Relationship Between Seals and Media in Industrial Applications

In the industrial sector, the relationship between seals and media is of paramount importance. The primary function of seals is to prevent media leakage and ensure the normal operation of equipment. Whether in the oil, natural gas, chemical, or water treatment industries, seals play an indispensable role. They not only protect equipment from media erosion but also prevent harmful substances from leaking into the environment, ensuring personnel safety and environmental protection. However, the performance of seals depends not only on their own materials and design but also on the properties of the media. Factors such as the chemical nature, temperature, pressure, and abrasiveness of the media can significantly affect the performance of seals. Therefore, selecting the appropriate seals to cope with different media conditions is key to ensuring the long-term stable operation of equipment.

The chemical compatibility between seals and media is the primary factor to consider when selecting seals. If the material of the seal reacts chemically with the media, it may lead to a decline in the seal's performance or even damage. For example, certain chemicals can corrode the material of the seal, causing it to lose its sealing capability. Therefore, when selecting seals, it is essential to ensure that the material of the seal is chemically compatible with the media.

Take polytetrafluoroethylene (PTFE) as an example. This material has excellent chemical stability and can resist the erosion of most chemicals. Therefore, PTFE is commonly used in seals that come into contact with corrosive media such as strong acids and strong bases. However, even PTFE has its limitations. For instance, its performance may deteriorate at high temperatures. Therefore, in high-temperature environments, other more suitable materials, such as carbon graphite, may need to be selected.

The particles in abrasive fluids can significantly affect the performance of seal elements. These particles may enter the interior of the seal elements, causing wear and damage. This is especially true for seal elements located at the bottom of equipment, where only a portion of the load applied by the gland can be transmitted to the bottom, making these elements more susceptible to the effects of particles.

For example, in some industrial equipment, fluids containing suspended particles can evaporate and crystallize on the side of the packing near the external air. This phenomenon not only causes wear on the packing but may also lead to problems with the actuator. Therefore, when dealing with abrasive fluids, special attention must be paid to the wear resistance and erosion resistance of seal elements.

Pressure is another important factor that affects sealing performance. According to Bernoulli's equation, the flow rate variable is proportional to the square of the pressure variable. This means that as pressure increases, the difficulty of sealing also increases significantly. For example, the sealing difficulty of a 1500-pound class valve is much higher than that of a 150-pound class valve.

In high-pressure applications, the design and material selection of seal elements must be able to withstand high pressures. For example, O-rings are commonly used in non-severe conditions below 400℉, but in high-pressure environments, more robust materials such as carbon graphite may need to be selected. Moreover, the design of seal elements must also be able to adapt to changes in pressure. For instance, the "self-tightening" characteristic of a sealing ring allows it to maintain good sealing performance even when pressure suddenly increases.

Temperature is another key factor that affects the performance of seals. Different sealing materials are suitable for different temperature ranges. For example, high molecular polymers such as PTFE and aramid fibers are suitable for environments below 550℉, while carbon graphite packing is commonly used in high-temperature environments above 550℉.

In high-temperature environments, the material of the seal element may deteriorate. For example, in extremely high temperatures above 850℉, carbon graphite packing and the effective components used to enhance the sealing performance of the material may deteriorate in an oxidizing atmosphere. To address this situation, measures such as increasing the distance between the valve cover and the stuffing box to reduce the impact of high-temperature media on the packing can be taken. Additionally, installing parts with low thermal conductivity, such as ceramic washers, can lower the temperature of the seal element.

In low-temperature and cryogenic conditions, the material of the seal element may become brittle and lose strength. Therefore, the choice of suitable sealing materials is limited. For example, if the media is liquid ammonia, BAM type approval is usually required, meaning that the cleanliness of the seal element must meet the standard.

The operating frequency and stroke length of valves also affect sealing performance. Some valves may cause the entire equipment or even the entire building to vibrate when they are opened or closed. Some valves operate over a million times a year, while others rarely operate. Therefore, when selecting seals, it is necessary to consider the operating frequency and stroke length they can withstand.

