WHAT IS A MECHANICAL SEAL
A mechanical seal is a device for isolating two volumes on rotary machines (operating or driving), in which one or more rotating axles are in motion, at the point where the shaft exits the machine itself, connecting to a propulsion system of any nature.
Take an operating machine, centrifugal or volumetric (a pump, a compressor, a turbine). The machine’s internal volume, often understood as the process fluid, must be isolated from its external environment.
The mechanical seal has the task of being installed at the point where the shaft exits the machine the operating machine and connects to the motion-generating system (e.g. an electric motor, combustion engine), preventing the process fluid from escaping from the machine itself.
SELECTING PRINCIPALS
Mechanical seals consist of four basic elements:
1. A rotating ring
2. A stationary ring
3. An elastic element
4. A dragging element
The mechanical seal has many variations and configurations depending on its application. The distinctions that can be made are:
Dependent or independent of the direction of rotation.
Dipendente o indipendente dal senso di rotazione. A characteristic example is the conical spring mechanical seal, in which the motion of the rotor (shaft) is transferred directly to the seal itself, bringing it into rotation. In this specific case, the seal will only function if it is produced in the correct direction of rotation. Mechanical seals with several springs (multi-spring), leaf springs, or bellows (mostly metal) can, on the other hand, work in both directions of rotation
Balanced or unbalanced The most important of the principles that characterize a mechanical seal is whether its geometric structure is balanced. A balanced mechanical seal is one in which, during the design phase, the operating pressure that must be satisfied has been taken into account, thus attempting to ensure that the pressure in the seal chamber helps to close the sliding faces. Balancing is carried out by calculating the difference of the surfaces placed under pressure (positively, if it’s in favor of thrust, and negatively, if for example the seal must be applied externally to the process). The calculation of balancing can be summarized by the following formula:
B = (Do2 – Db2)/(Do2 – Di2) for external mechanical seals
B = (Db2 – Di2)/(Do2 – Di2) for internal mechanical seals
Components or cartridge seals. This distinction is also one of the main choices to make when selecting a mechanical seal. All mechanical seals consisting of a rotating part (elastic part, static seal, sliding ring) and a static part, usually consisting of a sliding ring and a secondary seal, belong to the category ‘with loose components’. This type is usually used on large production machines, such as pumps and compressors, where the use of the machine itself is very well defined. The seal, in this case, will be mounted inside the machine. Loose component seals can undergo certain distinctions if they are to be used in a “multiple” configuration. They can therefore become tandem (in international jargon referred to as ‘face-to-back’) or opposed (‘back-to-back’).
Cartridge mechanical seals represent the technical evolution that mechanical seals have developed over the years. Within this particular configuration, ‘single’ or ‘double’ cartridge seals (in which two or more sealing systems are inserted) can be distinguished. Once this initial selection has been made, cartridge seals can follow standards that represent unification in dimensions (ANSI/ISO), that refer to sizing indications (ASME) or, in the case of seals used in the “Oil & Gas” sector, that are defined in all their design aspects in relation to pressures, speeds, temperatures and configurations (API682).
PRINCIPLES OF OPERATION
Let us give the example of a component seal below in order to go into detail about its fundamental parts.
Here we describe a component seal, single, with a conical spring (thus dependent on the direction of rotation) and unbalanced. The rotating part consists of a spring element (1), the o-ring or ‘secondary seal’ (2) and the rotating case (3). In this particular example, the rotating case also contains the sliding insert or primary seal element (4). Opposed to the rotating part is the stationary part, consisting of a monolithic ring (5) and an o-ring or “secondary seal” (6).
The operation of the mechanical seal will therefore be given by a rotating part which, dragged by the rotating shaft, will revolve over the stationary ring which, unlike the rotating part, will be locked inside the seat, specially made on the machine component (e.g. a cover) where it will be installed. The two sliding faces will create a barrier that will prevent the process fluid (inside the machine) from coming into contact with the external environment.
