air vs. nitrogen

[dirt bike dave]

Well, I guess the air in the reservoir could act as an accellerant if your hot shock got pierced by a bullet from a high powered rifle or machine gun. A .50 cal tracer bullet from a P-51 Mustang MIGHT make an air filled reservoir explode a little more forcefully than if you had a non-combustable gas in there.

Like I say, in theory, nitrogen is safer. ;)

 

[James]

Like I say, in theory, nitrogen is safer. ;)

I believe you but will test air for three more years and let you know if I explode.

 

[dirt bike dave]

James - you really like to live life on the edge!

Please show some common sense and at least make sure your gas tank vent is connected to a source of inert, dry gas at atmospheric pressure. Afterall, fuel fumes mixing with air in your tank are highly explosive. Also, the moisture in the air could be absorbed into the fuel, hurting performance and eventually corroding your carb, fuel lines, reeds, reed block, cylinder, piston, gaskets, crank case, exhaust pipe and silencer.

 

[James]

I have one of those fancy aluminum vent cap thingies and some aluminum foil, all should be well.

Thanks much for your concern, suggestions, and humor. :)

 

[bedell99]

Ok guys say you have water vapor in your resivor. The bladder remember is not that big in volume. Go out and take compressed air that contains water moisture. This can vary widely depending on the amount of humidity that day and temp. and fill up your shock. Now at room temperature this water vapor (70 F) puts about .4 psi on the air. if it is vapor state. Not that much right. Not doing much harm. Raise the temperature and see what happens that same amount vapor at 150F puts about 4psi on the air, still not that much. Raise the temp to 250F and it puts an additional 30psi on that same air. Still not impressed raise the shock temp to 300F and you put a whopping 80psi more pressure in the shock. Now I know most people can't get a shock to these temps but I know at the end of moto. I can't place my hand on my shock body without getting burnt. In most case you are right James, I know I don't ride like RC but there are some people out there that can go pretty darn fast for long periods of time and instead of have 150psi of nitrogen which will expand when you heat it up and the forces of moisture in your bladder all acting on the circlip at the bottom it can give way. It is not out of the question to put 250psi of pressure in your shock. If you take the area of bladder at bottom where the circlip is (2" dia.) and multiply it by the pressure you can get pressure in the 800lb range. That is a lot of force and can really hurt some one if the clip decides not to stay in that day. Will it ever happen? probably not, at least not at our speeds. Is it something we will not notice, probably not but for the $10 it make me feel warm inside.

Erik

 

[ellandoh]

every good story ive ever heard could be made GREAT , simply by adding the words "and then it exploded" to the end :)

 

[James]

Erik, I'll trust your math and say good points. If I were faster and had better endurance, this may be more of an issue...I don't know. I'll volunteer to be the guinea pig on the air in the bladder deal for the forseeable future. Unfortunately, I may never go fast enough to blow up and I suppose I shouldn't let any real fast guys ride my bike for their own safety (luckily none of my friends are that fast and nobody wants to ride 2 strokes anymore :) )

Another thought that crossed my mind is that I think my shock video says to remove the valve stem to let the nitrogen out and it also says to use compressed air for the initial bleed. Obviously, they recommend you use nitrogen for the final fill, but I wonder about the air that is in the bladder before you add the nitrogen? How can one ever be sure that NO moisture or oxygen is in the bladder?

 

[dirt bike dave]

Erik - I'm no expert on the subject, but I think at 175+- psi and 70 degrees F, there will be no water in a vapor state in the reservoir. All the vapor will have condensed into droplets due to the high pressure, and it will remain as water drops unless either the pressure decreases or the temps rise A LOT.

What temp does water vaporize at 175 psi? Get to that temp and then add 100 degrees more F, yeah, I agree you will see a pressure rise in the reservoir. By then your shock might be melting.


Edited to Add: I did a little checking, and it looks like a rule of thumb is for every 1 psi increase, the boiling temp of water increases by about 3 degrees F above the 'normal 212*. So water in your shock at 175 psi won't be boiling until it gets to about 737*F. No doubt you will see some pressure increase before that point...

Like I've said, Nitrogen is better in theory.

 

[bedell99]

Dirt bike dave I see how what you are saying but it doesn't work like that. Water vapor can exist at almost any temperature and pressure. It changes drastically as temp either increases or decreases. Cold air can hold hardly any water moisture while warmer air can hold alot. For a living I design steam systems for processes and heating. I constantly use 2 charts in my line work. The first is called a pyschometric chart and the second is the steam tables. According to tables at at an absolute pressure of 175 psi the tempature of the steam would be 370F.(Remember guys this is for a fixed volume) In the industryf or process we use mostly 15psi steam and this around 250F. A power plant for example will use super critcal steam which is around 1800 psi to turn it turbines. The temp of this steam is around 600F. Steam is a fun thing to use but it can be real dangerous. Dave your logic is a bit off though. What you are referring to is boiling temps exposed to atmospheric pressure. As most people know water will boil at different pressures. For example at altitude it will boil at a lower temperature due to less atmospheric pressure. Just like if I put a bowl of water in a container and pressurized it it would boil at higher temperature.

