Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

Подождите немного. Документ загружается.

472 Chapter 4.6: Demountable Seals

for

Flanges and Valves

in the presence of ozone from high-voltage equipment or natural sources, espe-

cially when compressed. Table 3 shows that the permeation constant for water

through nitrile is several times as high as for a fluoroelastomer. However, nitrilc

0-rings are widely used because they are inexpensive.

Butyl rubber is only moderately permeable to atmospheric gases, including

water. It is sometimes used as a low-cost seal material for unbaked systems.

Polyurethane may be chosen for seals because of its exceptional toughness and

wear resistance, specific chemical resistance, or its slightly better resistance to ra-

diation damage as compared to other elastomers. Ethylene propylene is average

in most respects, but similar to polyurethane in radiation resistance.

Silicone rubber is not used for general vacuum sealing. Its mechanical proper-

ties are poor, and the permeation of atmospheric gases is much higher than for

other elastomers. However, it is used in some applications because it will with-

stand short-term exposure to temperatures as high as 300°C without complete fail-

ure.

Perfluoroelastomers have good mechanical properties, including tear strength,

abrasion resistance, and lubricity. They can be used to higher temperatures than

the other elastomers, and are average as regards permeation. Chemical resistance

is excellent. However, perfluoroelastomer seals are so expensive that they are

used only when their special advantages justify the cost.

PTFE and PCTFE are not elastomers and have little application in static seals.

0-rings with an outer layer of PTFE and an elastomer core are available [15].

PCTFE is commonly used for the nosepiece and body seals in small valves for

gas handling. PCTFE is a semicrystalline material, and has low gas permeation.

Both have excellent chemical resistance.

Polyimide has been used in vacuum valves intended for baking to 300°C. It is

very hard and requires large seal forces. It is also hygroscopic. Hait [9] discussed

the merits of polyimide for UHV seals. In the form of thin sheet, polyimide has

been used for sealing flanges [16].

It is clear why a fluoroelastomer is so commonly used for vacuum sealing. It

withstands temperatures adequate for the bakcouts required to remove adsorbed

water from system walls, the life is good, the compression set moderate, and it

is among the better materials as concerns permeation. It is, however, easily dam-

aged by nuclear radiation.

4.6.2.4 Design Considerations When Using Elastomer Seals

Proper finish of contacting sealing surfaces and defining the compression of the

seal are just as important as selection of the material. Most static seals are in

the form of 0-rings made of elastomer materials. Sessink and Verster [17] studied

the conditions necessary for sealing with elastomer 0-rings and found that the

4.6.2 Polymers Used in Vacuum Sealing 473

important consideration was a contact pressure at the metal to polymer interface

of 15 kg/cm-. The proper compression for a fluoro or nitrile elastomer 0-ring

should fall in the range of about 20-25%, including extreme cases arising from

the tolerances of the minor diameter of the 0-ring and of the compression de-

termining feature. Greater compression decreases seal reliability by aggravating

compression set and, particularly for nitrile, cracking. The compression recom-

mendations in the handbook of one of the large 0-ring manufacturers are not nec-

essarily correct

[18].

The seal force per unit length required to produce 20 to 25%

compression is dependent on the minor diameter of the 0-ring and the hardness

of the material. For an 0-ring of -|--inch nominal minor diameter, hardness 75

Durometer, the required compressive force is about 15 lb per linear inch of seal.

The manufacturers' handbooks have some force vs. compression data for other

cases [19,20]. The optimum seal contact surface on a flange or valve seat is not

mirror smooth. Sealing is more reliable against a surface with fine concentric

grooves. Good results are obtained using a face-turned surface cut with a ^-inch

radius insert and a feed of about 0.003 inch. A surface made this way will have

approximately a 32-microinch finish measured across the grooves, and will be

free of roughness in the direction of the grooves. It is not desirable to use abra-

sives to finish seal surfaces because of the smearing of the surface, which may

embed abrasive particles, and because the finish marks are not as easily controlled

as are the marks from turning.

An 0-ring must not be twisted or cut, either when inserted into a groove, or

during use. Grooves should be made with radiused edges to prevent seal damage.

In a "radial seal" the confinement is radial, and the sealing action is on the

I.D. and O.D. of the 0-ring. Assembly of the components of a radial seal is diffi-

cult. Improper assembly may shear off part of the 0-ring. Poor design of dynamic

seals may result in short seal life. For example, when an 0-ring is used as a valve

nosepiece seal seating against a conical female seat the 0-ring tends to rotate

nonuniformly during closure. Repeated cycling leads to twisting and early seal

failure.

