Ammonium Bisulfide Corrosion | Materials And Corrosion Control
| Damage Mechanism | Ammonium Bisulfide Corrosion |
| Damage Description | · Aggressive corrosion occurring in hydroprocessing reactor effluent streams and in units handling alkaline sour water
· Hydroprocessing reactor effluent piping may be subject to erosion-corrosion by ammonium bisulfide · The metal loss is dependent on the ammonium bisulfide concentration, partial pressure of H2S, velocity, presence of hydrocarbon, and alloy corrosion resistance. Carbon steel performs well below 2 wt % NH4HS concentration but starts corroding with increasing concentration. The velocity dependence is also sensitive to NH4HS concentration · NH4HS salt deposits may lead to under deposit corrosion, fouling and plugging unless flushed away with wash water · In hydroprocessing reactors, NH4HS precipitates out of the gas phase in the reactor effluent stream at temperatures below about 150°F (66°C), depending on the concentration or partial pressure of NH3 and H2S |
| Affected Materials | Alloys ranked in increasing resistance are: carbon steel, 300 Series SS, duplex SS and nickel base alloys |
| Control Methodology | Use good REAC design (balanced, symmetrical), alloy selection done to meet the process conditions & process monitoring. Erosion-corrosion is best mitigated by using more corrosion-resistant alloys and/or altering the process environment to reduce corrosivity· NH4HS concentration, H2S partial pressure, velocity and/or localized turbulence, pH, temperature, alloy composition and flow distribution are all critical factors. · Maximum 20 ft/s has been taken to be the rule of thumb for carbon steel but this depends also on NH4HS concentration · For Incoloy 825 equipment, up to 40 ft/s maximum velocity is acceptable. · Oxygen in the wash water injected into hydroprocessing reactor effluent can lead to increased corrosion and fouling · Good wash water quality is essential & contaminants such as O2, CO2, NH3, chlorides, cyanides and Fe should be measured & minimized. Boiler feed water is a good source of water since it has been de-oxygenated but may still contain chlorides. |
| Monitoring Techniques | · Develop a plan to inspect specific areas of vulnerability, noting highly localized damage has been reported
· Determine ammonium bisulfide content through regular sampling and calculation. · Frequent UT scanning and/or RT profile thickness of high and low velocity areas. · UT downstream of control valves at high NH4HS concentrations. · EC inspect non-magnetic air cooler tubes. · IRIS, RFEC and flux leakage inspection of steel air cooler tubes. · Monitor water injection facilities and flow meters to ensure proper operation. |
| Inspection Frequency | · Frequent UT scanning and/or RT profile thickness of high and low velocity areas.
· Perform piping inspections at 3-year intervals but more frequent inspections (6 – 12 months) may be carried out at water injection points or other known problem areas. |
| KPIs | · Monitor wash water quality
· Target oxygen level at 50 ppb maximum & pH 7 to 9 · Maintain velocity limits of maximum of 20 ft/s for CS and 40 ft/s for Incoloy 825 · Regular NDT at vulnerable locations |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #7)
· API RP 932 · SAER-8942 |
Additional Information:
Hydroprocessing is meant to desulfurize and denitrogenize only; to desulfurize, denitrogenize, and modify the quality of the product; or desulfurize, denitrogenize, and crack the heavier feed into lighter products. In all the cases, H2S and NH3 are produced because this is how the process desulfurizes and denitrogenizes. For a fixed feed that does not change, the amount of H2S and NH3 produced depends on the properties of the feed; they are not parameters for corrosion engineers to specify. Changes can arise only as a consequence of changing the feed; that we can interfere by establishing that the unit is designed for certain feed and that changes leading to increased H2S and NH3 would require Management of Change approval.
Chloride sources for the process is either the hydrogen make-up or from the feed. Chloride can come into the unit when hydrogen is from catalytic reformer units or CCR chloride component is used in those processes to activate the catalyst and those compounds decompose to HCl that can be carried by the make-up hydrogen. There are usually chloride guards to remove the
chloride out of the hydrogen produced in catalytic reformer units or CCR before using it anywhere else.
Chloride can come with the feed and the most damaging one is the organic chloride that is not removed in the desalters at the crude units and can therefore makes its way to the hydroprocessing units.
Boiler feed water should have zero chloride but may have some caustic that is added to control its pH. Some refineries uses amine to achieve this but the most common chemical used to control the pH of boiler feed water is caustic (NaOH) that can have some chloride.
The parameter to control in BFW is oxygen. If condensate is used instead of BFW, CO2 may become a problem too. Both O2 and CO2 can make BFW corrosive to carbon steel.
Cyanide could be a very “Bad Actor” because it interferes with the iron sulfide layer that protects carbon steel from NH4HS and H2S wet corrosion. The result is increased corrosion rates but cyanide is more famous because it increases the hydrogen that enters the carbon steel so H2 blistering, HIC/SOHIC and even wet H2S stress cracking potential increased significantly. But cyanide is more likely produce in thermal cracking process like FCC, Cokers, and Visbreakers so its presence in hydroprocessing units is more related to where the feed or feed components are coming from rather that controlling a process parameter within the hydroprocessing unit.
The concentration of H2S and NH3 depends of the sulfur and nitrogen content in the feed.
If they are high, then there will always be enough produced H2S and NH3 to form NH4HS salts that will plug the tubes whether in air coolers or normal heat exchangers. NH4HS salts are not corrosive to carbon steel and to other common materials used in refineries if there is no presence of water. But because of plugging the tubes, the most economical way to deal with this is to use wash water. The introduction of water, however, makes the stream corrosive. It is only then that distribution and configuration of inlet and outlet piping begins to matter the most (REAC design, balanced, symmetrical)
The water removed from the cold flash separator D-140 in the J80 HCU in RTR typically contain NH4HS concentration of 2 to 5 wt%, most typically should be about 3.5 wt%.
The design should be based on this. Controlling NH4HS concentration is nothing to do with corrosion but rather to detect abnormal operating conditions or unpredictable changes in the feed.