API RP 939-C:2009 pdf download

API RP 939-C:2009 pdf download.Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries.
McConomy [6J developed corrosion rate vs temperature plots for several alloys. These curves represented average corrosion rates from industry data collected in the 1960s primarily from crude heater tubes. In subsequent years, these curves were modified (lowered) because it was found that they were in most cases too conservative for crude unit piping and pressure vessels [3] These newer curves are referred to as the modified McConomy curves, which are reproduced in Figure B.1. There is significant scatter in the data, so these curves should be used with the knowledge that much higher or lower rates are possible (possibly by a factor of 10).
The curves illustrate the beneficial effect of chromium as an alloying element; the higher the Cr content of the steel, the lower the sulfidation rate, i.e. 9Cr will corrode less than 5Cr, which corrodes less than carbon steel. The exact reason for the beneficial effect of Cr is not clear, but it is believed to be related to the stability and protective nature of the iron-chromium-sulfide scales formed.
6.2.2.2 Sulfur Content and Speciation
The modified McConomy curves are based on a 0.6 % total S content in the crude or fraction and are accompanied by a corrosion rate multiplier for lower and higher sulfur contents. In general, the higher the S content of the stream, the more corrosive the stream will be. This effect is less pronounced than the temperature effect. However, as discussed in Annex B, the situation is more complex. Most crude oils contain a range of sulfur species including H2S, mercaptans, elemental sulfur, polysulfides, thiophenes, aliphatic sulfides and aliphatic disulfides [7], [8] and each has a different reactivity or affect on corrosion rate. Some companies have performed analyses of crude oil and its fractions for various sulfur compounds and have developed predictive models based on a combination of laboratory and field experience. Nonetheless, it is very difficult for a refinery to accurately assess a crude oil (unknown to them) for corrosion based on total sulfur content alone or a breakdown of the sulfur species.
6.2.3 Complicating Variables
6.2.3.1 Nature of the Hydrocarbon Phase
Sulfur compounds tend to concentrate in heavier liquid fractions, but given the same sulfur content and temperature, light gaseous hydrocarbons appear to be more aggressive. This may be due to different sulfur species, or could be a function of the heavier hydrocarbon wetting the surface and retarding the sulfidation corrosion reaction.
6.2.3.2 Si Content
Carbon steels with low-silicon (< 0.10 %) content can corrode at an accelerated rate when exposed to H2-free sulfidation corrosion conditions. Refer to Annex C for data showing the relative corrosion rates of high- and low-silicon content steels. In some applications, carbon steel will appear to be adequate based on measured corrosion rates until failure occurs at some undocumented or unidentified low-silicon component. Figure 3 and Figure 4 relate to a failure where there were differences in silicon content amongst the various components and the lower Si content steel components corroded at a greater rate. 6.2.3.3 Flow RegimeNelocity Carbon and low-alloy steels form a sulfide scale in sulfur-containing streams, which can retard the sulfidation rate. Under high shear stress conditions, the scale can be removed (particularly if naphthenic acid is present) causing accelerated corrosion. One example of high-velocity (high shear stress) effects was the severe sulfidation corrosion of carbon steel balance lines connected to atmospheric bottoms feed pumps. At a flow velocity reported to be at least 180 ft/s (60 mIs), the sulfide protective scale did not form and thus severe corrosion was noted. Components in the same system exposed to lower flow rates did not corrode severely. No flow or very low flow can also be detrimental. This can contribute to stratification of corrosive species, and cause preferential attack. In some cases, such as a column bottoms, low velocity or long residence time can allow more H2S to evolve.