Mechanical Seal Principles Manual - Chapter 1

CHAPTER 1 MECHANICAL SEAL Purpose and Parts

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Chapter 1 Mechanical Seal Purpose and Parts Introduction Mechanical seals continue to evolve using today’s technological advances. However, the purpose and the basic parts of a mechanical seal have not changed since its inception. This chapter will explain the purpose of mechanical seals along with their basic parts and respective functions.

Objective Upon completion of this chapter you will be able to describe the purpose of a mechanical seal, the various mechanical seal parts and their function.

MECHANICAL SEAL Purpose and Parts

Introduction Objective The

The Mechanical Seal Purpose Mechanical seals are a very common sealing device used extensively extensively throughout industry today. This lesson will define a mechanical seal and describe its purpose.

They may leak flush water or they may leak product, but they still leak. This fact is the number one reason why mechanical packings are being replaced by mechanical seals.

The packed stuffing box Before mechanical seals, the attempts to control leakage of product around reciprocating or rotating shafts meant restricting the shaft and stuffing box wall clearance. This was accomplished by packing a soft, resilient material around the shaft in what is typically referred to as a stuffing box. Compression packings, referred to as mechanical packings, are still used in many applications because of their low initial cost, availability, familiarity, and ease of installation.

Another problem with mechanical packings is that they will cause shaft or sleeve damage given enough time. Even the newer materials will eventually fret the shaft or sleeve.

However, there are issues with mechanical packings. They can be expensive to maintain and in some cases result in excessive product losses to the environment. This high potential expense is often the result of improper packing installation or poor equipment condition. But the fact still remains with few exceptions, “All packings must leak to work properly . properly .”

CHAPTER 1

Mechanical Seal Purpose The packed stuffing box

5 Figure 1 The packed stuffing box


The Mechanical Seal Purpose The purpose of a mechanical seal

The purpose of a mechanical seal Mechanical seals were developed to address the disadvantages of and problems with compression packings. The purpose of a mechanical seal is to reduce or, in most cases, eliminate leakage of product or other fluids to the environment. A mechanical seal consists of two extremely flat surfaces, called faces, held together by product pressure and spring force to prevent product from escaping to the environment. Visible leakage that comes from compression packing is usually eliminated. Non-visible leakage ( i.e., fugitive emissions ) is often reduced by mechanical seals in order to meet the environmental laws of local, state, and federal regulatory agencies. Compression packings just cannot be used to comply with these environmental laws.

Mechanical seals that are applied correctly can reduce the operating and maintenance costs of most plants. However, a higher level of training is required for engineering and maintenance personnel in order to ensure mechanical seal reliability. It is important to note that initial installation costs for seals may be higher than compression packings. It is also important to realize increasing system reliability means that mechanical seals must be applied correctly and the seal may require custom designing for a certain application.

Mechanical Seal

6 Figure 2 The purpose of a mechanical seal

The Sections of a Mechanical Seal All mechanical seals are constructed similarly. They all can be distilled down to three basic sets of parts: the primary seal rings, the secondary seals, and the metal parts. This lesson will define these parts and explain their function.

Mechanical seal construction Almost every mechanical seal is constructed of the same three basic sets of parts. In no particular order, the parts are as follows: • Primary seal rings • Secondary seals • The metal hardware

The primary seal rings The primary seal rings are a set of two extremely flat surfaces held together by process and spring pressure to prevent product from escaping. In a mechanical seal, one ring must rotate with the shaft while the other ring does not rotate. These rings are commonly referred to as the rotary seal ring and the stationary seal ring or seat, respectively.


The Sections of a Mechanical Seal Mechanical seal construction

Of course, each of the above is comprised of many parts which are discussed in the following sections.

The primary seal rings

Primary Seal Rings

7 Figure 3 The primary seal rings


The Sections of a Mechanical Seal The secondary seals The metal parts

The secondary seals All mechanical seals will use some type of secondary sealing device to eliminate leakage at all other areas outside the primary seal rings. The two main places secondary seals are used is between the mechanical seal and the equipment shaft or sleeve; and between the seal gland and pump stuffing box face. These sealing devices can take many forms. They can be any one of the following: • O-rings • Gaskets • U-cups • V-rings • Teflon* wedges • Molded rubber boots • Chevrons • Square packing

The metal parts Mechanical seals also have plenty of metal hardware. Typical hardware may include the following, just to name a few: • Shaft sleeves • Gland rings • Collars • Compression rings • Pins • Springs • Bellows • Drive lugs • Snap rings • Seal ring holders

*Teflon is a registered trademark of DuPont Dow

Metal Parts

Secondary Seals

8 Figure 4 The secondary seals

Figure 5 The metal parts

Primary Seal Rings CHAPTER 1

Understanding the function of the primary seal rings is the basis for all mechanical seal discussions. This lesson will define seal face flatness, describe how an optical flat works and try to explain what takes place between these rings.

