An unloader valve is a device that is placed on the bottom of an air compressor and is used to protect the compressor from the hazards associated with improper operation. It is also commonly used to help with settling compressor pressure, or sealing off lines from compressor oil and gas lines. One of the uses of an unloader valve is to help provide control over how the compressors are run so that the engine will not overheat and damage itself. An unloader valve is typically used in both internal and external applications, but can be used on just one or the other if the situation calls for it.
The most common application for an unloader valve is in compressors. The controls that are used to place an unloader valve on either side of the compressor will assist the operator in determining which area of the compressor will accept the compressed air, or, in the case of a flatbed, the compressed propane gas. Because there are only a few unloader valves that are required on every line, most of the time, an unloader valve is left exposed to the elements. Although an unloader valve does have an exterior that can be protected with a cover, this does not always happen, and the valve can eventually begin to rust. When this happens, it is very easy to remove the valve cover and take out the valve, or, it is also possible to get the valve out without first covering it.
If the air compressor has a compressing source (i.e. a drive system), it is usually possible to run wires from the unloader valve to the compressor. If the compressor has a negative pressure, or a manifold pressure that needs to be reduced before the compressor can be started, it is necessary to run a line that will release the pressure of the unloader valve so that the compressor can run normally. The lines that are used to release compressed propane pressure should be kept clean and well-maintained as the unloader valve will get dirty over time. The lines may need to be cleaned at least once a week, although this is not usually necessary.