Manifold Gas Pressure

Empire_2

As usual I always go with Manufacturers specs and that goes for any product. If you take your CO/Free Air/Excess air readings and increase the Manifold pressure, you will find that it is very capable of maintaining an acceptable combustion reading. Considering "in shot" burners where we cannot adjust Primary air at the burner burner (80% ers), You can actually get more out of it with OK comb. readings. BUT, like I stated always go with Man. specs.

Mike T.

Darrell

Manifold Gas Pressure

I've been checking the manifold gas pressure on every boiler I do a yearly on...I see them all over the place. I got into a discussion with the local gas supplier serviceman today. I see mostly WMC and they consistently run at 4.5inches even though the gas valve is marked to be 3.5inches. So I set them down to 3.5 which also drops the flue stream CO from 25-30 down to 12 or 15. The gas dude said that I should be adjusting the manifold pressure for lowest CO. I thought I should be adjusting the manifold pressure to the design parameter...3.5. What say you? I know that on power burners like a conversion burner I can fine tune the combustion by adjusting the manifold pressure, but it seems a bad idea with an engineered manifold and burners to tamper with it.

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ttekushan_3

My impression is that

lowest CO is with the burner off.(!) The manufacturer's nominal rating of 3.5 is always a good place. If you are looking to drop CO further even though its already <100ppm, you will usually start to see excess air rise to the point where high losses ruin the gross efficiency numbers. If excess air looks ok and CO is well under 100ppm, then I'd say stay with the 3.5.

Just my thoughts.

-Terry

Glen

I would

adjust to spec as you have done. And then monitor combustion analysis. Do you have altitude considerations?

Mitch_5

I always adjust the pressure to mfg spec

I do an analysis before and after as long as the other readings are within spec I stay at the 3.5"

Mitch S.

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Unknown

In addition to checking

pressure you should if possible using the gas meter clock the burner to see if it is firing at rated input. That along with a complete combustion analysis (especially O2, CO and stack temperature along with draft) will keep you safe.

It is also important to keep in mind that in order many times to get equipment to fire at its full rated input many well trained (emphasis on well trained) gas combustion techs will increase pressure slightly to bring the burner to its designed maximum firing rate and efficiency. In that case you may find gas vales adjusted above 3.5 inches water column. They should however tag the valve so the next person going in wil not reset it.

Jim Davis_3

What is assumed by gas pressure??

When manufacturers chose the gas pressure they have listed they were assuming that the btu's in the gas is always at the maximum. They also assume that the maximum btu's in the gas can be converted to 100% usable energy. They assume that every gas burner orifice is accurate within .00018" area. 3.5" is not the only pressure used by manufacturers today, some use 3.8" and as high as 4". 3.5" is a safety pressure used by manufacturers to protect themselves and enables them to sell equipment to those that are not quite skilled in equipment setup or testing and or to compensate for field conditions they will never see.

To lower gas pressure just to lower CO is only going to reduce the efficiency of the equipment and could cause more wear and tear because of low flue gas temperatures.

Gas pressure should be adjusted to reduce O2, increase temperatures into proper operating ranges to provide the maximum amount of heat transfer. CO readings are used to limit how far we can go. A furnace or boiler making 70ppm of CO could be 30% more efficient than one making only 10ppm. Equipment needs to be tuned for output not input. Nothing in the mechanical world can be truly tuned properly for input. It is only because so few know how to do it correctly that the old antiquated method is being used.

Glen

the last two posts -

add important considerations - as I have been told by those in the know that NG calorific values can be as high as 1100 btucf and as low as 900. Which makes me wonder - as gas meters are calibrated for a set specific weight of gas, would this lead to artificially high/low clocked consumption rates; and further - the unofficial allowance by local gas inspectors that +/- 10% of rated capacity is OK - pending solid combustion analysis values? I had this same discussion a few years back when I was asked to clock/analyze several atmospheric boilers derated for high altitude. Combustion values were mediocre at published manifold pressures and the clocked input was >20% lower than I expected - yet the manufacturer claimed that those particular boilers were just fine as the orifices & pressures were to their spec and I should not worry about it. And as manufacturers instructions carry more weight than the gas code (if to a higher spec) - the client eventually had to live with the setup.

