• This presentation is made by

• Mohamed El-keik on 17/10/2016

• Email: mohamedelkeik@gmail.com

• Pardon me if there are any wrong information .

• as I am a chemical engineer, and Seals are not my specialty and I had no workexperience at the time it was made :D .

• but I simply made it as a quick reminder for me, from the couple of references (andother people presentations, credit to them and thank you for sharing your work ) I readabout mechanical seals in hope it will be useful one day.

• And I decided to share it hopping that it may benefit someone else .

• Also it was a part of a presentation on pumps so it will focus on pump sealing .

Sealing systems for rotating shafts

INTRODUCTION

• All rotating shafts require some form of a seal allowing motion of the shaft from some external device (motor) while sealing process fluid so no leaks occur between the shaft and the body .

• The general term for this mechanism is sealing.

But How to allow a moving shaft to pass through what needs to be an impenetrable barrier to some fluid ?

• we simply can make the clearance between the shaft and the containing body (housing) very small which will reduce leakage but also as the shaft rotates high friction between metals will lead to wearing of the shaft and body very quickly leading to replacing these expensive parts so that’s will not be a practical solution .

• A more traditional solution ( mechanical packing ) will be to wrap the shaft in a flexible material that maintains a close fit to the shaft without binding its motion. Thus eliminating the metal to metal contact and seal the annular space between the shaft and the body .

• the traditional packing material for a ship propeller shafts is flax. Some form of lubrication is usually provided so this packing material does not impose excessive friction on the shaft’s motion .

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• In the case of a ship’s stuffing box, a little bit of water leakage is not a problem since all ships are equipped

with pumps to pump out collected water over time. However, leakage is simply unacceptable in many

industrial applications where we must minimize Fugitive emissions ( any unwanted escape of process

substance into the surrounding environment ) usually from leaks around pump and valve shafts.

• So another form of sealing is needed to meet more strict process requirements ( mechanical seal ) .

• But in an overall view we can see that all seals have the same functions :

1. Minimize or prevent* leakage according to application requirements.

2. Minimize power loss results from friction.

3. Protect the expensive parts from wearing like the shaft and the housing.

• The previous functions are translated into benefits to the process

1. Economical : wear reduction / sometimes the sealed fluid can be expensive and thus losing it will

lead to higher operational cost .

2. Environmental : as regulations now put more strict emission limits .

3. And finally safety & health : where the sealed fluid can be toxic , hazardous , flammable or

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I. MECHANICAL PACKING (GLAND PACKING)

• Gland packing is the traditional solution to seal against fluids and yet it is very effective and desirable for many applications where small leakage does not posses problems .

• A series of pieces or “rings” are installed around the shaft into the stuffing box and they are compressed tightly by the packing gland so that they create a difficult leak path for the liquid to leak with out binding the shaft motion .

• Some of The packing materials used are Teflon (PTFE) & graphite .

• Mechanical packing components :

1. Stuffing box

2. Packing rings

3. Lantern ring

4. Packing gland

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• the stuffing box houses the mechanical packing components , where packing rings are placed and a gland (end plate) provides the compressive force by tightening the gland plate bolts for squeezing and pressing them down the shaft.

• The narrow passage, between the shaft and the packing housed in the stuffing box, provides a restrictive path to the liquid which causes a pressure drop and thus prevents leakage.

• When packing is dry it can damage the shaft thus It is good practice to tighten the gland just enough to allow for a minimal leak through the packing. This slight leakage of the liquid acts as a lubricant as well as a coolant to absorb the heat generated in the packing due to friction with the shaft .

• Periodic retightening of gland necessary for wear compensation.• Lantern ring can be provided for cooling /lubrication . • The minimal leak through the packing ( which is used as a lubricant ) cannot be allowed for hazardous and

toxic liquids, but then gland packings are also not used in such applications.• When toxic or corrosive liquids are handled, it is necessary to insure complete sealing of the stuffing box.

Leakage of such liquids is a hazard to the plant personnel and can also be detrimental to the outer surface of the pump and foundation. It can also result in the loss of a valuable product.

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Packing rings

Lantern ring

Packing gland

Gland bolts

Shaft

Stuffing box

Leakage path Retainer

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Retighten gland bolts to reduce leakage that results from wear of packing rings with time

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Stuffing box assembly

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The stuffing box is a chamber or a housing that serves to seal the shaft where it passes through the pump casing

lantern rings are rings with holes drilled along its circumference. Used in high pressure services

The gland is used to keep packing rings squeezed and pressing them down the shaft.

