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

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392

Rg.l.

Chapter

4.1:

Selection Considerations for Vacuum Valves

Pneumatically actuated right-angle poppet valve.

This valve has polymer nosepiece and bonnet seals, and uses a bellows for the transfer of ac-

tuation motion into the body. The spring within the bellows, concentric with the stem, closes

the valve when air pressure is removed from the pneumatic cylinder. (Courtesy of

the

HPS Di-

vision of MKS Instruments, Inc., Boulder, Colorado.)

valves a movable plate containing a seal element is forced against a surface in the

valve body to shut off the gas flow. The seal member can be either a polymer or a

metal. Two common valve types are the poppet valve and the gate valve. A pop-

pet valve is shown schematically in Figure 1. In this valve, the seal plate or nose-

piece is attached to a stem that moves perpendicularly to the sealing surface. Clo-

sure and sealing are made by pushing the nosepiece onto the valve seat. To open

the valve, the stem is lifted, pulling the nosepiece away from the seat. Poppet

4.1.2 Valves

for

Shutoff

393

valves are available as right-angle valves where the ports are at 90 degrees to each

other, or as "in-line" valves where the ports are spaced 180 degrees apart. In some

in-line valve designs the gas path is straight through, while in others it makes sev-

eral bends within the valve. Figure 2 illustrates these different gas paths.

A gate valve is shown schematically in Figure 3. Here the ports are in-line, and

the seal plate is part of a gate or carriage assembly that moves in and out of the

gas flow path. To make the seal when the gate is positioned between the ports, a

means of expansion is usually provided to force the seal plate against a sealing

area concentric to one of the ports.

In most poppet valves, the stem provides the location and guidance of the nose-

piece. This is also true in some smaller gate valves, but as the port size of a gate

valve increases, this becomes impractical. Instead, the gate assembly is usually

guided by the valve body, and some folding linkage is used to decrease the over-

all length of the valve.

Standard poppet valves are made in port sizes from 0.25 to 10 inches. Gate

valves with port sizes from 0.625 to 20 inches are readily available, but both types

of valves have been made in larger

sizes.

Gate valves with rectangular

ports,

some-

times called slit valves, are useful for applications such as handling wafers in semi-

conductor fabrication or other flat stock processed in vacuum.

Gate valves which use a means of gate expansion, have more moving parts

in vacuum and are more complicated than poppet valves, because the seal plate

must move in two directions. Other disadvantages include higher cost than a pop-

pet valve, and the gate expansion may produce mechanical jarring unacceptable

in some applications. Gate valves without expanding gates are commercially

available and have less mechanical jarring on closure. Advantages of gate valves

include relatively thin bodies, a large conductance when open, and an optically

straight-through flow path. Some gate expansion mechanisms mechanically latch

in the closed position, allowing the valve to remain closed and leak-tight in the

event of a power or compressed air failure. Advantages and disadvantages of pop-

pet and gate valves are summarized in Table 1.

Diaphragm valves (Figure 4) and butterfly valves (Figure 5) are sometimes used

in vacuum systems. In the diaphragm valve, the diaphragm is clamped at its edge

and the stem is attached to its center. The diaphragm is often made of a polymer,

and to make the seal, it is pushed against the valve body. These valves are made

for vacuum service with port sizes up to 2 inches. Butterfly valves rotate a circu-

lar vane to open or close the gas flow path. To form a seal in the closed condition,

an 0-ring is mounted in the periphery of the vane. "Scuffing" or twisting of the

0-ring can prevent a leak-tight seal. To help overcome this problem, some butter-

fly valves cam the seal disk into position at the end of its rotation.

Fig.

"In-line" poppet valve body styles and flow paths.

TO ACTUATOR

Figure 2(a) illustrates a body where the ports are "in-line," but the attached tubing is offset. In

Figure 2(b) the ports are truly in-line, without an offset. The flow path is not "optically straight

through." Figure 2(c) illustrates an optically straight through in-line valve body. This style is

useful for transfer into vacuum systems or for optical applications.

4.1.2 Valves for Shutoff

395

Fig.

Manually aauated gate valve.

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This valve has a polymer nosepiece seal and a metal bonnet seal. Actuation motion into the

body is sealed with a bellows. The gate expands and latches the valve closed when the end

of the carriage assembly contacts the body, pushing the ball bearings out of their detents

(VATLOCK mechanism, U.S. Patent No. 4,052,036). (Reprinted with the permission of

VAT Vakuumventile AG, Haag/SG Switzerland.)

Valve

Type

Table I

Comparison of Poppet and Gate Valve Advantages and Disadvantages

Advantages Disadvantages

Poppet Simple operating mechanism.

Few moving parts within the vacuum

enclosure.

Ports can be in-line.

Lower purchase price.

Most designs can tolerate opening with a

pressure differential.

