**1. Overview** Water is the most widely used medium in various processes. While selecting seals for cold water is generally straightforward, hot water is a difficult medium to seal. Providing cost-effective and reliable seals for hot water has long been a challenge for the sealing industry. Hot water has a high vaporization pressure, making it prone to flashing or vaporization when flowing over the sealing surface. Furthermore, hot water has poor lubrication properties. These characteristics make standard seal designs difficult to meet the requirements of hot water operating conditions. **2. Traditional Mechanical Seal Solutions for Hot Water Operating Conditions** Traditional mechanical seals used in hot water operating conditions absolutely cannot use hot water to directly flush the sealing surface. The hot water must first be cooled to reduce the flushing fluid temperature to below 65–820°C. The most commonly used seal flushing solutions for hot water operating conditions are API Plan 23 and API Plan 21. API Plan 23 involves drawing hot water from the stuffing box, passing it through a heat exchanger, and then flushing the sealing surface. The heat exchanger lowers the temperature of the hot water before it is reinjected into the stuffing box and flushes the sealing surface. The flushing fluid circulation is typically powered by a built-in pump ring, also known as a pump efficiency ring. API plan 21 draws pumped hot water from the pump outlet, cools it in a heat exchanger, and then injects it into the sealing cavity to flush the sealing surface. API plan 21 is generally less efficient than API plan 23 because it requires cooling the higher-temperature hot water from the process piping, typically necessitating a larger heat exchanger under similar operating conditions. Its advantage is that the flushing fluid does not require a pump ring for power. Another flushing solution for hot water sealing is API plan 32. This solution uses a single, lower-temperature, clean, soft water stream to inject into the sealing cavity and flush the sealing surface. Its flushing pipeline is simple and reliable, but its disadvantages include high flushing water consumption and the potential dilution or cooling of the hot water in the process piping, which is unacceptable to many users. Alternatively, a cooling water jacket can be added outside the sealing cavity to lower the hot water temperature. All of the above flushing schemes require reducing the temperature of the hot water in the sealing cavity to a level acceptable to the standard seal design, so that the seal can achieve an acceptable service life; and require an additional cooling water supply, which not only leads to a large consumption of cold water, but also causes rapid seal failure if the cold water supply is interrupted during operation. At the same time, the use of these flushing schemes also faces the problem of scaling in heat exchangers, jacket cavities, etc., which reduces the reliability of the sealing system in actual operation. The following introduces two sealing methods suitable for hot water conditions that do not require cooling water - the "Lube Groove" and "Hydropad" mechanical seals. [b]3. "Lube Groove" Seal[/b] In a standard mechanical seal, there is a very thin liquid film between the two sealing end faces. During the generation and formation of a stable liquid film, the physical properties of the fluid are in a critical state. When the fluid flows through the sealing surface, the pressure decreases and the temperature increases. The pressure decrease is caused by the pressure difference between the inner and outer edges of the sealing surface, and the temperature increase is caused by the shear force on the fluid in the liquid film and the frictional heat generated by the mechanical contact between the sealing surfaces. The liquid film prevents excessive mechanical contact between the sealing faces, thereby reducing frictional heat and wear during operation. If the liquid film vaporizes near the edge of the sealing face, most of the sealing surface will lose the support of the sealing liquid film, causing rapid seal failure. "Lubrication grooves" are narrow grooves tangentially carved into the sealing face, not penetrating the inner or outer edges. These grooves improve the pressure distribution of the fluid as it flows across the sealing face. This helps maintain a stable liquid film between the sealing faces and prevents vaporization at the liquid film. Because hot water has a high vaporization pressure, it easily vaporizes rapidly when flowing across the sealing face; therefore, controlling the pressure at the sealing contact surface is crucial. When the sealing faces are parallel, the pressure drop in the liquid film is essentially linear, gradually decreasing from the sealing cavity pressure on the outer side of the sealing face to atmospheric pressure at its inner edge. Vaporization occurs when the fluid pressure in the liquid film gradually decreases to equal the vaporization pressure of the liquid. By creating lubrication grooves on the sealing face, a relatively high-pressure area is formed on the sealing face. The pressure distribution of the liquid film decreases linearly from the outer edge (OD) of the sealing face until it enters the lubrication groove. The fluid pressure remains constant as it passes through the lubrication groove, then decreases linearly again from the lubrication groove until it reaches the inner edge (ID) of the sealing face. This change in pressure distribution restricts the vaporization of the liquid film within the annular region covered by the lubrication groove. Currently, lubrication groove type seals are manufactured by Flowserve. Their cartridge seals U, QB, and BX can all utilize lubrication groove type sealing surfaces. The following are the applications of "lubricating groove" seals in hot water applications: Maximum seal size: 5 inches (127 mm) Maximum outer edge linear velocity: 66 