Problem No. 31: Insulation of high-pressure flanges in a urea plant.

The intermediate product in the production of urea from carbon dioxide and ammonia is ammonium carbamate. Ammonium carbamate is very corrosive under the synthesis conditions in a urea plant. It is good practice to minimise the number of flange connections in the high pressure urea synthesis section. However some flange connections will be unavoidable. Typically two types of flange connections are applied in the high pressure urea synthesis section: ring type joints and lens ring joints. Once a flange connection leaks it is almost impossible to stop the leak by re-tightening the bolts. Due to the leak a crevice is created in which the oxygen dissolved in the carbamate containing liquid will be depleted and passive corrosion will become active corrosion. Active corrosion rates of stainless steel can be >52 mm per year and this means that in the case of a leaking flange connection one has to stop the plant immediately and service the leaking flange connection. This raises the question whether the flange connection should be insulated.