In mechanical engineering, an end-face mechanical seal (often shortened to mechanical seal) is a type of seal used in rotating equipment, such as pumps, mixers, blowers, and compressors. When a pump operates, the liquid could leak out of the pump between the rotating shaft and the stationary pump casing. Since the shaft rotates, preventing this leakage can be difficult. Earlier pump models used mechanical packing (otherwise known as gland packing) to seal the shaft. Since World War II, mechanical seals have replaced packing in many applications.
An end-face mechanical seal uses both rigid and flexible elements that maintain contact at a sealing interface and slide on each other, allowing a rotating element to pass through a sealed case. The elements are both hydraulically and mechanically loaded with a spring or other device to maintain contact. For similar designs using flexible elements, see radial shaft seal (or "lip seal") and O-ring.
Mechanical seal fundamentals
An end-face mechanical seal consists of rotating and stationary components which are tightly pressed together using both mechanical and hydraulic forces. Even though these components are tightly pressed together, a small amount of leakage occurs through a clearance that is related to the surface roughness.
Components All end-face mechanical seals have rotating elements, stationary elements, and include five basic components: API Standard 682, Fourth Edition, 2014, “Pumps – Shaft Sealing Systems for Centrifugal and Rotary Pumps,” American Petroleum Institute, Washington D.C. seal ring mating ring secondary sealing elements springs Encasing
The seal ring and mating ring are sometimes referred to as the primary sealing surfaces. The primary sealing surfaces are the heart of the end-face mechanical seal. A common material combination for the primary sealing surfaces is a hard material, such as silicon carbide, ceramic or tungsten carbide and a softer material, such as carbon. Many other materials can be used depending on pressure, temperature and the chemical properties of the liquid being sealed. The seal ring and mating ring are in intimate contact, one ring rotates with the shaft and the other ring is stationary. Either ring may be rotating or stationary. Also, either ring may be made of hard or soft material. These two rings are machined using a process called lapping in order to obtain the necessary degree of surface finish and flatness. The seal ring is flexible in the axial direction; the mating ring is not flexible.
Seal ring By definition, the seal ring is the axially flexible member of the end-face mechanical seal. The design of the seal ring must allow for minimizing distortion and maximizing heat transfer while considering the secondary sealing element, drive mechanism, spring and ease of assembly. Many seal rings contain the seal face diameters, although this is not a requirement of the primary ring. The seal ring always contains the balance diameter.
The shape of the seal ring may vary considerably according to the incorporation of various design features. In fact, the shape of the seal ring is often the most distinct identifying characteristic of a seal.
Mating ring By definition, the mating ring is the non-flexible member of the mechanical seal. The design of the mating ring must allow for minimizing distortion and maximizing heat transfer while considering ease of assembly and the static secondary sealing element. The mating ring can contain the seal face diameters, although this is not a requirement of the mating ring. To minimize primary ring motion, the mating ring must be mounted solidly and should form a perpendicular plane for the primary ring to run against. Like seal rings, mating rings are available in many different shapes.
Secondary sealing elements Secondary sealing elements are gaskets which provide sealing between the seal ring and shaft (or housing) and the mating ring and shaft (or housing). Typical secondary sealing elements include O-rings, wedges or rubber diaphragms. The secondary sealing elements (there may be a number of them) are not rotating relative to one another. The secondary sealing element for the mating ring is always static axially (although it may be rotating). Secondary sealing elements for the seal ring are described as being either pusher or non-pusher in the axial direction. The term pusher is applied to secondary seals that must be pushed back and forth by the movement of the shaft or primary ring whereas non-pusher secondary seals are static and associated with bellows seal rings.
Springs In order to keep the primary sealing surfaces in intimate contact, an actuating force is required. This actuating force is provided by a spring. In conjunction with the spring, axial forces may also be provided by the pressure of the sealed fluid acting on the seal ring. Many different types of springs are used in mechanical seals: single spring, multiple springs, wave springs, and metal bellows.
Encasing The term "Encasing" is used to describe various devices which hold the other components together in the desired relationship. For example, a retainer might be used to package the seal ring, secondary sealing element and springs into a single unit. Another example of encasing is the drive mechanism which is necessary to prevent axial and rotational slippage of the seal on the shaft.
Classifications
There are a number of different ways in which “seals” may be classified. Sometimes a reference to a “seal” may be to a sealing system whereas other times the reference is to a device such as a gasket, an O-ring, compression packing, etc. In this article, the reference is to an end-face mechanical seal.
