This article will explain what a mechanical seal is and the key design features that make them work.
A mechanical seal is simply a method of containing fluid within a vessel (typically pumps, mixers, etc.) where a rotating shaft passes through a stationary housing or occasionally, where the housing rotates around the shaft.
When sealing a centrifugal pump, the challenge is to allow a rotating shaft to enter the ‘wet’ area of the pump, without allowing large volumes of pressurized fluid to escape.
To address this challenge there needs to be a seal between the shaft and the pump housing that can contain the pressure of the process being pumped and withstand the friction caused by the shaft rotating.
Before examining how mechanical seals function it is important to understand other methods of forming this seal. One such method still widely used is Gland Packing.
Gland packing is a braided, rope like material that is packed around the shaft - physically stuffing the gap between the shaft and the pump housing.
Gland packing is still commonly used in many applications, however increasingly users are adopting mechanical seals for the following reasons;
Mechanical seals are designed to overcome these drawbacks
A basic mechanical seal contains three sealing points.
The stationary part of the seal is fitted to the pump housing with a static seal –this may be sealed with an o-ring or gasket clamped between the stationary part and the pump housing.
(Highlighted in red below, left the stationary part and right the rotary portion)
The rotary portion of the seal is sealed onto the shaft usually with an O ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft.
The mechanical seal itself is the interface between the static and rotary portions of the seal.
One part of the seal, either to static or rotary portion, is always resiliently mounted and spring loaded to accommodate any small shaft deflections, shaft movement due to bearing tolerances and out-of-perpendicular alignment due to manufacturing tolerances.
While two of the sealing points in a seal design are simple static seals, the seal between the rotating and stationary members needs a little more consideration. This primary seal is the basis of all seal design and is essential to its effectiveness.
The primary seal is essentially a spring loaded vertical bearing - consisting of two extremely flat faces, one fixed, one rotating, running against each other. The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. In this way a seal is formed to prevent process leaking between the rotating (shaft) and stationary areas of the pump.
The surfaces of the seal faces are super-lapped to a high degree of flatness; typically 2-3 Helium light-bands (0.00003” / 0.0008mm).
If the seal faces rotated against each other without some form of lubrication they would wear and quickly fail due to face friction and heat generation. For this reason some form of lubrication is required between the rotary and stationary seal face; this is known as the fluid film
In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces. This film can either come from the process fluid being pumped or from an external source.
The need for a fluid film between the faces presents a design challenge – allowing sufficient lubricant to flow between the seal faces without the seal leaking an unacceptable amount of process fluid, or allowing contaminants in between the faces that could damage the seal itself.
This is achieved by maintaining a precise gap between the faces that is large enough to allow in a small amounts of clean lubricating liquid but small enough to prevent contaminants from entering the gap between the seal faces.
The gap between the faces on a typical seal is as little as 1 micron – 75 times narrower than a human hair. Because the gap is so tiny, particles that would otherwise damage the seal faces are unable to enter, and the amount of liquid that leaks through this space is so small that it appears as vapor – around ½ a teaspoon a day on a typical application.
This micro-gap is maintained using springs and hydraulic force to push the seal faces together, while the pressure of the liquid between the faces (the fluid film) acts to push them apart.
Without the pressure pushing them apart the two seal faces would be in full contact, this is known as dry running and would lead to rapid seal failure.
Without the process pressure (and the force of the springs) pushing the faces together the seal faces would separate too far, and allow fluid to leak out.
Mechanical seal engineering focuses on increasing the longevity of the primary seal faces by ensuring a high quality of lubricating fluid, and by selecting appropriate seal face materials for the process being pumped.
When we talk about leakage we are referring to visible leakage of the seal. This is because as detailed above, a very thin fluid film holds the two seal faces apart from each other. By maintaining a micro-gap a leak path is created making it impossible for a mechanical seal to be totally leak free. What we can say, however, is that unlike gland packing, the amount of leakage on a mechanical seal should be so low as to be visually undetectable.
A recent case study indicated - Upgrading from packing reduces water usage and operating costs.
If you want to find out more about increasing the operating life of your seals, watch our video series...
2019-04-16
In the world of mechanical engineering, seals feature predominantly. They are important devices because they function to prevent or limit any form of leakage of fluids. Furthermore, they have a crucial role when it comes to system performance seals are very common in applications that have components that do not have movements that are significantly different from each other. They are also used in applications where there is a significant movement resulting from rotation or reciprocation. The function of a mechanical seal is to provide a seal at the inlet or outlet of a rotating shaft. Usually it is used to prevent a high pressure fluid from leaking into a low pressure fluid.
