presentation on mech seal by prashant

MECHANICAL SEALSBy : Prashant Dhakate

Piping Batch :47

INTRODUCTION

• What is mechanical seal ?

Mechanical seal are leakage control devices, which are generally found on rotating equipments like pumps and mixers to prevent fluid escaping into the environment.

NEEED OF MECHANICAL SEAL : TO MINIMIZE LEAKAGE TO PREVENT TOXIC FLUIDS ESCAPING TO ATMOSPHERE TO REDUCE POWER LOSS

CONVENTIONAL SEALING : GLAND PACKING

• Before the advent of mechanical seals, pump users relied primarily on “rope” or braided style packing to achieve a “seal” around the shaft. A series of pieces or “rings” were installed into the pump “stuffing box” and they were compressed tightly so that they created a difficult leak path for the liquid to negotiate in order to leak to atmosphere.

SEALING THE LIQUID

• Early packing styles did not seal very well. In fact, until recently, braided packing styles required varying amounts of leakage for lubrication. If leakage was not permitted to occur, the packing would literally “burn up” and often cause severe damage to the pump shaft. Even with adequate leakage for lubrication, pump shaft wear was a commonly expected occurrence and as the shaft wore it would in turn, cause poor shaft packing life.

• As leakage becomes more excessive, the gland is tightened to reduce leakage.

MECHANICAL SEAL VS GLAND PACKING

Mechanical Seal Gland Packing

Leakage Extremely small or noneCertain leakage is necessary to avoid seizure

LifeLong with appropriate design and material

Comparatively short as it wears and periodical tightening is required

Wear of shaft and sleeveSpring load keeps faces in contact if faces wear

Periodical tightening required and must be replaced if wear of shaft or sleeve becomes excessive

Power LossSmall as frictions area and coefficient are small (Energy Saving Type)

Big as friction area and coefficient area are large

Operating Range (Pressure Temp.)

Applicable widely with appropriate materials and design

Limited application due to keeping leakage small

CostRunning cost is low as maintenance is not necessary

Running cost is high due to necessary maintenance

TYPES OF MECHANICAL SEAL

BASED ON ARRANGEMENT BASED ON DESIGN

SINGLE SEALS

• The most common seal configuration is to mount a single seal inside the pump cavity (Figure 1).

• Such a seal is chosen because it is reasonably leak-free. They are used frequently in pumps and machines in which lubrication or cooling by the product itself can be counted on.

• However, when abrasive particles from the liquid get between the seal faces, liquid from the pump under pressure will start to ooze, ultimately resulting in dripping.

• The single seal is also vulnerable to damage caused by cavitation, which, in turn, causes excessive vibration. The seal surfaces may suffer chemical attack and become roughened, which speeds up the abrasion process and shortens the life of the seal so that leakage will occur prematurely. And, finally, with the seal inside the pump chamber, seal components (often metal) are subject to attack from the pumped fluid

Fig. 1

SINGLE SEAL

INSIDE MOUNTED• SEAL INSIDE THE SEAL

CHAMBER• Inside mounted = pressure

on outside diameter of parts

OUTSIDE MOUNTED• SEAL OUTSIDE THE SEAL

CHAMBER• Outside mounted = pressure

on inside diameter of parts• Outside-mounted seals are

considered to be used for low-pressure applications since both seal faces, the primary ring and mating ring, are put in tension. This limits the pressure capability of the seal.

SINGLE SEALS

INSIDE MOUNTED SINGLE SEAL OUTSIDE MOUNTED SINGLE SEAL

DUAL SEALS

• Dual seals can be either pressurized or non-pressurized. This is in reference to the artificial environment that is provided to exist “between” the seals.

• A non-pressurized dual seal, also known as a “Tandem” arrangement, means that the inner, or primary seal is functioning as would a single seal. It is subject to stuffing box conditions, i.e. stuffing box pressure, process liquid to lubricate the faces and usually immersion of seal components in the process liquid. The secondary, or outside seal runs in a non-pressurized “Buffer” liquid that is supplied from an outside source, typically a nearby supply tank.

• In a non-pressurized dual arrangement, the outside seal is primarily there as a containment device in the event that the inside or primary seal is lost. A “Back up” or safety mechanism.