Erik

 

[dirt bike dave]

Thanks for the info, Erik.

 

[ML536]

Erik, I'm no expert on the subject, either, but as you note, higher pressures require more heat before the water changes state from a liquid to a gas. Doesn't your data above suggest that at 175 psi, water will remain liquid (and not boil/become steam) until it reaches 370 degrees?

Also, the gas (whether it be pure nitrogen or air) will want to expand as it gets hotter, as temperatures rise, so will the pressure inside the shock. So at 370 degrees, the pressure inside the shock, regardless of the gas with which it is filled, will be greater than 175 psi that was in the shock at room temperature. Because the heat increased the pressure in the shock, doesn't this mean that even at 370 degrees, the water present in the shock will be in liquid, not gaseous form?

It's been a long time since I've had chemistry or physics classes, so maybe I'm looking at this issue incorrectly. It just looks to me like the water present may have some effect on the pressure in the shock, but only at temperatures of 370+ degrees.

 

[bedell99]

You are on the right track ML536. Rememember that for a constant volume, closed system that p2=p1(T2/T1). (Ideal gas law), so its a direct relationship. As temp goes up so will pressure as long as the volume is constant. But the v olume is not constant. It actually decreases as the fluid in the shock body compressesthe bladder. So it actually gets worse. Not only is the volume getting smaller, the temp is increasing and the temperature is constanly getting higher due to friction. So say we have the ideal gas law PV=nRT wher n and R are constants(n is the number or moles and R cis the gas consant). Say the pressure stays the same at 175 and the volume is decreased by 1/2 the temperature will actually be increased by double. This is for an ideal gas. Now this is where it gets real fun and is really hard to model. There are logerthmic realtionships that model this but I'm not smart enough to them.

Next it is almost impossible to get a 100% saturated steam at all times. What we call the water that gets unsuspended out of the "steam" is called condensate. Now in a perfect world we would have no air in steam system, just like in a perfect world we would have no water in shock body but it happens and it much less as pressure and temperature go up in a constant volume(pipe in my case and also we use equipment such as dearators to get the steam as pure as possible). So yes to your answer according to the tables at 180 psi and 373.13 F in a constant colume system all the water should be steam, but that is ideal. Conditions are never ideal. So there will always be a small amount of "condensate" in the suspended in the system. One last thing though remember as temp goes up it allows more water to be saturated into the air.


Side note: When designing a steam system for a power plant, condensate is a really big deal, I mean really big deal. Now you have to undersatnd we put "traps" in the piping to catch this condensate because we don't want it flying around in the pipes. Now you have to understand on high pressure steam ,velocities get really up there and the water droplets will actually be like speeding bullets. You do not want that hitting elbows in pipes becuase eventuall it will blow and that is real bad. You don't have to worry about this in shock bladder because there is no flow and no chance that the water droplet can get up to any real velocity but I just thought that it was interesting.

Erik

 

[ML536]

Thanks, Erik. That was interesting. It's discussions like this that make this site so interesting to me. I've been thinking about this, and had a few more questions/observations. Would the following be true?

If the air was absolutely dry with no moisture, wouldn't it expand at the same rate as absolutely dry nitrogen? And the additional expansion that takes place when there is some water trapped in the air occurs when water that is otherwise condensate (in droplets) changes state and becomes water vapor (a gas?). If so, it would seem that the point at which the rate of expansion changes for non-dry air versus a dry gas would be the boiling point of water, at that pressure. If the boiling point of water at 180 psi is 370 degrees, wouldn't that be the point at which the increased rate of expansion of non-dry air become problematic? (assuming that the volume was constant, which you pointed out above does not, due to the expansion of the oil and resulting decrease of the volume of the bladder).

As you noted, because the volume of the bladder is decreasing as the shock gets hotter, the pressure and temperature inside the bladder will rise even higher than it would if the bladder's volume remained constant. As the reduction in volume is caused by the expansion of the oil, this would seem to be true regardless of what was used to pressurize the bladder.

But when the pressure increases, the boiling point of water also increases. So if the pressure was 180 psi at room temperature where the boiling point is 370, and the pressure increases with heat to some higher pressure, the boiling point (and the point where the presence of water condensate becomes problematic) will be even higher. If I'm looking at this correctly, and the boiling point is the temperature where the change of phase from liquid condensate to gas in non-dry air becomes a problem, then the rising pressure accompanying the rising temperature would cause the boiling point also to go up, in effect "chasing" the rising temperature and minimizing, rather than increasing, the effect of the condensate on the rise in pressure.