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Packing with lantern ring Normal Packingv01MME-K

The lantern ring

• When pumps are handling dirty or high-pressure liquid, lantern rings are used. These are rings with holes drilled along its circumference and has torturous path making it difficult for the pumped fluid to escape .

• Function : cooling , lubrication and provides extra sealing .

• Extra sealing :By supplying ‘back pressure’, which aids in impeding the entrance of abrasive and corrosive material into the stuffing box. Abrasives and corrosives will damage the shaft or sleeve, and disintegrate the packing.

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Lubrication & cooling• Water which is directed from

the discharge side of the pump and ported to the center of the lantern ring where it enters into the shaft through the lantern ring holes and provides cooling and lubrication .

• In dirty services water can be pumped into the ring from other sources .

• Oil, water, grease, or any liquid or substance compatible with the fluid are forced under pressure into the packing through the lantern ring .

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Mechanical packing

• Advantages • Disadvantages

• Inexpensive • Adjustable in operation• Maintainable• Wide range of materials for multiple applications

• Requires lubrication • Constant monitoring & adjustment • In-precise adjustment leads to wear on shaft .• Higher power consumption due to friction • Product loss .

• Gland packings work on principle of controlled leakage for proper life ( has to leak to perform ) .

• The product loss resulting from this leakage can be quantified as follows,

Leakage rate quantum

One drop every five seconds 550 ltrs/year

Two drops per seconds 5500 ltrs/year

Steady stream leakage 40000 ltrs/year

• Product loss = above quantities * cost/lt of liquor

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II. MECHANICAL SEALS • Mechanical seals are simply another mean of controlling leakage where other means (packing) are deemed

to be less capable of performing the task adequately .

• Advantages of mechanical seals over mechanical packing :

1. Lower mechanical losses

• One of the largest of these mechanical losses comes from the frictional drag of the shaft running against packing. A mechanical seal, on the other hand, has considerably less mechanical loss, thus improving efficiency.

2. Less Sleeve Wear

• the sleeve under the packing is subject to wear, especially if the pumped liquid has any abrasives in it or if the packing is tightened too much. With a mechanical seal, this wear of the shaft sleeve is eliminated.

3. Reduced Maintenance .

4. Zero or limited leakage of product meeting process requirements .

5. Ability to seal higher pressures and more corrosive environments.

6. The wide variety of designs allows use of mechanical seals in almost all applications.

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HOW IT WORKS ?• Mechanical seals are simply a sealing device depends on the idea of transforming the leakage path from

being parallel to the shaft to perpendicular to it and hence shaft will not be subjected to wear and small running clearance between two sealing faces can be achieved .

Leakage path parallel to the shaft Leakage path perpendicular to the shaft

• To do so mechanical seals forms a running seal in a plane perpendicular to the shaft consist of two highly polished surfaces running adjacently.

• One surface being connected to the shaft and the other to the stationary portion of the housing. The polished surfaces which are of dissimilar materials are held in continual contact by a spring, forming a fluid tight seal between the rotating and stationary members with very small frictional losses .

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• The fluid try to exist from the high pressure red region ( inside the process ) to the low pressure light blue region ( atmosphere ) to do so the fluid losses energy in form of pressure loss to overcome the restriction ( tiny fluid path ) imposed by the seal and escape . So it’s pressure decreases as it moves down the sealing faces , and it also gains energy ( heat ) results from friction between seal faces until it reaches a point where it is vaporized (the vaporization point in a perfectly designed seal is at the outer tip of the seal faces ) and the leakage is so minute that actual droplets of liquid are not detected. Instead, the leakage is a gas or vapor.

• The fluid ( leakage ) running between sealing faces acts as a lubricant

But What happens to the fluid in that perpendicular leakage path between those rotating sealing faces ?

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Components of a mechanical seal .

• All mechanical seals are constructed of three basic sets of parts.

2. A set of secondary seals known as shaft pickings and insert mountings such as 0-rings, and V-rings.

1. A set of primary seal faces: one rotary and one stationary.

3. Mechanical seal hardware for attaching, positioning and maintaining face to face contact including : gland rings, collars, compression rings, pins, springs and

bellows.

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Shaft

Seal follower

Stuffing box

Retainer Spring Static seal Dynamic seal

Liquid

Bolt for locking

Gasket

Secondary seal

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C. Between the rotating element and the shaft Sealing on the shaft (secondary seal ) B. Between the stationary

element & the seal chamber (secondary seal )

D. The seal gland to the stuffing box

A. Between the seal faces ( primary seal )

• The previous components serve the function to seal leakage across four points :

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A. Between the faces (rotary and stationary) of the seal ( primary seal ).• The primary seal is achieved by two extremely flat, lapped (lapping is a polishing process) faces which create

a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage.

