By Dan Herring

Furnace interior view showing vacuum-furnace power feedthru arrangement

This is the 11th in a series of articles in our Vacuum Heat-Treatment Series. This part discusses vacuum valves, penetrations and flanges found on most vacuum vessels; where they are used, how they operate and a little about how they should be maintained.

Fig. 1. Plethora of valves, seals, penetrations, feedthrus & flanges on a module quench chamber.

Valves intended for vacuum service are subjected to a variety of special conditions (Fig. 1), ranging from high and ultrahigh vacuum levels to low, high and ultrahigh pressures, differentials in pressure and differentials in temperature as well as variable frequencies of mechanical operation. They can be supplied in a number of configurations: ball valves, gate valves, butterfly valves, needle valves, isolation valves, pressure-relief valves and control valves just to name a few. The type of valve in use is typically identified by its design or function, and each type can be actuated in a variety of ways (manually, electro-magnetically, pneumatically, electro-pneumatically or via electric motor). Position indicators and limit switches located on the valves are common.

Here’s a summary of the most common types of valves.

Ball Valves

Application

Fig. 2. Pneumatically operated ball valve (photo courtesy of A&N Corporation)

Ball valves (Fig. 2) offer straight-through, unimpeded flow with a minimal valve body footprint. Because of their unique design, ball valves are less sensitive than other vacuum valves to particulate contamination and, therefore, are especially useful in "dirty" vacuum applications. For example, ball valves are almost always used to isolate scrubbers and traps downstream of mechanical pumping systems.

How They Work

Ball valves are made of a body, stem, ball and end caps. The ball is sealed within the body by end caps, creating a vacuum-tight central cavity. The valve is opened and closed by turning the stem 90 degrees back and forth (1/4 turn). The sealing seat for the ball wipes the ball clean as it is opened and closed. This last features provides the "self-cleaning" action that makes this valve fairly robust to particle-rich effluent streams. Pneumatic actuators are available.

Gate Valves

Fig. 3. Gate valve (Photo courtesy of A&N Corporation)

Application

Gate valves (Fig. 3) provide straight-through, unimpeded flow in relatively large diameters. These valves are characterized by their low flow resistance and compact size. They are ideal for space-constrained applications that require maximum flow conductance. Typical inside diameter sizes range from 1-12 inches (25 mm-300 mm) with port terminations normally available in multiple flange families (ISO, CF and ASA). These valves can come with either manual or pneumatic actuators. For ultrahigh vacuum applications, a copper bonnet seal version with CF port terminations is typical.

How They Work

A central carriage or gate is raised and lowered by the actuator within the body. In the open position, the gate retracts completely from the tube aperture, allowing unrestricted flow. In the closed position, a sealing ring is compressed against the surface on the inside of one of the body ports. Depending on the direction of movement of the gate, a distinction is made between rebound valves, shuttle valves and rotary vane valves. Typically, they can seal against a differential pressure of no greater than 1 mbar (0.015 psi).

Poppet and Angle Valves

Fig. 4. Poppet valve (Photo courtesy of A&N Corporation)

Application

Poppet valves (Fig. 4) are typically sealed and can operate in vacuum ranges down to 10-8 Torr. These valves are made of a body, actuator and internal poppet. A pneumatic, bellows-sealed actuator transports the poppet linearly. Rubber plugs or small plates that seal against blade-shaped valve seats are also used for extremely small valves. Valves are typically supplied in several body styles: 90-degree angle, in-line and angle in-line. Port terminations are commonly available in ISO-LF/QF, CF and Tube Ends. All-metal versions typically have copper bonnets and valve seats.

How They Work

Poppet valves are made up of a body, actuator and internal poppet. The (manual or pneumatic) actuator transports the poppet linearly. When the poppet is moved toward the top of the valve, the internal body cavity is open to the system and flow is unimpeded (maximum conductance). Once the actuator moves the poppet to the bottom of the valve, a sealing ring is compressed onto the sealing surface of the lower body of the valve. This action creates a vacuum-tight seal and stops the flow.

Butterfly or Plate Valves

Fig. 5. Butterfly valve

Application

Butterfly or plate valves (Fig. 5) are commonly used to isolate the pumping system of a vacuum furnace from the main vessel. They are subjected to repeated cyclical operation, and their simple design ensures a high degree or reliability.

How They Work

Butterfly or plate valves use a sealing plate that is swung open by some sort of lever or tilted open by means of a simple rotary motion, with the valve plate remaining in the valve opening. Plate valves, due to their design, are used to close very large openings.

Other Types of Valves

Valves used on vacuum furnaces have many special purposes, including:

Fig. 6. Ultrahigh-voltage ceramic-to-metal feedthru (photo courtesy of Pfeiffer Vacuum)

Feedthrus

When electrical power, optical signals or mechanical movement must be transmitted from outside the vacuum vessel to inside the vacuum vessel, feedthrus are usually involved. They are classified in the following general types:

Rotary Motion Feedthrus

Continuous motion and high-speed rotational movement are best conveyed through rotary feedthrus. Application examples include fans mounted inside the heating chamber and seals for rotational part movement.

Electrical Feedthrus

Electrical feedthrus come in all shapes, sizes and applications; from the need to pass thermocouples into the chamber to high-voltage and/or high-current power feedthrus. The choice of electrical feedthrus (Fig. 6) depends to a great extent on the amount of current and voltage that must pass through the connection. Feedthrus with glass-to-metal seals are best suited for high-voltage and low-current devices. Ceramic insulation offers the greatest stability (mechanical, thermal).

Mechanical Feedthrus

Gases as well as liquids (e.g., hydraulic fluid) can be introduced into the vacuum vessel by means of either permanently welded connection points or via flange type pipe connections.

Site Glasses

Fig. 10. Combination site port and light illumination (photo courtesy of J.G. Papailias Co.)

These are windows into the vacuum process and are normally positioned on the vessel so that a particular event (e.g., a load transfer from the heating chamber to the quench) can be observed. Site glasses typically consist of safety glass held in place by a flange and sealed vacuum-tight by means of an “O” ring. Sight ports can also be provided with an arrangement whereby half the window is see-through glass and half is a light designed to illuminate the area under observation (Fig. 10).

Often when observing directly into the heating chamber, an optically dense baffle of some type is provided to block line-of-sight radiation from damaging the seals holding the glass in place when not making observations.

Flanges

Two types of connections are found on vacuum equipment: detachable (flange type) and non-detachable (connection type). The latter typically are attached to the vacuum vessel by brazing, welding, or soldering. Mechanical strength and resistance to changes in temperature and pressure are key requirements.

Detachable joints typically involve some type of fixed flange and clamping arrangement. Since leakage is a major concern, stainless steel reinforced hoses are generally preferred over thick-walled rubber or thermoplastics.

Conclusion

Without specialized valves, feedthrus, flanges and penetrations, modern vacuum equipment would be plagued with both operational problems and leaks. These devices allow our vacuum systems to operate in a range that permits heat treating of today’s demanding products.

Next Time: Part 12 of this series discusses leak testing and leak detection and also reviews the choices available for leak repair.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. The Vacuum Technology Book, Volume I, Pfeiffer Vacuum (www.pfeiffer-vacuum.com)
2. A & N Corporation (www.ancorp.com)
3. Mr. George Papailias, J. G. Papailias Co., Inc. (www.papailias.com), private correspondence.