SHAFT SEALING TECHNOLOGY REVIEW… … AND WHAT'S NEW?

For as long as the technology to transport and control the flow of fluids has been around, the challenge of how to manage leakage has confronted designers and manufacturers of pumps, valves and agitators along with those responsible for their maintenance and upkeep.

Seals are mechanical devices used to control leakage of liquids, solids or gases. The level of technology and effort required to solve a particular shaft leakage problem depends on an analysis of the key variables involved. It is important to have a clear understanding of the leakage tolerance the process will allow. This will obviously take into account the importance of process yield and cost of material as well as the potential for a disaster (e.g. the risk of explosion and flammability) at varying rates of leakage. This is because, in any application other than that involving static seals, absolute zero leakage does not exist and one can only approach zero leakage within the economic parameters and the risk level that the process can tolerate. For instance, one can accept a relatively high leakage rate inflating a bicycle tire with air, but the tolerable level of leakage when pumping gasoline would be considerably lower.

There have been various attempts to define zero leakage. For instance, General Electric's Advanced Technology Laboratories defined zero leakage as less than 10-8 atmospheric cm3/sec. of helium and NASA's Manned Spacecraft Center accepted zero leakage as less than 1.4 x 10-3 standard cm3/sec of GN2 at 300 psig and ambient temperature. The fact remains that both definitions accept a rate of leakage, they only differ in the level of leakage they allow.

Of equal importance to the selection of the sealing technology required, is the consideration of the impact of the leaks on the work site. Poor housekeeping increases the risk to environmental protection and worker health and safety. In some cases, even minor but regular leaks can produce significant cumulative negative effects on the environment and work site health and safety. The level of tolerance to such risks should also be clearly understood and evaluated when selecting the type of sealing technology.

 

Types of industrial seals

To manage rotating or reciprocal shaft leaks, there are many sealing types and materials available. Devices for rotary shafts come under the axial and radial class of seals; these include mechanical seals, braided packing, lip ring seals and split ring seals. This family of seals represents over 80% of all shaft seals in industrial use today and it is designed to work through direct contact between the sealing components and the shaft. There is another family of seals that does its work without direct mechanical contact between the seal components and the shaft; rather it is designed to allow a controlled leakage rate and uses external forces to control the clearance gap through the leak path. The types of seals in this family include visco, labyrinth, bushing and magnetic seals.

 

Types of industrial seals

By far the most common sealing devices used are the braided packing and mechanical seals. The earliest packing in domestic or industrial use probably dated back to about the thirtieth-century BC, where hemp cordage was adapted to deal with joint leakage in bamboo piping systems. The use of modified jute and cotton cordage as packing material subsequently came into use in China, India and later in Europe. Today, braided packing comes in a wide variety of materials, designs, braiding styles and finishes. Some are compounded with different grades of lubricants and others come laminated with other fabrics and materials into different seal formats, such as spirals, rings or coils.

 

Leakage occurs when there are gaps and voids at the joint. The key characteristic of seal packing, therefore is its ability to flow and fill the gaps and voids at the contact surfaces, using external compression force to overcome the counteracting internal pressure of the system. The use of braided packing does not stop leakage; it is designed to only limit the rate of leakage to an acceptable level.

 

The selection process for the proper packing should consider its ability to maintain seal effectiveness under the dynamic condition of the rotating shaft, the continuous elevated temperature due to the compression and the corrosive and chemical action of the fluid stream being handled. In other situations, it is also important to consider the risk of cross-contamination that could arise using certain packing material, relative to the fluid stream being handled.

 

The ability of the packing material to flow under the compression forces is typically measured by its hardness or durameter. Industrial sealing materials are available in durameter numbers from 35 to 95. The lower the durameter rating, the softer the material and the easier the packing would flow under relatively low compression forces; however it may not provide sufficient seal rigidity and strength. On the other hand, while a harder material would provide more rigidity, it may not flow enough to fill the gaps and voids or the risk of heat buildup and scoring the shaft and other components would be unacceptable. It is generally accepted that industrial packing should have durameter numbers in the range of 60 to 90.

 

In most cases where braided packing is used, the seal design must include the use of cooling water (or another compatible fluid) to lubricate and reduce heat buildup due to the compression forces. Without sufficient cooling, the packing material would usually fail prematurely and the seal compromised. It is estimated that for a typical process pump with a 3" shaft, cooling water consumption would be between 3 GPM and 5 GPM. This comes to over 1.5 million gallons of cooling water per pump in a year. In many jurisdictions, this pump and seal design must also consider the capital and running cost of water treatment and discharge.