Inside or Primary seal

Outside or Secondary Seal

Immersed in process liquid in the stuffing box

Buffer fluid warmedby seal generatedheat returns to thebuffer supply tank

Cool buffer fluid from the buffer supply tank entersvia the inlet port

DUAL SEALS UNPRESSURISED

DUAL SEALS UNPRESSURISED

• Since the outside or secondary seal runs in a non-pressurized clean lubricating liquid, it will generally last for an extended period of time. When the inside or primary seal fails, the leakage through the faces will be contained by the secondary seal until the pump can be shut down for seal replacement.

• Failure indication and shutdown devices can be attached to the buffer supply so that the pump operators know when the primary seal has failed.

DUAL SEALS PRESSURISED• When pumping volatile liquids, hazardous, corrosive, abrasive, etc. it is sometimes

necessary to insure that the process liquid does not enter the atmosphere or the artificial environment created for the seal or even the seal faces.

• Pressurizing the artificial environment, 20 to 30 psi. above the pump stuffing box pressure will prevent process liquid from crossing the primary seal faces. Instead, boundary layer film liquid is supplied to the primary seal by the artificial environment or “Barrier”.

• External fluid lubricates both sets of faces.

• Leakage to the atmosphere is external fluid.

• In a pressurized dual seal, the outboard or secondary has the tougher job of the two. It operates sealing high barrier pressure while the inboard or primary seal has clean lubricating liquid applied at differential pressure of only 20 to 30 psi.

• Now let’s look at the environmental controls for operating dual seals.

Pressurized Dual Seal

Artificial Environment“Barrier” System

Non-Pressurized Dual Seal

Artificial Environment“Buffer” System

PUSHER TYPE

• The basic difference between pusher and non-pusher types have to do with the dynamics of the shaft packing or O-ring and whether or not it moves as the seal wears.

• As the seal faces wear down over time, they must be closed to compensate for lost face material. If the shaft O-ring must move when this compensation takes place, it is pushed forward by the components of the seal and by stuffing box pressure. If the seal is configured with a “dynamic” O-ring of this type the seal is called a pusher type.

Illustrated here is a common pusher seal. As the seal springs and other pressures in the stuffing box are exerted on the seal, closure of the faces is achieved.

Rotating face and dynamic O-ring.

Hard Stationary Face

Closing forces exerted on the seal faces

As the softer carbon face wears down, the rotating face must move to maintain face closure.

Minute particles of carbon and solids from the process liquidthat migrate across the seal faces build up on the shaft.

This build up will ultimately cause the seal to “hang up” and in most cases, failure will occur well before the seal is actually “worn out”.

NON-PUSHER

• There are seal types that have no dynamic O-rings. All O-rings are “static” and the seal components compensate for face wear without “pushing” any sealing points.

• One of these types is called a “Bellows Seal”. A disadvantage of this style seal is that its thin bellows cross sections must be upgraded for use in corrosive environments

• Metal bellows are constructed by welding “leaflets” into a series of “convolutions”. This series of convolutions is referred to as the “Bellows Core”.

• SEAL HANGUP due to clogging of spring or dynamic elastomeric materials which is a matter of concern in PUSHER TYPE SEAL is avoided in non pusher type

Metal bellows

Carbon rotating face

Hard stationary face

The bellows core expands to compensate for face wear.

BALANCED & UNBALANCED SEAL• When speaking of “Balance” in reference to mechanical seals,

we are not talking about Mechanical or Rotational Balance. Instead, we are referring to Hydraulic Balance.

• Since mechanical seals are subject to stuffing box pressure, this pressure is utilized to achieve and maintain seal face closure in a non-balanced seal.

• If stuffing box pressure is very high, typically over 100psi., then the closing force may be too great to allow the “Boundary Layer Liquid” that lubricates the faces to be sufficient and the faces will wear prematurely.

• A balanced seal compensates for higher pressures by locating the seal faces such that stuffing box pressure has less effect on face closure.

