thumb|right|300px|Elements d<sup>1</sup> and a<sup>1</sup> bear and slide on each other, creating a seal at their interface. One group of parts is connected to the rotating shaft and the other to the machine's case. The spring keeps the elements tight against each other, maintaining the seal and allowing for wear.

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:

  • 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.

Design features

The individual components of end-face mechanical seals may be designed to include features such as: 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.thumb|right|The seal components may be conveniently pre-assembled into a cartridge for ease of installation.

Rotating vs stationary springs

Either the seal ring or the mating ring may be the rotating element. Seals with rotating seal rings are said to be "rotating" seals; seals with stationary seal rings are said to be "stationary" seals. Because the springs are always associated with the seal rings, sometimes the distinction is made as "rotating springs" versus "stationary springs". For convenience, rotating seals are used in most equipment; however, stationary seals have some advantages over rotating seals. In small, mass-produced seals for modest services, the entire seal may be placed in a package which minimizes shaft and housing requirements for the equipment. Stationary seals are also used to advantage in large sizes or at high rotational speeds.

Single vs multiple

When classifying end-face mechanical seals by configuration, the first consideration is whether there is only one set of sealing faces or multiple sets. If multiple sets are used, are the sets configured to be unpressurized or pressurized.

Tandem seals

thumb|right|Multiple seals may be oriented in Face-to-Face, Face-to-Back or Back-to-Back directions.A tandem seal consists of two sets primary sealing surfaces with the space in-between the two seals filled with a compatible low pressure fluid called the buffer fluid. This buffer fluid/space may be monitored to detect performance of the assembly. Unfortunately, the definition of “tandem seal” was often stated in a confusing manner. In particular, a tandem seal was usually described as two seals pointing in the same direction; that is, in a face-to-back orientation. This orientation is not necessary to the function of the configuration and the API chose to use the term Arrangement 2 instead of tandem in the API 682 standard.

Double seals

A double seal consists of two sets primary sealing surfaces with the space in-between the two seals filled with a compatible pressurized fluid called the barrier fluid. This barrier fluid/space may be monitored to detect performance of the assembly. Unfortunately, the definition of “double seal” was often stated in a confusing manner. In particular, a double seal was usually described as two seals pointing in the opposite direction; that is, in a back-to-back orientation. This orientation is not necessary to the function of the configuration and the API chose to use the term Arrangement 3 instead of double in the API 682 standard.

Seal piping plans

An end-face mechanical seal generates heat from rubbing and viscous shear and must be cooled to assure good performance and reliable operation. Typically, cooling is provided by circulating fluid around the seal. This fluid, known as a flush, may be the same as the fluid being sealed or an entirely different fluid. The flush may be heated, filtered or otherwise treated to improve the operating environment around the seal. Collectively, the flush and treating systems are known as piping plans. Piping plans for mechanical seals are defined by American Petroleum Institute specification 682 and are given a number. Some piping plans are used for single seals and some only for multiple seals. Some piping plans are intended to provide a means of monitoring the seal. Some sealing systems include more than one piping plan. See the table below for a summary and description of piping plans. His design was originally called a "Cooke Seal" and he founded the Cooke Seal Company. Cooke's seal (which actually did not have a means of drive) was first used in refrigeration compressors. The Cooke Seal Company was a sideline product for Cooke and he sold the company to Muskegon Piston Ring Company where it became the Rotary Seal Division. Muskegon Piston Ring sold its Rotary Seal Division to EG&G Sealol who were later acquired by John Crane Incorporated.

The first commercially successful mechanical seal to be used on centrifugal pumps was probably made by the Cameron Division of the Ingersoll-Rand Company. The Cameron seal was installed in a number of centrifugal pipeline pumps in 1928.

Mechanical seals in the 1930s often used a face combination of hardened steel versus leaded bronze. Carbon-graphite was not widely used as a seal face material until after World War II. Soft packing was used as secondary sealing elements. The O-ring was developed in the 1930s but not used in mechanical seals until after World War II.

In the late 1930s, probably about 1938 or 1939, mechanical seals began to replace packing on automobile water pumps. The famous Jeep of WWII used a rubber bellows seal in the water pump. After WWII, all automobile water pumps used mechanical seals.

In the mid-1940s pump manufacturers such as Ingersoll-Rand, Worthington, Pacific, Byron Jackson, United, Union and others began to make their own mechanical seals. Eventually most of these companies got out of the seal business but the Byron Jackson seal became the Borg-Warner seal (now Flowserve) and the Worthington seal was sold to Chempro (now John Crane - Sealol).

Cartridge seals were used on a regular basis by 1950; this convenient packaging of seal, sleeve and gland was probably developed by C. E. Wiessner of Durametallic about 1942.

In 1959, John C. Copes of Baton Rouge, LA filed for a patent on a split seal and was awarded Patent #3025070 in 1962. In the Copes design, only the faces were split. Copes chose to provide custom split seals which he manufactured himself so very few of his split seals were produced.

The Clean Air Act of 1990 placed limits on fugitive emissions from pumps. Seal manufacturers responded with improved designs and better materials. In October, 1994, the American Petroleum Institute released API Standard 682, "A Shaft Sealing Systems for Centrifugal and Rotary Pumps”. This standard had a major effect on the sealing industry. In addition to providing guidelines for seal selection, API 682 requires qualification testing by the seal manufacturers. API 682 is now in its 4th Edition and work has begun on 5th Edition.

There has been much consolidation in the mechanical seal industry. Among the major manufacturers:

  • John Crane (Smiths Group of Great Britain) includes Sealol (Rotary), Flexibox, Safematic, Ropac;
  • Flowserve includes BW/IP (Borg-Warner), Durametallic, Five Star, Pacific Wietz;
  • EagleBurgmann includes Eagle, Burgmann.

Today, in addition to face patterns such as spiral grooves and waves, materials have been developed that have special surfaces to promote hydrodynamic lift. Lasers can be used to etch microscopic, performance enhancing textures on the surface of the seal face. Piezoelectric materials and electronic controls are being investigated for creating truly controllable seals. The application of specialized seal face patterns, surfaces, and controls is an emerging technology that is developing rapidly and holds great promise for the future.

See also

  • Labyrinth Seal
  • Dry gas seal

References

  1. Bloch, Heinz P. and Budris, Allan R., "Pump User’s Handbook Second Edition", CRC Press, 2006.
  2. Lebeck, A. O., "Principles and Design of Mechanical Face Seals", New York: Wiley-Interscience, (1991).