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Preface: When purchasing new pumps, mixers, or other types of rotating equipment, companies often ask, “How long can my equipment operate before it needs to be disassembled for maintenance?” ”In some cases, it may be advisable to upgrade its existing pumps in order to improve their reliability, productivity, and efficiency, or to meet federal or local emission regulations. The problems are usually the same. In many industrial pump applications, the seal is the most critical component. Due to unacceptable leakage levels, or in some cases level or pressure alarms, the seal usually forces the pump to stop operating. Although seals are often considered to be the cause of equipment failure, the real reasons usually lie in the pump’s operating and mechanical conditions or the environmental control system of the seal. Companies that have successfully implemented plans to increase the mean time between repairs (MTBR) have achieved an average seal life of three years or more, thereby reducing the lifetime cost (LCC) of the pump systems. The LCC prediction for pump applications may vary depending on the seal MTBR. In situations with a high risk of failure, it is prudent to seek expert advice, as errors in life expectancy estimates can have costly consequences. The key to predicting any sealed MTBR is to understand the factors most likely to affect sealing performance. The key performance indicators of mechanical seals are usually leakage rate and power consumption. Predict the life of the mechanical seal? Predicting the service life of mechanical seals is not an easy task, as the sealing performance at the macro level is influenced by many phenomena at the micro level. The reliability of sealing depends to a large extent on its ability to maintain a stable liquid film in the gap between the mating surfaces, while minimizing the duration and degree of mechanical contact in the friction areas of these surfaces. Excessive contact may overheat the material ； Insufficient contact can lead to high leakage rates. The working principle of the mechanical seal surface, one of the key components, is very similar to that of bearings. The main difference is that the lubricant is usually the liquid being pumped itself, which under pressure, and sometimes at high temperatures, can be dirty, volatile, viscous, toxic, or explosive. Obviously, the sealing surface is the most vulnerable component in any mechanical seal, but other components such as auxiliary sealing elements (O-rings, bellows) or metal components like springs, drive pins, or fixing screws can also affect the seal’s lifespan under conditions of excessive movement or high temperature and pressure. This article focuses on the factors affecting the lifespan of sealing surfaces. To reasonably assess the expected service life of an end-face seal, it is necessary to have some basic knowledge regarding the functions of mechanical seals. Remembering the five key performance factors can help determine the optimal replacement cycle for mechanical seals. Seal surface wear: First of all, it should be understood that excessive leakage of the seal rarely occurs, unless the seal surface has been completely worn out. If this happens, the narrower of the two surfaces will wear down to the point where it loses contact, creating a path for the process fluid to leak into the atmosphere. In fact, the normal wear rate of the sealing surface is extremely low, so it is not a typical cause of excessive leakage. Worn seals may be found in high-pressure applications and in fluids that cause abrasive wear of the sealing surface material. In most cases where it is necessary to disassemble and inspect the sealing components, the sealing surfaces do show signs of damage. In these cases, although the wear on the narrower sealing surface still leaves sufficient material, the friction area of the surface is damaged, resulting in a leakage level that is unacceptable. These signs of damage can vary – from being barely visible to the naked eye to severe damage such as cracked sealing surfaces. Common damage symptoms include pitting, bubbling, chipping, scratches, and thermal shock cracks. The normal replacement of mechanical seals is an exception, as in such cases the equipment is likely to be taken out of service for reasons unrelated to the seal or merely for preventive maintenance purposes. Symptoms of excessive leakage in mechanical seals are clear evidence of deeper problems existing within the equipment, process, or control system. Types of seal failure: Based on MTBR, seal failure can be classified as early-stage, wear-related, or random. Statistically, random failure (also known as accidental failure or mid-life failure) is the main type. Initial failures are usually caused by incorrect seal selection, installation, or startup, and with the widespread use of mechanically sealed assemblies in modular designs, such initial failures have practically been eliminated. After operating for a considerable length of time, random failures often occur unexpectedly, characterized by a rapid change in the sealing leakage behavior from normal to excessive. Under operating conditions, this may manifest as a change from an invisible leakage amount to dripping in high-pressure or high-speed applications, or from a steady drip to a small amount of leakage. Random failures are difficult to predict, as they are usually the result of transients caused by process operations or equipment, which the sealing surface material cannot tolerate. Transient operating conditions may be intentionally induced, or they may result from component failures in the pump or accidental process deviations. Verifying whether the seal can handle expected transients is a key step in lifetime assessment, as transient conditions are likely to have a destructive effect on the sealing surfaces. Compared to cyclic (unstable operating) applications, essentially continuous, steady-state applications are relatively easier. In fact, it can be safely assumed that as long as the lubrication film at the sealing surface remains stable, the sealing surface will remain intact for almost forever; thus, the seal will not become a limiting factor in the equipment’s service life. When estimating the lifespan of mechanical seals, application constraints, minimum leakage requirements, and the sealing environment must also be taken into account. In \"Centrifugal Pump Foundation – How Long Can a Mechanical Seal Last (Part 2)\", the other three key factors that affect seal life and the need for subsequent replacement will be discussed. In many industrial pump applications, the seal is the most critical component. Due to unacceptable leakage levels, or in some cases level or pressure alarms, the seal usually forces the pump to stop operating. Although seals are often considered to be the cause of equipment failure, the real reasons usually lie in the pump’s operating and mechanical conditions or the environmental control system of the seal. Five performance factors affect the service life of mechanical seals and help determine the optimal replacement interval for such seals. They can also increase the service life of the seal and the mean time between