Download ppt - Centrifugal Pumps Seals

CENTRIFUGAL PUMPS SEALS

Centrifugal Pumps Seals

Sealing

The proper selection of a seal is critical to the success of every pump application.

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

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


Sealing Basics

There are two basic kinds of seals:

Static seals :

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

Gaskets.

O-rings .


Sealing BasicsDynamic seals :

Which are used where surfaces move relative to one another. where a rotating shaft transmits power through

the wall of a tank.

Through the casing of a pump or through the housing of other rotating equipment such as a

filter or screen.


Sealing Basics


Sealing Basics

Pump manufacturers use various design techniques to reduce the pressure of the product trying to escape such:

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

of the impeller.

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

As there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the

atmosphere. Such sealing devices are typically either compression packing or end-face mechanical seals.


Sealing Basics

Packing Seals:The most common arrangement for gland packing used in pumps is the solid packed stuffing box

It is important not to overtighten the packing gland so that all

leakage is prevented, as it is this small amount of liquid, which lubricates the packing and so

reduces friction and wear.


Sealing Basics

Packing Seals:A replaceable sleeve is often

fitted to the shaft in the region of the stuffing box.

This simple type of packing arrangement should not be used when the pump is operating with

suction lift conditions.

Because air may be drawn into the pump through the stuffing

box. Causing the pump to lose suction.

Why?

How to solve this problem?


Sealing Basics

Packing Seals:Pumps designed to operate under suction lift conditions use a sealing

or injection type of packing arrangement.

This uses a liquid to help seal the packing gland and prevent air being drawn into the

pump.

Why?

This sealing liquid comes from either

the discharge side of the pump or from an

external source .


Sealing Basics

Packing Seals:If the sealing liquid is supplied from the pump, then the stuffing box is

said to be internally sealed. If the sealing liquid supplied from an outside source, then the stuffing box is said to be externally sealed.

A sealing ring cage that often called a

lantern ring is usually used to distributing the sealing liquid within the

stuffing box.


Sealing Basics

Packing Seals:Lantern rings are usually made of brass or bronze and are normally

positioned mid-way in the stuffing box, with an equal number of packing rings at each side.

If the pump is to handle liquids

containing sand or grit, then the sealing

system should be of the external type.


Sealing Basics

Packing Seals:The pressure of the sealing liquid must be greater than the pump

suction pressure.

Why?


Sealing Basics

Packing Seals:A typical packed stuffing box

arrangement consists of:

Five rings of packing

A lantern ring used for the injection of a lubricating and/or

flushing liquid.

A gland to hold the packing and maintain the desired

compression for a proper seal


Sealing Basics

Packing Seals:A flow from 40 to 60 drops per

minute out of the stuffing box must be maintained for proper

lubrication.


Sealing Basics

Packing Seals:

Internally sealed


Sealing Basics

Packing Seals:

Externally sealed


Sealing Basics

Packing Seals:

Externally sealed

When pumping slurries or abrasive liquids, it is

necessary to inject a dean lubricating liquid

from an external source into the lantern ring .

A flow of from 0.2 to 0.5 gpm is desirable and a

valve and flowmeter should be used for accurate control.


Sealing Basics

Packing Seals:


Sealing Basics

Packing Seals:By a quench type gland. Water,

oil, or other fluids can be injected into the gland to

remove heat from the shaft, thus limiting heat transfer to the

bearing frame.

This permits the operating temperature of the pump to be

higher than the limits of the bearing and lubricant design.

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


Packing MaterialsThe materials commonly

used to make packing include cotton, asbestos

and flax.

They are usually woven or braided to form a

continuous 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 pumps

having differing stuffing box dimensions.

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


Packing MaterialsIn addition to the natural

packing materials, there is also a range of synthetic

and metallic packing materials, which may be

more suitable for high temperature applications or for certain types of pumped

liquid.

Many of the synthetic packing materials are made in the form of a "V" or chevron section and are installed with the open part of the "V" facing the liquid being

pumped. In this position, the pressure of the liquid in the pump tends to expand the packing and helps it to seat on the shaft.


Packing InstallationIt is important that the correct number of packing rings are used when inserting

packing 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 lantern ring must be deducted from the total depth of the stuffing box).


Packing Installation


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

The rings should be dipped in oil before insertion (If the packing is not of the lubricated 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 is required.

It is normally permitted to add one extra ring of packing to a stuffing box that has a slight leak, but a note should be made when the ring is added so that

more rings are not added at a later date.

Packing Installation


Mechanical Seals

When leakage of the pumped liquid is not acceptable, packing seals are not suitable and one of the many different types of mechanical seals is used

instead.

A mechanical seal is a sealing device, which forms a running seal between rotating 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.


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

Mechanical Seals


The Basic Mechanical Seal


Principles of Operation:A. Stationary Seal Ring – This is

usually made of tungsten carbide or stainless steel and fits into the

seal plate.

B. Stationary Seal Ring Seal – This is an O-ring which prevents

leakage of liquid between the stationary seal ring and seal

plate.


Principles of Operation:C. Rotating Seal Ring – This is

secured to the shaft and is pressed against the stationary

seal ring by the combined action of the spring and the pressure of

the liquid. The material used is normally carbon.

D. Rotating Seal Ring Seal – This is also an O-ring , which

prevents leakage the rotating seal ring and the shaft.


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

spring which helps to maintain contact pressure between the

faces of the rotating and stationary seals.

F. Thrust Collar – This is secured to the shaft by a grub screw and

takes the reaction thrust of the spring. The position along the shaft where the thrust collar is

mounted will control the amount of thrust exerted by the spring

upon the sealing faces.


