Monitoring mechanical seal performance to reduce fixed costs, Solutions! Online Exclusives, June 2003

Monitoring mechanical seal performance
to reduce fixed costs

By Ron Mumbray, Lino Dimichino, Rick Hull, Kevin Midgley, and Raphael Urteaga of Anchor Seals, Inc.

In the pulp and paper industry, strict control over emissions is critical, and problems can be costly. By providing tighter seals and reducing friction, mechanical seals can be a great solution—but how do you choose the right one for the job?

A mechanical seal is a spring-loaded device that forms a seal between the rotating and stationary parts of different types of equipment, such as pumps, mixers, or reactors. Typically, this equipment will consist of a rotating shaft and a stationary housing. The mechanical seal’s purpose is to minimize leakage of fluids and gases. Depending on the manufacturing process and application, these materials could include hazardous chemicals, process water, or even food.

Any industry or manufacturing operation that needs to pump material from one place to another probably has some use for a mechanical seal. In addition to pulp and paper manufacturing, some of the more prominent industries that use mechanical seals are chemical processing, pharmaceutical, refineries, food processing, and steel manufacturing.

The materials used in the design of a mechanical seal are determined by the operating conditions. The body of a mechanical seal is typically made from stainless steel. The wearing, or contact face of the seal, can be made from a variety of corrosion-resistant materials, such as carbon, glass-filled Teflon®, tungsten carbide, and silicon carbide. The other face, the hard face, can be made from ceramic, niresist, tungsten carbide, or silicon carbide. One face rotates, while the other remains stationary. Typically, the nature of the process, the pressure and velocity of the pump, and the temperature the seal is operating in will determine the seal face material. There is also usually a shaft or sleeve packing. The material used in this element of the seal could be "O"- rings, Teflon wedges, metal bellows, rubber bellows or elastomers such as Viton®, EPR, Neoprene, or Grafoil® packing.

Choosing a mechanical seal
There are many options, designs and materials when it comes to mechanical seals. The configuration and features that are most effective depend on the application. Perhaps the simplest, yet most traditional type of mechanical seal is the component seal, which is often mounted on the inside of the pump housing, or stuffing box.

Single cartridge mechanical seals are used in processes using non-hazardous, non-corrosive materials. A single cartridge seal is usually located outside the pump housing, and is not exposed to the material or product being pumped.

Double cartridge mechanical seals are mounted separately on the same shaft, outside of the housing, or stuffing box, and provide maximum sealing for potentially hazardous materials, such as slurries, acids, and volatile organic liquids.

Single and double cartridge seals can be designed with a number of features. They come with either single or multiple springs, or bellows. In addition, both the single and double cartridge seals are available in a split design, where the seal comes in two parts that are assembled to surround the shaft. These are mostly seen on large split case pumps.

Tighter control of shaft leakage is the single most important benefit to using mechanical seals. In industries where strict control over emissions is critical—and costly—mechanical seals are the answer. In addition, the reduced friction that occurs when using mechanical seals reduces wear, heat generation and energy consumption.¹

These questions will help identify the proper mechanical seal for a given application and process:

• What is the temperature range for the process?
• What is the pressure? Suction? Discharge? Stuffing box?
• Is the product viscous? Does it crystallize, solidify, freeze or build up film?
• Is the product corrosive?
• What kind of pump is used in the process?
• What size is the pump? What is the pump’s speed?
• What are the shaft and sleeve sizes?
• Does the pump cavitate or run dry?

The best way to specify the right mechanical seal is to work in cooperation with a knowledgeable representative from your mechanical seal supplier. Take some of the information you’ve developed from the list above, and work with your sales representative to identify the right product and design features.

Generally speaking, a basic, single-cartridge balanced mechanical seal is adequate for applications where stuffing box pressure is less than 300 pounds per square inch (psi), the temperature is less than 400°F, and the shaft size is from 1 inch to 4 inches. Conditions in excess of these parameters, along with other requirements, might necessitate a special seal design. Double cartridge seals are best in processes where temperature and emission control are critical factors. This is typically the case when working with material that is toxic, radioactive, explosive, or is categorized as a pollutant.²

Mechanical seals vs. conventional packing
Mechanical seals are very different from conventional packing. Conventional packing requires a lubricant so as not to burn up while the shaft is turning on the equipment. Often the material being pumped serves as the lubricant. However, as a result, packing needs to leak a little to function properly. Mechanical seals, on the other hand, can achieve practically a leak-free environment. With tighter environmental emission regulations placed on manufacturers, this becomes even more important.

Packing requires a higher level of attention and maintenance than mechanical seals. Packing must be adjusted and repacked periodically. What’s more, packing tends to wear on pump parts, cutting the shaft or sleeve. With a static "O"-ring mounted to the shaft, mechanical seals will not damage the shaft or sleeve.

