Solenoid valves for vacuum applications
There is a lot of misunderstanding about the requirements for solenoid valves in vacuum applications. In this article, the requirements for solenoid valves are clarified. For this purpose, firstly the basic principles and definitions of pressure are explained.
Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed. The SI unit used for pressure is the pascal [Pa] and it equals one newton per square meter. The standard air pressure (defined as one atmosphere, or 1 atm) at sea level is defined as 101.325 kPa, which makes pascals a somewhat cumbersome unit to deal with. In hydraulic applications it is more common to use the metric unit [bar], which is equal to 100 kPa (approximately 10 tons per square meter), or the imperial unit [psi] (pounds per square inch, approximately 690 kg per square meter). Perfect vacuum is defined as a space void of all matter, and by definition the absolute pressure of a perfect vacuum is zero, however perfect vacuum is technically unattainable in practice.
Graphical representation of the relation between different pressure definitions.
There are several references to which pressure is measured, and these are:
- Absolute pressure (Pabs), measured relative to a perfect vacuum.
- Gauge pressure (Pgauge), measured relative to the atmospheric surrounding ambient air pressure, sometimes expressed in barg (bar gauge).
- Differential pressure (Pdiff), measured between two points in a system.
- Vacuum pressure (Pvac), measured relative to atmospheric pressure.
Solenoid valves for vacuum applications
When choosing the right valve for a vacuum application, several factors should be considered, such as:
- circuit function (2-way, 3-way, etc),
- flow rate (Kv-value),
- design principle (direct, semi-direct),
- pressure differential across the valve in all scenarios,
- leakage rate,
- response time.
As an example, automated pick and place machines used in the electronics industry do not require large flow rates, but do require a high, consistent vacuum in order to pick up components reliably, and a fast response time of the valve in order to achieve high speeds. A suction control application for a vacuum chamber does not typically require a fast response time, but may require a high flow rate in order to allow the chamber to be brought to a vacuum quickly.
Not all solenoid valve types can be used in vacuum applications, but it is a misconception that none of the standard solenoid valves are suitable and that only specially designed vacuum valves can be used. Universal direct acting or semi-direct acting solenoid valves are generally well suited for vacuum applications, since they do not require a minimum pressure differential. Indirect acting solenoid valves need a large enough pressure differential between the inlet and the outlet port to operate, which makes them unsuitable for (low) vacuum applications. On the other hand, if the pressure differential across the valve is guaranteed by system design to be greater than the minimum required at all times (for example 0.5 bar), these valves could in theory be considered. However, in practice it is recommended to consider (semi-)direct valves only. There is an exception for externally piloted (pneumatic) valves, as will be described later on.
2-way solenoid valves
A 2/2-way solenoid valve has two ports and two positions (open and closed) and can be NC (Normally Closed, opens when energized) or NO (Normally Open, closes when energized). Examples of suitable (semi-)direct operated solenoid valves that are suitable for vacuum, are the CM-D, ST-D, ST-S and DF-S series.
3-way solenoid valves
A 3/2-way valve has three ports and two positions and have different circuit functions, such as NC, NO, diverting or universal. For example, to control a suction gripper, a 3/2-way is required. One port connects to the suction gripper, one port to the vacuum line and one port to the atmospheric pressure. The direct operated 3/2-way valves of the JP Fluid Control TW-series can be used for vacuum applications.
The JP Fluid Control TW-series is a suitable 3/2-way valve for vacuum.
Pneumatic solenoid valves (3/2, 4/2, 5/2-way)
Solenoid valves for pneumatics often have a spool design and are usually indirect acting. When they switch, the valve releases a small amount of air that is used for the pilot operation into the surroundings. The valve can be piloted using the mains pressure supply (internally piloted) or by using a separated external pressure supply (externally piloted). Internally piloted valves are the most common and are not suitable for vacuum applications. Externally piloted valves can be suitable for vacuum, provided that they are supplied with an external compressed air source.
It is important to note that most solenoid valves have a specified flow direction. This means that the vacuum side must be connected to the outlet port of the valve, allowing air to travel from the higher pressure inlet to the lower pressure outlet when the valve is open.
Ultra high vacuum systems have strict leakage requirements for closed valves, and require specially designed solenoid valves in order to maintain vacuum over time.
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