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

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402 Chapter

4.1:

Selection Considerations for

Vacuum

Valves

4.1.4.3 Seals Within Valves

Valves that provide shutoff have either a metal or a polymer seal between the

valve plate and the body. Polymers are inexpensive, but may be damaged by the

process, have higher permeation and outgassing rates, can be a source of particu-

lates,

and usually are not suitable for UHV pressures. Bakeout temperatures of

valves are usually limited by any polymers used—either within the vacuum, or

for pneumatic seals of the actuator. Metal-sealed valves often have higher bake-

out temperatures, but some of these valves cannot be baked in the closed position.

For a more complete discussion and comparison of sealing materials, the reader is

refered to Chapter 4.6 on sealing.

0-rings used as gate or nosepiece seals must be properly retained, or they may

come out of their groove during operation. Suitable 0-ring grooves have a trape-

zoidal cross section and are vented. Polymer seal materials can also be bonded di-

rectly to the valve plate. Valves with metal gate or nosepiece seals are more ex-

pensive, because the operating mechanism is more complicated and better surface

finishes are needed on sealing surfaces. The number of closures that can be ob-

tained with metal plate seals depend on the application and the care during the use

of the valve. Metal seals are more susceptible to damage by particulates than are

polymer seals.

Many valves have a demountable body seal, often called the bonnet seal, which

allows the components within the vacuum to be removed for cleaning and service.

The bonnet flange seal can be an 0-ring or other shaped elastomer, or a metal

seal. On some valves, metal and polymer seals can be used interchangeably. Valves

for UHV applications may eliminate the body seal by using an all-welded con-

struction for the body.

Many valves use a metal seal for the bonnet, and a polymer for the valve plate

seal. This has the advantages of a metal seal for the static body seal without the

increased cost and complexity of

sealing mechanism for a metal plate seal. With

this arrangement, the polymer is immersed in vacuum. As explained in Chap-

ter 4.6, an 0-ring in vacuum is much less of a gas source than an 0-ring exposed

to atmosphere.

4.1.4.4

Flanges,

Tubulations,

and Valve Conductance

valve is attached with flanges, it can be removed for service or for modification

of the system. Flange systems are described in Chapter

4.2.

In some cases, valves

are permanently installed by welding or brazing direcdy to the system piping.

The conductance of the valve and any associated piping should be considered

4.1.4 Valve Construction 403

Table 2

Measured Conductances of Angle Valves as a Function of Pressure

Angle Valve

Dimensions

(inches (mm))

Inlet

Diameter

1.26(32)

1.97(50)

2.56 (65)

3.94 (1(X))

Axial

Length

5.39(137)

6.10(155)

7.28(185)

9.25 (235)

At lO'^torr

Conductance (L/sec)

At lO'^torr

160

340

At 10"' ton-

100

170

850

—

torr

500

1000

4500

20,000

Source: Based on information given by H. A. Steinherz, in Handbook of High

Vacuum

Engineering

(Reinhold, New York, 1963), p. 21.

in system design. Formulas and a comprehensive discussion of conductance cal-

culations in general can be found in Roth [1] or Herman [2]. The literature does

not contain many papers on the conductances of valves. Conductances of butter-

fly valves were calculated and measured by Hunmier, Halter, and Grossl [3].

Angle valve conductances are frequently taken to be "similar to an elbow of the

same nominal size." Steinherz gave some measured values for the conductance of

several sizes of angle valves

[4].

These are tabulated in Table 2. These data illus-

trate how the conductance of a component depends on the pressure-flow regime.

Rozanov, Shchenev, and Akimov [5] calculated angle valve conductances using

Monte Carlo methods. They also modeled these valves analytically, as a series of

tubes,

apertures, and baffles. Actual valves were then experimentally tested, and

good agreement to both models was found. This work showed that clearance

around the bellows was important for high conductance. More recently Hablanian,

Nuzzi, and Pflanz measured the conductance of angle valves where the body di-

mensions had been optimized for high conductance [6]. They also found that

clearance between the body and the bellows had more effect on conductance than

the valve opening. In a valve where the nosepiece cleared the sideport, conduc-

tance did not increase during the last 20% of the valve's opening stroke. Rozanov

and associates also observed that after a certain valve opening, conductance did

not greatly increase. The advantage of designs with a shorter stroke is that bellows

life is significantly increased. Hablanian and associates examined an electro-

mechanically operated valve without a bellows, and found that its conductance

was approximately 90% of an elbow of the same nominal size.

