Gas valve brain teaser

Bob Sweet

At 5000' to produce 100000 btu with a content of 1000 btu/btu I need a boiler with an output of 125000 btu. With bob's new thread and a btu content of 847 btu/btu I'll need a boiler to produce 148000 btu to produce 100000 btu output, Yikes.


Sure am glad Man. J gives a mighty big cushion!!

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Unknown

If I build a boiler at sea level in NY

and orifice it for ten thousand feet altitude, and set the gas regulator for 3.5" w.c., what gas pressure will you see on the manifold when the boiler is started up on the job at that altitude?

This illustrates the old saying, "The pressure was set at the factory."

Noel

Mitch_4

about

2.8" - 3" I'm guessing

Mitch

Gene_3

it's a trick question

if you set the reg for 3.5" you must be at location and have the gas on, soooo, you will see 3.5"

Unknown

You missed the point.

If it is set at sea level, with one full atmosphere pushing on the back of the diaphragm, along with the spring, and you deliver the boiler to the job site at 10,000 feet of altitude, what will the manifold pressure be BEFORE you adjust it?

No trick question, and it is as clear as I can make it.

Noel

Gene_3

hmmm

I see references to changes required after 2,000 ft and every 1,000 ft after to the orifices but no mention of what you speak other than setting the pressure for the manifold.

I'm not sure how off it will be but it will no doubt need re adjustment. 10,000 is up there.

Robert O'Brien

2.42" WC ?

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Techman

Here goes

I say 3.5"w.c.

GW

i

am in the 3.5 camp

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Plumdog_2

Excellent Question

I have asked what effect does altitude play on Manometer readings and have gotten no good answer. By the way I sometimes install gas burners at altitudes that high; and they can be a real bear to get right.

Timco

Proper orifice sizing for altitude requires us to know the specific gravity of the gas as well...

Tim

Bob Sweet

2.10 in. W.C.



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Bob Harper

shot at it

Some people figure the 10,000ft. mark as a magical reference where atmospheric pressure is about 25% of sea level according to some. Another source quotes roughly 1 inch of Hg drop for every 1,000 ft. If I decrease my pressure by 10 inches Hg and convert that to inches of water column, I calculated 270.83 wci. Now, this is 66% less than sea level. If I multiply 0.66x 3.5 wci, I get 1.155 wci manifold pressure at altitude. A crude interpolation off the orifice sizing charts in the Gas Engineer's Handbook seems to amount to about 13,000 BTUs or 39%underfired using a #30 D.M.S. orifice off 0.6 sp. gr. NG. With primary aeration kept constant or higher, the possibility of flashback and yellow tipping become apparent, not to mention incomplete combustion and all the nasties that go with it.

Similar factors that can pervert manifold pressures include plugged vent limit caps, common venting of two regulators, downsizing vent lines, restricting vent terminations, plugging the main operator orifice inside the valve, and defective line pressure regulators such as MP regulators at the meter, LP second stage regs. and MP off a 2 psi system.

How'd I do Noel?

Mark Hunt

Ahhhhhhhhhhh BUT!!


He is not sizing for altitude, is he? The factory is setting the burner.

Noel's point......Factory is in Long Island, they do not know where the boiler will end up. So the gas valve is set for "sea level" and ends up in Denver.

Is the boiler "Denver ready" out of the box??

Mark H

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Timco

absolutely not. Being @ 4200', I always size the orifice & check manifold pressure (derate for altitude). Most units I get say " adjusted for high altitude" and usually say that means 4000' or 5000'. But this is orifice...3.5 is 3.5...

Tim

Christian Egli_2

Dense question

Let's see if I'm sticking my foot in the mouth.

Since calling the tech line always comes up with a fault on my part, it must be that the factory can't ever be wrong. Thus, pressure settings at the factory are good everywhere.

The atmospheric pressure sits on the spring side of the diaphragm in exactly the same way it crowds the space at the pressure regulator's exit. In short, the spring sets a pressure differential with the atmosphere and this difference stays constant regardless of what the atmosphere does.

I don't think scuba divers fiddle with the knobs on their bottles as they go deeper under water - where the pressure gets heavier.

In any event, you'd go measuring this pressure with a water tube device which would be affected on the open side in the same way the regulator diaphragm is - thus no stretchy water level change.

But, in case this is a trick question, I'll say it depends.

Next, altitude adjustments have nothing to do with gas pressures directly, it's simply that the air we breathe in Denver is rarified and less dense than low level air. What's important is that there is less air in spite of it being the same volume as everywhere else.

