seals1.pdf

7/27/2019 seals1.pdf

http://slidepdf.com/reader/full/seals1pdf 1/6

Mechanical Seals & Packing: Video Based Educational Packages Page 1 of 6

file://C:\WINDOWS\...\Mechanical Seals & Packing Video Based Educational Packages.ht 7/13/03

MECHANICAL SEALS &

PACKING: Getting the job done...

Mechanical Seals: video based educational package produced

by pump people for pump people...

I. The Pump Stuffing Box

1. Stuffing Box Components:

In appreciation of the variables involved in modern process

pumping most manufacturers offer stuffing boxes with ample

room to incorporate packing or mechanical seals with the

ports required for internal or external flush injection, to

utilize auxiliary devices, and jacketing to accommodatetemperature requirements. Knowing how to properly apply

packing, flush arrangements and auxiliary devices will prolong both pump and packing

life.

a. Compression Packing: An understanding of compression packing & the pump stuffing

box will lessen downtime while improving process performance.

b. Throttle & Throat Bushings: The use of throat and throttle bushings will result in a

controlled stuffing box pressure environment which is essential to packing life.

c. Lantern Rings & Seal Cages: Arranged properly lantern rings and seal cages will

contribute to decreasing stuffing box pressure, removal of solids, and sealing between the

packing and pump sleeve on static lift or suction under vacuum conditions.d. Packing Glands: Glands come in a variety of designs each with a specific purpose to

complement a given pumping application.

2. Controlling Box Pressure:

To obtain maximum packing or seal service life it would be desirable

to seal the fluid wishing to escape through the pump stuffing box at

a minimal pressure. To control and reduce the pressure in the

stuffing box centrifugal pump manufacturers will incorporate

balancing holes or pump out vanes in impeller design...

a. Impeller Pump-Out Vanes: To reduce the volume of pumpage attempting to exit astuffing box, pump out vanes are use at the rear of an impeller to reduce the pressure at

flow seen at the stuffing box bore.





II. Mechanical Shaft Seals

b. Impeller Balancing Holes: Realizing the pumpage leaves the impeller vane tips and a

portion seeks a path behind an impeller, balancing holes permit a percentage of this fluid

to return to the impeller eye (suction pressure), both reducing bearing loads and reducing

stuffing box pressure.

c. Positive Displacement Pumps: Internal and external porting from discharge to suction to

reduce suction pressure. In many pump designs this recirculation is required to cool and

lubricate tight tolerance sleeve bearings and bushings.

3. Packed Stuffing Box Configurations

Successful packing application depends largely in part to

the proper application of the lantern ring, throttle

bushing and the incorporation of internal and external

flush arrangements. Positioning and control of these

components and auxiliary systems will ensure maximum

pump and packing performance.

a. Abrasive Duty: Suitable packing material combined with proper positioning of the

throttle bushing(s), and lantern ring(s), with flushing where applicable will result in

extended packing and pump life.

b. High Temperature Applications: When faced with high pumpage temperatures stuffing

box and gland cooling is essential. Many fluids will vaporize when seeing the reduction of

pressure within the stuffing box and when exiting to atmosphere. External box cooling and

jacket offer alternatives to control flashing and vaporization.

c. High Pressure Sealing: The use of throttle bushings and lantern rings appropriately

positioned will decrease the volume of fluid wishing to enter the stuffing box resulting in a

reduction of stuffing box pressure. A bleed from a lantern ring to pump suction, whenappropriate, can also reduce and help control stuffing box pressure.

d. Vacuum Conditions:

4. Packing Installation & Troubleshooting

Packing installation and troubleshooting is an art form, rather

engineered science. Only those who thoroughly understand the

application and purpose of the components and auxiliary devices

incorporated into a packed stuffing box arrangement will be able to

effectively apply compression packing.

a. Cut Packing & Installation: While a "no brainer" task to most individuals, cutting

packing is an important contribution to packing life. Remember; a good butt cut is

superior to a poor miter cut, even though miter cut packing is a preferred method.

