Basic Knowledge of Mechanical Seals

　　 (1) The concept of mechanical seals:

　　1. A mechanical seal is a device used to seal the fluid medium between a rotating shaft and the machine housing.

　　2. A mechanical seal is a device designed to prevent fluid leakage, consisting of at least one pair of end faces that are perpendicular to the axis of rotation and maintain close contact with each other while sliding relative to one another, under the combined action of fluid pressure, the elastic force (or magnetic force) of a compensating mechanism, and auxiliary seals.

　　 (2) Applications of Mechanical Seals: Mechanical seals are commonly used in rotating fluid machinery such as pump shafts, compressors, reaction vessels, reaction agitators, and fans. They are also employed in devices like gearboxes, valves, rotary joints, and ship propeller shafts.

　　 (3) Main components and functions of mechanical seals:

　　 1. Main components: The mechanical seal consists of five main parts:

　　(1) The compensation ring and the non-compensation ring form the primary sealing surface, also known as the friction pair.

　　(2) The elastic element primarily constitutes the loading, compensation, and buffering mechanisms;

　　(3) Auxiliary seals (such as O-rings, skeleton oil seals, rectangular seals, V-shaped seals, etc.);

　　(4) Transmission mechanism: A component connected to the rotating shaft and rotating coaxially with it;

　　(5) Anti-rotation mechanisms: such as anti-rotation screws, pins, and flat keys.

　　 2. Functions of the main components:

　　(1) Compensated loop and non-compensated loop:

　　a. The compensation ring—also known as the sealing ring with axial compensation capability—can be either a rotating ring (also called the moving ring) or a non-rotating ring (also called the stationary ring). In general, compensation rings are made of softer materials and have narrower end faces compared to non-compensation rings.

　　b. Non-compensating seal ring—refers to a seal ring that does not have axial compensation capability. It can be either a rotating ring (also known as a moving ring) or a non-rotating ring (also known as a stationary ring). In general, non-compensating rings are made from relatively hard materials and have wider end faces than compensating rings.

　　The mating end faces of the compensated and non-compensated rings together form the primary sealing end face, which plays the main sealing role and is a key component of mechanical seals.

　　In summary: Simply put, a sealing ring that can undergo axial movement during operation is generally referred to as a compensating ring, while a ring that cannot move axially is called a non-compensating ring. A ring that can rotate circumferentially along with the shaft (or sleeve) is known as a moving ring, whereas a ring that cannot rotate circumferentially with the shaft (or sleeve) is called a stationary ring. It’s important to note that a compensating ring is not necessarily a moving ring, and a non-compensating ring is not necessarily a stationary ring—these two types must be distinguished based on the specific mechanical seal design.

　　(2) Elastic Element and Spring Seat: These components form the loading, compensation, and cushioning mechanism, ensuring that the end faces of the mechanical seal remain tightly coupled after installation. They provide timely compensation when wear occurs and act as a buffer under conditions of vibration and axial movement.

　　〈1〉Requirements for elastic elements:

　　a. The magnitude of the elastic force generated by the elastic element must be sufficient to overcome the frictional resistance when the auxiliary sealing ring of the compensation ring slides along the shaft (or sleeve).

　　b. The magnitude of the elastic force generated by the elastic element must also be sufficient to prevent the compensating ring and the non-compensating ring from being drawn into the sealing cavity when the sealing cavity is under vacuum.

　　C. However, excessive elastic force can exacerbate wear on the sealing end faces, affecting the performance of mechanical seals and shortening their service life.

　　<2> Types of elastic elements: Elastic elements can be single cylindrical helical springs, conical helical springs, or multiple small cylindrical helical springs arranged around the circumference; alternatively, they can be paired leaf-type corrugated springs or butterfly springs, among others.

　　<3> Spring seat: A component used for the axial and radial positioning of springs; it typically also serves to transmit or counteract torque.

　　(3) Auxiliary sealing rings: These serve as auxiliary seals and include compensation ring auxiliary sealing rings, non-compensation ring auxiliary sealing rings, gland sealing rings, and sleeve sealing rings, among others.

　　a. Compensating ring auxiliary sealing ring: Used to seal leaks between the compensating ring and the shaft (or sleeve), it is classified according to its cross-sectional shape into O-rings, V-rings, wedge-shaped rings, U-shaped rings, rectangular rings, and others.

　　b. Non-compensating ring auxiliary sealing ring: Used to seal leaks between the non-compensating ring and the shaft, sleeve, gland (or sealing cavity). Depending on their cross-sectional shapes, they come in forms such as O-rings, V-rings, rectangular rings, L-shaped gaskets, and rectangular gaskets.

