how does a mechanical seal work free sample

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.

Packing needs to press against the shaft in order to reduce leakage – this means that the pump needs more drive power to turn the shaft, wasting energy.

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.

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.

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.

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.

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.

Mechanical seals touch nearly every aspect of industrialized society. Wherever a rotating shaft moves fluid, mechanical seals play a key role in sealing process fluids in, keeping contaminants out, or both.

A few basic components and principles in mechanical seal design contribute to a working seal at the interface of the rotating shaft and stationary pump/mixer/seal-chamber housing. Mechanical seals are usually end-face seals or rotating-face seals, but in some designs they can be circumferential or even a hybrid of lip-type seals. In either case, the following components are common to all mechanical seals:

Stationary primary sealing element:fixed to the stationary housing of the pump, mixer or other equipment through which the rotating shaft passes and seals against the rotating primary sealing element

The more common end- or rotating-face mechanical seal designs feature mating faces as the primary sealing elements. Rings of ceramic, carbide, carbon or composites of these materials are lapped flat in the range of less than 1 micron on an axial end face. These lapped faces run against each other, one rotating with the shaft and the other stationary with the equipment housing.

The sealed fluid migrates between the flat faces and forms a stable fluid film at this interface. During shaft rotation, the face materials heat up, wear and degrade quickly without a lubricating fluid film between them. The sealed fluid creates this thin lubricating film.

In a lip-seal-type mechanical seal, a thin film of sealed fluid also lubricates the sealing interface. Rather than two flat rings, the sealing interface is a polymer material deflected against a hard material. This material could be a hardened, coated or plated metal, ceramic, or carbide face or sleeve. One of these elements rotates with the shaft while the other is stationary with the equipment housing.

Leakage is a function of the mathematical cube of the film thickness, so to minimize leakage, the gap at the sealing interface must be kept at a functional minimum. Closing forces are used to optimize this design parameter throughout the operating range of the mechanical seal.

The initial closing force ensures that the seal will function properly from startup. In end- or rotating-face mechanical seal designs, the initial closing force is provided by a spring component, which can be a single coil spring, multiple coil springs, a deflected bellows unit (elastomer or metal), or formed or flat springs. Initial biasing forces also can be created by magnets, compressed elastomers or any other means of applying a closing force between sealing elements. In a lip-type mechanical seal, the initial closing force is typically from the deflected polymer of the lip-type seal or a garter spring for less resilient materials.

The sealing elements must be secured to the rotating shaft and stationary housing of the equipment being sealed. O-rings, gaskets and other elastomer seals stop leakage at these interfaces.

A static secondary seal stops leakage between components that do not move relative to each other. One example is the interface between a sleeve and a shaft, where both rotate but do not move relative to each other. A dynamic secondary seal, on the other hand, stops leakage between components that move relative to each other. An example is a spring-mounted seal face, where the face is free to move as the spring deflection allows, and the secondary seal will stop leakage between the seal face and the component to which it is resiliently mounted.

A lip-type mechanical seal may only require static secondary seals because the deflection of the lip-type seal accommodates equipment operating motion. All effective end- or rotary-face mechanical seals require at least one dynamic secondary seal. This is because the mating faces of the sealing interface are rigid materials that cannot comply with any equipment shaft/housing misalignments, thermal growth and shaft end-play. The dynamic secondary seal will accommodate the relative motion between at least one of the seal faces and the component to which it is mounted.

Mechanical seals are used with many process fluids. Each fluid has different lubrication qualities, but a thin, lubricating film at the sealing interface is always needed. A film that is too thick will increase leakage and may allow particulate between the faces, which will increase wear from abrasion. A film that is too thin will generate heat and cause materials to degrade. Keeping the sealing interface cool and clean will promote longer seal life.

Seal design can influence film thickness by balancing the closing forces on the sealing interface in such a way that the sealing interface does not become overloaded as process pressures increase. A closing force that is too high will lead to a fluid film at the sealing interface that is too thin, generating detrimental heat.

Another way to influence film thickness is to design surface features at the sealing interface that promote hydrodynamic lift between the rotary and stationary sealing elements. This can help create a purposeful separation at the sealing interface that results in a thicker fluid film that provides cooling and decreases face wear.

Primary seal material selection can influence seal life as well. Chemical or process compatibility is just one consideration. Harder materials are more resistant to abrasive processes, but if both sealing elements are hard materials, the wear characteristics may be less desirable in a nonabrasive application.

Using one sealing element made of a softer material and/or one that contains lubricating components such as graphite decreases friction for starting and incidental contact. The use of composite hard faces will also reduce friction by providing microscopic reservoirs of system fluid at the interface.

Thermal conductivity of materials will dissipate heat away from the sealing interface, promoting seal life. Material toughness also can play a dominant role in mechanical seal life. The inherent material surface texture may also play a role in promoting desirable film thickness.

Note that many seal failures result from failed secondary seals that have exceeded chemical compatibility, pressure or temperature limits. Metal parts must be compatible to avoid corrosion, and springs and other hardware must hold up in service.

Process and seal environmental controls greatly contribute to a cool, clean lubricating film at the sealing interface. If the process fluid is a slurry mixture, process pressure will drive the particulate-laden fluid into the sealing interface, resulting in abrasion and accelerated wear.

Environmental controls, such as a restriction bushing and clean flush, can isolate the mechanical seal from the harsh process so the seal is mostly sealing the cleaner, cooler flush fluid. In other cases, the pump product may crystallize, abrading the sealing interface and causing premature wear. Product crystallization can be prevented by using temperature controls, quenching the atmospheric side of the sealing interface, or using a double seal with a buffer or barrier fluid.