repairs (MTBR), thereby reducing the life cycle cost (LCC) of the pump. Sealing application limitations: The third factor is the application limitations of mechanical seals, which are typically defined by pressure, speed, and temperature, and are prone to misunderstanding. Most seal manufacturers state that operating a seal under one of the extreme operating conditions will result in a minimum service life of less than two years, whereas some types of seals (such as those specified in the API 682 standard) have a service life of at least three years. Although the release constraints are reliable for steady-state operation, they may be exaggerated or unreliable for cyclic (unsteady-state) operation. API 682 seals may be an exception, as it specifically addresses this drawback by incorporating a series of cyclic tests to determine whether a particular seal type is suitable for a range of services. Generally, there are no fixed criteria for how much specific transient pressure, speed, temperature, gas or solid retention, or vibration a seal can withstand. Expert advice is needed in this regard. Seal surface deflection: The amount of deflection of the seal surface during transients depends on several factors, including the scale and speed of the transient, as well as the stiffness or strength of the seal surface. Most importantly, as operating conditions change, the sealing surface deforms, which may improve or reduce the lubrication of the sealing surface. A reliable seal is one that has a low sensitivity to expected or unexpected transients. The sensitivity of a seal can be determined by its ability to maintain more or less parallel sealing surfaces throughout all possible transients. Sensitivity can also be controlled by properly selecting materials and environmental control systems. For example, seals with hard/hard combinations, such as tungsten carbide (WC) and silicon carbide (SiC), are the preferred choice for dirty fluids or in high-pressure applications in some cases; they are highly susceptible to complete or partial loss of the liquid film. When these materials collide with each other, any type of damage is usually irreversible, and it only gets worse over time. New sealing surface materials and processing techniques, such as diamond coatings, hold promise for making significant progress in this field. On the other hand, hard/soft composite surfaces using carbon graphite as the wear-resistant material possess unique advantages; they can withstand higher loads and longer periods of insufficient lubrication. Most importantly, it is easier to recover after damage to the carbon graphite surface. Compared to hard carbon-graphite, silicon carbide is generally considered to be one of the materials with the highest loading capacity among existing options; it can withstand dry operation best, making it the preferred material combination for many applications. Application limitations: Undoubtedly, using larger shaft diameters, higher speeds, as well as higher temperatures, pressures, and solid contents makes the applications more difficult and dangerous. The strength of the sealing surface and its ability to dissipate heat effectively must be optimized accordingly. In many demanding applications, it is wise to consider custom-engineered mechanical seals, which are designed specifically to handle abnormal conditions or predefined operating modes of the equipment. Different seal surface designs and lubrication techniques can be chosen to extend the service life of the seal surface. For most applications, the seal life can exceed three years. API 682 and the Hydraulic Institute publication “Mechanical Seals for Pumps: A Guide to Applications” provide good guidelines for specifying engineering seals. Leakage requirement: The fourth factor is that all face seals must allow for a slight amount of leakage in order to maintain a sufficient lubrication film under all operating conditions. Since the seal operates under these favorable lubrication conditions, energy consumption is reduced to a minimum, thereby making the operation of the seal more economical. The leakage amount is usually a few milliliters per day or less, and it is invisible. For special applications involving high pressure, high speed, or large shaft diameters, this figure can be much higher. For calculations regarding the expected leakage amount, please consult the sealing supplier. Mechanical seals can meet today’s emission regulations in the vast majority of applications. Predicting the leakage and friction behavior of any given application with high accuracy can help establish operator guidelines for normal, suspicious, and faulty sealing behavior. When operating conditions such as pressure, temperature, or speed change, the leakage rate increases or decreases. This means that the leakage rate depends on the way the pump operates and on the degree to which the sealing surfaces respond to any transient operating conditions. The key to a low and consistent leakage pattern is maintaining the flatness of the sealing surface. The key performance indicators (KPIs) of sealing may vary depending on the sealing (end) surface technology and materials used. Any given application usually has several solutions, which vary greatly in terms of cost and benefits. Sealing suppliers usually promote unique features to enhance or optimize the lubrication of the sealing surfaces. Some OEMs advocate a hard/hard combination for high-pressure applications, while other OEMs insist on a hard/soft combination. Everyone’s ability to handle abnormal events varies, and the best choice is not always obvious. Sealed environment Finally, the reliability of a seal depends on its operating environment. By selecting and applying the appropriate system from various methods, it is possible to manipulate or control the operating environment – which is almost as important as sealing itself. Most seals require a flushing system to remove the heat generated by the seal. If that system fails, it could cause problems. ““A cool seal is a happy seal” is true for most applications. Some fluids may change state as pressure or temperature changes. If not addressed when selecting the sealing and control systems, it may cause problems with the sealing. Environmental control systems can be used to prevent or minimize the negative effects of certain transients, eliminate frictional heat at sealing surfaces, reduce the presence of solids in the flushing fluid, and improve operating conditions to prevent fluid evaporation or freezing. Most importantly, they minimize the impact of failures on the surrounding environment. Various flushing protocols are included in ANSI, API, and ISO standards. The diversity of seal types and materials, along with various flushing options, makes seal selection a very challenging task. Useful tips and tools, such as the FSA seal life-cycle cost estimator, can be found at www.fluidsealing.com. This tool can compare various seal types and systems to help select a sealing system effectively. Choosing the “most effective sealing technology” requires a thorough understanding of the factors that influence sealing performance… not only under normal operating conditions, but more importantly, under abnormal conditions.