• one face is held stationary in housing and the other face is fixed to, and rotates with, the shaft. and held in contact using a combination of hydraulic force from the sealed fluid and spring force from the seal design. the spring pressure holds the primary and mating rings together during shutdown or when there is a lack of liquid pressure .

• The mating surfaces of the seal faces are made of dissimilar materials One of the faces is usually a non-galling material such as carbon-graphite. The other is usually a relatively hard material like silicon-carbide. Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent adhesion of the two faces. The softer face usually has the smaller mating surface and is commonly called the wear nose.

• The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces.

• Flatness of the faces determines the quality of the seal & is measured in “Light Bands” (refers to the variations in the surface of the face & equivalent to 0.000011 inch ).

Wear nose

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B. Between the stationary element and the seal chamber housing ( secondary seal ).

C. Between the rotary element and the shaft ( secondary seal ) .

D. Between the seal gland and stuffing box .

• The secondary seal in this locations achieves sealing by 0-ring, V-ring or gaskets

V ring O ring

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Mechanical seal lubrication .

• IF the seal faces were rotated against each other without some form of lubrication they would wear and quickly fail due to face friction and heat generation. For this reason some form of lubrication ( fluid film ) is required between the rotary and stationary seal face.

• In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces. This film can either come from the process fluid being pumped or from an external source.

• The need for a fluid film between the faces presents a design challenge in allowing sufficient lubricant to flow between the seal faces without the seal leaking an unacceptable amount of process fluid, or allowing contaminants in between the faces that could damage the seal itself.

• This is achieved by maintaining a precise gap (≈1micron ) between the faces, that is large enough to allow in a small amounts of clean lubricating liquid, but small enough to prevent contaminants from entering the gap between the seal faces and reduce leakage through it ( is so small that it appears as vapor – around ½ a teaspoon a day on a typical application ).

• This micro-gap is maintained using springs and hydraulic force to push the seal faces together, while the pressure of the liquid between the faces (the fluid film) acts to push them apart.

• Without the pressure pushing them apart the two seal faces would be in full contact, this is known as dry running and would lead to rapid seal failure.

• Without the process pressure (and the force of the springs) pushing the faces together the seal faces would separate too far, and allow fluid to leak out. v01MME-K

• Mechanical seal engineering focuses on increasing the longevity of the primary seal faces by ensuring a high quality of lubricating fluid, and by selecting appropriate seal face materials for the process being pumped .

• Seal flush port

• The gland or seal housing in many seals contains a port for injecting liquid, either from the pump discharge or from an external source .

Why flushing plan is required?1. To lubricate and cool mechanical seal.2. To remove foreign particles.3. To remove carbon deposition and engrossed

materials on mating surfaces.4. Prevent seal from dry running.5. To maintain the operational parameters of

mechanical seal.

Flush arrangements• It refers to the various methods used to lubricate,

cool and remove deposits and heat in mechanical seal. And they referred to as API seal plan

• Single seal (01, 02, 11, 13, 14, 21, 23, 31, 32 and 41)

• Double seal (52, 53A, 53B, 53C and 54)

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• Mechanical seal types and classification

• Mechanical seals can be classified by arrangement and configuration

By arrangement

Single

Inside mounted

Outside mounted

Dual

Duel pressurized

Duel unpressurized

( tandem )

Face to face & back to back

By design

Pusher

(single & multi spring)

Balanced

Non-balanced

Non pusher

(metal bellows)

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Inside vs outside

Single inside Single outsideD

escr

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on • They called inside seals because the seal parts are

located inside the seal housing.• They are called single seals because there is one

dynamic seal face .

• They called outside seals because the metallic rotary unit parts are exposed to the atmosphere, only the insert seal ring and secondary seals are exposed to the product ( non-metallic components )

• thus can be used in corrosive service .

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tage • The seal is submerged in the liquid making it easier to

flush and carry away heat .• Can be balanced to withstand high seal environment

pressures.

• Hardware items of the seal don’t come in contact with liquid.• Easier to access for adjustment and trouble shooting.

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• Hardware items of the seal come in contact with liquidwhich may be corrosive.

• cannot be adjusted without dismantling theequipment unless they are cartridge

• lower pressure limit as the pressure of the sealed liquid pushesthe two seal faces apart, rather than forcing them together.