　　(4) Transmission mechanism:

　　<1> Function: To transmit torque.

　　<2> Form of the transmission mechanism:

　　a. In rotary mechanical seals, multi-spring structures commonly employ transmission methods such as convex-round concave pits, pins, and forks; the transmission mechanism is typically arranged on the spring seat and the compensation ring. In single-spring structures, the spring itself often serves both as a compression coil and as a hooked element to provide transmission. Alternatively, a impeller can be used in conjunction with a removal ring to clamp the shaft sleeve, while tightening screws on the drive ring can also serve as the transmission mechanism.

　　b. In stationary mechanical seals, the rotating ring is commonly driven by means of a flat key, pin, or similar components.

　　(5) Anti-rotation mechanism: This mechanism serves to counteract torque and has a structural configuration that is the reverse of that of the transmission mechanism.

　　 (4) Mechanical Seal Sealing Principle and Operating Conditions:

　　1. The performance of mechanical seals primarily depends on the primary seal.

　　2. The sealing principle of the primary seal: When a mechanical seal operates at high speed, an extremely thin fluid film forms between the sealing end faces, providing lubrication and generating mutual attractive forces to ensure its proper functioning. Therefore, the common belief that no leakage will occur between two sealing surfaces unless there is liquid present is incorrect. The thickness of this fluid film typically ranges from 6 thousandths to 6 ten-thousandths of a micrometer (μm). Mechanical seals that operate without any liquid on the sealing surfaces are similar to dry gas seals used in compressors.

　　In short: If there is no liquid between the two sealing surfaces, dry friction between them during high-speed rotation of the mechanical seal will generate significant frictional heat, causing the seal temperature to rise sharply. This rapid temperature increase will quickly lead to carbonization of the auxiliary sealing ring, while the sealing surfaces themselves will develop cracks due to the high temperatures, ultimately resulting in deformation of the entire mechanical seal assembly.

　　Function of mechanical seal cooling water: The primary function of mechanical seal cooling water is to remove the frictional heat generated by the main sealing surfaces during high-speed operation, thereby maximizing the service life of the mechanical seal.

　　3. Operating conditions for mechanical seals:

　　(1) Modular single-face mechanical seal:

　　a. Under no circumstances is dry running permitted when there is neither conveyed medium nor cooling water inside the pump cavity.

　　b. In the absence of cooling water but with a medium present, the mechanical seal can operate for a short period of time. This is because, in a single-face seal, only one set of sealing surfaces is immersed in the fluid being conveyed; thus, no dry friction occurs when both sealing surfaces are in operation. However, prolonged operation under these conditions will significantly shorten the service life of the mechanical seal. The primary reason is that, while the sealing water and the skeleton oil seal remain intact, the frictional heat generated at the sealing surfaces cannot be dissipated from the sealing cavity, causing the temperature of the entire mechanical seal to rise rapidly.

　　C. If there is no fluid being conveyed but cooling water is present, the mechanical seal can still operate for a short period of time. This is because the cooling water flows from inside the mechanical seal into the space between the two sealing rings, ensuring that no dry friction occurs during operation of the mechanical seal.

　　(2) Modular double-face and multi-face mechanical seals: Absolutely prohibit operating the mechanical seal without cooling water.

　　a. Under no circumstances is it permissible to operate a mechanical seal when the pump cavity contains the conveyed medium fluid but no cooling water. This is because, in such a situation, the sealing surfaces of the sealing rings in the medium-fluid lubrication group are lubricated by the conveyed medium fluid, whereas the sealing surfaces between the sealing rings in the cooling-water lubrication group remain completely devoid of any liquid lubrication, leading to dry friction. In a short period of time, this dry friction generates significant frictional heat, causing the temperature to rise sharply and rapidly damaging the mechanical seal. Therefore, packaged double-end-face mechanical seals must never be operated without cooling water.

　　b. Under conditions where the pump cavity is empty of conveyed medium but filled with cooling water, the mechanical seal can be operated for a short period. This is because the cooling water flows from inside the seal into the space between the two sealing faces; during operation, neither sealing face will experience dry friction, thus preventing rapid wear and damage to the mechanical seal. However, under no circumstances should the mechanical seal be operated in this manner unless absolutely necessary.

　　C. Absolutely prohibited when there is neither conveyed medium nor cooling water inside the pump cavity.

　　Running the mechanical seal in this way will cause the entire mechanical seal to burn out in a very short time.

　　(5) Classification of Mechanical Seals: There are many methods for classifying mechanical seals. Depending on factors such as structural arrangement, end-face specific pressure, source of initial closing force, and end-face combination type, mechanical seals can be categorized into various types with different names.