b. Running-In & Packing Adjustments: With the evolution of mechanical seals and sealless

magnetic drive pumps most individuals capable of performing these tasks correctly have

retired taking their expertise. Most damage to pump sleeves initiates at conception during

start-up after a new or replacement packing task has been completed.

c. Troubleshooting:





III. Auxiliary Mechanical Seal Systems

1. Operating Principles & Fundamentals

Since the development of the basic mechanical seal introduction of new

and innovative seal technologies has enabled mechanical seal

installation on virtually any fluid handling application. To sort through

which seal design will provide optimum performance a thoroughunderstanding of mechanical seal principles and fundamentals is

mandatory.

a. Pusher & Non-Pusher Seal Designs: Pusher seals, while generally less expensive than

non-pusher seal designs, will have a tendency to "hang-up" on the pump shaft when

handling fluids which coke or crystallize as the secondary sealing member which must

accommodate for travel as the seal faces wear is unable.

b. Seal Driving & Spring Compression: The rotary portion of a mechanical seal is either

positive or friction drive. Incorporating an improper driving arrangement on a given

application will result in premature and catastrophic failure.

c. Balanced & Unbalanced Seals: This difference in seal design will make the difference inseal performance. An unbalanced mechanical seal seeing high pressures has the fluid film

between the seal faces reduced due to high hydraulic face loading resulting in overheating,

rapid face wear, and premature seal failure.

d. Inside & Outside Seal Mounting: While inside mechanical seals are a preferred method

outside seals can be used when fitting a pump with a shallow stuffing box which cannot

dimensionally accommodate an inside seal.

2. Mechanical Seal Configurations

a. Double Mechanical Seals: When the use of a an appropriate single mechanical seal

becomes to expensive and when the pumpage dictates the use of an artificial sealingenvironment double seals are used as an economical and performance alternative.

b. Seals In Tandem: Carcinogens and other hazardous materials require "zero leakage".

Tandem seals will permit a fail safe seal operating configuration enabling the

implementation of alarms, shut-downs and other warning and safety components.

c. Cartridge Seal Designs: The critical nature of many pump installations prohibits and

limits downtime for seal replacement. Many seals require complex settings during

installation and the time required for proper installation is simply not available. Cartridge

seals accommodate these scenarios by providing the complete seal pre-assembled and

readied for installation offering repair of failed seals at convenience.

3. Fluid Characteristics & Seal Application

a. Process Fluid Behavior: Prior to selecting a mechanical seal it is imperative process fluid

characteristics be identified. In most cases it is the fluid which will determine materials of

construction, seal design, auxiliary components require, etc., to ensure expected seal

performance.

b. Seal Pressure - Velocity Limitations: Mechanical seal designs and seal faces require

cooling and lubrication to function properly. The hydraulic pressure acting on the seal

faces and the rotating speed of the rotary seal will generate heat. It is this seal generated

heat that limits various seal designs and materials.





IV. Mechanical Seal Troubleshooting & Failure Analysis

1. Flush Fluid Temperature Control&

Cleaning Devices

Most mechanical seals require the fluid to cool and lubricate

between he sealing faces to remove seal generated heat. Manyauxiliary devices and flush arrangements, applied properly can

accomplish this task to ensure adequate seal performance.

a. Controlling Flush Fluid Temperature: Flush fluid temperature is critical to the life of a

mechanical seal. There are many methods for seal flush cooling; internal & external, using

water or air cooled heat exchangers.

b. Cyclone Separators: Abrasives will have a dramatic effect on seal face materials

selection and the price of a given seal. Cyclone separators installed in a discharge to seal

flush by-pass line can remove most abrasives directing them back to suction or to waste,

while providing the seal faces with clean pumpage.

c. Filtration Systems: An alternative to removing abrasives from a seal flush system wouldbe to filter the debris. Most filtration methods are used on dead-ended sealing systems

where the sealing fluid is once through. On continuous flush systems arrangements are

made to clean the filter which generally incorporates a dual on/off line filtration system.

