Solving Control Valve Leakage in Petrochemical Refineries

In petrochemical refineries, control valves play an indispensable role. They are not only numerous and diverse in type but also face numerous challenges during chemical production. Because the media involved in chemical production are mostly highly corrosive, toxic, or flammable and explosive, any leakage of control valves can have unimaginable consequences. This not only leads to serious waste of raw materials, energy, and products, but also causes severe environmental impacts, and may even trigger major safety accidents. Therefore, thoroughly understanding the causes of control valve leakage and taking effective control and preventive measures is crucial for ensuring the safe operation of petrochemical refineries.

Main Causes of Control Valve Leakage

In petrochemical refineries, leakage of control valves has always been an important factor affecting production safety and efficiency. To effectively solve this problem, it is first necessary to thoroughly analyze the main causes of control valve leakage. Only by identifying the source of leakage can targeted measures be taken for control and prevention. The following discusses four aspects in detail: valve body leakage, valve stem leakage, valve body connection leakage, and internal leakage.

1. Valve Body Leakage

Valve bodies are usually manufactured through casting processes, which can easily produce casting defects such as sand holes. Sand holes can cause media to seep from the inside of the valve. Although this type of leakage is usually small in flow, it can be detected in time through hydrostatic testing. Therefore, hydrostatic testing is an important method for detecting valve body sand hole leakage and can effectively prevent leakage caused by sand holes.

2. Valve Stem Leakage

The valve stem is an important component of the control valve, responsible for achieving the opening, closing, and regulating functions. However, stem leakage is relatively common, mainly due to improper design and material selection. If the stem is stuck in a certain position, the valve cannot close properly or seal tightly, resulting in media leakage. This type of leakage not only affects production efficiency but may also pose safety hazards. Therefore, in design and material selection, the strength, toughness, and corrosion resistance of the valve stem must be fully considered to ensure stable operation in complex chemical environments.

3. Valve Body Connection Leakage

Sealing at valve body connections is one of the key aspects of control valve sealing. The seal between the valve body and the bonnet usually uses either flange connection sealing or threaded connection sealing. Flange connection sealing is suitable for control valves with larger nominal diameters, while threaded connection sealing is suitable for smaller diameters. However, regardless of the sealing method, if the type of gasket is unreasonable, the material quality is substandard, the material size does not meet sealing requirements, the flange sealing surface is poorly machined, or the threaded connection tightness and bolt fastening are inadequate, leakage may occur at the valve body connection. Therefore, selecting appropriate gasket materials and sealing methods, and ensuring the machining quality of sealing surfaces and connection tightness, are crucial to prevent leakage at valve body connections.

4. Internal Leakage

Internal leakage of control valves is mainly caused by inadequate valve closure, usually occurring at the sealing surface of the valve seat. Specific causes may include unreasonable structural design of the control valve, manufacturing and construction process issues, such as dimensional errors of a component exceeding allowable limits. In addition, deformation of the valve seat sealing surface or the presence of small solid impurities in the media can also result in inadequate closure, causing internal leakage. Internal leakage typically manifests as seepage, with flow that may be small or large depending on the situation. Therefore, optimizing the design and manufacturing of control valves, improving the quality of valve seat sealing surfaces, and ensuring media purity are effective measures to prevent internal leakage.

Principles for Control Valve Leakage Prevention

The principles for controlling and preventing control valve leakage involve taking a series of effective measures to minimize leakage as much as possible and reduce it to the lowest level, thereby extending the valve's service life. This not only helps improve the lifespan and utilization of media within the refinery but also effectively reduces production costs, decreases environmental pollution, and ensures production safety. Reducing leakage and extending service life largely depends on reasonable design and selection of control valves, high-quality valve products, advanced installation and construction techniques, and the correct choice of sealing methods. Therefore, to solve and control leakage issues, it is necessary to start with optimizing the design and selection of control valves.

Optimization of Control Valve Design and Selection

Optimizing control valve design and selection is a systematic project involving valve type selection, valve design and manufacturing, and material selection.

