We used a certain company’s anhydrous ammonia analysis tower bottom gate valve to corrode after 6 months of use in production, causing internal leakage. Let’s discuss how to choose the valve material in the ammonium phosphate solution pipeline.
The company’s synthetic ammonia production uses ammonium phosphate solution to absorb NH3 in the coke oven gas after desulfurization, and the ammonia phosphate-rich liquid after ammonia absorption in the analysis tower is heated to a boiling state to remove acid gases such as H2S, HCN, and CO2, and then steam is used to remove tar. Desorption, the ammonia is freed from the rich ammonium phosphate solution to produce 18% concentrated ammonia water. After systematic characterization and analysis of the chemical composition, metallographic structure, macro and micromorphology of the gate valve, and the composition of the fouling material, this paper determines the main reason for the gate valve failure and the failure mechanism and proposes corresponding preventive measures.
2. Overview of failed gate valves
The material of the failed gate valve is SUS304 stainless steel, the model is Z41Y-300Lb DN 80. The working condition of the analytical tower bottom gate valve is 196～198 ℃ ammonium phosphate (H3PO430%, NH37%, H2O steam), the density of the medium is 1.2～1.25, and the working pressure is 2.0 MPa. After the gate valve was disassembled, the surface of the gate was uneven, and a large amount of reddish-brown and black corrosion products and green powder were visible on it. It can also be seen on site that there are a lot of green crystals at the connection between the valve body and the valve cover and the valve body. Take one vertical and horizontal sample from the shutter (Figure 1). One side of the longitudinal sample is the inner surface of the gate. The transverse sample is the cut surface cut from the part shown in the figure, and the arc surface is the outer surface of the gate.
3. Failure characterization and analysis
3.1 Metallographic observation
Observed after the sample is polished and polished, the vertical and horizontal samples have scattered point oxides and a small amount of orange-red titanium nitride inclusions. These simple oxides and nitrides that do not have the ability of plastic deformation are brittle inclusions. For brittle inclusions with low deformation rate, in the process of steel processing and deformation, the deformation of the inclusions is very small compared with the steel matrix. Due to the significant difference in deformability between the inclusion and the steel matrix, stress concentration will inevitably occur at the interface between the inclusion and the steel matrix, leading to microcracks or the inclusion itself cracking.
Large loose holes can be seen on the transverse specimen (the cross-section of the ram specimen). The existence of porosity is more harmful. Due to the presence of porosity in the casting, its mechanical properties are significantly reduced, which may make it a source of fatigue and fracture during use. If there is porosity in the steel, it will also reduce its mechanical properties. However, since the porosity can generally be reduced or eliminated during the hot working process, the effect of the porosity on the properties of the steel is smaller than that of castings. There is serious porosity in the metal, which has a certain influence on the surface roughness after machining.
The vertical and horizontal microstructures of the samples were observed after etching. They were all composed of white austenite and gray-black ferrite. Irregularly distributed strip ferrite on the austenite and black dot-like carbides were precipitated. None see the exception.
3.2 Chemical composition analysis
Take sample materials for chemical composition testing (Table 1). From the analysis of the test results, the content of C and Cr in the gate material is higher than the 316L standard, while the content of Ni is lower than the 316L standard. Similar to 304 compositions. Therefore, it can be determined that the gate valve actually uses 304 steel instead of 316L steel. 304 steel has good heat resistance, low-temperature strength, and mechanical properties, but its corrosion resistance is not as good as 316L stainless steel.
3.3 Fouling analysis
X-ray fluorescence spectrometer was used to analyze the chemical composition of the green crystals in the gate valve. The mass fractions of the chemical elements and oxides are shown in Table 2 and Table 3. The NHO analyzer was used to analyze the deposits on the surface of the gate valve, and the mass fractions were 46%O, 411%N, and 2166%H, respectively. It can be seen that the powder contains high Fe, O, and S elements. S, N, and H come from the impurities HCN and H2S contained in the ammonium phosphate solution. Among the red-brown, black corrosive, and gray-green substances covered on the gate surface, the main components are Fe2O3, FeS, ammonium phosphate, and other impurities, respectively.
4 . Cause of failure
4.1 Material quality
According to the analysis of the chemical composition of the material, the material used for the failed gate is SUS304. The corrosion resistance of SUS304 is lower than that of SUS316L steel. Metallographic analysis shows that there are inclusions and looseness on the inner and outer surfaces of the ram. Their presence affects the mechanical properties and process performance of the material and has a certain effect on the roughness of the sealing surface of the ram, which is closely related to the failure of the ram.
