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A crucial step in preventative maintenance in pumps is finding and using the correct seal for the job. Mechanical seals are used to seal the rotating shaft in pumps and other equipment. With so many mechanical seal designs on the market, it can be difficult to identify which kinds to use. A.W. Chesterton Company&#;s mechanical seal design utilizes 5 design principles to maximize its sealing capabilities. These principles have been refined by Chesterton since the company brought its first mechanical seal to market in . After all the lessons learned from successes and failures, they have determined the Five Key Features that a mechanical seal should have to reduce unplanned maintenance and downtime. To provide the greatest mean time between failures (MTBF) and avoid downtime, they utilize these five design principles in their sealing devices.
Typically, within early mechanical seal designs, the springs were submerged within the fluid. However, this can lead to trouble when fluids with dirt or other contaminates interact with the springs. When the contaminates collect within the spring, it cannot correctly maintain the alignment necessary for a complete seal. Chesterton&#;s mechanical seal design protects the springs within the mechanical seal from the fluids being pumped.
The second design feature and open secret of Chesterton&#;s mechanical seal design is balance. On the mechanical seal, there are two main sides, the fluid side, and the atmospheric side. To prevent moisture from leaking out of the seal, both sides put equal pressure on each other to trap the fluid. Many seals will often see that the fluid side exerts greater pressure than the atmospheric side. Not only does this reduce the quality of the seal, but it can also ruin the pump with time. Balanced seals reduce the seal ring area on which the hydraulic pressure of the liquid in the pump acts. This allows for better lubrication that results in lower heat generation, face wear, and power consumption. Balanced seals will generally have higher pressure ratings than unbalanced seals.
Fretting on a pump shaft refers to the way that a material is worn away on a shaft by an elastomeric material. An example may be a 316 stainless steel shaft sleeve that is damaged by a Viton O-ring. Stainless steel protects a shaft from corrosion by forming a chrome oxide layer on the surface. The dynamic O-ring wears that protective surface away and it then reforms. Over time the chrome oxide gets imbedded into the O-ring and works like sandpaper removing even more material. The result is a groove worn into your expensive sleeve and a leaking mechanical seal. Chesterton seals utilize designs that have any dynamic O-rings operating against a non-metallic surface, usually the sacrificial face.
Additionally, some manufacturers offer seals that are self-fretting, rather than shaft-fretting. What this means is that these seals do not fret or damage the shaft, they harm seal itself. While this protects the pumps, the O-rings still work against metal, reducing the lifespan of the seal itself.
Some mechanical seal designs utilize an inserted seal face, typically made of carbon or graphite inserted into a metal holder. However, the disadvantage to this technique is these non-metallic materials often react to heat differently than the metal of the seal. The result is the face of the seal is easily deformed- leading to leaks and early replacement of the seal. Proper mechanical seal design uses monolithic seal faces without using a holder and inserted face. By using Finite Element Analysis (computer modeling), Chesterton&#;s monolithic seal face designs are made more efficiently than ever.
Mechanical seals can either use rotating or stationary springs. With rotary mechanical seals, it is important that the stuffing box face is perpendicular to the shaft for the faces to stay closed. There will always be some resulting misalignment from installation and parts tolerances so this alignment cannot be assured. To compensate for the misalignment, the springs must adjust each time the shaft rotates to keep the seal faces closed. At motor speeds this adjustment happens thousands of times per minute (for rpm, adjustments occur) which is not only difficult to accomplish, the springs will fatigue and fail causing seal failure. Rotary seals are simple in design which makes them inexpensive
In contrast, a stationary seal is designed in such a way that the springs do not rotate with the pump shaft; they remain stationary. Because the springs do not rotate, they are unaffected by the rotation of the shaft or how fast the shaft rotates. The springs do not need to correct or adjust with each rotation; they adjust for misalignment only once when installed. Chesterton uses this as the fifth design requirement, and this provides greater life to their mechanical seals.
Using these five design principles, Chesterton has updated its 150 seals to the new seal. The new design uses a monolithic seal face and changes where the dynamic O-rings are placed to be a non-fretting seal. Chesterton unveiled this updated seal in the US in September of . However, Chesterton also has one additional add-on for this new seal. Starting at the end of October , Chesterton will be releasing the L. The L has all the same design capabilities of the , the difference between the two is the installation process. The L utilizes a lock ring mechanism to simplify installation requiring the installer to tighten only one screw to connect the seal to the shaft.
To request a demonstration for the L mechanical seal, click here. Learn more about Northwest Pump&#;s mechanical seals by contacting us here, emailing us at , or by calling 1-800-452-PUMP.
