Pressure Maintaining Valve Information

[Taso1000]

Hi All,

In my quest to ensure I have dry hpa air for my pcp guns I had some questions concerning pressure maintaining valve set points.  In my search, pretty much from scuba forums,  I found that the recommended set points were between 140 and 160 bar, 2030 - 2320 psi.

A couple weeks ago when I charged my first scba cylinder the pmv valve on my Alpha filter opened at about 1700 psi.  I thought that maybe I should set the pmv higher and that maybe a higher set point would have benefits.

So today I found an article that answered some of my questions.  http://thedecostop.com/forums/showthread.php?10085-Understanding-SCUBA-Compressors-and-Filtration

This is an excerpt from the article concerning water filtration:

First on the list is water.
On an analysis for Scuba air (grade E) and OCA (Oxygen Compatible Air) water is expressed
as a dew point and should be -50 degrees Fahrenheit or below or 67 ppm (parts per million).
For those not familiar with the term dew point, this is the temperature at which moisture in
the air condenses to water droplets. If you can visualize 67 parts per million of anything, your
doing better than me. In the last segment, I explained that water was not compressible and
there are filter media that will reduce the percentage in the air. We will talk about which
media to use later. For now the important thing to understand is that we must remove as
much moisture as possible before it gets to the filters.
The first step is to force the air through the last moisture separator at as high a pressure as is
practical. The higher the pressure, the more moisture will separate out in the separator for a
given temperature. How high should we go? When we run air through the separator at 58.8
psi (5:1 compression), the air will have near 100% humidity since some moisture will have
fallen out. At ambient pressure this air will have 20% humidity. In a house this would be
considered dry. If we crank the back pressure valve up to 279.3 psi (20:1 compression), after
the separator we will still have 100% humidity, but the ambient pressure humidity has
dropped to 5%. You’re off the scale as far as houses are concerned. If we crank the back
pressure valve up to 1,455.3 psi (100:1 compression), the ambient pressure humidity has
dropped to 1%. The Sahara desert would be envious. At a back pressure of 2,925.3 psi (200:1
compression), the ambient pressure humidity has dropped to 1/2%. Nothing in nature is this
dry. At a back pressure of 4,395.3 psi (300:1 compression), the ambient pressure humidity
has dropped to 1/3%. While 1/3% humidity is impressive, it’s not worth running your
compressor at 4,395 psi all the time to achieve. In my book ½% of anything is nothing. So my
back pressure valve is set at 3,000 psi and has been for the last 20 years (8,000+ compressor
hours). If you want to set you valve at 1,455.3 psi, go ahead, but your filter will have more
moisture to contend with and your compressor runs more than 60% of the time above this
pressure anyway.
The second step is to reduce the dew point in the separator. A lower dew point means more
moisture condensing and falling out in the separator. The best possible scenario would be to
submerge the third stage inter cooler coil in a bucket of ice water. This unfortunately isn’t
practical. So, keep your compressor clean. Run it outside in open air. Pump high pressures if
you can, when the temperature is coolest, early morning and late evening. Fill your banks
completely on cool days. Enough about water.

From this article I learned that the higher you set the pmv the more moisture is squeezed out.  I also stumbled on the information, from another thread: https://www.gatewaytoairguns.org/GTA/index.php?topic=120428.0 , that molecular sieve needs to be above 1590 psi to perform its best. 

So I have a different question now:  are there any cons to setting the pmv set point to  2,925.3 psi or higher? 

Also would it be better to place the molecular sieve filter after the pmv, to separate it from the pre set point moisture, instead of before it?  I could put a shutoff or one way valve after the Alpha to keep the molecular sieve from being contaminated by atmosphere while not in use.

I think the pmv it a great find for water removal.  There have been suggestions of mimicking a pmv with a shutoff and letting the system build pressure then intermittently letting the hpa pass to your cylinder.  That's would be lot of time wasted sitting in front of my Shoebox when an adjustable set point pmv only costs $90.

Thank you!

