Classification of Mechanical Seals

Seal Fundamentals Classification of Mechanical Seals FSA Knowledge Series

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Abstract This Fluid Sealing Association Knowledge Series training presentation introduces the different configurations of mechanical seals. This section includes seal types: ▪

By arrangement

▪

By design

The wide variety of seal types is due to the diversity of applications: machinery, fluids, and processes

Selection of the best type is not always easy and straight forward as there are usually multiple choices. The most effective choice is usually a compromise between economical and technical factors.

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Mechanical Seal Classification

Arrangement

Classification

Mechanical Seal Classification

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Single Seal

Dual Seal

Pressurized

Unpressurized

Face-to-Back

Inside Mounted

Face-to-Face

Outside Mounted

Back-to-Back

Face-to-Back

Inside Mounted

Outside Mounted

Single Mechanical Seal Arrangements Inside Mounted

Outside Mounted

▪

Seal assembly mounted inside the seal chamber

▪

Seal assembly mounted outside the seal chamber

▪

Sealed pressure is on the outside diameter of the seal faces

▪

Sealed pressure is on the inside diameter of the seal faces Stationary Mating Ring and Housing

Stationary Mating Ring and Housing

Process Fluid

Rotating Seal Assembly 5

Process Fluid

Inside Mounted is the most common arrangement

Rotating Seal Assembly

Dual Mechanical Seal Selection Dual seals are used when one or more of the following criteria cannot be achieved by using a single seal design: ▪

Emission control of process fluid to the environment under normal operating conditions (e.g. EPA emission limits)

▪ ▪

Emission of process fluid under upset conditions or failure of the primary seal Acceptable life (time between repairs) as a result of inadequate lubricating properties of the pumped medium: too many solids, too close to vapor pressure, too low viscosity in combination with pressure and speed, or too high-pressure differential for one set of faces

The choice between a pressurized and un-pressurized seal is not always easy and must be evaluated on a case-by-case basis.

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Dual Mechanical Seal Classification Dual seal can be classified based on the pressure between the two seals within the seal assembly relative to the pressure in the seal chamber.

Pressurized Seals ▪ Pressure is above the seal chamber pressure (typically min. 30 psi/2.4 Bar) ▪ External fluid lubricates both seal face pairs ▪ Leakage to atmosphere is the external fluid ▪ Also called a “Double Seal” as traditionally they are used in a back-to-back arrangement Unpressurised Seals ▪ Pressure is below the seal chamber pressure (typically atmospheric pressure) ▪ Inner seal faces lubricated by pumped fluid, outer by the external fluid ▪ Leakage to the atmosphere is external fluid, possibly mixed with small amounts of pumped fluid ▪ Also called a “Tandem Seal” as traditionally they are used in a face-to-back arrangement 7

Dual Mechanical Seal Arrangements Dual mechanical seals can be arranged in 3 different arrangements:

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▪

Face-to-Back

Can be pressurized or unpressurised

▪

Back-to-Back

Commonly pressurized only

▪

Face-to-Face

Commonly pressurized only

Mechanical Seal Classification by Design Mechanical Seal Classification

Pusher Mechanical Seal

Component Seal

Unbalanced

Single Spring

Non-Split

Cartridge Seal

Balanced

Multiple Spring

Partly Split Fully Split

Rotating Spring

Spring in Process

Stationary Spring

Spring in Atmosphere

Secondary Seal

O-ring PTFE Wedge Spring Energized Polymer

Bellows Mechanical Seal

Welded Bellows Component Seal

Unbalanced

Rotating Bellows

Metal

Cartridge Seal

Balanced

Stationary Bellows

Elastomer

Formed Bellows PTFE 9

Mechanical Seal Classification by Design Pusher versus Bellows

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O-ring Pusher

Elastomer Bellows

Wedge Pusher

Metal Bellows

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Defined by the secondary seal type: o-ring or polymer wedge versus bellow, rubber or metal

▪

Application fields of each seal type overlap

▪

Most apparent distinction is the pressure limit

▪

Acquisition cost can vary widely

Mechanical Seal Classification by Design Unbalanced versus Balanced Ao

Ac

Ao Ac

Unbalanced

Balanced

𝐴𝑐 <1 𝐴𝑜

𝐴𝑐 >1 𝐴𝑜

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▪ ▪

High closing forces Low leakage

▪

Reduced closing forces

▪

Reduced power consumption

▪

For pressures up to 14 Barg/200 psig

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For pressures up to 200 Barg/3000 psig

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Not recommended for volatile liquids

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Always recommended for volatile liquids

Mechanical Seal Classification by Design Seal Face Patterns and Texturing

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▪

Examples include hydro-grooves, wavy faces, tapered faces

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Intended to increase opening forces in order to improve lubrication.

▪

Friction is reduced at the expense of a higher leak rate

Mechanical Seal Classification by Design Rotating versus Stationary Flexible Element

Rotating Flexible Element

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Stationary Flexible Element

▪

Mechanical seals with stationary flexible elements (bellows or spring) seals are recommended for equipment with high speeds (tangential velocities greater than 25 m/s (5000 ft/min).

▪

Stationary spring seals are also more suitable for machinery with inherently larger tolerances, such a heavy-duty slurry pumps and older pumps which have looser tolerances.

Mechanical Seal Classification by Design Cartridge Seals

Setting Spacers

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Seal is pre-assembled with sleeve and flange in one unit

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Easy to install

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Spring load is preset

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No measurements during installation

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May be factory tested with air, water or oil

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More costly when compared to noncartridge seal

Seal Sleeve

Sleeve Drive Collar Flange (Gland Plate) 14

Mechanical Seal Classification by Design Split Seals

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▪

Seal is axially split

▪

Does not require disassembly of the pump to install - reduces down time

▪

Leaks more then a conventional seal

▪

Split seal are more costly compared to conventional seals

Containment Seal Classification • •

Intended to contain the leakage of the primary mechanical seal.

May provide redundancy if Clearance the primary seal fails.

Fixed Bushing

Solid Floating

Segmented

Abeyance Seal

Containment Seals Types

Compressed

Gland Packing Uncompressed Contacting

Lip Seal

Dry Sliding Mechanical Seal 16

Non-Contacting

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