Introduction to Mechanical Seals

What is a mechanical seal?

It is the element used to prevent fluid / gas leaks at the point where the shaft passes from the wet end to the dry or atmospheric end. The mechanical seals are used in bombs, compressors and other types of rotating equipment.

What are the elements that make up the Mechanical Seal?

Tracks or faces (rotary track and stationary track), O’rings-joints (dynamic and static) and compression mechanism (springs, bellows, blades, etc).

What types of Mechanical Seals are there?

• simple
• Double
• internal
• external
• stationary
• Rotary
• Balanced
• unbalanced
• component seals
• cartridge seals
• Single springs
• Multisprings

What materials are Mechanical Seals made of?

The mechanical seals They can be made of various materials. As for the metal parts of a seal, they are commonly made of stainless steel. In stamps of lesser manufacture, we can find foundry. Exotic alloys such as A276, A20, etc. are also used.
On the other hand we have the materials of the tracks or faces. These can be carbon, stainless steel, bronze, silicon carbide, tungsten carbide, ceramic, etc.
Finally, the materials of the gaskets and O’rings must be considered: Ethylene Propylene, Nitrile, Buna, Viton, Aflas, Teflon, Grafoil, etc.
Both the materials of the metallic parts and of the tracks and O’rings-joints must be selected based on the type of fluid / gas that must be sealed.

Single Cartridge Seal

How does a Mechanical Seal work?

A mechanical seal It has a static part and a dynamic part. The dynamic or rotary part is the one that rotates together with the axis. There is the rotary track. The static part is the one that is subject to the fixed parts of the equipment and where the stationary track is housed. tracks of a mechanical seal they work with each other: one revolves around the other. Both must be flat and free of irregularities. The mechanical seals they have three sealing points: the primary seal (between the tracks); the secondary seal (the dynamic O’rings); finally, the joints or static O’rings.

What information do I require to select a mechanical seal?

The mechanical seals They are manufactured and designed depending on the fluid or gas that I want to seal. In order for mechanical seal manufacturers to select the mechanical seal suitable, they need to know the following data: Fluid or gas, RPM, temperature, working pressure. When we talk about fluid / gas, the products that compose it, PH, etc. must be indicated.

Why do track materials vary?

Double Cartridge Seal

As indicated above, the manufacturing materials of a mechanical seal they are selected based on the fluid / gas that we want to seal.
The tracks of mechanical seals are classified in hard courts and soft courts: We can classify silicon carbide, tungsten carbide, ceramics, etc. as hard tracks. As soft tracks, we have carbon, bronze, stainless steel, etc.

If we have to seal a fluid with suspended solids, for example water with sand, we should think about using a seal with hard tracks: tungsten carbide or silicon carbide, since if we use soft tracks, the sand would damage them causing the leak of the fluid to the atmospheric side.

In other cases, we may need to seal acid-chemical products, where we must use tracks made of silicon carbide, carbon, ceramic, materials which have great resistance to these types of products.

What is the difference between a single mechanical seal and a double mechanical seal?

Constructively the mechanical seals simple have a set of tracks (called internal) and the double mechanical seals they have two sets of tracks (inner tracks and outer tracks).

The simple mechanical seal is lubricated by the fluid/gas itself being sealed (sometimes lubrication from an external source is used) while a double mechanical seal It is lubricated with an external fluid.

When should I use a single mechanical seal and when should I use a double seal?

API 682 Cartridge Seal

When I must seal fluids at low temperatures, non-toxic, non-abrasive, non-contaminating, etc., we can use a single mechanical seal. When we need to seal abrasive fluids (fluids with suspended solids), dangerous fluids – flammable, toxic, polluting, etc. – fluids at high temperatures, fluids that crystallize / carbonize in contact with the atmosphere, etc., we must use double type mechanical seals. However, there are different designs and types of mechanical seals, so perhaps we can use a single mechanical seal sealing a fluid with high temperature, etc.
Both the seal model (single-double, cartridge-component, rotary-stationary, balanced-unbalanced, etc.) as well as the materials of the races, metallurgy and O’rings-gaskets must be selected based on the type of fluid / gas that we want to seal.