Taso

[rsterne]

I assume the concept of a "pressure maintaining valve" is to have the oil/water separator running at a high pressure ALL the time, so that the saturated air on the output side is at a higher pressure, and hence contains less water vapour per unit volume.... If you had this setup, and it was tight with no leaks, then you would have the added advantage of not losing the HPA in the separator, except when you bleed it.... Of course any leakage backwards into the compressor would defeat the purpose....

If you also had a dessicant filter on the output side, then yes, that should be on the downstream side of the PMV.... This is so that it has less water to remove from the saturated air.... If you are filling a tank to 4500 psi, and you had the PMV set at 3000 psi, you would only need to remove 1/3 of the remaining water to bring the humidity below the saturation point at 4500 psi (at constant temperature)....

If you are using a dessicant filter on a ShoeBox between the shop compressor and the inlet of the ShoeBox, then it is likely that a PMV would have no advantage, other than to keep a separator on the output side pressurized to conserve air between fills.... However, that would keep the O-rings in the ShoeBox pressurized all the time, and may increase leakage and/or shorten their life....

I suppose a PMV may be as simple as a spring-loaded check valve that only opens when the inlet pressure exceeds the set point.... There might be an advantage to running it at 5000 psi, if the compressor and the separator were safe at that level.... because then when you fill a 4500 psi tank you would be filling it with air saturated at 5000 psi, so only about 90% humidity at 4500 psi (at constant temperature).... During use, the PMV would "chirp" open on each compressor pulse, allowing a small amount of 5000 psi air to pass, expanding to whatever the tank pressure was at that moment.... Just a thought....

Note that with such a setup, pressure sensing for any auto-shutoff for the compressor would have to be on the outlet side of the PMV.... If it was before it, the compressor would shut off at 4500 (assuming that is the setpoint), and no air would reach the tank.... OR, if you increased the auto-shutoff above 5000 psi so that the compressor kept running, it could overfill the tank until the switch eventually shut off the compressor (at 5500 or whatever you had it set at)....

Bob

[Taso1000]

Mr. Sterne,

Thank you for your response.  It's taken me a long time to wrap my brain around all this.  Dew points, relative humidity are hard to understand.  Parts per million is a little easier.  To me it's like remainders, if that makes sense. 

The way that I understand it the higher the pressure the less moisture air can hold.  The same goes with temperature.  The higher the temperature the more moisture air can hold. 

Lets take my Shoebox setup as an example.  I initially thought that removing moisture between the shop compressor and the Shoebox would have the most benefit.  The membrane drier I stumbled upon researching refrigerated air driers will remove moisture to a -4°F dew point.  At 75°F that's 4.25% relative humidity.  This is before I knew what a pressure maintaining valve was and how it could be used.  But I will get to that later.

I still feel, in my situation, that the ~100 psi moisture removal is beneficial because I don't have the capacity to capture the condensate on the output side of the Shoebox.  I created a water trap on the output of my Shoebox to collect condensate out of hpa fittings.  Lloyd created a thread on how to estimate how much water was in compressed air and I used enough fittings, a couple ounces per my estimate at my 4% rh input, to hold this condensate.  If I wasn't removing moisture on the 100 psi line I would need a larger high pressure vessel to collect it.  A vessel of this type recently popped up from China.  I had asked the self anointed airgun scientist if he could sell me one from the Carette compressor he sells.  He never responded.  So I went with my fitting route.

After I bought my Alpha filter I researched what the pmv did and it became clearer what member Guy Kuo was saying in my Shoebox thread.  The high pressure wringing of the air is very effective and not as expensive as I thought.  That's what happens when one doesn't grasp what is going on. 

So the pmv, set at 3000 psi, bringing down the relative humidity to .5% is pretty good in my opinion.  A concern it still have is, is it still low enough to not have water and corrosion in my scba cylinders at 4500 psi?

So since molecular sieve has such little capacity, I knew to protect it from ambient air.  Then I thought if I ran the pmv before the Alpha I could extend the life of the molecular sieve on the input side.  I was puzzled as all the scuba compressor schematics showed the pmv after the molecular sieve.

So in answer your 5000 psi set point thought:  I guess it's possible but like the author of the article inferred, there's a point of diminishing returns. 