The primary seal rings The primary seal rings consist of two extremely flat surfaces held together by process and spring pressure to prevent product from escaping. In a mechanical seal, one ring must rotate with the shaft. This ring is commonly called the rotary seal ring. The second ring does not rotate and is commonly called the stationary seal ring. Dissimilar materials are commonly used for the rotary and stationary seal rings. One of the seal rings is usually a softer ring relative to the other. Because the ring is softer, it will wear as the mechanical seal rotates. The softer seal ring contact surface or “face” is always more narrow than the harder seal ring face material. As the narrow, softer face wears on the seal ring, it maintains contact with the harder face throughout the life of the mechanical seal. The narrow soft face can be on either the rotary or stationary seal ring. Its location depends only on seal design and type. When the sacrificial, narrow, softer face has worn down completely, the mechanical seal life has expired. One can see a similarity between mechanical seal face wear and tread wear on an automobile tire. When the seal face wears down, leakage is likely, and it‘s time for a new seal. Illustrations showing a new soft face made of carbon and a worn soft face are shown in Figures 8 and 9 .

MECHANICAL SEAL Purpose Primary Seal Rings

and Parts

Primary Seal Rings

Figure 6 The primary seal rings

The primary seal rings

Harder, Wider Seal Ring

Narrow, Softer Seal Ring

Figure 7 Typical mechanical seal showing the narrow soft face and the hard wide face

New Carbon Figure 8 New soft face made of carbon. Note: the long “ nose”

Worn Carbon Figure 9 Worn soft face. Note: the shorter “nose”

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Primary Seal Rings Seal face flatness Flatness defined

Seal face flatness A mechanical seal consists of two seal rings whose faces have to be extremely flat. There are numerous factors that determine rate of fluid flow between the seal faces, however, the distance between the two face surfaces is the factor that has the greatest influence. This means that it is vital that the distance between these two faces be minimized. To achieve optimum seal face flatness, the seal faces must be lapped and polished. The first step is to lap or create a flat surface. This surface is then polished to achieve a reflective finish. To ensure that a seal ring has the proper face flatness, specialized equipment is necessary to measure it.

Figure 10 Primary seal rings

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Flatness defined Flatness is a term that describes a level surface that has no elevations or depressions. We use terms like waviness, or concave and convex surfaces to describe the condition when we refer to the mechanical seal faces. It is this flatness that is of the most concern to us. Testing has shown that if the faces are separated by a space of about two microns or more, the seal faces will show visible leakage and, depending upon the separation, let solids penetrate that might score or in some way injure these lapped faces. We just said that the seal faces should be separated by two microns or less to seal properly. Considering that the human eye can, at best, see items that are forty microns or greater, it stands to reason that we cannot detect the proper face flatness by ourselves without help. Some understanding of the proper terminology is required to discuss how we measure distances this small.

What is a Helium Light Band? To understand how we measure such small distances we have to know that it is a characteristic of light to travel in waves. These light waves can interfere with each other, causing the light to disappear. This appears as a black band on the surface of the measured surface. It results from the interference of the wavelength going and coming from the reflective surface of the piece being measured. When you discuss visible light, color and wavelength mean the same thing, so to make the measurement we use a monochromatic or single wavelength light source ( mono means one, and chromatic means color). Any color (wavelength) could be used, but most companies use a pink color that comes off a helium gas light source. This color has a wavelength of just about 0.6 microns (0.000023 inches). This monochromatic light operates using a very simple law of physics. This law is that if two lights with identical wavelengths interfere with each other the result is blackness, not light. Please review the two illustrations Figures 11 and 12 .

1 Helium wavelength


0.0000232 inches ( 0.00059 mm )

Primary Seal Rings What is a Helium

Figure 11 Helium wavelength

Light Band?

Helium Light Source

Reflected Light Cancel Points 1/2 Helium Wavelength = 1 Helium Lightband = 0.0000116 inches ( 0.000295 mm ) Figure 12 Helium light reflecting off a surface causing light waves to cancel and black lightbands to form

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Primary Seal Rings What is an Optical Flat?

