Vacuum Relief Valve Adjustment

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DMACJR
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Vacuum Relief Valve Adjustment

Post by DMACJR »

Okay, just throwing this out there to gather information, does anyone know what amount of movement, of the vacuum relief valve adjustment screw, will result in what amount of change in vacuum, with the old system, using the venturi tube system (two venturi horn install) Any insight would be greatly appreciated.
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Bruce Fenstermacher
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Re: Vacuum Relief Valve Adjustment

Post by Bruce Fenstermacher »

Assuming you have two 4" venturi you will never get more than about 4" of mercury. Oh ok, if you look at the gauge from the side it can look like 4-1/8" :D

Assuming you are not running AN gyros, you need all the vacuum you can get and so you could simply remove the relief valve. The relief valve was to adjust from 4" to 3" for the AN gyros. Personally I'd run it adjusted to give all you can get.

What are you trying to do?
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Re: Vacuum Relief Valve Adjustment

Post by DMACJR »

Bruce Fenstermacher wrote:Assuming you have two 4" venturi you will never get more than about 4" of mercury. Oh ok, if you look at the gauge from the side it can look like 4-1/8" :D

Assuming you are not running AN gyros, you need all the vacuum you can get and so you could simply remove the relief valve. The relief valve was to adjust from 4" to 3" for the AN gyros. Personally I'd run it adjusted to give all you can get.

What are you trying to do?
When I bought my bird it only had an older AN type DG, which works fine and the original T&B and it too works fine, originally it had the old style AH, but it was removed many years ago, but I found the plumbing capped off, so I installed a more modern AH and an in-line main filter (had NONE) and all items work just fine, but I wanted to kick the vacuum amount up a bit more as with a master gauge installed, it indicates, in straight and level, a very steady but not quite 3 1/2" and would like to get a bit more from it and where that crazy valve is mounted (straight up from the center console tunnel mounted hanging from the glare shield) is gonna be a bitch to adjust and probably has not been adjusted in many years......yes it has 2 each 4" venturi horns......
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Re: Vacuum Relief Valve Adjustment

Post by lowNslow »

The modern style DGs and AHs require 5 inches of vacuum to operate properly. The 3.5 inches you are getting is fine for AN gyros but weak for modern stuff. You could either go to all AN gyros and adjust as Bruce suggested but if you want to run new stuff I would recommend you convert to 9 inch venturis, They bolt right in using the same hole pattern as your 4 inch venturis.
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Re: Vacuum Relief Valve Adjustment

Post by Bruce Fenstermacher »

This is a relief valve meaning it sets a leak at a given pressure set by a spring and the adjusting screw. As it is now, you can pretty much turn the adjuster all the way one way and test. If you went the wrong way turn it the other way. I think you can turn the adjustor all the way out which you don't want to do so I'd just go a turn or so in that direction until you can get 4" in level flight. Your Ah should be the first thing in the line and will use most of the vacuum and what' left shouldn't hurt your AN DG. Your gauge might not be reading the vacuum at the end of the line depending on if it is attached to the AH or DG.

As Karl states the 9" venturi(s) are the way to go and then you will need to adjust your relief valve to 5" and will still be over driving the AN gyro.
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Re: Vacuum Relief Valve Adjustment

Post by 170C »

I just got my DG back from an overhaul and am wondering what others may experience when they do a 360 deg turn. Do most venturi powered DG's go back to the original heading? I don't remember if mine did or not before the OH. It isn't doing so now.
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Re: Vacuum Relief Valve Adjustment

Post by mongo2 »

I am just finishing up a total restoration of a 170A , and have (2) 9' venturi's and regulator with a modern AH , DG and gauge.. Whereas I have no base line of knowing that the system is working, other than the overhaul of the instruments,( plane hasn't flown since 92, and I got it as a project) does anyone know of a way to test the integrity of the system on the ground, i.e provide enough flow across the venturi's to simulate flight?, or even just test from the regulator to the instruments.. thoughts anyone?

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Re: Vacuum Relief Valve Adjustment

Post by Bruce Fenstermacher »

Rick, when I'm dealing with the vacuum system I've often asked myself the same question. Could I hook a shop vac to the back of the venturi(s) and suck air through it or maybe use the shop vac exit to blow air through the venturi(s)?

And the answer is probably. But why chance hurting anything and there would always be doubt if it did work at all how accurate a simulation it would be.

I've, so far, resisted the temptation to experiment. But one of these days I've pick up one the the spare venturi I have laying around the garage and coincidentally have a few hours to kill and the experiment will be on. But it won't be on my aircraft and installed gauges.

Of course another way would be to build a regulated vacuum pump and bypass the venturi(s). But then after that trouble you'd only be setting the regulator and I don't think that's worth the trouble.
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Re: Vacuum Relief Valve Adjustment

Post by hilltop170 »

I don't see why you couldn't use a leaf blower aimed at the ventures and see what happens. Most leaf blowers are in the plus or minus 150mph range so it should not hurt anything if you start several feet away and get closer in steps as you watch the results.
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Re: Vacuum Relief Valve Adjustment

Post by mongo2 »

I like the leaf blower idea, as I could start far away and have someone watch the gauge... I tried briefly with the shop vac . but there was no result. as there is just not enough velocity and flow across both venturi's... I could only pull from one at a time...It sure would be nice to know that everything is" tight" and right before the maiden voyage... I can always get to them later , but now is the best time..I'll see what I can do this weekend, and post the results... thanks gent's

