2. Corrosion analysis of the inlet and outlet pipelines of the acid pump
2.1 Corrosion analysis of the inlet pipeline
According to the requirements of PetroChina's alkylation unit corrosion control, when carbon steel is used, the flow rate of sulfuric acid should be controlled at 0.6 m/s to 0.9 m/s. Through the calculation of process equipment parameters, it can be calculated that the flow velocity of sulfuric acid in the cross-section of the pipe of DN100 is 0.3399 m/s, and that of the cross-section at the flange of DN40 is 2.449m/s. Through comparison, it is found that the flow rate at the flange is much faster than the recommended flow rate of 0.9m/s, and 2229-P-702 is an intermittently operating pump. From the beginning of alkylation to June 2019, the operating time is about 1120 hours, and the annual corrosion rate is close to 39mm/a; the design is flawed. The cross-section of the inlet pipeline of 2229-P-701AB is the same as that of the 702 pump, and the operating flow is 2.2m³/h. By calculating the flow velocity of the inlet pipeline, the cross-sectional flow velocity at the inlet flange of DN40 is 0.22m/s under normal conditions, which is much lower than 0.6m/s. The pickling system is not adopted for the alkylation unit, so the actual working conditions of 2229-P-701AB deviate from the designed working conditions. The actual process demand flow rate (0.9m³/h) is much lower than the design flow rate (2.2m³/h). As a result, the temperature of the sulfuric acid during the operation of the pump is too high, and the maximum temperature of the outlet pipeline reaches 65°C.
2.2 Corrosion analysis of outlet pipelines
The viscosity of 98% concentrated sulfuric acid at room temperatures is 25.8×10-3Pa·s (The water viscosity is 1×10-3Pa·s.). According to the sulfuric acid flow rate v, density ρ, viscosity η, and tube radius r, the 25°C concentrated sulfuric acid can be obtained. The Reynolds number in the acid pipeline of DN40 is as the following:
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When the temperature of concentrated sulfuric acid reaches 70℃, the maximum Reynolds number is 1000. Since the Reynolds number is less than 2300, the flow state is laminar. When the motion law of viscous fluid is discussed, compressibility can still be ignored, but its viscosity must be considered. In the straight pipe section, the flow rate of concentrated sulfuric acid is slow, and this area is called the slow flow area. However, when concentrated sulfuric acid flows in the pipeline, there is partial resistance. Partial resistance is that when the fluid passes through the pipe fittings and valves in the pipeline, the boundary layer separates and generates vortices due to partial obstacles such as changes of diameters and directions, resulting in energy loss. At the same time, the velocity of the part where there is the vortex is increased. This area is called the turbulent flow area. There are many areas where partial resistance exists in the alkylated acid pipeline, such as the pump outlet flange, elbow, reducing tee, gate valve and globe valve. All the corrosion and leakage parts of the alkylated acid pipeline are concentrated in these parts.
At the flange of the outlet of the pump, sulfuric acid flows in the suddenly enlarged pipe. When the fluid flows from a pipe with a small diameter to a pipe with a large diameter, the main stream first shrinks and then expands, forming a vortex between the corner of the pipe wall and main stream. The vortex continues to rotate under the force of the main stream. The passivation film in the pipe wall is unstable due to the friction between the surrounding solid wall and other fluid particles, causing the straight pipe section behind the pump outlet flange to be continuously scoured, corroded and perforated. For tees, when fluid flows from a straight pipe to a branch pipe, because the cross-sectional area instantly increases and the stream expands sharply, the root of the branch pipe where the medium flows is scoured. The main flow flows to the main pipe due to the inertia effect, and the expanded stream flows to the branch pipe after passing through the root of the branch pipe. In the above process, friction and energy loss are caused by continuous scouring. A stable passivation film cannot be formed at the root of the branch pipe, resulting in corrosion and perforation. The loss of concentrated sulfuric acid flowing in the bend consists of three parts. One part is the loss along the way caused by tangential stress; another part is the loss caused by the vortex, and the other part is the loss caused by the double spiral flow formed by the secondary flow. These vortices cause severe corrosion of the concentrated sulfuric acid on the outer side of the outer bend. As for socket welding elbows, since the outer elbow side is almost at a right angle, the internal cross-section changes variously, which will form many complex vortices and corrode the outer bend side.
The vortex of the elbow Corrosion on the inner bend side of the elbow
When the valve is open, there are many vortices caused by partial resistance inside the valve. The materials of valves of alkylation and acid pipelines are all carbon steel, so inner corrosion easily happens, causing the valve to fail to be closed or valve plate to fall off. In order to eliminate the risk of leakages of sulfuric acid, the overall acid pipeline's material has been upgraded from 20# steel to 316L, and leakages never happen. Through thickness measurement, it is found that the annual corrosion rate is up to 0.1mm/a.
| Chemical composition |
Carbon (%) |
Chromium (%) |
Nickel (%) |
Molybdenum (%) |
Copper (%) |
Manganese (%) |
Silicon (%) |
| 904L |
0.02 |
23 |
28 |
5 |
2 |
2 |
1 |
| 316L |
0.03 |
18 |
14 |
3 |
0 |
2 |
1 |
| 20# |
0.24 |
0.25 |
0.25 |
0 |
0.25 |
0.65 |
0.37 |
| 10# |
0.13 |
0.15 |
0.25 |
0 |
0.25 |
0.65 |
0.37 |
The above table is the chemical composition table of the acid related pipeline material in the alkylation. It can be found that the most severely corroded 20# steel has the highest carbon content. Coupled with the peeling of the original passivation layer, the carbon content is high, resulting in more and more corrosion cells formed by carbonization in the steel and substrate. Moreover, concentrated sulfuric acid is used as the electrolyte, which makes the corrosion faster.
3. Conclusion
In order to prevent the corrosion and leakage of the pipeline by concentrated sulfuric acid, the following measures should be taken:
(1) Use acid resistant materials or steels with low carbon content in the sudden flow parts, such as lining PTFE, 316L, 904L, etc.
(2) If it is a must to use carbon steel pipe fittings, it is necessary to eliminate the sudden flow part in the pipeline as much as possible, such as using elbows with large radius to reduce variable diameters, or the electron migration caused by metal corrosion being able to be inhibited and the occurrence of corrosion being avoided or weakened through cathodic protection technology.
(3) In the pipeline's welding process, the bottom layer is not allowed to have obvious bumps so as to ensure the smooth formation of the welded seam inside the pipeline.
(4) Ensure that the sulfuric acid's temperature in the pipeline is close to 25°C at room temperatures, and the flow rate is less than 0.9m/s.