Initially, a mechanical seal will cost significantly more than packing. However, when all of the maintenance and operation costs are taken into consideration, mechanical seals are generally more cost-effective. With packing, there are costs involved in repacking the pump at least twice a year and downtime for repacking. Because packing wears and cuts into the sleeve of the pump, there are potential costs in replacing any worn sleeves. There are also costs from the loss of product that comes from increased leakage with packing. If the product being pumped is hazardous, it must have piping to run to a drain and be treated. What’s more, because of the higher rate of friction, packing consumes more electricity than mechanical seals. If maintained properly, the life cycle of a mechanical seal can be two to three times that of packing.

Energy savings alone can be significant. Presume that 10 percent of a 30 HP motor’s horsepower is used in friction against the packing, and one HP equals one kW of energy. The packing then uses three kW per hour of electricity, and at a rate of 8 cents per kW, times the number of hours in a year (8760), the cost of energy using packing is $2,102.40. A mechanical seal uses only 1/6 of the electricity, which would result in a cost of $350.40. In that situation, for one motor, the cost savings in terms of energy alone then is $1,752.00.³

Mechanical seal and pump failures
Typically, the first symptom of a mechanical seal failure will be some level of leakage. This could range from a small tight spray on the outside of the seal that can be felt on the hand to a full spray out of the back of the seal to a continual drip. Another common symptom is a squealing sound that results when the seal cannot maintain a barrier/buffer fluid. Depending on the type of seal and the American Petroleum Institute (API) piping plan, symptoms could also include leaking of process water into the product or material being pumped. This may dilute the process, or it must be treated further down into the system to separate contaminants.

The largest cause of seal failure is loss of the liquid film, or lubricant, between the faces of a mechanical seal. This usually results from lack of maintenance on the seal’s lubricating parts. Sometimes a seal fails because it wasn’t the right type of mechanical seal for the application. Other causes include improper assembly or installation, cavitation, and improper operation of the pump itself.

Losing lubrication to the mechanical seal faces can result from a number of factors. Some of these include dry start up, suction loss, a plugged flush orifice, increased temperature, the wrong balance, or contamination in the cooler, water jacket, or flush lines.⁴

Pump problems are more frequently the cause of trouble than the seal failing on its own. A mechanical seal is probably the weakest link in the process "chain." As a result, the seal functions like an electrical fuse—the seal will go first to save the pump. Usually, a seal failure is an indication that something is not functioning properly in the pump or the process. Therefore, any upset in the pump or system could result in a seal failure. If the right seal is specified for a given application, failures are mostly related to a pump or process issue. If a seal is installed on a pump, and when the pump is turned on the seal does not leak, it should remain that way for at least two years.

There are many pump problems that could cause a seal to fail:

• The shaft is bending due to side load on the impeller
• The valves are not opened in the proper sequence
• Strain on the pipe
• The pump is out of alignment

In addition to pump problems, sometimes a system issue may be the cause of seal failures. The characteristics of the fluid, the environment, temperature variations, the valve/piping arrangement, filters or strainers, and maintenance practices can all be causes of system, and therefore, seal failure.
Improper maintenance can also contribute to mechanical seal failure. Excessive pump shaft movement, which would cause a seal to fail, can occur when the pump is not routinely leveled and aligned, when there is excessive pipe strain, when the entire rotating assembly in addition to the impeller is not balanced, when the shaft and bearings are damaged during sleeve removal, and when damage occurs to the impeller when it is removed. Mishandling or improper installation of the seal itself, and use of the wrong lubricant, can also be a factor in mechanical seal failure. Plus, if the bearings are not properly maintained, if they are over-greased, or if the oil becomes contaminated, the seal can fail.⁵

When examining a failed seal, look for these "tell-tale" signs:

• Evidence of corrosion
• Unusual wear patterns on the seal faces
• Evidence of rubbing or wear on components that should not be touching
• Discoloration of any of the seal components, especially metal parts
• Springs, set screws and drive lugs that are missing or loose
• Product attaching to a rotating component⁶

It may sound morbid, but examining the physical evidence on a failed mechanical seal is much like a coroner’s autopsy in a murder case. The type of wear or marks on a seal can provide solid clues about what’s going on in the system. For example, if there is scoring on the seal face, it could be an indication of contamination, abrasive particles between the seal faces, or a dirty environment. If there is chipping on the seal face, it could be a symptom of vibration or cavitation. Blisters on the seal face could be an indication that the seal is incompatible with the process. Perhaps the speed, temperature or pressure of the process requires a different type of seal.

