Mechanical Seal Basics

After completing a recent training class, I had opportunity to ask our customer what were some of the highest cost failures they experienced. The answer? Mechanical seal failures. Mechanical seals come in a wide variety of configurations and manufacturers. The cost of these seals can range from $ to $ per inch of shaft diameter. These are very close tolerance and will not withstand misalignment for long if at all. A high percentage of mechanical seal failures are due to vibration induced by misalignment.

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While researching several mechanical seal manufacturers to gain some insight as to what their tolerances were (they are specific to configuration and are provided with the mechanical seal), I ran across the following very good article on mechanical seal basics.

BACK TO BASICS: MECHANICAL SEALS EXPLAINED

Posted by : SuperSailor 30 June

 

INTRODUCTION

Because mechanical shaft seal failures are the number one cause of pump downtime, we decided to dedicate this column to mechanical seal basics.

Years ago, most pump shafts were sealed using rings of soft packing, compressed by a packing gland, but this type of shaft seal required a fair amount of leakage just to lubricate the packing and keep it cool. Then came the development of the &#;mechanical seal,&#; which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces (one stationary and one rotating). Even though these mechanical seal faces also require some (very small) leakage across the faces, to form a hydrodynamic film, this leakage normally evaporates and is not noticeable. Most pump shafts today are sealed by means of mechanical seals. However, because of the delicate components used for this new sealing method, mechanical seal failures are the greatest cause of pump down time. This begs for a better understanding of this seal type and its application.

 

THE BASICS

Figure 1

 

 

 

 

 

Figure 2

Mechanical seals are leakage control devices, which are found on rotating equipment such as pumps and mixers to prevent the leakage of liquids and gases from escaping into the environment. Figure 1 above shows a typical centrifugal pump, which highlights its constituent parts, including the mechanical seal.

A mechanical seal consists of 2 principle components. One component is stationary and the other rotates against it to achieve a seal (Figure 2). There are many types of mechanical seal, ranging from simple single spring designs to considerably more complex cartridge seal types. The design, arrangement and materials of construction are essentially determined by the pressure, temperature, speed of rotation and product being sealed (the product media).

THE DESIGN

 

 

 

 

 

Figure 3

By way of example, a simple mechanical seal design has 7 components (Fig 3):

1. Stationary component; commonly referred to as the seat.

2. Stationary component sealing member.

3. Rotating component.

4. Rotating component sealing member.

5. Spring.

6. Gland plate.

7. Clamp ring.

 

THE SEALING POINTS

A mechanical seal has 4 main sealing points:

I. The seal between the rotating (3) and stationary faces (1). This is known as the primary seal.

II. The seal between the stationary member (1) and stuffing box face, i.e. Gasket (2).

III. The seal between the rotating member and shaft or shaft sleeve (4). This is known as the secondary seal and may be an o -ring as shown, a v -ring, a wedge or any similar sealing ring.

IV. The seal between the gland plate and stuffing box, this is usually a gasket, or o -ring.

3 of the 4 main sealing points need little explanation, but consideration is required for the sealing point between the rotating and stationary components (faces). This primary seal is the basis of a mechanical seal design, and is what makes it work. The rotating component (3) and stationary component (1) are pressed against each other, usually by means of spring force.The mating faces of both components are precision machined (lapped) to be extremely flat within 2 light bands, which is an optical method of measuring flatness).
This flatness minimizes leakage to a degree where it is essentially negligible. In fact, there is leakage between these faces but it is minute and appears as a vapor. (For immediate consideration)

Spring compression (usually) provides initial face pressure. This pressure is maintained when the seal is at rest via the spring(s) thus preventing leakage between the faces

 

FLUID FILM

If the mechanical seal faces rotated against each other without some form of lubrication they would wear out (and the seal would fail) due to face friction and the resultant heat generated. So, lubrication is required which for simplicity, is supplied by the product media. This is known as fluid film and maintaining its stability is of prime importance if the seal is to provide satisfactory and reliable service.

