Mechanical Seals - an Introduction

Topics Covered

Background

Classes of Seals

Materials Selection

Surface Finish

Spiral Technology

Summary

Background

The importance of seal design is widely understood in the process, aerospace, automotive and other industries. Often, when a seal fails, a considerable amount of money is involved in replacing it and in the most extreme cases process equipment worth tens of millions may have to be shut down until the seal is replaced.

In many applications, mechanical seals provide a safeguard against hazardous materials escaping into the environment. Another important consideration in seal design is energy consumption since the frictional properties of the seal often have a big impact on the amount of power consumed by the machinery on which it is used.

Classes of Seals

There are four basic classes of seals which cover the vast majority of applications:

•        Traditional contact seal that is lubricated by the process fluid flowing through the equipment.

•        Seals that are cooled and lubricated by the fluid separated from the process.

•        Seals that run dry with the lubricant derived from the seal material.

•        Seals lubricated by gas which can either come from the process or be introduced externally.

Materials Selection

For every seal application there is an optimal pair of rubbing materials that provides the longest life and the lowest operating cost. This rubbing pair must be selected from scores of seal nose materials and an equally large number of mating face materials. The right choice of materials for the mating pair can easily increase seal life fivefold in those applications where wear is the controlling factor.

Surface Finish

Seal rings are usually received from the supplier in machined form and face preparation operations are then performed to achieve the required finish. A very flat and smooth finish is normally required to prevent leakage.

Depending on the seal type and application, materials such as carbon and silicon carbide are commonly used seal materials (see figure 1). The mechanical properties of some seal face materials are given in table 1.

carbon graphite and silicon carbide seal rings

Figure 1. Assortment of carbon/graphite and silicon carbide seal rings

Table 1. Mechanical seal face material properties

Seal material

Density (g.cm-3)

Hardness sclero.

(Knoop)

Comp. Strength

(MPa)

Thermal

Cond (W.m-1.°C/RT)

Temp limit (°C)

Resin impreg

carbon P-658RC

1.8

 

95

 

235

 

9

 

260

Antimony impreg

carbon P-7465

2.3

95

275

16

375

Salt impreg

carbon P-4229

1.9

100

290

16

480

Siliconised graphite

PE-8148

2.0

(2000)

85

50

260

Reaction bonded silicon carbide PR-9242

3.1

(2400)

2750

145

1370

Silicon carbide/graphite composite PG-9723

2.8

Na

550

150

550

Sintered silicon carbide

Hexoloy SA

3.1

(2800)

3900

125

1650

Tungsten carbide

6% Ni bonded

15.0

(1800)

4000

100

900

Alumina 99.5%

3.9

(1200)

2600

35

1500

Spiral Technology

A recent development is spiral technology. This technique provides a dry running, non-contacting double seal cartridge that ensures zero emission of process fluid to plant atmosphere. Clean dry buffer gas, normally plant nitrogen, is injected into the seal chamber at just 1 to 2 bar above the process pressure.

At the inboard seal, the spiral groove pattern on the mating ring/seat pumps the gas towards the ungrooved portion of the face. This compressed gas cushion serves as a sealing dam to block escape of the fluid being sealed.

The material used in the seal is still critical because there is contact when the unit starts and shuts down. Carbon is nearly always used because of its self-lubricating properties.

Summary

Seal manufacturers are meeting the demands for longer life, zero emissions and low power consumption. The wide ranges of seal materials available today provide a wealth of opportunity for optimising seal performance. Careful attention to material selection will pay dividends in reduced maintenance cost and downtime.

 

Primary author: Lawrence Thorwart

Source: Materials World, vol. 2, pp. 519-21, 1994.

 

For more information on Materials World please visit The Institute of Materials

 

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