Mechanical seals are elements manufactured to provide a physical and watertight barrier. The main advantages of using mechanical seals over other sealing systems are that they prevent leakage, reduced friction, shaft wear and consequent power loss, among other benefits.
But what should be taken into account when deciding on the most suitable mechanical seal material in each case?
There are different types of mechanical seals, but in order to offer the right sealing solution for each application, it is important to choose the correct type of seal and the correct materials. The wrong choice of materials could end in a quick sealing failure or in a drastic reduction of the seal lifespan.
So which mechanical seal material should be used in each case?
In order to know what kind of materials the mechanical seals have to be made of, three elements have to be taken into account:
- The components of the mechanical seal.
- The design of the mechanical seal.
- The operating conditions of the application where the mechanical seal is going to be incorporated.
The following are the main components and materials available for the manufacture of mechanical seals:
Types of materials for mechanical seals
There are many materials for mechanical seals depending on the needs of each application and the characteristics of the element that requires the sealing. The main components and materials available for mechanical seals are:
Contact surfaces
Depending on whether the fluid is clean or particle-laden, whether it has lubricating properties or even whether it is non-existent (as in agitators/reactors), the following materials can be chosen for the mechanical seal faces:
- Alumina: It has purity up to 99.5% aluminium oxide content, one of the hardest existing materials. It has the highest hardness of all the materials available for making mechanical seal faces but it is a brittle material. It can be found in any type of application, even in the chemical industry, as it has a very high chemical resistance.
- Silicon carbide: This is a synthetic material with very high hardness and chemical resistance properties, although it is still a brittle material. It is the most versatile option because it can be combined with another silicon or tungsten carbide face.
- Tungsten carbide (Widia): with this material for mechanical seals we combine hardness with resistance, being able to work with the most aggressive applications, such as waste water treatment.
- Graphite: is considered a soft material and should always be combined with another hard rubbing face. It has self-lubricating properties, which allows it to run dry under certain conditions. In Lidering we have different types of graphite suitable for different working conditions; high speed or pressure, dry working, food certification…
- PTFE: exceptionally, contact faces can be manufactured in PTFE, taking advantage of its very high chemical resistance and self-lubricating properties. Its use is almost exclusively for applications related to the chemical industry.
Secondary seals
There are different materials available for the manufacture of secondary seals for mechanical seals, such as O-rings, flat seals, PTFE shims, etc. Some of the most common materials for the manufacture of secondary seals in mechanical seals are:
- NBR: very versatile with reduced chemical resistance and a maximum temperature of approx. 90 °C.
- EPDM: good behaviour with alkaline products and ideal for working with hot water or steam. Not generally compatible with mineral oils or greases.
- FPM: good chemical resistance and ideal for working with mineral oils up to a temperature of 200 °C. Not compatible with hot water or steam.
- AFLAS®: this is a registered trademark for a type of fluoroelastomer. This material offers high temperature resistance up to 230°C, very good resistance to chemicals and oils.
Springs and other metal components
As mentioned in previous posts, the material of springs and other metal parts must be resistant to interactions with working fluids, which are often chemically aggressive. For this reason, when manufacturing mechanical seals, it is important to choose from a wide variety of metal alloys.
Lidering has specific alloys to manufacture these mechanical seal components:
- 316L Stainless Steel: This is our standard material for fabrications in our Production Centre. It is a high quality stainless steel of European origin.
- Duplex and Super Duplex: very high quality steel ideal for working with chemically aggressive fluids.
- Hastelloy: Ideal for small components such as springs.
