Air compressor questions

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Old 06-07-14, 10:34 PM
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Air compressor questions

Hello everyone,

I purchased an Air compressor and I have a few questions for those of you who are familliar with this kind of tool.

1. My compressor is an Oil compressor that has a flow rate of 5.2 CFM @ 40 PSI. It comes with a 5 gallon tank and is supposed to be able to get 125 PSI pressure. When the tank fills to 90 PSI, the compressor shuts off. How do I set it to go to 125 like it's supposed to? I read the manual and saw nothing regarding this. I have read that the cut off and kick in pressures can be adjusted on compressors... I just don't know how to do it. My compressor is a Mastercraft, here is the add ans spec sheet: Mastercraft 5G Compressor, Matte | Canadian Tire

2. Since the tank is so small, I got a used compressor tank of 11 gallons to increase air reserve. The tank has something loose in, it. Probably rust or débris. Before I hook it up I would like to do some kind of restoration to it. Can I poor in rust converter and then drain it? Afterwards it would need to be washed and drained again. Can I poor in some kind of paint to protect the metal against humidity and rust?


Any help would be greatly appreciated!

Daniel
 
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Old 06-08-14, 04:45 AM
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Welcome to the forums Daniel!

Have you adjusted the regulator? It's the knob between the 2 gauges.
I'd be leery of the used tank. There is no way to 'restore' the inside of an air tank.
 
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Old 06-08-14, 10:43 AM
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Well, tanks CAN be cleaned and sealed to some extent. But the cost of materials may be more than a new tank in better condition. I've used this a couple of times and it does work. Kreem | Fuel Tank Liner

The cleaner they talk about is basically phosphoric acid IIRC. You could probably just use Metal Prep from a paint store. The hard part will be agitating it. An 11 gal compressor tank is a lot bigger and heavier than a M/C gas tank.

As to the pressure adjustment, is it new? I'd take it back and exchange it. I see in the reviews, someone else had the same issue as you...or is that you?
 
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Old 06-08-14, 10:57 AM
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I've cleaned fuel tanks before but the issue with air tanks is the amount of psi. I believe in keeping air tanks clean but once they go so far they can become dangerous.
 
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Old 06-08-14, 11:12 AM
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Shoot, I forgot to mention that! I agree, any major pitting and it should be replaced.
 
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Old 06-08-14, 04:03 PM
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The knob between the 2 gauges adjusts the pressure to the hose (and Tools). It doesn't affect the pressure Inside the tank nor the cut in/cut out point.
 
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Old 06-08-14, 04:07 PM
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There is no way for me to know the condition Inside the tank. Could be very good or it could be rusted. All I know is that there is noise for débris when I move it around. I will try to get the débris out to see what it is.

I use rust converter all the time and it does a great job at turning rusty surfaces into no-rusty ones. I don't see why it would not work Inside an air tank.

I paid 25$ for the used tank. A new one is 80$ around here. So even if I spend say 10-20$ for products to remove rust and coat it, I'm still ahead. Plus this tank came from a defective compressor, so it's much better quality than the portable tire inflator tanks that sell for 80$.
 
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Old 06-08-14, 11:05 PM
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I use rust converter all the time and it does a great job at turning rusty surfaces into no-rusty ones. I don't see why it would not work Inside an air tank.
The problem is when the rust has caused a thinning of the metal and this thinning WILL cause the steel to have less strength. Pressure vessels are generally designed for a burst pressure that is about five times the working pressure but unless the tank has an ASME label you cannot know for certain what the design working pressure may be.

Even if you get the rust out and are successful in "converting" the remaining rust you cannot be certain of the strength of the tank without testing. The best test would be with an ultrasonic thickness tester but few people other than a pressure vessel inspector would have one or know how to use it. You could do a hydrostatic test by filling the tank completely with water and then using a pump to add more water to a pressure of 1-1/2 times the setting of the safety valve and then doing a hammer test on the welded seams and nozzles (connections) but I still wouldn't consider it safe.

If you cannot see into the tank then the nozzles must be pretty small, no larger than 1/2 inch pipe and if they are smaller then additional storage volume may be a moot point as far as releasing the volume needed for the air tools. I suspect the pressure regulator is either a 1/8 inch or 1/4 inch size and if the former it will not pass enough air volume for much more than an air brush. A standard 1/2 inch impact wrench needs a flow rate of 16 CFM or more for proper operation. Also, an additional 11 gallons of storage is maybe an additional 30 seconds of run time on an impact wrench.

As for changing the cut-out/cut-in pressure settings there is a pressure switch that is adjustable. Look for a box connected to the tank with pipe or tubing as well as having the electrical cords for the motor and power cord. The cover is usually held on with a single screw or acorn nut. Remove the cover and there are often instructions on how to change the settings. Generally the nut over the main spring is turned clockwise to increase cut-out pressure and turned the opposite direction to lower the cut out pressure. Some switches will have a "fixed differential", the difference between cut-in and cut-out and other switches will have a separate adjustment for the differential.
 
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