Selection and Application of Chemical Control Valves

In the chemical industry, control valves play a crucial role, much like the regulators of blood vessels in the human body, precisely controlling the flow of various media to ensure the smooth and stable operation of chemical production processes. However, the chemical production environment is complex and ever-changing, with corrosive media omnipresent, posing significant challenges to the selection and application of control valves. Statistics show that approximately 60% of chemical equipment failures are caused by corrosion. An improper selection of control valves can lead to equipment damage at best, and accidents or even disasters at worst. Therefore, mastering the selection points of control valves under different corrosive media conditions is key to safe production in the chemical industry.

In chemical production, selecting the right material for control valves is essential because they often operate in corrosive environments. The wrong choice can lead to equipment damage or accidents. Understanding the characteristics of different corrosive media and choosing suitable materials is key to ensuring long-term stable operation of control valves.

Sulfuric acid is a widely used, highly corrosive medium. Its corrosive effect on materials varies significantly with concentration and temperature. For concentrated sulfuric acid (above 80% concentration, below 80°C), carbon steel and cast iron show good resistance but aren't suitable for high-temperature, flowing acid. High-silicon cast iron and high-alloy stainless steels (like Alloy 20) are better choices, despite the manufacturing challenges of high-silicon cast iron. Fluoroplastics, with their excellent resistance, make fluoroplastic lined valves a cost-effective option.

Hydrochloric acid is highly corrosive to most metals, including stainless steels. However, high-silicon cast iron with molybdenum performs well in environments with 50°C and 30% hydrochloric acid. Non-metallic materials like rubber-lined and plastic pumps (e.g., polypropylene, fluoroplastics) are ideal for conveying hydrochloric acid, maintaining stability in harsh conditions.

Most common metals corrode quickly in nitric acid, but stainless steel is widely used due to its resistance. Ordinary stainless steels (304, 321) resist all concentrations at room temperature. Molybdenum-containing stainless steels (316, 316L) offer no significant advantage and may even perform slightly worse. For high-temperature applications, titanium and titanium alloys are preferred due to their superior corrosion resistance.

Acetic acid is highly corrosive, especially to ordinary steel. Stainless steel, particularly molybdenum-containing 316, is effective and can handle high-temperature and dilute acetic acid vapor. In harsh conditions like high-temperature, high-concentration acetic acid or the presence of other corrosive media, high-alloy stainless steel or fluoroplastic pumps are better choices.

Steel shows good resistance to sodium hydroxide solutions up to 30% concentration and 80°C. Many plants use ordinary steel at 100°C and below 75% concentration, despite increased corrosion, due to cost benefits. Ordinary stainless steel isn't significantly better than cast iron for alkaline solutions and isn't recommended if the medium contains sodium chloride. For high-temperature alkaline solutions, titanium, titanium alloys, or high-alloy stainless steels are widely used.

Most materials show minimal corrosion in liquid ammonia and ammonia water, except for copper and its alloys. Material selection is flexible in processes involving these media, but copper and its alloys should be avoided.

Ordinary steel has a low corrosion rate in sodium chloride solutions and seawater but typically requires protective coatings. Various stainless steels have low uniform corrosion rates but can suffer local corrosion due to chloride ions. 316 stainless steel is widely used for its good corrosion resistance.

Control valves are the terminal control components in chemical automatic control systems. They regulate flow by changing the travel of the valve core, thereby altering the resistance coefficient of the valve. During the selection process, it is necessary to fully understand the chemical process, clarify the physical properties of the controlled media (such as composition, temperature, viscosity, density, inlet and outlet pressures, and flow rates), and master the structure and technical characteristics of the control valves.

The flow characteristic of a control valve refers to the relationship between the relative flow of the medium controlled by the valve and the relative displacement of the valve opening. When selecting the flow characteristic, it is necessary to follow the principle that the flow characteristic of the control valve is opposite to the characteristics of the controlled object and the controller, in order to achieve linear control and reduce control difficulty.

Linear Flow Regulation Characteristic: This characteristic is suitable for situations where the medium pressure difference is relatively stable and the pressure change fluctuation range is small. In the chemical process system, the flow control is linear, the internal pressure loss of the system is mainly controlled by the control valve, the control valve has a small interference with external disturbances when working, and the adjustable range of the control valve is relatively small.

Percentage Regulation Characteristic: This characteristic is suitable for situations where the flow range regulated by the control valve is large and the pressure difference acting on the control valve has a large variation range. In the automatic control process, the pressure loss of the system medium is large, and the control valve often works at a small opening.

The entire control system requires the control valve to operate with low energy consumption to ensure the normal operation of the system. However, these two characteristics cannot coexist in one system. If the system selects a control valve with a smaller opening under the same flow conditions, although the other resistances in the system will not increase, the total resistance in the system will increase, resulting in a larger pressure drop across the control valve. If the medium in the system is pressurized by a pump, an increase in system pressure will require the use of high-power pumps and motors, causing greater energy loss in the control valve. Therefore, in the selection of control valves, it is necessary to comprehensively consider the characteristics of the control valve itself and the impact of its pressure drop on the entire chemical plant system.

