Il materiale comunemente utilizzato per i tubi dei condensatori

The condenser is important auxiliary equipment in the thermal generator set. The condenser is generally composed of neck, casing, water chamber, tube bundle, tube plate, support rod, steam baffle, air cooling area, hot well and other parts, which is the key equipment to determine and affect the load and thermal efficiency of a steam turbine. The heat exchange tube, as the main heat transfer component of the condenser, is the key component of the condenser. With the increase of suspended solids, chloride ions and sulfur ions in the cooling circulating water, there is a higher requirement for a condenser cooling pipe.

Condenser heat exchanger pipe should have excellent heat transfer performance, good corrosion resistance, erosion resistance and wear resistance, but also should have good strength and stiffness, as well as economic and good processing performance. The materials of condenser heat exchange pipe are mainly copper alloy pipe, Austenitic stainless steel pipe, Ferrite stainless steel pipe, Duplex stainless steel pipe, titanium and titanium alloy pipe. The copper alloy pipe mainly includes military brass pipe (C26800), tin-brass pipe, aluminum-brass pipe, nickel-copper pipe, etc. Stainless steel grades mainly include Austenitic stainless steel tube TP304, TP316L, TP317L and Ferrite stainless steel grades TP439, TP439L, and duplex stainless steel tube 2205, 2507, titanium and titanium alloy tube mainly includes GR1, GR2, GR5, etc..

Different materials of the heat exchange pipe because of its own characteristics and cost factors, its application scope and working conditions are not the same. The corrosion in the Condenser is always an important problem in boiler accidents in power plants. The condensers of power plants in offshore areas generally use Cu-Zn tubes and Cu-Ni alloy tubes. The corrosion resistance of the latter is better than that of the former, because the thermodynamic stability of Ni is close to that of Cu, and the nanoscale compact and stable surface film will be generated on the surface in water or air. Therefore, the Cu-Ni tube in high saltwater (or seawater) and dilute acid, alkali medium is not easy to corrosion. But once there is an attachment on the surface of the copper tube, pitting will occur. Pitting corrosion is autocatalytic and latent, which will bring great damage. The condenser tube blockage and leakage frequently occur in the offshore area due to seawater backfilling, corrosion, dirt and other reasons. Yongxiang operates the generator set. Why is the brass condenser tube so easy to corrode? It depends on the type of corrosion. The corrosion of copper alloy condenser tube is affected by many factors, and the corrosion types are various, mainly including the following items:

Corrosione selettiva

Poiché il tubo di rame del condensatore è composto per lo più da una lega di rame e zinco, il potenziale dello zinco è inferiore a quello del rame, per cui lo zinco è facile che diventi l'anodo della batteria che si corrode, in modo che lo zinco si dissolva selettivamente per corrodere il tubo di rame. La teoria e la pratica dimostrano che il processo di corrosione del tubo di rame è strettamente legato alle prestazioni del film protettivo sulla superficie del tubo di rame. Se non si forma un film protettivo iniziale denso, è più probabile che si verifichi la corrosione del tubo di rame. Se non c'è un trattamento iniziale di rivestimento con FeSO4 sul tubo di rame del condensatore, è anche facile che si verifichi una corrosione locale da dezincatura.

Corrosione delle elettrocoppie

Coupling corrosion may occur when two different metal materials come into direct contact in a corrosive medium. In the condenser, the copper alloy condenser tube material is different from the carbon steel tube sheet material in the cooling water potential, there is the possibility of galvanic corrosion between them. The potential of the condenser copper tube is higher than that of the tube plate, which will accelerate the corrosion of the tube plate. But because the thickness of the carbon steel tube plate is larger, generally 25~40mm, the galvanic corrosion won’t affect the safe use in clean freshwater, but in the environment with a high salt concentration of water galvanic corrosion is more likely to occur.

Corrosione da vaiolatura

Questa corrosione è incline a verificarsi sulla superficie della pellicola protettiva del tubo di rame. Poiché l'acqua di raffreddamento contiene Cl e l'ossidazione di Cu generata da Cu+ per generare CuCl instabile, può essere idrolizzata in Cu2O stabile e rendere la soluzione di acidificazione locale la corrosione delle apparecchiature termiche. Se il tubo di rame del condensatore non viene pulito nei tempi previsti, i depositi superficiali irregolari favoriscono la corrosione e alla fine portano alla perforazione per corrosione. Nel funzionamento del tubo di rame del condensatore in frequenti start-stop, il cambiamento di carico è maggiore, l'impatto del vapore di scarico della turbina ad alta velocità, il ruolo del tubo di rame da stress alternato, facile da rendere la membrana superficiale di ottone rottura, produrre corrosione locale, la formazione di buche di corrosione pitting, ridurre il limite di fatica del materiale, e perché la concentrazione di stress alla corrosione, pitting fondo è facile da rompere, sotto l'erosione di NH3, O2 e CO2 in acqua, la frattura è gradualmente ampliato.

Erosion corrosion

This type of corrosion can occur on both the waterside and the steam side, mainly in the waterside. Suspended solids, sand and other solid granular hard objects in circulating cooling water impact and friction on the copper tube at the inlet end of the condenser. After a long time of operation, the inner wall of the front section of the copper tube at the inlet end is rough. Although there is no obvious corrosion pit, the surface is rough, the brass matrix is exposed and the copper tube wall becomes thin. The anodic process of erosion and corrosion can be said to be the dissolution of copper, and the cathodic process is the reduction of O2. The high flow rate will hinder the formation of stable protective film, is also the cause of erosion-corrosion, the general flow rate is not more than 2m/s.

Corrosione da NH3

L'NH3 in eccesso entra nel condensatore con il vapore e si concentra localmente nel condensatore. Se contemporaneamente è presente O2, in quest'area si verifica l'erosione di NH3 sul lato vapore del tubo di rame. La sua caratteristica è l'assottigliamento uniforme della parete del tubo e l'erosione dell'NH3 si verifica facilmente quando il contenuto di ammoniaca nell'acqua raggiunge i 300 mg/l. La condensa nel foro del deflettore è troppo fredda e la concentrazione di ammoniaca disciolta aumenta, causando anche l'erosione della striscia anulare di ammoniaca nel tubo di rame.

Cricca da corrosione sotto sforzo

When the condenser copper tube is not installed properly, vibration and alternating stress will occur in the operation of the copper tube surface to destroy the protective film and corrosion, finally, produce transverse crack to break the copper tube. This is mainly due to the relative displacement of grains inside the copper tube under the action of alternating stress, and the formation of anodic dissolution in the corrosive medium, mostly occurring in the middle of the copper tube.

Corrosione microbica

Microorganisms can change the medium environment in local areas of the condenser wall and cause local corrosion. The electrochemical corrosion process of metal in cooling water is promoted by the biological activity of microorganisms, which generally occurs on the carbon steel tube plate at the inlet side of the condenser. Cooling water often contains bacteria that thrive on Fe2+ and O2, called iron bacteria, which form brown slime. The anoxic conditions at the bottom of the slime provided a suitable environment for the survival of anaerobic sulfate-reducing bacteria. The combined action of iron bacteria and sulfate-reducing bacteria promotes metal corrosion. Operating temperature on the high side, the corrosion scale inhibitor and water quality and operating temperature are not appropriate, inadequate dosage or concentration fluctuations in the scale, will cause the condenser tube wall local Cl – easy through scale layer, caused the corrosion of the metal matrix, and the corrosion of metal ion hydrolysis, leading to higher medium H + concentration of algae and microbial activities also cause increased acidity of medium, The passivation film on the metal surface is destroyed and the metal matrix is further corroded.