The primary purpose of these mechanical oil seals is to reduce, prevent or limit any potential flow that may arise between components of a machine. They serve in locations where machine designs have pressurized fluids that need containment within defined areas such as hydraulic cylinders.
Whether you are thinking about the oil seal or the mechanical seal, one thing that should come to mind is that they serve a rather similar role. Mechanical and oil seals prevent fluids that have a tendency of seeping to other parts of the machine that keeps the machine components properly lubricated. These seals are very popular when it comes to various mechanical engineering applications. The question that lingers is do they have any differences? To be in a better position of understanding the difference between oil seal and mechanical seal we are going to look at features that make each one of them distinct from the other.
Oils seals are also known as rubber oil seals, dirt seals or oil gaskets. These are devices used in preventing leakages associated with lubrication of oil in mechanical components or equipment. They function by closing spaces that exist between moving and stationary components. Furthermore, these seals are useful in precision bearings and they help in maintaining lubricants, which prevents corrosive moisture that may seep into the bearings.
Oil seals exist in various shapes and sizes. This depends on the nature and capacity of particular equipment in use. The seals have been manufactured in such a way that they fit into different types of valves, pipes or other openings that might allow leakage of lubrication of oils. The difference between oil seal and mechanical seal is that they are useful in two main functions, that is, they help to hold the oil in place. In addition, they facilitate the lubrication of machine moving parts thus ensuring that no lubricant leaks. Dirt and harmful contaminants at times leak into various parts of mechanical equipment. For this reason, engineers use oil seals to ensure that these harmful contaminants do not get into the equipment.
Regarding the mechanical seal, the basic description of it is that it’s a method used to contain fluid within a mechanical vessel, for example in pumps and mixers. Essentially, the pumps and mixers among other equipment of this nature consist of a system that comprises of a rotating shaft that passes through various stationary housing. In some cases, the housing rotates around the shaft.
The difference between oil seal and mechanical seal is that they are very useful when it comes to sealing rotating equipment. Engineers use them in sealing the process side of different equipment like agitator and pump. Mechanical seals come with a certain degree of variations. For instance, we have double seals and single seals. All these variations depend on the applications one may choose to use. A good example for water and other liquids that require low-pressure applications the single seal is the one that is commonly used. On the other hand, engineers use double seals in cases where there is slurry media, hazardous media, and even high-pressure applications.
When it comes to design the difference between the oil seal and the mechanical seal (mechanical seal vs oil seal) is that mechanical seals have three sealing points. Engineers fit the stationary part of the seal to the pump’s housing using a static seal. While they use the use an O ring to seal the rotary portion to the shaft. This makes the mechanical seal itself be the existing interface between the static and rotary portions respectively. When mounting, one part of the seal maintains a resilient stance. This can be either static or the rotary portion to facilitate movement and accommodate any tiny shaft deflections that may arise.
The fluid film is another major aspect that presents the difference between oil seal and mechanical seal. Essentially, most of the mechanical seals require a certain level of lubrication. The necessity of having lubrication is made possible by the presence of a thin film of fluid between the seal faces. The source of the film comes either from an external source or from the pumped process fluid. Even though this tends to work, one of the key challenges associated with this arrangement often arises. This challenge revolves around how to allow sufficient lubricant to flow without having the unacceptable or risky amount of process fluid or contaminants leaking. Nevertheless, engineers agree that it is possible to curb this challenge by ensuring that there is a precise gap between the faces. The gap must be large enough to allow adequate amounts of lubricants and at the same time, small enough to restrict any amount of contaminants from entering the seal faces.
There are differences between oil seal and mechanical seal in its usefulness. Oil seals play a critical role in preventing seeping in of contaminants that are potentially hazardous getting into the machine. They are safeguard mixing of two liquids, for example, water and oil. Oil seals are sturdy and long lasting due to their sturdy nature. They have low rates of wear and tear promotes system durability. On the other hand, machine seals are useful in that they reduce bearing contamination because they ensure that the lubricant does not get affected by any potential seal leakages. Their inboard springs reduce the need for day-to-day maintenance requirements. They also reduce rates of corrosion that face plant equipment especially the product is contained in a pump. Lastly, they are friendly to the environment since they play a role in cutting down waste product as well as reducing the need for necessary cleanups.