What is an Optical Flat? To measure seal face flatness a precision ground and polished clear glass of optical quality is required. This type glass is called an optical flat. Optical flat glass is lapped flat on at least one side to a certain accuracy standard. This working standard is a maximum of 0.000004 inches (4 mK) or 0.1 microns The optical flat is placed on the piece to be measured. The monochromatic light is aimed at the piece and this light reflects off of the piece back through the optical flat causing interference light bands. If the distance between the optical flat and the piece we are measuring is one half the wavelength of helium, or an even multiple of the number, a dark band is formed. This is referred to as a helium light band and because it is one half the wavelength of helium it measures 0.3 microns, or 0.0000116 inches. To understand this measurement we might mention that the smallest object that can be seen with the human eye is 40 microns or 0.0015 inches. Another way to understand this measurement is to know that the average coffee filter is in the range of 10 to 15 microns or 0.0004 - 0.0006 inches. Experienced seal people know that this means that solids cannot penetrate between the seal faces unless they open.

Figure 13 Monochromatic light and optical flat

Figure 14 Optical flat under a monochromatic light showing helium light bands on a mechanical seal ring

We check flatness of our seal face by comparing the pattern we see to a chart that is supplied by the measuring equipment manufacturer. A sample of this chart is shown in Figure 15 .

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Figure 15 Typical flatness interpretation chart showing light wave band pattern guide

Reading Light Bands When using an optical flat there are two methods that can be used to determine face flatness. The two methods are the wedge and contact methods. • Wedge Method – This method is usually used when the surfaces of the work and the flat are nearly parallel. The flat will contact the work at one point. Use a tissue at this point between the flat and the work.Read the bands in two directions by changing the pressure point by 90 degrees. The amount of the curve indicates flatness. If the band curves across two adjacent bands then the piece is flat within 23.2 mK. • Contact Method – This is the best method for Ring Shaped Work. The optical flat rests on the highest points of the work. Establish an imaginary line parallel to the bands in the center of the piece. Count the bands between one side of the line, then on the other side of the line. Divide the largest number by the smallest number. The result indicates the flatness in light bands. Localized distortions are measured by taking an imaginary line across the light bands.

Contact Point O p ti c a l F la t

Count the number of light bands the line crosses. This indicates the flatness in light bands. Silicon carbide, tungsten carbide and ceramic seal faces are less likely to be out of flat than carbon. Flatness is a good indicator of wear on the wide face of the seal.


Primary Seal Rings Reading Light Bands

Contact Method 1 Light Band 0.0000116 inches 0.3 microns

Contact Method 2 Light Band 0.000023 inches 0.6 microns

Figure 17 Flatness interpretation chart showing the contact method for various rings


CHAPTER 1


Work

Work

Work

Wedge Method

Wedge Method

Wedge Method

Figure 16 Flatness interpretation chart showing the wedge method for various symmetric pieces of work


Work

Wedge Method

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Primary Seal Rings Flatness Readings Rules of Thumb Seal face

Flatness Readings – Rules of Thumb Hard seal faces should read less than three light bands for seal faces with a mean diameter up to four inches. There should be no visible leakage. Leakage is always subject to definition, but three light bands of flatness will allow a mechanical seal to seal a vacuum down to a measurement of one torr (one millimeter of mercury). Carbon graphite faces relax after lapping. Although lapped to less than one light band by the seal manufacturer, you will see readings as high as three light bands if you check the faces. These faces should return to flat once they are placed against a hard face that is flat.

lubrication The asperity theory

Most large seal manufacturers use finite element analysis techniques to design these faces. Some repair and smaller seal facilities supply, replace or repair these faces with no provision for keeping them flat during temperature and pressure transients. Carbon is a flexible material. It can go out of flat easily. It should go back flat again when it presses against the hard face. Some seal companies lap faces concave or convex on purpose. That is why three helium light bands is often the specification. Tests done with two hard faces (they do not relap easily) show that visible leakage starts to occur at about five helium light bands.

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It is not a good idea to relap carbon graphite faces. Imbedded solids are pushed even further in, causing scoring and wearing of the hard face. Remember carbon cannot wear a hard face, only foreign material stuck in the carbon can do that, and relapping cannot remove it.

Seal face lubrication Mechanical seals typically require a layer of gas or liquid lubrication between the rotary and stationary seal faces. Seal face lubrication is crucial in maintaining seal life and reducing energy consumption. Even though mechanical seals have been in operation since the early 1900’s in one fashion or another, it is still not known what actually happens between seal faces. There are at least five common theories of what may be happening between the faces. These are explained in more detail as follows. The asperity theory This theory was proposed by the Battell Memorial Institute back in 1963. They were commissioned by the U.S. Air Force to find out once and for all what was happening between seal faces. Battell made one of the faces out of glass and photographed the result. The test was run on a carbon graphite face running against this glass face. The sealing medium was MIL7808 oil, a high grade turbine oil. Battell observed that the faces were separated by vapors coming from the asperities in the seal faces. Figure 18-A describes the seal face lapped flat. Being a mixture of carbon and graphite, the graphite transfers Figure 18-C to the hard face, leaving asperities (roughness of surface). Unlike other materials that tend to wear smooth, these asperities continue to appear as the faces wear ( graphite is a natural lubricant ). Battell observed vapors coming from the asperities Figure 18-B . The British picked up on this idea and came out with vapor phase seals in which the seal faces were heated to vaporize the fluid.