For example, the stroke of control valves is usually short, while the stroke of large electric valves may be longer and more frequent. In such cases, the seal element must be able to withstand frequent operation and longer strokes. Moreover, valves in a steady-state temperature environment are easier to seal than those in a temperature-changing environment. Therefore, when selecting seals, it is necessary to consider the range of temperature changes they can withstand.

Among all the concerns, the requirements for sealing performance are undoubtedly the most important. Different industries have different requirements for sealing performance. For example, in the water treatment industry, a certain degree of visible leakage may be allowed. However, in some other industries, visible leakage may be a significant issue. For invisible leakage, it is usually limited to detection by conventional factory methods.

The requirements for fugitive emissions from seal elements are even stricter. Fugitive emissions are usually in a non-visible state, and the unit of measurement is parts per million (PPM). As standards become increasingly stringent, the requirements for fugitive emissions from seal elements are also becoming higher. Some fluids are extremely hazardous, such as carcinogens, and some are even lethal in trace amounts. Therefore, for these dangerous fluids, additional preventive measures must be taken, such as backup systems, dual-seal systems, and leakage holes between the two systems for monitoring. Bellows-sealed valves with backup sealing systems can be used for such hazardous fluids.

The interaction between media and seals is the key to sealing performance. The pressure, temperature, chemical nature, and abrasiveness of the media all affect the performance of seals. Under the action of media pressure, the stress distribution on the main contact surface is approximately a quadratic parabolic curve, with the peak stress still maintained at the midpoint of the contact surface, but the curves on both sides are asymmetrical.

When the media pressure increases from 1 MPa to 2 MPa, the peak stress on the main contact surface increases from 7 MPa to 9 MPa. This is because the sealing ring produces a greater secondary compression under higher media pressure in the axial direction, and the increase in peak stress is greater than the increase in media pressure. As the media pressure increases, the contact width of the main contact surface slightly increases. With the contact center point as the center, the stress value decreases gradually from the peak point to both sides, and increases with the increase of media pressure. The contact width of the side contact surface also increases with the increase of media pressure.

Therefore, the secondary compression of the sealing ring by the media pressure is the main factor in generating stress on the side contact surface. The peak stress on the main contact surface increases with the increase of media pressure and is always higher than the media pressure. Therefore, in situations where media pressure suddenly increases, the sealing ring can still prevent media leakage. Due to the "self-tightening" characteristic of the sealing ring and the ideal parabolic distribution of contact stress, it has the ability to adapt to changes in media pressure and good sealing performance, which is why it is widely used in mechanical seals.

There are various categories of sealing media, including oil, water, gases, etc. These media play a key role in transmitting power in mechanical equipment. Sealing media should not come into direct contact with the components of the equipment, so seals are needed to isolate the media from the components.

When selecting seals, it is necessary to consider whether they are suitable for use in these mechanical devices and the conditions of the media. For example, in the case of oil seals, the applicable sealing media are generally mineral lubricating oils, greases, and synthetic lubricating oils, greases. They can also seal hydraulic oils used in industrial production, high fire-resistant hydraulic oils, and low-lubricity silicon oils. In special cases, they can also seal corrosive media with low lubricity, such as acids, alkalis, and organic solvents. The compatibility of seal materials with the media is also very important. For example, oil seals made of fluororubber material have better corrosion resistance and high-temperature resistance than those made of NBR material. In conditions of no lubrication, pure dryness, and multiple media, it is recommended to use oil seals with PTFE sealing lips, which have sufficient lubricating ability and can greatly reduce the wear of the lip edge. A single oil seal should not be used to seal two different media, as a large number of chemicals can increase the impact on the performance of the oil seal material. Therefore, the compatibility of oil seals with sealing media should be determined through laboratory tests.

In the operation of industrial equipment, the interaction between seals and media is key to ensuring the normal operation of the equipment. Seals not only need to prevent media leakage but also maintain their performance and reliability in various complex working conditions. From chemical compatibility to abrasiveness, from pressure to temperature, and from operating frequency to sealing performance requirements, each factor has a profound impact on the performance of seals. By properly selecting the materials and design of seals, we can effectively improve sealing performance and ensure the safe operation of equipment. In practical applications, selecting the appropriate seals requires a comprehensive consideration of various factors. The combination of laboratory tests and field experience can help us better assess the interaction between seals and media.