End-face mechanical seal
An end face mechanical seal, also referred to as a mechanical face seal but usually simply as a mechanical seal, is a type of seal utilised 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 (a.k.a. "lip seal") and o-rings.
Mechanical seal fundamentals.
A mechanical end face seal has five functional components, these are: seal ring, mating ring, secondary sealing elements, a means of actuation, and a means of drive. The seal ring and mating ring are sometimes referred to as the primary sealing surfaces.
- The primary sealing surfaces are the heart of the device. A common material combination consists of a hard material, such as silicon carbide, Ceramic or tungsten carbideand a softer material, such as carbon. Many other materials can be used depending on the liquid's chemical properties, pressure, and temperature. The seal ring and mating ring are in intimate contact, one ring rotates with the shaft, 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 machining process called lapping in order to obtain the necessary degree of flatness.
- The secondary sealing elements (there may be a number of them) are not rotating relative to one another. Usually the secondary sealing elements are o-rings, PTFE wedges or rubber diaphragms.
- In order to keep the primary sealing surfaces in intimate contact, an actuation force is required and is commonly provided by a spring. In conjunction with the spring, axial forces may also be provided by the pressure of the sealed fluid. The primary ring is axially flexible and is in contact with the spring. The mating ring is not axially flexible.
- The primary sealing surfaces must be the only parts of the seal that are permitted to rotate relative to one another, they must not rotate relative to the parts of the seal that hold them in place. To maintain this non-rotation a method of drive, sometimes called the drive mechanism, must be provided.
Seal face technology
Mechanical seal face geometry is one of the most critical design elements within a mechanical seal. Seal face properties such as: balance diameter, centroid location, surface area, surface finish, drive mechanism, and face topography can be altered to achieve specific results in a variety of liquids. Seal face topography refers to the alteration of an otherwise flat seal face sealing surface to one with a three-dimensional surface.
Seal categories
All mechanical seals must contain the four elements described above but the way those functional elements are arranged may be quite varied. Several dimensional and functional standards exist, such as API Standard 682 - Shaft Sealing Systems for Centrifugal and Rotary Pumps, which sets precise configurations and sizes for mechanical seal used in Oil & Gas applications.
Mechanical seals are generally classified into two main categories: "Pusher" or "Non-Pusher". These distinctions refer to whether or not the secondary seal to the shaft/sleeve is dynamic or stationary. Pusher seals will employ a dynamic secondary seal (typically an o-ring) which moves axially with the primary seal face. Non-pusher seals will employ a static secondary seal (either an O-ring, high temperature graphite packing, elastomeric bellows or metal bellows). In this case, the face tracking is independent of the secondary seal which is always static against the shaft/sleeve.
A "cartridge seal" is a prepackaged seal that is common in more complex applications and were originally designed for installation in equipment where a component type seal was difficult due to the equipment design. Examples of this are horizontally split and vertical pumps. In 1975 the A W Chesterton Company designed the first cartridge seal that fit pumps with varying stuffing box bore sizes and gland bolt patterns. To accomplish this the seal utilized internal centering of the stationary parts and slotted bolt holes. This "generic" cartridge seal could be manufactured in higher production quantities resulting in a cartridge seal that could be used in all applications and pumps types. Cassette seals, patent no. 6,685,191 introduced by Gold Seals, Inc., utilize a replaceable inner "cassette" mounted in the cartridge end plate or gland, while modular cartridge seal systems makes it possible to replace only the parts subject to wear, such as sliding faces, secondary seals and springs, while keeping the seal's hardware (gland, sleeve, bolts). Cartridge seals can suffer from clogging due to the bigger space occupied inside the stuffing box, leading to dense or charged fluids not moving enough to centrifugate the solid particles.
Gap seals are generally used in bearings and other constructions highly susceptible to wear, for example, in the form of an O-ring. A clearance seal is used to close or fill (and join) spacing between two parts, e.g. in machine housings, to allow for the vibration of those parts. An example of this type of seal is the so-called floating seal which can be easily replaced. These seals are mostly manufactured from rubber or other flexible but durable synthetic materials.
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.
| API Plan | Description |
|---|---|
| 01, 02, 03 | Internal system for single seals |
| 11, 12, 13, 14 | Simple recirculation system for single seals |
| 21, 23, 31, 41 | Recirculation system with auxiliary equipment for single seals |
| 52, 53A-C, 54, 55, 74 | External system for dual seals |
| 32, 62 | External injection system |
| 65A/B, 66A/B | Leakage containment and management |
| 61, 71 | Connections only (plugged) |
| 72, 75, 76 | External control and leakage management system for containment seals |
| 99 | Miscellaneous, requires a sketch |
Component seals
Usually these are considered to be disposable since refurbishing the metal parts and replacing the wearable items isn't economical.