The one problem with this theory is it does not explain how we are able to run ceramic against ceramic or tungsten carbide against tungsten carbide. These materials do not have asperities on the seal face.

Carbon Graphite

A

Carbon Graphite

B

C

Figure 18 Asperity theory

The pressure drop theory This theory has some similarities to the asperity theory. It assumes, that as asperities develop, the fluid goes through a series of pressure drops across the face until all pressure is lost at the atmospheric side of the seal faces. In addition, a meniscus of fluid forms on the inside diameter of the face and is held there by centrifugal force.

The pressure wedge theory This theory is the one we use when discussing mechanical seal balance. It claims that the faces are running on a film of liquid that produces hydrodynamic forces, keeping the faces apart. The liquid is forced between the faces by a combination of pressure and capillary action. The pressure drop across this “wedge” is assumed to be linear for most applications. This means that as the fluid travels from the process pressure side ( high pressure) to the atmospheric side (low pressure) the pressure drops by the same amount for every 0.0001inches or 0.01mm we move across the seal face. However, the drop may not be linear across the wedge and seal manufacturers need to be aware of this to properly design mechanical seals.


Primary Seal Rings The pressure drop theory The pressure wedge theory

Process Pressure Side

e c a F l a e S

o p r D r e u s e s r P a r e i n L

Pressure

Rotating Face

Stationary Face

Miniscus held by centrifugal force

Non-Linear Pressure Drop Atmosphere Pressure Side Figure 20 Pressure drop across the pressure wedge from the process pressure side to the atmospheric pressure side

Figure 19 Pressure drop theory

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Primary Seal Rings The dry running theory The three band theory

The dry running theory This theory proposes that no lubricant is necessary because the seal is running on a combination of carbon versus graphite. The hard face is there to provide a surface for the graphite to stick to. While there is evidence that seals run well in solvents and hot water that provide no lubrication, this theory does not explain how hard faces can be run in these non-lubricating applications.

The three band theory This theory uses the observation of three distinct areas, or bands, between some seal faces as its basis. A band of liquid forms on the outside area, a vapor band in the middle and a dry band on the inside diameter. All of the conditions noted previously have been observed when using mechanical seals. The observation of a certain phenomenon or theory may be a function of the face material and the fluid being sealed. Tribological researchers really do not know. The only thing we know for sure is that something is happening. Whenever we install seals we try to keep the faces immersed in liquid. Seals immersed in lubricating fluid seem to last longer, as the faces do not wear as quickly. One day we will be able to explain what happens between the faces and design seals with extremely long lives.

Liquid Vapor Dry

Figure 21 Three band theory

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Secondary Seals CHAPTER 1

In a mechanical seal there are numerous “secondary” seals that function to keep the liquid from leaking to the atmosphere. This lesson will describe the types and functions of various secondary seals. It is very important to understand the limitations of each type of seal so that it will be used properly.

The gland seal The gland seal is a static seal. A static seal is a seal between two surfaces that have no relative motion to each other. It functions to provide sealing between the gland and the face of the stuffing box. The gland seal is usually a gasket or an o-ring and can be made of many different materials. As with all secondary seals, this seal needs to be compatible with the fluid being sealed.


Secondary Seals

Dynamic Shaft Seal The gland seal

Figure 23 Typical dynamic shaft seal

The shaft seal

Soft Seal Face Wears

Gland Seal

Shaft Seal is Moved by Springs Figure 24 Dynamic shaft seal being moved by the springs to the right as the soft seal face wears

Figure 22 The gland seal

The shaft seal The shaft seal is the part between the mechanical seal and the shaft ( or sleeve ) that prevents fluid from leaking along the shaft out to the atmosphere. The shaft seal can come in various types and configurations. Some common ones are o-rings, V-rings (chevrons ), U-cups, wedges, and boot-type seals.

