About Mechanical Seal2

Apr 18, 2023 Flowserve Sanmar Limited

Mechanical Seals


Typical Centrifugal Pump

1. Suction 2. Impeller 3. Discharge 4. Stationary casing 5. Shaft6. Mechanical Seal 7. Gland 8. Bearings


Stuffing Box Pressure• The pressure acting on the stuffing box which

must be sealed.• It is a function of pump impeller design and the

presence or condition of wear rings.• Impeller design and Stuffing box pressure

– Back vanes - Open impeller• Suction + 25% of differential = Stuffing box pressure

– Balance Holes - Closed impeller• Suction + 10% of differential = Stuffing box pressure

– Double Suction• Suction pressure = stuffing box pressure


Viscosity – Generally the higher the viscosity, the better the lubricating

characteristics

– A product with a viscosity of 20,000 SSU (5000 cp) can be sealed with standard seals

– Beyond 5000 cp, the drive unit has to be beefed up to compensate for the increased shear occurring at the sealing faces.

– For high viscous applications:

• Single seal with external flush

• Double seal with external flush

– The viscosity & lubricity of water decrease with increased temperature. Above 82C (viscosity 0.35 cp) seal faces can contact each other and wear rapidly, unless specially designed


Specific Gravity – Specific Gravity = Density of a substance

Density of Water at 4C– Specific gravity is used to estimate the lubricity of

light hydrocarbons– Specific gravity below 0.68 (Heptane), require one

face as carbon and special seal face design with higher balance to contain fugitive emissions


Vapour Pressure – The pressure at which a liquid will flash into vapor at

given temperature– If seal generated heat causes the sealing liquid to boil

in seal chamber pressure, the liquid film between the seal faces will vaporize.

– Solution :• Increase seal chamber pressure with proper bypass flush

line and throat restriction device.

• Cooling the seal chamber. Seal chamber should be kept at least 15C below the boiling point of the product at the seal chamber pressure


Cavitation – Operated under cavitating conditions for sufficient length of

time:• Pitting of impeller vanes and pump volute

• Short bearing life and heavy loading

• Shaft breakage and other fatigue failures in the pump

• Short seal life

– Damages to mechanical seals can be :• worn pins and pin slots

• Broken springs

• shaft fretting

• Chipping of carbon faces


Cavitation – High speed video shows that seal face temperature excursions occurred when

gases were drawn across the seal faces and by the formation of gas bubbles in the seal chamber near the seal faces during pump cavitation.

Seal Performance during cavitation :

Zones Normal PumpOperation

Pump CavitationMode

Seal chambertemperature (C)

65 65

Shaft sleevevibration, mils p-p

1.9 2 to 8.4

Seal facevibration, mils p-p

<2.0 13 to 35

Seal facetemperature ( C )

67 to 74 166 to 427


Vibration – Pusher Seals :

• Vibration should be kept below 0.32 inches/second

• At levels beyond 0.32 inches/second and upto 0.60 inches/second seal damage and leakage will start.

– Bellows seals :• Axial vibration of the bellows seal end face

– Caused by out of perpendicularity

– Dampened by the hydraulic closing force

• Axial vibration of the bellows leaflets– Caused by the vibration of the

• Stick-slip torsional vibration

• Radial vibration of the bellows seal face end piece


Typical Stuffing Box with compression packings

ImpellerStuffing BoxPacking Pusher

Fasteners

Compression Packings


Disadvantages of Compression Packings

Power consumption is HIGH. The ratio of power consumptionof Mechanical Seal to compression packing is 1:8

Leakage is very high and unpredictable. Varies with compressionon the packings. The ratio of Leakage of mechanical seal to compression packing is 1:600No automatic wear compensation

Fretting on sleeve or shaft


Compression Packing Replaced with Mechanical Seal


Typical Unbalanced Seal

Compression Unit

Collar

Set screwSprings Drive Pin Gland

Insert Mounting

Insert

SealringShaft Packing

Compression ring

Hardwares: Collar, Compression ring, Gland, SleeveSeal Faces : Sealring & InsertSecondary Packings : Shaft packing, Insert Mounting, Sleeve &Gland Gaskets


5 Paths of LeakageGland Gasket

Shaft Packingand Sleeve

Shaft Packingand Sealring

Insert Mounting

Seal Faces


Materials : Hardwares

• 300 series Stainless Steel like 316ss, 316Lss

• 400 series Stainless Steel like 410ss

• Duplex Stainless Steel

• Alloy 20

• Hastalloy B & C

• Monel

• Inconel

• Nickel

• Titanium


Materials : Seal Faces

• Carbons :– # 5 Carbon (273C)– #17 Carbon (320 C)– Antimony Impregnated Carbon (204 C)– #18 Carbon (Salt Impregnated - SB 200 only)– Special Grades



• Tungsten Carbide :– TungCar 62-6 (Ni bonded) - 400 C– TungCar 62-1 (Cobalt bonded) - 400 C

• Silicon Carbide :– Silicon Carbide 1(Reaction bonded) - 427 C– Silicon Carbide 2 ( - Sintered) - 427 C



• Peramic :– Pure Aluminum Oxide - 177 C

• Weak in Thermal shock

• Good Chemical resistance

• Durchrome – Chromic Oxide layer on metal body - 177 C

• Glass Filled Duraflon– in TB seals

• Temperature limit of 71C as seal face


Shaft Packings

Viton - 204CNeoprene - 107CBuna N - 107CEPR /EPT - 177CKalrez - 315C

Pure Teflon - 177CGlass Filled Teflon - 232CTeflon Encap. “O” ring - 204C

Fully Interchangeable Shaft Packings


API Seal Code

About Mechanical Seal2

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