404

Chapter

4.1:

Selection Considerations for Vacuum Valves

4.1.5

SPECIALTY VALVES

Some vacuum valves are designed for specific applications. For instance, it may

be desirable to rough a process chamber slowly to reduce damage to fragile items

like wafers or to decrease particle counts. Simple vane-type valves are available

that automatically perform this function and are illustrated in Figure 7. These are

Fig.

Small automatic valve for "soft starts."

TD ROUGHING

MANIFOLD

D-SDFT*

ROUGHING VALVE

ROUGHING PUMP

The inset shows a small spring-operated valve for automatic soft roughing to reduce turbu-

lence during system roughing. Installed in the roughing line, initial gas flow closes the valve

and pumping continues around the periphery of the vane. When the flow has decreased, the

valve springs open for greater conductance. (Reprinted with the permission of the HPS Divi-

sion of MKS Insruments, Inc., Boulder, Colorado. Auto-Soft is a registered trademark of the

HPS Division of

MKS

Instruments, Inc., Boulder, Colorado.)

4.1.5 Specialty Valves

405

Fig.

Poppet valve incorporating a bypass for soft starts.

1st stage

SLOW PUMP VALVE

orifice controlled flow

(a)

Initial roughing is through a small orifice as shown in Figure 8(a). After preliminary roughing,

the main poppet is opened for increased conductance, as in Figure 8(b). In the valve illustrated,

the bypass orifice can be closed with a poppet mechanism for

shutoff.

(Reprinted with the per-

mission of the HPS Division of

MKS

Instruments, Inc., Boulder, Colorado.)

installed in a flange joint of the roughing line, and the gas movement at the start

of pumpdown closes the vane. Once the gas flow has decreased, the valve auto-

matically springs open for greater conductance.

Poppet valves that include this soft start function are also available. Figure 8

illustrates the flow paths in these valves. Initial roughing is through a bypass with

a small orifice, and the nosepiece is opened later in the pumpdown. Such valves

are sometimes called two-stage valves. The bypass contains a small poppet mech-

anism to provide shutoff and can be integral with the body. In some cases, pump-

down characteristics can be varied to suit a particular application and system by

adjusting the small poppet mechanism.

Another specialized vacuum valve is designed to provide isolation and venting

of mechanical pumps in the event of a power failure. This helps to maintain vac-

uum in the system and prevent system contamination by oil backing up from the

406

Chapter

4.1:

Selection Considerations for Vacuum Valves

Fig.

Valve

for

automatic system isolation

and

pump venting

power loss.

VACUUM

CHAMBER

VACUUM

"SENTRY®

HIGH VAC

PUMP

VACUUM

SENTRY®

MECHANICAL PUMP POWER ON MECHANICAL PUMP POWER OFF

(a) (b)

Figure 9(a) illustrates normal, power-on operation. In Figure 9(b), power to the pump and the

solenoid valve has been interrupted. Atmospheric pressure has closed the valve, isolating the

system, and vented the mechanical pump. (Reprinted with the permission of

the

HPS Division

MKS

Instruments, Inc., Boulder, Colorado. Vacuum Sentry is a registered trademark of the

HPS Division of

MKS

Instruments, Inc., Boulder, Colorado.)

pump. Use of this valve is illustrated in Figure 9. The valve is installed near the

pump inlet, and its control solenoid is wired in parallel with the pump motor. When

power fails, the valve closes very quickly, isolating the system from the pump.

The pump side of the valve is automatically vented to prevent oil backup and for

easier restarting of the pump. After power is restored, the valve does not open un-

til the pump side has been evacuated, minimizing any pressure burst to the system

on opening. It is important that the valve be installed with the proper flow orien-

tation, so the pump side, not the system side, is vented on closure.

Some applications may require a modified or custom valve. Common modifi-

cations to a standard valve include the addition of ports for pumpout or gauge in-

stallation. Special materials may also be used to achieve a low magnetic perme-

ability or for increased chemical resistance. Gate valves can be modified to include

optical windows in the gate or water-cooled gates for metallurgical processes.

Valves for nuclear accelerators can be very specialized, providing rf shielding of

the body cavity, or closing times on the order of milliseconds to protect equipment.

Table 3 summarizes the conmion valve types that have been discussed in this

section.

