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  • Mechanical SealsTechnical Manual

  • THE FUNDAMENTALSThe mechanical seal ............................................................................1The liquid film ....................................................................................3Leakage ............................................................................................4Degree of freedom..............................................................................4Balancing ratio ..................................................................................5Unbalanced seals ................................................................................6Balanced seals....................................................................................7

    THE CONFIGURATIONSSingle internal seal............................................................................11Single external seal ..........................................................................11Back-to-back double seal ..................................................................12Tandem double seal ..........................................................................13Dual seal ........................................................................................14Face-to-face seal ..............................................................................14

    THE SELECTIONCooling system and API planes ..........................................................19Selection of mechanical seals ............................................................19Clean, not harmful, neutral, not flammable products ............................21Fluids crystallizing when in contact with atmosphere ............................21Acid products ..................................................................................22Hot liquids ......................................................................................22Aqueous solutions prone to solidify or produce sediments ......................23Toxic, poisonous or highly viscous fluids ..............................................24Abrasive fluids ..................................................................................24Flammable fluids ..............................................................................25Hot water ........................................................................................26

    THE TYPESSingle seal with single spring..............................................................29Bi-directional seals ..........................................................................30Seals with protected springs ..............................................................31Elastomeric bellow seals ....................................................................32PTFE bellow seal ..............................................................................33Metal bellow seals ............................................................................34External seals ..................................................................................35Cartridge seals ................................................................................36Stationary seals ................................................................................37

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  • THE MATERIALSSeal face materials............................................................................41Graphite ..........................................................................................41PTFE................................................................................................42Stellite ............................................................................................42Chromium steel ................................................................................42Ceramic ..........................................................................................42Tungsten carbide ..............................................................................43Silicon carbide ..................................................................................43TAB. I - FLUITEN code seal face materials ............................................44TAB. II - Seal face characteristics........................................................45TAB. III - Fisical and mechanical property seal face ..............................46Secondary seal materials ..................................................................47Elastomers ......................................................................................47Nitrilic rubber ..................................................................................47Fluoroelastomer ................................................................................48Ethylene Propylene............................................................................48Perfluoroelastomers ..........................................................................48Silicone............................................................................................48Neoprene ........................................................................................48Aflas................................................................................................49Non elastomeric materials ..................................................................49PTFE (Polytetrafluoroethylene) ............................................................49FEP (Fluoruro of ethylene e propylene) ................................................50Grafoil and asbestos free ..................................................................50TAB. IV - FLUITEN code secondary seals ..............................................50Metallic parts....................................................................................51TAB. V - FLUITEN code metallic parts ..................................................51

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    The mechanical seal

    Mechanical seals have the purpose

    of preventing leakage of a fluid

    (liquid or gaseous) through the

    clearance between a shaft and the

    fluid container. (Fig.1)

    The main components of a

    mechanical seal are the seal rings

    on which a mechanical force is

    acting, generated by springs or

    bellows , and an hydraulic force, generated by the process fluid pressure.

    The seal ring which rotates with the shaft is called the "rotary ring" ; the

    seal ring fixed on the casing of the machinery is called the "stationary ring".

    Secondary seals are required to perform static sealing between rotary rings

    and shafts and also between stationary rings and the casing of the machinery.

    Elastomeric O-Rings are usually used as secondary seals but alternative

    systems can be used, as described in the following sections. (Fig.2)

    Fig. 1

    Stationary Ring Gasket

    Rotary Ring GasketStationary Ring

    ROTARY SHAFT

    Rotary Ring

    Spring

    STUFFING BOX

    Fig. 2

  • Typically mechanical seals are installed on pumps and mixers. (Fig.3 & 4)

    On both of the above set-ups, the installation of a suitable device is required

    to seal the fluid contained in the casing.

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    Fig. 3

    Fig. 4

  • The liquid film

    In order to minimize the amount of friction between the seal rings an efficient

    lubrication is required.

    Seal faces can be lubricated by the process fluid or, with double mechanical

    seals, by a proper auxiliary fluid (see chapter relevant to configurations).

    An stable and complete layer of lubrication greatly affects the performance

    and the life of a mechanical seal. (Fig.5)

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    Fig. 5

    Hydraulic forcegenerated by pressure in

    stuffing box

    VAPORIZATION OFTHE LIQUID FILM

    DRY-RUNNINGSITUATION

    Opening forcesgenerated by liquid

    film pressure

    STABLE LIQUIDFILM

  • In order to insure good lubrication and sufficient cooling of the seal rings, the

    correct selection of a mechanical seal shall take into consideration the

    following parameters:

    Process fluid temperature

    Vaporisation pressure at operating temperature

    Process fluid characteristics

    Shaft speed

    (see also chapter relevant to selection)

    Concepts and principles above discussed are valid for all mechanical seal

    operating with a liquid fluid. Dry-running seals and gas-seals operate on

    different principles and shall be considered further on.

    Leakage

    All mechanical seals produce leakage.

    The reason lies in the previously discussed theory of lubrication ; it is obvious

    that a stable lubrication layer means a certain amount of leakage.

    Leakage can be calculated and depends on several factors as rotational

    speed, fluid pressure and characteristics, and balancing ratio. But the

    equipment on which the mechanical seal is installed can have some influence

    on it too. Often leakage is so reduced that it cannot be detected

    (vaporisation).

    Degree of freedom

    The elastic components of a mechanical seal (spring or bellow, gaskets) are

    of paramount importance for good performance.

    The gasket mounted on the seal r