Volumetric proportioning devices in burners don't account for density on a continuous basis, thus factory settings have to either change the orifices or change the 3.5 pressure difference with the atmosphere to make for the proper combustion ratio. Rarefied air calls for rarefied gas.

Mark Hunt

NOTE


Noel did not mention what the incoming pressure was at altitude.

Would this make a difference?

Mark H

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Unknown

I don't know

I don't know the answer.

It's this type of thing that we talk about over lunch at the factory. I just wanted your opinions.

On pressure differential, the differential is not between atmosphere and atmosphere, as I see it. It's atmosphere plus spring vs 3.5" of back pressure on the valve. If you change the atmospheric pressure, but expect the back pressure to run at the same number, do you have to adjust the spring?

I don't see how this is a trick question.

It doesn't come up in conversation, because the code requires that the pressure be adjusted when the appliance is first put into service. That's why the factory doesn't need to check the pressure on gas valves from Honeywell, out of the box.

Setting pressures is done in the field, and I was wondering what you are finding.

Noel

Empire_2

The combination gas valve has 1 function. It steps down or reduces a higher pressure to a lower pressure in "WC for firing purposes. Although it should still be checked, the 3.5 adj. at sea level should remain the same at any given rise in elevation. However it is the orifice size on the equipment that must be reduced due to the increase in air volume at higher elevations.

Air expands at aprox. 4% per 1000 ft. of elevation. Your new 150M furnace at given elevation whether it be 2k, 4k, above sea level etc. will now be over fired. Since the air shutters for primary air cannot be increased in most cases (In shot Burners) At elevations above 2000 feet, the volume of air required to supply enough oxygen for complete combustion exceeds the max. air inducing ability of the burners.

PS: this is from one of my books. I even learned something.

Mike T.

Techman

Hello up there!!!!

Hi ,high Wallies.Can any of you 10,000'inhabitents tell us lowlanders if you brothers up there have problems with making any major pressure changes to the gas valves?Or do you do just a little tweeking?

Bob Sweet

Dont see 10000' but do get up to

7800'. The set up parameters recommended by manuf. that I've seen only spec for up to 4500', so set up has been a little learn as you go.

What I've seen is that the co2% and the co ppm need to be adjusted, sometimes it can be quite an ordeal zeroing the burners in, the fan speed as well has to be adjusted.



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Plumdog_2

Well yeah

Sometimes there are problems. If you take the 4% per thousand and reduce the input rating, select the correct size orifice, and set the pressure at the factory recommended level, most times they are OK. But then with Nat. Gas, some locales have diluted gas (about 20% diluted). So at 8000 feet, reduce the input by 32% but subtract 20% from that for the already diluted gas; and then use 12% for your total reduction. With LP there are no dilutions being used as far as I know. Still, sometimes you get flashback and or draft problems. Many installers and even Gas Co. guys sometimes leave these things at factory configuration, and they will soot up and burn the wiring off the controls. Bad news. The new equipment with the sealed combustion air/gas metering valves are a vast improvement over atmospheric burners.

Empire_2

This is Great....:-)

Techman, I did not imply that I had the true answer, but that is why I quoted from a book and gave credit to same. This is an interesting topic and even after I posted, I was wondering to myself,..." Since the pressure at higher alt. is greater, why wouldn't that have an effect on the valve out-put itself???,. Hum. Shouldn't that have made me lower the Manifold pressure itself to (at least,. compensate for the opposing pressure) make all things even out as far as "WC goes?.... Sorry just thinking to myself out loud:-) there.

Plumbdog....Also: the book I Quoted from is from, 1976, which I love to read, but I know there has been more knowledge since then, but some things remain the same. The 4% in this case is just a figure (granted from 1976) That gave a reference to altitude and not quality, current altitude/climate changes, and overall new technology that has been obtained since then.

I hope that I din not come off as the for most expert, because that was not my intention. Only to contribute and help...:-)

Mike T.

Christian Egli_2

Less air, less gas and rarified elephants

The lighter atmosphere of Denver

Pressure regulators can be built to do two things while receiving upstream gas and delivering it further downstream. 1) either maintain a set upstream pressure, or 2) maintain a set downstream pressure. The first ones are the ordinary safety relief valves, the second, the ones we're interested in.

