3/8" NPT couplings to 3/8" rubber air hose

by Robert
(Apopka, Florida)

Hi I recently purchased a sandblasting cabinet and pressure pot. After setting it all up and plumbing in all my air lines I attempted to sandblast an axle. My pressure was great for about oh 3 seconds and then there was substanial drop. I contacted the Company that maunfactured the Cabinet and I was told on two seperate calls that every fitting leading to this cabinet and pressure pot has to be larger than than 1/4". So here is what I have to give you an idea. 80 gallon compressor rated @ 23 CFM @ 90 or 100psi, cant remember which. From the compressor air outlet on the tank is a 3/4" quarter turn valve. This valve shuts off the air supply to the shop line. From the air out side of the valve I reduced it to 1/4" with a 3/4" to 1/4" reducer. Then I put a 1/4" galv. nipple due to the filter/regulator having 1/4" NPT in/out ports and mounted the filter regulator. From the air-out side of the filter I have another 1/4" NPT galv nipple to a female coupling. From there I have a 4' or so piece of 3/8" rubber air line with male connectors on each end that connects to another female coupling on the wall which feeds a 3/4" hard air line. This line dead ends into a 90 degree fitting with another 1/4" npt female coupling. This dead end coupling is where I have my Sandblasting cabinet and pressure pot set up. The total length of line from the compressor to the cabinet is about 20 or so feet. Just to let you know the line Tees off about 3' from the compressor and feeds the inside of my shop which is 16'x 12'. There are 3 stations with hanging coil line inside the shop with 1/4" couplings. And yes some of these cheap couplings do leak air when nothing is connected to them. Now back to the original issue. So to comply with the maufacturers recommendations this is what I was going to do and want to know if I am doing the right thing. I removed the reducer from the 1/4 turn valve and replaced it with a 3/4" - 3/8" reducer with a 3/8 nipple into my new filter that has 1/2" npt inlet and oulet. I used 1/2" to 3/8" reducers on each side of the inlet and outlet to accept the 3/8" nipples. Now I am at the point where I need to get 3/8" couplings. I seen industrial type and mechanic type with MPT and FPT, I do not know the difference between the four. Once I fiqure which type to use and what MPT and FPT mean I plan to put these from the filter outlet to the hardline using a 4' or so 3/8" rubber "whip" hose and then at the end of the line connection for the blasting equipment only. All other conection will be left 1/4" "std" couplings. And I say "std" loosely but hopefully I explaiined it well enough for you to get a mental picture. My thought is that by increasing all the fittings from the 1/4" npt to 3/8"npt that will directly feed the blasting equipment I will be supplying the blasting equipment with the air it needs. Sorry to be so long winded but it can get costly troubleshooting these issues... Thanks alot.


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Bill says...

Robert, excellent description. Thanks.

Visit "Pressure Drop In Compressed Air Systems" linked from the site map page. You need to read this to get a better idea as to how pipe diameter, length and the addition of fittings all combine to reduce effective compressed air flow and pressure at the end.

In supplying equipment from the compressor tank, bigger lines are always best.

After you have read the page of info, if you have a specific question, please just ask.

You might also want to read the pages on couplers, one on connectors and another on fittings, all linked from the site map too.

Cheers,

Bill