natural gas 2" pipe 10 psi 1

[PNA]

we have a customer that has a 2" diameter sch 40 pipe at 10 psi, it runs well over 100' with lots of elbows, etc.

then it is stepped down to 1-1/4" diameter reducer to a pressure regulator, to drop the 10 psi down to 7" wc (which we recommend doing it in 2 stages) and the outlet is 1-1/4" to stepped up to 2" diameter

we need 3000 cubic feet per hour at 7" wc in order to have enough btu's to heat up our furnace

we originally had a 3" diameter tee down to 2" diamter manifold (short section, less than 5')

this was removed by the gas fitter, and replaced with a 2" diameter tee

what happens is we fire 1 burner, then afterwards we fire the other 2 burners.

we get a huge drop in temperature in the furnace when that happens

is it lack of supply?

 

[PNA]

can a 2" schedule pipe supply 3000 cfh of natural gas, at 10 psi?

 

[IRstuff]

There appear to be several on-line calculators for this sort of thing.

TTFN

FAQ731-376

 

[BigInch]

we originally had a 3" diameter tee down to 2" diamter manifold (short section, less than 5')

this was removed by the gas fitter, and replaced with a 2" diameter tee

This is not clear.
I don't have any idea what was originally 3". Was it the 2" 100 ft run with lots of elbows that was 3", or are you just saying the "T" was 3". If it was the T that was 3" that is now 2", where was(is) that connected? Did you have the 3" T connected to the 2" after the regulator.

Try giving a more accurate description or post a diagram.

100 ft of 2" will only supply about 1200 CFH, so that is certainly ONE problem, even if you didn't have a lot of elbows. Your long run should be at least 3". That's borderline if you had the available pressure to do it. Probably a 3.5" or even 4" would be more appropriate depending on available pressure. The rest of the piping being short, would probably supply the 3000 CFM, but the velocity I think might still be too high at that 7" WC (its 0.25 psig) Dropping that flowrate from 10 psig down to 0.25 psig may be problem #2, because the velocity would go much higher. Are you blowing the gas straight out the stack unburned?
Velocity is about 40 fps from a 2" outlet. May be a bit high for burning. If you're going through smaller burner tip holes, it would be much too high.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[zdas04]

Gas flows from high pressure to low pressure. That is all it does. If your regulator is set at 7 inWC then the pressure drop in your pipe will reduce the pressure at the burner to something less than 7 inWC. You didn't say how far it was from the regulator to the burners. Let's pretend that it is 100 ft. In that case 7 inWC at the regulator and 3,000 CFH in the 2-inch should give you 5 inWC at the burner. If the pipe is 100 ft of new steel, then you would need 9 inWC at the regulator to get 7 inWC at the burner.

10 psi to 7 inWC is not a very big drop for a commercial regulator, I sure wouldn't ever do it in two steps (twice as much moving equipment to maintain).

By the way 3000 CFH in 2-inch with 10 psig at the foot has 10.039 psig 100 ft upstream (velocity is less than 20 ft/s). The pipe sizes are fine.

David

 

[whammett]

Your undersized. Assuming everything is ideal from the utility or supplier, and everthing downstream of the piping is in order. What I don't understand is going from a 2" diameter pipe steping down to a 1-1/4" diameter pressure reducer. By lowering the diameter, you actually increase the pressure and velocity of the gas going into a pressure REDUCING regulator. Sounds like its a very bad design.

I would consider completely redesigning the system.

"Scientists dream about doing great things. Engineers do them." -James Michener

 

[BigInch]

Yes the velocity in the 2 or 3" is ok, but at the release point to atmospheric pressure and burn, I think its too high.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[zdas04]

whammett,
That design is very common. The reasoning (which I find hard to fault) is that a 1-1/4 regulator costs about half what a 2-inch regulator costs, and since your goal is to reduce pressure the smaller pipe just aids in the goal. Normal practice and not a bad one.

You probably need to go back and review your incompressible flow arithmetic--greater velocity is the opposite of higher pressure. Reducing the pipe size increases velocity and DECREASES pressure into the regulator. In this case the reduction is infinitesimal, but it is in the right direction.

David

 

[PNA]

1) online calculators for the most part are not very good, i don't need to calculate losses from the meter to the connection point, we stipulate that a "supply" was provide 3000 CFH @ 7" w.c., we provided a 2" x 3" x 2" connection point to our furnace manifold, the tee, is close to center of the manifold and is 8' long

2) our connection point was removed and a 2" x 2" x 2" tee put in its place by the gas fitter, from there a short run (less than 10') of 2" to the pressure regulator, reduced to 1-1/4" and the inlet is 1-1/4" up to the 2" supply (it is 14" w.c. coming out and 10 psi going in)

3) 10 psi to 7 inWC is not a very big drop for a commercial regulator, I sure wouldn't ever do it in two steps (twice as much moving equipment to maintain). every time someone goes from 10 psi to 7" w.c. we have a very difficult time with how the furnace operates, once they put in a 10 to 2, and a 2 to 7" everything is peachy, nice steady flow

4) we arent undersized, someone else provided the regulator and supply

5) That design is very common. The reasoning (which I find hard to fault) is that a 1-1/4 regulator costs about half what a 2-inch regulator costs, and since your goal is to reduce pressure the smaller pipe just aids in the goal. Normal practice and not a bad one. (well we have sold hundreds of furnaces and it always bites the customer in the rear, cause cheaping out doesnt work) we also include payback period, even if a 1-1/4 costs $500, and a 2" costs a $1000, why not cut the reducers out of the equation and go same pipe size to same pipe size

6) from the charts i have researched, i dont see anything that states at 10 psi, thru a 2" pipe, you will x amount of cfh of natural gas, unless i am missing something

 

[PNA]

thankyou for everyones time and information

just a quick observation, i have seen that some say its more than enough and some say its not

 

[Latexman]

My observation on that is it must have been the perfectly clear description you gave that most commented on, like "it runs well over 100' with lots of elbows, etc."

