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    Back To Basics: Mechanical Seal

     

    INTRODUCTION

    A mechanical seal is a device used to prevent fluid leakage through the rotating shaft. Rotating shaft equipment may be a pump, compressor, agitator vessel, and agitator reactor. Predetermined compression is applied to the seal mating component. A small leakage is allowed in the seal chamber that forms a thin film of fluid between the seal ring and the mating ring. If this barrier fluid film breaks then seal leakage begins. Mechanical seals are used to prevent leakage of fluids, that are corrosive, precious, and hazardous to the atmosphere. The Mechanical seal has the following components:

    (1)Seal ring

    (2)Mating ring

    (3)Secondary seal or wedge

    (4)Spring

    TYPE OF SEALS

    (1)Inside Seal

    (2)Outside Seal

    (3)Single Seal

    (4)Double Seal

    (5)Tandem Seal

    INSIDE MECHANICAL SEAL

    The seal mounted inside the stuffing box of the pump is called inside the seal. Installation and maintenance of the seal can only be done by complete disassembly of the pump.

    single mechanical, seal parts are indicated
    Single Mechanical Seal, Seal parts are indicated.

    OUTSIDE MECHANICAL SEAL

    Seal mounted outside the stuffing box is called outside seal. This seal can be installed easily. It can be removed quickly for cleaning. These seals are used when stuffing box depth is small.

    Mechanical Seal Mounted outside Stuffing Box

    SINGLE MECHANICAL SEAL

    A single mechanical seal can be mounted inside or outside the stuffing box. A rotary head having the seal ring is mounted on the pump shaft against the mating ring that is placed in the stuffing box gland.

    DOUBLE MECHANICAL SEAL

    In double seal, two seals are placed in the stuffing box in two arrangements. (1) Seals face each other. (2) Seals away from each other.

     

    Double Mechanical Seal Mounted away from each other.

    TANDEM MECHANICAL SEAL

    In this type of seal arrangement two inside seals are placed in the stuffing box, one after another. As shown in fig.

    INSTALLATION OF MECHANICAL SEAL

    Trouble-free running of the mechanical seal can be achieved, by proper installation of the seal. For proper installation of seal the following checks are to be done:

    EQUIPMENT ALIGNMENT

    (1)Radial movement of the shaft to be checked for(deflection and run out). This can be checked with a dial gauge indicator while rotating the shaft by hand. When the dial gauge indicator reads .003inch(.08 mm) at seal location, this condition to be corrected. This misalignment may be due to playing in the bearing or bending in the shaft.

    (2)Axial movement of the shaft to be checked. This is done by the dial gauge indicator. If the dial gauge indicator reading is .010inch(.26 mm), this should be corrected. This could be due to excessive play in the thrust bearing. Bearing to be replaced for correction.

    (3)Stuffing box face run out to be checked regarding shaft by dial gauge indicator. If the reading of the dial gauge is more than .002inch(0.5) that of the shaft size. This condition to be corrected by machining the stuffing box face.

    (4)Stuffing box bore concentric to be checked with the dial gauge indicator. Stuffing box concentric should be within .005inch(.13 mm). If the dial gauge indicator reading is more than (.13 mm), the stuffing box bore concentric to corrected by machining the bore size.

    EQUIPMENT CONDITION CHECK UP

    (1)Stuffing box must be cleaned from any foreign matter to avoid damage to the seal.

    (2)All scratches or burrs to be removed from the stuffing box cavity to seal them perfectly by ‘O’ ring or gasket.

    (3)Any hole or crack in the stuffing box to be corrected by plugging.

    (4)All the gasket seating surfaces must be well polished.

    (5)Shaft must be well polished with sandpaper and all sharp edges or burrs must be removed.

    (6)Shaft sleeve also must be free from sharp edges and burrs.

    (7)Shaft must be smoothly polished in the seal seating area.

    MECHANICAL SEAL OPERATION RELIABILITY

    Mechanical seal operation reliability is dependent on the conditions imposed on the seal assembly during the running of the seal. The fluid film is formed between the faces of the seal to be maintained all the time to get a reliable service from the seal. Any condition that breaks the fluid film will lead to the dry running of the seal and causes immediate seal failure. To keep the fluid film intact between the seal running faces following methods are used to maintain the condition favorable for seal running

    (1) FLUSHING

    A fluid is injected into the stuffing box to spray or jet on the seal faces. The fluid injected is the same fluid that is being sealed. Injecting fluid pressure must be higher than the pressure in the stuffing box. Injected fluid mixing with the fluid to be sealed is permitted. Seal flushing provides the cleaning of the seal faces and cooling of the mating faces.

    (2) QUENCHING

    A fluid is introduced on the outside of the seal mating faces. This provides the cooling of the mating faces. It controls the temperature at the mating faces of the seal to maintain the thin film stability. In sub-zero temperature application quenching media is used as steam to prevent fluid solidification and crystallization due to a drop in temperature.

    Factors affecting the seal performance:

    (1)Lubrication of seal faces is achieved by proper flushing plan.

    (2)Circulation of sealing fluid in the seal chamber removes heat generated by mating faces.

    (3)Solids/Slurry in the circulating fluid entrap between the seal mating faces cause premature seal failure.

    (4) The Property of fluid that vaporizes, solidifies, or crystallizes on account of change in temperature affects seal performance if a proper API plan is not implemented. Some seal flushing plans are shown below:

     

    Mechanical Seal Flushing Plan 11
    Mechanical Seal Flushing Plan 11

    Mechanical Seal Flushing Plan 14
    Mechanical Seal Flushing Plan 14

    INFORMATION REQUIRED FOR SEAL PROCUREMENT

    (1) STUFFING BOX DETAILS:

    > Sleeve OD (Outside diameter)

    > Shaft OD

    > Stuffing Box Bore

    > Stuffing Box Depth

    > Bolt Circle diameter

    > Collar diameter

    > Sleeve extension

    > Nearest Obstruction

    > Raised Collar

    > Stuffing box cover(Jacketed/ Nonjacketed)

    (2) PUMP DATA:

    >Pump make
    >Type

    >Model

    >MOC

    >Existing Seal

    (3) OPERATING PARAMETERS:

    > Suction Pressure

    >Discharge Pressure

    >Total Head in meters

    >Speed in RPM

    >Direction of rotation(CW/ACW)

    (4) FLUID TO BE HANDLED:

    >Fluid

    >Pumping Temp.

    >Max.Temp.

    >Specific GR.at PT.

    >Viscosity at PT.

    >Boiling point

    >Freezing point

    >Fluid description(clean,dirty,abrasive,slurry,toxic)

    >% of solids

    >Particle size

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