Before you can correct mechanical seal failure, you need to understand what has caused the failure. Often, the seal itself will provide tangible evidence of what is occurring in the pump. By inspecting the seal’s rotary equipment and faces, one can tell if the cause of the failure is related to the pump or the seal. Good record keeping can identify process problems and eliminate future seal failures.

If a seal fails, it could result in a variety of costs. First, there is the loss of revenue due to the loss of product, both from leakage and ruined batches of material. Also, there is the loss of revenue due to the downtime to take apart and repair the pump, and replace the seal. Then, there are the costs associated with changing out the pump. It could take as long as 24 hours to change out one pump depending on the size. The pump might need to be rebuilt, costing thousands of dollars. And, the seal itself will need to be rebuilt and/or replaced. Large leaks could lead to costs involved with violating environmental permits. So, there’s a lot at stake when pumps fail.

Mechanical seal tracking and monitoring
The first thing you’ll need is the strong commitment of the plant’s management and maintenance personnel toward building a tracking system that will result in reduced costs. Secondly, you’ll want to work with a mechanical seal supplier that offers excellent service. You can then establish a "Seal Team." The group should have strong technical expertise, and be interested in cooperatively working together toward the benefit of the plant’s efficiency. Team members should include:

• Key plant management personnel from both maintenance and operations
• Engineering personnel
• Purchasing personnel
• Representatives from the facility’s mechanical seal supplier

The team can then establish its own mission and "next steps." More than likely, the first item the team should address is a complete survey of the plant, identifying where mechanical seals are and/or should be used. Then, the team can identify the "top ten" highest cost pumps in the facility. A method of tagging old and new equipment should be agreed upon, and the team should determine what information regarding the process will need to be monitored, and how that data will be collected.

No special equipment is involved; a mechanical seal supplier accustomed to establishing and developing tracking systems will have the right tags and devices. However, a good software platform can make the team’s job much easier. A software program written and designed specifically for mechanical seal tracking provides timely information that is pertinent to the cost-saving goals of the team.

As pumps and seals are maintained, repaired, put into service, or taken out of service, all of that information is recorded by the plant’s maintenance personnel, and forwarded on to the mechanical seal supplier. The supplier maintains a comprehensive database that is crossreferenced with all of the seals and pumps in the facility. On a regular basis, reports that "red flag" problem areas are reviewed. The team then goes through a "fishbone" process analysis to identify potential problems and causes. It is critical toward solving the process problem that the team relates the physical evidence—the condition of the failed seal—to the application and system data that is recorded in the database.

For example, take an instance where a plant has twenty Gould 3196MT pumps all using the same 1.750" seal. If eighteen of the pumps run for 14 months and two of the pumps run for three months each, chances are there is another variable that is causing the reduced operating time. The team can then examine other issues, such as operating conditions, temperatures, fluid viscosities, and other "evidence" to help pinpoint the cause of the problem.

An effective mechanical seal tracking system will enable a facility’s maintenance personnel and management to make smart, technically-sound decisions about their manufacturing operations. What’s more, it holds the seal supplier accountable for its products. And, rather than regarding the frequent replacement of seals as a part of the process, and therefore budget, an effective tracking system will enable the plant’s management to pinpoint trouble spots, reduce maintenance costs, improve efficiency, reduce downtime, and save money.

CASE HISTORY:
Saving money at the U.S. Steel Clairton Works
Located 20 miles south of Pittsburgh, Pennsylvania, the Clairton Works of U. S. Steel Corporation is the largest coke plant in the United States. Clairton Works is comprised of 12 coke oven batteries that annually convert about 6.6 million tons of coal into 4.7 million tons of furnace coke for the steelmaking process. That translates to about 13,000 tons of coke per day. Approximately 1,700 people work at theplant.⁷

The coking process generates several chemical by-products. More than 225 million cubic feet of coke oven gas is generated per day. The coke oven gas is processed to recover coal chemicals through a series of unit operations for specific by-product/chemical recovery. Through a variety of recovery, and separation processes, the plant produces tar, anhydrous ammonia, light oil, and sulfur that is processed for sale to customers.⁸

The problem
At more than $430,000 per year, expenditures for mechanical seals at the Clairton facility had historically been high. It had been plant practice that if a seal failed, it was simply replaced, and forgotten, until it failed or leaked again.

Senior purchasing agent George Scull recognized this as both a problem and an opportunity when he was assigned responsibility for seal procurement at the plant in 1995. "I was frustrated that we were spending large amounts on mechanical seals, and the real issues weren’t being addressed," said Scull. "Too often, failed seals were being replaced without identifying the root cause of the problem."

