Bimetallic tubes is a process of two different materials unified into a single homogenous metallic tube. Usually, the inner material is a corrosion resistance alloy (Copper, Aluminum, Brass, Titanium) whereas the outer material is high-stress resistant alloy (Stainless steel, Carbon steel, Titanium), depending on the application.
Bimetallic tubes is a unique product which has got immense applications across the market. Through our distinct capabilities, we can provide tubes with two different materials like SS+MS, Copper+MS, Brass+MS in a combination of OD+ID or ID+OD as per customer requirement. This enables addressing the end use at a relatively lower cost as the costlier material is rolled over the mild steel as a thin layer or cladding. Applications include General engineering, ornamental/decorative and numerous others.
Traditionally, thick wall titanium tubes have been used in high temperature (U to 250 C), high-pressure condition for shell and tube heat exchangers used for the production of urea. To suit both these conditions, thick titanium tubes need to be used to withstand the high pressure and the cost of the heat exchangers multiply exponentially due to the high cost of titanium.
The solution is to use a thin thickness of titanium, up to 0.7mm, on the inner pipe and wrapped with a stainless steel tubes 2.3mm wrapped on the outer side, hence maintaining the thickness, but reducing the price of the tubes drastically.
The best-known production processes include those developed by Snamprogetti, Stamicarbon, Toyo Engineering, and Urea Casale. In the process, liquid ammonia and gaseous carbon dioxide are converted in a reactor under pressure (150 – 200 bar) at high temperature (180 – 190°C) to ammonium carbamate, urea, and water.
Modern energy-efficient processes foresee the extraction of the carbamate, not transformed into urea, in a stripping column working at the same pressure as the reactor while the off-gas from the stripper is condensed in the carbamate condenser also working at the pressure of the reactor and the stripper. Condensation heat is recovered as steam and ammonium carbamate heating at approximately 180 – 250°C and 150 bar is extremely aggressive to materials. The customary steels of 304 L / 316 L type cannot withstand such conditions.
For this very reason, special “urea” steels have been developed: DMV 316 L Urea Grade (UNS S 31603). Its increased nickel content (more than 13 %) gives it improved resistance to corrosion and, in conjunction with a small addition of nitrogen secures austenitic stability.
The bimetallic tubes for urea strippers have been developed to prevent the corrosion process. These consist of two concentric tubes: an external tube in DMV 25.22.2 having a minimum thickness of 2 mm, and an internal tube in zirconium Gr. 702 having a minimum thickness of 0.7 mm. The two tubes are fabricated separately according to Snamprogetti specifications, then they are assembled and drawn.
The dual material used in the bimetallic tubes enhances its resistance to corrosion and thus does not affect the Urea formation process. There are many advantages of using bimetallic tubes over a stainless steel tube. Only approved suppliers can produce bimetallic tubes.