Selection of Valves for Harsh Working Conditions in Coal Chemical Industry

The CO conversion process is an important purification step in the coal chemical industry. In the conversion process, to reduce the NH₃ content in the conversion condensate system, a condensate stripping system is established. The low-temperature conversion condensate has a high NH₃ content and needs to be sent to the stripping tower to remove dissolved NH₃. The low-temperature conversion condensate enters the stripping tower after undergoing heat exchange in the tower top condenser. The top condensate flows back to the top of the tower through the top condenser and the reflux pump. The non-condensable gas is sent to the sulfur recovery unit, while the condensate at the bottom of the stripping tower is sent to the gasification unit for recycling. The stripping system process is illustrated in Figure 2, and the parameters of the low-temperature condensate in the stripping system are shown in Table 2.
 
Table 2 Parameters of low-temperature condensate in stripping system

 

The conversion process condensate contains CO₂, NH₃, H₂S, H₂, CO, Cl⁻, and other components, with a complex composition and unknown corrosion mechanisms. Currently, there is no unified analysis or understanding of the corrosion mechanisms of the condensate in the conversion process. In terms of the degree of corrosion, the equipment and pipelines of the stripping system are currently the most severely corroded, particularly the pipeline after the condensate reflux pump and the pipeline from the gas at the top of the stripping tower to the tower heat exchanger. The tower reflux pipeline frequently experiences corrosion and penetration, and the valve also corrodes. The valve at the top of the stripping tower leaks after two weeks of use. Figure 3 shows a photo of the valve after corrosion damage.


Figure 2 Flow Chart of the Stripping System


Figure 3 Internal Corrosion of Stainless Steel 321 Stripping Valves
 

Despite the corrosion caused by H₂S, H₂, and CO₂, the conversion process has not mitigated the stripping system’s corrosion, transitioning from the use of austenitic stainless steel 0Gr18Ni10Ti (ASME grade 321) in the early stages to equipment and valves made of 022Cr17Ni12Mo2 (ASME grade 316L) later. Research suggests that the corrosion and damage to valves in the stripping system pipelines primarily arise from the following factors:

(1) Cavitation
The outlet medium at the top of the stripping tower is in the gas phase. In the tower top condenser, it transforms into stripping tower condensate after heat exchange. During the gas-liquid phase change, the two phases coexist, leading to cavitation. When multiple corrosive media coexist, they can cause severe cavitation in the valve.

 

(2) Cl⁻ and CN⁻ Corrosion on Metals
Due to the internal structure of the valve, there are numerous "gaps" and "dead corners." Additionally, the presence of Cl⁻ and CN⁻ disrupts the dynamic balance between dissolution and reformation of the passivation film on austenitic stainless steel in the condensate, making it easier for the valve body and valve cavity to develop small corrosion pits at contact points with the condensate, compared to ordinary straight pipes. These pits can then evolve into pitting nuclei, exacerbating pitting and crevice corrosion.

 

(3) Ammonia Corrosion
Ammonia forms stable complex ions with metal ions, reducing the standard electrode potential between metal ions and elements, thereby making the metal more susceptible to oxidation and corrosion.

 

(4) H₂S Low-Temperature Corrosion
H₂S low-temperature corrosion refers to the corrosion of metal by hydrogen sulfide at temperatures below 200°C. It primarily includes three types of corrosion: chemical, electrochemical, and stress corrosion cracking. The first two types of corrosion affect carbon steel and low-alloy steel, while stress corrosion cracking also occurs in austenitic stainless steel.

 
In recent years, based on production experience and exploration by a coal chemical enterprise, the valves in the stripping system were replaced with steel-lined ceramic valves, solving the leakage and short lifespan issues of the previous stainless steel stripping valves and significantly extending the valves' service life. After 15 months of use, the ceramic valves in the stripping system showed no significant corrosion or leakage, as shown in Figure 4.


Figure 4 Use of Stripping Ceramic Ball Valves
 

High-pressure oxygen from the air unit is primarily used as a catalyst for the gasification furnace in the gasification unit. The operating pressure is 7–8MPa. The temperature is room temperature, and the valve pressure class ranges from Class 600 to Class 1500. The primary valve types include globe valves, ball valves, and check valves.
 

