Why is 0.5 in WCG pressure drop so important?

NRT_Rob

Hey all,

Learning new things, and getting into gas design procedures a bit. Being a wethead, pressure drop and flow rate calculations are not exactly new to me. I can figure out what the pressure drop is from A to B in a piping system with gas and I've seen the different design methods.

What I can't figure out for the life of me is why some sources keep referring to some code specified "allowable pressure drop" on a gas piping system. WHY would anyone care what the pressure drop in a system was, presuming it was calculated correctly and resulted in the appropriate gas volume at acceptable pressures at an appliance?

Do I really need to size gas pipe systems for 0.5" drop (low pressure) on propane?? Even if I can calculate a 3.0" drop that is well within an appliances min-max inlet pressure parameters at the required cubic feet per hour?

thanks! hope everyone is doing well.

Rich_49

Welcome to the world of gaining knowledge Rob . I did what you are doing 8 years ago and what I found was enlightening , it also allowed me to be more profitable on my gas pipe jobs while ACTUALLY abiding by the INTENT of the code . You must be aware of where your install is and available pressures there . Allowable Pressure drop is key to this . Remember . " the intent of this code is to minimize the hazards associated with storage and distribution " . Smaller pipe means less gas and fuel to an already unfortunate event should that happen . Why have more gas in your building than you require . Been doing it for years and have not had a single occurrence of there being too little gas for everything to operate . Also remember that we size to .60 SG which loosely translated means @ 60*F . Check out " The application of temperature and / or Pressure Correction factors in Gas measurement . Combined Boyle's - Charles' gas Laws . In my particular area I always wondered why I would deliver 7" to an appliance that required 5" or less . Now I know and you will too .

NRT_Rob

Thanks for the pointers Rich!

I checked out that paper and I think I'm seeing that on temperature variance, we could see a 10% reduction or less at higher (typical) outdoor temps of the gas SG (if I read it correctly, SG goes up with lower temps, more gas per foot when it's more dense?). So I can see having some safety factor there to avoid starving an appliance on a hot day, looks like 10 or maybe 15% should be safe. Good so far?

What I'm not getting (I think?) is if I have 11" wc at the beginning of a run and, say, 8" wc on the appliance end, that's 3" wc which violates the 0.5" wc prescriptive standard, which is, I think, only present in SOME codes...

But in this case, let's say I"m using boiler with minimum inlet of 3.5" and maximum inlet of 14", and I've calculated those drops at the CFH the boiler actually requires at full fire. I'm delivering 8" WC which should be, I think, far beyond any necessary safety factor above the minimum inlet pressure there... I'd be all set, right?

Am I missing something there? Am I crazy? Would a code official really call foul on that?

Thanks again for sharing Rich.

SWEI

You need to know the minimum supply pressure likely to be available. Surely there must be software that would calculate all this, including diversity/demand factors and minimum pressures tolerated by each appliance.

bob_46

Rob, I think you might be mixing up requirements for propane vs. nat gas. Propane usually requires 11" W.C. in the burner manifold there is no regulator at the appliance. Nat gas is usually 7"W.C. at the meter regulated down to 3.5"W.C. in the burner manifold. There are exceptions.

NRT_Rob

Maybe? The Elite Firetube mod/con I am looking at is specifying a minimum 3.5" and max 14" inlet pressure.. that's for natural gas, but it talks about propane and doesn't provide any alternate spec for that in the manual.

Just cross checked with the Triangle Tube Trimax 110 and it specifies, specifically for propane, a minimum inlet pressure of 5" and maximum of 13". So I think my example still is relevant here.. if I deliver 8", after a full flow pressure drop, I should be ok even though the drop from tank to boiler was 3" from the baseline 11", right?

Rich_49

IFGC states that there are 3 sizing methods available to the designer , YOU .
402.3 Sizing
1. Pipe sizing tables or sizing equations in accordance with Section 402.4 .
2. The sizing tables included in a listed piping system's manufacturer's installation instructions .
3. Other approved engineering methods .