Component seals are produced in high volumes so the end price is low in comparison to cartridge seals.
The majority of mechanical seal manufacturers offer seals that are dimensionally interchangeable with each other. The only difference being material quality and price. Also component seal is expensive to assemble as it will be assembled on the pump.
Tandem and double seals
Since almost all seals utilize the process liquid or gas to lubricate the seal faces, they are designed to leak. Process liquids and gases containing hazardous vapors, dangerous toxic chemicals or flammable petroleum must not be allowed to leak into the atmosphere or onto the ground. In these applications a second "containment" seal is placed after the primary seal along the pump shaft. The space in between these two seals is filled with a neutral or compatible liquid or gas (generally nitrogen) called a buffer seal (unpressurized) or barrier seal (pressurized).
In a tandem seal [face-to-back], the seal will leak into the buffer fluid contained in the unpressurized cavity commonly known as thermosiphon pot. If the cavity registers a dramatic increase in pressure or fluid level, the operator will know that the primary seal has failed. This can be achieved by using pressure/level switches or transmitters. If the cavity is drained of liquid, then the secondary seal has failed. In both instances, maintenance will need to be performed. This arrangement is commonly used when sealing fluids that would create a hazard or change state when contacting open air. These are detailed in API 682 [Currently 3rd Edition] Piping Plan 52
In a double seal [Generally Back to Back], the barrier liquid in the cavity between the two seals is pressurized. Thus if the primary seal fails, the neutral liquid will leak into the pump stream instead of the dangerous pumped fluid escaping into the atmosphere. This application is usually used in gas, unstable, highly toxic, abrasive, corrosive, and viscous fluids. These are detailed in API Piping Plan standards #53a, 53b, 53c; or 54. Plan 74 may also be considered a double seal piping plan, although it is used exclusively when describing a dry gas barrier seal support system. The barrier fluid used in a Plan 74 system is simply a gas, not a liquid. Typically, nitrogen is used as its inert nature makes it advantageous due to mixing with the process stream being sealed.
Tandem and double seal nomenclature historically characterized seals based on orientation, i.e., tandem seals mounted face-to-back, double seals mounted back to back or face-to-face. The distinction between pressurized and unpressurized support systems for tandem and double seals has lent itself to a more descriptive notation of dual pressurized and dual unpressurized mechanical seal. This distinction must be made as traditional 'tandem seals' can also utilize a pressurized barrier fluid.
Origins and development
The mechanical seal appears to have been invented by George J. Cooke (Patent #1545080, “Seal for Rotating Shafts”) in 1923. 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 1930's 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 1930's but not used in mechanical seals until after World War II. In the late 1930's, 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-1940's 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. By 1954, mechanical seals were used with such regularity in the refining and process industries that the American Petroleum Institute included seal specifications in the first edition of its Standard 610, ACentrifugal Pumps for General Refinery Services@.
By 1956, many of the conceptual designs and application guidelines that are in use today had been developed. Commercially available designs included both rotating and stationary flexible elements, balanced and unbalanced hydraulic loading, rubber and metal bellows, and a wide variety of spring designs and types. Secondary sealing elements included O-rings, wedges, U-cups and various packings. Carbon-graphite was widely used as a seal face material; the mating seal face was often cast iron, Ni-resist, 400 series stainless steel, Stellite or aluminum oxide although tungsten carbide was coming into use. Stainless steel was widely used for springs, retainers, sleeves and glands. Single and multiple seal arrangements were used as necessary to accomplish the required performance.
In 1957, Sealol introduced the edge welded metal bellows seal. Previously, metal bellows seals had used a formed bellows which was much thicker and stiffer. 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, AShaft 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
References
- * End Face Mechanical Seals and related technologies
- * What is a Pump Seal?
- * Mechanical seal patent from 2004
- * Mechanical seal patent from 1925
- API Standard 682, Fourth Edition, 2014, “Pumps – Shaft Sealing Systems for Centrifugal and Rotary Pumps,” American Petroleum Institute, Washington D.C.
- Bloch, Heinz P. and Budris, Allan R., "Pump User’s Handbook Second Edition", CRC Press, 2006.
- Elonka, Steve, "Take a Look at Today's Mechanical Seals", Power, 1956.
- Lebeck, A. O., "Principles and Design of Mechanical Face Seals", New York: Wiley-Interscience, (1991).
- Miller, Arthur H., 1992, "People, Products and Progress: The Durametallic Story", Allegan Forest, Michigan: Priscilla Press.
- Schoenherr, K. S., "Design Terminology for Mechanical End Face Seals", Society of Automotive Engineers Transactions, Vol. 74, Paper Number 650301, (1966).