feet/second (20 meters/second) Maximum speed: 3000 rpm Minimum speed: 750 rpm Maximum seal chamber pressure*: 15 Bar Gauge pressure Maximum seal chamber temperature: 175 °C Seal material: Graphite, silicon carbide, hard alloy Recommended piping options: API Plan 11 or API Plan 02 Note*: The minimum seal chamber pressure should be maintained at 2 Bar above the vaporization pressure of the hot water. Typically, for ease of machining, "lubricating grooves" are engraved on a softer sealing surface. The exact number, length, and depth of the "lubricating grooves" are determined by the dimensions of the sealing end face. [b]4. "Hydrogen Buffer" Seal[/b] A "hydrogen buffer" is a groove or tongue-and-groove formed radially inward from the outer edge on either the moving or stationary ring sealing surface. Its depth can range from a few thousandths to 0.125 inches (i.e., 3 millimeters). In a typical seal, the sealing surface is a uniform annular shape. Grooving the sealing surface causes deformation in the circumferential direction due to mechanical and temperature changes. Mechanically, the grooving weakens the local strength of the sealing face; thermally, the grooves cause heat to dissipate at different rates along the circumference of the sealing face. These two factors cause wavy deformation of the sealing face. Because the liquid film is very thin, even small deformations on the sealing surface can significantly affect the formation and stability of the liquid film. Wavy deformation facilitates the extension of the liquid film between the sealing faces. The contraction and expansion of the liquid film thickness along the circumferential direction generates a fluid thrust that separates the sealing faces. This fluid thrust reduces mechanical contact between the sealing faces, heat generation, and wear. The magnitude of the fluid thrust depends not only on the wavy deformation of the sealing face but also on the operating characteristics of the seal and the physical properties of the fluid. The relative velocity between the sealing faces is also an important factor in generating fluid thrust. The greater the velocity, the greater the fluid thrust. Fluid properties, such as specific gravity, viscosity, and vapor pressure, significantly affect the liquid film. Lower specific gravity and viscosity result in lower fluid thrust. Due to these characteristics, "liquid buffers" can be used for media such as hot water, light hydrocarbons, and hydrofluoric acid. When determining whether a "liquid buffer" is needed, the following factors should be considered: 1) Vaporization pressure: When the pressure in the sealing cavity is lower than the liquid's vaporization pressure plus 25 Psi (i.e., 1.7 Bar), temperature should also be considered. 2) Pressure difference: When the pressure difference between the inner and outer edges of the sealing surface is very low or very high (greater than 250 Psi, i.e., 17 Bar), a "liquid buffer" should be considered. This is because when the fluid pressure is very low, the liquid pressure cannot overcome the spring force acting on the sealing surface, making it difficult to form the required liquid film. Conversely, when the fluid pressure is very high, the liquid closure pressure on the sealing surface is also very high, similarly making it difficult to form the required liquid film. 3) Liquid specific gravity: The liquid specific gravity should be less than 0.52. Currently, both "liquid buffer pads" and "lubricating grooves" are cartridge seals. Flowserve's BAW series and John Crane's LASERFACE series are of this type. [b]5. Comparison of Mechanical Seal Types for Hot Water Applications[/b] A comparison of mechanical seal types for hot water applications is shown in Table 1 below. Table 1 Comparison of Mechanical Seal Types for Hot Water Applications Sealing Surface Type Traditional Mechanical Seal "Lubricating Groove" Mechanical Seal "Liquid Buffer Pad" Mechanical Seal Sealing Surface Structure Type Plane Opening on a softer sealing surface Closing Groove Opening a groove or tongue and groove on any sealing surface facing outwards Flushing Scheme API Plan 21, 23, 32 API Plan 02, 11 API Plan 02, 11 Cooling Water Required Not Required Sealing Cavity Pressure Pc Unlimited Pc Pc 17 Bar Note: Pc – Sealing cavity pressure; Pv – Vaporization pressure of the medium; ΔPc – Pressure difference between the inner and outer edges of the sealing surface. **6. Application Example** In the styrene project of SECCO Company in Caojing, Shanghai, a condensate pump has a temperature of 1170°C, an inlet pressure of 1.6 Bar, an outlet pressure of 7.2 Bar, a calculated sealing chamber pressure of approximately 3.2 Bar, a vapor pressure of 1.41 Bar, and a specific gravity of 0.951. The original design used API Plan 23. This design required an auxiliary piping system, a heat exchanger, and a cooling water consumption of 0.7 m³/hour. After careful research and comprehensive comparison with the client, we decided to adopt the "QB Lube" type mechanical seal, with a flushing scheme of API Plan 11. This saved approximately 30% on equipment purchase costs and approximately 6000 RMB/year on operating costs (cooling water consumption only). Because the "QB Lube" type mechanical seal does not require cooling water, it avoids seal failure due to accidental water interruption and reduced sealing system reliability due to scale buildup on the heat exchanger. This type of seal has been used in Europe and the Middle East for nearly 40 years. Besides their successful applications in hot water conditions, "lubricating groove" and "liquid buffer pad" seals have been widely and successfully used in special operating conditions such as liquefied air, cryogenic liquefied gas, light hydrocarbons, and hydrofluoric acid. Because they partially replace dry gas seals in these special conditions, and their purchase, operating, and operational costs are significantly lower, their application is expected to become increasingly widespread.