As for safety I would have to verify every fitting if it was ok to work at 5000 psi.  I know I made sure of at least 4500 psi and I know some fittings were rated at 6000 psi.  I don't know what pressure the Alpha is rated or if my pressure switch can be set that high. 

Do you think there would be enough benefit to change the set point to 5000 psi?

Thank you Bob!

Taso

[rsterne]

If the air is saturated at 4500 psi when the tank is filled, and the tank is then stored at a lower temperature, then at least in theory SOME liquid water will condense out in the tank.... However, as soon as you use some of the air and the pressure in the tank drops, that water will once again "evaporate"....

I only know of one way to guarantee no liquid water is in your tank.... You must reduce the ABSOLUTE humidity to less than what would be 100% relative humidity at 4500 psi at your lowest storage temperature.... The best thread about this is by AlanMcD....

https://www.gatewaytoairguns.org/GTA/index.php?topic=139845.0

The table attached to the bottom of the first post shows you exactly what is required.... At 4500 psi at 20*C (68*F) you must have less than 0.06 g/m³.... That is the equivalent of a dewpoint of -48*F at ambient conditions.... Reading through Alan's thread should answer all your questions....

Bob

[Taso1000]

Bob,

Your idea was so elegant, I couldn't see the forest for the trees.  lol  I apologize.  If we compress to 5000 psi and remove the condensate, filling a scba cylinder to 4500 should not produce any liquid water as long as the temperature is kept similar and not much colder. 

My thinking is that the electronic pressure switch on my Shoebox would need to be set higher that the pmv setpoint.  So if we set the pmv to 4900 psi and the compressor cutoff to 5000 psi that should work.

I have seen in the Shoebox Max user manual that the mechanical shutoff will initiate between 4000 and 5000 psi.  This makes me think the Shoebox can handle going to 5000 psi safely.

I think you could fill an scba cylinder this way but would have to manually shut off the cylinder valve when it's pressure reaches 4500 psi. 

I do have a gold regulator I use for tethering to the cylinder to shoot and I have one of the blue Ninja fill stations.  If I had to I could
 change fittings and connect the gold regulator or I could use the Ninja if it can be made to work.  The Ninja doesn't work like the gold regulator, or I'm not using it right, so I don't know if it could be integrated.

Another thought that I had is if I get this idea to work do I still need the Alpha filter?  There was a graph I saw about desiccants that showed that molecular sieve and the others don't work well under about 5% relative humidity.  I may be confusing relative and absolute humidity again so maybe it would still provide more drying?

Thank you,

Taso

[rsterne]

You aren't worried about the relative humidity, but the absolute humidity.... On the output side of any compressor, the relative humidity will usually be 100%, unless the air going in is VERY dry.... Look at that chart in AlanMcD's thread and try and wrap your brain around absolute humidity and how that relates to the dew point.... If you can get the dew point to below -50*F you will never see any liquid water in your tank....

Bob

[Taso1000]

Bob,

With all due respect, I get what absolute humidity is.  I am not saying that in a snarky tone, just clarifying. 

What will the resultant dew point be from compression to 4900 or 5000 psi?  I expect the absolute humidity to be less than .06 grams/meter³ and the dew point to be less than -48°F as in Alan's chart.

I had a calculator that i can't seem to find now.  I tried this one, www.howelllabs.com/resources/dew-point-conversion-calculator but I can't get the right results for some reason.  On top of that I can't figure out how to reset all the fields.   

So if we compress past 4500 psi to 5000 psi, we trap and bleed all the condensed water out and our "Hot air charge out of "hot" Shoebox" air temp is 120°F.  Once the scba cylinder cools to room temperature, say 70°F, would the extra 500 psi we temporarily compressed to give us enough leeway to keep and moisture from condensing?

If yes, there is no need for the molecular sieve right before the air cylinder.  Correct? 

Why has no one done this before?  I get that the scuba guys have been compressing are for the most time and are the experts.  Maybe it's because they only compress to 3500 psi and are also concerned with air that is breathable. 

That was my original question.  Why are pmv set points only 140 - 160 bar and are there any cons for us air gunners to setting the pmv to 2,925.3 psi or higher?

Thank you,

Taso

[rsterne]

Here are the specs on different classes of breathing air....