Jim Davis_3

BTU of fuel at high altitudes is a big problem. Most equipment is way over derated and underfired. Fuel value and air density are unknowns. This is why setting up equipment by combustion analyzers is the only accurate method to use. It isn't any different at sea level. O2, CO and Flue Temperatures measure reality and true mechanical operation. Performance of equipment should fall within the same parameters everytime it is set up. No two pieces of equipment are exactly alike nor is any installation. Using one set of standards to handle a million possibilities is ridiculous. Unfortunately that is the skill level of the majority of the industry. It is not the manufacturers fault because they are doing the best they can. But to those who have elevated their knowledge and actually have skills, know the correct way to do things in the field. Their customers are the ones that are getting something in return rather than just exchanging money.

Darrell

I work within 100 feet of sea level with 1050-1100 gas so everything ought to be just about text book. I looked at a 30 year old Hydrotherm today...a buyer's inspection...that had an odd sound to it rather than the characteristic WHOMP of the single stage gas valve. Turns out it was turned down to 1.5 inches for who knows what reason.

On fixed orfice and shutter burners I was taught to set the manifold pressure to the stated pressure on the valve and you get what you get for numbers because the engineer designed it that way. On a fully adjustable burner, I was taught to set the Manifold pressure by clocking the meter and adjusting the firing rate to design , and then adjust the O2, CO2, CO by tuning the air adjustment until the CO bottoms out and begins to go back up, then give it a bit of fudge for the dirt that will accumulate. Check for smoke and call it good. So, I'm gathering from y'all that I could be doing some of the same thing for the fixed burners and maybe get alittle more efficiency out of them and less byproduct by adjusting the manifold pressure within certain limits by watching my Bacharach wind down the O2 rather than the CO as long as the CO isn't too bad.

But, and this is the cruxt of the discussion with the gas dude, what is the meter supposed to be set too? Because, if the meter is set to, say 5 inches, it will give less btu's per cubic foot than if it was set to 6 inches...so, to be really accurate on a combustion test I'd have to first check the meter delivered, (house side), pressure, then clock the thing under the load, make any adjustments to the manifold pressure for design firing rate, then check the O2, CO2, CO numbers.

Or, am I making too big of a deal out of this?

I need to charge more!

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Glen

You are not making -

too much of a deal about this. And you're probably right about your hourly fee (or flat rate for that matter). Good gas techs are hard to find - especially those with their mind wide open. Because as the above posts states - not all things are created equal. I've never worked at SL - that would be a treat - mostly 3000' +. Here's an example - this at a local resort @ about 6000' on LPG - 3 boilers side by side - two are a dream to tune each year - the third gives me a hard time each and every time. Spectacular boiler, equally fine burner - all matched, but one completely different.

Glen

No - I dont think

you are making too big a deal about this. Here's an example - three identical boiler with matching burners; two tune like a dream each year, the third always make me work for my money. 6000' elevation - on LPG. Second pic is from the roof at the Chateau looking at a frozen Lake Louise and the glacier behind it. I might be there for the boilers - but the view always takes my breath away.

Jim Davis_3

First thing here is that the btus per cu. ft. does not change with the gas pressure from the meter. Pressure only determines how many cu. ft. can be delivered. Kind of like the gas company that told a contractor once that if you turn up the street pressure you will be stealing gas because the meter can only measure cu. ft. at 5". Talk about the school of misinformation.

Engineers design equipment on paper, they don't build it. Equipment is designed to meet a certain standard to pass a lab test and nothing more. The conditions the equipment is operated in a lab is never duplicated in the field. But that is never the piece that we get. We get equipment that is assembled by factory workers with parts that come from all different manufacturers that are never exactly the same. Close but not the same. Then the equipment is handled and shipped and bounced and banged etc. which means it is not exactly perfect anymore.