SEAL BALANCE

To reduce the axial face contact force which allows to seal high pressures, i.e. up to 3000 psig with one set of faces.It is the ratio (k) of 2 geometric areas: the closing (Ah) and opening area (Ac)K= Ah / AcFor unbalanced seals k = 1For balanced seals k = 1

BALANCED AND UNBALANCED SEAL

UNBALANCED sealAh>Ac BALANCED SEAL

Ah < Ac

BALANCED & UNBALANCED SEALS

BALANCED SEAL

• Reduced closing forces• Reduced power

consumption• For pressure up to 3000 psig• Always recommended for

volatile liquids

UNBALANCED SEAL

• High closing forces• Low leakage• For pressure up to 200 psig• Not recommended for

volatile liquids

COMPONENTS OF MECHANICAL SEAL

1. Stationary seal face(Rigidly mounted)

2. Rotating seal face (Flexibly mounted)

3. O-rings, gaskets (Secondary seals)

4. Springs/ retainers (Compression devices)

MATERIAL OF CONSTRUCTION –mechanical seal components

1. Primary seal facesa. Ceramicb. Carbon • Resin impregnated-corrosive application• Antimony impregnated – non corrosive applicationc. Tungsten carbide• Nickel binder• Cobalt binderd. Silicon carbide

MATERIAL OF CONSTRUCTION –mechanical seal components

2. Secondary seals• Elastomeric material like nitrile, butyl,

neoprene• Non elastomeric materials like TEFLON, PTFE

3. SpringsSS304, SS316, Haste alloy

SEALING FACES• The mechanical seal faces are obviously the most critical

sealing point of a mechanical seal assembly.

• In order for a “seal” to be achieved, the faces must be very flat and is to be laid perpendicular to the shaft. This is achieved by machining the faces, then “lapping” them to a fine finish.

• Flatness is measured in “Light Bands”. After lapping, the faces are placed on an “Optical Flat”, a clear glass surface where a monochromatic light is shined on the face. This single wavelength light will produce an image of rings or lines on the face. Each ring/line is “One Light Band”. Each light band is equivalent to .000011” or eleven millionths of an inch. This refers to the variations in the surface of the face.

SEALING THE LIQUID

• Like packing, the mechanical seal “faces” must also be lubricated. With proper application and design however, the leakage is so minute that actual droplets of liquid are not detected. Instead, the lubricating liquid will vaporize as it crosses the seal faces and the leakage is a gas or vapor.

Leakage point 2:Sealing on the shaft Leakage point 3 :O.D. of the stationary

Leakage point 4: Betweenseal gland and the The stuffing box

Leakage point 1: the seal faces

LEAKAGE POINTS IN MECHANICAL SEAL

Leakage points in mechanical seal

• Leakage point 1: Between the sealing faces. Leakage stopped by maintaining a thin film of fluid between the rotating and stationary seal faces

• Leakage point 2: Between the shaft and rotationary seal face. Leakage stopped by secondary seals like O-ring.

• Leakage point 3:Between the stationary seal face and gland. Leakage stopped by secondary seals like O-rings, gasket etc

• Leakage point 4: Between the stuffing box and seal gland. Leakage stopped by secondary seals like gaskets

FACTORS GOVERNING SELECTION OF MECHANICAL SEAL

Liquid: Identification of the exact liquid to be handled is the first step in seal selection. The metal parts must be corrosion resistant, usually steel, bronze, stainless steel, or HasteAlloy. The mating faces must also resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered. Stationary sealing members of Buna, EPR, Viton and Teflon are common.

Pressure: The proper type of seal, balanced or unbalanced, is based on the pressure on the seal and on the seal size.

Temperature: In part, determines the use of the sealing members. Materials must be selected to handle liquid temperature.

Characteristics of Liquid: Abrasive liquids create excessive wear and short seal life. Double seals or clear liquid flushing from an external source allow the use of mechanical seals on these difficult liquids. On light hydrocarbons balanced seals are often used for longer seal life even though pressures are low.

Reliability and Emission Concerns: The seal type and arrangement selected must meet the desired reliability and emission standards for the pump application.