Dynamic Seal

Static Shaft Seal

Figure 25 Static shaft seal plus another secondary seal that is dynamic and moves to the right as the soft seal face wears

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Secondary Seals O-ring seals

O-ring seals An o-ring is a sealing ring with a circular shaped cross section. O-rings come in many different sizes and cross sections depending on the application. They are very common in mechanical seals and have two distinct advantages over most other secondary seals. • It is impossible to install an o-ring the wrong way. Think about it. You can’t do it. • An o-ring can seal both positive pressure and vacuum. This is important if the pressure in the stuffing box can fluctuate between these two extremes.

V-ring (Chevron) seals

O-ring Seal

V-ring (Chevron) seals The V-ring, or Chevron, is a sealing device that requires constant loading in order to seal properly. The V-ring must be oriented so the “V” opens toward the fluid pressure. If the V-ring is installed backwards, the pressure in the stuffing box could force the fluid underneath the ring and leak to atmosphere. Unlike o-rings, Chevrons can only seal in one direction. In other words, they can seal either positive pressure ( when installed as illustrated below ) or vacuum (when installed backwards), but not both. Most V-rings are loaded by the springs and the process pressure and are usually dynamic shaft seals.

V-ring Seal

Figure 26 O- ring seal Figure 27 V-ring ( Chevron ) seal with “ V” open to process pressure

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U-cup seals The U-cup seal is another sealing device that requires constant loading in order to seal properly. The cup must be oriented so the “U” opens toward the fluid pressure. If the U-cup is installed backwards, the pressure in the stuffing box could force the fluid underneath the ring and leak to atmosphere.

Wedge seals This type of secondary seal is a wedge, usually made of Teflon, that is spring-loaded and mates behind the rotating primary seal ring. The spring and process pressures keep the wedge in contact with the shaft. The wedge must be oriented so that it provides a leak-free seal when exposed to this process pressure.

Like Chevrons, U-cups can only seal in one direction. That is, they can seal either positive pressure or vacuum, but not both. Most U-cups are loaded by the springs and the process pressure and are usually dynamic shaft seals.

Like Chevrons and U-cups, wedges can only seal in one direction. Again, this means they can seal either positive pressure or vacuum, but not both. Because of the tendency for Teflon to“cold-flow”, almost all wedges need to be loaded by one or more springs along with the process pressure. And in almost all mechanical seals that use the wedge, they are usually dynamic shaft seals.


Secondary Seals U-cup seals Wedge seals

U-cup Seal

Wedge Seal

Figure 28 U-cup seal

Figure 29 Wedge seal

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Secondary Seals Boot-type seals Additional secondary seals

Boot-type seals This type of secondary seal usually consists of a rubber boot and a large single-coil spring that loads the boot against the back of the rotating primary seal ring. The boot is made of any number of rubber materials and provides two things for the mechanical seal. • The boot is the shaft seal and prevents leakage along the shaft or sleeve. • The boot, along with the spring, provides the drive mechanism that attaches to the shaft and rotates one of the primary seal rings. Drive mechanisms will be discussed later in this chapter.

Additional secondary seals Additional o-rings or secondary seals may be located throughout a mechanical seal design. Typically, these seals will allow for radial or axial movement of the primary seal rings while still eliminating leakage. These seals can be dynamic or static in nature. As we said earlier, all secondary seals must be compatible with the product and operating temperature so it will not deteriorate and produce leakage.

Because boot-type seals are loaded by the spring and do not move in relation to the shaft, they are considered to be static shaft seals. Also, because the boot has to adhere to the shaft to work, almost all mechanical seals that use this type of secondary seal and need to be replaced require a completely new seal, as they cannot be rebuilt very easily.

Figure 31 Mechanical seal with numerous secondary seals, including the shaft and gland seals

Boot-type Seal Figure 30 Boot-type seal

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Mechanical Seal Hardware As mentioned earlier, mechanical seals also contain a multitude of other parts and hardware, mostly made of metal. Understanding the type and function of this hardware is important to ensure proper mechanical seal operation. This lesson will describe the various pieces of hardware found in most mechanical seals and define the function of each.

The gland The gland holds the non-rotating parts of the mechanical seal, including the stationary seal ring. The gland is also called the stationary holder, gland plate, end plate, or flange. The gland is mounted to the seal chamber, by means of various types of bolts, to prevent the stationary parts from moving. It also provides an opposing surface to mount the static seal to the seal chamber as previously discussed.


Mechanical Seal Hardware The gland

Gland

21 Figure 32 The gland


Mechanical Seal Hardware The spring mechanism Large single coil springs Multiple small springs

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The spring mechanism The spring mechanism is a machined component that stores energy and, when required, releases that energy. The spring provides the force to hold the rotary and stationary rings together when the seal is not pressurized. The fluid pressure in the seal chamber provides the majority of the closing force holding the seal rings together. The spring mechanism may be located on the rotary, stationary, or both depending on mechanical seal design. There are basically five types of spring mechanisms. All, except one, are metal parts. The one that is made of an elastomer must be compatible with the product and operating temperature so it will not deteriorate and produce leakage.

