30079_22.pdf

CHAPTER 22

SEAL TECHNOLOGY

Bruce M. Steinetz

NASA Lewis Research Center

Cleveland, Ohio

22.1 INTRODUCTION

629

22.3 DYNAMICSEALS

638

22.3.1 Initial Seal Selection

638

22.2 STATICSEALS

629

22.3.2 Mechanical Face Seals

642

22.2.1 Gaskets

629

22.3.3 Emission Concerns

644

22.2.2 O-Rings

634

22.3.4 Noncontacting Seals for

High-Speed/Aerospace

Applications

22.2.3 Packings and Braided

Rope Seals

637

646

22.3.5 Labyrinth Seals

650

22.3.6 Honeycomb Seals

653

22.3.7 Brush Seals

654

22.1 INTRODUCTION

Seals are required to fulfill critical needs in meeting the ever-increasing system-performance re-

quirements of modern machinery. Approaching a seal design, one has a wide range of available seal

choices. This chapter aids the practicing engineer in making an initial seal selection and provides

current reference material to aid in the final design and application.

This chapter provides design insight and application for both static and dynamic seals. Static seals

reviewed include gaskets, O-rings, and selected packings. Dynamic seals reviewed include mechanical

face, labyrinth, honeycomb, and brush seals. For each of these seals, typical configurations, materials,

and applications are covered. Where applicable, seal flow models are presented.

22.2 STATICSEALS

22.2.1 Gaskets

Gaskets are used to effect a seal between two mating surfaces subjected to differential pressures.

Gasket types and materials are limited only by one's imagination. Table 22.1 lists some common

gasket materials and Table 22.2 1 lists common elastomer properties. The following gasket character-

istics are considered important for good sealing performance. 2 Selecting the gasket material that has

the best balance of the following properties will result in the best practical gasket design.

Chemical compatibility

Heat resistance

Compressibility

Microconformability (asperity sealing)

Recovery

Creep relaxation

Erosion resistance

Compressive strength (crush resistance)

Tensile strength (blowout resistance)

Shear strength (flange shearing movement)

Removal or "Z" strength

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Table 22.1 Common Gasket Materials, Gasket Factors (m) and Minimum Design Seating

Stress (y) (Table 2-5.1 ASME Code for Pressure Vessels, 1995)

Min.

Design

Seating

Stress y,

psi

Gasket

Factor

Gasket Material

Sketches

Self-energizing types (O-rings,

metallic, elastomer, other

gasket types considered as self-

sealing)

Elastomers without fabric or high

percent of asbestos fiber:

Below 75A Shore Durometer

75A or higher Shore Durometer

Asbestos with suitable binder for

operating conditions:

Vs in. thick

Vi6 in. thick

!/32 in. thick

Elastomers with cotton fabric

insertion

0.50

1.00

200

2.00

2.75

3.50

1.25

1600

3700

6500

400

Elastomers with asbestos fabric

insertion (with or without wire

reinforcement):

3-ply

2.25

2200

2.50

2-ply

1-ply

2900

3700

2.75

Vegetable fiber

1.75

1100

Spiral-wound metal, asbestos

filled:

Carbon

Stainless, Monel, and nickel-

base alloys

Corrugated metal, asbestos

inserted, or corrugated metal,

jacketed asbestos filled:

Soft aluminum

Soft copper or brass

Iron or soft steel

Monel or 4%-6% chrome

Stainless steels and nickel-base

alloys

Corrugated metal:

Soft aluminum

Soft copper or brass

Iron or soft steel

Monel or 4%-6% chrome

Stainless steels and nickel-base

alloys

2.50

3.00

10,000

2.50

2.75

3.00

3.25

3.50

2900

3700

4500

5500

6500

2.75

3.00

3.25

3.50

3.75

3700

4500

5500

6500

7600

Table 22.1 (Continued)

Min.

Design

Seating

Stress y,

psi

Gasket

Factor

/77

Gasket Material

Flat metal, jacketed asbestos

filled:

Soft aluminum

Soft copper or brass

Iron or soft steel

Monel

4%-6% chrome

Stainless steels and nickel-base

alloys

Grooved metal:

Soft aluminum

Soft copper or brass

Iron or soft steel

Monel or 4%-6% chrome

Stainless steels and nickel-base

alloys

Solid flat metal:

Soft aluminum

Soft copper or brass

Iron or soft steel

Monel or 4%— 6% chrome

Stainless steels and nickel-base

alloys

Ring joint:

Iron or soft steel

Monel or 4%-6% chrome

Stainless steels and nickel-base

alloys

Sketches

3.25

3.50

3.75

3.50

3.75

5500

6500

7600

8000

9000

5500

6500

7600

9000

10,100

3.25

3.50

3.75

4.25

4.00

4.75

5.50

6.00

6.50

8800

13,000

18,000

21,800

26,000

5.50

6.00

6.50

18,000

21,800

26,000

• Antistick

• Heat conductivity

• Acoustic isolation

• Dimensional stability

Nonmetallic Gaskets. Most nonmetallic gaskets consist of a fibrous base held together with

some form of an elastomeric binder. A gasket is formulated to provide the best load-bearing properties

while being compatible with the fluid being sealed.

Nonmetallic gaskets are often reinforced to improve torque retention and blowout resistance for

more severe service requirements. Some types of reinforcements include perforated cores, solid cores,

perforated skins, and solid skins, each suited for specific applications. After a gasket material has

been reinforced by either material additions or laminating, manufacturers can emboss the gasket

raising a sealing lip, which increases localized pressures, thereby increasing scalability.