Good luck,
Latexman

 

[racookpe1978]

Just an innocentquestion from a bystander here:

Why the "incompressible flow" assumption above for the software/calculator?

Would the fact he's got a reducer in the line mean you have to calculate based on compressible flow?

 

[katmar]

The 100' of 2" pipe at 10 psig is fine at 3000 cfh. Assuming your 3000 cfh is split equally to the 3 burners (i.e. 1000 cfh per burner) then the 2" manifold legs to each burner are also fine. The only section that is marginal is the 2" section from the reducing station to the manifold at 3000 cfh. Is it possible to move the sensing point for the reducing station closer to the manifold so that it controls the pressure at the manifold, rather than right at the reducer? If you can do this you will factor out the pressure drop in that section.

Although I have said that the 2" section from the reducing station to the manifold at 3000 cfh is marginal, it is not seriously undersized and I would be surprised if it was the cause of the "huge drop in temperature in the furnace" that you report. You should check the rated capacity of the reducing valve, and also check for debris left behind in the pipes and valves from construction. Debris has accounted for the majority of the start up piping problems I have experienced in my career.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ione]

zdas04,

The gas velocity (3,000 CFH in a 2" pipe) should be approx 35.5 ft/s. Anyway I agree the line is ok

 

[zdas04]

The "incompressible flow" assumption for gases causes a lot of confusion. All of the readily solvable fluid flow equations started with the Bernoulli Equation. One of the assumptions in the derivation of that equation is that density is constant (i.e., the fluid is not compressed). Actually, the gas density is always changing some, but the change is often small enough to be insignificant and can be assumed to be zero. This assumption is generally taken to be valid if the downstream pressure is between 90% and nearly 100% (for incompressible flow it has to be less than 100% or there is no flow) of the upstream pressure.

For compressible flow there are different equations, but none of them are as tidy as Bernoulli, and all are special cases. For example, if velocity is above 0.6 Mach then the change in density with changes in velocity is huge (at 1.0 Mach the density of a low-pressure stream has increased several orders of magnitude) and can't be disregarded.

ione,
My calculations show that velocity at the regulator at 10 psig is just over 20 ft/s. I guess we have different assumptions. I'm assuming the 3000 CFH is 3000 SCFH which may be the difference.

David

 

[katmar]

Good point made by zdas04 on assumptions. All my calcs were done on the same basis as his - i.e. the 3000 cfh are at 14.7 psia and 60 F. I think this is standard practice for end-users of gas, but I should have mentioned it in my earlier post. Any slight changes in "standard" conditions - eg 14.5 psia or 68 F will not change the conclusions.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

The online calculators are not going to be based on the equations typically used for LOW PRESS gas flow.

The low pressure gas flow tables may be more appropriate; if you are not specifically using one of the low pressure flow equations yourself. This one from

http://www.engineeringtoolbox.com

http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html

Assuming 100' w/ "lots of elbows" = 150 ft equivalent length
This table would imply 3" pipe.

Capacity of Pipe (MBH ? CFH)
Pipe Pipe Length (ft)
Nom ID(in) 10' 20' 40' 80' 150' 300'
1/2 0.622 120 85 60 42 31 22
3/4 0.824 272 192 136 96 70 50
1 1.049 547 387 273 193 141 100
1 1/4 1.380 1200 849 600 424 310 219
1 1/2 1.610 1860 1316 930 658 480 340
2 2.067 3759 2658 1880 1330 971 686
2 1/2 2.469 6169 4362 3084 2189 1593 1126
3 3.068 11225 7938 5613 3969 2898 2049
4 4.026 23479 16602 11740 8301 6062 4287
5 5.047 42945 30367 21473 15183 11088 7841
6 6.065 69671 49265 34836 24632 17989 12720
8 7.981 141832 100290 70916 50145 36621 25895


"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[PNA]

My observation on that is it must have been the perfectly clear description you gave that most commented on, like "it runs well over 100' with lots of elbows, etc."

Good luck,
Latexman

your right, but before the pressure regulator is well over 100' with the aforementioned, even though they are seeing 10 psi just before the regulator, what volume would they really be seeing?

 

[PNA]

I thank everyone again for their input.
I agree, debris is a concern for sure.

From what i have gathered, i think maybe a flow meter would determine if there is enough flow? Not just calculations.

Regards

Paul

 

[zdas04]

BigInch,
What upstream pressure does your chart use? I've seen these charts use a wide range of pressures. I've also seen charts from different people who got different flow rates for the same pressure. The things are built by Engineers after all.

David