According to metallographic analysis and morphological observation, it can be seen that there is corrosion on the surface of the gate. Since the deposits on the surface of the gate contain S, N, and H, this indicates that the H2S and HCN present in the medium participate in the electrochemical process of corrosion. The molar ratio of 30%H3PO4 and 7%NH3 in the medium is 1.35. From the relationship diagram of the pH value and the molar ratio of the ammonium phosphate solution, the pH value of the medium in which the valve is located is about 513, which is in an acidic state. The corrosion of the gate material by the medium is an electrochemical reaction process.
Anode: Fe →Fe2 + + 2e
Cathode: 2H+ + 2e →2H →H
The secondary reaction process is
Fe2 + + S2 – →FeS ↓Black precipitation
Fe2 + + 2OH– →Fe (OH) 2 ↓
4Fe (OH) 2 + O2 →4H2O + 2Fe2O3 ↓Red-brown precipitation
Fe2 + + 2PO3 –4 ︱→Fe3 (PO4) 2 ↓Yellow-white to beige precipitation
Fe2 + + 2CN – →Fe (CN) 2
Fe (CN) 2 + 4NH4CN → (NH4) 4 [ Fe (CN) 6 ] ↓ White precipitation
After ferrous hydroxide is formed, it immediately reacts with oxygen in the air or in the solution, so it quickly turns gray-green, and finally red-brown iron oxide. The white ammonium ferrocyanide, because the solution contains trace oxygen, easily oxidizes ferrous iron to ferric iron, forming iron blue NH4[ Fe Ⅱ (CN) 6Fe Ⅲ]. The color intensity of iron blue is very high. The color is almost black, so in addition to red-brown iron oxide and black iron sulfide, the corrosion products may also contain trace amounts of ferrous phosphate, ammonium ferrocyanide, and iron blue. When the gate valve is opened and closed, the lifting gate can scrape off the corrosive substances on the sealing surface, resulting in the gradual thinning of the gate surface, resulting in internal leakage.
From the observation of a large number of green crystals inside and outside the gate valve on-site, it can be seen that the cause of valve leakage is also scaling. Due to the different reaction conditions of H3PO4 and NH3 in the production process, NH4H2PO4, (NH4) 2HPO4, and (NH4) 3PO4 are generated respectively. The chemical stability of the salt gradually weakened. The three pure ammonium phosphates are all white crystalline substances, (NH4) 3PO4 is unstable, and it separates out ammonia at room temperature. Under normal working conditions, the medium (30% H3PO4, 7% NH3, and H2O steam) runs stably in a pipeline with a temperature of 196 to 198 ℃ and a working pressure of 210MPa. When the ram corrodes and leaks, the medium runs at room temperature and normal pressure. At this time, the solubility of ammonium phosphate decreases, and ammonium phosphate crystallizes out of the supersaturated solution. When the molar ratio of NH3/ H3PO4 is 1.35, impurities such as Fe, Al and Mg are formed when ammoniated to a certain degree of neutralization (Fe, Al) NH4 ( HPO4 ) 2 · 0.5H2O, Mg ( NH4 ) 2(HPO4) 2 · Complexes such as 4H2O and MgNH4PO4·H2O make the color of the crystal green. The crystal is filled between the gate and the valve body, and it is not flushed in time when opening and closing, causing excessive squeezing of the sealing surface of the gate, making the sealing surface more uneven. This makes the sealing surface of the gate more prone to electrochemical corrosion, leakage caused by corrosion, and more ammonium phosphate crystallizes out of the solution.
Through the analysis of the reasons for the damage of the gate, it can be known that the selection of the wrong valve material or the use of unqualified materials for the valve leads to corrosion and fouling are the main reasons for the leakage of the valve in a corrosive environment. In order to prevent similar accidents, it is recommended to use SS316 for the valve in the ammonium phosphate solution pipeline. SS316L contains higher Ni and added Mo, although the price is higher. Ni is an element that expands the austenite region. It is usually used to form and stabilize the austenite structure. Adding an appropriate amount of Ni can significantly improve the corrosion resistance of this type of steel in non-oxidizing and weakly oxidizing media. At the same time, the content of H2S and HCN at the outlet of the absorption tower should be strictly controlled so that the coke oven gas after desulfurization reaches the standard and enters the analysis tower.