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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