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Gas pipe size (26 Posts)
Gas pipe sizeGood eve. I am finally starting meaningful gas training(i.e. Timmie's training manuals). I have a few basic questions. Number one is what is the reason behind gas pipe sizing? In water supply the main concern is volume of water. If the pipe size is to small then the volume of water that can pass through that pipe will be limited. Water is not compressible and therefore only a limited amount can pass through any given pipe.Is the same concept true with gas? Are the limits on how much gas is allowed for a given pipe based on the volume of gas that can pass through that pipe? Or maybe gas works differently. Gas is compressible(I think). Are the sizing charts designed to eliminate friction caused by undersized pipes with will result in an unexceptable pressure drop?Meaning that if a given gas pipe is too small the needed volume of gas will be able to pass through but the friction will create an unacceptably low gas pressure. Please help set me strait. Thanks in advance for sharing your wisdom!
Pipes:My third world understanding is the two is that it is all about pressure drop. Water pipes must be sized so that the pressure drop at the outlet and gas piping is the same.
Gas piping sizes are based on the flow pressure shouldn't drop much if at all, when it is at rest or being used.
The difference is semantics
ThanksPlease correct me if I am wrong. I am under the (uneducated) understanding that If you have for example a 1" water line and at some point the line is reduceded to 1/2" and then increased to 1" then everything down stream of the 1/2" will have the voulume/presure of a 1/2" pipe. On the other hand with gas it is possiable to have short lengths of smaller id pipe upstream and the larger downstream pipe will be adequate because short lengths of small pipe only create minimal pressure drop. Not sure if I am making any sense. Please let me know in either case(that's how we learn) Thanks
Up, down and Up:If you take your example of the 1" pipe, reduced to 1/2", then increased to 1", and you had a hose drain on the end (1/2"), if you ran the hose slowly, the hose "sees" a 1/2" pipe, even if it was fed with a 2" pipe. When you increase the flow, the hose will always "see" a 1/2" pipe and the amount of water that can flow through it. If you put a 1" pipe on the end of the "train", it will still only "see" the 1/2" flow because of the restriction. The pressure up-stream of the restriction may drop slightly but the pressure after the restriction will drop considerably under high flow.
Gas is the same way. Same principle, different medium.
gas and water are both fluidsso the same basic physics applies. Pressure, viscosity, and friction interact with velocity to produce pressure drop. Once the pressure drops below a certain point, appliances don't function well (imagine your showerhead with 10 PSI water.)
lets keep it simplewe want to make sure we get sufficient gas at the point of use (the equipment) so we have to know the following:
Length of pipe and allowance for fittings
What are you using for an allowable loss
What method we will use (Longest Run or Branch)
Gas pressure before the gas valve
Simple rule do not reduce size and then increase size as that becomes a resistance to flow. Betterr if a new appliance is being added to run a separate line to that appliance.
Gas Pressure lets say at the outlet of the gas meter you have 6" W.C. or if it is LP 10" at the outlet of the second stage regulator. The farthest appliance is say 40 feet then your pressure at the appliance under a full load condition should be the same as it was at the point of origin the meter or regulator. If not then you probably have not calculated the pipe size correctly. Any restriction in size along the way will reduce the final pressure.
exampleI have 1" main coming in from street. The main is reduced to 3/4" for 2 elbows and a 6" nipple. The swivels are both 3/4". After the swivel the pipe size is increased to 1" and travels 50' to the boiler room where there is an 80 mbtu boiler and 65 mbtu water heater. There is also a oven on first floor and dryer on 2nd floor. My understanding is that I need a 1" line for this load. Is the fact that there is short run of 3/4" pipe in the begging of the run an issue? Thank you.
meterWhere's the meter? It has to be sized correctly, then you deal with everything after it.
Lets start at the beginnngIs the main coming in to the building low or high pressure that is inches pressure is low, pounds pressure is high.
Assume that the utility knows what they are doing to get you the right pressure at the outlet of the meter, it should be somewhere in the range of 6" W.C. to maybe 10" depending on the system as to it being low pressure or high pressure.
From that point add up all the BTU's required in the building. Make sure the meter is sized to handle that load.
Using the longest run method size the main line and then each of the branches. You have to use that longest run as the base for all the others.
The test is that with everything in the building running there is no drop in pressure anywhere.
BTUSRight now there is a single 3/4" main with a 2rd floor dryer and 1st floor oven(aprox 80 mbtu). The old oil boiler is going out and a gas boiler and water heater going in (aprox 130 mbtu). The meter is an ac -120. The idea is to T off right after the outlet of the meter and run a 1" gas main which will serve both the boiler and water heater. Please let me know what else I need to know. Thank you. Not sure if it matters but at one point in time there was a second meter on the incoming 1" line which is now locked. I think the home used to be a 2 family and it has since been converted to a one family. Thank you again.