Class 1 = -70*C (-94*F).... This is 2.6 PPMV, or 0.002 g/m³.... Dew Point at 4,500 psi = -22.6*C (-9*F)
Class L (SCBA) = -53.8*C (-65*F).... This is 24 PPMV, or 0.02 g/m³.... Dew Point at 4,500 psi = 2.8*C (37*F)
Class D = -45.5*C (-50*F).... This is 67 PPMV, or 0.05 g/m³.... Dew Point at 4,500 psi = 18.2*C (65*F)
Class 2 (SCUBA) = -40*C (-40*F).... This is 127 PPMV, or 0.1 g/m³.... Dew Point at 4,500 psi = 28.8*C (85*F).... but at 3,000 psi typical for SCUBA, DP = 22*C (72*F)

I wrote a bit about this in Reply #9 of this thread.... https://www.gatewaytoairguns.org/GTA/index.php?topic=140456. .... You can see that I concluded in that thread that a dew point of -50*F at STP is "dry enough" for our use.... Using the calculator I linked in that thread.... http://www.dew-point.com/dew-point_calculator.asp ....

If we input 5000 psi at 20*F, we get 67 PPMV, which is 0.05 g/m³, or a dew point at 1 atmosphere of -45.4*C.... If we then reverse the calculation, and use 4500 psi at 67 PPMV, we get a dew point at 1 atmosphere of almost the same at -45.5*C, and at line pressure of 18.2*C.... If I understand that correctly, cooling the 5000 psi air just 2*C while expanding to 4500 psi would mean it is still saturated.... If that is the case, then we would not be getting the desired result.... In fact, expanding it to 3000 psi only drops the dewpoint at line pressure to 11.9*C.... What this means is that temperature drop is more important in the formation of liquid water than a drop in pressure....

As we often find, a little knowledge is a dangerous thing.... It would appear that my suggestion of setting a PMV to 5000 psi and then expanding the air to 4500 would only work at constant temperature.... Even a few degree drop in temperature in the 4500 psi tank would still cause water to condense out.... As Alan stated, it is the absolute humidity, in PPMV or g/m³, that is important....

Bob

[Taso1000]

After a little more research and conferring with member GuyKuo I have decided to add another pmv set at 3000 psi before my Alpha filter to wring more moisture out of my compressed air before it hits the molecular sieve.  The molecular sieve is staying in the loop as it's still needed.

I will report my findings once I have integrated the pmv and I fill another tank.  Hopefully I will have some captured water for all my efforts.   

Thanks,

Taso

[rcontrera]

LOL ... you guys are making this too hard.  Who cares what the dew point is as long as you are doing your best to remove as much moisture as you can?

ANY compressor is going to be able to compress the air.  And, since water or oil isn't compressible, it becomes a bigger "chunk" of the air that gets pumped out.  So, to get air out, we put a gizmo inline that makes the air change direction a LOT which makes the uncompressed chunk of moisture fall free from the air flow.  There are two things that can increase the amount of moisture that is knocked free. 
1.  Increase the air flow - the faster the air is flowing across the deflectors, the more moisture is going to get knocked loose.
2.  Increase air density - the thicker the air is before going into the separator, the easier the moisture will come loose

Once you have purchased your compressor, you are pretty much locked in to the air flow rate so nothing you can do for #1.  But changing #2 is easy ... install a pressure maintaining valve.  A simple PMV will restrict the air flow until pressure gets up to about 1800 psi.

To further process air, we add a filter chamber that is filled with a real fine filter medium called molecular sieve.  Basically, they are little balls of material that has micro pores that trap moisture molecules that get past the moisture separator.  To increase the efficiency of THIS filter we need to:
1.  Increase the dwell time - this allows the air to spend more time flowing across the media, increasing the chance of moisture getting trapped
2.  Increase air density -  denser air means the moisture chunk sticks in the media better

So, assuming you don't have an adjustable size filter tower, the only other adjustment you can add is a PMV.

  So ... this is really a no brainer ... install one PMV on the outlet of your filter so that it can maintain back pressure on the entire filtration system.

