Continuous wash water injection in gas recovery units (GRU) has always been recognized as a best practice method for corrosion control. Among other functions, wash water injection in these systems traps cyanides and carries them away with the stream, thereby efficiently reducing the negative impact of cyanides on the system.
The main damage mechanisms likely to impact GRUs are sour water corrosion (SWC), hydrogen induced cracking (HIC), and stress oriented hydrogen induced cracking (SOHIC). All these damage mechanisms are closely related to the presence of H2S in an aqueous environment. Presence of cyanides in the system exacerbates these corrosion and cracking tendencies, since cyanides are known to inhibit the combination of atomic hydrogen to form molecular hydrogen (H2) gas. If this combination reaction does not take place, the steel wall becomes saturated with atomic hydrogen. The permeated atoms may eventually combine at microstructural defects and cause wet H2S damage, which could lead to cracking and through-wall penetration.
The case study described in this paper discusses failures that occurred in the wash water injection system. These failures caused the GRU to operate in the absence of wash water which meant that cyanides were not efficiently trapped or removed. The onset of hydrogen permeation was detected and measured using field inspections and hydrogen permeation probes. Condition monitoring was also performed downstream in the amine recovery unit (ARU) by tracking the level of heat stable salts. These measures helped reactivate the wash water injection program in a timely manner and avoid potential future failures downstream in the GRU compression systems.
Gas recovery units (GRU) remove components such as elemental sulfur in a series of stages. Continuous wash water injection has always been a very important method for corrosion control in the GRU. Figure 1 shows a simplified process diagram of a GRU plant in a refinery, specifically highlighting the main lines and equipment associated with the wash water injection system. The wash water system consists of water from the sour water stripper unit; this stream is combined with boiler feed water to increase the available volume. However the boiler feed water does not have the capabilities to trap cyanides. The GRU discussed in this paper is downstream of a delayed coker unit (DCU).