Rotating face mechanical seal
Encyclopedia
Seal (mechanical)
A mechanical seal is a device which helps join systems or mechanisms together by preventing leakage , containing pressure, or excluding contamination...
utilised in rotating equipment, such as pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
s, mixers
Industrial mixer
Industrial Mixers and Blenders are used to mix or blend a wide range of materials used in different industries including the food, chemical, pharmaceutical, plastic and mineral industries. They are mainly used to mix different materials using different types of blades to make a good quality...
, blowers
Centrifugal fan
A centrifugal fan is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing...
, and compressors
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
. 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
Radial shaft seal
Radial shaft seals are used to seal rotary elements, such as a shaft or rotating bore. Common examples include strut seals, hydraulic pumps seals, axle seals, power steering seals, and valve stem seals. Early radial shaft seals utilized rawhide as the sealing element, and many elastomeric seal...
(a.k.a "lip seal") and o-ring
O-ring
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.The O-ring...
s.
Mechanical seal fundamentals
A mechanical seal must contain four functional components: 1) Primary sealing surfaces, 2) Secondary sealing surfaces, 3) a means of actuation and 4) a means of drive.1) The primary sealing surfaces are the heart of the device. A common combination consists of a hard material, such as silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, Ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
or tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
, embedded in the pump casing and a softer material, such as carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in the rotating seal assembly. Many other materials can be used depending on the liquid's chemical properties, pressure, and temperature. These two rings are in intimate contact, one ring rotates with the shaft, the other ring is stationary. These two rings are machined using a machining process called lapping
Lapping
Lapping is a machining operation, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.This can take two forms...
in order to obtain the necessary degree of flatness, which is measured through the interference of helium light bands.
2) The secondary sealing surfaces (there may be a number of them) are those other points in the seal that require a fluid barrier but are not rotating relative to one another. Usually the secondary sealing elements are o-rings, PTFE wedges or rubber diaphragms.
3) In order to keep the two primary sealing surfaces in intimate contact, an actuation force is required and is commonly provided by a spring. In conjunction with the spring, it may also be provided by the pressure of the sealed fluid.
4) 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 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, centroidCentroid
In geometry, the centroid, geometric center, or barycenter of a plane figure or two-dimensional shape X is the intersection of all straight lines that divide X into two parts of equal moment about the line. Informally, it is the "average" of all points of X...
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. Flowserve Corporation issued the first patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
in 2007 for applying micro-topography to mechanical seal faces using an excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
.
Seal categories
All mechanical seals must contain the four elements described above but the way those functional elements are arranged may be quite varied. The standards of modern mechanical seals are widely defined by APIAmerican Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
Standard 682 - Shaft Sealing Systems for Centrifugal and Rotary Pumps.
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
Metal bellows
Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape .Bellows technology of the 20th and 21st century is centered on metal...
). 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
Stuffing box
A stuffing box is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.-Boats:...
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. In 2000 Gold Seals, Inc. (acquired by Chesterton in 2001) invented an iteration of the cartridge seal called a cassette seal. This seal utilized a replaceable inner "cassette" mounted in the cartridge end plate or gland simplifying the repair of the cartridge seal.
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
Since the rotating seal will create heat from friction, this heat will need to be removed from the seal chamber or else the seal will overheat and fail. Typically, a small tube connected to either the suction or the discharge of the pump will help circulate the liquid. Other features such as filters or coolers will be added to this tubing arrangement depending on the properties of the fluid, and its pressure and temperature. Each arrangement has a number associated with it, as defined by American Petroleum InstituteAmerican Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
"API" specifications 610 and 682.
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.
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 [Unpressurized]" or "barrier [Pressurized]" fluid.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, 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 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
The mechanical seal was invented by George Cook and was originally called a "Cook Seal". He also founded the Cook Seal Company. Cook's seal (which actually did not have a means of drive) was first used in refrigeration compressors. The Cook Seal company was a sideline product for Cook and he sold the company to Muskegon Piston Ring Company where it was renamed as The Rotary Seal Division of Muskegon Piston Ring Co. Muskegon Piston Ring sold the Rotary Seal Division to EG&G Sealol who in turn was largely acquired by John Crane Industries of Morton Grove, IL.In 1990, the world market for mechanical seals was estimated at $1 billion.