There are many process considerations other than abrasion that might prevent a cool, clean lubricating film at the sealing interface. If the sealing fluid has a low vapor point, for example, flashing can result. Flashing occurs when the sealed fluid changes from liquid to gas at the sealing interface, expanding quickly and forcing the sealing elements apart until the pressure and temperature are relieved, only to have the sealing elements collapse back into contact. Mechanical damage to the sealing contact surfaces quickly results in seal failure. No lubricating film is established. Operators must incorporate process controls and ensure proper mechanical seal selection to prevent such upsets. There are many other process conditions that require special attention such as fluids that harden, are toxic, must be kept anaerobic, are part of food or water supply, or present another specific constraint.

Seal environmental controls are often overlooked, resulting in surprisingly short seal life. Many seal failures of this type happen in cool water applications. Cool water is an effective sealing fluid for creating a stable lubricating film at the sealing interface, but failure to apply proper seal environmental controls can lead to seal failure.

Many cool water applications fail prematurely because they are vertical, with the seal installed at a high point in the system where air is trapped. Without properly venting the air out of the seal chamber area, the mechanical seal seals air, not cool water. This is a dry-running condition that generates heat and quickly degrades the materials at the sealing interface.

A common environmental control used in vertical applications is a recirculation line from the seal chamber to pump suction, but in some cases the seals run dry for too long before the fluid replaces the air in the seal chamber.

Poor equipment conditions—caused by bad bearings, cavitation, excessive impeller loads and misaligned shafts—result in excessive motion, vibration and mechanical shock to the mechanical seal. These conditions cause greater stresses, more heat and more opportunity for abrasives to enter the sealing interface.

Mechanical seals are designed to handle a range of motions and conditions, but they are just one machinery component in a larger system. Understanding the basics of mechanical seals and how they may be adapted for different application requirements is critical for choosing the best seal for the job and ensuring optimal system reliability.

We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.

The mechanical seal is one of the most important components of a pumping system. As the name suggests, the seal is a simple component that forms a barrier between the motor and the volute of a pump, protecting the motor against leakage.

Leakage is death to any mechanical instruments and pumps are no exception. Fluid leakage often results in corrosion of the casings, sleeves and bearings. Corrosion left unattended over a period of time will will degrade the construction material of the pump. Fluid leakage that enters the motor shaft can short circuit the motor.

Naturally, these problems will impede proper pump functioning and eventually could stop the pump from running altogether. Companies often spend a lot in terms of money, wasted manpower and lost operational time to fix leakage.The mechanical seal is designed to prevent that leakage from ever happening. Mechanical seal shaft failure is the number one cause of pump downtime according to WaterWorld magazine.

Submersible wastewater pumps, such as sewage pumps, are particularly susceptible to the dangers of leakage as their operation depends on being surrounded by water that may contain potentially corrosive or clogging waste solids. This water can accumulate in the motor casing and obviously a submersible pump cannot be drained without interrupting operation.

A wide variety of seal types are available for any number of applications. The type of seal most commonly used in sewage pumps is an end face mechanical seal.

In an end-faced seal two ringed “faces” or seal heads rest flat against each other (but are not attached) in the seal chamber, which is located between the volute (the “wet end” of the pump) and the motor. An actuator, such as a spring, presses the faces close to each other.

The rotating motor is inserted through the two ringed faces and attached to the impeller. As the motor shaft rotates, the upper seal (closer to the motor) rotates with the shaft. The bottom seal closer to the volute remains stationary.

This action creates a sealing interface which keeps the water in the volute and prevents leakage. A minimal amount of water might escape the sealing interface but this liquid essentially acts as a lubricant for the seal and will eventually evaporate from heat.

All the components of an end faced mechanical seal work in unison to prevent leakage and are equally important to proper functioning. The main components are:

1. The primary seal faces that rest against each other. The primary seal faces are typically made of durable materials such as silicon carbide, ceramic carbide or tungsten. Certain materials work better for certain applications. For instance, silicon carbide is resistant to acidic liquids, less so to alkaline liquids. Generally, face materials should be of high hardness and should have the ability to slide on each other.

2. Secondary seal surfaces or faces. The secondary faces surround the primary seal faces, but do not rotate. The secondary surfaces hold the primary faces in place and create an additional barrier. Secondary faces can come in a variety of forms – examples include o-rings, elastomers, diaphragms, mating rings, gaskets and wedges. The secondary face also allows for shaft deflection and misalignment.

3. Actuator or a means of pressing the seal faces together and keeping the entire seal properly aligned to the shaft. Often (but not always) a loaded spring. The actuator is mounted above the seal face closer to the motor while the motor shaft passes through the spring.

Mechanical seals are precise, sensitive and temperamental instruments. Even seemingly minor mishandling can negate the seal’s functionality. Therefore Pump Products highly recommends leaving the mounting and installation of mechanical seals to qualified technicians.

Before you actually handle your mechanical seal, be sure to wash your hands thoroughly. Because the faces are meant to be extremely flat, even small particles from the oil of human hands can damage the surface integrity of the faces and render the seal useless. Make sure to wipe the seal itself with an alcohol solution, in case another person touched the seal faces during the packing or shipping process.

The following is a basic guide to replacing a defective mechanical seal. Each seal should come with its own specific instructions, but this is overview covers the most essential parts of the mounting process.

2. Carefully remove the old seal head, taking care not to scratch the motor shaft. Take note of how the seal was mounted; the new seal will be mounted in the same manner.

Mechanical seals are classified by construction type and the construction type is expressed through a letter code. The seal listing code will designate the construction material of each component. For example, here is a construction code guide from U.S. Seal:

The construction materials of the seal will in turn inform what specific seal is suited for your specific pumping application. You can consult a material recommendation chart to best choose the right mechanical seal.

