Metals in Severe Conditions – Symposium on Materials of Construction in Fertiliser Plants. Session 1

Materials of Construction for the Nitric Acid Process:

The many improvements in materials and design which have been incorporated into the plants have substantially increased availability. The costs involved in this development work and the use of more expensive materials have been fully justified by longer vessel lives and a reduction in the number of unforeseen breakdowns.

In a very corrosive environment such as a nitric acid plant, modifications to design and materials which extend the lives of vessels can bring to light other problems which were previously masked by premature failure of the older vessels. It is therefore important to carefully analyse failures and to monitor sensitive vessels so that the design is continually reviewed.

Although we have a good background knowledge of existing materials, there will always be new developments to consider and assess, and this is regarded as an ongoing task.

W R Harker and K B Moxon, Fisons Ltd (Fertilizer Division), Levington, Ipswich, UK.

17 pages, 2 figures, 2 tables, 3 references.

The Case for Good Distribution in Steam Reforming Furnaces:

A steam reforming furnace contains a lot of expensive nickel alloy components including the reformer tubes, and where fitted, outlet pigtails and outlet manifolds. In operation these components are subject to extremely arduous conditions. The reformer tubes for instance may be subject to an internal pressure of some 30 bar and a temperature in the region of 900^oC.

At room temperature the material usually employed for reformer tubes, the 35/25 Ni-Cr range, has a yield stress of 250 MPa but at 900^oC its stress to rupture in 100,000 hours is only 23 MPa (lower boundary). A temperature rise of 10^oC at this level reduces the design life by 32 percent.

Mal-distribution has been reported by some operating companies to the extent that reformed gas outlet temperature at one end of the reformer differs in temperature by 100^oC from that at the opposite end. Such a high temperature scatter can only result in a large reduction in the potential life of reformer tubes, outlet pigtails and outlet manifolds – or, in the absence of realistic engineering, to the inclusion of expensive design margins.

Scale can have a multiplying effect on mal-distribution, but even with the present vogue for large single stream plants, the proper adoption of the principles of distribution presented herein can restrict the temperature scatter to ± 10^oC with consequent safe reduction in design margins.

K Nuttall and A R Reid, Humphreys and Glasgow Ltd, London, UK.

16 pages, 7 figures.

Experience of Pumps for Severe Duties in Urea Plant Service:

A brief review of the main production process is given, with particular reference to the isobaric loop of the Snamprogetti process, in order to identify the severe service conditions where High Pressure pumps are used. The types of pumps which have been installed during the past decade in Snamprogetti plants are listed, and the special features of design, materials of construction, and installation to ensure trouble free operation in severe conditions are described.

R Borromei, F Granelli and M Rex, Snamprogetti, Milan, Italy and Basingstoke, UK.

20 pages, 6 figures, 6 tables.

Waste Heat Boilers for Reliable Service in Ammonia Plants:

In spite of the fact that a substantial number of different waste heat boiler designs are available in the market, too many unnecessary shut-downs of ammonia plants are still caused by waste heat boilers.

A new waste heat boiler design has recently been developed by Haldor Topsoe A.S. with special attention paid to improved reliability of operation under the severe conditions of ammonia production.

H Stahl, Haldor Topsoe A.S., Denmark.

24 pages, 19 figures.