Pressure Relief Valves (pictures)

eddiepotts · April 12, 2011, 10:24:46 AM

I have found lately I would get up to 90# of air then it would slow way down and take a little while to reach the full 120#. I found the pressure relief valve on the air dryer was lifting. I tapped on the pin and it slowed way down and air shot up like normal. After a minute it started to blow again but held pressure. I pulled the valve after the bus bleed down the next day to clean and look at it. What I found was a rusted and pitted ball check in it. I can see it has 150# stamped on it but not a CFM. Now I see the things come in all kinds of CFM ratings. What do we need? They cost from $8 to $80. The one thing that I will change is that it points down so any moisture that gets on top of the ball sits there and causes it to rust.. These should be nondirectional so I will install it back straight up. This may be something some of you want to look at.

robertglines1 · April 12, 2011, 12:41:01 PM

mine sets horizontal and is 150# That's what it had in it and I replaced with same. Bob

eddiepotts · April 12, 2011, 12:44:43 PM

Bob, where did you get it and what CFM is it? I went to Napa and they sell one for an air dryer but it does not have any info on it. Plus they did not have one in the store so I did not want them to order it until I was sure it was what I needed.

robertglines1 · April 12, 2011, 12:49:38 PM

I got mine at local brake supply. don't know cfm. would think it would be limited by size of opening in pop off I think the od thread is 1/8 inch pipe thread fine. Is that like yours? Any where that sell truck air brake supplies

eddiepotts · April 12, 2011, 01:14:39 PM

I think mine is 1/4. It is somewhat limited to CFMs but it should make a difference. Even Napa has allot of differant ones being changed be CFM. I know our compressors are high and you would not want to use one from a small pancake compressor like you have in the garage. Even mine blowing fairly good, the compressor was able to keep up. If under rated I would think you would still blow lines.

robertglines1 · April 12, 2011, 01:23:41 PM

1/4 inch od would be 1/8 id pipe thread by id I have one laying out I will go ck it to make sure.

robertglines1 · April 12, 2011, 01:46:10 PM

It takes a 1/4 inch id for 130 to 170 lbs pressure for proper flow rate. I couldn't find old one. but spec is for output of 750 air compressor. Hope this helps. I just took mine to brake supply and they matched it up. sorry couldn't be of more help Bob

eddiepotts · April 12, 2011, 01:55:37 PM

I was thinking mine was 1/4 ID but I have to see when I get home. I don't have a truck store close to take it to. I was hoping to find specs and maybe buy it from one of the supply houses I deal with. Or Napa

bevans6 · April 13, 2011, 04:56:18 AM

Look on your wet tank, it may have a pressure relief valve. That's where they are stock in a lot of air brake systems. If it has a pressure relief valve set at the DOT standard 150 PSI then you don't need another one on the air dryer and you could just put in a plug. That's what I would do, anyway.

My MC-5C has one on the wet tank, fwiw.

Brian

Oonrahnjay · April 13, 2011, 06:52:24 AM

Quote from: bevans6 on April 13, 2011, 04:56:18 AMLook on your wet tank, it may have a pressure relief valve. (snip)

My bus is quite different from N American buses in a lot of ways but the pressure relief valve is about 9" from the compressor. No matter where any blockage may be in the system, the compressor is protected. I like that.

eddiepotts · April 13, 2011, 08:21:42 AM

Here are some pictures if anyone is intrested in how they are built. If I can find a new ball today I will just change it and fix this one.

bigjohnkub · April 13, 2011, 08:35:23 AM

You can get the ball at any Bearing supply store.

Big John

luvrbus · April 13, 2011, 09:56:52 AM

Cfm are not going to be a big part of the relief valve on the dryer your compressor is probably going to be a 600 to 750 that is 10 to 15 cfm fwiw they just started that crap a few years ago buy a 150 # valve and you will be ok it is about pressure not CFM's

good luck

eddiepotts · April 13, 2011, 10:39:59 AM

Clifford, You may be right but like a small pin hole in a tank would not stop equipment from rupturing if you have a large volume going in. So what good would a relief be if it could not overcome CFMs and save your equipment. It would change it from a safety device to a warning device. At any rate I went down to our machine shop and found a big bucket of balls that someone has saved. Sure enough I found what I needed and all is working again. It will be back on the bus tonight and it would be great if it would hold air for more than 24hrs now.

luvrbus · April 13, 2011, 11:45:27 AM

My point pull the ring on a 3 cfm valve and your 12 cfm air compressor on the bus won't air the bus up, for years the relief valve and the tank drain valves were the same unit now they figured out how to get you for 2 valves

good luck