The above chart is a guide to identifying and sizing the appropriate mechanical seal for your pump. Because seals are specifically engineered instruments, making sure that the seal is properly sized for a specific pumping system and application is critical. Manufacturers often make specific recommendations for the type of material to use for an application as well – a recommendation chartis helpful.

Back to back: Two rotating seal rings are arranged facing away from each other. The lubricating film is generated by the barrier fluid. This arrangement is commonly found in the chemical industry. In case of leakage, the barrier liquid penetrates the product.

Face to face:The spring loaded rotary seal faces are arranged face to face and slide from the opposite direction to one or two stationary seal parts. This is a popular choice for the food industry, particularly for products which tend to stick. In case of leakage, the barrier liquid penetrates the product. If the product is considered “hot”, the barrier liquid acts as a cooling agent for the mechanical seal.

Users can choose different material for this double mechanical seal 208, matching for different liquid conditoncarbon, silicon, and tungsten carbide for this mechanical seal as seal face, if for high temerperature, we suggest to choose rubber seal viton for the rubber parts.

Lepu seal make this dual mechanical seal for many years, and offer professional suggestion when client need this grundfos seal, so we are your reliable specialist for grundfos mechanical seal.

Double mechanical seals are commonly used in the following circumstances:If the fluid and its vapors are hazardous to the operator or environment, and MUST be contained

Guangzhou Lepu machinery CO., LTD becomes one of the leading mechanical seal supplier in south of china, we focus in designing and manufacturing mechanical seal for many kinds of famous brand pumps, our mechanical seal cover many kinds of industry like food, petrol chemical, paper making, sea ship, and so on.

Machine lapping is the production method of lapping Mechanical Seal Faces. Kemet Lapping Systems are used in both manufacturing and reconditioning processes. The following techniques are applicable to Bench Mounted and Free Standing Machines, as well as Kemet Hand Lapping Systems.

The machine size to be used would normally be determined by the largest seal to be lapped. For example, a Kemet 15 machine could lap seals up to approximately 125mm diameter (5”), whereas a Kemet 24 machine would be required if seals up to 200mm (8”) diameter were to be lapped. In most cases, if the following procedures are adopted, it should be possible to produce high quality reflective surfaces flat to less than 2LB.(0.0006mm)

Many materials used for the manufacture of mechanical seals are now too hard to be lapped using old fashioned conventional abrasives like aluminium oxide and silicon carbide. A Kemet composite lapping plate with a diamond slurry will generate a good cutting rate, and a highly reflective surface finish meaning a secondary polishing stage is not needed.

Kemet Flat Lapping Systems are available to fit most Lapping Machines up to 72” diameter, but for the majority of seal repair workshops 15” or 24” machines are adequate.

Any conventional Lapping Machine can be converted to a Kemet Diamond System by simply substituting the existing plate with a suitable Kemet Plate and fitting the appropriate Kemet Spray System. Existing control rings are still used, but require chamfers on their outer edges to assist diamond impregnation of the Kemet Plate. Once a machine is converted the lapping/polishing process is fully automatic.

As the processing capability of the Kemet System is so efficient, very little is used in the way of Diamond slurry. The high consumable usage, common to conventional lapping does not occur and the disposal of this waste material is therefore no longer a problem.

Flat Mechanical Seals can only be produced on flat lapping plates. It is therefore essential to regularly check plate flatness. This is the most important skill to master when operating a flat lapping machine.

The dispenser should be set to spray at the above intervals with the individual air regulators at the back of the Diamond Dispenser both set to 1/2bar (7.5 PSI). The lapping plate should first be charged for 10 minutes by running the machine and applying the above spray quantities.

It is important to ensure when lapping or polishing that the lapping plate is neither too wet nor too dry. If the plate is too wet, the diamond becomes flooded by lubricant and the stock removal will be very low; if the plate is too dry, the parts can come into direct contact with the lapping plate, and this will negatively affect the resulting surface finish and cause the plate to heat up.

Seals can then be lapped by placing them inside the machine’s control rings and applying a pressure. If multiple parts are to be lapped inside the same conditioning ring, it is essential that they are the same thickness (+/- 0.5mm). To keep the parts in position during lapping either a work-holder, (sometimes called a nest), should be manufactured. This is typically a 3mm disc of Tufnell with the profile of the parts to be lapped machined into it. Alternatively the pressure weights themselves can be faced with a Kemet Facing Kit. This special material has non-skid properties that hold the parts in position during lapping without the need for a work-holder.

Single seals may be allowed to run freely inside a control ring, but it is important to apply a weight to obtain optimum stock removal. This weight should be balanced. An ideal weight is a large steel ball that can simply be placed on top of the seal. It is also recommended that a rubber band be placed around single seals as this gives them drive within the control rings, thus avoiding tracking on the lapping plate. .

If a large seal is to be lapped on a relatively small lapping machine, then it is possible to remove a control ring and substitute the seal, which will then run in the Roller Yoke Assembly.

During any lapping operation, the parts being lapped will mirror the shape of the lapping plate. It’s for this reason that lapping plates are generally maintained in an extremely flat condition (typically less than 0.004mm) so that the parts being lapped will mirror this flatness.

For some applications, a convex surface is required, so in these instances, the lapping plate must be conditioned so that it is concave before you begin lapping parts. A concave lapping plate will create a convex part because the part mirrors the plate.

The amount of concavity needed will depend on the convexity required, and the size and surface area of the parts to be lapped.To create a concave lapping plate, position all the conditioning rings as far towards the centre of the plate as is possible.

Once this concavity has been generated, clean the plate and rings, and then charge the plate with the appropriate Kemet diamond slurry for the application.

