Gas valve brain teaser

[Deleted User]

Air...

Good ol' all American dry air. How do I know this? All gas in the Denver area comes from 3 central processing plants on the plians, east of Denver. Back in 1977, one of the plant operators fell asleep at the switch, and the air to gas ratio got over run, They filled the city of Denvers gas mains with compressed air, which as we all know doesn't burn for crap. They were hiring anyone with matches to go out and relight pilots for 2 days. I was one of the sub contractors hired to do the dirty deed. Back then it was known as Public Service Company. It is now known as Xcel Energy Corp out of Minnescrotum. Averge btu content for gas in Denver is 830 btu/cu ft. Average but content at the well head in wells surrounding Denver, including wells within the city limits (DIA) of Denver is 1,050 btu/cu ft.

An artilce relating this information can be read at

http://www.oildompublishing.com/PGJ/pgj_archive/April03/gas quality403.pdf

Another example, up in the mountains there is a little town caled Vail. People like to take advantage of gravity there, and the owners of hte mountin like to take advantage of people who like to take advantage of gravity there. No charge coming down, but getting to the top is EX$PEN$IVE. Some kind of European invetion I never really got in to , but I digress. In Vail (altitude 8,200' ASL) the btu content of natural gas is HIGHER than it is in Denver Colorado (5,280', 830 btu/cu ft) because they don't have the compressor facilities on the western slope that they have on the Eastern slope. Same utility, different gas source. In that case, the utility went through the whole city and reorificed all gas fired appliances to deal with the higher btu content gas. I inadvertently installed an appliance in that system about 15 years ago and dang near killed a bunch of constrution workers with CO. I had not reorificed the boilers to deal with the higher BTU (1,050) gas. I didn't know...

So, long story short, the manufacturers of natural gas appliances think we have 1,050 btu gas here in Denver, and we don't. The boilers, in most cases have to be double derated. Once for the atmospheric conditions, and once for the chincy gas.

The above infomration applies only to that gas which has gone through one of three Xcel Energy Corps eastern colorado processing plants. If you are north or south of Denver, and are not serviced by Xcel Energy, you ARE dealing with high btu natural gas and MUST changes the orifices in accordance with accepted standards.

Now (WHEW), having said all that, let me tell you of my experiences with derating/re-oricing atmospheric appliances.

In the case of older inshot atmospheric burners, I find that you should follow the standard theory of deration using 4% per 1000 feet above sea level, but if you adjust for the derated gas, it makes for a good clean burn. In other words, don't assume what the manufacturer assumes for caloric content of your gas. KNOW what you are dealing with FIRST, and compensate accordingly.

Then, there's the case of Buderus atmospheric burners. I have in more than one case, forced (increased manifold pressure, clocked meter) the burners into a sealevel operating condition, which is considered OVER firing the appliance at this altitude, and have witnessed cleaner burning than with the standard deration factors applied...

Splain that one Lucy...

ME

Plumdog_2

Good question

The density would be related to the amount of compression that the gas is under. I am not sold on the concept of 3.5 being 3.5 at the outlet of the regulator regardless of atmospheric pressure. Wierd things happen to atmospheric burners at high altitude if they are fired in the configuration supplied by the factory (usually). The highest NG burner I set up was at 9000+ feet. I have set up LP burners at 10,500 feet, and sometimes major adjustments are required. My own boiler (atmospheric at 8250 feet) is derated from 165k and clocks the NG meter at 117K with very good readouts on the analyser. That is about 71% of sea level build.

Plumdog_2

and so......

when the gas is in the orifice it is at 3.5 inches and when it leaves the orifice it assumes atmospheric pressure, becoming less dense. But it is fed in at ideal pressure for sea level, and escapes at a rate too great for altitude. The gas is now less dense, having less btu content per cubic foot, but there is too many cubic feet for the available cubic feet of air to get a correct ratio.

Christian Egli_2

The best for the tourists

Like you, Mark, the touristy attraction of places such as Vail, for me, center all on how to keep warm.

For my local natural gas company, the BTU content per cubic feet is defined at sea level, not at the particular atmospheric air pressure I live at. My gas is labeled as 1000 BTU per cubic foot, the cubic foot at sea level. It's obvious, a cubic foot of gas is something else taken at different pressures, a common definition is needed.

[My gas comes to me at 11 inches of water column, that's how it goes through the gas meter, but I do not know what sort of cubic feet the gadget reads in, whether they are related to sea level or kept at my atmospheric height. I don't know, because on the gas bill, there is a fudge factor to multiply the meter reading by, and no explanation to be gotten.]