Spring-closed (or open) types automati-

cally close (open) with power failure,

placing the valve in a known state.

Some valves incorporate a soft-start

bypass feature.

Gate Thin body.

Good conductance.

Many designs lock in closed position.

Short, optically straight-through path

for use in load locks or other transfer

applications.

Valve body can be bulky, especially for

large port sizes.

Lower conductance than a gate valve.

Gas path with in-line ports may not be

optically straight through.

Some designs may not tolerate opening

against a pressure differential.

Operating mechanism is typically more

complicated, with more moving parts

within the vacuum enclosure.

Designs with expanding and locking gate

mechanisms may produce unacceptable

jarring or particulate contamination.

Higher purchase price.

Hg.4.

Manually operated diaphragm valve.

B=2ri

L ^

Hz-z//

7///:^

The diaphragm is clamped and sealed between the body and bonnet at its periphery. The stem

is attached to the center of the diaphragm. For

shutoff,

the diaphragm is forced against the

valve body, isolating the inlet and outlet

ports.

(Reprinted with the permission of

VAT

Vakuum-

ventile AG, Haag/SG Switzerland.)

4.1.3 Valves

for

Control

397

Rg.5.

Butterfly valve in a partially open condition.

The valves illustrated are actuated with a stepper motor for precise control of the vane angle.

This configuration is useful for closed-loop pressure control. When the vane is sealed by an

0-ring, butterfly valves can provide shutotf. (Reprinted with the permission of MKS Instru-

ments, Inc., Andover, Massachusetts.)

4.1.3

VALVES FOR CONTROL

Valves can also be used to control pressure in a vacuum system. There are two

types of control valves—those that regulate the adnriission of gas, and those that

control the pumping or removal of gas from a system. These are also referred to

as upstream and downstream control valves, respectively. Some control valves

do provide

shutofif,

although it is a misapplication of a control valve to use it for

shutoff only.

398 Chapter

4.1:

Selection Considerations for

Vacuum

Valves

The upstream/gas admission valves are frequendy referred to as leak or meter-

ing

valves.

One common type is the needle valve, where a tapered needle is moved

into an orifice to vary the gas

flow.

Another type resembles a poppet or diaphragm

valve, but the nosepiece seal uses hard materials such as stainless steel or saphire

against a soft metal. Polymers are rarely used because creep of the polymer would

cause large changes in conductance. Mechanical reduction is frequently used in

the drivers of these valves to obtain the minute motion of the nosepiece necessary

to accurately control the gas

flow.

Metering or gas admission valves may not open

quickly, and often have only a small conductance in the "full open" case. Piezo-

electric leak valves are also available. They do not have mechanical linkages and

require only a voltage input as the control signal. Response times are typically

several milUseconds.

Leak valves are very sensitive to contamination. The gases being controlled

should be dry, or a "liquid seal" may form. A hquid seal occurs when adsorbed

material completes a seal between the valve nosepiece and the seat. Then, as the

nosepiece is retracted, there is no conductance through the valve until the Uquid

pulls away, breaking the seal. This gives a sudden opening, without any control.

Because of the small motions and openings used, leak valves are susceptible to

damage from particulates. A vacuum-grade filter made of sintered metal installed

on the supply side of the valve can prevent debris from entering the valve.

Downstream control valves or throtding valves are usually mounted near the

high-vacuum pump, and regulate the gas flow into the pump. Butterfly valves are

often used, because the vane angle can be varied to regulate the flow. When used

for control, the vane may not have a seal for complete

shutoff.

If an

0-ring

seal is

used in the vane, control can be difficult as the vane begins to move. Other down-

stream control valves move blades across the port to vary the size of the valve

opening and conductance. A valve of this type is illustrated in Figure 6. The ad-

vantage of this valve is that the blades do not protrude past the body, minimizing

the need for clearance in the adjacent components. Specially designed poppet and

gate valves are also available for control purposes.

4.1.4

VALVE CONSTRUCTION

Valves differ greatly in method of actuation, means for transferring the actuating

motion into the valve body, and the seal materials used. After a valve type has been

selected, these points need to be considered.

4.1.4 Valve Construction

399

Fig.

Multiblade valve for pressure control

I—X

(a)

Flat vanes or blades are mounted on radii of

the valve port. A mechanism within the body

rotates the

vanes simultaneously, changing the

conductance of the valve. Three vane posi-

tions are illustrated in Figures 6(b) to 6(d).

Remote actuation is usually with a stepper

motor. While more complicated than a butter-

fly valve, the vanes do not protrude beyond

the valve body. (Reprinted with the permis-

sion of MKS Instruments, Inc., Andover,

Massachusetts.)