This is what got me thinking about this years ago . How can someone feed a larger BTU requirement using smaller pipe that has a higher "w.c.p.d per foot than steel ? To hell with beating around the bush Rob . Use the series summation method sizing out of the Gastite manual for steel also , steel has a lower P.D per foot than CSST . There it is , it's recognized and is CSA approved engineering . Our State agency that is in charge of code enforcement recognized this after many arguments with inspectors . Iron pipe pressure drop tables at loads between 10 K and 15 million are located in the back of that text .
What we do here .
8" wc at meter , 1/2" drop through meter at low pressure and at a specific load (usually750) higher than 750 requires larger meter or 2" PD through meter be recognized . Total load is say 450, 000 . First tee has water heater and boiler on that segment and that tee is 40 feet T.E.L from meter
They are 200,000 BTU s. 1" Iron has a pressure drop of .024 per foot at 450 K .
7.5" - 40' x .024 = 7.404" at tee . I take the required minimum at fixture and subtract from 7.404 to determine what size and how long that segment can be . 7.404" - 5" = 2.404 . Call it 2.41 for safety . Your A.P.D is 2.41" wc . 3/4 pipe has a pressure drop of .017 per foot at 200K . 2.41" / .017=141.75' . There you go you can run 3/4" steel up to 141. feet and suffer nothing . All the segments get sized through the main and the secondary piping for the remaining load . You'll never be short of fuel and you'll save lotsa time and money . Caution on meter sizing and equipment . Years ago we beat our heads on the wall when called to a few houses that had Rinnais and the like installed which when it got cold out took these systems that were sized for 400k to 560 and the water heaters were locking out on lack of pressure , lots of 415 meters got switched out to Rockwell 750s really quickly .
Hope this helps you Rob .

NRT_Rob

cool, thanks Rich, that was really helpful. Now... got a good source for the CFH/Pressure drop per foot tables for COPPER like gastite has for their CSST and steel pipe? I can find everything else regarding copper but not the pressure drop per foot at a given CFH flow rate.

thanks again!!

Ironman

Rob,
The problem with having too much variance in pressure is this: the regulator in the appliance's gas valve sets the difference in pressure between the incoming and outgoing (manifold) pressure. If it's set to give 3.5" manifold with 8" incoming and the incoming dropped to 5", the manifold pressure would also drop a corresponding amount. The appliance would be under-fired.
2 stage regulators greatly reduce this problem, but some appliances/systems may not always have them.

Rich_49

Bob ,
If it's sized for maximum flow and load why would anything drop ? Again , I have had not one problem sizing this way in roughly 8 years or longer . The fixtures will receive designed for flow at full flow but will receive a bit more gas at less than design , kinda like hydronic systems

NRT_Rob

maybe to put it another way, if the appliance says "minimum inlet pressure" is 3.5", isn't that the minimum value I am to deliver to the valve? That doesn't necessarily say what the pressure after the appliance gas valve is, just what the pressure should be delivered to the valve... no?

SWEI

Optimizing this requires math that is, um, non-trivial. The good news is "we have software for that." Fire sprinkler designers routinely make use of hydraulic flow calculation and modeling software that understands things like ring and grid mains, multiple sources with varying pressures, and fairly complex failure modes that span systems.

Utilities also employ network modeling and analysis in day-to-day operations.

NRT_Rob

well, it's not that bad. Tracpipe, for example, has "pressure drop per foot" charts for their product as well as S40 pipe at different CFH flow rates. I could go straight to the hydraulic resistance calculations if I needed to, but I'm lazy and CSST pipe makes that a little more complicated... not sure how to reflect corrugation's effect on inside diameter effectively in those calcs.

Has anyone seen a 'pressure drop per foot' chart for copper? I don't mind using a little chart lookup to automate this in excel.

thanks again for everyone's help.