Creating the concavity can take a long time, so if in production, we recommend keeping a plate specifically for this convex face generation. Ideally a dedicated set of conditioning rings would be kept with the plate.

Parts were lapped for 10 minutes on the diamond lapping machine and then lapped by hand on the Kemet Copper plate until they were polished. Hand lapping took up to 30 seconds. As the parts were spring loaded the parts had to be put on top of an optical flat to get a light band reading. rather than putting an optical flat on top of the seal.

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Flushing of mechanical seals, on large industrial pumps, is used to extend the life of the seal by cleaning and/or cooling the seal. There are many methods for implementing seal flush, but all either recirculate fluid from various other points on the industrial pump (usually the output) or use an external source for the flushing stream.

In either case a step-up in pressure, and/or flow control may be required, and a small Micropump pump is often used for that purpose. Micropump gear pumps are well suited for this application because they provide a well-controlled, smooth flow, and are built of materials that provide chemical capability and the temperature range require for most applications.

The PSS Shaft Seal is a mechanical face seal. The sealing surface is created between the flat surfaces of the rotating stainless-steel rotor and the stationary carbon flange. The stationary carbon flange is attached to the front side of the bellows with hose clamps, and the back end of the bellows fits over the stern tube and is secured with hose clamps. The stainless-steel rotor is fitted on the shaft in front of the carbon flange. The stainless-steel rotor compresses the bellows before the rotor is secured to the shaft with set screws. This compression (pre-load) maintains constant contact between the two flat faces of the stainless-steel rotor and carbon flange, allowing the PSS to compensate for the variable fore and aft movement due to propeller thrust. In addition, the carbon flange is over-bored to the shaft diameter allowing it to float around the shaft and thus compensate for most misalignment and vibration problems. The stainless-steel rotor is sealed to the shaft by two O-rings recessed into the collar"s bore. These O-rings rotate with the shaft and stainless-steel rotor and do not experience wear during operation.

An end face mechanical seal is a device intended to prevent or minimize leakage from a vessel through the clearance around a rotating shaft entering that vessel. Perhaps the most common example is the end face mechanical seal used in the water pump of an automobile engine.  Most pumps used in petroleum refineries, chemical plant and pipelines also use end face mechanical seals.

The simplest possible mechanical end face seal consists of a shoulder on a rotating shaft which rubs against a stationary case. This concept is shown in Figure 3.

As with packing or any bearing material, lubrication and cooling are required to prevent heat buildup and wear. Hydraulic pressure tends to force fluid between the faces and provide a lubricating film but the face separation must be kept very small to minimize leakage.  Cooling is provided by the surrounding liquid.  The conceptual design shown in Figure 3 is very simple, but it demonstrates the basic principle of the end face mechanical seals.  Of course, it has functional drawbacks which must be addressed.

A major shortcoming of the conceptual design shown in Figure 3 is that there will be always be some shaft movement during operation of the equipment. This movement is a result of manufacturing tolerances, vibration, hydraulic forces and wear.  Shaft movement may either decrease face separation, causing gross face contact and wear, or increase face separation, causing increased leakage.

Seal face leakage is governed by many variables, but the dominant variable is face separation. A variation of a few micro-inches (millionths of an inch) in the face separation can cause significant changes in leakage.  Unfortunately, shaft movement can amount to several thousandths of an inch.

A practical approach to overcoming shaft movement is to mount one of the seal faces in a flexible manner so that it can move axially. Obviously a desirable feature, this flexibility is a prerequisite for effective seal design.

Figure 4 shows an improved seal as compared to Figure 3. In Figure 4,  The sealing shoulder on the shaft has been removed and replaced with a component which is not permanently attached to the shaft.  This component is called the primary ring.  The “face” of the primary ring rubs against the mating ring.  Since the primary ring and the shaft are two separate parts, an additional sealing device must be used to prevent leakage between the shaft and primary ring.  The flexibly mounted primary ring can compensate for the small variations in movement on the axial plane.  It can also adjust for seal face wear.  Figure 4 is a very simple mechanical seal but it illustrates the concept used by the majority of mechanical seals.  Of course, some additional components are required to preload the faces, transmit torque and provide ease of installation.

Figure 5 shows a more complete mechanical seal including a replaceable mating ring, “O-ring” gaskets, springs, setscrews and various other hardware. As will be seen later, the design of these components may vary considerably according to the required service for the seal.  In addition, the assembled components themselves may be arranged and oriented in various ways to accomplish varying degrees of sealing, reliability and redundancy.

In a mechanical seal, the primary leakage path is between the seal faces.  Naturally, increasing the face separation increases the leakage. In fact, as will be shown later, doubling the face separation can increase the leakage rate by a factor of eight!  This relationship between leakage and face separation provides a powerful incentive for minimizing face separation.  In modern mechanical seals, face separations are so small (on the order of a few microns) that the leakage rate is affected by the surface roughness.  The effective face separation is a combination of the surface roughness of the two mating parts and the fluid film thickness. This concept is illustrated in Figure 6.  The seal manufacturer can control the initial surface finish by lapping and polishing.  A typical seal face is flat to within 23 millionths of an inch.  This degree of flatness is so small that refracted light rays must be used to measure it.  The fluid film is established during initial start-up of the seal by hydraulic forces.

The principle of establishing a fluid film is essential to all seal designs. Most mechanical seals are designed to operate in liquids; these seals require a liquid film.  Designing a seal to operate on a gas film is much more complex.  Whether the film is gas or liquid, it reduces the gross contact between the rotating component and the stationary component.  It also provides lubrication to reduce friction and wear.  Without a stable fluid film, gross rubbing contact could damage the faces.