What’s more, Scull believed there were too many different suppliers being called on to provide sealing solutions for the facility. "I’m a big believer in single sourcing," said Scull. “With one supplier, a manufacturing plant can command better pricing based on volume. But more importantly, by partnering with one supplier, there’s a lot of opportunity for value-added service that comes with it."

The solution
Scull and a group of key personnel from Clairton’s operations and maintenance groups visited with their mechanical seal distributor, and the manufacturer. They also visited and benchmarked what other chemical plants had done to reduce costs.

Eventually, Scull recommended formation of a Seal Team. Along with that, Scull suggested consolidating mechanical seal purchases through one distributor. The facility selected Anchor Seals, Inc., because of the company’s willingness to work in partnership with the plant to help solve process problems, and reduce costs. Also, Anchor Seals developed a software program that would help manage the tracked information and help focus the plant’s efforts.

After carefully reviewing Scull’s proposal, the plant managers agreed to form a Seal Team. The group started by establishing tagging, monitoring, and tracking procedures. They also developed a system for reporting problem pumps and seals. Today, the Clairton Works’ Seal Team consists of approximately a dozen representatives from different functions within the plant, and a representative from the mechanical seal distributor. Run by shift manager Joe Hann, the team meets on a monthly basis to review problem seals and pumps, potential causes, solution ideas, successes and failures.

The results
Scull says the real measures of the effectiveness and cost savings associated with the Seal Team’s efforts are the average seal run life and the average cost per day. When a seal is taken out of service for any reason, its number of days in use is totaled to arrive at the run life. The seal’s original cost is then divided by the run life to get a seal cost per day.

In 1997, the Clairton facility’s average seal run life was102 days. By 2001, that number tripled to an average of 386 days. As a result, average seal cost per day decreased from $31 to less than $7 in 2001. That’s a reduction of more than 78 percent.9

Another indicator of success is the number of pumps repaired per year. In 1996, over 330 pumps were repaired, and by 2000, that decreased by 66 percent to 130 per year.10 When the Clairton plant’s Seal Team was formed in 1996, expenditures for replacement mechanical seals were $433,538, including those seals that were put into inventory or bought as spares. By 1999, that number dropped to $269,536.11 In 2001, mechanical seal expenditures increased to $410,464, but as Scull explains, that is only because the company began to see the potential for further cost-savings, and began to upgrade certain seals.

Scull says that there are many other instances of cost savings and efficiency that have not been so easy to track, such as reduced downtime, lower maintenance costs, and less product loss. Overall, Scull estimates the annual savings to be well in the hundreds of thousands of dollars.

ENDNOTES

1. European Sealing Association, Bowerham House, The Grove, Lancaster LA1 3AL,United Kingdom, ESA web site, found at http://www.europeansealing.com/divisions/mechanical_seals.htm
2. McNally, Bill; The McNally Institute, 1986 S. Belcher Rd., Clearwater, Florida, 33764; Bill McNally’s Centrifugal Pump and Mechanical Seal Reference Manual, Section TN005"Mechanical Seal Selection" found at http://www.mcnallyinstitute.com/CDweb/narratives/tn005.htm
3. "Mechanical Seal Basics for Maintenance," PPC Mechanical Seals, Section 1, pg. 5
4. "Mechanical Seal Basics for Maintenance," PPC Mechanical Seals, Section 12, pg. 2-3
5. McNally, Bill; The McNally Institute, 1986 S. Belcher Rd., Clearwater, Florida, 33764; Bill McNally’s Centrifugal Pump and Mechanical Seal Reference Manual, Section GT004"Maintenance Practices That Cause Seal and Bearing Problems".
6. McNally, Bill; The McNally Institute, 1986 S. Belcher Rd., Clearwater, Florida, 33764; Bill McNally’s Centrifugal Pump and Mechanical Seal Reference Manual, Section ST007"Inspecting the Individual Seal Components for Damage".
7. U. S. Steel web site; http://www.ussteel.com/corp/facilities/clairton/index.htm
8. U. S. Steel web site; http://www.ussteel.com/corp/facilities/clairton/by.htm
9. U. S. Steel, Clairton, Pa., Works
10. U. S. Steel, Clairton, Pa., Works
11. U. S. Steel, Clairton, Pa., Works

Teflon ® is a registered trademark of E.I. du Pont de Nemours and Company. Only DuPont makes TEFLON®.

Viton ® is a registered trademark of DuPont Dow Elastomers L.L.C. All rights reserved.

Grafoil ® is a registered trademark of UCAR Company.

This white paper was developed by Ron Mumbray, Lino Dimichino, Rick Hull, Kevin Midgley, and Raphael Urteaga of Anchor Seals, Inc., with the assistance of Maurer Communications, Inc.

For more information, visit www.anchorseals.com.

Copyright © 2002 Anchor Seals, Inc. All rights reserved.