Material Considerations for Control Valves in Harsh Environments

 
Control valves are pivotal terminal control components in the process control industry, whose primary function is to regulate the flow of fluids to compensate for load disturbances, thereby keeping the controlled process as close to the set point as possible. Due to their significance in industrial automation, the design and manufacturing of control valves must rigorously consider various operating conditions, especially in harsh environments such as high temperatures, high pressure differentials, high flow rates, and cavitation, where material selection and performance are crucial.

Material Importance

 
The selection of materials for control valves directly affects their performance and lifespan. When designing control valves, various factors must be considered, such as the material's creep, thermal expansion rate, oxidation resistance, wear resistance, and heat treatment temperature. Under high-temperature conditions, the properties of materials can change significantly, thus necessitating particular attention to the following aspects.

1. Creep and Fracture at High Temperatures

 
At high temperatures (such as 427°C), creep and fracture of materials are the main factors of failure. For instance, carbon steel may undergo a transformation of its carbide phase into graphite in a long-term high-temperature environment, affecting the material's strength. For austenitic stainless steel, its applicable temperature is typically above 528°C, but only applicable when the carbon content exceeds 0.4%. Therefore, in high-temperature applications, it is necessary to calculate the tensile strength, creep, and high-temperature aging parameters of the valve body material.

2. Material Hardness and Thermal Expansion

 
Under high temperatures, the hardness and thermal expansion coefficient of materials will affect the performance of the valve internals. For example, different linear expansion coefficients of the valve body and seat may lead to jamming, so it is necessary to appropriately increase the clearance during design to ensure normal operation under high temperatures. The thermal expansion coefficient of the material, the difference in thermal hardness of the guide parts, as well as elastic and plastic deformation, also need to be comprehensively considered.

Design Considerations Under High-Temperature Conditions

 
Under high-temperature conditions, the design of control valves requires special attention to the performance of materials and the structure of the valve to ensure its reliability and long-term operation under extreme conditions. The following are the main considerations for design under high-temperature conditions.

1. Oxidation Resistance

 
At high temperatures, the oxidation resistance of materials is very important. Materials that can withstand high temperatures, such as austenitic stainless steel, hard alloys, and special alloys, are usually chosen for their better high-temperature stability. During temperature cycling, it is necessary to avoid repeated oxidation of the material surface, which can lead to issues such as scale formation.

2. High-Temperature Aging Impact

 
High-temperature aging can lead to changes in the physical properties of materials, such as a decrease in toughness and intergranular corrosion. Valve internals may experience annealing and a reduction in hardness when used at temperatures reaching or exceeding the heat treatment temperature. Therefore, when selecting materials, it is necessary to ensure that their hardness remains within a safe range under high-temperature conditions.

3. Sealing Design

 
Under high-temperature and high-pressure differential conditions, the sealing design of the valve body and seat is particularly important. High-temperature resistant sealing materials, such as flexible graphite or composite materials, should be chosen, and attention should be paid to changes in the coefficient of friction. The seat sealing surface structure that undergoes periodic temperature changes can adopt a self-centering wedge structure to achieve automatic centering and compensation, thereby improving sealing performance.

Material Requirements for High Pressure Differential and High Flow Rate

 
Under conditions of high pressure differential and high flow rate, the design and material selection of control valves become particularly important. Since these operating conditions can impose great stress and wear on the valve, the following aspects require special attention.

1. Material Characteristics

 
Under high pressure differential and high flow rate conditions, even at room temperature, the characteristics of materials must be evaluated to meet the requirements of the operating conditions. When the pressure differential exceeds 15 bar, it should be considered to adjust the material of the valve core and seat from 316 stainless steel to higher requirements, such as Stellite alloy cladding or 420QT, etc. For weakly corrosive media, materials such as 420QT (quenched and tempered) can be selected.

2. Erosion and Cavitation

 
High pressure differential and high flow rate can lead to severe erosion or cavitations, which can cause significant damage to the valve internals. The material requirements for the valve body and internals are high, and it is necessary to consider using stainless steel surface nitriding treatment (HRC70) to improve erosion resistance and extend service life. The valve design also needs to add multi-layer cages behind the valve to reduce pressure step by step, change the material of the valve body and internals, or increase the flow area of the valve to reduce flow velocity, thereby reducing cavitation and erosion problems.

Material Plastic Deformation and Cavitation Issues

 
In the design of control valves, plastic deformation and cavitation are two important material issues, especially under conditions of high temperature, high pressure differential, and high flow rate. These issues can significantly affect the performance and service life of the valve, so they require special attention and resolution.

1. Plastic Deformation

 
Plastic deformation is a phenomenon where the metal surface is worn due to friction with other materials, which can lead to valve jamming, damage to the sealing surface, or increased friction. High temperatures soften the metal, increasing the tendency for plastic deformation. The presence of larger particles in the fluid can also exacerbate plastic deformation, so when selecting materials, their plastic deformation characteristics should be considered, and measures should be taken in the design to reduce the impact of plastic deformation.

2. Cavitation and Flash Vaporization

 
Under liquid conditions, cavitation and flash vaporization can cause structural damage to the valve and adjacent pipelines, and increase vibration and noise. Measures such as adding multi-layer cages behind the valve, changing the material of the valve body and internals, and increasing the flow area of the valve should be taken to mitigate the impact of cavitation and flash vaporization on the valve.
 
The design and material selection of control valves need to comprehensively consider the impact of high temperature, high pressure differential, high flow rate, and cavitation on the material performance. Choosing the appropriate materials, reasonably designing clearances, and taking effective measures to prevent cavitation and flash vaporization can significantly improve the service life and reliability of the valve. Designers should refer to relevant standards and data to ensure the best performance of the control valve under various operating conditions.