O dan Ba ​​Gyfuniad Penodol o Dwr Porthiant Crynodiad Ocsigen Toddedig a pH A Mae 316 o Wain Gwresogydd Dur Di-staen yn Dioddef Llif-Cyrydiad Cyflym (FAC) ar 200 gradd mewn Dŵr Porthiant Boeler Cyflymder Uchel-

Diddymu'r Ffilm Goddefol mewn Dŵr Tymheredd Uchel Purdeb Uchel

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). Mae'r papur hwn yn asesu'r trothwyon ocsigen toddedig a pH ar gyfer cychwyn FAC ar 316 gwain ar 200 gradd.

Mecanwaith hydoddedd tymheredd uchel o ffilmiau goddefol

Mewn-dŵr tymheredd uchel (200 gradd) mae'r ffilm oddefol ar 316 yn haen ddeuol o gromiwm ocsid (Cr₂O₃) a asgwrn cefn cromiwm haearn (FeCr₂O₄). Mae'r ocsidau hyn yn sensitif iawn ar pH ac ocsigen toddedig am eu hydoddedd. Pan fydd wedi marw (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) mae cromiwm ocsid yn cael ei hydoddi fel cromite (CrO 2-) neu cromad (CrO 4 2-). Ar pH isel (<6.0) the film dissolves into Cr3+ ions. High flow velocity (>3-5 m/s) yn cael gwared ar y rhywogaethau toddedig yn gyson ac yn atal yr atdafiad ac yn cynnal cyfradd hydoddi uchel. Mae'r egni actifadu ar gyfer FAC o 316 tua 40-60kJ/mol, hy mae'r gyfradd cyrydiad yn dyblu bob 15-25 gradd uwchlaw 150 gradd.

Trothwyon FAC meintiol o 316 ar 200 gradd mewn Dwr Cyflymder Uchel

Isod dangosir cyfraddau cyrydiad ac ymddygiad teneuo waliau ar gyfer 316 o ddur di-staen mewn prawf dolen reoledig ar 200 gradd gydag amrywiadau mewn ocsigen toddedig, pH a chyflymder llif.

Ocsigen Toddedig (ppb) pH ar 200 gradd Cyflymder Llif (m/s) Dominyddol Ocsid Hydoddedd Cyfradd FAC (mm/yr)Amser i 0.5 mm Colli Wal (oriau) Argymhellir ar gyfer Gwasanaeth 200 gradd<5 (deaerated) 6.5-7.5 Any Minimal <0.02 >25,000 Bydd<5 7.5-8.5 <3 Minimal <0.02 >25,000 Bydd<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 Diddymiad cromite 0.15-0.40 1,250-3,500 Annerbyniol<5 >10.5 Any Rapid chromite >0.40 <1,250 Unacceptable <5 5.5-6.5 >3 Cr3+ hydoddedd 0.10-0.30 1,700-5,000 Annerbyniol<5 <5.5 Any Rapid Cr3+ >0.30 <1,700 Unacceptable
5-20 (low) 8.5-9.5 >3 Hydoddedd llai 0.03-0.08 6,000-17,000 Ymylol
20-100 (moderate) 8.5-9.5 >5 Ffilm goddefol wedi'i sefydlogi<0.02 >25,000 Yes 20-100 9.5-10.5 >5 Some stabilization 0.02-0.05 10,000-25,000 Acceptable >100Unrhyw un Cwbl oddefol<0.02 >25,000 Bydd
Mae'r FAC o 316 yn ddibynnol iawn ar gyflymder llif.

Cyflymder llif yw'r ffactor graddio sy'n trawsnewid hydoddedd ffilm goddefol yn deneuo waliau wedi'i fesur. Mae'r tabl isod yn dangos y ddibyniaeth ar gyflymder ar 200 gradd C, amodau gwanhau, pH 9.2 (parth ymylol).

Cyflymder Llif (m/s) Straen Cneifio Wal (Pa, amcangyfrif) Cyfradd 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
Manyleb Dylunio a Chemeg Dŵr Ymarferol Gwresogyddion Dŵr Porthiant ar 200 gradd

Mae 316 o wresogyddion caeedig ar 200 gradd mewn dŵr porthiant boeler cyflymder uchel yn cael eu hamddiffyn rhag cyrydiad cyflymu llif gan y gofynion canlynol.

Math o Ddŵr Porthiant Targed Ocsigen Toddedig (ppb) pH a argymhellir, 200 gradd Cyflymder Llif Uchaf ar Wain (m/s) Oes Disgwyliedig Gwresogydd (blynyddoedd)
Pob therapi anweddol (AVT) - lleihau 2-5 8.0-8.5 3.0 8-10
Pob-triniaeth anweddol (AVT) - yn ocsideiddio 50-100 8.0-8.5 5.0 10-15
Oxygenated therapy (OT) 100-300 8.0-8.5 6.0 >15 
Triniaeth ffosffad 5-20 8.5-9.0 3.0 5-8 (monitro)
Any treatment with pH >9.5 N/A >9.5 <2.0 5-10 (316 marginal)
Adnabod Maes o FAC ar 316 Gwain Gwresogydd

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 neu<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 (os yw'n gydnaws â'r system gyffredinol), neu i gyfyngu ar gyflymder llif trwy osod y gwresogydd mewn ffordd osgoi neu lonydd yn dda. Gellir dileu tueddiad FAC trwy newid i ditaniwm neu Alloy 825.

Crynodeb: Cemeg Dŵr Porthiant a Rheoli Cyflymder i Atal FAC

Mae cyrydiad cyflymedig llif 316 o wresogyddion wedi'u gorchuddio â dur gwrthstaen mewn dŵr bwydo boeler 200 gradd yn digwydd o dan amodau gwan (DO<5-10 ppb) in combination with alkaline pH >9.5 neu pH asid<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 >Bydd 100 ppb yn sefydlogi'r ffilm goddefol ac yn osgoi FAC. Mae'r fethodoleg a roddir yma yn ymwneud ag ocsigen toddedig, pH, a chyflymder llif i gyfraddau FAC mesuradwy ar 200 gradd, ac mae'n caniatáu i'r prynwr ddiffinio cemeg dŵr a pharamedrau hydrolig a fydd yn cynnal y ffilm oddefol ar 316 o weinau gwresogydd mewn gweithrediad dŵr porthiant boeler cyflymder uchel.