The Line Comes Off The Pressure Switch


Everything is working well on the compressor when all of a sudden the line blows off the pressure switch on the compressor. If you've had this happen to your air compressor, the logical step is to push the line back in the hole. That may or may not work. Here's why this happens and how to fix it.

To better understand why the line comes off the pressure switch, it is useful to know what the line is and how it is attached.

The line to the pressure switch commonly runs over from a fitting on the compressor tank.

The purpose of this air line is to allow compressed air that may be trapped over the compressor piston to escape when the compressor stops. The air that may be trapped over the piston is vented through the compressor unloader valve, and it is the unloader valve that this line commonly connects to.

The photo shows a typical pressure switch with a side unloader valve.


The line-comes-off-the-pressure-switch - side unloader valve

The line from the tank fitting will typically insert where shown by the red arrow. This type of air line is often copper or metal, and normally uses a compression fitting to attach the line. It is not common for this line to self-disconnect from the unloader valve, unless the attaching nut is loose. Reinserting the line and tightening the nut is normally all that is needed should this line comes off the pressure switch.

It is the pressure switch with a bottom-inserted air line that seems to most frequently self-disconnect. This type of pressure switch typically had a hole in the bottom plate into which a plastic air line was inserted. Inside the hole was an instant fitting which was an integral part of the switch and the unloader valve inside the pressure switch.

Over time and with frequent compressor use, the heat generated by the compressing of air would travel throughout the compressor, even to where the plastic air line was inserted into the fitting inside the pressure switch. That continuous exposure to heat would harden the plastic tube. The collett inside the fitting could not longer "grab" the surface of the plastic tube due to this hardening, and air pressure inside the tube would then blow it out of the fitting.

It seems to me, based on the newer design of unloader tube to bottom-connected fittings, that the pressure switch manufacturers have recognized this frequent problem and resolved it by moving the unloader tube fitting outside of the pressure switch - to a cooler area - and one that may allow a more robust connection to the plastic tube. This newer design can be seen in the next image.


The line-comes-off-the-pressure-switch - bottom-unloader-valve

Fix The Line Comes Off
         The Pressure Switch



Knowing why the line blows off the bottom of the switch is fine, but how is it fixed?

Fairly simply, actually. Since there is some slack in almost all unloader tube lines, remove the tube from the pressure switch if it isn't already off, and cut off about 1/2" from the end of the line.

This will remove the heat-hardened part of the tube, leaving a fresh, and softer tube surface to be inserted into the fitting, and hopefully to fit snugly and firmly to prevent blow off of the tube.

If there isn't enough slack in the tube to allow sufficient to be cut off, then it's time to replace the tube. I don't recommend that you use poly tube as it may weaken when exposed to the compressor heat. Visit an industrial fluid power supply shop and ask for a length of polyurethane tube, or any other higher heat resistant tube, to be use instead. You should be able to get one long enough (add 6" so your line has sufficient slack) for a couple of dollars.

Here is the compressed air fitting forum page if you would like to ask a question.