Choosing the Right Seal

A seal is a device that joins two things together to prevent leakage of liquid or gaseous fluids. While they are often used to seal pipes together, they are also suitable for anti-vibration applications. Seals are used in machines, tools, vehicles, windows, air-conditioning systems, and more. They can be used in dynamic (moving) or static (stationary) applications. They are available in a variety of materials, depending on the application. They are available in elastomer (such as rubber or silicone), plastic, metal, leather, felt, brass, or fibrin.

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  • How to choose the right seal

    A seal is characterized by its ability to withstand:

    • Pressure
    • Temperature
    • Movement
    • The speed of moving parts (for moving applications)
    • The environment

    There are several factors to consider when choosing the right seal:

    Type of material

    Seals can be made of metal, rubber, plastic, composites (carbon, resin, fiberglass) or PTFE.

    • Metal seals are particularly effective at high temperatures and pressures.
    • PTFE seals are suitable for extreme pressures.
    • Plastic, which is much more sensitive, is not recommended for high temperatures and pressures.

    Type of seal

    There are different types of seals, depending on the application: static seals (O-rings and flat seals) and dynamic seals (lipped and felt seals).

    • An O-ring is ideal for use in environments subject to high pressures and temperatures, with or without moving parts. It can handle moderate speeds but is much better suited to static applications.
    • A lipped seal should be used in a low-pressure environment and is particularly effective for sealing rotating or sliding parts.

    Size of seal:

    If the seal is fitted inside a threaded tube, the size indicated by the manufacturer actually corresponds to the diameter of the thread expressed in millimeters or inches. This in turn determines the inside and outside diameter of the seal.

    The table below shows the corresponding sizes of seals (thread diameter) in inches and millimeters, and their inside and outside diameters.

    Thread size in inches Thread size in mm Inside diameter in mm Outside diameter in mm
    1/4 8/13 8 12
    3/8 12/17 9 or 9.5 or 10 14.5 or 15
    1/2 15/21 12 18
    3/4 20/27 or 21/27 16 or 18 24
    1 26/34 22 30
    1” 1/4 33/42 30 38
    1” 1/2 40/49 34 44
    2 50/60 45 or 46 55 or 57

    Source: ooreka article

  • Why choose an O-ring?

    O-rings are the most common seals on the market and are widely used in hydromechanics because they offer several advantages.

    • They’re lightweight, inexpensive, low-maintenance, and don’t take up too much space.
    • They are easy to install.
    • Depending on their material, they can withstand high temperatures.
    • They can be used for both static and dynamic applications, provided that speed and pressure are moderate.

    However, these can also have drawbacks, and measures should be taken to correct any problems:

    • Pressure and force can damage them, so don’t hesitate to protect them with sealing rings (one or two rings are enough).
    • The compression of an O-ring after assembly must not exceed 20%.
    • Finally, beware of gap extrusion because pressure compresses the seal. The higher the pressure, the harder the elastomer must be to avoid gap extrusion. Adding sealing rings helps correct this problem.
    NORELEM O-ring

    NORELEM O-ring

  • Why choose a lipped seal?

    Lipped seals are used for dynamic applications and have several advantages:

    • They are particularly suitable for applications requiring rotary motion, such as rotating parts.
    • They can also be used to seal sliding parts.
    • They are compact.

    However, they do have a few drawbacks. They are extremely sensitive to pressure and should therefore only be used at low pressures, below 1 bar. You should also be aware of any eccentricity problems, which can lead to leaks. Make sure that the shaft is coaxial with the seal. In other words, the center of the shaft and the center of the seal must be aligned.

    HUTCHINSON lipped seal

    HUTCHINSON lipped seal

  • Why choose a flat seal?

    Flat seals are used for static applications and offer several advantages:

    • They are available in a wide range of sizes, shapes, and materials.
    • There are as many flat seals as there are applications.

    Flat seals are used in a wide range of industrial sectors.

    The automotive industry uses large, thin seals known as gaskets. On the one hand, they ensure resistance to even the most extreme temperature variations, as well as to oils and fuels. They also provide a seal between two mechanical components (clutch housing, oil pan, etc.).

    The food industry uses rubber seals of varying diameters, often reddish/orange in color, to seal food jars and bottles. These single-use seals can no longer be used once the container has been opened.

    In plumbing, fiber seals, which swell slightly when in contact with water, are most commonly used. They seal the connections between the valve and the pipe. We strongly recommend changing the seal every time you disassemble the valve. For gases, CNK blue seals (a mixture of rubber, nitrite, and aramid) are recommended, because the presence of gas does not allow the seal to swell.

    DONIT flat gaskets

    DONIT flat gaskets

  • What material should I choose for my seal?

    There are various materials available for seals. To choose the right one, you’ll need to consider operating conditions including temperature, pressure, and chemical compatibility.

    Metal seals are ideal for high pressures and temperatures. Metal, semi-metal, and metal-plastic seals are available. The higher the metal concentration, the more resistant the seal will be to high temperatures.  PTFE seals are generally special seals designed for applications requiring greater thermal or chemical resistance.  Composite seals are available in carbon, resin, or GRP (glass-reinforced plastic).  The advantage of rubber and plastic seals is that they are highly flexible. They are suitable for a wide range of applications (piping, heat exchangers, etc.). Depending on the type of rubber, the seal may be recommended for certain chemicals or environments, and may not be compatible with others.

    The table below summarizes the different types of rubber and plastic and their compatibility with their operating environment.

    Temperature Compressive Strength Abrasion Resistance Gas Air Mineral Oils Acids Fuels Water
    Nitrile -50°C to 120° Y A Y N Y Y Y Y
    Fluorocarbon -20° to 100° Y A Y Y Y Y Y Y
    Silicone -55° to 250° A A N Y A A Y N
    Polyurethane -30° to 100° Y Y Y N Y Y Y N
    Ethylene -55° to 125° A Y N Y N N A Y
    Chloroprene -40° to 100° N A N Y Y Y Y Y
    Butyl -40° to 120° N A Y Y N Y A Y

    Y: Yes
    A: Average
    N: No

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