During the use of control valves, flashing and cavitation are two common phenomena that can severely affect the normal operation of control valves.

Flashing is a liquid spray sand phenomenon that mainly occurs in the valve body of the control valve and the downstream part of the fluid pipeline in the chemical plant. It can cause extremely severe erosion problems on the inner surface of the control valve and the chemical plant pipeline, while also reducing the flow capacity of the control valve. Therefore, during the selection of control valves, it is necessary to pay sufficient attention to the phenomenon of flashing.

Cavitation is a phenomenon where a liquid medium, when passing through the constriction in the control valve, vaporizes due to the high pressure of the fluid, causing the liquid medium passing through the control valve to turn into a gaseous medium. Cavitation produces a noticeable hissing sound during its formation, and in severe cases, it can produce a clacking sound in the control valve. Since the pressure generated by cavitation is variable and accompanied by violent vibrations, it can cause great damage to the valve body components inside the control valve and greatly affect the flow performance of the medium. Therefore, cavitation is also a phenomenon that should be avoided in the use of control valves in chemical plants.

To avoid the occurrence of flashing and cavitation, the following measures can be taken for control: During system design, try to locate the control valve at the lowest position in the system to increase the inlet and outlet pressures of the control valve; install globe valves or orifice plates upstream and downstream of the control valve to change the original installation pressure drop characteristics of the control valve; equip the control valve with special anti-cavitation internals to enhance its ability to combat these adverse phenomena; and when selecting control valves, prefer those with higher material hardness to increase their resistance to impact and corrosion.

Cavitation and noise are the main influencing factors when control valves control high-pressure difference media, among which valve noise is an important noise source in chemical production. Therefore, in the system design process of chemical plants, it is necessary to select low-noise control valves and adjust the operating conditions of the valves to reduce the impact of noise generation.

The causes of noise generation mainly include the following: mechanical vibrations, vibrations at natural frequencies, and instability of the valve core. These factors can all lead to varying degrees of noise generation during the operation of the control valve, affecting the normal progress of chemical production.

To reduce the impact of noise, the following measures can be taken:

Select Low-Noise Control Valves: In the selection process of control valves, prioritize those with low-noise characteristics to reduce noise generation at the source.

Adjust Operating Conditions: Reasonably adjust the operating conditions of the control valve, such as controlling the flow velocity and pressure of the medium, to reduce noise generation.

Implement Sound Insulation Measures: Install sound insulation equipment around the control valve, such as sound insulation hoods and walls, to reduce the impact of noise on the surrounding environment.

Optimize System Design: In the system design process of chemical plants, reasonably arrange pipelines and equipment to avoid resonance phenomena, thereby reducing the propagation of noise.

Selecting the appropriate chemical control valve is just the first step. Correct installation and commissioning are equally crucial, as they directly relate to whether the control valve can operate stably and efficiently in the chemical system.

Installation Position: Choose the appropriate installation position based on the type of control valve and its usage requirements. For example, for control valves that need to avoid flashing and cavitation, they should be installed at the lowest position in the system as much as possible.

Installation Direction: Ensure that the installation direction of the control valve is correct, so that the flow direction of the medium is consistent with the designed flow direction of the control valve, avoiding the control valve from failing to work properly due to incorrect installation.

Pipeline Connection: When connecting the control valve to the pipeline, ensure that the connection is secure and the seal is reliable, to avoid leakage problems caused by pipeline vibration or loose connections.

Adjust Valve Core Stroke: Adjust the stroke of the valve core of the control valve according to the actual working conditions, to ensure that the control valve can accurately regulate the flow within the required flow range.

Calibrate Control Signal: Calibrate the control signal of the control valve to ensure that the control signal is consistent with the actual movement of the control valve, improving the control accuracy of the control valve.

Check for Leaks: During the commissioning process, carefully check for any leakage in the control valve, promptly identify and handle any leakage issues, and ensure the sealing performance of the control valve.

The application of control valves in the chemical industry is extremely extensive, and their selection and application directly relate to the safe production and economic benefits of chemical production. When facing various corrosive media, it is necessary to scientifically and reasonably select the materials and structure of control valves based on the characteristics of the media and the working conditions, to avoid equipment damage or even accidents caused by improper selection. At the same time, it is essential to fully consider various factors such as the flow characteristics of control valves, energy consumption and pressure drop, flashing and cavitation, noise control, and installation and commissioning, to ensure that control valves can operate stably and efficiently in chemical systems. Only in this way can the smooth progress of chemical production be guaranteed, creating more economic and social benefits for enterprises.