Pressure Question

[ghancock]

Ok,  I have a problem with my pressure system that confuses me a bit and need some help.

1) Everything off and on RAM air, 3000psi steady with no fluxuations.

2) I engauge Auto air switch just to blow air around and don't hit the pressurization and air instantly begins to blow into the cabin (oh so refreshing), however, the pressure guage drops to around 2000-2500 PSI which I would expect because we've relieved that wall that it was pressing against and now it is running through pipes and turbine to get things moving.

3) Sometimes I can fly for an hour like this, sometimes only a few minutes, but at some point the presssure guage starts jumping from 0 to 3000 and settings closer to zero.  Air stops blowing as hard and I shut down system for fear something is about to go sideways.

The confusion though is the zero part.  To loose all pressure (and there are NO leaks) like that I would think a valve or something has opened up and is just dumping fluid without turning any turbines and without having any pressure to withstand it.  Problem is, I haven't been able to find any such valve that would do something like this.  I think you have the hydraulic manual on the 680FP but if not I can upload it.  Just wondering if one of you hydraulics guys might be able to point me to a cause.

Thanks,

Glenn

[Willis]

Ok,  I have a problem with my pressure system that confuses me a bit and need some help.

1) Everything off and on RAM air, 3000psi steady with no fluxuations.

2) I engauge Auto air switch just to blow air around and don't hit the pressurization and air instantly begins to blow into the cabin (oh so refreshing), however, the pressure guage drops to around 2000-2500 PSI which I would expect because we've relieved that wall that it was pressing against and now it is running through pipes and turbine to get things moving.

3) Sometimes I can fly for an hour like this, sometimes only a few minutes, but at some point the presssure guage starts jumping from 0 to 3000 and settings closer to zero.  Air stops blowing as hard and I shut down system for fear something is about to go sideways.

The confusion though is the zero part.  To loose all pressure (and there are NO leaks) like that I would think a valve or something has opened up and is just dumping fluid without turning any turbines and without having any pressure to withstand it.  Problem is, I haven't been able to find any such valve that would do something like this.  I think you have the hydraulic manual on the 680FP but if not I can upload it.  Just wondering if one of you hydraulics guys might be able to point me to a cause.

Thanks,

Glenn

Just got home and read your post.  It can be  A. a single point failure B. multipoint failure or C. a combinatin of two or more hydraulic components "Talking" to each other.

1.  single point:  intermittant contact with the selector switch, which closes the shutoff valve.
2.  each hyddraulic pump has an isolation valve, that may be intermittant.
3.  The Flow control valve, the relief Valve and the Pressure reducing valve may be talking to each other (system pressure pulses cause the relief valve and flow control valve to open slightly/ in on off cycles.

If your system goes to zero, then the Pressure Reducidng valve is wide open and dumping sysetm pressure back to the reservoir.

Do you have  the schematic for your 680FP?  Fig. 3-1?

[ghancock]

I just posted the service and parts manual for hydraulic section and pressurization in a separate thread in this section.

As for you points:  Can we eliminate point 2 since the pressure holds steady at 3000psi when in RAM air position?  Or would those valves only come into play when you turn on the system?

Glenn

[Willis]

Thanks for posting the Service and IPC's.  I was missing those.

The isolation valves are for before startup and after shutdown unless there is a REAL reason to shut them off in flight.

Im looking at the accumulator/Pressure Reduction Valve.  (Its a true Control valve, not a relief valve, but there is one of those too).

Let me think about this a while. I need to absorb all this new data.

IF there was an issue with the Accumulator, say it leaks down over time, Im not sure how this would affect your system.

Right off, I think its wierd (but not impossible) to have a relief Valve upstream of a control valve.  They ususally are parallel to protectd the system from spikes and cold starts.

[Willis]

I thought of one other thing, the CB for the Hydraulic Isolation valves.

The only way for the system to go to ZERO pressure is for the pumps to run dry or all the fluid is being dumped to bypass. 

There is no by pass valve in the system, just a flow control valve and Im not sure how/what it is controlling.  (yet)

Just for grins,  find out if the isolation valves are on the same or different Circuit breakers.

considering the intermittant nature of this, Im betting on the CB(s), or the drum switch that controls the system.