 

Common packing materials include natural rubber, neoprene, butyl and silicone rubber, Teflon, graphite braiding and certain thermoplastics such as Kel-FTM. Certain manufacturers also offer derivatives of basic material such as the use of chemically blown Cellular Rubber, Buna-N and Buna-S and yet others also add carbon, graphite, PTFE (polytetrafluoroethylene) and a selection of other lubricants and additives. These materials are chosen to provide a formulation to respond to different operating conditions, such as the durameter number required, level of mechanical stress, shaft speed, stuffing box temperature and pressure. At the same time, one also has to pay attention to chemical compatibility to the fluid stream being handled, such as resistance to acids, alkalis, solvents and water under the operating temperature and pressure. Most manufacturers have comprehensive selection charts to help designers and end-users.

 

Mechanical seals

Mechanical seals saw their first applications in the automotive industry for sealing engine coolants and water systems and have since gained wide acceptance in the petrochemical, pulp and paper, utility and other chemical processing industries. Mechanical seals are specified for applications where braided packing is not able to perform reliably and economically. These include operating conditions that specify temperatures up to 1800oF, service pressure up to 5,000 psi or near absolute vacuum, shaft speed up to 50,000 RPM and compatibility to highly unusual chemical environments. Mechanical seals do perform, but they come at a price. It is estimated that, depending on the style and features, mechanical seals are priced at between $700 to $1,200 per inch of shaft diameter. And installation and maintenance will require technical people with a level of training and experience much greater than those handling braided packing; and maintenance parts will also cost more.

 

Mechanical seals do their work using two primary sealing surfaces, one mounted on and rotates with the shaft, the other installed on a stationary surface, such as on the inside face of the gland plate. Under normal operation, the primary seal ring attached to the rotating shaft contacts and rubs against the primary counterseal face of the stationary ring. Continuous seal contact is achieved through the use of mechanical or hydraulic axial forces; springs and elastomeric devices are often used. Mechanical seals also require a combination of secondary rotational and stationary seals to assure sealing effectiveness. The choice of material for the primary and secondary sealing components would follow a decision path similar to that discussed under braided packing.

 

The continuous rubbing action at the seal interface produces heat and requires cooling and lubrication to cut the risk of premature failure, costly downtime and maintenance. As with braided packing, the seal design must include the use of continuous cooling water (or other compatible fluid). While the amount of cooling required here is generally less than that for braided packing, it is estimated that in many applications, the consumption of cooling water would amount to over 3 GPM or nearly a million gallons of cooling water to treat and discharge per year.

 

New-generation injectable sealing compounds

Over the past ten years or so, a new sealing device has emerged and has been quietly gaining acceptance in a select range of applications. The new device is generally referred to as an injectable fluid seal and is designed to be a permanent replacement for braided packing and mechanical seals. It is made with a blend of finely divided colloidal fibers, impregnated with synthetic lubricants. The combinations of fibers and lubricants are designed to satisfy selected ranges of operating temperature, pressure, shaft speed and chemical compatibility.

 

The injectable fluid seal is installed with specially prepared gaskets, referred to as "restrictors", installed at the impeller and gland-end of the stuffing box. Kem-A-Trix^TM (Lubricants) Inc. of Champlain, New York is the original inventor of the device and Kem-A-Trix^TM specifies different "restrictor" materials for compatibility to various operating conditions.

 

When installed in pump, valve and agitator systems, the injectable fluid seal forms a rigid yet compliant lubricated sealing plug that requires only minimal axial pressure to achieve a near zero leakage seal. The use of the injectable fluid seal device, therefore, does not cause any heat buildup nor does it add any abrasion or wear to the shaft and other parts of the sealing system. The injectable fluid seal is designed to run without the use of cooling water.

 

After initial breaking-in and as the injectable fluid seal material settles in and flows to fill the gaps and voids, more sealing material would be required to maintain the seal. To achieve this, one simply injects a small amount of the sealing material on-line (with the use of a specially designed injection tool) and without interrupting the production process. Experience shows that for many applications and with only minor additions on-line, the injectable fluid seal is able to maintain seal effectiveness for much longer and trouble-free than that using braided packing or mechanical seals.

 

The injectable fluid seal represents significant capital savings over the use of mechanical seals, and counting longevity, over that for braided packing as well. The device has shown that, once installed, it will also contribute to productivity improvement and savings in maintenance and repair expense.

 

The injectable fluid seal device has been well accepted in the pulp and paper, building materials, petrochemicals and chemical processing industries. Generally, this technology is compatible to processes with stuffing box temperature of between 0ºF and 500ºF, pressure of under 1,050 psi, shaft speed up to 3,600 RPM and pH of between 0 and 14. The manufacturer will have more resources to evaluate specific applications and recommend specific products and procedures.

 

copyright June 2001 Issue of Fluid Handling Systems