Materials of Construction for the Nitric Acid Process

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°C.

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°C its stress to rupture in 100,000 hours is only 23 MPa (lower boundary). A temperature rise of 10°C 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°C 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°C with consequent safe reduction in design margins.