Say we set our downstream pressure at 3.5 inH2O. Stick a hose in the valve outlet, leave the other end open to the atmosphere and measure the water column, The elephantesque weight of the atmosphere is obviously sitting on the top of the open and taller column.

On the other branch of our U tube, we have a shorter column because whatever is sitting on it is even heavier than our previous atmosphere load. The payload here, which builds up to the set pressure is the addition of one elephant (who squeezes himself into the pressure regulator through the little hole) and whatever push the spring has.

All stylized into a purest U shape sketch with uneven water levels, the spring takes up the free space between the two level difference, and it maintains this difference for all times regardless of what weight changes the atmosphere goes through - as the atmosphere sits on top of both U tube branches.

So, I still say 3.5 at the factory is good everywhere, unless springs go all mushy with altitude.

Note also, that the upstream gas pressure has an influence on what comes out of the regulator. For instance, oxygen bottles that start their full life at more than 3000 PSI, make the regulator that jumps from that high down to nothing go all wobbly when the jump is only from 5 PSI high - when the bottle goes flat. Some regulators need readjustments as the bottle empties, but there are dual regulators that make that weakness go away. This effect comes from where the upstream pressure elbows itself in across the small piston surface - which is always small compared to diaphragm area and thus, only shows a response to a grossly huge input pressure change.

Whatever high minded people there are in gas industry, I would not expect home deliveries of natural gas to come at 3000 PSI, thus I wouldn't expect any significant effect on downstream pressure relating to upstream variations.

Here is a zoological sketch I attach. I hope it all makes sense, it is head scratching. Are you having doughnuts tomorrow at coffee time? I've been thinking about this all day; a double doughnut ration might be in order. Good fun question, Noel.

Techman

This is great

Hay Mike T.I like your info ,it makes me think out loud,sometimes TOO loud.I do like learning !

[Deleted User]

Christians right...

3.5 is 3.5 regardless.

What does change is the btu content of natural gas.

That, bufered by the fact that the local utility blends AIR into the gas to derate it to protect themselves from people dragging sea level appliances up here and gassing them selves with CO. As the supplier of the fuel, they ARE responsible for its use, so they take command.

Problem is, all boiler manufacturers think we're dealing with 1050 gas up here, and we're not. It's 1050 at the well head, and in a few systems in the mountains, but for the most part its been stepped on by about 20%. (830 btu/cu ft.)

3.5" is 3.5."

Nice cartoon Chris. Ever consider getting a job as a graphics illustrator?

ME

bob_50

The answer

The answer to Noel's question is 3.5''w.c. The regulator uses atmospheric pressure as a refrence. The pressure in the manifold will be 3.5" regardless of altitude. The REAL question that has an effect on the operation of the appliance is what is the ABSOLUTE pressure in the manifold. At sea level the pressure in the manifold is 410.984"w.c.absolute at 10,000' the pressure in the manifold is 280.7"w.c. absolute. bob

Bob Sweet

3.5 is 3.5

3.5 worked fine and still does in some instances if your changing orifices to compensate for altitude, but if the manuf. does not require orifice change out which seems to be the way manuf. are going, than manifold pressure has to be adjusted to compensate for altitude. So 3.5 would be over fired, imo.

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Empire_2

Mark.

If the fuel Co. derates their gas with whatever,??? What are they using? I am sorry , but never heard of such a thing. Mercaptian is an additive that gives Nat. gas it's unique odor. Other than that, what have I been missing????

In my opinion the BTU content of NG is the same as long as there is no impurities etc. BTU/Cu Ft is as listed from area to area, which I might add is the same. It is the oxygen that causes the higher temps if not dratted. Not the nat. gas itself. The burners can only handle so much to burn properly and with the added volume of air for combustion the derate,....has to be done. Not to decrease the Btu value of the gas being supplied, but to maintain proper air to gas burn ratio for complete combustion.

Mike T.

bob_50

btu

mike, I wish i was at home in chi with my library i would give you the equation to calculate the btu content of nat gas at different altitudes. You are correct when you say the gas co does not change btu content at different locations in the mountains. The laws of physics change the btu content at different altitudes.

Empire_2

Bob

I can understand that. Was looking for clarification, thanks. When you get a chance let me know. I guess I am learning, but I will just watch and read responses. Something just does not ring true here.

PS: SO tired of these code/PCWI letters I have to type...:-) Was OK for a while, not I have to type them in every time I have a thought....

Mike T.

Unknown

OK, then is this true?

If I change the pressure on the atmospheric side of the diaphragm, the spring and outlet pressure will still operate the same?

