Yong Heng low-pressure relief valve hissing - "problem" fixed

[airgunfans]

Just today I observed something odd on my YH. It's working fine but a few seconds after it is shut down, I noticed some faint hissing sound coming out from the low-pressure relief valve ( the one with a red plastic cover and a ring ). The hissing stops once the high pressure end is bleeded.

The only reason I can think of is the check valve between the first and second stage leaking. When the compressor is running, air is constantly being sucked away by the second stage so excessive pressure cannot be built up in the first stage. Once the compressor stops running, air on the high pressure side can flow back to the low-pressure side , apparently quite slowly in my YH resulting in pressure build-up in the low pressure end and the relief valve being activated.

What do you guys think ? Is this the likely reason ? anything I may have missed ?

 

[Bruce54]

I have had my YH for almost 2 years now and I can't remember when it started doing the same thing. It hasn't had any effect on the function of the compressor as far as I can tell. Have had no breakdowns, other than a temprature gauge (it didn't work right out of the box). I will follow this thread with great interest to see what comes out.

 

[DirtyDovi]

I guess I'll have to follow this thread just out of curiosity..

I haven't run across anything like this, however, the way that I run my compressor, I'm not sure that I ever would..

How are you guys starting and stopping your comps?

My [and many others' methods] 

*Loosen / Open both the side screws w/ the red knobs while the comp is Off
*Turn the comp On, Then Tighten / Close both screws to start building pressure slowly / gradually
[Doing this will let the compressor motor start up with no initial load on it - also great for people who's comps pop breakers]
*Whether or not your comp shuts off automatically or you shut it off manually - Loosen / Open those side screws again - At least the one on the right.. 
When manually shutting off - Open those screws or the right screw First - Then shut off the comp.. 
When using auto-shut off - As soon as the comp shuts off - Open the screws [or right screw] to quickly bleed off the remaining high pressure..

**I usually crack the right screw open a few times randomly anyways - Just to help prevent excess moisture build up in the lines / system

Again, that method just helps start and stop the comp with as little to no pressure in the system as possible, and seems to work great..
And if doing that, there's no air left over in the system [that could hiss/leak out]

 

Sam -

 

[Hepotter]

 

 

[Queequeg]

Mine does this intermittently too. Haven't had an opportunity to see if it's something to do with shutdown sequence that causes it to only happen sometimes. Doesn't seem to affect fill time.

 

[Eaglebeak]

I think this is normal. Mine has done it since new. The first time it happened I thought that something had blown. If you release the pressure before shutting the unit down, as per the instructions, it won't happen. This is one of the reasons why I disabled the auto stop feature. If it is leaking slowly in normal operation, try pulling the ring while it is running to clear anything that may have jammed in the seat of the valve.

 

[airgunfans]

I pulled my YH apart to upgrade the first-stage piston last week and I took the chance to checked out what causes the hissing at the low-pressure relief valve after the compressor is shut down.



Between the outlet of the first stage and the inlet of the second stage, there is a reed valve sitting at the top of the second-stage cylinder :

The valve works like this : When the second stage piston goes down, air is sucked into the cylinder and the inward air flow pushes the flap of reed valve downward to open the valve. In the up stroke, the flap is pushes in the opposite direction and the valve is closed.

The reed valve is just a piece of thin metal sitting on top of a hole. There is not any O ring or things alike so it should not seal too well. Immediately after the compressor is shut down, the pressure in the second-stage cylinder is still high so air can be driven back to the moisture trap of the first stage via small leaks in the reed valve. When the pressure so created in the moisture trap is high enough, the pressure relief valve will be vent.

What I don’t understand is why this did not happen over the last year but only recently. Does it indicate some kind of degradation in the reed valve ? I cannot see any visible signs. Anyway the filling time is not affected so I think I will just leave it like that for now.

 

[airgunfans]

I have got some spare time today so I decided to dig into this issue trying to find out the cause.

Upon close examination , I noticed that the metal surface the reed valve sits on is not very smooth : 

\

I polished the part around the inlet hole ( where the reed valve sits on ) to a mirror finish : 

Just when I was about to re-assemble it, I found that the center piece ( made of steel ) of the cylinder head is removable : 

There is another reed valve on the other side of the center piece guarding the outlet of the second stage. The purpose of this valve is to prevent high pressure air in the stainless tube from getting back to the second-stage cylinder. The valve is VERY dirty as you can see : 

I then cleaned it and put everything together before firing it up again. DA DA ! no more hissing at the first-stage relief valve after the compressor is shut down ! I concluded that the cause is the dirt accumulated in the reed valve preventing it to close fully. The polishing work could not be the fix because this symptom did not exist when the compressor was new. There has been a report on youtube that dirt accumulated here have caused failure to build up pressure so may be you guys will check this part out in your next rebuild. 

 

[srkhan]

how to open " another reed valve on the other side" ?? my one is stuck ! cant get out