Large single coil springs Large single coil springs were one of the first spring mechanisms used in early mechanical seal designs and are still used in a wide range of applications today. They are found in older seal designs, but they work very well when used properly. However, there are several limitations to this spring. • They have a tendency to distort at high surface speeds. This means that large seals at high rotational speed can be affected by this problem. • There is a large axial and radial space required. Because there is just one spring, it has to be of sufficient mass to provide the proper spring load. • There is a need to stock a different size spring for each seal size. • The large coil springs, by design, cannot provide even closing pressure for the entire seal ring. This could cause uneven seal face wear and premature failure.

Large Single Coil Springs

Figure 33 Large single coil spring

Multiple small springs Many of the newer mechanical seal designs incorporate multiple small springs as the spring mechanism. These small springs operate better in high speed mechanical seal applications as well as low speed applications. Because of their quantity, these small springs are not prone to distortion. Consequently, they exert an even closing force on the seal ring at all times. Unlike the large single coil spring, the multiple small springs may be used with a wide range of shaft sizes. Also, because of their size, they do not require as much axial and radial space as large coil springs.

Multiple Small Springs

Figure 34 Multiple small springs

Metal bellows Metal bellows are another form of spring mechanism used in mechanical seals. The welded metal bellows is formed by welding separate thin (~0.005 inches or 0.13 mm) plates of metal together to form the bellows assembly. This one-piece unit provides the spring loading required to maintain face contact. Because metal bellows mechanical seals are often designed without elastomers ( i.e., no o-rings ), they are typically used in high temperature applications.

Thin ( 0.005 inches or 0.13 mm ) Welded Metal Plates that Form the Bellows


Vibration Damper Mechanical

Figure 35 Typical welded metal bellows mechanical seal with vibration damper

Seal Hardware Metal bellows

Metal Bellows

23 Figure 36 Metal bellows. Note: this seal uses no elastomer


Mechanical Seal Hardware Some common problems with welded metal bellows seals Finger springs

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Some common problems with welded metal bellows seals Welded metal bellows seals work well when applied properly. However, there are some inherent problems with them that are listed below. • Welded metal bellows are very sensitive to vibration ( either harmonic or slip-stick ). Vibration problems can be recognized by a cracking of the bellows near the end-fittings. Vibration damping is a serious problem at extreme temperatures because the shaft and the vibration damper ( notches on the inside diameter of the bellows) are growing at different rates. The shaft is usually growing faster. If the vibration damper causes the seal to fail by sticking to the shaft, rub marks will be present on the shaft. • Some bellows materials are not very corrosion resistant. This can cause problems if the seal is cleaned with an acid or a solvent because the bellows is very thin. • Recirculation lines can act as a sand blaster and rupture the thin bellows. • If the product that is being sealed has a tendency to harden or set-up between the seal faces, the bellows can twist and rupture because the faces have stuck together. Remember the bellows is only 0.005 inches ( 0.013mm) thick. • When using welded metal bellows in high temperature petroleum products ( or other organics), the proper environmental controls must be used or coking will cause the seal to fail. Coking is the build-up of a hard black organic residue caused by over-heating. This coke builds up on the inside diameter of the seal and can fill the spaces between the metal bellows plates to stop the bellows from acting like a spring. * Hastelloy C is a registered trademark of Haynes International Incorporated ** Inconel is a registered trademark of International Nickel Company

Stainless steel should not be used as a metal bellows material because of the possibility of chloride stress corrosion (to be discussed later). Better materials like AM350, Hastelloy* C or Inconel** 718 should be used instead. • Welded metal bellows seals have limited use in slurry applications. The seal can fail from bellows rupture due to wear or corrosion. It can also fail when the slurry clogs the bellows. • Because the most common failure for these seals is bellows breakage, metal bellows seals are very costly to repair. If this occurs, the whole bellows requires replacement. •

Finger springs Finger springs (sometimes called leaf springs ) are spring mechanisms that are typically located on the outside of the mechanical seal gland. These springs are a new design that provide the force required to hold the stationary and rotary seal rings together. Because these springs are outside the mechanical seal, they are much less prone to clogging from the product.

