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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

End-suction centrifugal pump1. 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 accommodate temperature 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.

Double suction split-case pump2. 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 a stuffing box, pump out vanes are use at the rear of an impeller to reduce the pressure at flow seen at the stuffing box bore.
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.

High pressure/temperature packing configuration

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, when appropriate, can also reduce and help control stuffing box pressure.
d. Vacuum Conditions:

Compression packing
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:

II. Mechanical Shaft Seals

Single inside pusher mechanical seal 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 thorough understanding 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 in seal 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 sealing environment 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 varioAmerican Seal designs and materials.

III. Auxiliary Mechanical Seal Systems

By-pass flush arrangement1. 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. Many auxiliary 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 would be 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.

IV. Mechanical Seal Troubleshooting & Failure Analysis

1. Start-Up & Stand-By ConsiderationsVaporization at mechanical seal faces

Unfortunately during a process start-up most pump manufacturers do 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 is imperative 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 to crumble 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 sleeve directly 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 usually shortens 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.

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"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 Microsoft and/or its suppliers, All rights reserved.