[AlanMcD]

Ray,

What you say is basically true, but you are comingling two different things in your one term of "moisture": 1) condensed water, or droplets, no mater how small (including at least three water molecules stuck together via a hydogen bond - the thing that makes water a liquid instead of a vapor once more than two are bonded together; two are referred to as a water dimer) and 2) free water molecules, or what most people call water vapor (individual water molecules not held together via hydrogen bonds).

And yes, the two things you are referring two do work to "dry" the air - first, physically removed the droplets/liquid via spearators, and then reduce the remaining water vapor to level below any anticipated future condensation level for the compressed air.  Once that water vapor is removed from the air, it does not come back at any point in time - it maintains that level of absolute humidity (but do note that if the air tank temperature falls below the dew point for that air charge, small amounts of water vapor will condense to liquid, but having no where to go it will evaporate back into the air when the temperature later rises - the amount fo water molecules does not change as it is a closed system). So once the air is "dried" to the desired level, it will maintain that level of dryness.

I do agree that for the use of a Shoebox compressor, many including Tsao is making it more complicated than it need be.  In his system, he is doing both things twice - once before the Shoebox, and then again after it.  I contend that once is enough, and that it can be done properly before the air enters the Shoebox. And I have proved, at least to my satisfaction that this is true, by checking my Guppy tank for signs of condensed water after having had more than 350 cubic feet of air compressed into it and found none https://www.gatewaytoairguns.org/GTA/index.php?topic=139773.msg1407075#msg1407075

But it does matter that we "dry" the air to the right level, and knowing and understanding the relationship between dew points and absolute humidity levels is important for figuring this out.  Of course once somebody has figured it out and "showed a way" that works, all one needs to do is to follow a proven path - understanding the path is optional.  After all, we don't all need to know how do design, build, and program a computer to use one . . .

All that said, we all have hobbies, and we all go overboard on them from time to time, so more power to Tsao!  Keep experimenting!

[plinker81366]

heres a screenshot on one of the scuba forums why pmv are set at around 2000 psi

[Taso1000]

Eumel,

Thanks for that.  Can you provide the web link?

Taso

EDIT:  Disregard Eumel.  I found it:  https://www.scubaboard.com/community/threads/rix-sa-6-needs-priority-valve-owner-needs-guidance.540338/page-2

[txrpls]

I'm confused. Where does the PVM go in the food chain? between the filters and the tank? On the alpha, it looks like it goes before the filler. If you left it like this it seems like it would keep the pressure on the compressor. This would, in turn, put added stress on the compressor if you left it under pressure. What am I missing?

[rsterne]

No matter where you put it on the output side of the compressor.... if you don't bleed it off when you are finished, it will keep the compressor under pressure.... at least back to the check valve on the output side of the last stage of the compressor (there has to be one).... As long as that seals, and there are NO leaks, the pressure between the check valve and PMV should stay at the PMV setpoint.... Chances are the compressor itself will leak down past the rings or seals.... I would be concerned with a ShoeBox that the final O-ring might stay under pressure and deform.... which might cause it to leak prematurely....

Bob

[Taso1000]

Mark,

I've attached two pictures of my "experimental" setup.  Circle #1 in the second picture is the pmv on the Alpha filter as it came from Airtanks for Sale.  As I read up on what the pmv does I thought about installing a pmv with a higher setpoint in the area of circle #2 before the Alpha filter.

Taso

[txrpls]

Thanks for the clarification on the location. Do you have a source for the PVM?

[Taso1000]

I ordered this one and it should be arriving tomorrow.

https://www.nuvair.com/store/1-4-npt-6000-psi-back-pressure-regulator-pressure-maintaining-valve.html

Taso

[Back_Roads]

 I am just dropping a handy shoe box tip, I prefer to keep the cover on mine while it is running ,, to ease the removal of it I ran one of the cover screws on each side on the bottom holes in from the inside , so the threads stick out, leave the top screws out,, then I just slip the cover over the holes and it snaps in place 

[txrpls]

Thanks, Taso. Did you order any fittings from them? I am planning on installing a drain valve on the bottom of my gold filter and gauge and hose on the top with the PMV.

It wouldn't allow me to quote you because of the link? Maybe because I'm a noob?