• they are subject to exposure to dust and other environmentalcontaminants.

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Dual seals• Dual mechanical seals are used for extremely tough services, where it is desirable to completely eliminate the possibility of leakage of

sealed fluid . they include two dynamic seals, either mounted in face to face, back-to-back arrangement or other, In either cases, they have a fluid zone between the two seals & according to this fluid zone configuration they are sub-classified into :

Dual pressurized Dual pressurized gas(Non-contacting)

Dual unpressurized(Tandem)

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• The fluid zone between the two seals is at a pressure higher than the sealed liquid thus the sealed liquid can not enter this region .

• The fluid between the two zones is called a barrier fluid

• If the outside dynamic seal were to leak, the barrier liquid, rather than the pumped liquid, would leak out to the environment.

• If the inside dynamic seal were to leak, the barrier fluid would leak into the pumped liquid .

• The two dynamic seal faces are always kept completely free of the pumped liquid .

• In a Gas Lift-Off seal, the faces theoretically never contact.

• Dual pressurized gas mechanical seal is a combination of two seals with a barrier gas (N2 , clean air . Co2 etc. ) injected between the two seals at pressure 25 to 30 psi higher than pumped liquid pressure.

• The gas flows through holes in faces which separates the seal faces results in non-contacting faces at both dynamic seal faces.

• As the seal operates, an envelope of gas surrounds the seal faces keeping process liquid out.

• The fluid zone between the two seals is at apressure lower than the sealed liquid.

• The fluid between the two zones is called a bufferfluid

• The idea of the unpressurized dual seal is that if the primary seal fails, the operator of the pump can plan an orderly shutdown to repair the primary seal, with sealed fluid escaping to buffer fluid and then sealed by secondary seal rather than escaping to environment.

• Ordinarily, no pumped liquid reaches the secondary seal because the primary seal keeps the liquid, so the secondary seal runs in a non-pressurized clean lubricating liquid (buffer fluid), so it will generally last for an extended period of time.

• Usually, the buffer fluid is monitored for pressure, level, ph, conductivity, or some other convenient variable, to signify a failure of the primary seal.

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•Fe

atu

re• When pumping volatile liquids, hazardous, corrosive,

abrasive, etc. It is sometimes necessary to insure thatthe process liquid does not enter the fluid zonebetween 2 seals . Thus Pressurizing this zone higherthan the sealed liquid will prevent process liquidfrom crossing the primary seal faces.

• Pumped liquid can’t leak into the environment.

• Less mechanical losses due tolesser friction .

• without liquid contaminationof the process liquid like incase of pressurized dual seal .

• If primary seal fails, enduser can plan torepair/replace seal inadvance without anyproduct escaping to theenvironment.

• The difference between un/pressurized seals , Is In a pressurized dual seal, the outboard or secondary has the tougher jobof the two. It operates sealing high barrier pressure while the inboard or primary seal has clean lubricating liquid applied atdifferential pressure of only 20 to 30 psi. and it is vice versa in unpressurized .

Barrier Fluid Characteristics of Barrier/Buffer Fluid Buffer Fluid

• Safe to use, handle inexpensive & Compatible with seal materials & process fluid .

• Good flow qualities at operational temperatures (including very low temperature service).

• Nonflammable.• Good lubricity.• Non-foaming when pressurized.• Good heat transfer properties.• Low gas solubility.

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Dual seal, with dynamic seals mounted in back to face.v01MME-K

Inside or Primary seal

Outside or Secondary Seal

Immersed in process liquid

in the stuffing box

Buffer fluid warmed

by seal generated

heat returns to the

buffer supply tank

Cool buffer fluid

from the buffer

supply tank enters

via the inlet port

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The equipment

can then be started

and process suction

opened allowing

liquid into the

stuffing box.

Gas is supplied

to the inlet port.

UTEX DUAL CO-AXIAL pressurized GAS SEAL

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Pusher vs. Non-pusher• Both pusher and non-pusher types can be either shaft mounted or cartridge assemblies.• The basic difference 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.

Pusher type Non pusher type

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• 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. Then it is called a dynamic 0-ring and the seal type is a pusher .

• Can be a single spring or multi-spring design

• Non-Pusher type or Bellow seals have no dynamic secondary seal under the movable seal ring instead it is located in the retainer. & the seal components compensate for face wear without “pushing” any sealing points. Then it is called a static 0-ring and the seal type is a Non-pusher .

