Effective Mechanical Seal Performance Monitoring By: Ron Mumbray, Rick Hull, Kevin Midgley, Raphael Ortega December 2003
A2C2 --
There aren’t many industries where minimizing product loss
and completely eliminating emissions are as critical as the pharmaceutical
industry. As a result, the use of mechanical seals in production facilities is
increasing, since the nature of mechanical seals enables leakage elimination.
Mechanical seals are not inexpensive items—as any maintenance manager will tell
you—and frequently mechanical seals are replaced at pharmaceutical plants on a
regular basis. Yet, if mechanical seals are failing too frequently, it could be
an indicator of a process problem, rather than a parts problem. Oftentimes,
monitoring the performance and replacement of mechanical seals can provide a
pharmaceutical facility with data to help identify and solve these types of
process problems. With that in mind, we have developed some “questions and
answers” that will help you understand the role of mechanical seals in a
manufacturing process, and how perhaps you can set up a “Seal Team” to begin
improving your plant’s efficiency.
I. Mechanical Seal Basics
What is a mechanical seal?
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 range from
hazardous chemicals to process water to food.
What industries have the most use for mechanical seals?
Mechanical seals can be found in hundreds of various applications. Any
industry or manufacturing operation that has a need to pump material from one
place to another probably has some use for a mechanical seal. Some of the more
prominent industries where mechanical seals are used include chemical
processing, pharmaceutical, refineries, food processing, pulp and paper
manufacturing, and steel manufacturing.
What are mechanical seals made of?
The materials used in the design of a mechanical
seal are determined by the conditions under which it will be operating. 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.
What are the different types of mechanical
seals?
There are many options, designs, and materials when
it comes to mechanical seals. The configuration and features right for you will
depend on your 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.
The 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.
What benefits are there to using mechanical
seals?
Tighter control of shaft leakage is the single most
important benefit. 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
What information is needed to determine the right seal for my
application?
Here are some questions that 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?
*the product corrosive?
*kind of pump is used in the process?
*size is the pump? What is the pump’s speed?
wWhat
are the shaft and sleeve sizes?
*the pump cavitate or run dry?
*Which mechanical seals are best for a given application?
The best method of specifying 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
How do mechanical seals differ from regular
packing?
Like day and night. 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.
Is there a life cycle cost benefit to mechanical
seals over regular packing?
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 the most cost-effective way to efficiently
control and seal fluids in a manufacturing process. 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.
Let’s look at the energy savings alone. 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. The cost savings in terms
of energy alone then is $1,752.00.3
II. Mechanical Seal and Pump Failures
What are the initial symptoms indicating there may
be a problem with the seal?
Typically, the first symptom 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 separate contaminants.
What can cause a mechanical seal to leak or fail?
The loss of the liquid film, or lubricant, between
the faces of a mechanical seal is the largest cause of seal failure. This
usually results due to a 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.
What can cause the seal to lose lubrication?
Losing lubrication to the mechanical seal faces can
result from a number of factors. Some of these include dry start up, suction
loss, plugged flush orifice, increased temperature, the wrong balance, or
contamination in the cooler, water jacket, or flush lines.4
Are there instances where the problem could be the
pump rather than the seal?
Actually, this is more frequently the case 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 there’s something
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.
Can improper maintenance cause a mechanical seal
to fail?
Absolutely. 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
What should I look for 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
What can a failed mechanical seal indicate about
the manufacturing process?
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.
What options are there for correcting mechanical
seal failure?
Before you can correct mechanical seal failure, you need to understand what
has caused the failure. By approaching a problem with a concerted, team effort
toward identifying the root cause, much of the guesswork can be taken out of the
process. Often times, 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.
What costs are typically associated with pump and
seal failures?
If a seal fails, it could result in a variety of
costs. First, there’s the loss of revenue due to the loss of product, both from
leakage and ruined batches of material. Also, there’s 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.
III. Mechanical Seal Tracking and Monitoring
How do you set up a mechanical seal tracking
system?
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 dedicated to providing excellent service—one that
understands that prolonging the life of a seal actually enhances their
business.
You’ll then want to 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. The team’s members should include:
*Key
plant management personnel from both the maintenance and operations sides
*Engineering people
*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.
Are special equipment or processes
involved?
Not really. 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. By
utilizing a software program written and designed specifically for mechanical
seal tracking, the Seal Team will have the benefit of timely information that is
pertinent to the cost-saving goals of the team.
How does the process work?
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
cross-referenced 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.
Will it take a lot of time to manage the process?
Under normal circumstances, the Seal Team should
meet on a monthly basis. Outside of that, the only time required is for
recordkeeping on the part of the maintenance personnel in the plant.
What kind of information will be useful?
The more information, the better. Some of the
information you’ll want to keep track of includes:
*Serial number
*In-service date
*Type of seal
*Seal construction material
*“O”-ring type
*Cost of seal
*Seal and pump location
*Current seal life
*Maintenance history (work done, hours spent, etc.)
*Replacements and rebuilds
*Out-of-service date
*Type of pump
*Pump speed
*Size of shaft or sleeve
*Pump operating temperature
*Fluid being pumped
*Operating conditions in the plant
*Reservoirs
*Flow meters
Again, a software platform designed to track mechanical seal performance,
like AnchorSoft, will not only effectively collect this information, but it can
also generate a variety of reports that will help narrow down the root cause of
the seal and/or pump’s failure. For example, AnchorSoft generates several useful
reports, including:
Mechanical Seal Report: Inventories every seal in the facility, and details the seal type,
face combination and “O”-ring, and provides installation date and service
life.
Pump/Seal History Report:
Relates seal performance to pump performance, and details specific pump data,
such as pump maker, size of shaft/sleeve, gland, pump speed, temperature and
fluid type.
Seals in Service Report:
This will indicate the cost per day per seal, the average seal cost per day by
unit, and the average seal days in service by unit. This information helps the
team manage each unit’s process, and reduce costs.
Seals Out of Service Report: Shows seals removed from each unit, the reasons for removal, and
helps track cost per day, and average service time by unit and cost per day by
unit.
Rebuilt Seal Report:
Tracks previous service time, current service time, price of the seal new, and
price rebuilt. The report shows you total savings of using re-built vs. new.
What can I do with this information?
By comparing the information collected in the
tracking system with the physical evidence on a failed seal or pump, the Seal
Team can begin to diagnose the root cause of the failure. What’s more, as
corrections are made in the process, the seal tracking system will allow the
team to establish the life cycle benefit of the various pumps and seals. This is
done by taking a “total cost” approach to the mechanical seals tracking system.
The team will examine the costs involved with new seals, labor, parts,
inventory, and the cost of downtime.
How does the mechanical seal tracking system translate into reduced
costs?
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.
2 McNallyl. “Bill McNally’s Centrifugal Pump and Mechanical Seal
Reference Manual,” The McNally Institute, 1986 S. Belcher Rd., Clearwater,
Florida, 33764, Section TN005 “Mechanical Seal Selection.” Website:
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