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Re: Vacuum Relief Valve Adjustment

Post by lowNslow »

Why not just go on a test flight see how the gyros are doing and check the vacuum gauge for proper adjustment of the relief valve. Nothing like real world conditions (minus the IFR in this case).
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Re: Vacuum Relief Valve Adjustment

Post by hilltop170 »

mongo2 wrote:I like the leaf blower idea, as I could start far away and have someone watch the gauge... I tried briefly with the shop vac . but there was no result. as there is just not enough velocity and flow across both venturi's... I could only pull from one at a time...It sure would be nice to know that everything is" tight" and right before the maiden voyage... I can always get to them later , but now is the best time..I'll see what I can do this weekend, and post the results... thanks gent's

RC

You could mount the leaf blower on a step ladder so it would have a steady position for a long enough time to get stabilized readings. Then it could be moved to get the desired effect. It will be interesting to hear your results. Be sure and wear your ear plugs!
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Owned from 1973 to 1984.
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Re: Vacuum Relief Valve Adjustment

Post by DaveF »

I think testing the integrity of the system means checking for leaks and checking the regulator. Other than that, just fly it. You can plug the inlet filter lines and pull suction with your mouth on one of the venturi hoses (plug the other one). Use the suction gauge to watch for leakdown. I've tried a vacuum pump (see below) and shop vac as test suction sources, but neither can pull 5" at the approximately 3 cfm required to run the instruments.

To check the regulator I used one of these pumps http://www.walmart.com/ip/FJC-FJC6909-3 ... p/25461285. The picture below shows a messy workbench with the vacuum pump plumbed to a suction regulator, a Cessna vacuum gauge, which reads 5.2" Hg, and an automotive SW vac gauge.

Yesterday I test-flew my new two-venturi instrument vacuum system. I have two 9" "super" venturis, a regulator, gauge, and two "modern" instruments: an AI and DG. Results from the test flight show that I get more than 4.75" suction on takeoff and all level flight above 75mph. At 100mph cruise I get max regulated suction. Suction only falls off when the power is pulled back and there's less prop wash. Unfortunately, that's the case when flying an approach. I was able to raise the suction to 4.8" by flying the approach at 90mph or by using 30 degrees of flaps, which required more power. The instrument suction is very responsive to prop wash.

If you're worried about over-sucking your instruments, put someone with long arms in the right seat and have them block a venturi by hand if the suction goes too high. It doesn't take much of an obstruction on the outlet to kill the vacuum.
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Re: Vacuum Relief Valve Adjustment

Post by mongo2 »

All great ideas and info, thank you...Flying the aircraft right now, is not currently an option , as it is the final resto stages and I am almost ready to turn on the master...engine still needs run-in after overhaul , paperwork etc.. but I have a vac pump and a leaf blower, so I'll let you know how it turns out...now, the answer to the regulator adjustment question is : FROM AEROTECH 2H3-X model

Description
The 2H3-[ ] vacuum regulator is designed to maintain desired vacuum to the aircraft gyro instruments via a spring-loaded diaphragm. All 2H3 valves operate in the same basic manner, with model number variations ( - [ ] ) denoting number of connecting tubes, tube diameter and/or vacuum adjustment range.
A foam filter provides protection to the internal valve and system components.
Operation
Vacuum created by the air pump enters the valve cen-ter body, then bleeds through the diaphragm orifice to the top of the diaphragm. The outer bottom portion of the diaphragm is open and exposed to ambient air pressure located in the engine compartment or cabin.
With regulator spring tension normally set for 4.7” to 5.0” Hg., any increase in pump vacuum will lift the dia-phragm upward off it’s seat allowing excessive vacuum to be diluted by incoming atmospheric air.
Increasing spring tension by turning the adjustment screw will result in higher system vacuum in the valve body and higher differential pressure across the air pump. The result will always be shorter pump life.
NOTE:
The gyro instrument vacuum gage does not
record air pump vacuum !
Common Problems
A damaged or deteriorated diaphragm, loose valve cover or rivets, or a broken adjusting spring will result in loss of desired vacuum control.
Loss of system regulation caused by carbon particles wedged under the valve diaphragm is another com-mon occurrence following air pump failure.
Remember!
If the regulator needs adjustment
Your system needs maintenance
2H3-[ ] Vacuum Regulator
Construction
& Operation
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Re: Vacuum Relief Valve Adjustment

Post by mongo2 »

So the results were mixed.....The leaf blower with one 9" venturi , one guage, no regulator and one instrument (AH) provided just enough vacuum to spool up the instrument to the point to where I could hear it, and see it set the horizon, as well as rock the wings and see it respond... Then I hooked it to the DG, and the results were about the same, as far as seeing it "lock" cage and then hear it spool down when the vac was removed.... at no point did I even register anywhere near green on the vac gauge..but I am satisfied that the venturis work, the gauge works, and the regulator (bench check) , as well as seeing the instruments respond... there is just no way with that leaf blower , right on the single venturi, that enough air can be moved to produce very much vac, but again there was just enough to satisfy
my curiosity...now I'll just set the regulator, by adjusting the screw clockwise to increase vacuum to where I want it ........ the last thing I am questioning of is the plumbing of the 2 venturi's... there are (2) 3/8 lines to a T , then a single 3/8 adapted up to the 5/8 inlet of the regulator...then (2) 3/8 lines out , one to each instrument....does anyone see an issue with that?, specifically the inlet side?

Rick
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