　　1. Classified according to the initial closing force source: elastic-force mechanical seals and magnetic-force mechanical seals.

　　(1) The initial closing force of elastic-force mechanical seals originates from the elastic forces generated by springs, metal bellows, PTFE bellows, rubber bellows, and the like.

　　(2) The initial closing force of the magnetic mechanical seal comes from the magnetic force generated by magnets and other magnetic components.

　　2. According to structural arrangement, mechanical seals can be classified—based on whether the compensation ring relies on contact and sliding against an auxiliary sealing ring mounted on the shaft (or sleeve)—into sliding-type and non-sliding-type mechanical seals.

　　(1) Sliding mechanical seal:

　　〈1〉Definition of a sliding mechanical seal: A mechanical seal in which the compensation ring is supported on the shaft (or sleeve) by means of an auxiliary sealing element (such as an O-ring, V-ring, or wedge ring), and its compensation function is achieved through contact and sliding. The initial closing force for such a seal is typically provided by a spring.

　　〈2〉Classification of Sliding Mechanical Seals:

　　A. Sliding mechanical seals can be classified according to the spring structure type and quantity as follows:

　　a. Single-spring design: This type of mechanical seal commonly uses a spring as the transmission mechanism, featuring a simple structure. However, the transmission is not very reliable, and the spring force is unevenly distributed, making it unsuitable for high-speed operation.

　　b. Multi-spring structure: This type of mechanical seal features a set of multiple small springs arranged in a circumferential pattern. The entire mechanical seal exhibits uniform spring force, making it suitable for high-speed operation. However, the springs cannot be used for transmission purposes, and the overall structure is relatively more complex than that of a single-spring mechanical seal.

　　c. Leaf spring structure: This mechanical seal features a compact design, a short axial length, and a small compensation range.

　　B. Sliding seals, classified according to the spring’s position, can be divided into two structural types: built-in and external.

　　a. Spring-embedded mechanical seal—refers to a mechanical seal in which the spring is located on the high-pressure side and comes into contact with the fluid on the high-pressure side; it is also known as a spring-mounted mechanical seal.

　　b. External-spring mechanical seal—refers to a mechanical seal in which the spring is located on the low-pressure side of the seal and does not come into contact with the fluid on the high-pressure side; it is also known as a rear-spring mechanical seal.

　　C. Sliding mechanical seals, classified according to their mode of motion (i.e., whether the compensation ring rotates with the shaft), can be divided into rotating and stationary mechanical seals.

　　a. Rotary mechanical seal—refers to a seal in which the compensation ring rotates together with the shaft (or sleeve).

　　b. Stationary mechanical seal—refers to a seal in which the compensation ring does not rotate along with the shaft (or sleeve).

　　Rotary and stationary seals can be further classified, based on whether the end face of the sealing ring (whether it’s a compensating ring or a non-compensating ring) that remains relatively stationary faces the main machine’s working cavity, into internal-mounted and external-mounted seals.

　　(a) Internal mechanical seal—A mechanical seal in which the end faces of the sealing rings, located in a relatively stationary state, are oriented toward the working cavity of the host machine. It can be either a sliding-type or a non-sliding-type seal.

　　(b) External mechanical seal—refers to a mechanical seal in which the end faces of the sealing rings, located in a relatively stationary state, are oriented away from the main machine’s working cavity. This type of seal can be either a sliding seal or a non-sliding seal.

　　Features of internal and external mechanical seals: (1) With an internal mechanical seal, the seal itself is invisible from outside the main machine; (2) In an external mechanical seal, the outer circular portion of the sealing ring is generally exposed to the atmosphere on the low-pressure side, allowing for direct visual monitoring of operating conditions such as leakage and wear on the sealing surface.

　　Defects of externally mounted bulk-type mechanical seals: Compared to internally mounted mechanical seals, externally mounted bulk-type mechanical seals distribute forces less rationally. The sealing ring is subjected to tensile stress from the medium, and the force exerted by the medium acts in the opposite direction to that of the elastic element, creating a tendency for the sealing surfaces to open and resulting in greater leakage.

　　Principles for the Use of External and Internal Mechanical Seals: (1) External mechanical seals are suitable for applications where the fluid medium pressure is less than 0.5 MPa and the size of the main machine cavity is restricted. (2) Internal mechanical seals can be used under all conditions, especially when the fluid medium pressure is equal to or greater than 0.5 MPa; in such cases, it is advisable to use internal mechanical seals whenever possible. This is because the direction of leakage in an internal seal aligns with the centrifugal force direction of the fluid inside the pump, resulting in a smaller leakage volume.