It is rare and in facr I have never found one place in the country that maintains 1050 to 1100 btus per cu. ft. of gas. It is always lower.

Although we are discussing gas pressure here there is something even more important that should always be addressed before gas pressure and that is primary and secondary air. If these are not controlled then no gas pressure is safe!! What controls primary and secondary air?? The draft in the chimney and indoor pressure variances. Unless these are controlled no amount of gas pressure is correct. Draft in chimneys and mechanical room pressures are constantly changing unless we control them. Therefore there is never any guarantee of how much air we have to burn the fuel. Air shutters don't control air they just limit how much is primary versus secondary. If fact they were never intended to be used on natural gas appliances with atmospheric burners. LP Only!! That is why in the past many manufacturers didn't include them until you got the conversion kit. Some manufacturers knew this would confuse some contractors so they just put them on regardless.

Just talked to a contractor in Denver last night. He was working around 8000 ft. He increased the gas pressure on the furnace he was working on and bottomed out the regulator. The furnace was still underfired a delivering lower than normal air temperatures. It was a 90%. This furnace should supply 130 degree air and it was only 120. O2 should be under 9% and it was still over. The furnace had never been derated and the orifices were still the larger ones used at sea level. Can't imagine what the operation would have been if it was derated.

If you know how to calculate btu's delivered, less than 1% of all furnaces made would be operating within any acceptable ranges at the factory settings. But then how many even know what acceptable ranges should be??

Darrell

I don't get how a higher pressure cubic foot of gas doesn't contain more btu's at the step down pressure. If I get a propane bottle at say 10 psi it will fire my torch a given length of time. If that bottle is given to me at 5 psi it will not fire the same torch as long. So, if the house meter regulator is set at five inches, then I will get so many cubic feet of gas measured through the meter at five inches...that will in turn be stepped down to 3.5 inches giving me more cubic feet after the step down than before. If that same meter is set to six inches and then stepped down to 3.5 inches I will get more realized cubic feet after the step down because the primary side was denser. That's why the gas company is so adamant about setting their regs to 5 inches instead of six, otherwise why worry with one inch? Yes, when I clock a meter, I am counting the cubic feet passing at five inches, but the chart, or the formula, takes into account the step down regulator pressure...or am I missing a step? I may be about to learn something...but I'm sitting down and t is the weekend...so speak slow and use little words.

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bob_50

If you have

two natural gas fired boilers with atmospheric burners one at sea level and the other at 10,000 feet. A manometer connected to each burner manifold reads 3.5"WC. Is the BTU per CU FT content of the gas the same in both manifolds?

Glen

Here's a thought -

and I'm sure Jim will correct the thinking if off base. When using a high pressure meter set, and we clock an appliance, we always compensate clocked flow for the higher pressure. (atmospheric pressure in psi times output pressure in psi divided by atmospheric pressure again in psi) If a meter set is calibrated for 7 inches but only puts out 5 could we then not also compensate our clocking by this reduced pressure? It would make sense then for reduced performance realized by the appliance. Just a thought -

Glen

it's physics -

and the btu content of the higher elevation cf gas would be lower. It's the same as if you take an empty soda bottle (plastic)and open it at 10 000 feet, close it and then transport it to SL - the bottle collapses. The reverse is true going the other way.

Plumdog_2

And that is the problem

in particular with atmospheric burners. As long as that gas is compressed to an exact pressure, and contained within a pipe, a cubic foot remains a cubic foot. But as it approaches and escapes the orifice, it becomes larger; and this effect is more pronounced at high altitude. Not to say that more or less caloric value is happening, but that the available air to obtain a proper mix is reduced, cubic foot per cubic foot.
The technology in modern auto fuel injection can measure the MASS of the air, and meter the fuel in correct proportion, and clock the products of combustion in real time.