Ironman

Rich said:

Bob ,
If it's sized for maximum flow and load why would anything drop ? Again , I have had not one problem sizing this way in roughly 8 years or longer . The fixtures will receive designed for flow at full flow but will receive a bit more gas at less than design , kinda like hydronic systems

Rich,
I was was saying IF there's an excessive pressure drop such as 3" because that's the number Rob stated. Not saying your method would produce that. Just trying to show that a pressure drop on the incoming side will cause a pressure drop on the outlet.
If there were only one appliance on the line, then the pressure drop wouldn't vary and it could be calculated in. But, when there are multiple appliances, or a modulating burner, the drop must be minimized.
Fluctuating inlet pressure is an issue for modulating, negative pressure gas valves. I do a lot of work in Staunton, Va which was one of the earliest cities to get natural gas. The downtown section still has lines under the streets that are over 100 years old. Because of this, it's a low pressure zone and the gas co. keeps the pressure at or below 12" w.c. In the early a.m., when everyone turns up the heat or uses hot water, I've seen the street pressure drop to as low as 3". Try running a mod/con on that. In fact, manufacturers have told us not to install them in low pressure zones.

Rich_49

Bob of course one would have to pay attention to the area 's specific characteristics , utility practices and available pressures . I happen to be in an area where the pressures are as reliable as one could wish for . This fluctuating pressure you speak of will take place no matter what , as different appliances turn on and off , the most we can all do is the best we can all do . At the end of the day those pressure fluctuations are gonna happen at all the gas valves and at the end of my day I must protect the well being of those within the built environment . If lessening the amount of an explosive gas within their dwelling can accomplish that , then that is what I will do wherever I am as long as the equipment can still operate safely and conditions allow .

RobG

I always design for a .5" pressure drop. I rarely have to worry about fluctuating inlet pressures and the material and labor cost is minimal if you do a 2 or 5 PSI system and regulate it down at the fixture(s). I would much rather thread a hundred foot of half inch and pay for a regulator than thread a hundred foot of two inch and not have a regulator.

JMHO
Rob

Rich_49

How bout if you could thread a hundred foot of 3/4 and not have elevated pressure inside the envelope Rob ? Regulators also have OPDs , is there such a thing as an acceptable amount of NG , LP inside the envelope ? Are you one of the four guys in the country that spends the extra dollars and pipes them to outside ? Most don't because there is an acceptable amount by code .
Let's look at another scenario . Fire starts in the house with 2# - 5# and that aluminum regulator and its internal parts become compromised ? Would you like to venture a guess at the potential outcome ? I have contemplated that and will stick with having as little volume and pressure inside the envelope as possible .

RobG

Rich said:

How bout if you could thread a hundred foot of 3/4 and not have elevated pressure inside the envelope Rob ? Regulators also have OPDs , is there such a thing as an acceptable amount of NG , LP inside the envelope ? Are you one of the four guys in the country that spends the extra dollars and pipes them to outside ? Most don't because there is an acceptable amount by code .
Let's look at another scenario . Fire starts in the house with 2# - 5# and that aluminum regulator and its internal parts become compromised ? Would you like to venture a guess at the potential outcome ? I have contemplated that and will stick with having as little volume and pressure inside the envelope as possible .

So you won't install an elevated / high pressure system? I personally have never seen two or even five pounds of pressure blow out a steel or copper pipe much less a vent limiter. You are talking about using the code to your advantage to downsize the piping however I am also doing the same. If it wasn't allowed it would not be listed as an alternative pipe sizing method.

Rich_49

Cannot seem to find where I stated that 2# - 5# would blow out a steel or copper pipe . Could you direct me to that statement or how you came to the conclusion that this is what I meant ?
- Codes also allow us to deliver 140* water to lavatory faucets Rob , but should we ? We discuss many installed to code systems that don't perform right here on this site , we install differently because we know better and there is a better way . Should we not do what we do because a code says you don't have to go that far ? Codes as it has been pointed out many times on THE WALL are just " The worst possible job you can install without the inspector being required to FAIL your installation " .
Code says that you can have 2 CFM of gas venting inside the envelope also , I say NO WAY . My favorite code of all is the
" Code of Hammurabi " . I bet there was not a whole lotta discussion about what everyone should do back then .

bob_46

Good thread, which one of you guys can piss the farthest . I think NFPA says 2.5 CUBIC FEET PER "HOUR" in a ventilated area .

RobG

" Code of Hammurabi " Man you are getting deep, I had to "Wiki" that one. I think we shall agree to disagree and call it a day.

Rich_49

That's good with me Rob .