On Long Island, the gas will contain 1000 BTU per cubic foot, because it was defined that way. In order to power a burner at a rate of 100,000 BTU/h, we need to observe a flow (through the set orifices and the 3.5 inH2O manifold to local atmosphere pressure jump) of 100 cubic feet per hour. Things will warm up and we'll all be happy.

The atmospheric pressure is what it is in New York and if we travel to Denver, we'll only be left with 80% of it. The mountain air is rarified and less dense and contains less stuff. To live fully, we have to breathe harder and longer and deeper and otherwise have a bigger chest and bigger lungs. No matter where we live, we still need the same amount of oxygen; if the available air is stretched thin, we'll have to suck more of it in.

Same is true of boilers.

Filling a ziplock bag full of gas in Denver will show it only weighs 80% of what it does down here, and thus, only contains 80% of its heat content. Without any fiddling of any kind, the standard definition of 1000 BTU/ft3 gas at sea level pressures yields 800 BTU/ft3 at Denver altitudes, this with no change in composition. And I guess a local utility would sell you the stuff labeled as 1000 BTU/ft3 - sea level standard, like mine does here in Dayton.

Next problem, your new boiler, labeled as 100,000 BTU/h, with no changes from the sea level settings will not pour out 100,000 BTU/h in heat at high altitude. Remember, the factory flow rate was set according to orifice and manifold pressure at 100 cubic feet per hour. This flow rate does not change as we move boilers up and down hills, it only changes when the manifold pressure is fiddled with or the orifices are fiddled with.

With nothing changed to the sea level adjustments, feeding 100 cubic feet of Denver altitude gas will only render 80,000 BTU/h. Not enough if you really wanted 100,000 BTU/h. As such I would expect 1) lack of output performance and 2) some combustion problems, particularly with excess secondary air supply.

The logical thing to do, since the interest is in having a full 100,000 BTU/h output is to either increase the manifold pressure or increase the orifice diameter, or both, in order to get an increased gas flow rate by 20%. Along with some air shutter adjustments.

You'll now be burning gas at about 125 (125 - 20% = 100) cubic feet per hour which contain about 800 BTU/ft3 at Denver level, which turns out to be the (125 * 800 = ) 100,000 BTU/h we wanted in the first place.

The utility can now look you straight in the eyes while you complain about having had to buy 125 cubic feet of gas while in New York, a 100 would have sufficed, by saying your high altitude gas is derated, discounted and diluted to 800 BTU/ft3, you'll pay the same price per BTU as on the sea shore. In spite of how different it all looks, you seem to breathe exactly the same composition gas as we do as expressed in BTU per cubic foot at sea level. No cheating. Just labeling the gas content at local altitude takes the bite out of racketeering billing complaints.

[By the way, my local utility also dilutes air into the gas pipes, mainly when instead of real natural gas, it delivers up propane (and such other petroleum gases) that have to be diluted to yield the 1000 BTU/ft3 - sea level]

In Vail, special tourists call for special gas, or so they believe.

Perhaps, all they do is label their gas at sea level heat content without attempting to explain the rate thing and attempting to give localized altitude adjusted heat content per cubic foot. Which is what I suspect.

And since Vail lives with a 70% rarified atmosphere, to get the same heating effect with the rarified gas and the rarified air, you now need to burn much more of the stuff - volume wise. To get our earlier wished for 100,000 BTU/h, we need to go through about 143 cubic feet of Vail rarified gas at 700 BTU/ft3 as observed at Vail altitude, thus still delivering the (143 * 700 1000 BTU/h for gas that can still be labeled as 1000 BTU/ft3 at sea level.

For this to happen, of course, you need reorificed burners, no doubt, and operating a sea level appliance on 70% fire unchecked, would possibly bring some kinds of combustion problems.

I don't know to what extent a sea level burner can handle the extra volume of deflated high altitude gas with a reorificed manifold, and if so, when it becomes necessary to buy a boiler that would be oversized on the beach, just to get a useful derated mountain performance.

Do you high altitude guys do that? say you need 100,000 BTU/h to fill your heat loss, so, you'll buy a sea level labeled boiler at 143,000 BTU/h just so on 70% rarefied atmosphere it will give out 100,000 BTU/h, just out of the box and with minor air shutter adjustments??

As you can see, there is no rarefied typing problems for me here in Dayton... Thanks for reading along. *** Sorry for delaying this post, it got stuck the other day. Bob's other cool answer thread reminded me to salvage it.

bob_46

Don't go nuts

Bob, you only have to derate once.

Bob Sweet

I see my error

I was thinking about it all day, something didn't seem right about my equation. But I see it now!

Thanks bob

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