Novel steel specified for Iranian urea plant

A Sandvik Materials Technology UK product story

Edited by the Engineeringtalk editorial team Apr 15, 2003

The high-pressure vessels in the new Assaluehy urea production plant in Iran are being constructed in Sandvik Material Technology's recently developed Sandvik Safurex steel grade.

The high-pressure vessels in the new Assaluehy urea production plant in Iran are being constructed in Sandvik Material Technology's recently developed Sandvik Safurex steel grade.

The licensing company and specifier for this latest technology plant is the leading process system designer, Stamicarbon of Holland.

All stainless steel materials for high-pressure equipment are being supplied by Sandvik.

Although Sandvik Safurex materials have been installed, tried and tested in several plants around the world, the Assaluehy plant is the first green field site, where Stamicarbon has introduced Sandvik Safurex as the preferred steel grade, for use in all high pressure equipment, throughout the stripper, pool condenser, reactor and scrubber.

Developed in co-operation with Stamicarbon, Sandvik Safurex has many advantages over other steel grades for urea service but, in particular, has the ability to passivate at a lower oxygen level.

It, therefore, has the key benefit of reducing the level of oxygen content in the process.

With the injection of less oxygen in the form of air, there is less inertia in the system, contributing to increased manufacturing capacity.

Sandvik Safurex also has the benefit of extremely high strength.

Urea is made by the reaction of ammonia and carbon dioxide under a pressure of 150-220bar, in a reactor, at temperatures in the range 180-200C.

Corrosive conditions are severe in all high-pressure parts of the process, where the intermediate component, ammonium carbamate is present.

The oxygen content is of the utmost importance.

If there is no oxygen present, stainless steels tend to corrode actively and at high rates, whereas only small amounts of oxygen are sufficient to keep them in a passive state.

In principle, to reduce the oxygen content, all material in contact with ammonia carbamate needs to be in Sandvik Safurex grade.

If it is, the risk of high corrosion rates, is considerably reduced.

Disrupted oxygen feed causes a devastatingly high corrosion rate in conventional urea grade materials.

Corrosion rates 100 times higher have been experienced.

Since the process involves toxic ammonia, the improved safety factor alone prompts the use of Sandvik Safurex.

Sandvik Safurex also has improved resistance to stress corrosion cracking, which can occur with other steel grades, if chloride content is higher, than is recommended, on the steam side, in the high pressure carbamate condenser.

Sandvik Safurex resists much higher concentrations at higher temperatures and, in practical terms, means that stress corrosion cracking need not be a problem if the grade is used.

Due for delivery from Sandvik early in 2003, the Assaluehy pressure vessels are due for fabrication by Summer 2003.

The government run plant is designed to produce 3250 tonnes of urea per day.