The primary disadvantage of this seal type is that it is prone to secondary seal hang-up and fretting of the shaft or sleeve, especially when the seal is exposed to solids. A pusher seal type should not be selected if the secondary seal is likely to hang-up. Can small deposits of solids form ahead of the secondary sealing member?

 

MECHANICAL SEAL TYPES

There are multiple designs available for the mechanical seal configuration. Understanding how they work will help the readers select the appropriate type for their application.

They are:

• Conventional
• Pusher
• Non-pusher
• Unbalanced
• Balanced
• Cartridge

 

PUSHER SEALS incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at   the seal faces, to accommodate wear and to assist in the absorption of shaft misalignment.

Advantages are that they are inexpensive and commercially available in a wide range of sizes and configurations.

NON-PUSHER OR BELLOWS SEAL does not have a secondary seal that must move along the shaft or sleeve to maintain seal face contact. In a non-pusher seal the secondary seal is in a static state at all times, even when the pump is in operation. A secondary sealing member is not required to make up the travel as the rotary and stationary seal faces wear. Primary seal face wear is typically accommodated by welded metal or elastomeric bellows which move to assist in the compression of the rotary to stationary seal faces.

The advantages of this seal type are the ability to handle high and low temperature applications (metal bellows), and that it does not require a rotating secondary seal, which means it is not prone to secondary seal hang-up or shaft/sleeve fretting. Elastomeric bellows seals are commonly used for water applications.

The disadvantages are that thin bellows cross sections must be upgraded for use in corrosive environments, plus the higher cost of metal bellows seals.

 

 

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Contact us to discuss your requirements of bellows mechanical seal supplier. Our experienced sales team can help you identify the options that best suit your needs.

CARTRIDGE SEALS have the mechanical seal pre-mounted on a sleeve (including the gland). They fit directly over the shaft or shaft sleeve, and are available in single, double, and tandem configurations. Best of class pump users give strong consideration to the use of cartridge seals.

The advantages are that this seal configuration eliminates the requirement for seal setting measurements at installation. Cartridge seals lower maintenance costs and reduce seal setting errors.

The primary disadvantage is the higher cost, plus in some cases they will not fit into existing stuffing box/seal housings.

 

MECHANICAL SEALS ARRANGEMENTS

Single seals do not always meet the shaft sealing requirements of today&#;s pumps, due to the small amount of required leakage when handling toxic or hazardous liquids; suspended abrasives or corrosives in the pumpage getting between the seal faces and causing premature wear; and/or the potential for dry operation of the seal faces. To address these situations, the seal industry has developed configurations which incorporate two sets of sealing faces, with a clean barrier fluid injected between these two sets of seal faces. The decision to choose between a double or single seal comes down to the initial cost to purchase the seal vs. the cost of operation, maintenance and downtime caused by the seal, plus the environmental and user plant emission standards for leakage from the seal.

The more common multiple seal configuration is called a Double (dual pressurized) seal, where the two seal face sets are oriented in opposite directions. The features of this seal arrangement are:

• Potentially five times the life of a single seal in severe environments.
• The metal inner seal parts are never exposed to the liquid product being pumped, which means no need for expensive metallurgy; especially good for viscous, abrasive, or thermosetting liquids.
• The double seal life is virtually unaffected by process upset conditions during pump operation.

 

The other multiple seal configuration is called a Tandem (dual unpressurized) arrangement, where the two individual seals are positioned in the same direction. This seal arrangement is commonly used in Submersible wastewater pumps, between the pump and motor, with oil as the barrier liquid. The typical features of this seal arrangement are:

• The pressure between seals is lower than the seal chamber pressure (typically atmospheric)
• The external fluid only lubricates the most outside set of faces
• Pumped fluid lubricates most inside faces
• The outside seal serves as a safety seal or containment device
• Leakage to the atmosphere is external fluid, possibly mixed with small amounts of pumped fluid.