Metallic Gaskets. Metallic gaskets are generally used where either the joint temperature or load

is extreme or in applications where the joint might be exposed to particularly caustic chemicals. A

good seal capable of withstanding very high temperature is possible if the joint is designed to yield

locally over a narrow location with application of bolt load. Some of the most common metallic

gaskets range from soft varieties, such as copper, aluminum, brass, and nickel, to highly alloyed

steels. Noble metals, such as platinum, silver, and gold, also have been used in difficult locations.

Metallic gaskets are available in both standard and custom designs. Since there is such a wide

variety of designs and materials used, it is recommended that the reader directly contact metallic

gasket suppliers for design and sealing information.

Required Bolt Load

ASME Method. The ASME Code for Pressure Vessels, Section VIII, Div. 1, App. 2, is the most

commonly used design method for gasketed joints where important joint properties, including flange

thickness, bolt size and pattern, are specified. Because of the absence of leakage considerations, it

should be noted that the ASME is currently evaluating the Pressure Vessel Research Council's method

for gasket design. It is likely that a nonmandatory appendix to the Code will appear first (see dis-

cussion in Ref. 3).

An integral part of the AMSE Code revolves around two gasket factors:

1. An m factor, often called the gasket-maintenance factor, is associated with the hydrostatic

end force and the operation of the joint.

2. The y factor is a rough measure of the minimum seating stress associated with a particular

gasket material. The y factor pertains only to the initial assembly of the joint.

The ASME Code makes use of two basic equations to calculate bolt load, with the larger calculated

load being used for design:

W ml = H + H p = - G 2 P + 2TTbGmP

W m2 = H y = TTbGy

where W ml = minimum required bolt load from maximum operating or working conditions, Ib

W m2 = minimum required initial bolt load for gasket seating (atmospheric-temperature con-

ditions) without internal pressure, Ib

H = total hydrostatic end force, Ib [(TrM)G 2 P]

H p = total joint-contact-surface compression load, Ib

Hy = total joint-contact-surface seating load, Ib

G = diameter at location of gasket load reaction; generally defined as follows: When b 0 <

1 A in., G = mean diameter of gasket contact face, in.; When b Q > 14 in., G = outside

diameter of gasket contact face less 2b, in.

P = maximum internal design pressure, psi

b = effective gasket or joint-contact-surface seating width, in.

b = b 0 when b 0 ^ 1 A in.

b = 0.5Vb 0 when b 0 > 1 A in.

2b = effective gasket or joint-contact-surface pressure width, in.

b Q = basic gasket seating width per ASME Table 2-5.2. The table defines b 0 in terms of

flange finish and type of gasket, usually from one-half to one-fourth gasket contact

width

m = gasket factor per ASME Table 2-5.1 (repeated here as Table 22.1).

y = gasket or joint-contact-surface unit seating load, per ASME Table 2-5.1 (repeated here

as Table 22.1), psi

The factor m provides a margin of safety to be applied when the hydrostatic end force becomes

a determining factor. Unfortunately, this value is difficult to obtain experimentally since it is not a

constant. The equation for W m2 assumes that a certain unit stress is required on a gasket to make it

conform to the sealing surfaces and be effective. The second empirical constant y represents the

gasket yield-stress value and is very difficult to obtain experimentally.

Practical Considerations

Flange Surfaces. Preparing the flange surfaces is paramount for effecting a good gasket seal.

Surface finish affects the degree of scalability. The rougher the surface, the more bolt load required

to provide an adequate seal. Extremely smooth finishes can cause problems for high operating pres-

sures, as lower frictional resistance leads to a higher tendency for blowout. Surface finish lay is

important in certain applications to mitigate leakage. Orienting finish marks transverse to the normal

leakage path will generally improve scalability.

Flange Thickness. Flange thickness must also be sized correctly to transmit bolt clamping load

to the area between the bolts. Maintaining seal loads at the midpoint between the bolts must be kept

constantly in mind. Adequate thickness is also required to minimize the bowing of the flange. If the

flange is too thin, the bowing will become excessive and no bolt load will be carried to the midpoint,

preventing sealing.

Bolt Pattern. Bolt pattern and frequency are critical in effecting a good seal. The best bolt

clamping pattern is invariably a combination of the maximum practical number of bolts, optimum

spacing, and positioning.

One can envision the bolt loading pattern as a series of straight lines drawn from bolt to adjacent

bolt until the circuit is completed. If the sealing areas lie on either side of this pattern, it will likely

be a potential leakage location. Figure 22.1 shows an example of the various conditions. 2 If bolts

Fig. 22.1 Bolting pattern indicating poor sealing areas. (From Ref. 2.)

cannot be easily repositioned on a problematic flange, Fig. 22.2 illustrates techniques to improve

gasket effectiveness through reducing gasket face width where bolt load is minimum. Note that gasket

width is retained in the vicinity of the bolt to support local bolt loads and minimize gasket tearing.

Gasket Thickness and Compressibility. Gasket thickness and compressibility must be matched

to the rigidity, roughness, and unevenness of the mating flanges. An effective gasket seal is achieved

only if the stress level imposed on the gasket at installation is adequate for the specific gasket and

joint requirements.

Original gasket:

Redesigned gasket

gasket identical

to casting flange

Fig. 22.2 Original vs. redesigned gasket for improved sealing. (From Ref. 2.)

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