Isolation Valves

Pfeiffer Vacuum offers a variety of different valve types. The area of application is defined, among things like the process sequence, the process temperature, the ultimate vacuum to be achieved, or the gas composition. In the following passages you will get an overview of our valves types, seals and sealing mechanisms as well as actuator types. You will also get tips and answers to the most important questions before buying a valve.

Valve types

This first section will help you select the valve with regard to the following requirements:

• What is the purpose of the valve?
• What does the installation space look like?

Angle valves have the advantage that the gas flow is diverted only once. Inline valves allow the valve to be installed simply instead of an intermediate piece (in a straight pipe), as the connecting flanges are opposite each other. Angle and Inline valves usually operate with a bellows as an external seal. An edge welded bellows or hydroformed bellows allows the linear movement of the valve plate against the housing, the bellows is sealed against the housing with a static elastomer seal. Another design uses a sliding elastomer ring on a piston rod (linear feedthrough) as an external seal, which allows faster switching frequency and higher gas flow at a slightly lower tightness. These valves operate at high differential pressures up to over 1 bar and can be used, for example, as a "switch" in backing-vacuum or to open a bypass into the main chamber. Diaphragm valves operate on a similar principle, but enable lower gas conductance values. Depending on the process the number of cycles until maintenance differs.

Ball valves are very robust and can be opened and closed even at several bar differential pressure. Inexpensive KF versions have leakage rates of 10^-6 mbar l/s and a temperature resistance of only 80 °C. The "machined" version can be used in the HV ranges and up to 150 °C. Three-way ball valves can be used as mixer tap, but are not free of overlap (i.e., gases can be mixed during turning).

Gate valves also have this inline design with a much shorter installation length. If space is limited, the gate valve is the suitable solution. It also allows completely free unrestricted gas flow (has an optically free passage). Gate valves are built for high or ultra-high vacuum. Most of them they can only opened at a low differential pressure around 30 mbar and are used in airlocks or upstream of pumps.

Butterfly valves and diaphragm valves offer a (functional) compromise between inline valve and gate valve at significantly reduced cost. The installation lengths are also shorter than for the inline valve, but longer than for the gate valve, while at the same time not having a completely (optically) free passage. The choice of nominal sizes is limited. Butterfly valves flip a plate across the passage to close. The high-vacuum version is used in front of the turbopump to control gas flow in "downstream" operation by varying the cross-section. With some add-ons you can automate this. In our control valve section we have several models.

Gas dosing valves can be used to set a low gas flow. Automated versions are used for "upstream" control. Pfeiffer Vacuum supplies two-stage valves with "soft pump" for smooth venting.

Seals and sealing mechanisms

This section will help you select the valve with regard to the following requirements:

• What temperatures is the valve exposed to?
• What gases is the valve exposed to?
• What pressures is the valve exposed to?

A distinction is made between internal and external leakage rates. The internal leakage rate indicates the tightness at the closed valve position. The external leakage rate describes the tightness of the housing against atmospheric pressure. While the external leakage rate can be minimized to UHV-compatible levels by metallic seals and bellows, the internal sealing is usually done with elastomers. Elastomer seals in the form of O-rings are most commonly used in valves. The elastomer on the valve plate (usually FKM) and the actuator type determine the bake out temperature for these valves. The excellent chemical properties and good permeation and outgassing characteristics make FKM-sealed valves suitable for UHV when accurately processed.

Diaphragm seals are made from elastomers such as FKM. The functional principle: the edge of the diaphragm is attached to the valve body, whereas the center is connected to the movable diaphragm plate. A saddle-tipped protrusion in the direction of flow contacts the depressed diaphragm, blocking the flow. The elastomer seals are ideally suited for use in backing to high vacuum and for temperatures up to approx. 100 °C.

Metal seals do not contain elastomers therefore they are UHV compatible. They can be baked out to 300 °C and can be used in vacuum up to 10^-12 mbar.

Actuator types

This section will help you select the valve with regard to the following requirements:

• What functions should the valve have?
• Does the valve have to fit into an existing plant?

Manual valves with hand wheel or quick-closing/-opening are mostly suitable in the basic version for high temperatures up to 150 °C. They are not limited by electronic components or sensitive supply lines, they are only limited in temperature resistance by the seal.

Pneumatically actuated valves are requiring an oil-free compressed air supply, which provides the switching power. Frequent standard is "pressure less closed" ("normally closed"). However, double-acting pneumatic cylinders are also used.

A small control valve in the compressed air supply line is electrically switched with a voltage according to your specification. The pilot valve can be located directly on the valve or in a valve terminal in a central control system.

For position indicators different technologies are used: Micro switch, reed switch and bipolar junction transistor. They indicate the open or closed position. A micro switch is a mechanic switch that means it is triggered mechanically. The reed switch is an electrical switch operated by an applied magnetic field. A permanent magnet in the actuator of the vacuum valve is responsible for the magnetic field. The bipolar transistors (NPN and PNP) can be used as "ON/OFF" solid-state switches by biasing the transistors at the base terminal differently than with a signal amplifier.
Electromagnetic valves are opened directly with an electromagnet. The switching capacity limits these valves to small diameters up to DN 40.