Seal Spring
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 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.
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.
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.
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 mechanical 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.
About the Author
Ningbo Yinzhou Guowei, mechanical seal Component Factory is a professional engaged in the design, development, manufacture, sale of various cartridge seal and flygt seals business. Production and testing are well-equipped with a high-quality staff.
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