　　Rotary and stationary mechanical seals, according to the compensation ring, are positioned at the back side farthest from the end face.

　　On the high-pressure side or the low-pressure side, mechanical seals can be further classified into high-backpressure types and low-backpressure types.

　　(a) High-back-pressure mechanical seal—refers to a sliding seal in which the back side of the compensation ring, located furthest from the end face, is on the high-pressure side. There are roughly four possible scenarios (details omitted).

　　(b) Low-back-pressure mechanical seal—refers to a sliding seal in which the back side of the compensation ring, located furthest from the sealing end face, is on the low-pressure side. (Omitted)

　　Summary of the structural features of high-low back-pressure mechanical seals:

　　(1) In low-backpressure mechanical seals, the springs are all located on the low-pressure side, which prevents them from coming into contact with the high-pressure-side sealing fluid. Since the high-pressure-side sealing fluid is often the sealing medium itself, this arrangement effectively avoids the problem of spring corrosion caused by the medium. Therefore, mechanical seals designed for highly corrosive environments frequently adopt a low-backpressure structural design.

　　(2) The high back-pressure mechanical seal, on the other hand, is the opposite.

　　B. Non-sliding mechanical seals are used less frequently and are uncommon. Typically, bellows-type seals...

　　A mechanical seal is a non-sliding type of seal.

　　Classification of Non-Sliding Mechanical Seals:

　　(a) According to their mode of operation, they are also classified into rotating and stationary structures. The rotating type is further divided into internally mounted internal-flow and externally mounted external-flow types.

　　(b) According to their corrugated tube structure, they can be further classified into pressure-formed metal corrugated tubes, welded metal corrugated tubes, rubber corrugated tubes, PTFE corrugated tubes, and other types.

　　Among these: (1) Mechanical seals with metal bellows formed by pressure molding and those formed by welding are commonly used in high-temperature and high-pressure service conditions; (2) mechanical seals with rubber bellows are suitable for use with clear water at ambient temperature; (3) mechanical seals with PTFE bellows are often employed in applications involving highly corrosive acidic or alkaline fluids.

　　3. Classification by Specific Pressure on the End Face: The classification principle is based on whether the specific pressure on the end face equals zero—that is, whether the sealing end faces are in contact with each other—resulting in two categories: contact mechanical seals and non-contact mechanical seals.

　　The concept of specific pressure on the mechanical seal end face refers to the force exerted by the elastic force generated by the sealing spring itself, acting on the unit area of the effective sealing ring when the spring is at its normal operating height.

　　(1) Contact-type mechanical seals: These are mechanical seals in which the sealing end faces are in contact with each other and the specific pressure on the end faces is greater than zero. The magnitude of the specific pressure on the end faces of a contact-type mechanical seal depends on the value of the load coefficient K; therefore, they can be further classified into balanced and unbalanced mechanical seals based on whether the load coefficient K is less than 1 or greater than or equal to 1.

　　A. Balanced mechanical seal—refers to a seal with a load coefficient value K < 1.

　　B. Non-equilibrium mechanical seals—refers to mechanical seals with a load coefficient value K ≥ 1.

　　(2) Non-contact mechanical seal: A mechanical seal in which the sealing end faces do not come into contact with each other and the face-specific pressure Pc is zero.

　　Features: The sealing surfaces of the contact-type mechanical seal are in a state of liquid friction, ensuring a tight seal.

　　There is a continuous hydraulic fluid film between the sealing surfaces; in contrast, non-contact mechanical seals do not operate under liquid friction conditions and lack a continuous hydraulic fluid film between the sealing surfaces. Instead, they rely on the adsorption force generated during high-speed rotation to achieve sealing—such as dry gas seals used in compressors and other similar applications.

　　4. Classified by the number of end faces: double-end-face, multi-end-face, and tandem mechanical seals.

　　(1) Double-face mechanical seal: A mechanical seal consisting of only two pairs of sealing faces, which can be further classified into axial double-face and radial double-face structures.

　　A. Axial Double-End Face Seal: Divided into axially opposed and axially parallel structures.

　　a. The axial back-to-back arrangement is what we commonly refer to as a “back-to-back” sealing structure;

　　b. Axial Opposed Type: Refers to a double-end-face seal arranged in an axially opposed configuration (i.e., both sets of sealing rings are positioned adjacent to each other, with the spring seats distributed on either side of the seal).

　　B. Radial Double-Seal: Refers to a double-seal arrangement positioned radially (e.g., the 204 and 212 designs used on reactors).

　　(2) Series mechanical seals: Refers to seals consisting of two or more single-face seals arranged in the same direction (details omitted).