1" mainThe 1" main for the boiler/water heater will be aprox 50 feet long. Thank you.
ac120i believe that meter is rated for 120000 btus
BN read my post abovegive me the break down in BTU's of each appliance, then the length of pipe to the furthest appliance. The meter is sized okay. All you have to do is worry about what goes in after the outlet of the meter.
IS THIS LOW PRESSURE GAS COMING INTO THE BUILDING OR HIGH PRESSURE?????????? ANSWER THIS PLEASE!!!!!!
Low pressureThe gas is low pressure. 1.dryer=35000 btu. 40 feet from meter, 20 feet high.
2. oven= 50000 btu. 40 feet from meter 10 feet high.
3. boiler=100 mbtu. 50 feet from meter
4. water heater between 50 and 75 mbtu. 50 feet from meter
There is a 3/4" line feeding oven and dryer. The boiler and water heater don't exist yet. They are replacing oil boiler. The plan is to T off after meter and run 1" line to boiler and water heater and leave current 3/4" line for the oven and dryer.
Very goodTotal on boiler and WH is around 175,000 BTU's at a pressure drop of .3 with inlet pressure less than 2lbs and a run of 50 feet then 1" pipe will handle 215,000 BTU's. Table 6.2(a) Schedule 40 Metallic Pipe in NFPA 54.
Now total up your appliances
WH 75,000 (why so large?)
Oven I assume you mean range which may be more than 50,000 example 4 top burners 10,000 each that is 40,000 plus oven 25,000 is 65,000
Total is around 325,000 BTU's so that meter is too small to handle full load.
THANKS!What is max that I can put on that meter? I do have flexibility with the water heater. The 75 mbtu was assuming I put in 75 gallon water heater and was meant as most conservative estimate. I could opt for a smaller water heater or maybe even a coil on the new boiler.
Your AssignmentThe meter is rated for 250 cfh. How many btu's in cu ft of NG?This post was edited by an admin on September 14, 2012 5:22 PM.
btu totalI just added up numbers and I am getting total of 275. 100+75=175. 175+65=240. 240+35=275. Please correct me if I am wrong.
RightIs the meter big enough?
Why such a big water heater?How many people will be using the water? A 50 gallon is typical for a residence or less.
Make sure the BTU's you are counting are taken directly from the appliance rating plates.
A 250 cubic foot meter will allow about 256,250 BTU's figuring 250 times 1025 (amount of BTU's typically found in a cubic foot of gas).
By the way the larger meter will use a larger meter bar with probably a 1" outlet.
water heaterI haven't spoken with homeowner about there hot water needs yet. Around here people have between 40 and 75 gallon. Thank you.
alternativesI think is is highly desireable to have a new temperature compensated meter properly sized installed to match the load. However, there are alternatives. Around Philly, our utility, PECO, often installs a 12.2 wci supply pressure into the home for larger loads while continuing to use the previously undersized piping. However, you will need to check with all the attached appliances and possibly turn down some standing pilots. I recently responded to a complaint about an odor from an air conditioner. Turns out they forgot to turn down the pilot in one of three boilers in this big house. The ~12 wci was blow-torching the pilot causing flame impingement on the flame shield and one burner tube making CO and aldehydes, which is what they were smelling (not the CO).
If the meter is old or not temp. compensated then it is probably due for replacement anyway. Note that some meters are installed without a MP regulator btw it and the street. This can result in wide swings in inlet pressures depending upon how that neighborhood was piped.
The bottom line is, you must supply the requisite inlet pressure UNDER LOAD as stated on the rating plate and in the IOM manual. That means you really should fire up every appliance and measure the dynamic or flow pressure under max load at each appliance or at least the furthest and closest to the meter.
For those who do not knowthe reason for problems with heating system pilots is that the flow through gas valves to pilots is not regulated, they are on line pressure. Just about every other pilot/main burner is regulated by the control regulator on the appliance. Water heaters (storage type) use a total regulation gas valve so the pilots on those is regulated.
On heating systems the problem in many low pressure areas is that local utilities used to guarantee 7" W.C. to 10" W.C. now many of them say only 5" W.C. Conversely the pressure in those areas may be all the way from 5" W.C. to 10" W.C. o0n natural gas low pressure systems.Adjustments to pilots gets tricky unless you plan on going out and checking them everyday. Take a gas pressure ahead of the valve when you are there if you are fortunate enough to have around 6 to 7" W.C. then adjust the pilot according to that. It is also a good idea to use a multimeter and check millivolts (Thermocouples 25 to 30) or microamps ( Intermittent pilots 2 to 10 microamps).
At the end of the process also do a combustion analysis.
For those who do not know what temperature compensation is all about. The meters have a bi-metal compensator inside the meter which measures at all times as though the temperature is at 60 degrees F. This is for accuracy of measurement both for the utility and the customer. Meters by the way never run fast they run slow always to the customers advantage. Most meters have identification of temperature compensation on the front of the meter.
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