Operation is a word used to describe if a valve is normally open (NO), normally closed (NC) or universal (U). NO and NC refers to the state of a 2-way solenoid valve when de-energized or off. NO, NC or U is used to describe the state of a 3-way valve when it is de-energized or off. Below is a table that describes operation modes of 2-way and 3-way valves.
These operational modes do not apply to 4-way valves since a 4-way functions the same as two, 3-way valves where one is NC and the other NO.
Media are the types of fluid that flow through the valve. Typical solenoid valve media are air, inert gas, fuel gas, water, oil and steam. Media are a very important consideration in solenoid valve specification because the materials of the valve must be compatible with the media. If the valve internal materials are not considered, internal corrosion can take place.
Media temperature also is an important consideration, and in selecting the right valves, it is useful to know what minimum and maximum media temperatures are. High temperature media such as steam can have adverse effects on the valve's coil temperature rise.
Regardless of what the media are, however, the valves need to be clean and free from debris left from piping installations. When using a valve with a gas media, such as compressed air, installing a filter before the inlet of the valve is recommended. When using a valve with liquid media, a strainer is recommended. In all cases, installers should check with the manufacturer for recommended filtration values, which should vary depending on the specific solenoid valve used.
Size may seem obvious when choosing a solenoid valve, but a few considerations need to be noted beyond a pipe thread connection. Size has a direct effect on media flow, a fact that is not determined by pipe size but by the size of the orifice in the valve that the media must flow through. Flow requirements can be specified in any of the following units: gallons per minute for water or liquids, standard cubic feet per hour for gases, pounds per hour for steam. However, Cv (Imperial Flow Coefficient) and Kv (Metric Flow Coefficient) are by far the most common method of specifying valve flow. Care should be taken when specifying flow-solenoid valve manufacturers are not consistent in the way they test and rate the flow of the valves. If flow is a crucial part of a specification, it is best to test that flow in the application before making a final selection.
Pressure rating of a valve is the maximum differential pressure applied into the pressure or inlet port when the valve orifice is closed that the valve can withstand without significant leakage. But there’s more to the situation than that. This maximum pressure is usually tested at the valve's minimum-rated temperature and the valve’s maximum-rated temperature for a large number of cycles. This is another point of inconsistency in the valve manufacturing world. Not every maker pressure-rates solenoid valves the same way. If the valve has third-party approval, there is consistency; however, if there is not a requirement for third-party approval such as UL, not all valves are equal.
The main differences between manufacturers' pressure ratings are factors of safety. If maximum pressure is a crucial specification point, those specifying should check with the manufacturers to see how they rate their products. Be aware that many valves have minimum pressure ratings as well-you must have at least a specific amount of pressure for the valve to operate correctly. This is most common in low-power pilot valves. For these valves, air pressure is used to create movement in the valve. Without this minimum pressure, the valve can be energized, but nothing will happen. If a minimum pressure is not specified, the application should be checked to ensure that the minimum system pressure will not fall below the solenoid valve’s rated minimum pressure.