AD-9 Air dryer questions

[TheHubbardBus]

My gradual rebuild of our air brake system continues (lol).


We're still purging constantly from cut-out to cut-in (once pressure drops to cut-in it stops). Governor is new, check valve is new, & control line from it to the air dryer is good to go. Suspect purge valve assembly itself now.


I've decided to overhaul the whole things, to include the purge valve assembly & dessicant cartridge. But I have a couple issues / questions:


1) Hard seat vs soft seat purge valves: What's the difference? Are they interchangeable? Which should I get?


2) I CANNOT get this damn dessicant cartridge removed from the cover. Using a strap wrench now with a foot on the cover to try to turn it, but nada. Any tricks to this? Called Bendix & they say it might be over-torqued and/or cross-threaded. I've seen the threads on a new cartridge - don't see how anyone could cross-thread - but I've seen some stupid stuff before. My bench vice was next on the list of options, but it's literally 1/2" too small to grip the cartridge.


If I can't get the cartridge off, I may be looking at a whole 'nother dryer

[TheHubbardBus]

Got if figured out. Just answering my own questions in case it helps someone else.


Putting the cartridge in a vise (went to the corner auto shop as mine wasn't large enough) & turning the case cover got the thing to break, but not without heroic effort. Threads were not cross-threaded. The single bolt securing the cartridge to the dryer cover goes into a blind bolt-hole that was filled with oil. Think a large part of the problem removing it was it being hydro-locked.



Also called Bendix, and according to the tech rep I spoke with (at least with AD-9s) the soft-seat purge valve assembly is essentially just a better-sealing variant as opposed to the hard-seat. Totally Interchangeable.

[Booyah45828]

If they've never been changed, or haven't been changed in a while, they can be a complete bear to get off.

No clue on the difference in check valves. I'd make sure everything is spotlessly clean when installing the new one.

[TheHubbardBus]

Got if figured out. Just answering my own questions in case it helps someone else.


Putting the cartridge in a vise (went to the corner auto shop as mine wasn't large enough) & turning the case cover got the thing to break, but not without heroic effort. Threads were not cross-threaded. The single bolt securing the cartridge to the dryer cover goes into a blind bolt-hole that was filled with oil. Think a large part of the problem removing it was it being hydro-locked.



Also called Bendix, and according to the tech rep I spoke with (at least with AD-9s) the soft-seat purge valve assembly is essentially just a better-sealing variant as opposed to the hard-seat. Totally Interchangeable.

[TheHubbardBus]

Originally Posted by Booyah45828

If they've never been changed, or haven't been changed in a while, they can be a complete bear to get off.

No clue on the difference in check valves. I'd make sure everything is spotlessly clean when installing the new one.

@Booyah45828, Thank my friend
Yeah, a bear it was. Bendix rep had me scared thinking the cartridge could be cross-threaded. I was so stoked to see that wasn't the case.
Didn't even know the old cartridge was missing a plastic flapper valve over the vent till I picked up the new one to compare. Guess that was the unidentified residue I found inside the case.
Will do on cleaning. Going to blow everything out w/ WD-40 & then compressed air before putting her back together.
I'm 99% sure this will be the end of my issues. The o-rings on the purge valve were old, flat, & hard. I was actually able to stop the purging once by pressing on the shoulder bolt w/ my finger.



One question for you if you don't mind me asking: The main (large, flexible) discharge line coming from the compressor connects to an npt fitting with a beveled brass face that matches the contact area on the hose end. Is that metal-to-metal interface all that seals the connection? Should I apply anything to the threads, and how tight should that connection be? I tried looking up info for those types of connections, but I don't know what they're called, so... (If my explanation isn't clear, I can provide pictures)

[Jolly Roger bus 223]

any NPT fitting need teflon tape and or pipe joint compound.
in my world we use both on every npt(national pipe thread) joint the combination of both helps alot on high pressure systems.
anything besides npt like a flare connection should not need anything special for fitting to fitting.
a little bit of lube if it is sqeaking while tightening and type of lube is dependent on what the pipe is used for.
If you are cutting and flaring your own tubing then of course the correct flaring tool whether single or double flare is decided by the fitting(single flare,double flare,JIC,AN) and so many more.

[Booyah45828]

Originally Posted by TheHubbardBus

Going to blow everything out w/ WD-40 & then compressed air before putting her back together.
I'm 99% sure this will be the end of my issues. The o-rings on the purge valve were old, flat, & hard. I was actually able to stop the purging once by pressing on the shoulder bolt w/ my finger.

Sounds good, it doesn't have to be surgical clean, but make sure there isn't dirt, grime, or corrosion in there, that will cause those o-rings to not seal and leak.

Originally Posted by TheHubbardBus

One question for you if you don't mind me asking: The main (large, flexible) discharge line coming from the compressor connects to an npt fitting with a beveled brass face that matches the contact area on the hose end. Is that metal-to-metal interface all that seals the connection? Should I apply anything to the threads, and how tight should that connection be? I tried looking up info for those types of connections, but I don't know what they're called, so... (If my explanation isn't clear, I can provide pictures)

That's likely just a flare type fitting, whether it be AN, JIC, JIS, etc.