Jim Davis

First let me say that some people at the gas company are one filiment short of a light bulb. A cu.ft of gas is a quantity not necessarily a volume or area. Gas meters measure flow not pressure. If the pressure is increased the flow is increasedand the meter spins faster. Gas metersare fairly accurate up to 28" of pressure when it comes to measuring quantity. Under pressure a cu.ft. of gas takes up less area but it is still a cu.ft. of gas. Your propane tank has twice as much propane in it at 10psi than it does at 5 psi. It get compressed under pressure but it is the same btus. If you take a loaf bread that weighs 1 lb. and crush it, it is still llb and one loaf of bread. No matter how fast I push it past the cashier, I still get charged for one loaf of bread. Just trying to keep this as simple as possible and I am not trying to act like this is a dumb question, because it is a great question and it has come up before with much of the same confusion.

Mitch_5

Just to through something our are we not looking at only

one piece of the puzzle. The difference in the atmosphere itself. Not just the fuel source but actual differences
in the atmosphere itself between high elevations and sea level.

Mitch S.

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Jim Davis

That has more to do with set up not btus in fuel

At any altitude the btu per cu.ft. in the fuel is the same at any pressure. Gas pressure does not change the btus per cu.ft.

But, all the more reason things need to be set up with a combustion analyzer versus clocking meters and settling for a fixed gas pressure.

Unknown

As always Jim

you are absolutely correct. The gas meter has very little to do with BTU or any other part of the overall combustion process it is just a cash register that measure gas flow and most are indexed for inches water column measurement. There are however meters that can be used to measure at pounds pressure. It is by many local utility commision requirements accurate to with in = or - 2%. Meters also typicall run slow so it is to the customers advantage.

Unknown

About all clocking the

meter will do for you is find out if you are in the ball park.

Another thing Jim mentioned BTU content of the fuel being all over the place. Here in the Rhode Island area it is almost all the time around 1025. In recent years however in contact with the local gas supply personnel I have found that some mix of LNG causes it to go as high as 1100 to 1300 BTU per cubic foot.

That will cause you to be way over fired and sooting and CO are definitely an issue.

Plumdog_2

Interesting paper from American Meter

The way a meter measures gas is very interesting; as it compensates for pressure, which can vary a little even with temperature. Check Out This! http://www.americanmeter.com/profiles_files/amco_basicgaslaws.pdf
There is some bad science, though, in regards to the way cubic feet are regarded. In my mind a cubic foot is exactly that-a cubic foot; but when speaking of gas, it is a caloric amount, not a size.

bob_50

The BTU content

per CU FT is directly proportional to the pressure. The two boilers in my previous post both have a measured manifold pressure of 3.5" WC. The pressure in inches water ABSOLUTE is 410.984" WC for the boiler at sea level and 280.7"WC for the boiler at 10,000 FT.

bob_50

Plumdog

If you have a CU FT of gas that contains 1000 BTU's at some pressure. Try to picture a wire frame CU FT that contains 1000 1/4 20 wing nuts equally spaced. Gases always expand to fill the container. Now cut the pressure on the CU FT in half. The wire frame CU FT will expand to TWO CU FT. Now you have 1000 wing nuts occupying TWO CU FT equally spaced. When we change pressure we don't add or subtract any wing nuts. Cut the new TWO CU FT wire frame in half so you have two wire frames of one CU FT each. How many wing nuts are in each frame?

Darrell

OK,now I'm learning, help me make money with it. A couple of you obviously are in the combustion business big time and are very knowlegable...so...hypothetical situation:

I'm looking at a brand new boiler fresh out of the box. A contractor/installer has hired me to do the first fire and set-up. It is a WMC atmospheric burner with gravity draft. From step one through data card what should I do? I have all of the pertinent digital equipment to measure everything.

Now, I'm down the street mounting a conversion burner on an oil boiler.

Next I'm going to do a setup on an induced draft, negative pressure gas valve...step me through the set up.