For service on your mechanical seals by our Chesterton certified pump technicians, contact us here, us at , or call 1-866-577-.
Mechanical seal designs come in a wide variety. These designs differ in the way they are driven, the choice of secondary seals, and the location of the springs. Mechanical seals have all been designed to meet certain criteria in terms of application, cost, fit, ease of installation, and capability to seal. There are, however, a few basic characteristics that are common to any good mechanical seal design.
Many early mechanical seal designs placed the spring inside the process fluid. Most products (process fluids) that are sealed are not very clean. When the spring mechanism of the mechanical seal is immersed in this unclean fluid, dirt collects between the springs. This situation eventually impacts the spring&#;s ability to respond to movements and vibrations, and the ability to keep the seal faces closed. Over time, clogging of the springs will cause premature seal failure.
The ideal design offers springs on the atmospheric side of the mechanical seals. The springs will be protected from the process fluid and their ability to work will not be impeded.
The pressure from both the seal springs (Ps) and the hydraulic pressure of the liquid in the pump (Pp) provide a compression force that keeps the seal faces closed. Balanced seals reduce the seal ring area (Ah) on which the hydraulic pressure of the liquid in the pump (Pp) acts.
By reducing the area, the net closing force is reduced. This allows for better lubrication that results in lower heat generation, face wear, and power consumption. Balanced seals typically have higher pressure ratings than unbalanced seals.
For more information, please visit seal manufacture.
The balance ratio (B) of a mechanical seal is given by the ratio of the hydraulically loaded area (Ah) and the sliding surface area (As):
Mechanical seals can be designed with inserted seal faces or with monolithic seal faces. In both cases, the sacrificial seal face is often made from carbon/graphite. This material offers good running properties but is relatively weaker from a mechanical standpoint than other options. Inserted face designs use a metal rotary holder to transmit the shaft torque to the seal face.
The disadvantage of this inserted face design is that the face and holder material have different coefficients of thermal expansion. This changes the net interference force between both parts when they are exposed to heat from the process fluid or face friction. The seal face deforms, which results in leakage and accelerated wear.
More modern seals are equipped with monolithic seal faces that are made out of only the seal face material itself. The torque transmission is applied directly to the seal face. This is possible if the geometry of the seal face is designed in a particular shape to give it the strength to handle the torque through its geometric design. These monolithic seal face designs have been made possible through the use of Finite Element Analysis (computer modeling).
Monolithic seal faces provide a more stable fluid film between the faces, and they do not deform in operation compared to inserted faces (or to a much lesser degree). Therefore, they are more commonly used nowadays when reliability and low emissions are vital.
All mechanical seal designs have at least one secondary seal that interacts with the dynamic movement of the flexible mounted face. This secondary seal moves with the springs to keep the seal faces closed and is defined as the dynamic secondary seal. During operation of a rotary design, springs will keep the seal faces closed. They adjust with each rotation for any misalignment from installation and parts tolerances. As the springs compensate, the dynamic secondary seal moves back and forth, twice per revolution. This rapid movement prevents the protective chrome oxide layer (the layer that protects the metal) from forming. Erosion of this unprotected area under the dynamic secondary seal will cause a groove to develop. Eventually this groove becomes so deep that O-Ring compression is lost and the seal leaks. In most cases, fretted shafts must be replaced to achieve an effective seal.
Non-fretting seals are designed so that the dynamic secondary seal rides on a non-metallic surface, usually the sacrificial face.
A rotary mechanical seal has the spring mechanism in the rotating section (A) of the mechanical seal.
With rotary mechanical seals, it is important that the stuffing box face is perpendicular to the shaft for the faces to stay closed. There will always be some resulting misalignment from installation and parts tolerances. The springs must adjust with each rotation to keep the seal faces closed. This adjustment becomes more difficult at higher speeds.
In contrast, a stationary seal is a mechanical seal designed in such a way that the springs do not rotate with the pump shaft; they remain stationary. Because the springs do not rotate, they are unaffected by rotational speed. The springs do not need to correct or adjust with each rotation; they adjust for misalignment only once when installed.
Rotary seals are simple in design which makes them inexpensive. They are suitable for lower speeds only. Stationary seals are more complicated to design but are suitable for all speed ranges. Because of design complexity, stationary seals are more commonly configured as cartridge seals rather than component seals.
Marco Hanzon is Vice President of Global Marketing for A.W. Chesterton Company. He has been an active member and past chairman of the Mechanical Seal Committee of the European Sealing Association. Marco's experience includes working as an In-Field Support Engineer for mechanical seals.
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