End-face mechanical seal

Elements d¹ and a¹ 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.

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 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 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 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 or tungsten carbide, embedded in the pump casing and a softer material, such as carbon 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 in order to obtain the necessary degree of flatness.

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.

3) In order to keep the two primary sealing surfaces in intimate contact, a means of actuation must be provided. This 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 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 API 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, or elastomeric bellows). In this case, the face tracking is independent of the secondary seal which is always static against the shaft/sleeve.

John Crane Inc., based in Morton Grove, IL, creates various mechanical seals including cartridge seals. John Crane has a live-loaded mechanical packing cartridge seal — the Type LMPC seal — for static or dynamic sealing applications. The one-piece seal is mechanically loaded in the axial direction by a gland follower and can be installed easily in new or existing equipment without any modifications. Designed to perform in applications ranging from high to low temperature and pressure, the seal's compact design makes it usable in all types of process equipment such as mixers, agitators, positive displacement pumps, centrifugal pumps, hydraulic cylinders, valves and reciprocating pumps. In the past, there was no stand-alone unit for these kinds of sealing applications. However, the Type LMPC's one-piece cartridge design makes mounting simple, fast and accurate.

The LMPC's live-loading feature allows for in-service self-adjustment of the packing load with the aid of Belleville disc springs. This eliminates frequent manual gland bolt adjustment and increases intervals between manual retorquing. Further, the Type LMPC is very adaptable and can be customized to any equipment.

Mechanical seals serve the basic yet important function of preventing leakage by closing a gap or making a joint between two surfaces that have relative motion. In a pump, the shaft moves in relation to the housing unit, requiring a device such as a mechanical seal to dynamically fill the void between those two surfaces. In order to prevent any liquid leakage under demanding conditions, the dynamic sealing interface of a new seal is lapped flat within millionths of an inch.

A "cartridge seal" is a prepackaged seal that is common in more complex applications. Cartridge seals were originally designed for installation in equipment where a component type seal was difficult due to the design of the equipment. 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. 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, this heat will need to be carried away 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 Institute "API" specifications 610 and 682.

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 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 liquid or gas called a "buffer" or "barrier" fluid.

In a tandem seal, the seal will leak into the buffer fluid contained in the unpressurized cavity. If the cavity registers a dramatic increase in pressure, operator will know that the primary seal has failed. 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 Piping Plan 52

In a double seal, 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 it's 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.

John Crane Inc. was founded in 1917 as Crane Packing Company, the company established several facilities throughout the United States, Canada and England. John Crane had been in the Mechanical seal business for many years prior to this acquisition and in fact had long since exceeded Sealol in overall market size. John Crane now offers mechanical seals under the brand names of Sealol and Flexibox.

Today, John Crane's main competitors are Flowserve, ULTRASEAL Selos Mecânicos, AESSEAL, Eagleburgmann and A.W. Chesterton Co.

Others, manufacturers include Flex-a-seal, Global Seals Direct, Vulcan, First 4 Seals, Roplan and ULTRASEAL Selos Mecânicos.

In 1990 the world market for Mechanical Seals was estimated at $1 billion.

See also

• Stuffing box
• O-ring
• Labyrinth Seal
• Clearance Seal

External links

• Ultraseal Selos Mecânicos Mechanical Seals for all Industries
• John Crane: mechanical seals for all industries
• Information on Mechanical Seals from Flowserve
• Information on Mechanical Seals
• FAQ at mcnallyinstitute.com
• Basics of Mechanical Seals by Problem Solving Products, Inc.
• billi dichtungstechnik gmbh (Germany)