Mechanical seals may be classified by their design features and the arrangement of those features. The Design category includes the details and features incorporated into a single primary ring/mating ring pair. The Arrangement category includes the orientation and combination of the primary ring/mating ring pair.  Figure 7 illustrates the classification of mechanical seals.

The Design classification considers the details which enter into the features of these components. Some examples of these features are balance, face treatment, rotating element, springs, secondary sealing elements and drive mechanism.  In general, these design features are not completely independent; that is, emphasis of a particular feature may also influence other features.  For example,  selection of a particular secondary sealing element may influence the shape of the primary ring.

By definition, the primary ring is the flexible member of the mechanical seal. The design of the primary ring must allow for minimizing distortion and maximizing heat transfer while considering the secondary sealing element, drive mechanism, spring and ease of assembly.  Many primary rings contain the seal face diameters, although this is not a requirement of the primary ring.  The primary ring always contains the balance diameter.

Balance.   The term “balance” is frequently referred to as the relationship of hydraulic forces on a seal but it is actually a geometric ratio.  Balance ratio is defined as the ratio of the hydraulic closing area to the hydraulic opening area. This ratio is customarily expressed in a percentage.

Figure 8 illustrates the concept of balance. In a seal, hydraulic pressure acts on the back of the primary ring; the resulting force pushes the faces together. This force is called the closing force and this area the closing area.  Similarly, any pressure between the seal faces creates an opening force which tends to separate the faces.  Therefore, the face area is also called the opening area.  The balance ratio is simply the ratio of the closing area to the opening area.

As shown in Figure 8, the area above the seal face outside diameter is disregarded when the closing area is computed. This area is not considered because the pressure is the same all around it;  consequently the contribution of the resultant of the hydraulic forces on this area is zero.

When the closing area is reduced, the closing force is reduced proportionally; this feature can be used to advantage when designing a seal. However, for a seal shape such as shown in Figure 8, the closing area will always be greater than the opening area. In order to make the closing area less than the opening area, the shape can be changed as shown in Figure 9.

The seal shown in Figure 8 is said to be an “unbalanced” seal. Its balance ratio is greater than 100% because of the necessary clearance underneath the mating ring.  Typical balance ratios for unbalanced seals range from 120 to 150%.  The seal shown in Figure 9 is said to be a “balanced” seal; its balance ratio is less than 100%.  The balance ratio of “balanced” seals is typically from 65% to 90%.

The distinction between “balanced” seals and “unbalanced” seals is simply that balanced seals have a balance ratio less than 100%.  A seal with 99% balance ratio is balanced, a seal with 101% balance ratio is unbalanced.

For a given pressure, balanced seals have less face load than unbalanced seals. Therefore, balanced seals are normally used in higher pressures than unbalanced seals.

Primary ring shape.The shape of the primary ring may vary considerably according to the incorporation of various design features.  In fact, the shape of the primary ring is often the most distinct identifying characteristic of a seal.  Figure 10 shows four examples of typical primary ring shapes.

Figure 10a represents a primary ring associated with elastomeric bellows seals. This primary ring has been optimized to take advantage of the elastomeric bellows and a large, single spring.

Figure 10b represents a primary ring with an inserted seal face. Insert faces must be designed with care because temperature differentials can cause differential expansion between the adaptor and the primary ring.  Insert designs can also have problems associated with mechanical stress and distortion.

Figure 10c and 10d show how the shape of the primary ring is influenced by the secondary sealing element. Figure 10c is a primary ring designed to work with a wedge.  Figure 10d is designed to work with an O-ring.  Also, Figure 10c shows an unbalanced shape while Figure 10d is a balanced shape.

Face Treatment.The most common seal face design is a plain, flat surface but there are many special treatments designed for specific applications.  Figure 11 shows some of the more common face treatments.  The plain, flat face is most common.  In general, face treatments are a means of modifying the pressure distribution between the seal faces.  The most common objective is to increase the opening force and thereby reduce the magnitude of the mechanical contact.  Face treatments may be considered to produce hydrostatic or hydrodynamic forces.

The simplest face treatment is a plain face that is not flat. This design produces forces that are primarily hydrostatic. Figure 11b shows an example of a face that is lapped so that it tends to touch the mating ring at the inside diameter.  This means that the leakage path is converging.  Although this is a simple concept, it is actually difficult to lap the required taper with the desired accuracy.  Too little taper will allow some rubbing to occur at the ID which will change the taper.  Too much taper will cause gross separation of the faces with the resulting high leakage.

Figure 11c shows another type of hydrostatic face treatment — the hydropadded face. Hydropads are recesses which are machined into one face — usually the face with the softer material.  After machining the recesses, the remainder of the face is lapped flat.  The size, quantity and location of the hydropads is such that the faces still rub; therefore, leakage is low.  The depth of the hydropads is sufficient to allow for long life in spite of the rubbing.  Hydropads actually show some slight hydrodynamic effects, especially in viscous liquids.

Spiral grooves, shown in Figure 11d, produce hydrodynamic forces and generally are used to cause non-contact operation. Spiral grooves also produce a pumping effect.  The grooves may be oriented so that pumping is either with or against the pressure differential.  With a suitable design, the pumping effect can overcome the hydrostatic leakage effect.

Rotating Element. Although most illustrations have shown the primary ring to be rotating with the shaft, either the primary ring or the mating ring may be used as the rotating element.  Seals with rotating primary rings are said to be “rotating” seals; seals with stationary primary rings are said to be “stationary” seals.  Because the springs are always associated with the primary rings, sometimes the distinction is made as “rotating springs” versus “stationary springs”.