I know most boilers are close enough out of the box, and never get a thourough check up...but it seems to me a good thing to be able to do it right...and then to test an older installation will also tell me much about what is going on be it right or wrong.

Or, if you know of a publication that tells me this tell me where to get it. I'd love to come back East and take the gas classes...but, not this year.

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Plumdog_2

I see what yer sayin

But for purposes of measuring gas, a cubic foot is a nominal (in name only) value. Gas can be delivered in tiny pipes at high pressure or large pipes at low pressure, either way nobody cares about the physical dimensions. Your two foot by one foot by one foot wire cage with a thousand wing nuts is still just a "cubic foot".

Glen

what this boils (sic) down to -

is utilization. We take a known volume of a product which has varying characteristics - mix it with another gas also of unknown proportions - and light it! And we are now in an age where "it looks good" does not cut it. So we haul out our manometers (of varying calibrations) hook in our analyzer - which may or may not be up to date - and have at it. Add to that geographic locale, eg. I would tune a boiler/burner differently that is 1 hour away vs, one that is eight hours away or in the artic. Which is to say - we have a great deal of latitude with todays first class burners. And the complexity is increasing - zero mix governors are becoming the norm on todays wall hung high efficient boilers - for good reason. But that also means our diagnostic equipment must keep up. EG - for day to day use I use a pair of UEI diff manometers - which are just fine for atmospheric burners but I have found that the +/- 0.05 inwc calibration allowance is not good enough for other burners. Even our analysers are subject to varying calibration requirements - they are not all subject to the same methodology or tolerances. So then - we can discuss combustion perfection and I can send you a calibrated EZ1 burner that tests beautifully in my test chamber - and you can throw it in your favourite boiler - only to retune for your circumstances. Which is what appeals to me greatly - we take our lessons in physics and apply it to real day to day applications. We may have a diagnostic checklist a yard long - but that just gives us a bit of flexibility to make the most of this business.

Jim Davis

You don't have to come east to take a class, I do them all over the country.www.nationalcomfortinstitute.com.

A Atmospheric boiler should run between 6%-9% O2 at any altitude. The flue gas temperature should be 270 degrees hotter than the water temperature once the boiler water is above 100 degrees. The CO should be under 100ppm and stable. Unfortunately anything atmospheric has inherent design flaws that don't make them the most efficient or safest in the field until we fix them. In 1993 AGA Labs spent a week in the field with myself and students. We tested 18 atmospheric appliances and found all to be operating unsafe when installed according to manufacturers specs and code. A few years later AGA stopped certifying equipment all together. Hmmm?

Unknown

I have any of the

books you will need. I suggest starting out with My Fundamentals of Gas Volume I and II. I also have a series of three manuals on Testing Designed Gas Equipment. I also have a manual on Gas Conversion Burners. Just get in touch with me at gastc@cox.net and I will send you the catalogs. If you come east to Rhode Island we have training available on all of this or get to Jim Davis Classes when in your area Jim will get you going in the right direction.

Darrell

Thankyou

Thankyou to all of you who tried to help me settle this in my head. What I am trying to do is develope a standard procedure for a yearly inspection on a gas boiler that I can follow step by step on every boiler with the goal of giving the customer good value, reliability and safety, and to keep me focused so I can get 'er done and get on to the next one...profitability. This procedure will be incorporated into an existing checklist that I perform anyway and give to the customer...just like you get at the car mechanics for your tuneup. There aren't any classes close for awhile, but I think the text books will be exactly what I need.

Again, thankyou for taking me seriously.

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Glen

I would add -

several other books:
Electricity for Refrigeration, Heating & AC - by R Smith
Industrial & Commercial gas burner and systems - by John Dutton (Centennial College Press)
Fundamentals of Gas Utilization also by John Dutton
and finally the Combustion & Oil burning equipment by Lanthier.
As far as developing a checklist - I can fax you a copy of mine - it's not completely inclusive - but covers boilers, direct fired, and other process burners. Leans toward the com/ind side.