STANDARD FOR MECHANICAL SEAL

• ISO 21049 / API 682 – PUMPS SHAFT SEALING SYSTEMS FOR CENTRIFUGAL AND ROTARY PUMPS

Category 1For non - ISO 13709 Seal Chambers

Temperatures from -40 °C (-40 °F) to 260 °C (500 °F)

Absolute Pressures up to 22 bar (315 psi )

ISO 21049 / API 682 – Seal categories

Category 2For ISO 13709 Seal Chambers (Table 1 only)


Absolute Pressures up to 42 bar (615 psi)


Category 3Must meet full specification (incl. documentation)

For ISO 13709 Seal Chambers (Table 1 only)


Absolute Pressures up to 42 bar (615 psi)


Type A

• Balanced

• internally - mounted-

• cartridge

• pusher

• multiple springs

• flexible element rotating

• secondary sealing by elastomer o-rings

ISO 21049 / API 682 – Seal types

Type B

• Balanced

• internally- mounted

• cartridge

• non - pusher ( metal bellows )

• Flexible element rotating

• secondary sealing by elastomer O - RINGS


Type C

• Balanced

• internally - mounted

• cartridge

• non - pusher ( metal bellows)

• flexible element stationary

• secondary sealing by flexible graphite


ISO 21049 / API 682 covers 3 Arrangements:

Arrangement 1: one seal per cartridge assembly

Arrangement 2: two seals per cartridge assemblywith a containment seal chamber at pressure lessthan the seal chamber pressure

Arrangement 3: two seals per cartridge assemblywith externally supplied barrier fluid

ISO 21049 / API 682 – Seal arrangements

ISO 21049 / API 682 covers 3 DESIGNS: :

• Contacting Wet Seals (CW)

• Non -contacting Seals (wet or dry, NC)

• Containment Seals

( contacting or non-contacting, CS)

ISO 21049 / API 682– Sealing methods

ISO 21049 / API 682 covers 3 orientations for

Arrangements 2 & 3:

• Face-to- Back

(arr.2 default selection & arr. 3 wet seals)

• Back-to- Back

(arr. 3 default for gas seals; option for wet seals)

• Face-to- Face

(arr.3 optional selection for wet or gas seals)

ISO 21049 / API 682– Sealing methods

FLUSHING FOR MECHANICAL SEAL

WHY IS FLUSHING REQUIRED IN MECHANICAL SEALS?

• In mechanical seal heat is generated between the rotating and stationary seal face. If more amount of heat is generated between the faces cause the mechanical seal failure. In order to avoid the seal failure flushing fluid is injected between the seal face.

FLUSHING PLANS

There are different types of flushing plans used depend upon the conditions. These are following condition for selecting suitable mechanical seal flushing plan for both the supplier and vendor.

• Nature of working fluid (pumping fluid)such as toxic, corrosiveness, hazards chemicals.

• Suspended solid particles in the pumping fluid.• Temperature of pumping fluid• Vapour pressure of pumping fluid

API FLUSHING PLANS• PLAN 01 AND 02 : Internal circulation for single

seals• PLAN 11,12,13 AND 14 : Simple recirculation

system for single seals• PLAN 21,23,31 AND 41 : Recirculation system with

auxiliary equipment for single seals• PLAN 32 AND 62 : External injection systems• PLAN 52,53,54 AND 74 : External system for dual

seals• PLAN 72,75 AND 76 : External control system for

containment seals

CODING OF MECHANICAL SEAL AS PER standards

• API 682 (3rd edition) specifies different Sealing Systems using as parameters: MECHANICAL SEAL CODE are specified as per API 682 in the form of a code in the following order

1. Seal category-2. Seal arrangement type3. Seal type4. Flushing plans

For Example: a Mechanical Seal designated as: C2 A1 A 3161 is a Mechanical Seal according to

Category 2Arrangement 1 Seal type A31,61: up to 41 bar; 400°Csingle seal/cartridgebalanced, inboard, pusher type sealAPI plan 31 and 61

CODING OF MECHANICAL SEAL AS PER standards

• API 610(8th edition) is still being used for the "API Plans" (refer below) as well as the API Code (ref. below)

API Code: 1st letter B = balanced

U = unbalanced 2nd letter S = single-acting

T = double-acting (tandem/ unpressurized)D = double-acting (pressurized with barrier pressure)

3rd letter type of coverP = plain, without throttle bushT = throttle bush with quench, leakage and / or drainage connectionA = additional/ auxiliary seal (has to be stated)

4th letter secondary seal materials F-FKM5th letter face materials

Example for sealing and material code:BSTFO = balanced, single-acting mechanical seal with throttle bush, with dynamic and static secondary seals (O-rings) made of FKM and seal faces/ secondary seats made of tungsten carbide against silicon carbide

THANK YOU

presentation on mech seal by prashant

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