1. Selection of Valve Type

When selecting a control valve type, comprehensive optimization should be considered from the perspective of process requirements and design specifications. Factors such as valve purpose, medium temperature, pressure, flow rate, pressure drop, and corrosiveness directly affect valve selection. For high-temperature, high-pressure, and strongly corrosive media, valves resistant to high temperature, high pressure, and corrosion should be selected. For low flow rate and low pressure drop media, relatively simple valve structures can be chosen. In addition, the materials used to manufacture the valves should be selected according to the medium's temperature and corrosiveness. In practice, in addition to meeting process requirements and design specifications, various specific conditions should be considered so that the control valves match operational conditions as much as possible and meet usage requirements to the fullest extent.

2. Valve Design and Manufacturing

The design and manufacturing quality of control valves directly affects their sealing performance and service life. During the design process, the structural rationality of the valve must be fully considered to ensure precise fit and sealing performance between components. For example, the design of the valve seat sealing surface should minimize deformation and improve sealing reliability. During manufacturing, strict control of component dimensional accuracy and surface quality is necessary to ensure the sealing performance meets design requirements. In addition, using advanced manufacturing processes and equipment to improve valve quality is an important part of optimizing valve design and selection.

3. Material Selection

Material selection is a key factor in optimizing valve design and selection. Because the media involved in chemical production are highly corrosive, toxic, or flammable and explosive, it is essential to select materials that are corrosion-resistant, high-temperature resistant, and wear-resistant. For strongly corrosive media, materials such as stainless steel or alloy steel can be chosen. For high-temperature media, ceramics or high-temperature alloys may be used. In addition, suitable sealing materials, such as flexible graphite rings or fluoropolymers, should be selected according to the nature of the medium and operating conditions to improve sealing performance and service life.

Optimization of Control Valve Sealing Technology

In petrochemical refineries, sealing performance is one of the key factors ensuring normal valve operation. Optimizing sealing technology can effectively reduce leakage risk and significantly extend valve life.

1. Optimization of Packing Seals

Traditional soft packing relies on axial pressure from the packing gland, generating radial contact stress between the stem and packing, and between packing and packing box sidewalls, to achieve sealing. However, this sealing method has disadvantages: the axial force must be large, increasing friction torque and wear, and soft packing wears quickly, requiring frequent tightening or replacement. Therefore, using suitable packing or packing combinations can improve reliability and extend valve life. For example, combining flexible graphite ring packing provides better sealing than using only flexible graphite rings. In China, single flexible graphite rings are still commonly used, whereas abroad, combination packing methods have begun to gain popularity with good results.

2. Optimization of Valve Body Connection Seals

Sealing at valve body connections is a static seal, and it must meet the following requirements: withstand rapid changes in temperature and pressure; allow multiple disassemblies without damage; simple and compact structure with minimal metal consumption; not sensitive to vibration or impact; and suitable for all working media. In recent years, O-ring seals have been widely applied due to their simple structure, ease of manufacture, and good sealing performance. Compared with traditional groove or flat gasket seals, O-rings have the advantage of providing sufficient radial compression after assembly without axial loading, which reduces flange size and valve weight. Flat gasket seals, which are easy to remove during disassembly and have good manufacturability, remain a commonly used method. Flat gasket materials can be selected based on process parameters and fluid properties, such as aluminum, copper, 1Cr18Ni9Ti, or rubber asbestos sheets. Fluoropolymers require caution because of cold flow properties that may cause issues if the seal is poorly designed.

Optimization and Treatment of Valve Stem

The valve stem is a critical component for transmitting motion and achieving valve opening, closing, and regulation. As it functions as a moving part, load-bearing part, and sealing element, it must have sufficient strength and toughness to meet operational requirements and support the valve's regulating role. Materials resistant to medium and packing corrosion with good process performance, such as stainless steel or alloy steel, are commonly used. To further improve wear and corrosion resistance, surface treatments such as hard chrome plating or nitriding may be applied. These treatments effectively prevent stem corrosion and erosion by the medium, controlling stem leakage.

Conclusion

Control valves play a crucial role in petrochemical refineries. Leakage can lead to waste of raw materials, energy, and products, severe environmental pollution, and major safety incidents. Therefore, thoroughly understanding leakage causes and implementing effective control and preventive measures is essential. Optimizing valve design and selection, improving sealing technology, enhancing stem corrosion and wear resistance, and strengthening regular maintenance and inspection, along with proper operator procedures, can effectively reduce leakage risk, extend valve service life, increase production efficiency, and ensure safe operation.