So, leaving the issue of corrosion alone, and considering only the effect on performance filling the bladder with non-dry air from a compressor instead of dry nitrogen, wouldn't the following statements also be true:

1. Non-dry air will contain a certain amount of water, which will vary depending on the humidity and air temperature the day the air was compressed. Air from a hot and humid day will contain more moisture than air on a cold and dry day.

2. The non-dry air, when it is placed under the high pressure in a shock absorber, will cause some of the water vapor present in the air to condense and become liquid condensate.

2a. Some of the water may remain as a vapor at room temperature, and thus will be in a gaseous state even at room temperature.

3. The liquid condensate will cause a greater rate of expansion than a dry gas, such as nitrogen, because at some temperature, the condensate will change phase from liquid condensate to a gas. Any water that was in a gaseous state as water vapor at room temperature will expand at the same rate as a gas, and as such will not really affect the rate of expansion.

4. The point at which pressure will increase more rapidly in non-dry air will be the boiling point of water at that pressure.

5. Below the boiling point, the rate of expansion between dry and non-dry gas will be approximately the same.

5. The boiling point of water at 175-180 psi is around 370 degrees.

6. As the shock heats up, the volume available for the bladder to fill will decrease due to the expansion of the shock oil, causing the pressure to rise even higher than it would have risen due to the increased temperature alone.

7. With the increased pressure, the boiling point will rise even higher.

8. Due to the number of variables (amount of humidity, rate of expansion of the shock oil, etc.) it is difficult to predict with any accuracy when the condensate will boil, spiking the pressure and causing a problem. But there is some (albeit high) temperature where the non-dry air's condensate will change to gas and spike the pressure up even higher than it would have been with dry gas.

9. While that temperature cannot be calculated with any accuracy, it almost certainly will be a temperature over 370 degrees if there is 175 psi of pressure in the shock, as that is the boiling point at that pressure.

Thanks for bearing with me. I know that was long-winded. I just wanted to explain my thought process, and to make it easier to identify a flaw in my thinking and assumptions.

Mike

 

[Jaybird]

As mentioned, we have to realize that dry air will react almost identically to nitrogen in the shock.

I think you are spot-in with your observations, Mike.

One thing to consider...if you use non-dry air (forget the explosion part folks...we arent talking oxygen here) then you are definately introducing water, be it vapor or condensate, into the shock body.
Nobody has mentioned what this H2o will do when the ambient temps reach say...<32 degrees f.

Jeremy had a picture a few years back of a shock that had exploded wide open...wonder what caused it? I don't think the definitive answer ever was posted...

 

[WWR]

Dont forget, ambient air is approximately 75% nitrogen in the first place. If you are filling the reservoir in a place with very low humidity, the amount of moisture being introduced is extremely small.

 

[marcusgunby]

What a long thread LOL, do you realise you can get different grades of nitrogen??? that will add another few pages to the discussion :) my thoughts are this, dry air is fine and you wont notice a difference, nitrogen is better and is also cheap to get, so why mess with anything else?

 

[bedell99]

Dry air is perfect. Most machines that use compressed air will go thru an air dryer. Perfect example a prinitng press where humidity is very important.(OR should i say I a specified humidity at a specified temperature. The reason I think you don't want to use the components of air(oxygen and hydrogen) is there ability to explode near a spark and it is pretty dangerous to have those tanks in a motorcycle shop.

Erik

 

[marcusgunby]

LOL N2 is perfect, dry air is a decent alternative.

 

[Jaybird]

Having the equipment to provide nitrogen fills means you can fill with nitrogen.

Having the proper equipment to provide clean, dry, air means you can also use the air to run your power tools. And at the benefit of keeping the power tools alive longer than they ever would have with regular compressed air.

 

[motometal]

As mentioned, we have to realize that dry air will react almost identically to nitrogen in the shock.

I think you are spot-in with your observations, Mike.

One thing to consider...if you use non-dry air (forget the explosion part folks...we arent talking oxygen here) then you are definately introducing water, be it vapor or condensate, into the shock body.
Nobody has mentioned what this H2o will do when the ambient temps reach say...<32 degrees f.

Jeremy had a picture a few years back of a shock that had exploded wide open...wonder what caused it? I don't think the definitive answer ever was posted...

that shock blew out because the piston ring wore very thin, and a pc of it came loose and jammed in the compression orifice.

Since there is a bladder in the reservoir, I don't think you would be introducing any moisture into the actual shock body and oil, regardless of the quality of the air used.