One such method of classification considers design features or the configuration in which these features may be used. Classification by Design accounts for the details and features incorporated into a single seal ring/mating ring pair. Classification by Configuration includes the orientation and combination of the seal ring/mating ring pair.Schoenherr, K. S., "Design Terminology for Mechanical End Face Seals", Society of Automotive Engineers Transactions, Vol. 74, Paper Number 650301, (1966).
Design features The individual components of end-face mechanical seals may be designed to include features such as: face treatments such as hydrodynamic features, pads, etc. balance ratio pusher or bellows spring design hardware for containing assembly and providing drive mechanism considerations for secondary sealing element. In general, design features are not completely independent; that is, emphasis of a particular feature may also influence other features. For example, selection of a particular secondary sealing element may influence the shape of the seal ring.
Face treatment The most common seal face design is a plain, flat, smooth surface but there are many special treatments intended for specific applications. The most common objective for the face treatment is to reduce the magnitude of mechanical contact. In general, face treatments provide a means of modifying the pressure distribution between the seal faces through hydrostatic or hydrodynamic topography. Seal face topography refers to the three dimensional aspects of the seal face surface.
Balance ratio In addition to the spring force, the seal faces are pushed together through pressure acting hydrostatically on the geometry of the seal. The ratio of the geometric area tending to close the seal faces to the area tending to open the seal faces is called the balance ratio.
Pusher or bellows seal ring Pusher seals employ a dynamic secondary sealing element (typically an O-ring) which moves axially with the seal ring. Bellows seals employ a static secondary seal (such as an O-ring, high temperature graphite packing, or elastomeric bellows and axial movement is accommodated by contraction or expansion of the bellows.
Spring design Many different types of springs are used, including: relatively large single coil springs, multiple sets of small coil springs, and wave springs. A formed or welded metal bellows can also act as the spring. Corrosion, clogging and movement are major considerations when selecting a spring design.
Hardware In addition to retaining the other components, the seal hardware includes the drive mechanism which is necessary to prevent axial and rotational slippage of the seal on the shaft. The drive mechanism must withstand the torque produced by the seal faces while also allowing the seal ring to move axially. In addition to torque, the drive mechanism must withstand the axial thrust produced by hydrostatic pressure acting on the components. The various types of drive mechanisms include: dent drive, key drive, set screws, pins, slots, snap rings and many more. Typically, the retainer for the seal ring might include set screws, a dent or slot drive, recesses for the spring and a snap ring to complete the assembly. In contrast, mating ring hardware might be only a pin or slot to prevent rotation. Corrosion is a major consideration when selecting seal hardware.
Considerations for secondary sealing elements Both the seal ring and mating must accommodate secondary sealing elements. In some designs, various retainers, sleeves and other components may also include secondary sealing elements. Whereas a simple O-ring might require only a groove for fitting, some secondary sealing elements (for example, packing) might require mechanical compression. Although O-rings are available in many elastomers, sometimes an elastomer might not be compatible with the fluid being sealed or might be considered too expensive. In such cases, a secondary sealing element might be manufactured from perfluoroelastomer and shaped in the form of a wedge, V or U.
Configurations Although all end-face mechanical seals must contain the five elements described above, those functional elements may be arranged or oriented in many different ways. Several dimensional and functional standards exist, such as API Standard 682 - Shaft Sealing Systems for Centrifugal and Rotary Pumps, which describes the configurations for used in Oil & Gas applications. Even though the scope of API 682 is somewhat limited, it may be extended to describe end-face mechanical seals in general. Configuration refers to the number and orientation of the components in the end-face mechanical seal assembly. For example, springs may be rotating or stationary. Single or multiple pairs of sealing faces may be used. For multiple seals, the individual pairs of sealing faces may be similarly oriented or opposed. Containment devices such as bushings may or may not be used as part of the configuration.
Component vs cartridge The basic components of an end-face mechanical seal may be installed directly onto the shaft but a popular approach is to pre-assemble the components into some sort of package for ease of installation.Miller, Arthur H., People, Products and Progress: The Durametallic Story, Priscilla Press, Allegan Forest, Michigan, 1992.United States Patent #6685191, "Mechanical Seal for Rotating Shaft", February 11, 2000 When the components are pre-assembled onto a sleeve and gland plate, the complete assembly is called a cartridge seal. This complete assembly can be easily slid onto the shaft and bolted in place thus reducing the potential for installation errors. Some cartridge seals use regular component seal parts whereas other cartridge seals might use specific purpose parts. API 682 specifies that only cartridge seals are acceptable to the standard.
Rotating vs stationary springs