We aren't using an open pipe on the outlet, we are using an orifice that is within the size range of the gas control. We have a FIXED outlet pressure, not an open U-tube.

I'll put a hose on the vent of the regulator and use a vacuum pump to haul some of the air pressure out of the regulator. Will the diaphragm remain uneffected? I'll only draw it down a little bit, if you like.

Hypothetically, that is.....

Noel

Plumdog_2

Sorry

But they really do change the btu content of the gas. The appliance is firing at a lower rate because of atmospheric pressure being lower, hence a lesser amount of oxygen per cubic foot of air. The ratio of air to gas remains the same, but there is less air, so there has to be less gas. Same with normaly aspirated gas engines. You just plain lose power because of lack of fuel/air. Add turbo or supercharge and then it's a different story.

bob_50

Noel

the OUTLET of the orfice sees the same pressure as the atmospheric side of the diaphragm

bob_50

plumbdog

in Noel's example the absolute pressure in the burner manifold at sea level is 410.984 inches water, at 10,000 ft. it's 280.7 inches water. Do you think the density of the gas is the same in both manifolds?

Unknown

exactly

the orifice sees the atmospheric pressure, but the regulator doesn't see that, it only sees outlet pressure.

The gas passing through the orifice sees the differential pressure of atmospheric pressure and the pressure in the manifold pipe.

The valve sees a differential pressure between manifold pressure and atmospheric (plus spring) pressure.

If the manifold pressure remains nearly constant, but the atmospheric pressure changes from the factory atmospheric pressure, why wouldn't that have any effect?

If it has no effect, why is the back of the diaphragm even vented? Perhaps to not allow a build-up or decrease in pressure?

Noel

bob_50

Noel

First let me say Egli's description is hard to improve on. In one of your posts you mention connecting a hose to the vent opening on the regulator and connecting it to a vacuum pump. Do this,fire the boiler with a manometer connected to the manifold. Adjust pressure to 3.5"wc. Connect hose to the regulator vent and stick it in your....mouth. Watch manometer as you suck or blow. If you suck the manifold pressure goes down if you blow it goes up. The regulator is mataining manifold pressure at 3.5''wc. ABOVE WHATEVER THE PRESSURE IS IN YOUR MOUTH.

Marty

confused

If the diaphragm wasn't vented how could it move ? I see gas company regulators they get Iced over vents all the time they will not open up and when anything fires off incoming pressure really drops off and sometimes disapears. Say at 10k feet incoming gas pressure was at 10" and at sea level incoming gas was 10" The diaphragm would be in a slightly different position yet the outlet pressure would remain the same.

Unknown

So

if the pressure changes on the back of the diaphragm, the manifold pressure changes?

Noel

Unknown

New direction

How about this theory.

Measured with a water manometer, it would be the same at both altitudes. Same thing if you used your mechanical manometer, and I used mine.

But if I brought my sea level mechanical manometer to 10,000 feet, it would no longer read zero when I wasn't connected to anything.

If I re-zero the needle at altitude, the manifold pressure will read what I set it for at sea level.

If this works out to be the case, then maybe it did turn out to be a trick question, after all. It just came to me from thinking about the mouth idea...

Noel

Christian Egli_2

All you need are flippers

More doughnuts, more words. I hope I'm not trampling over anyone with my elephants here.

My sketch showed the regulator valve neatly piped into a water filled U tube, but who does that at home? (I know... don't ask... This would leave some wiggle room for the atmosphere to squeeze in. But when we're doing some real boilering, there is no open tube, so how would air pressure play a role?

I still say the omnipotent atmosphere plays an inescapable role at all times, everywhere, whether or not you connected a U tube to the downstream pressure regulator. Even a plugged hole couldn't deny its relation to the outside.

By definition, we determined that the 3.5 inches of water level difference would be with regards to the atmospheric pressure, that alone says the atmospheric pressure is at play. By design, the pressure after the regulator and inside the gas manifold will be the set 3.5 inH20 above the atmosphere, of course, in absolute pressure we'll be wherever the atmosphere pushes us, plus 3.5. Next, the jump we observe at the orifices is the one from the atmosphere plus 3.5 to just plain atmosphere, always a constant 3.5 jump. Meanwhile, if we want to define the relative 3.5 to something else, we can try, but we'll be soon back to the starting point.

