Centrifugal Pump Seals

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CENTRIFUGAL PUMPS SEALS


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Centrifugal Pumps Seals

SealingThe proper selection of a seal is critical to the success of every pumpapplication.

For maximum pump reliability, choices must be made between thetype of seal and the seal environment.

A sealless pump is an alternative, which would eliminate the need for adynamic type seal entirely.


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Sealing BasicsThere are two basic kinds of seals:

Static seals :

Which are employed where no movement occurs at the Juncture to besealed. Typical static seals are:

Gaskets.

O-rings .


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Sealing BasicsDynamic seals :

Which are used where surfaces move relative to one another.where a rotating shaft transmits power throughthe wall of a tank.

Through the casing of a pump or through thehousing of other rotating equipment such as afilter or screen.


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Sealing Basics


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Sealing BasicsPump manufacturers use various design techniques to reduce thepressure of the product trying to escape such:

1. The addition of balance holes through the impeller topermit most of the pressure to escape into the suction sideof the impeller.

2. The addition of back pump-out vanes on the back side ofthe impeller.

As there is no way to eliminate this pressure completely, sealingdevices are necessary to limit the escape of the product to theatmosphere. Such sealing devices are typically either compression packing or end-face mechanical seals .


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Sealing BasicsPacking Seals:The most common arrangementfor gland packing used in pumpsis the solid packed stuffing box

It is important not to overtightenthe packing gland so that allleakage is prevented, as it is thissmall amount of liquid, which

lubricates the packing and soreduces friction and wear.


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Sealing BasicsPacking Seals: A replaceable sleeve is oftenfitted to the shaft in the region of the stuffing box.

This simple type of packingarrangement should not be usedwhen the pump is operating withsuction lift conditions.

Because air may be drawn intothe pump through the stuffingbox. Causing the pump to losesuction.

Why?

How to solve this problem?


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Sealing BasicsPacking Seals:Pumps designed to operate under suction lift conditions use a sealing or injection type of packing arrangement.

This uses a liquid tohelp seal the packinggland and prevent air being drawn into thepump.

Why?

This sealing liquidcomes from either the discharge side of the pump or from anexternal source .


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Sealing BasicsPacking Seals:If the sealing liquid is supplied from the pump, then the stuffing box issaid to be internally sealed . If the sealing liquid supplied from anoutside source, then the stuffing box is said to be externally sealed .

A sealing ring cagethat often called alantern ring is usuallyused to distributing thesealing liquid within thestuffing box.


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Sealing BasicsPacking Seals:Lantern rings are usually made of brass or bronze and are normallypositioned mid-way in the stuffing box, with an equal number of packingrings at each side.

If the pump is tohandle liquidscontaining sand or grit,then the sealingsystem should be of the external type.


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Sealing BasicsPacking Seals:The pressure of the sealing liquid must be greater than the pumpsuction pressure.

Why?


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Sealing BasicsPacking Seals:

A typical packed stuffing boxarrangement consists of:

Five rings of packing

A lantern ring used for theinjection of a lubricating and/orflushing liquid.

A gland to hold the packing and

maintain the desiredcompression for a proper seal


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Internally seal ed


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Externally sealed


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Sealing BasicsPacking Seals:By a quench type gland. Water,oil, or other fluids can beinjected into the gland to

remove heat from the shaft,thus limiting heat transfer to thebearing frame.

This permits the operatingtemperature of the pump to behigher than the limits of thebearing and lubricant design.

The same quench gland can be used to prevent the escape of a toxic or volatileliquid into the air around the pump (smothering gland)


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Packing MaterialsThe materials commonlyused to make packinginclude cotton , asbestos and flax.

They are usually woven or braided to form acontinuous length of square section.

The lengths of packing are often impregnated with graphite to assist in reducing

friction and are available in a range of different section sizes to suit pumpshaving differing stuffing box dimensions.

Packing lengths are often reinforced with wire strands which strengthens thematerial and helps it to retain its shape.


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Packing MaterialsIn addition to the naturalpacking materials, there isalso a range of syntheticand metallic packingmaterials, which may bemore suitable for hightemperature applicationsor for certain types of pumped liquid.