Figure 37 Finger springs. Note: springs are external to the seal

Rubber bellows The rubber bellows, although not made of metal, also acts as a spring mechanism when used with a single coil spring. Because it is a bellows, it has speed limitations. Like all elastomer parts, it must be compatible with the product and operating temperature so it will not deteriorate and produce leakage. The rubber bellows is required to bond to the shaft to work properly. Silicone grease should never be used to install this bellows. Rubber bellows seal designs were one of the first mechanical seal designs available to industry over 50 years ago. Due to its longevity, and low cost, this is a popular seal choice for original equipment manufacturer ( O.E.M.) pump companies.

Spring

Stationary Seal Ring

Rubber Bellows

Shaft Surface Adhered to Bellows

Rotating Seal Ring

Figure 38 Rubber bellows with single coil spring bonded to a shaft to perform its shaft seal and spring mechanism duties

Some common problems with rubber bellows seals Some of the more common problems with rubber bellows are listed below: • Many of the existing rubber bellows are made of Buna-N rubber. This rubber has a finite shelf-life and is easily attacked by sunlight and ozone. • Mechanical seals using a rubber bellows often fail due to bellows breakage. This failure is usually catastrophic in nature. This is different from o-ring seal failure because o-ring problems usually begin slowly and gradually deteriorate, allowing for time to schedule repairs. • The rubber bellows suffers from repair problems because the elastomer bellows bonds to the shaft in order to obtain a proper seal and drive mechanism. During a repair the bellows usually must be scraped clean from the shaft after the mechanical seal has been removed. • Installation of this seal type is often difficult. The rubber bellows location is critical and there is no way to set it. The only recourse to this issue is to relocate the spring, however, this does not always solve the problem. • The rubber bellows must be lubricated to slip it over the shaft, however, once installed the rubber bellows must bond to the shaft. Most other mechanical seals use silicon grease to lubricate the elastomer. Silicon grease should never be used with a rubber bellows seal because it will not allow the bellows to bond to the shaft. The shaft will spin through the bellows, thereby causing premature failure.


Mechanical Seal Hardware Rubber bellows Some common problems with rubber bellows seals

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Mechanical Seal Hardware The shaft sleeve

The shaft sleeve The shaft sleeve is a cylindricalshaped piece of metal or composite material placed over the shaft, usually inside the stuffing box. Some common reasons for using a shaft sleeve are listed below: • Probably the most common is to provide protection from wear to the shaft due to mechanical packing. • The second most common is to provide protection from wear to the shaft due to mechanical seals. • Shaft sleeves are often used to provide the proper spacing for the impeller. • They also can be used to provide a “step” in the shaft to achieve hydraulic balance for the mechanical seal ( to be discussed later ). • Some sleeves are installed because the fluid is extremely corrosive and it would be costly to make a shaft from the sleeve material. • Lastly, cartridge mechanical seal designs use a sleeve as an integral part of the seal.

Hook Shaft Sleeve

Figure 39 Common shaft sleeve used to protect the shaft and provide impeller spacing

Shaft Sleeve

26 Figure 40 Shaft sleeve used to provide hydraulic balance in a mechanical seal and protect the shaft

The drive mechanism The drive mechanism is the part of the mechanical seal that provides positive contact to the rotating shaft. This mechanism, once secured to the shaft, allows the rotating parts of the seal to rotate with the shaft.

Probably the most common drive mechanism is a group of set screws. However, other mechanisms such as a clamp, rubber boot (vulcanized or bonded to the shaft) or o-ring drive are also used.


Mechanical Seal Hardware The drive mechanism

Clamp

Vulcanized Rubber Boot

Figure 41 Clamp drive mechanism

Figure 43 Vulcanized rubber boot drive mechanism

Set Screw

O-ring Drive

27 Figure 42 Set screw drive mechanism

Figure 44 O-ring drive mechanism


Review Questions 1 through 10

Review Questions 1. For the most part, all packings must leak to work. a. True. b. False. c. It depends.

6. Typical mechanical seal metal hardware includes all of the following: Shaft sleeves, Gland rings, Teflon wedges, Pins, Springs, Bellows. a. True. b. False.

2. The purpose of a mechanical seal is to reduce or eliminate visible leakage to the environment. a. True. b. False.

7. Which statement is not correct? a. Dissimilar materials are commonly used for the rotary and stationary seal rings. b. One of the seal rings is usually a softer ring relative to the other. c. The softer seal ring is always wider than the harder seal ring material. d. Because the ring is softer, it will wear as the mechanical seal rotates.

3. Mechanical seals are constructed of: a. Primary seal rings. b. Secondary seals. c. Metal hardware. d. All of the above. 4. Which statement is correct? a. In a mechanical seal, all the seal rings rotate with the shaft. b. The ring that rotates is commonly referred to as the stationary seal ring. c. The seal ring that rotates is commonly referred to as the rotary seal ring. d. The seal ring that does not rotate with the shaft is called the rotary seal ring.