4.1.6 Installation Considerations for Vacuum Valves

407

Table

Primary Use and Features of Commonly Available Vacuum Valves

Valve Type

Primary Use or Function Features/Comments

Poppet

Gate

Diaphragm

Butterfly

Iris/Blade

Leak or Needle

Pump Protection

Soft Start

Shutoff/isolation

Shutoff or control

Control

Gas admission

Soft pumping

Usually inexpensive and reliable; available with metal

seals for UHV applications; bodies with "in-line"

ports available; may have control as well as shutoff

capabilities.

Bodies are frequently thin and ports are in-line; avail-

able in a wide variety of sizes and with either elas-

tomer or metal seals.

Very low cost, usually only available in port sizes less

than 2.0 in; diaphragms are frequently elastomer, al-

though metal ones are used in small valves suitable for

gas handHng.

Manually actuated valves are inexpensive; motor-

driven types are suited to closed-loop pressure control.

For downstream pressure control; often motor driven.

For controlled admission of minute quantities of gas

into vacuum systems; many designs can provide shut-

off of the gas supply.

Isolates the vacuum system and automatically vents the

pump in the event of

power failure.

Slows initial pumping of

chamber to reduce particu-

lates.

This may be the only function of the valve or it

can be incorporated in another valve.

4.1.6

INSTALLATION CONSIDERATIONS

FOR

VACUUM VALVES

The location of a valve in a system requires thought. Pressure differentials across

the valve or loading due to atmospheric pressure can prevent a valve from sealing

or opening and may shorten its lifetime. The valve location and orientation should

be chosen to minimize the formation or trapping of contamination. Mild contam-

ination may prevent a valve from sealing, while gross contamination can prevent

a valve mechanism from

operating.

Some valves are available with heating mantles

to help prevent condensation of process products. Care should also be taken to see

that the valve is not subjected to temperatures higher than those reconmiended by

the manufacturer. Overtemperature can decompose polymer seals or other parts

and cause the mechanism to seize up. If there could be loose objects in the system,

the valve should be oriented so these will not find their way into the valve. The lo-

cation should also allow enough space for service access, including removal of

the operating mechanism.

408 Chapter

4.1:

Selection Considerations for

Vacuum

Valves

Care must be taken to see that valves are not damaged during installation. Seal

surfaces on flanges should be protected. Valve bodies have been destroyed by us-

ing bolts that are too long in flanges with tapped bolt holes. Any valve can be

damaged by undue stress, although gate valves are more susceptible. Long runs

of piping or other heavy objects attached to the valve body are a common source

stress.

To prevent damage in such cases, stress relief must be provided.

Finally, the manufacturer's recommendation for maintenance should be fol-

lowed. This is especially true with metal-sealed valves and other valves subjected

to bakeout, high temperature, or continuous use where the driver or operating

mechanism may need periodic lubrication.

REFERENCES

A. Roth,

Vacuum

Technology,

2nd ed (Elsevier Science, Amsterdam, 1982).

A. Herman, Vacuum Engineering Calculations, Fonnulas ami Solved Excercises (Academic

Press,

San Diego, 1992).

G. Hummer, G. Halter, and M. Grossl,

Vacuum,

41 (1990) 2126.

H. A. Steinherz, Handbook of High

Vacuum Engineering

(Reinhold, New York, 1963), p. 21.

5. L. N. Rozanov, V. V. Shchenev, and Yu. D. Akimov, Sov. Phys.

Tech.

Phys., 22(10) (1977)1249.

Originally published in

Zh.

Tekh.

Fiz., 47 (October 1977) 2151-2156.

6. M. H. Hablanian, F. J. Nuzzi, and

L. Pflanz, /

Vac.

Sci.

TechnoL,

A9(3) (1991) 2062.

CHAPTER

4.2

Flange and

Component Systems

Neil T. Peacock

HPS Division

ofMKS

Instruments,

Inc.

4.2.,

INTRODUCTION

Demountable flanges have made today's vacuum systems possible by allowing

nearly anyone to quickly and easily design and assemble a complex vacuum sys-

tem. Until the early 1960s, building a vacuum system required skilled personnel.

Smaller laboratory systems were frequently made of glass, and construction or

modification required a scientific glassblower. Larger systems were often assem-

bled by sweating or brazing copper plumbing fittings and adapting other compo-

nents to vacuum service. Producing clean, reliable, easy to modify, and leak-free

vacuum systems was difficult. Standardized flanges and components with flanges

designed for vacuum service have eliminated many of these problems.

Many flanges are part of a "system," with components like Tees and crosses

forming modular "building blocks." This allows for easy modification or sub-

stitution of

components.