Many of the synthetic packing materials are made in the form of a "V" or chevronsection and are installed with the open part of the "V" facing the liquid beingpumped. In this position, the pressure of the liquid in the pump tends to expandthe packing and helps it to seat on the shaft.
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Packing InstallationIt is important that the correct number of packing rings are used when insertingpacking into the stuffing box. This can be achieved by one of the following:

1. By counting the number of rings previously removed.

2. By checking the manufacturer's recommendations.

3. By measuring the depth of stuffing box (the width of the lanternring must be deducted from the total depth of the stuffing box).


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Packing Installation


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To insert each ring of packing separately and push it into position with thegland.

The rings should be dipped in oil before insertion (If the packing is not of thelubricated type).

The location of lantern rings (if it is used).

The thermal packing extension clearance.

Checking the shaft or sleeve when re-packing a stuffing box..

A total re-pack job should be done whenever more than one ring of packing isrequired.

It is normally permitted to add one extra ring of packing to a stuffing box thathas a slight leak, but a note should be made when the ring is added so thatmore rings are not added at a later date.

Packing Installation


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Mechanical Seals When leakage of the pumped liquid is not acceptable, packing seals are notsuitable and one of the many different types of mechanical seals is usedinstead.

A mechanical seal is a sealing device, which forms a running seal betweenrotating and stationary parts.

Advantages of mechanical seals over conventional packing are as follows:

1. Zero or limited leakage of product (meet emission regulations.)

2. Reduced friction and power loss.

3. Elimination of shaft or sleeve wear.

4. Reduced maintenance costs.

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


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6. The wide variety of designs allows use of mechanical seals in almostall pump applications.

Mechanical Seals


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The Basic Mechanical Seal


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Principles of Operation:A. Stationary Seal Ring This is

usually made of tungsten carbideor stainless steel and fits into theseal plate.

B. Stationary Seal Ring Seal This is an O-ring which preventsleakage of liquid between thestationary seal ring and sealplate.


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Principles of Operation:C. Rotating Seal Ring This is

secured to the shaft and ispressed against the stationaryseal ring by the combined actionof the spring and the pressure of the liquid. The material used isnormally carbon.

D. Rotating Seal Ring Seal Thisis also an O-ring , whichprevents leakage the rotating

seal ring and the shaft.


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Principles of Operation:E. Spring This is a single coil

spring which helps to maintaincontact pressure between thefaces of the rotating andstationary seals.

F. Thrust Collar This is securedto the shaft by a grub screw andtakes the reaction thrust of thespring. The position along theshaft where the thrust collar is

mounted will control the amountof thrust exerted by the springupon the sealing faces.


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Principles of Operation:G. Seal Plate This seals the

stuffing box and also houses thestationary seal ring.

H. Seal Plate Seal This may be

either an O-ring or a flat gasketwhich prevents leakage from the

joint between stuffing box andseal plate.


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Mechanical Seal Types :PUSHER:Incorporate secondary seals thatmove axially along a shaft or sleeveto maintain contact at the seal faces.This feature compensates for sealface wear and wobble due tomisalignment.

Advantage:

It's inexpensive Commercially available in a widerange of sizes and configurations.


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Mechanical Seal Types :PUSHER:Incorporate secondary seals thatmove axially along a shaft or sleeveto maintain contact at the seal faces.This feature compensates for sealface wear and wobble due tomisalignment.

Disadvantage:

It's prone to secondary seal

hang-up and fretting of the shaftor sleeve.


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Mechanical Seal Types :UNBALANCED:

Disadvantage:

Its relative low pressure limit.

Advantage:

It's inexpensive.

Leak less. More stable when subjected tovibration, misalignment, andcavitation.


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Mechanical Seal Types :CONVENTIONAL:Which require setting and alignmentof the seal (single, double, tandem)on the shaft or sleeve of the pump.

Although setting a mechanical seal isrelatively simple.


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Mechanical Seal Types :NON-PUSHER:The non-pusher or bellows seal doesnot have to move along the shaft or sleeve to maintain seal face contact.

Advantage:

its ability to handle high and lowtemperature applications.

its does not require a secondary seal (not prone to secondary sealhang-up).

Disadvantage:

Its thin bellows cross sections must be upgraded for use in corrosiveenvironments.


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Mechanical Seal Types :BALANCED:Balancing a mechanical seal involves asimple design change, which reducesthe hydraulic forces acting to close theseal faces.

Balanced seals have higher-pressurelimits, lower seal face loading, andgenerate less heat. This makes themwell suited to handle liquids with poor lubricity and high vapor pressures suchas light hydrocarbons.


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Mechanical Seal Types :BALANCED: Advantage:

Its higher-pressure limits .