Any other gadget beyond the plastic U tube still works the same way. The Bourdon tube wiggles because of the difference between it's inside pressure and the outside pressure it sits in - the atmospheric pressure. An inverted mercury filled tube would show us a measure in relation to a vacuum forming within the tube, this time absent of a link to the atmosphere, but how do you define the vacuum you indeed observe? You define it in relation to the atmosphere - the Torricelli tube was precisely invented in order to measure an atmospheric pressure, thus relating everything it does to the said atmospheric pressure.

Our 3.5 pressure difference remains the same, as we measure it relative to the atmosphere because we regulate it with springs relative to the atmosphere. And, of course, modifying any atmospheric pressure changes nothing to the game. Neither can we do anything to change the 3.5 inside pressure (other than by turning the screw) because if we could, it would mean we could change atmospheric pressure - which we can't.

Now, how about changing the atmospheric effect independently on both sides of the openings on my earlier sketch. Easy, leave, say, the opening by the spring clear for Denver elephants to enter, and on the open side of the U tube, pipe an extension hose all the way to Long Island, where the atmospheric pressure is much higher... With two open holes, opening at different places in the world, we'll hopefully see something else than the 3.5 factory dictated measure... Think about it... Think some more...

It is not possible to move around atmospheric pressure simply by way of a tube - or any other method - whatever you try, air inside the extension hose will build or subtract an equivalent atmosphere modifier as on the outside. If instead, you plan on just bottling a bit of New York air to carry over to the mountains, you'll still not have achieved anything because what's inside the bottle is measured relative to what's outside, change the outside, change altitude, change your relative measurements and you're back to being acted upon by the local conditions.

You traveled a lot, but you came right back to where you were initially. It's the communicating vases that Pascal played with so much while looking at pressure heads.

To better visualize all this, let's suit up in a rubber bathing costume the way divers do, then throw us overboard while we clutch a pressure regulator set at 3.5. The weight of the sea water is easier to imagine piling up on top of us as we drop like rocks down to the bottom of the oceans.

Watching the bubbles come out of the regulator is fun - at what depth level will the bubbling stop?

The weight of the ocean is placing a virtual finger on the regulator's outlet (or the orifices beyond the short manifold), thus demanding more absolute pressure for the bubbles to come out. That would seem obvious. What happens when we go deep enough to pass the 3.5 inches of water depth? See how you can test this at home: the dizzying depths of a bathtub are enough to see something happen, all you need is the rubber suit.

If the bubbles were to stop coming out of the regulator, what would you do? Easy, manually put the squeeze on the spring and force it to release more pressure. But will the bubbles ever stop popping out?

Remember now, as we are sinking down the Mariana rift along with the pressure regulator, won't we see all the weight of the ocean piling up on the spring side of the diaphragm, putting its squeeze along with the additive spring on what pressure release we'll observe? You can't deny the presence of the ocean on both the diaphragm side and the outlet side. This even if we had solidly plugged the outlet hole, the ocean's presence will not go away.

We could even float yards of hose from both the diaphragm compartment hole and the gas outlet, and still, nothing would change at the regulator level. And if we put our mouth to the regulator and suck at where the air bubbles come out, it won't change a thing about the 3.5 differential: the regulator is the point of no pressure change (relative to the sea pressure) you can suck all you want and as hard as you want, the regulator will happily resupply the manifold with gas at 3.5 inH20 above the outside. If instead you suck on the spring side hole, that will fool the regulator into adding the spring weight to a fake ambient pressure; however, if you follow the pressure differences from the diaphragm hole to your mouth to the local condition and back to the regulator outlet, you'll find the 3.5 never went anywhere, because for all your sucking, you can't change the static pressure applicable at the level we're floating at.

I think it's obvious that gas will never stop bubbling at a 3.5 relative pressure difference however deep we swim (this of course, providing our upstream pressure will be there to help at any depths we find ourselves) Human-like oceans have two hands to put one finger on the gas outlet and one fist on top of the diaphragm and since the two hands are attached to the same body, ocean depth plays no role in changing the relative additive properties of the spring within the regulator.

The same is true of our airy atmosphere. We live and walk at the bottom of an ocean filled with air; people on top of mountains simply live nearer the surface.

And the surface of this sea is only a few miles high, splash out of our atmosphere and you find yourself in void space where no aquatic creature can survive. I imagine Martians fishing along the banks hoping to catch one of us, hey... why is there a hook and a sinker attached to this doughnut?

Was this convincing? I still hope I didn't bite on a trick. Thanks all for enjoying and for reading all of this.