due to flow-accelerated corrosion (FAC), which is thought to be immune to this phenomenon (which occurs mainly in carbon and low-alloy steels). The FAC of 316 is different from the traditional FAC of magnetite (Fe3O4) layers on carbon steel where it is a direct dissolution of passive film by the combined impacts of high flow velocity, high temperature and unique water chemistry. The passive film on 316 dissolves at 200°C, when the dissolved oxygen is less than 5 ppb (deaerated) and the pH is greater than 9.5 or less than 6.5. Under such conditions wall thinning rates of 0.1-0.5 mm/year have been seen in high velocity zones (>5 m/s). Dan id-dokument jivvaluta l-ossiġnu maħlul u l-limiti tal-pH għall-bidu tal-FAC fuq 316 għant f'200 grad.

Mekkaniżmu ta 'solubilità f'temperatura għolja ta' films passivi

F'ilma ta' -temperatura għolja (200 grad ) il-film passiv fuq 316 huwa saff darbtejn ta 'ossidu tal-kromju (Cr₂O₃) u spinel tal-kromju tal-ħadid (FeCr₂O₄). Dawn l-ossidi huma sensittivi ħafna fuq il-pH u l-ossiġnu maħlul għas-solubilità tagħhom. Meta jitneħħa l-arja (DO<5 ppb) the corrosion potential is in the active or passive-transition area where the passive film is less stable . At high pH (>9.5) l-ossidu tal-kromju huwa maħlul bħala kromit (CrO 2-) jew kromat (CrO 4 2-). F'pH baxx (<6.0) the film dissolves into Cr3+ ions. High flow velocity (>3-5 m/s) tneħħi l-ispeċi maħlula b'mod kostanti u tipprevjeni r-repassivazzjoni u żżomm rata għolja ta 'xoljiment. L-enerġija ta 'attivazzjoni għal FAC ta' 316 hija madwar 40-60kJ/mol, jiġifieri r-rata ta 'korrużjoni tirdoppja kull 15-25 grad 'il fuq minn 150 grad.

Limiti FAC kwantifikati ta' 316 f'200 grad f'Ilma ta'-Veloċità Għolja

Hawn taħt huma murija r-rati ta 'korrużjoni u l-imġieba tat-traqqiq tal-ħajt għall-istainless steel 316 f'test ta' linja kkontrollata f'200 grad b'varjazzjonijiet fl-ossiġnu maħlul, pH u veloċità tal-fluss.

Ossiġenu maħlul (ppb) pH f'200 grad Veloċità tal-Fluss (m/s) Ossidu Dominanti Solubilità FAC Rata (mm/sena) Ħin sa 0.5 mm Telf tal-Ħajt (sigħat) Rakkomandat għal Servizz ta' 200 grad<5 (deaerated) 6.5-7.5 Any Minimal <0.02 >25,000 Iva<5 7.5-8.5 <3 Minimal <0.02 >25,000 Iva<5 7.5-8.5 3-5 Slight 0.02-0.05 10,000-25,000 Acceptable <5 8.5-9.5 3-5 Cr(OH)3 solubility 0.05-0.15 3,500-10,000 Not recommended <5 9.5-10.5 >3 Xoll tal-Kromit 0.15-0.40 1,250-3,500 Inaċċettabbli<5 >10.5 Any Rapid chromite >0.40 <1,250 Unacceptable <5 5.5-6.5 >3 Cr3+ solubilità 0.10-0.30 1,700-5,000 Inaċċettabbli<5 <5.5 Any Rapid Cr3+ >0.30 <1,700 Unacceptable
5-20 (low) 8.5-9.5 >3 Solubilità mnaqqsa 0.03-0.08 6,000-17,000 Marġinali
20-100 (moderate) 8.5-9.5 >5 Film passiv stabbilizzat<0.02 >25,000 Yes 20-100 9.5-10.5 >5 Some stabilization 0.02-0.05 10,000-25,000 Acceptable >100Kull Kwalunkwe Passiv bis-sħiħ<0.02 >25,000 Iva
Il-FAC ta '316 huwa dipendenti ħafna fuq il-veloċità tal-fluss.

Il-veloċità tal-fluss hija l-fattur tal-iskala li jittrasforma s-solubilità passiva tal-film f'tnaqqija tal-ħajt imkejla. It-tabella ta 'hawn taħt turi d-dipendenza tal-veloċità f'200 grad C, kundizzjonijiet ta' deaerated, pH 9.2 (żona marġinali).