2. Barrier Fluid Systems

a. Thermal Convection Pressure Pots: When using double mechanical seals the

introduction of a clean, cool barrier fluid at a pressure above the stuffing box pressure is

critical to seal life. A closed loop system is an economical maintenance free method for

creating this trifocal sealing environment depending on the application.

b. Closed Loop Circulators & Systems: When applications dictate, the use of a closed loop

pressure pot may not be acceptable. Closed loop circulators provide an artificial sealing

environment which provides clean, cool barrier fluid at all times. Closed loop circulators

can also be sized to handle several sealing requirements.

1. Start-Up & Stand-By Considerations

Unfortunately during a process start-up most pump manufacturersdo not provide adequate instruction as to proper seal starting

procedures and rarely is the seal supplier invited to the party.

Overlooking the mechanical seal during start-up can result in failure

rendering the pump useless.

a. Start-Up Procedures: Operating dry is the most common culprit when a pump with a

mechanical seal is first started. While many pumps are self-venting, this does not include

the stuffing box area. To avoid failure from running dry the stuffing box must be relieved

of air as well.

b. Stand-By Pumps: Many critical pump applications incorporate stand-by pumps. Many

mechanical seals incorporate elastomeric secondary sealing members which, when idle for

an extended period will have a tendency to take form, harden, or friction weld to the shaft.

When started these seals fail and excessive seal leakage will result.





2. Identifying Premature Failure & Corrective Actions

As with any piece of process equipment to avoid recurring failure and down time it isimperative the cause of the failure be addressed not merely the symptoms. Scrutinizing the

physical characteristics of failed seal faces and components will uncover the culprit and

assist in determining the corrective actions to be taken.

a. Chemical Attack: Leaves the parts appearing dull, honey combed, flaky, or starting tocrumble or break up. Weight and material hardness readings taken on the damaged parts

will be substantially lower than readings on the original parts.

b. Fretting Corrosion: One of the most common types of corrosion encountered in

mechanical seals. It only causes leakage at the secondary seals but damages the sleevedirectly beneath the secondary seal area. This area will appear pitted and shiny bright.

c. Leaching: Normally causes a minor increase in seal leakage and a large increase in the

wear of carbon faces. Ceramic and tungsten carbide faces that have been leached will

appear dull and matted, even though no coating is present on them

d. Erosion: Seal face may be eaten away or washed-out in one localized area. Erosion will

commonly occur on a stationary seal face until seal face distortion or break down occurs.

e. Heat Checking: Is indicated by the presence of fine to large cracks that seem to radiate

from the center of the seal face. These cracks act as a series of cutting and scraping edges

against carbon graphite and other seal face materials.

f. Vaporization: Any popping, puffing, or blowing of vapors at the seal faces is evidence of

vaporization. Vaporization does not frequently cause catastrophic failure, but it usuallyshortens seal life. Inspection of the seal faces reveals signs of chipping at the inside and

outside diameters and pitting over the entire area.

g. Oxidation & Coking: Leaves a varnish, a lacquer, or an abrasive sludge on the

atmospheric side of the seal. This can cause rapid wear of the seal faces or hang-up in both

pusher and non-pusher types of mechanical seals.

Other Pump & Seal Related Courses...

Process & Industrial Training Technologies6022 Harrison Ave., #3Cincinnati, Ohio 45248

Ph(513) 574-1666 | Fax (513) 574-1358

Return main menu "Pumps & Filtration On-Line"

Additional Information

"Pumps & Filtration On-Line" © 1996 is a publication of Process & Industrial Training Technologies, Inc., Cincinnati, Ohio. The material

presented within this site is for educational and information purposes only and the publisher makes no claim to warranty (expressed or

implied) to its content nor the validity of advertisement, articles, and opinions expressed by the authors. Microsoft Internet Explorer ©1996

Course Components

Component Pricing & On-Line Order Form




fil //C \WINDOWS\ \M h i l S l & P ki Vid B d Ed ti l P k ht 7/13/03

Microsoft and/or its suppliers, All rights reserved.

Documents

seals1.pdf