 

MECHANICAL SEAL SELECTION

The proper selection of a mechanical seal can be made only if the full operating conditions are known. Identification of the exact liquid to be handled is the first step in seal selection.

• Metal parts must be corrosion resistant, usually plated steel, bronze, stainless steel, or Hastelloy.
• Mating faces must also resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered.
• Stationary sealing members of Buna, EPR, Viton and Teflon are common.

Pressure: The proper type of seal, balanced or unbalanced, is based on the pressure on the seal and on the seal size.

Temperature: Can determine the use of the sealing members as materials must be selected to handle liquid temperature.

Characteristics of the Liquid:  Abrasive liquids create excessive wear and shorten seal life.

• Double seals, or clear liquid flushing from an external source, allow the use of mechanical seals on these difficult liquids.
• For best results with double (or tandem) seals handling abrasive, the inboard seal faces should be a hard material, such as silicon carbide vs. silicon carbide, while the outboard seal faces should have maximum lubricity, such as silicon carbide vs. carbon graphite.

 

CONCLUSIONS

The seal type and arrangement selected must meet the desired reliability, life cycle costs, and emission standards for the pump application. Double seals and double gas barrier seals are becoming the seals of choice. Finally, it should be noted that there are special single seal housing designs that greatly minimize the abrasives reaching the seal faces, even without an external water flush, but this is a subject for another column.

Mechanical seals are precision, dynamic devices that are used to seal process fluids from the environment and have been utilized by pump and other rotating equipment users for approximately 100 years. Today, the same four basic components and fundamental principles apply to all mechanical seals, but this technology has evolved over the past 100 years, so seal users have a variety of choices to meet their specific technical, commercial and reliability needs.

The purpose of this series is to give users a foundational background in the technology of mechanical seals from
which they can start to make informed decisions regarding the different
types and configurations available for today&#;s applications. First, however, we need to understand just how a mechanical seal works.

To understand how a mechanical seal works, we need to understand the basic components that make up the anatomy of any mechanical seal. Image 1 shows the cross section of two different types of mechanical seals, which show the same basic four components.

IMAGE 1: The cross section of two different
types of mechanical seals, showing the same
basic four components (Images courtesy of
A.W. Chesterton Company)

The components of a mechanical seal include the following:

1. Seal Face Pair

The seal face pair creates the primary sealing interface, which provides rotational, dynamic sealing within the mechanical seal. Depending on the seal configuration, there will be either one face pair known as a single seal or two face pairs, which are known as a double seal. Some mechanical seals may have more face pairs, but these seals are custom designed for a specific application. The face pairs are made from a variety of carbon/graphite materials, as well as hard ceramic materials such as silicon carbide, tungsten carbide or alumina ceramic. Each of these materials offers different operating characteristics and capabilities to the mechanical seal.

2. Secondary Seals

Each mechanical seal will also have a variety of secondary seals, which are used to prevent leakage from various other interfaces within the mechanical seal. Most of these secondary seals are static in nature, but one is dynamic to allow the
seal faces to dynamically track each other when the equipment is in operation. If the seal face pair loses dynamic tracking, there will be excessive leakage from the mechanical seal.

Secondary seals come in a variety of different elastomeric materials in various forms, including O-rings, gaskets, wedges, boots or metal/elastomer bellows, as shown in Image 2.

IMAGE 2: Mechanical seal showing the various secondary seals, including O-rings and a gasket

3. Springs & Loading Mechanisms

The third major component of any mechanical seal are the springs or other loading mechanisms used to maintain face contact during static, non-rotational times and dynamic tracking during operation.

These springs and loading mechanisms can take the form of a large diameter single spring, multiple smaller diameter springs, leaf springs, wave springs or a metal or elastomeric bellows. The proper function of these loading mechanisms is critical to the leakage control of any mechanical seal. Image 4 shows some of the different loading mechanisms that can be used.

 

 

IMAGE 4: Different types of loading mechanisms

4. Structural Components & Hardware

The last major components are the metal parts and hardware. These include the gland, sleeve, lock ring and the various pieces of hardware used in the mechanical seal.