8. To achieve seal face flatness, the seal faces must be lapped and polished. a. True. b. False. 9. Surface finish is a term that describes a level surface that has no elevations or depressions. a. True. b. False.

5. Mechanical seal secondary seals can be a. O-rings. b. U-cups. c. Rubber boots. d. All of the above.

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.

10. What gas is usually used in a monochromatic light to measure seal face flatness? a. Helium. b. Hydrogen. c. Argon d. Nitrogen.

11. is used to measure mechanical seal face flatness. a. Only the human eye. b. An optical flat. c. A very accurate straight-edge. d. A micrometer.

12. The methods used to measure light bands with an optical flat are the

a. b. c. d.

methods. contact and triangle. wedge and interference. interference and triangle. contact and wedge.

15. The asperity theory assumes that small holes appear on the soft face as the face wears providing places for lubricant to reside. a. True. b. False. 16. The pressure drop theory assumes that the asperities formed on the seal face act as an infinite number of pressure reducers to bring the process pressure to atmospheric pressure. a. True. b. False.

13. When does visible leakage start between two mechanical seal faces? a. 2 lightbands. b. 5 lightbands. c. 10 lightbands. d. 20 lightbands.

17. The pressure wedge theory assumes a film of liquid exists between the seal faces causing them to be pushed apart. a. True. b. False.

14. Which statement is correct? a. Mechanical seals typically require a layer of gas or liquid lubrication between the rotary and stationary seal faces. b. Seal face lubrication is crucial in maintaining seal life and reducing energy consumption. c. We really don’t know what happens between seal faces. d. All of the above.

18. What theory proposes that mechanical seal faces run without the fluid to act as a lubricant between them? a. The pressure wedge theory. b. The dry running theory. c. The pressure drop theory. d. The three band theory.



19. proposes the separation of the lubricated fluid into a liquid, vapor and dry are between the seal faces. a. The dry running theory. b. The pressure wedge theory. c. The pressure drop theory. d. The three band theory.

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CHAPTER 1 MECHANICAL SEAL Purpose

20. Which of the following is a static seal? a. The primary seal rings. b. The wedge seal. c. The gland seal. d. None of the above.

and Parts

a. b. c. d.

a primary seal. a secondary seal. a tertiary seal. None of the above.

26. Boot-type secondary seals are dynamic seals. a. True. b. False.


27. The gland holds the rotating parts of a mechanical seal. a. True. b. False.

21. Is the o-ring shown in the above illustration, static or dynamic? a. Static. b. Dynamic. 22. O-rings can be installed backwards and still work. a. True. b. False. 23. Can a V-ring seal be installed backwards and still work properly. a. Yes. b. No. 24. U-cup secondary seals can seal vacuum and high pressure. a. True. b. False.

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25. A wedge seal is

28. Which of the following stores energy and releases it as required? a. The gland. b. The drive mechanism. c. The spring mechanism. d. The o-ring. 29. What spring mechanism is limited in its ability to provide even face load. a. The bellows. b. The large coil spring. c. The set of multiple springs. d. The set of finger springs. 30. Multiple small springs are recommended for high surface speed applications versus a single coil spring. a. True. b. False.

31. What spring mechanism is constructed of thin metal plates welded together? a. Single coil spring. b. Multiple coil springs. c. Metal bellows. d. Rubber bellows. 32. What is the most common failure of bellows mechanical seals? a. O-ring failure. b. Bellows breakage. c. Bellows hang-up due to coking. d. Bellows corrosion. 33. What spring mechanism is usually mounted outside the gland? a. The large coil spring. b. The set of finger springs. c. The metal bellows. d. The set of multiple small coil springs.

36. Which of the following is a cylindrical-shaped piece of metal or composite material placed over the shaft to provide protection to the shaft? a. Metal bellows. b. The spring mechanism. c. The gland. d. The sleeve. 37. Which of the following is the part of the mechanical seal that enables the rotary seal ring to rotate? a. The spring mechanism. b. The gland. c. The drive mechanism. d. The shaft seal.



34. Rubber bellows seals are required to bond to the shaft in order to perform their functions. a. True. b. False. 35. If a rubber bellows is lubricated with silicone grease for installation, what usually happens? a. The shaft will spin inside the bellows. b. The bellows will not seal properly. c. The seal will fail prematurely. d. All of the above.

31 Answers – Located on the Inside Back Cover

Mechanical Seal Principles Manual - Chapter 1

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