• The bellows can be made of rubber, Teflon and metal. Rubber bellows are used for less critical applications , Teflon bellows are used for low pressure, moderate temperature acid services. Metal bellow seal is its ability to run at a very high

• Eliminates the problem of Seal hang up due to spring clogging or clogging of dynamic elastomers on the shaft.

• Single spring • Multi-spring design

Advantages: • less –CLOGGING• low spring constant Limitation:• Not recommended for very

high RPM applications Due to single spring design causes non uniform seal loading

• Require long axial space.

Advantages • Number of small springs are used on

the to give a uniform face loading. • compact in comparison with single

spring seal.• Due to uniformly loaded seal faces

the multi spring seals are a must for high rpm application.

Limitation:• more expensive.• spring clogging is more easily .

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Pusher type

Single spring design

Multi spring design

Stationary face

Rotating face

Retain

er

Spring

An

ti rotatio

n

pin

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

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

3. 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”.

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• Non pusher type

Metal bellows

Rotatory face

Stationary face

Static O-ring

• The bellows core expands to compensate for face wear.• Debris can build up without causing hang up.• This feature is probably the most notable selling point when comparing a bellows seal to a pusher type seal.

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Balanced and unbalanced mechanical seals

• 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.

• The purpose of a balanced seal is to seal against higher pressures and speeds than possible in an unbalanced configuration by balancing out the forces try to close the seal with those try to open it .

There are at least 2 forces closing the seal faces:

There are at least 3 forces trying to open the seal faces:

• A = The spring loaded face with an area of 2 𝑖𝑛2

• B = The stationary face held to the front of the stuffing box by gland "G"

• P = The hydraulic pressure in the stuffing box (i.e. 100 psi ).

• The mechanical spring force.• The hydraulic force caused by the

stuffing box pressure acting on the seal face area (pressure ×spring loaded face area).

• A hydraulic force is created any time there is fluid between the seal faces.

• A centrifugal force created by the action of the fluid being thrown outward by the rotation of the pump shaft.

• A hydrodynamic force created because trapped liquid is non compressible. ( Seal faces are lapped to within three light bands becauseslight waviness is enough to generate hydrodynamic lifting forces as we try to compress non-compressible liquid trapped between the lapped faces.

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B a l a n c e d U n b a l a n c e d

Bal

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The balance ratio of a mechanical seal is an area ratio and is related to the seal face load. Balance ratio is defined as the ratio of the closing area to the opening area.

Balance ratio < 1 Balance ratio > 1

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s • Less hydraulic force acting on seal faces.• Generate less heat.• Higher pressure limit.• Can handle liquids with poor lubricity and high vapor

pressure.

• Inexpensive.• Less leak.• Stable when subjected to vibration, misalignment and

cavitation.

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• Relatively higher cost.• Very minor leak is expected.

• Higher hydraulic force acting on seal faces.• Low pressure limit.

• How is that accomplished? Since the hydraulic closing forces were twice the opening forces (100 psi. Vs. 50 psi.) We have installed a stepped or shortened primary ring and sleeve inside the seal to reduce the closing area and reduce the closing force.

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Unbalanced Balanced

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• The faces of a balanced seal are located so that a portion of the face contact occurs inside the balance diameter resulting in reduced closing force due to stuffing box pressure.

• Most metal bellows seals are balanced

Ba

lan

ce L

ine

Face

ID

Lin

e

Face

OD

Lin

e

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C a r t r i d g e M e c h a n i c a l S e a l

Des

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Mechanical seal Pre-mounted on a sleeve and fit directly to shaft or shaft sleeve ( available single,double and tandem) .

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• No requirement for seal setting measurement for installation.• Eliminate seal setting errors.• Low maintenance cost.

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Mechanical Seal SelectionThe proper selection of a mechanical seal can be made only if the full operating conditions are known:

1. 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 Hastelloy. 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.

2. Pressure

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

3. Temperature

4. 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.

5. Reliability and Emission Concerns

• The seal type and arrangement selected must meet the desired reliability and emission standards for the pump application. Double seals and double gas barrier seals are becoming the seals of choice.

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O p e rat i o n a l e r ro r

1. Dry running.

2. Suction chocking.

3. Foreign material.

4. Material incompatibility.

5. Abnormal process parameters.

6. Flushing plan off.

M a i n te n a n c e e r ro r

1. Improper installation.

2. Shaft misalignment.

3. Shaft run out.

4. Failed bearing.

5. Unavailability of required flushing plan.

How Mechanical Seal Can Fail ?

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