Causes of failure of heat-resistant steel piping in Parex-1 units

Abstract

One of the causes of equipment failures in petroleum refineries is the nitriding of steels in ammonia media at high temperatures and pressures. At 350-600~ ammonia dissociates into hydrogen and nitrogen. The nitrogen, diffusing into the metal, forms brittle nitride layers that are susceptible to cracking in the course of long-term service. The process of saturation with nitrogen depends on the chemical composition of the steel, the temperature, and the duration of surface heating of the equipment. The dissociation constant of ammonia increases with increasing temperature, and the nitrogen saturation process is accelerated [1]. In the Parex-1 unit at the Kirishi Petroleum Refinery, after 10 years of operation, failures were noted in type KhM [chromium-molybdenum] heat-resisting steel pipelines carrying ammonia-containing media. These pipelines operate at 380400~ under a pressure of 1.8 MPa, so that we must assume the possibility of nitriding. Metallographic examinations to determine the causes of the pipeline failures were performed with a Neophot microscope (East Germany), on specimens cut from different sections of the piping (see Table 1). It was established that the pipelines transporting the desorbate and denormalizate were nitrided on the inside. The nitrided zone consisted of needles of a 7-phase (iron and chromium nitrates), segregated in the volume of grains of the c~-solution (Fig. 1). The brittle nitrided layer was susceptible to failure (Fig. 2). The depth of saturated nitrided layer, as revealed by microstructural analysis, was 0.8-1.4 and 0.5 mm, respectively, for the pipelines transporting desorbate and denormalizate. On the specimens cut from the pipelines transporting purge gas and gaseous feed mixture, no nitrided layer could be detected by microstructural analysis. The concentration of nitrogen varied through the thickness of the nitrided layer. The actual thickness of the layer could not be determined by microstructural analysis; the thickness was generally greater than the thickness of the diffusion layer as determined on the basis of the two-phase zone e~ + 3''. Therefore, we investigated the possibility of determining the thickness of the nitrided layer by means of layer-by-layer measurements of microhardness in a Microdur instrument manufactured by Krautkrfimer (East Germany). It can be seen from Fig. 3 that, for the specimens from the pipelines transporting desorbate and denormalizate, the maximum hardness (HB 400-700) is observed at depths up to 1 mm from the inner surface of the pipe; this is consistent with the thickness of the saturated nitrided layer (c~ + 3'') as determined metallographicalty. The greater hardness that is observed all the way to a depth of 4-5 mm indicates the presence of nitrogen dissolved in the a-phase at this depth.