[ghancock]

Here is a video of the gauge.  During the filming it didn't go below 500 but you can see what it is doing.  It will sit at around 2000psi for a while when I first turn it on and this happens after things warm up I guess.

https://youtu.be/FGDF_p1zCvk

Thanks for looking

Glenn

[Willis]

Okay, my friend.  IMHO, that is the best evidence for "Cross-talking" of two hydraulic components. Otherwise, the gauge would be rock steady.   The frequency of the "ripple" is the pump frequency and the pressure reducing Valve and accumulator.  think sympathetic vibration in a connected system.

Ive tried to explain this before with limited success.

One or more of the pumps puts out a pressure pulse.  Unless there is a dampening element in the loop, the regulating valves (which have springs) begin to respond to this frequency and open/close in a state that "Floats".  In this mode they cannot control anything.

Cure:  the dampening element in this system is the accumulator it takes excess pressure and stores it.  My guess the accumulator isnt working well.   When instantaeous flow is required, it provides it. Overhauling the PRV/accumulator will help this.

Test:  Cycle the landing gear during an event where the pressure is fluctuation like this.  the excess flow demand will pull down the accumulator and the PRV will be forced wide open to keep the pressure up.  the rest of the system will respond.  Im not sure how in this case; Im hoping it will force it back to normal.

At this point, it wont cost you anything to test, just the fuel to go flying for a while.

I wish I were there to put my hands on things.

bud

[ghancock]

Ok, will try the test but the question is where in the system is this accumulator?  Did you find it in the diagrams as to its location?

Thanks again,

Glenn

[Willis]

Im sorry,

Its in the Illustrated Parts Catalog  Vol. II

Sec.  VI

Page 6-47,   Figure 6-8.

It shows the auxiliary pump, the relief valve, Pressure Reducing valve and accumulator in the LH. Nacelle.

[ghancock]

Yep, I know exactly where all of that is.  In fact, when I first bought the plane the first thing that broke on the flight to TN was that T fitting on top of the accumulator that caused Morris to have to send me a used Accumulator that he had laying around.  So you're thinking the accumulator itself has an issue.  Seems like we just took the T off the top of the Accumulator Morris sent and replaced it on my existing one.  Maybe I should change the entire thing out?


Glenn

[Willis]

Im thinking an OH is pretty simple.  Mostly Orings.   springs dont generally fail.

I didnt see a spring in the accumulator assembly.  Generally the spring provides the motive force to push the piston and move the fluid out.  I have seem some accumulators that are charged with air/nitrogen instead of a spring.

If you have a spare one, I'd dissect it first.
 

[ghancock]

Update:  Hydraulic Accumulator was not charged to 600PSI so that is done now.  Afterwards I discovered the shop in TN not only did not charge it but they also did not tighten the reducer on top and the o-ring had fallen out.  Skydrol was oozing out all over the place.  It has been leaking a while but was slower so figured I'd get to it in due time.  I got that all fixed up today though.

As for the flux in the pressure, it hasn't changed.  After discussing with Bill Leff though he thinks the issue might be a clogged filter in the tank that isn't letting enough fluid through and therefore starving the pumps of their full potential.  I'm going to try to check that tommorrow and will report back.

Glenn

[Adam Frisch]

Normally the flux in the pressure is there because the membrane that separates the NOX from hydraulic in the accumulator is weak, as I recall it when explained to me. That was the case on the Aerostar, which has similar systems. If you want to talk to someone who knows all about that stuff, you could give Don Smith at Aerostar World Inc a call in Dothan, AL. Yeah, it's Aerostars, but they share the same Ted Smith philosophy in the hydraulics design. And Don knows his stuff cold. He used to build them in Santa Maria, CA, on the line, before he started servicing and ferrying them. I think he might be able to help if problem persists.

[ghancock]

Thanks for the info and will do that.  The confusing thing to me about the accumulator though is that it is outside of the pressurization system from what I can tell so not sure they are related.  Was also not aware the Aerostars use a Skydrol 3000psi hydraulic pump to power the super charger.  Very interesting indeed.

Will give him a call after I clean my filter.

Thanks again,

Glenn

[Adam Frisch]

No they don't use Skydrol, but I thought your system used Skydrol only for pressurization? Does it use it for all hydraulics?