Glands are typically supplied in a stainless alloy, but depending on the application requirements, several other alloys may be used, including alloy 20, duplex or super duplex, titanium, Hastelloy or Inconel. The hardware and fasteners include pins, lugs and screws, which are used to assemble and hold the seals components in place.

5. Setting Clips

The final component is the seal&#;s centering device or setting clips. These devices have two critical functions. First, they aid in the shipping of the mechanical seal to assure there is no internal movement that could damage the seal faces or other components. Secondly, and just as importantly, these centering devices set the intended spring compression to assure optimal seal operation and reliability. Image 6 shows the cross section of a typical cartridge seal&#;s structural components and hardware.

IMAGE 6: Cross section of a typical cartridge seal&#;s structural components and hardware

The second area that all mechanical seals share are the configurations offered by the various manufacturers. The seal configuration offered is dependent on the requirements of the application. There are both general configurations offered
as well as application specific configurations to suit specific industry needs. Most seal manufacturers offer a variety of these configurations.

Component seals

A component seal is one where the rotary component and stationary component are two separate units that must be assembled during pump assembly. This configuration dates to the first mechanical seals offered to the market. They are simple and low cost, but they typically do not have the same performance capabilities found in other configurations.

Cartridge seals

Cartridge seals are built on the concept that the seal components are assembled into a cartridge unit. This configuration offers greater reliability, as it dramatically simplifies the installation process, as well as reduces the time frame required. Cartridge seals are offered in single, double or tandem configurations to suit the specific application requirements
and offer higher operating capabilities. Image 7 shows the various cartridge configurations available.

IMAGE 7: Various cartridge configurations

Split mechanical seals        

Split mechanical seals have been available off the shelf for the past 40 years. As the name implies, they are fully split, which allows for installation and repair without the need for costly removal of rotating equipment when excessive seal leakage occurs. This technology has seen several evolutions over the past 40 years, which have resulted in improvements in the ease of installation as well as sealability, approaching higher performance cartridge seals. Image 11 shows the typical configuration.

 

 

IMAGE 11: Typical configuration of a split mechanical seal

Equipment & Application Specific Designs

Finally, there are many equipment or application specific seal designs that offer an optimized solution for challenging application. For example, slurry seal technology offers seal faces designed to limit the potential of pulling particulates into the sealing interface and are designed to be more robust than standard seals to handle the motion and shocks many slurry pump applications experience. Image 8 shows a cross section of a slurry seal design.

IMAGE 8: Cross section of a slurry pump
seal design

 

Gas seal technology is another adaptation for specific applications where an inert gas barrier is needed due to the application requirements. The seals are based on compressor gas seal technologies but are adapted specifically to pumps. This seal design does not rely on a liquid to provide the seal with its required lubrication and cooling, instead an inert clean/dry barrier gas such as nitrogen is used. One of the seal faces in a gas seal design will have face features that help to separate the faces while in operation, known as hydrodynamic lift. Some designs also include a degree of hydrostatic lift, which means the faces begin to separate before seal rotation starts. Image 9 shows a cross section of a gas seal design specifically for pumps.

IMAGE 9: Cross section of a gas seal design
for pumps

Mixer seals are a special category of equipment/application specific design that incorporate several different capabilities to meet the needs of the specific mixer/reactor/agitator application. These seals can include:

• extra motion capabilities
• dry running capabilities
• debris wells
• integral bearings for greater stabilization
• hydrostatic lift capabilities for slow shaft speed applications
• design for glass-lined vessels that cannot accept a standard metallic alloy due to the nature of the fluid sealed

Image 10 shows some of these features used in various mixer seal designs.

IMAGE 10: Features used in various mixer
seal designs

Today, plants have a variety of designs and configurations to choose from. Determining which to use can be difficult at times. This is where a trained sealing device specialist can help in the seal selection process. In next month&#;s edition, we will explore how a mechanical seal &#;seals&#; and what features to look for based on the application&#;s specific requirements.

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