Gas trapped in a hidden volume sealed to the outside, but with a slow leak to

the system interior, causes what is known as a "virtual leak." The symptoms in-

clude slow pumpdown and pressure bursts. To avoid virtual leaks, 0-ring grooves

should be vented by a narrow slot extending from groove to flange interior.

In most cases 0-rings will produce leak-free seals without the use of grease.

Grease is often a harmful contaminant, and it aggravates virtual leak problems,

particularly when used excessively. When grease is permitted, just enough grease

to make the O-ring surface shiny is adequate for static seals. However, when

0-rings are used in rotary or linear motion feedthroughs they must be well lubri-

cated to prevent

damage.

0-ring motion feedthroughs should be restricted to appli-

cations above about

"^ torr because of

the

exposed lubricated surfaces. Desorp-

474 Chapter 4.6: Demountable Seals for Flanges

and

Valves

tion characteristics of some vacuum greases have been described by Laurenson

[21].

The Fomblin® perfluorinated polyether oils and greases [22] have very low

vapor pressures and are chemically stable.

When fluoroelastomer 0-rings are intended for use at pressures below

10 ~^

torr they may be prebaked in vacuum at about 150-200°C to reduce the out-

gassing of water and other volatile material contained within the elastomer [23].

Occasionally 0-rings are cleaned with ethyl alcohol or fluorinated solvents. Bak-

ing is then necessary to remove the solvents completely. Metal seals are recom-

mended when lower pressures and cleaner systems are needed.

4.6.3

METAL SEALS

4.6.3.1 History of Metal Seals for Vacuum Joining

The history of metal seals is closely related to that of UHV. The first UHV-

compatible metal-sealed valves were designed by Alpert [24] at the Westing-

house Research Laboratories. The tubulations were small, about 12 nmi, and the

diameter of the valve orifice 6.3

nam.

Bills and Allen [25] developed a small metal-

sealed valve that had a negligible closed conductance. These valves were avail-

able commercially for many years [26].

In the late 1950s the demand for large all-metal vacuum systems for space and

plasma physics research led to the development of suitable components. In the

United States, NASA supported extensive studies of seals and sealing surfaces

[27,28,29].

The Stellarators for fusion research at Princeton Plasma Laboratory operated

under UHV conditions using gold wire seals [30], and Souchet discussed gold

wire seals used at CERN [31]. Gold wire seals are reliable, but the cost of the gold

is significant, and in large sizes the fully annealed, approximately 1-mm-diameter

gold wire is hard to handle.

Shear seal flanges using flat copper gaskets were first described by Lange and

Alpert [32], and were made commercially until about 1962. The shear seal prin-

ciple as illustrated in Figure 4 performed well, but the flanges were inconve-

nient because they were sexed. When the knife-edge flange design developed by

Wheeler and Carlson [33,34] was released by Varian Associates as the ConFlat®

flange

[35],

it quickly replaced all prior metal-sealed flanges. The seal detail is

shown in Figure 5. Varian Asociates held a patent [36] on the particular knife-

edge configuration used in the ConFlat®.

4.63 Metal Seals

475

Fig.

Seal detail of sliear-seai union first described by Lange and Alpert.

A copper gasket is partially sheared between male and female flanges.

Hg.5.

_^___

Seal detail of CF flange union showing a copper gaslcet captured between knife edges.

476

Chapter 4.6: Demountable Seals for Flanges and Valves

Fig.

Seal detail of a small face seal fitting showing a copper gasket indented

by radiused knife edges.

To escape this patent other manufacturers made flanges with the same bolt pat-

tern, outside diameter, and knife-edge diameter, but with radiused knife-edges.

They did not seal as reliably as the ConFlat®, and soon disappeared from the mar-

ket. The radiused knife-edge design, sketched in Figure 6, remains in use on small

face seal fittings available from various sources.

Although the patent has expired, the word ConFlat® remains a trademark. To

avoid using this name, flanges of ConFlat® type are often called "CF flanges."