For convenience, rotating seals are used in most equipment. Pump shafts are already made of a comparatively high grade material and manufactured to close tolerances. This makes pump shafts well suited for rotating seal applications.   Assembly of rotating seals can generally be done directly on the shaft or by using a relatively simple sleeves.  Figures 5, 8 and 9 all show rotating seals with stationary mating rings.

Stationary seals have some advantages over rotating seals. In small, mass produced seals for modest services, the entire seal may be placed in a package which minimizes shaft and housing requirements for the equipment.  Figure 13 shows a low cost stationary seal.   Stationary seals are also used to advantage in large sizes or at high rotational speeds.  Above 5,000 to 6,000 fpm, a rotating primary ring (which is flexible, by definition) may require dynamic balancing (for rotational imbalance) in order to operate in a stable mode.  A stationary primary ring does not require this balancing.  On the other hand, the stationary seal does require a close bore tolerance.  This close bore tolerance is usually a second manufacturing operation on most equipment.  Stationary seals sometimes incorporate special design features such as auxiliary liners, sleeves or adapters to help retain the mechanical advantages of the seal.  Figure 14 shows a stationary metal bellows seal used in high temperature centrifugal pumps.

By definition, the mating ring is the non-flexible member of the mechanical seal. The design of the mating ring must allow for minimizing distortion and maximizing heat transfer while considering ease of assembly and the static secondary sealing element.  The mating ring can contain the seal face diameters, although this is not a requirement of the mating ring.  To minimize primary ring motion, the mating ring must be mounted solidly and should form a perpendicular plane for the primary ring to run against.  Figure 15 shows some typical mating rings.

O-Ring. Figure 15a-c shows a mating ring which uses an O-ring. This is a simple arrangement but is limited to the temperature and chemical compatibility range of the elastomer.

Floating. Figure 15d is the floating ring type.  This design offers the flexibility of using teflon, grafoil, or an O-ring (Figure 15c) for the secondary sealing item.  If teflon or grafoil is used, a pin should be added to prevent rotation due to the low coefficient of friction of these materials.

Cup Mount.Figure 15e, the cup-mounted construction, offers a low-cost arrangement which can be used with surface finishes over 63 rms.  It is limited on high temperature applications and to lower pressure ranges because of the insulation effect of the cup.

Clamped-In.  Figure 15f represents a clamped-in mating ring design. A clamped-in design uses a series of gaskets to prevent leakage between the mating ring and the pump casing or gland.  This design has a wide temperature range since it can be used with spiral-wound metallic gaskets.

The secondary sealing elements are gaskets which provide sealing between the primary ring and shaft (or housing) and the mating ring and shaft (or housing). They are called secondary sealing elements because their leakage path should be secondary to the seal face leakage.  Loading by hydraulics or mechanical force makes the secondary seal tight in its confined area.  The secondary sealing element for the mating ring is always static axially (although it may be rotating).  Secondary sealing elements for the primary ring are described as being either pusher or non-pusher in the axial direction. The term pusher is applied to secondary seals that must be pushed back and forth by the movement of the shaft or primary ring.  A non-pusher secondary seal is a static seal for the primary ring.

Pusher type secondary seals have the disadvantage of damaging the surface to which they must seal. This damage, called fretting, is caused by the cyclic movement of the secondary sealing element as the shaft rotates.  In contrast to the pusher design, a non-pusher secondary sealing element could not cause any fretting.

On the other hand, this rubbing and dragging effect is also an advantage of the pusher design because it adds damping and therefore stability to the seal. This damping can make a significant difference in performance for some services.

Figure 16 shows examples of pusher, non-pusher and static secondary seals. The pusher design may use O-rings, wedges, etc.  The non-pusher is always some sort of bellows with a static section.  Mating rings use various static gasketing and O-ring designs.

Bellows. Figure 16a is a full convolution elastomeric bellows.  It offers the greatest possible flexibility to the front section of the primary ring.  The front section of this bellows has minimum contact with the shaft or sleeve, thus minimizing wear and hang-up.  The large tail section provides a considerable sealing area to compensate for imperfections in the shaft.  There are also are variations of the full convolution design  that use a half convolution.   Its ability to accept axial motion is reduced, due to the one-half convolution design.

Figure 16b is a bellows made out of teflon or glass-filled teflon combinations. This seal which is designed for the extremes of corrosive environments, provides the advantages of the non- contacting bellows convolution.  Because it is made out of teflon, support rings or a drive collar is required to attach the tail section or the bellows to the shaft.  Due to the requirements of flexibility, the convolutions must be considerably larger in cross-section than the typical elastomeric bellows design.

Figure 16c is an all-metal bellows style seal. It has the inherent advantage of flexibility associated with bellows design for high temperature applications.  Due to its all-metal construction, it offers considerable freedom of design since it is not restricted to the temperature and chemical limits of elastomers.  Metal bellows are constructed of individually welded metal leaves approximately .004/.012 thick.  A large number of leaves are required to provide the maximum amount of flexibility associated with other styles of bellows seals.  The mechanical closing force that is provided by the spring on other seal designs is accomplished by stressing the bellows from free height in this design.  Metal bellows seals are inherently balanced because of the manner in which the bellows becomes distorted when pressurized.

“V” Rings, “U” Cups and Wedges. Figure 16d is a wedge. Wedges are typically made of TFE material. They are considerably more flexible than the “V” ring or U-cup arrangement because they operate on the ball-and socket principal. Special manufacturing fits are not a requirement for wedges because of the shallow angle of contact between the primary ring and the shaft.  However, it does require a polished surface for effective sealing (32 rms with polish).