(b)

400 Chapter

4.1:

Selection Considerations for

Vacuum

Valves

4.1.4.1 Sealing of the Actuating Meclianism

The seal for the transfer of actuating motion into the valve body is usually either

an 0-ring or a metal bellows. In diaphragm valves, the diaphragm serves as the

seal between the valve interior and the actuating motion. When an 0-ring is used

to seal linear motion, the stem slides through the O-ring as the valve is opened or

closed. Although simple in conception, reliable sliding seals are difficult to make

and are not suitable for pressures below 10"^ torr. 0-ring seals used for rotary

motion perform better than those for linear motion. In either case, the O-ring will

be damaged very quickly unless the stem or rotating shaft has a polished surface

and the stem is well lubricated. The grease in a lubricated sliding seal may be a

contaminant, and at the least it will "carry in" gas every time the valve closes.

Also,

initial movement of the stem or shaft may cause a momentary leak past the

O-ring, producing a gas burst. Better elastomer seals that minimize these prob-

lems use two or three 0-rings in series, perhaps with pump out of the region be-

tween the 0-rings, and provide a reservoir for maintaining lubrication.

A motion feedthrough that overcomes the problems of the sliding O-ring seal

uses a metal bellows as illustrated in Figure I. One end of a bellows is fixed to the

valve body, while the other end is attached to the stem. Valves with bellows seals

may have a higher initial purchase price, but their vacuum performance is better.

Many are suitable for use into the UHV regime. After considerable use, the bel-

lows may fail, but valves are available with lifetimes in excess of 500,000 cycles.

Bellows used in valves are of either the formed or the so-called welded and

nested types. Welded ones are made by alternately welding thin rings together on

their internal diameter and outside diameter. The convolutions in formed bellows

are made by rolling and other forming techniques on lengths of thin-wall metal

tubing. Welded bellows are more expensive than formed ones. Also, in a com-

pressed welded bellows there is very little space between adjacent convolutions.

With formed bellows, space remains between adjacent convolutions in the com-

pressed condition. This is an advantage in systems where particulates are gener-

ated, because the formed bellows is less likely to be damaged. Valves with welded

bellows are frequently more compact because the welded bellows for a given

stroke is shorter than the formed bellows for the same stroke.

4.1.4.2 Actuation of Valves

Valves can be opened and closed manually, with a pneumatic cylinder, or with a

motor. The type of actuation is independent of the stem seal used.

Manual or hand actuation is usually the simplest and the least expensive. Fre-

4.1.4 Valve Construction 401

quently a screw arrangement is used for producing linear motion of the valve

stem. Another form of manual actuation sometimes found on smaller valves uses

a toggle lever and linkage to the stem.

Manual actuation is not practical for valves cycled frequently or for valves con-

trolled remotely, and pneumatic or motor actuation is then used. For pneumatic

operation, a pneumatic cylinder is built onto the valve, often with the piston at-

tached directly to the stem. The forces provided by the actuating cylinder and

mechanism determine how much overpressure the valve will seal against, and

what internal pressurization the valve can tolerate. The cylinder can be a double-

acting type, where the valve is both opened and closed with air pressure, depend-

ing on which side of the piston is exposed to the air

supply.

Single-acting cylinders,

where the compressed air either opens or closes the valve, are also used. A spring is

then used to move the valve in the opposite direction. Valves with the pneumatic/

spring combination have the advantage that in the event of a power or compressed

air failure, the valve goes to a known state. Whether this is open or closed de-

pends on the design.

Solenoid control valves are used to regulate the air to the pneumatic actuator

cylinder. In some installations, each valve might have its own solenoid, while in

others, multiple valves may be operated from the same solenoid. For critical clean

room applications, the solenoid or the pneumatic cylinder is vented to a "dirty"

area to minimize contamination. When the user provides the pneumatic control,

it is necessary to match the solenoid control valve specifications to the vacuum

valve's air supply requirements or operation may be adversely affected.

In the case of metal-sealed valves, manual actuation has advantages. Pneumatic

actuators tend to be large and expensive because of the higher sealing forces

needed. With many metal-sealed valves, it is necessary to reform the seal occa-

sionally. This is done by forcing the soft and hard elements of the seal together with

a force greater than the previously used sealing force. This procedure is easier with

a manual actuator, particularly when a torque wrench is used on the valve actuator.

When motor actuation is used, a screw or a rack converts the motor's rotary mo-

tion to linear motion. In the case of butterfly valves, a gear motor may be coupled

directly to the shaft carrying the vane. An advantage of motor operation is that

only electrical power need be supplied to the valve. A disadvantage can be that in

the event of a power failure, the valve will not go to a known state.

Remotely actuated control valves are often driven by a stepper motor. Together

with the valve controller, geared stepper motors can move the valve mechanism

by very small and precisely regulated

amounts.

Some manually actuated valves can

be used for control, but the ease of control depends greatly on the valve design.

Limit switches are available on most remotely actuated valves. These switches

provide an indication that the valve stem is at one end of its travel or the other and

can be used to tell if a valve has obeyed a command to open or close.