Mech.seal Hpcl

TOPICS TO BE COVERED

Why at all a Mechanical Seal?How does a Mechanical Seal work?

Types / Classification of seals.Materials of Construction.Environmental conditions.

Equipment condition.Trouble Shooting & Failure Analysis.

CENTRIFUGAL PUMPS – HORIZANTAL

CETRIFUGAL PUMPS – VERTICAL

AXIAL FLOW PUMPS

MIXED FLOW PUMPS. / JET PUMPS.

POSITIVE DISPLACEMENT PUMPS.

RECIPROCATING PUMPS / PISTON PUMPS

DIAGHPRAM PUMPS

SCREW PUMPS – SINGLE / DOUBLE / TRIPLE. / VANE

VACUUM PUMPS

SUBMERSIBLE PUMPS

TYPICAL END SECTION PUMP

Front End view of Pump.

Typical Stuffing box with Compression Packing

Impeller Stuffing Box

Compression Packing

(Gland Packing)Packing Pusher

(Gland)

Fastner

DISADVANTAGE OF PACKING.

Power Consumption is high. The Ratio of Power Consumption of Mechanical Seal to

Compression is 1: 8

Leakage is very high and unpredictable. Varies with the compression on the packing. The Ratio of Leakage of

Mechanical seal to compression Packing is 1 : 600

No automatic wear compensation.Fretting on Sleeve or Shaft.

Typical Mechanical Seal & Leakage Path.

Rotary Stationary

1. Gland to Pump face- Sealed with a Gasket. 2.Over the top

of the seal-’O’Ring

3. Along the Shaft

4. Under the Seal Ring 5. BETWEEN THE SEAL FACES

Stuffing Box PressureThe 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

Cavitations 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

Damages to mechanical seals can be :

•Short seal life•Worn pins and pin slots

•Broken springs•Shaft fretting

•Chipping of carbon faces

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 Pump

OperationPump Cavitation

Mode

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

Nomenclature of a Typical Seal

CollarSet Screw

Spring Pins

SpringsDrive Pins

Compression Ring

Compression Unit

Gland Ring

Insert Mounting

Insert

Seal Ring

Shaft Packing Gland Gasket

TYPES OF SEALSBased on mounting Outside seal

(Single sealInside seal

Based on arrangement

Single seal Double seal Tandem seal

Based on design

Balanced Unbalanced Special seal

Based on constructionPusher type

(spring design)

Stationary bellow

Bellow design

Metal bellowTeflon bellowElostamar

bellowSingle coil spring

Rotary bellow

Multispring

Dry running sealHigh pr seal

Gas seal

Custom built sealSlurry seal

Single Out Side Seal

EFFECT OF PRESSURE ON SEAL

BALANCED SEAL DESIGN FOR HIGH PR.

TANDEM SEAL

METAL BELLOW SEAL

MATERIALS OF CONSTRUCTION

300 Series Stainless Steel like SS 316, SS 316L

400 Series Stainless Steel like SS 410

Duplex Stainless Steel

Alloy – 20

Hastalloy B & C

Monel

Inconel

Nickel

Titanium

Seal Hardware Materials.

SEAL FACE MATERIALS

Other General Seal Face Materials

StelliteNi-ResistBronze

Secondary Packing Materials

Pure Graphite -400 C

Environmental Control

• Environmental control is a key factor for the proper functioning of Mechanical seal.

• (I.e) flushing / quenching / Cooling or heating / Jacketing / etc.,

API Plan – 02 (Jacket)

Flushing of Seal.

API Plan – 12

API Plan – 13 API Plan – 13 Re-circulation from Seal Chamber to SuctionRe-circulation from Seal Chamber to Suction

API Plan – 21 Bypass Flush through Heat ExchangerBypass Flush through Heat Exchanger

Close

Clearance

Bushing

Orifice (when specified)Heat Exchanger

API Plan 23 -- Heat ExchangerRe-circulation through Heat ExchangerRe-circulation through Heat Exchanger

Single Inside SealSingle Inside Sealwith Pumping Ringwith Pumping Ring

Pumping Ring

Circulating Device

API Plan - 31

API Plan – 32 External FlushExternal FlushSingle Inside SealSingle Inside Seal

Close

Clearance

Throat

Restriction /

LIP SEAL

Flow

MeterPressure Gauge

External

Flushing

Source

LIP Seal Assembly

API Plan - 51Vent Connection

API Plan - 52

API Plan - 53

API Plan – 54

API Plan - 61

API Plan - 62Flush and QuenchFlush and Quench

EQUIPMENT CONDITION

Shaft Whip -- Best limit 0.004” Check Condition of Radial Bearing

New Shaft Thrust bearing location.

Best Limit 0.004”

Care has to be taken while removing front casing.Max. 0.003”

Care has to be taken while removing the sleeve

Max 0.004”

Causes of Seal Failure

Failure Analysis

Failure due to Chemical Attack

Fretting Corrosion

Failure Due to Mechanical Action

High Spots

Seal Face Deflection

‘O’-Ring Extrusion

Anti Extrusion Ring.(with GFT)

Erosion

Drive Pin Wear

Failure Due to Heat Build up

Heat Checking

Vaporization.

BLISTERING

SPALLING

Oxidation & Coking

2PM 41D

11PM 6A

11P 10A

11PM 3B

11PT 2B

12PM 1B

16PM 4A

16PM 4A

42PM 17A

42PM 17A

12PM 1B

16PM 4A

CASE STUDYUnit: MEROXPump Item No. 16 P 04 A Amine Sump Pump.Pump make KSB – WKT 50 / 2Fluid handled: Acid Condensate / H2S / NH3& solvents.Operating Temp. 70o CMax. Discharge Pr. 8.0 Kg/ cm2Seal Size & Type 1.750” RA Outside sealSeal Faces Tung.Car / CarbonPacking Viton.Flushing Plan 11 & 13Due to urgent need and since there was no pump was available for the service, HPCL had a spare Hot Well pump, which met the operating parameters was installed.After few days of running the seal has failed. The pump was opened and the internals were checked. The main attack was to the secondary packing. Due to damage of packing the Carbon face also damaged.Remedial action was taken by modifying the seal and the packing were revised to Kalrez. The revised seal was installed about a week back, and the performance is being monitored.

MECHANICAL SEAL CHECKS

Check seal flushing, steam purging & stuffing box cooling for high temperature services.

Ensure flushing oil flow in the seal flushing line by sensing the temperature of the line. It should have the same temperature as that of the pump casing (incase no cooler is provided).

Ensure flushing line is insulated for hot services.

Incase seal is smoking, check for proper cooling water circulation to stuffing box, flushing flow is adequate.

Ensure LP dry steam to seal chamber as quench steam. Flow to be adjusted so that steam does not impinge on bearing housing

Contd

Ensure proper insulation to quench steam lines.

Incase of seal puffing, check for condensate in purging steam, contamination in product, inadequate flushing at seal, periodic vapor locking of pump. If no problem detected, inform rotating section for further check.

INTERACTIVE SESSION.

Use of Mech. Seal or Packing.

Can we convert packing to seal vis-a vis.

Carbon face to be Rotary or Stationery

Selection of seal type.