Food Processing Motors

Notes regarding Mechnaical Seals for Rotating Equipment:

Thursday, January 24, 2008
Effective Mechanical Seal Performance Monitoring
By: Ron Mumbray, Rick Hull, Kevin Midgley, Raphael OrtegaDecember 2003
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.
Notes: Teflon® is a registered trademark of E.I. du Pont de Nemours and Company
Viton ® is a registered trademark of DuPont Dow Elastomers L.L.C.
Grafoil ® is a registered trademark of UCAR CompanyPosted by Sajju Prince at 12:16 AM

Food Processing Motors

Thursday, January 24, 2008

Mechanical Seals for Food Processing Applications

Saturday, December 22, 2007

Food Processing Motors

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