4.6.3.2 Stored Energy

If a union consisting of a flange pair and seal is to remain leak tight when subject

to the differential motions caused by changing temperatures with the inevitable

temperature gradients, it is necessary that some element of the union store energy

elastically. In the case of seals using elastomers, the elastomer itself serves this

function. In CF flanges the deformation of the flange pair helps to maintain the

4.6.3 Metal Seals

477

seal [37,38]. The problem of seal integrity is more difficult as CF flanges grow

larger. Sixteen-inch CF flanges may not remain leak free after baking.

Slight changes are sometimes made in the details of CF flanges. In Europe, the

vertical face of the knife-edge is often made with a small positive angle.

It is possible to make a metal seal assembly that is itself elastic. The Helico-

flex® seal developed by CEFILAC [39,40] of St. Etienne, France, is an example

of one such seal. Illustrated in Figure 7, it is a form of metaUic 0-ring consisting

of a helical garter spring made endless by welding, a layer of hard metal to dis-

tribute the force from the discrete turns of

the

spring, and an outer layer of

softer

seal metal such as aluminum, copper, or silver. If the cross section of the Helico-

flex® seal is circular, the seal-to-flange contact area is larger than for a CF and a

correspondingly greater seal force is needed for the same seal metal. To reduce

Fig.

Helicoflex® seal developed

CEFILAC, St. Etienne, France.

The figure is from the patent drawing. In the figure (1) and (2) are the flanges, with seal sur-

faces (6) and (7). The components of

the

metal 0-ring are the garter spring (3), a liner (4), and

an outer layer of seal metal (5).

478

Chapter 4.6: Demountable Seals

for

Flanges and Valves

the seal force requirement, Helicoflex® "delta seals" are available with modified

surfaces providing a reduced contact area [41]. One advantage of the Helicoflex®

seal is that it can be made in shapes for use with non-circular flanges. Very large

sizes are also possible. The outer layer of seal metal can be chosen to suit the ap-

plication. A large number of Helicoflex® seals with silver outer layer were used

on racetrack-shaped ports 0.22 X 1.6 m on the TFTR (Toroidal Fusion Test Re-

actor) Tokamak at the Princeton Plasma Laboratory [42].

Welch and co-workers [43] evaluated several types of metal seals for possible

use in the AGS (Alternating Gradient Synchrotron) accelerator at Brookhaven

National Laboratories, and chose the Helicoflex® "delta seal." Unfortunately,

the Helicoflex® seal is not an ideal elastic element. The displacement vs. force

curve (hysteresis curve) for compression/decompression of a seal has a very open

loop [39].

4.6.3.3 Other Metal Seals

Many other metal seals are available conunercially. Aluminum seals of diamond

cross section for installation between flat-faced flanges as shown in Figure 8 are

available conmiercially. Unterlerchner

[44]

reported the results of

study at CERN

that used a large number of seals of this type. Sealing was reliable.

When CF flange knife-edges are damaged by careless handling, they may not

seal with the standard gaskets. Fend [45] dscribed a seal for use with CF flanges

as sketched in Figure 9. Seals are available in aluminum, copper, and nickel, and

it is claimed that these gaskets require only about 50% of the seal force of a stan-

dard CF gasket of the same material.

Rg.8.

Seal detail showing diamond cross section aluminum seal between flat-faced flanges.

4.6.3 Metal Seals

479

Fig.

Seal detail of a union between standard CF flanges, but using a corner-crush gasket.

The firm of

VAT

VAKUUMVENTILE AG [46] sells a family of reusable seals

with indium as the seal metal. They are intended for use with the ISO-KF flange

system. Bakeout temperature is limited to about 75°C, but they can be a very ef-

fective way to eliminate elastomers from an existing system with KF flanges.

4.6.3.4 Theory of Metal Seals

To obtain a metal seal between a hard metal and a soft metal, it is essential that

the soft metal be deformed plastically at the interface. Roth [47,48] investigated

the mechanism of hard/soft sealing. Armand, Lapujoulade, and Paigne [49] com-

pared theory and experiment for the leakage conductance of contacting surfaces

with microirregularities.

