Compressor Unloading issues

[Joseph Marasco]

Doosan XP 825, Cummins ISC power.

Mechanically the compressor works perfectly. We purchased this unit last summer with a plethora of issues, including a blown out drive coupling for the air end. I replaced the coupling and changed the separator, performed routine maintenance on the unit, and worked the bugs out.

Start up the engine, runs great, and bypasses air properly. Here's where the issues are. Once I activate the "Service Air" switch, RPMs build and we're compressing air. Once the service valve is closed, the airend starts to unload, but continues building pressure and overpowers the engine forcing shutdown. When I release system air pressure the engine fires right back up, no issues.
If I actuate the Service Air switch with the service valve closed, the unit moves directly into "Unload" and runs in bypass with no issues. Open the service valve, we're compressing as it should. Close the valve, engine slowly shuts down as in the last scenario. Now for the proverbial "wrench in the gears".... This is an intermittent problem.
One day the compressor functions flawlessly between service air and unload multiple times during our work day. The next day it will act as I described above.

Hopefully one of you can help me. Thanks in advance!

 

[Joseph Marasco]

Here's an update.
I spent most of the day today working on this machine. I noticed that when it was functioning "correctly" and in unloaded state, the RPM's were a little low. around 1000. The engine was shaking a bit and smoking a bit as well. A few times it would unload and run at 1200 rpm, when it did it would run much better. I checked the pick up tube and the supply hoses, ok. Since I didn't have access to a clear hose to see if I was sucking air, I replaced the lift pump with one we had in stock. That seemed to do the trick. I cycled the compressor 10 times or so with no failures. I guess when it is working on a job site tomorrow I'll know for sure.

 

[Joseph Marasco]

Ok. Here's another update on this compressor. Tuesday it went to work and apparently had issues staying running from the moment they got on site. I chaulked up the early morning hard start complaint to the temperature being so cold. There is no ether start system or intake air heater on this engine. Anyway, they got back and shared their issues with my other mechanic. I started it up and proceeded to hit the service air button. The machine functions normally. So I called the foreman and asked him to come to the shop and run through his normal procedure.

Having an extra pair of hands really helped. One thing i talked to him about was the machine "working" will have different requirements than it will just running in the yard. We hooked up one of the gunite machines ad ran through the process. Sure enough the machine slowly stalled out after the service air valve was closed. Not every time, but certainly more often. I still felt that it was fuel related so while he was working the controls, I was monitoring the machine. I felt that the throttle wasn't right and i found the bolt in the Heim joint at the throttle arm was loose. That could be the culprit. The throttle arm is controlled by an air piston which returns to idle with air pressure (sent by the unloader) and when the pressure is released the spring opens to full throttle. I tightened up the bolt and Eureka! it doesn't fall below 1400 rpms any longer. At full throttle I found the arm was not reaching the throttle stop bolt, thereby stopping the rpms at 1900. I fully adjusted the throttle linkage and air piston and now max throttle, 2000 rpms, is reached. The compressor produces a bit more pressure and even with an abrupt closure of the service valve, it returns to unload without fault.

I guess sometimes while we are looking for the big issues we can overlook the small ones. Learning experience for sure.