Veloċità tal-fluss (m/s) Wall Shear Stress (Pa, stmat) Rata FAC @ DO<5 ppb, pH 9.2 (mm/yr)Time to 50% of Original 1.5 mm Wall Loss(hours) Recommended Maximum Velocity for 10-Year Life <1 <10 <0.01 >75,000 Safe 1-2 10-20 0.01-0.03 25,000-75,000 Safe 2-3 20-35 0.03-0.06 12,500-25,000 Acceptable 3-5 35-60 0.06-0.12 6,000-12,500 Marginal (monitor) 5-8 60-100 0.12-0.20 3,750-6,000Not recommended 8-12 100-150 0.20-0.35 2,000-3,750 Unacceptable >12 >150 >0.35 <2,000 Not acceptable
Speċifikazzjoni tad-Disinn u Kimika Prattika tal-Ilma tal-Fedwater Heaters f'200 grad

316 ħiters magħluqin f'200 grad f'ilma ta 'għalf tal-bojler b'veloċità għolja huma protetti mill-korrużjoni aċċellerata tal-fluss bir-rekwiżiti li ġejjin.

Tip ta' Ilma ta' l-Għalf Ossiġenu Maħlul fil-mira (ppb) pH Rakkomandat, Veloċità tal-Fluss Massima ta' 200 grad f'Għat (m/s) Ħajja Mistennija tal-Heater (snin)
It-terapija volatili kollha (AVT) - tnaqqis 2-5 8.0-8.5 3.0 8-10
Kollha-trattament volatili (AVT) - ossidanti 50-100 8.0-8.5 5.0 10-15
Oxygenated therapy (OT) 100-300 8.0-8.5 6.0 >15 
Trattament tal-fosfat 5-20 8.5-9.0 3.0 5-8 (immonitorja)
Any treatment with pH >9.5 N/A >9.5 <2.0 5-10 (316 marginal)
Identifikazzjoni tal-Qasam tal-FAC fuq 316 Heater Sheaths

The FAC on a 316 heater will produce unique surface features that are different from conventional corrosion mechanisms in high temperature, high velocity feedwater. The surface is smooth, polished or "orange-peel" in appearance with no pitting or scale. The wall thinning is homogeneous at the high velocity zones (e.g. pipe entry, directly facing the flow). The thinning is often a steady drop of outside diameter throughout the heater length. Scanning electron microscopy (SEM) shows a smooth, featureless surface, with no attack at the grain boundaries. Water chemistry log will show pH >9.5 jew<6.0 and dissolved oxygen <5-20 ppb for prolonged durations. For such failures the answer is to adjust water chemistry to keep pH in the 8.0-8.5 range, and dissolved oxygen >20-50 ppb (jekk kompatibbli mas-sistema ġenerali), jew biex tillimita l-veloċità tal-fluss billi tpoġġi l-heater f'bypass jew kwiet. Is-suxxettibilità FAC tista 'tiġi eliminata billi tinbidel għal titanju jew Alloy 825.

Sommarju: Il-Kimika ta' l-Ilma ta' l-Għalf u l-Ġestjoni tal-Veloċità għall-Prevenzjoni tal-FAC

Il-korrużjoni aċċellerata tal-fluss ta 'ħiters magħluqin ta' l-istainless steel 316 f'ilma ta 'għalf tal-bojler ta' 200 grad sseħħ f'kundizzjonijiet ta 'deaerated (DO<5-10 ppb) in combination with alkaline pH >9.5 jew aċidu pH<6.0 and flow velocities >3-5 m/sec. Under these conditions, the passive film is dissolved as chromite or chromium ions, resulting in wall thinning rates of 0.1-0.5 mm/year. For high temperature feedwater service with 316 sheaths, engineers should stipulate pH of 8.0-9.0, dissolved oxygen >20 ppb (ideally 50-100 ppb) and a flow velocity below 3-4 m/s at the sheath surface. If high velocity (>5 m/s) cannot be avoided, oxidising water treatment (oxygenated treatment, OT) with DO >100 ppb se jistabbilizza l-film passiv u jevita FAC. Il-metodoloġija mogħtija hawn tirrelata l-ossiġnu maħlul, il-pH u l-veloċità tal-fluss ma 'rati FAC li jistgħu jitkejlu f'200 grad, u tippermetti lix-xerrej jiddefinixxi l-kimika tal-ilma u parametri idrawliċi li jżommu l-film passiv fuq 316 għantijiet tal-heater f'operazzjoni ta' feedwater tal-bojler b'veloċità għolja.