With the system concept, components can be quickly as-

sembled for a particular experiment, then disassembled and reused in another

arrangement. When these flanges and components are built to international or

industry standards, parts from different sources or manufacturers can be inter-

mixed.

In a few applications and installations, demountable joints are undesirable. Ex-

525.00 All rights of reproduction in any form reserved.

409

410 Chapter 4.2: Flange and Component Systems

amples include the handling of hazardous gases or materials that would damage

the joint or its seal. In these cases, components are usually welded or brazed

together.

4.2.2

SELECTING A FLANGE SYSTEM

When selecting a flange system, there are many points to keep in mind. The first

is whether the joint must be metal sealed, or if an elastomer is acceptable. Metal

and polymer seal materials are compared in Chapter 4.6. Next is the material for

the flange. Typically the chamber, the flanges, and components of a system are

made of the same material. Flange pairs of dissimilar metals should be avoided on

baked systems because differences in thermal expansion rates could cause leaks.

Also,

the material needs to be suitable for the method of attachment. When braz-

ing, the material must be compatible with the braze alloy. Welding is generally

straightforward when the flange and the component are the same weldable alloy.

For stainless steel flanges, parameters such as the ferrite numbers of the metals

being welded influence the weldability

[1-3].

Other flange selection considerations include the pressure range, with any over-

pressure requirements, and operating temperature extremes. How flange pairs are

held together is an additional consideration. Flanges are often bolted together, but

for frequently disassembled

joints,

or components moved from system to system,

a flange held together with an easily removed clamp may be advantageous. Band

clamps also facilitate assembly with remote handling equipment as required in

high-radiation environments.

Ranges for critical UHV applications are subject to additional considerations.

In these cases, vacuum remelt or forged material should be specified to help avoid

porosities and leakage paths. The hardness of the flange material is important for

flanges with knife edges, so that the flange is not damaged with repeated

use.

Some

specialized applications may require additional material specifications. For ex-

ample, 316 stainless steel is used when greater chemical resistance is required, and

low-magnetic-permeability stainless steel is often used in particle accelerators.

4.2.3

COMMON FLANGE SYSTEMS

Numerous flange systems are in current use. Features of the most widely used

ones are compared in Table 1. In this section, descriptions and basic dimensions

4.23 Common Flange Systems

411

Table

Comparison of Flange Systems

Range

Tubing

Sizes

Type Accommodated'

Beaded-Edge

Face-SeaH

ISO 2861/1^

(smaller ISO

series,

often

called ISO-KF)

ISO 1609/1986^

(larger ISO series,

multifastener type)

ASA (ANSI)

Knife-Edge''(CF)

Wire-Sealed

MESA or SEMI

|-" to

1.0"

2.0"

2.5"

12"

1.5"

12"

0.75"

10"

12"

to 18"

Rectangular

Primary

Seal

' Material''

Metal

Elastomer

Metal

Elastomer

Minimurr

Usable

Pressure

(torr)^

10-9

io-«

10-8

io-'»

10-"

10-8

Applications

Gas-handling lines, gauge attachment

Forelines, general high-vacuum use.

frequently modified systems

Pumps, high-conductance forelines.

general vacuum use, frequently modi-

fied systems

Found on high-vacuum pumps, older

vacuum systems

Baked systems, UHV systems

Bell jars or ports on large systems

Designed for semiconductor fabrication

equipment; ports are sized for passage

of 200-mm or 300-mm wafers

"Additional sizes may be available.

''Primary seal material refers to the designer's intended seal material. Both elastomer and metal

seals are available for some flange systems.

^With elastomer seals, the minimum usable pressure is often determined by outgassing of the seal

material. The values given are conservative. The maximum usable pressure for a

flange

depends on

how well the gasket is contained. In knife-edge and beaded-edge face seal flanges, the gasket is well

contained and large internal or overpressures are acceptable. Unless the

0-ring

outside diameter is

confined by an overpressure ring, ISO flanges will not tolerate internal pressure differentials much

greater than 5 psig.

'^Flanges of this type are sold under a variety of trade names.

* Refers to flanges made according to this ISO (International Standards Organization) standard.

These flanges are sold under a variety of names.

of common flanges are provided. The designation of flange sizes can be a source

of confusion. The outside diameter (O.D.) is sometimes used as the size, while

other times the largest tubing size a flange can accommodate is used. With ASA

(ANSI) flanges, the nominal pipe size used with the flanges in steam service is

still used as the flange size. When specifying and ordering flanges, enough di-

mensions should be given to identify the flange with certainty.