High-pressure, high-purity oxygen is an extremely dangerous medium. If particles such as rust are present on the inner wall of the pipeline, collision and friction can easily trigger explosions. Therefore, the selection, structure, materials, manufacturing, and construction of oxygen valves require special measures, and piping must also meet specific requirements to ensure the safety of oxygen pipelines and prevent collisions.

In addition, to prevent oxygen from entering the pipeline, oxygen check valves are installed in the pipelines connected to the oxygen system. If the oxygen check valve fails to isolate, oxygen entering other medium pipelines may cause combustion and explosions. An explosion occurred at the valve flange of the purge pipeline in the oxygen system at the gasifier center of a coal chemical enterprise during normal production. The specific explosion location in the oxygen pipeline is shown in Figure 5.


Figure 5 Explosion of Oxygen System Purge Pipeline
 

The design and use of oxygen valves in coal gasification units are primarily based on the following aspects:
(1) Selection of Valve Materials
Select valve materials that meet the requirements of relevant oxygen pipeline specifications based on the pressure, flow rate, and specific working conditions. The valve bodies of high-pressure oxygen valves in coal gasification units currently primarily use nickel-based alloys, such as Monel and Inconel.
(2) Valve Selection
If the valve selection is unreasonable, the oxygen flow rate in the channel may be too high, which significantly increases the risk of ignition and explosion. High-pressure oxygen valves primarily include globe valves, ball valves, and check valves. The friction between the gate and the valve seat of a gate valve can easily cause sparks, making it unsuitable for oxygen applications. Ball valves are fast-opening and fast-closing valves. Rapid opening in a high-pressure oxygen medium may cause combustion and should be avoided whenever possible. The sealing of clamp-type check valves is unreliable and can easily lead to pipeline leakage. Therefore, oxygen pipelines typically use oxygen-specific valves, which have special requirements regarding product structural design, material selection, manufacturing processes, degreasing, and anti-static measures, differing from ordinary valves.
(3) Piping Optimization
The European Industrial Gas Association standard, 'Oxygen Pipeline and Piping Systems,' classifies oxygen working conditions into 'impact conditions' and 'non-impact conditions.' High-pressure oxygen flows through the pipeline corridor into the gasification device and is ultimately transported to the gasifier. The direction of the oxygen flow changes suddenly, generating vortices, which creates 'impact conditions.' Numerous valves are present before the high-pressure oxygen enters the gasifier in the coal gasification device, which can easily cause turbulence and impact before and after the valves. The layout of the oxygen pipeline should comply with the relevant provisions of the oxygen specification for 'impact conditions,' meet the requirements for the straight pipe sections before and after the valve, and preferably use long-radius elbows with a bending radius greater than 1.5 times.
(4) Operation Process
During production operations, the manual oxygen valve should be opened slowly. When the high-pressure oxygen valve equipped with a small bypass valve is opened, the small bypass valve should be opened first to gradually pressurize the system. The main oxygen valve should be opened when the pressure difference across the oxygen valve is ≤0.3MPa to avoid adiabatic compression and a sudden increase in partial temperature.
 

In recent years, technical advancements resulting from the long-term stable operation of domestic coal chemical enterprises and the technological progress of equipment manufacturing have progressively resolved the technical equipment problems that have plagued coal chemical projects and enterprises. As the control components of the fluid transmission systems in the petrochemical and coal chemical industries, valves serve functions such as diversion, cutoff, regulation, and prevention of backflow. The safety and appropriateness of the valve material play a crucial role in the safe operation of the valve itself, the pipeline system, and the overall functioning of the entire facility. The manufacturing processes and standards for valves and pipes in China's coal chemical industry have achieved significant breakthroughs and advancements. Currently, the coal chemical industry relies on technological innovation to facilitate its upgrading and achieve high-quality development. It is anticipated that with the advancements in coal chemical technology, the technical standards of China's valve industry will further improve.