Lower seal face loading . Generate less heat.


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Mechanical Seal Types :CARTRIDGE:Which have the mechanical seal premounted on a sleeve including the glandand fit directly over the special model shaft or shaft sleeve (available single,double, tandem).

The major benefit, of course is no requirement for the usual seal settingmeasurements for their installation.

Cartridge seals lower maintenance costs and reduce seal setting errors.


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Mechanical Seal Arrangements :SINGLE INSIDE:This is the most common type of mechanical seal.

These seals are easily modified to accommodate seal flush plans and can bebalanced to withstand high seal environment pressures.

Recommended for relatively clear non-corrosive and corrosive liquids withsatisfactory' lubricating properties where cost of operation does not exceedthat of a double seal.


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Mechanical Seal Arrangements :SINGLE OUTSIDE:If an extremely corrosive liquid hasgood lubricating properties, an outsideseal offers an economical alternativeto the expensive metal required for aninside seal to resist corrosion.

The disadvantage is that it is exposedoutside of the pump which makes itvulnerable to damage from impact andhydraulic pressure works to open the

seal faces so they have low pressurelimits (balanced or unbalanced).


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Mechanical Seal Arrangements :DOUBLE (DUAL PRESSURIZED):This arrangement is recommended for liquids that are not compatible with asingle mechanical seal (i.e. liquids thatare toxic, hazardous, have suspendedabrasives, or corrosives which requirecostly materials).

The advantages of the double seal arethat it can have five times the life of asingle seal in severe environments. Also,

the metal inner seal parts are never exposed to the liquid product beingpumped, so viscous, abrasive, or thermosetting liquids are easily sealedwithout a need for expensive metallurgy.


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Mechanical Seal Arrangements :DOUBLE (DUAL PRESSURIZED):In addition, recent testing has shown thatdouble seal life is virtually unaffected byprocess upset conditions during pumpoperation. A significant advantage of using a double seal over a single seal.


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Mechanical Seal Arrangements :DOUBLE GAS BARRIER (PRESSURIZED DUAL GAS):Very similar to cartridge double seals... sealing involves an inert gas, likenitrogen, to act as a surface lubricantand coolant in place of a liquid barrier system or external flush required withconventional or cartridge double seals.

The gas barrier seal uses nitrogen or air as a harmless and inexpensivebarrier fluid that helps prevent product

emissions to the atmosphere and fullycomplies with emission regulations.

The double gas barrier seal should be considered for use on toxic or hazardous liquids that are regulated or in situations where increased reliabilityis the required on an application.


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Mechanical Seal Arrangements :TANDEM (DUAL UNPRESSURIZED):Due to health, safety, and environmental considerations, tandem seals havebeen used for products such as vinyl chloride, carbon monoxide, lighthydrocarbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous liquids.

Tandem seals eliminate icing and freezing of light hydrocarbons and other liquids, which could fall below the atmospheric freezing point of water in air (32 F or 0 C). {Typical buffer liquids in these applications are ethylene glycol,methanol, and propanol.)

A tandem also increases online reliability. If the primary seal fails, the outboardseal can take over and function until maintenance of the equipment can bescheduled.


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Mechanical Seal Selection :The proper selection of a mechanical seal can be made only if the fulloperating conditions are known:

1. Liquid.

2. Pressure. 3. Temperature .

4. Characteristics of Liquid .

5. Reliability and Emission Concerns .


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Seal Environment :STANDARD BORE STUFFING BOX COVER :

Designed thirty years ago specificallyfor packing. Also accommodatesmechanical seals (clamped seat outsideseals and conventional double seals.)


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Seal Environment :CONVENTIONAL LARGE BORE SEAL CHAMBER:

Designed specifically for mechanicalseals. Large bore provides Increasedlife of seals through improvedlubrication and cooling of faces.

Seal environment should be controlledthrough use of CPI or API flush plans.

Often available with internal bypass to

provide circulation of liquid to faceswithout using external flush. Ideal for conventional or cartridge singlemechanical seals in conjunction with aflush and throat bushing in bottom of chamber.


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Seal Environment :LARGE BORE SEAL CHAMBERS:

Enlarged bore seal chambers withincreased radial clearance between themechanical seal and seal chamber wall,

provide better circulation of liquid to andfrom seal faces.

Improved lubrication and heat removal(cooling) of seal faces extend seal lifeand lower maintenance costs.
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Centrifugal Pump Seals

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