In addition, including shear in the plastic deformation is conducive to effective

sealing. There are several ways to obtain shear of a seal. Spreading of a gasket

during clamping between flat surfaces causes some shear of seal metal with re-

spect to the flange surface. A slippage like displacement of a flange with respect

to the seal causes shear. Perhaps most important, as the knife-edge of a CF flange

enters the copper gasket there is considerable shear of the copper on the near-

vertical surface. Small imperfections such as scratches on the copper gasket dis-

appear in the steep portion of the imprint. This may be seen by intentionally

scratching a gasket with a scribe, compressing it fully between a pair of CF

flanges, removing the gasket, and inspecting it with a microscope [33,50]. The

scratch will be visible on the shallow-angle portion of the imprint, but will disap-

pear on the vertical. The disappearance of surface faults in the imprint is thought

480

Chapter 4.6: Demountable Seals

for

Flanges and Valves

to be a requirement for sealing. Exactly where the sealing occurs with a CF flange

remains an interesting question. CF flanges also seal reliably when the flange

faces and knife-edges have been glass bead blasted.

The situation is different when a radiused knife-edge is used. Today, such knife-

edges are found in face seal fittings from several manufacturers intended to be

used with tubing from j to

inch in diameter. Making up a seal using a scratched

gasket may not cause the scratch to fully disappear anywhere in the knife-edge in-

dentation. One illustration of the poorer sealing properties of the radiused knife-

edge is the fact that these fittings will not seal when the stainless surfaces are

glass bead blasted. A good way to make a leak of about

"^ to

10 ~^

atm cc/s is

to bead-blast a pair of these fittings and assemble the joint.

Seals between hard metal pairs were considered theoretically by Inbar [51]. An

interesting hard metal seal for use between flanges was described by Verheyden

and Kline [52]. It is doubtful that two clean hard metal surfaces can be brought

into such intimate contact with only elastic deformation that a leak-free seal would

be the result. Figure 10 represents metal-sealed angle and gate valves [46] where

the seal is said to be between hard-hard pairs without plastic deformation. How-

ever, one element of this seal is plated with a very thin film of silver to prevent

cold welding. Perhaps this silver film serves to fill the minute voids remaining

Fig. 10.

Sketch

hard-hard seal construction as used by VAT VAKUUMVENTILE

in metal-sealed valves.

The tapered seat, the nosepiece, and the conical washer are made of

hard

stainless steels. Note

that the washer must seal on both its I.D. and O.D.

4.6.3 Metal Seals 481

when the metal surfaces are brought into contact under high loading. Whatever

the mechanism, sealing of these valves is reliable over a long cycle life.

A few molecular layers of adsorbed oil or water between the interfaces of a

metal seal can cause an imperfect seal to appear to be leak free. This is known as

a "liquid seal," and the effect is particularly annoying when encountered in preci-

sion leak valves where it causes the conductance to jump abruptly as the valve is

opened. Because this is a common effect, and atmospheric water vapor is enough

to cause it, a metal seal should be considered tight only when it has been baked

and tested in a humidity-free environment.

4.6.3.5 Seal Metals

Many metals are used for vacuum sealing. In order of increasing seal load re-

quirement, the most conmion include indium, lead, gold, aluminum, silver, cop-

per, and nickel. Indium has a very low vapor pressure, and it even seals success-

fully to ceramics and glass. But the maximum operating temperature is limited to

about 70°C, and indium is subject to cold flow. If a seal is to last, the indium must

be contained. Lead has been used as wire or as plated coatings. Aluminum is used

for several types of seals, but is not a perfect performer. Perhaps the oxide film

prevents consistent metal to metal contact. That aluminum seals were not as reli-

able as copper was mentioned by van Heerden [50].

Silver is an excellent seal metal, but seals using it are expensive, and gold is

even more so. Copper gaskets are commonly used with CF flanges. When copper

gaskets are baked at 400°C, oxidation of the copper limits the number of bakeouts

to 20 to 30. Wikberg [53] tested silver-plated copper gaskets at CERN and found

that they would withstand at least ten 300°C bakeout cycles of

h duration with-

out tarnishing or peeling of the silver. Copper is not acceptable in some chemi-

cal environments present in semiconductor device fabrication. Nickel gaskets are

sometimes substituted in these cases. Even when nickel is fully annealed, the seal

force requirement is so great that it is difficult to avoid breaking the CF flange

bolts.

A dry film coating of molybdenum disulphide is helpful on bolts used with

CF flanges to obtain maximum clamping with minimum torque.

4.6.3.6 Review of Metal Sealing

Metal seals are required when the temperatures are high or low, when the out-

gassing and permeation that occur with polymer seals are not allowed, or in the

presence of sufficient nuclear radiation to damage polymers.

CF-type flanges used with copper gaskets are satisfactory for most circular