In addition to wedges, there are U cups and V rings.  Their  construction as a secondary seal offers limited amounts of flexibility and requires extremely close tolerances on the cross-section fits.  The “V” rings are generally manufactured in TFE or TFE-filled material, requiring pre-loading of the rings to activate the point contact on the lips.  Because of the “V” ring design, it can be used at high pressures but it is the least flexible, and most rigid of the secondary seal designs. Because of it’s construction, the “V” ring cannot flex to compensate for motion and it must be pushed along the shaft to take up wear at the seal faces.  Shaft and primary ring finish must be highly polished (15 rms) in order for this type of secondary seal to operate leak-free.

O-Rings. Figure 16e is the O-ring.  This is by far the simplest and most popular secondary sealing element.  It has been used successfully over a wide temperature range and in a variety of fluids.  O-rings are considered to be self-energizing seals and do not require much mechanical preloading.  This feature allows O-rings to be used at very high pressures.  O-rings are offered in a complete range of chemical resistant and general service compounds.  Buna-N, Neoprene, Ethylene-Propylene, Viton, and Kalrez are typical materials selected for a variety of service conditions.  On the other hand, Teflon does not make a good O-ring material, particularly if the O-ring is to be dynamic.

There are encapsulated O-rings. This is an attempt to obtain the chemical resistance offered by Teflon and the flexibility of the elastomeric O-ring.  Unfortunately, the result is dominated by the hardness of the Teflon outer shell.  The recommended surface finish of the shaft/sleeve is 15 rms.  The encapsulated “O” ring is sensitive to temperature fluctuations.  Because of these limitations, encapsulated O-rings are usually recommended only for static sealing.

In every mechanical seal there is always a need for keeping the faces closed in the absence of hydraulic pressure.   Generally, a mechanical device in some form of spring is used.  Figure 17 shows some of the different types of springs used in mechanical seals.

Single spring.A single spring seal has the advantage of comparatively heavy cross section coil which can withstand a higher degree of corrosion.  Another advantage is that single springs do not get clogged by viscous liquids.  The disadvantage of a single spring is that it does provide uniform loading characteristics for the faces.  Also, centrifugal forces may tend to unwind the coils.  Single springs also tend to require more axial space and a specific spring size is required for each seal size.

Multiple springs.Multiple springs are usually smaller than single springs and provide a more uniform load at the faces.  The same spring size can be used with many seal sizes by simply changing the number of springs that are used. Multiple springs resist unwinding from centrifugal force to a much higher degree than a single coil spring since the forces act differently.  The most obvious disadvantage of small springs is the small cross section wire.  This makes the smaller springs subject to corrosion and clogging.

Wave spring. The next form of spring generally considered is the wave type, simple described as a washer into which waves have been formed to provide a given amount of mechanical loading . The main reason for using this type of spring is that it requires even less axial space than the multiple spring design.  On the other hand, special tooling must be made for best manufacturing results.  Further, the tempering required on this design limits materials to those which are not as corrosion resistant as the high grade stainless and Hastelloy groups.  Also, when using wave springs, a greater change in loading for a given deflection must be tolerated.  That is, a great deal of force loss or force gain, with comparatively small axial movement must be expected.

Belleville washer.The Belleville washer is a very stiff spring; in fact, the normal problem with the Belleville washer is that the spring rate is to high.  To reduce the spring rate, the washers are stacked.

Metal bellows.  A metal bellows is actually a combination of a spring and secondary sealing element.  Welded edge metal bellows resemble a series of Belleville washers.  Formed bellows may be used to reduce the quantity of welding; however, a formed bellows has a much higher spring rate than a welded bellows.  The bellows thickness must be selected for resistance to pressure without an excessive spring rate.  The welding technique and bellows shape must be selected for maximum fatigue life.

The term “hardware” is used to describe the various devices which hold the other components together in the desired relationship.  For example, a retainer might be used to package the primary ring, secondary sealing element and springs into a single unit.  Another example of hardware is the drive mechanism which is necessary to prevent axial and rotational slippage of the seal on the shaft.

A drive mechanism is required because of the torque created between the seal faces. Both static and dynamic drives are required. The static drive is only required to hold an axial position and transmit torque.  The dynamic drive must transmit torque and allow for the axial flexibility of the primary ring.  Figure 18 shows some variations of drive mechanisms.  Dedicated devices made of strong materials are said to be positive drives and are generally preferred in the heavy duty seals.

Dent drive.The dent drive is an effective drive generally utilized because of its simplicity, comparative economy and because it can be made with simple tooling.

Key drive.This is one of the more rugged forms of drive. In high pressure, comparatively large size units, the ruggedness of this type of drive is in keeping with the balance of the sturdy features incorporated in seals for high pressure applications.

Set screws.This simple method of driving takes advantage of a common hardware item.  Set screws are probably the most common type of static drive mechanism.  They are not recommended for dynamic drive because the roughness of the threads can restrict axial flexibility of the primary ring.

Pins.Other positive drive designs include slotted pins, dowels or split roll pins.  In the mating ring, they are probably one of the simplest methods of insurance to eliminate the possibility of spinning.

Snap ring.Snap ring drives are sometimes utilized when axial space limitations prevent the use of other types of drive. These are generally limited to light duty services without corrosion.

Slot. This is another of the more rugged methods of engaging metal to metal, especially when the slots and mating ears are used in multiple forms.  It is one of the best methods to assure positive engagement between two parts that have relative axial motion between them.  Its basic design also allows for the high degree of flexibility generally incorporated in the primary ring.

Elastomer Drive.This is not a positive drive method and is generally limited to light duty seals; however, it is extremely simple, economical and effective for some services.

Spring drive.Another effective drive mechanism for light duty seals is to incorporate the drive into the springs.  Care must be taken in this design as to the direction of rotation, corrosion rate and spring rate.

