Cartridges & Mechanical Seals

GENERAL INFORMATION 152 A non-balanced mechanical seal has the following ratio: K = A/a > 1 and a balanced mechanical seal has a ratio of K = A/a > 1. In designing mechanical seals, K values of between 0.6 and 1.2 are used. A value under 0.6 favours the duration and life of the mechanical seal, but will not guarantee adequate sealing. • Conical spring seal: These seals are defined by the type of spring they use, and their function. They have an axial conical-shaped spring with one end fi- xed to the primary ring and the opposite end to the shaft. The spring maintains the tension between the contact surfaces and transmits the shaft movement to the rotating part of the mechanical seal. Conical spring seals are usually mechanical seals with a dependent Construction This classification is related to the characteristics and design of the different components that are used to make the mechanical seal. Each variation responds to special application needs or characteristics. rotation direction, i.e., they can only function properly in the direction of the rotation for which they are designed. They are normally supplied with the rotation direction to the right, and the left rotation option is only supplied if specifically ordered. Secondary seals are usually O-rings, since they require no bellows to transmit the movement. • Bellows seals: These mechanical seals have a secondary bellow seal on the shaft. There are bellows of different types made of different materials, and each one gives a different, specific performance: - Elastomer bellows seals: The bellows are made of elastomer ma- terial and are responsible for generating static sealing on the shaft, and also for transmitting the shaft movement to the mechanical seal. The spring mechanism that is normally used is an axial cylindrical spring, which means that the performance of these mechanical seals does not depend on the shaft rotation direction. - Metal bellows seals: The spring mechanism in these seals are metal bellows. Their main advantage is in how they perform: they are more resistant to abrasion and chemicals (particularly in combination with chrome-nickel alloys) than elastomer bellows, and have a higher temperature resistance (when secondary seals made of graphite are mounted), and prevent particles or traces of fluid from becoming trap- ped in the spring coils (the folds of the bellows act like a pumping ele- ment) and they permit the use of a “static O-ring” as a secondary seal. In other seals the O-ring or the bellows are in the primary ring of the seal, generating static sealing. However, their behaviour in real operating conditions is not strictly static and there may be: movement due to wear of the contact surfaces, vibrations or oscillations which lead to the axial shifting of the shaft or other factors that have a dy- namic effect on the O-ring. In the worst case, if combined with a poor surface finish of the shaft, this causes wear or damage in the O-rings. However, the “static O-ring” system permitted by this type of seal is through the bellow, which absorbs the displacements. The O-ring is at the opposite end of the bellows with respect to the primary ring and not at all affected by these defects. There are two different types of metal bellows seals, depending on the bellows construction characteristics. -Welded bellows seals: They are formed by ring plates welded to- gether along their perimeter at the ends, which is the most common type. -Rolled bellows seals: They are more complex in construction terms and is formed by an undulated metal tube. It has a larger elastic run and is often used in applications that involve demanding sterilisation processes, as there are no corners or closed angles where undesi- rable particles could accumulate, which makes them easy to clean. - PTFE bellows seals: PTFE is a plastic with high chemical resistance to practically all working fluids and a temperature resistance that is higher than most elas- tomers. These features make them extremely useful in certain applications. PTFE bellows are used for the same purpose as metal bellows. They have a more limited resistance to temperature and abrasion but higher chemical resistance. With respect to the secondary seals, it has a bushing or wedge that is also made of PTFE, which operates on a static basis. This is a very special type of seal that is used for working fluids that are extremely aggressive in chemical terms.