Principles of Operation:G. Seal Plate – This seals the

stuffing box and also houses the stationary seal ring.

H. Seal Plate Seal – This may be either an O-ring or a flat gasket

which prevents leakage from the joint between stuffing box and

seal plate.


Mechanical Seal Types :PUSHER:

Incorporate secondary seals that move axially along a shaft or sleeve

to maintain contact at the seal faces. This feature compensates for seal

face wear and wobble due to misalignment.

Advantage:

•It's inexpensive

•Commercially available in a wide range of sizes and configurations.


Mechanical Seal Types :PUSHER:

Incorporate secondary seals that move axially along a shaft or sleeve

to maintain contact at the seal faces. This feature compensates for seal

face wear and wobble due to misalignment.

Disadvantage:

•It's prone to secondary seal hang-up and fretting of the shaft

or sleeve.


Mechanical Seal Types :UNBALANCED:

Disadvantage:

•Its relative low pressure limit.

Advantage:

•It's inexpensive.

•Leak less.

•More stable when subjected to vibration, misalignment, and

cavitation.


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

Although setting a mechanical seal is relatively simple.


Mechanical Seal Types :NON-PUSHER:

The non-pusher or bellows seal does not have to move along the shaft or

sleeve to maintain seal face contact.

Advantage:

•its ability to handle high and low temperature applications.

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

Disadvantage:

•Its thin bellows cross sections must be upgraded for use in corrosive environments.


Mechanical Seal Types :BALANCED:

Balancing a mechanical seal involves a simple design change, which reduces

the hydraulic forces acting to close the seal faces.

Balanced seals have higher-pressure limits, lower seal face loading, and

generate less heat. This makes them well suited to handle liquids with poor

lubricity and high vapor pressures such as light hydrocarbons.


Mechanical Seal Types :BALANCED:

Advantage:

•Its higher-pressure limits .

•Lower seal face loading .

•Generate less heat.


Mechanical Seal Types :CARTRIDGE:

Which have the mechanical seal premounted on a sleeve including the gland and 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 setting measurements for their installation.

Cartridge seals lower maintenance costs and reduce seal setting errors.


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 be balanced to withstand high seal environment pressures.

Recommended for relatively clear non-corrosive and corrosive liquids with satisfactory' lubricating properties where cost of operation does not exceed

that of a double seal.


Mechanical Seal Arrangements :SINGLE OUTSIDE:

If an extremely corrosive liquid has good lubricating properties, an outside

seal offers an economical alternative to the expensive metal required for an

inside seal to resist corrosion.

The disadvantage is that it is exposed outside of the pump which makes it

vulnerable to damage from impact and hydraulic pressure works to open the seal faces so they have low pressure

limits (balanced or unbalanced).


Mechanical Seal Arrangements :DOUBLE (DUAL PRESSURIZED):

This arrangement is recommended for liquids that are not compatible with a

single mechanical seal (i.e. liquids that are toxic, hazardous, have suspended abrasives, or corrosives which require

costly materials).

The advantages of the double seal are that it can have five times the life of a

single seal in severe environments. Also, the metal inner seal parts are never exposed to the liquid product being

pumped, so viscous, abrasive, or thermosetting liquids are easily sealed

without a need for expensive metallurgy.


Mechanical Seal Arrangements :DOUBLE (DUAL PRESSURIZED):

In addition, recent testing has shown that double seal life is virtually unaffected by

process upset conditions during pump operation. A significant advantage of

using a double seal over a single seal.


Mechanical Seal Arrangements :DOUBLE GAS BARRIER (PRESSURIZED DUAL GAS):

Very similar to cartridge double seals ... sealing involves an inert gas,

like nitrogen, to act as a surface lubricant and coolant in place of a

liquid barrier system or external flush required with conventional or cartridge

double seals. The gas barrier seal uses nitrogen or

air as a harmless and inexpensive barrier fluid that helps prevent product emissions to the atmosphere and fully

complies 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 reliability

is the required on an application.


Mechanical Seal Arrangements :TANDEM (DUAL UNPRESSURIZED):

Due to health, safety, and environmental considerations, tandem seals have been used for products such as vinyl chloride, carbon monoxide, light

hydrocarbons, 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 outboard seal can take over and function until maintenance of the equipment can be

scheduled.


Mechanical Seal Selection :

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

1. Liquid.

2. Pressure.

3. Temperature .

4. Characteristics of Liquid .

5. Reliability and Emission Concerns .


Seal Environment :

STANDARD BORE STUFFING BOX COVER:

Designed thirty years ago specifically for packing. Also accommodates

mechanical seals (clamped seat outside seals and conventional double seals.)


Seal Environment :

CONVENTIONAL LARGE BORE SEAL CHAMBER:

Designed specifically for mechanical seals. Large bore provides Increased

life of seals through improved lubrication and cooling of faces.

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

Often available with internal bypass to provide circulation of liquid to faces

without using external flush. Ideal for conventional or cartridge single

mechanical seals in conjunction with a flush and throat bushing in bottom of

chamber.


Seal Environment :

CONVENTIONAL LARGE BORE SEAL CHAMBER:

Also excellent for conventional or cartridge double or tandem seals.


Seal Environment :

LARGE BORE SEAL CHAMBERS:

Enlarged bore seal chambers with increased radial clearance between the

mechanical seal and seal chamber wall, provide better circulation of liquid to and

from seal faces.

Improved lubrication and heat removal (cooling) of seal faces extend seal life

and lower maintenance costs.