Although all end face mechanical seals must contain the five elements described above, those functional elements may be arranged or oriented in many different ways. Several dimensional and functional standards exist, such as API Standard 682 – Shaft Sealing Systems for Centrifugal and Rotary Pumps, which describes the configurations for used in Oil & Gas applications. Even though the scope of API 682 is somewhat limited, it may be extended to describe end face mechanical seals in general.

Configuration refers to the number and orientation of the components in the end face mechanical seal assembly. For example, springs may be rotating or stationary.  Single or multiple pairs of sealing faces may be used.  For multiple seals, the individual pairs of sealing faces may be similarly oriented or opposed.  Containment devices such as bushings may or may not be used as part of the configuration.

Single seals.  The vast majority of all seals fall into the single seal category. In this category the seal can be mounted so that it is inside the process liquid or outside the process liquid.  Inside mounted seals are easier to cool and generally can seal higher pressures.  Also the direction of leakage is opposed by centrifugal force.  Inside mounted is by far the most popular, see Figure 19.

An outside seal is shown in Figure 20.  Outside seals have minimal contact with the process liquid.  This is an advantage in sealing corrosive liquids providing that overheating is not a problem.  A disadvantage of outside seals is that the direction of leakage is the same as centrifugal force.

Multiple seals.  Multiple seal arrangements can provide environmental controls and/or redundancy.  The most common types of multiple seal arrangements were previously called double and tandem but are called Arrangement 2 and Arrangement 3 by API 682.  The double seal emphasizes environmental controls while the tandem seal emphasizes redundancy.

The principal purpose of the tandem seal is to provide redundancy. The two seals are oriented so that the outer seal can accomplish the sealing task if the inner seal fails. A tandem arrangement is shown in Figure 21.  Because of this redundancy, the outer seal is sometimes called the “backup” or “safety” seal.  Tandem seals use a buffer fluid to lubricate and cool the outer seal.  The inner seal operates in the process liquid and is cooled by that liquid.  Any leakage from the inner seal must be contained by the outer seal and buffer system.  The buffer fluid is normally at a pressure less than the stuffing box pressure.  In order to keep the buffer pressure low, the buffer system is vented continuously.

Tandem seals (API Arrangement 2) are also used in “stages” (perhaps this is the origin of the name “tandem”)  when process pressures are extremely high. When tandems are used in stages, the buffer system is pressurized to some intermediate pressure, typically half the stuffing box pressure.

The double seal (API Arrangement 3) uses two primary ring/mating ring pairs oriented so that a pressurized barrier liquid is maintained between them as shownin Figure 22. On double seals, the inner seal seals between the process liquid and the barrier liquid.  Because the barrier system is at a greater pressure, any leakage is barrier liquid.  The outer seal seals between the barrier liquid and the atmosphere.  In most cases the barrier liquid is circulated and cooled to prevent heat buildup.

Notice that Figure 21 and 22 are identical.   The only differences are the design of the faces and balance diameter and these details are not shown.  This is somewhat controversial and many would describe both Figure 21 and 22 as being “tandem” because of the orientation of the components.    Traditionally, a “double” seal would have the primary rings in a back-to-back orientation; however, for purposes of operation, either face-to-back (shown) or back-to-back may be used.

Figure 23 illustrates face-to-back, back-to-back and face-to-face orientations.  Although Figure 23 shows rotating primary rings, the same concept and names applied to stationary primary rings.

The biggest problem with the double seals is maintaining the barrier liquid at a higher pressure than the stuffing box. It is generally recommended that a 20 psi or 10% differential in favor of the barrier be maintained at all times.

In the past, double seals were described as primary ring/mating ring pairs which faced in opposite directions.   A classic double seal is shown in Figure 24.  This is usually true but is not a requirement of the double seal arrangement.  Similarly, a tandem arrangement was frequently described as seal pairs facing the same direction, see Figure 21.  Again, this is not a requirement.  In a tandem arrangement, the buffer fluid is at the lower pressure and is continuously contaminated by leakage of the product across the primary seal.  In a double seal arrangement, the barrier fluid is at the higher pressure and the product fluid is continuously contaminated by leakage of the barrier fluid across the primary seal.

Double seals (API Arrangement 3) are used with API Piping Plans 53A, 53B, 53C or 54.   Tandem seals (API Arrangement 2)  are used with API Piping Plans 52and 55.

The term “adaptive hardware” is applied to hardware designed to simplify the incorporation of the primary ring and mating ring into the equipment which requires the mechanical seal. The most common examples of adaptive hardware are the sleeve and gland.

When the components are pre-assembled onto a sleeve and gland plate, the complete assembly is called a cartridge seal. This complete assembly can be easily slid onto the shaft and bolted in place thus reducing the potential for installation errors. Some cartridge seals use regular component seal parts whereas other cartridge seals might use specific purpose parts. API 682 specifies that only cartridge seals are acceptable to the standard.

Figure 23a shows the plain integral type gland plate which, if lapped on the sealing surface and gasketed perfectly to the face of the box without distortion, can serve as a sealing face against the rotating seal head. It is an item utilized on many compressor installations, showing that when applied correctly, even a gas such as Freon can adequately be sealed with this type of design.  It becomes a desirable feature, especially where axial room is at a premium.  It must be considered only where practical.  If a relatively large bolt circle in relation to the shaft size is inherent in the unit, this design may prove uneconomical.

Figure 23b is a plain end plate with a pressed-in seat element. Some seal manufacturers advocate this design and it can be utilized where its stresses, replacement features and economics can be tolerated.

Figure 21c shows a so-called plain gland plate but with an O-ring seat added. Where the use of the integral gland plate is not practical from the standpoint of replacement cost or because the seat material itself must be of