No need to put any sealant or tape on the threads, The seal happens at the beveled spot your talking about. The tube end will actually deform slightly to make the seal against the fitting.

I sometimes put grease on the threads so that it's easier to disassemble the next time, especially if it's a steel fitting. With brass, which they typically are, it's not as much of a problem. Steel fittings can be a complete bear to disassemble if they rust.

I'm not sure of torque specs, with flare fittings, I usually don't torque them. Hand tight with a wrench is what I do. It doesn't need to be overly tight, Just tight enough not to leak.

[Booyah45828]

Oh, and pictures are worth a thousand words. I'm fairly certain it's a flared fitting. That's typically what you'll find at a spot like that, but it won't hurt to verify.

[TheHubbardBus]

@Booyah45828, understood. But you guys are right about what fitting it is now that I know what to call it. Had no problem with it or any other hookups. Completely rebuilt dryer is in & hooked up with no leaks and...


IT'S STILL PURGING CONSTANTLY (between cut-out & cut-in).


Just to recap: New governor, completely rebuilt dryer (including check valve), all new fittings on everything (no leaks at any of them).



I turned off the key at 110 psi and the purging stopped instantly.

[Native]

Please bear with me on this since I do not know the proper lingo nor the specifics of pneumatics.


It sure sounds like you have an electronic solenoid/valve which controls the purging and this solenoid/valve is stuck causing the constant purging. It could also be a pressure sensor that tells the solenoid/valve to open. If you can find the components that control the purging, then find those that are electronic (wired) and disconnect the wire, you can see if it stops the purging. Either way, you would be one step closer to finding the problem.

[TheHubbardBus]

Thanks Native. Unfortunately the only electric components in the system are heaters, & they shouldn't be part of the problem (to my knowledge).



The purging is controlled by air pressure via governor, which itself is controlled by air pressure from the reservoir and the compressor.

[mmoore6856]

i would bet on a bad check valve as the only air that should be purging would be the air in the air dryer. that emptys out and purging stops unless air from the wet tank can keep going backwards because of a bad check valve . also the purge valve remains open until the governor shuts off the air to the purge valve at about 90 psi and the compressor kicks back on

[mmoore6856]

and there are no solenoids or electronic parts aside of the heater and thermostat

[TheHubbardBus]

Originally Posted by mmoore6856

i would bet on a bad check valve as the only air that should be purging would be the air in the air dryer. that emptys out and purging stops unless air from the wet tank can keep going backwards because of a bad check valve . also the purge valve remains open until the governor shuts off the air to the purge valve at about 90 psi and the compressor kicks back on

Thanks mmoore.

I don't think it's the check valve, as it was the 2nd thing I replaced after the governor trying to fix this issue.


I'm still learning all this stuff, but I'm starting to believe it might be the unloader mechanism of the compressor itself.

[Booyah45828]

Originally Posted by TheHubbardBus

Thanks mmoore.

I don't think it's the check valve, as it was the 2nd thing I replaced after the governor trying to fix this issue.


I'm still learning all this stuff, but I'm starting to believe it might be the unloader mechanism of the compressor itself.

That would be a new one for me. I'll have to look at a compressor and see if something like that is possible. I know I've never seen something like that happen, and have seen dozens of the check valve fail.

[Native]

Originally Posted by TheHubbardBus

Thanks mmoore.

I don't think it's the check valve, as it was the 2nd thing I replaced after the governor trying to fix this issue.


I'm still learning all this stuff, but I'm starting to believe it might be the unloader mechanism of the compressor itself.

That is a possible cause. I've been doing some reading ....


Bendix Advanced Troubleshooting Guide for Air Compressors.pdf


in particular pages 11 and 12 address "safety valve releases air". [Note: the name is Advanced Troubleshooting Guide for Air Brake Compressors]

[TheHubbardBus]

Thanks Native. It's not the safety valve, though. That's a pressure-blow-off valve separate from the purge valve.


I've been thinking about this all night, and I think I might have this figured out, but I'm going off memory, so I'm not sure.



After I fixed the major leak at the push-pull valve (parking brake), that's when I realized I had this problem. And that's when I replaced the governor. But at the time, I had absolutely no idea how anything in the air brake system worked, so I configured the new governor exactly like the old one, without thinking if any of it was incorrect.


Here's the thing: Again, if my memory is correct, I think whoever worked on this thing last had the unloader port plugged where it connects with the compressor! Which means I did the exact same thing. So with that plugged port, the compressor would never unload, right? It sticks in my mind (I think) because I thought it was odd that there was no direct airway connection between the governor & the compressor.


Memory's fickle. So we'll see. Plan on working on it Monday. Guess I'll know in the first 10 minutes if I'm right or not. I hope I am!

[mmoore6856]

double check the governor is plumbed correctly

[mmoore6856]

post a pic of the governor for me to verify

[EastCoastCB]

Originally Posted by mmoore6856

post a pic of the governor for me to verify

Mine looks like a total douche who likes smelling his own farts.