I need to find the pressure of Natural Gas that is being transported in a pipe at Point x (Px). The pipe diameter at x is Dx (known). The pipe branches out into two different diameters downstream of Point x. Lets assume the pressures downstream are Py (known) and Pz (unknown), and the corresponding Pipe diameters are Dy (known) and Dz (known) .
I know each of the pipe diameters (Dx, Dy, & Dz) and know one of the pressures (Py).
Is this enough information to calculate Px and Pz? I have meter at point x and I need to calculate the Temperature and Pressure correction of which I have table but I need to know the pressure at Point x.
If it helps to have real numbers Dx = 2", Dy = 1.5", and Dz = 1" and Py = 57 psi.
If I remember my high school physics correctly, Im pretty sure that the pressure will be constant throughout the system as long as nothing is flowing. If the gas is actually moving (eg because the appliances attached to one or both of the y and z branches are turned on) then the pressure drop will depend on how fast the gas is coming out. The diameter of the pipes will have only a negligible effect in that for an equal flow rate the pressure will drop more slowly with increasing distance from the source in the wider pipe.
Please note that I am not an engineer and I’m not sure what youre trying to do exactly. Im also a bit surprised at your 57 psi pressure. I could well be wrong, but to me that sounds a bit high for a domestic natural gas supply! (Thats about twice the overpressure you’d use to inflate your car tires.) Anyways, HTH.
If you are discussing a dynamic system, you neglected to provide the rate(s) of flow and the lengths of pipe between all points in question.
A gas engineering handbook should provide the formulas for good estimates to answer your questions.
It is an industrial application, and there is a guage at Py which is registering 57 psi. Gas is constantly flowing and the meter is recording ft3 of gas. It needs to be corrected for Pressure and temperature. The Gas company has provided me a table and 57 psi is well within the “boundries” of the table.
Basically what Springears said. If flow rates are low and the distance between points x and y is not too far (obviously ‘low’ and ‘not too far’ are relative terms, and this is where you need to check in a gas engineering handbook if youre not sure; fwiw id estimate you’d be OK for up to about 1 cu ft per minute and less than say 20 ft) then, heck you could probably use 57 psi for Px and you wouldnt be too far off. How accurate do you need to be anyway?
The compensation factor at is 57 psi is 4.7. At 50 psi it is 4.0. At our volume of Natural Gas and the current price of Natural Gas ~$7.00/MMBTU., the difference could be about 12 thousand dollars per month, 150K per year.
THIS IS NOT CORRECT PRACTICE! PIPE DIAMETER IS HIGHLY IMPORTANT! HIGH SCHOOL PHYSICS WON’T CUT IT! You will need to apply Bernoulli’s equation relating pressure drop (dP) to velocity, but velocity is related to pipe diameter and the roughness factor. Then, you have to account for losses due to your fittings. There may be some adjustment for compressible flow, or sometimes general assumptions simplify the calc. You need to be familiar with the Moody chart and techniques which simplify the process… - Jinx, M.E.
Jinx, I agree that this has moved well beyond high school physics at this point. Initially I was assuming low flow rates, in which case pipe diameter really shouldnt make much difference, especially if the distances are short. Conversely yes, pipe diameter (and roughness) affects ‘resistance’, which becomes an increasingly important factor with increasing flow rates. fwiw Im not sure if it would be appropriate to apply Bernouilli’s equation to a compressible fluid, especially one that may well be in turbulent flow. The Moody Chart Im not familiar with, but Id agree that some simplifying assumptions would probably help/be needed here. The bottom line is: Notfrommensa: if this question is worth thousands of bucks a month, then dude, I think you need to be talking to a qualified engineer, not a bunch of strangers on the internet!
If it’s worth umpity thousand bucks a month, then yeah, I’d check with a qualified expert.
However, if there is a substantial pressure drop along the pipe, then that pressure drop is flow driven. More flow, greater pressure drop; less flow, smaller pressure drop. Which means that you ought to see your 57psi fluctuate as your cfm fluctuates.
I assume you have peak and off periods of gas usage, where the difference in flow through your line is a factor of two or more? Check your pressure gauge at these two times. If your pressure stays constant at 57psi, that implies that the pressure drop through the line is negligible.
[sub][I *am* an engineer, but I’m not *your* engineer, nor do I specialize in piping flow problems, and it is unlikely that I’m registered in your state. Blah blah blah.][/sub]
As has already been said, for a problem worth thousands of dollars per month, this is not the place to sort it. Another thing to consider is failure:
E.g.
Even if the normal operating pressure is well within tolerance, what is the expected pressure if branch y or z becomes blocked? Is that within tolerance?
Is the tee you are talking about entirely passive, or is there a valve to provide a degree of flow control? If it is entirely passive, can the whole system cope with, for example, reverse flow in the event of supply failure. If there is a valve, what is the shutdown procedure?
If you are designing this system then you need to think about these factors and far more. If you are modifying an existing system, then they should already have been considered, but you should have existing designs and calculations to consult.
notfrommensa:
:smack: TEMPERATURE !
Gas temperature is one of the factors necessary to determine Standard Conditions.
( P1 x V1 ) / T1 = C, a constant, far a given gasseous ‘fluid,’ e.g. natural gas.
( P1 x V1 ) / T1 = C = ( P2 x V2 ) / T2
At the point the volume, V, is known, the meter flow rate, the temperature can be measured and the pressure determined from the above formulas.
Measure the temperature at Point x and knowing P2 and T2 you can determine V2.
Lastly this provides the data to calculate the “Standard Conditions PTV”
Well, he does say that “[the meter reading] needs to be corrected for Pressure and temperature,” so I imagine he’s aware of the importance of temperature. I was assuming (and perhaps this is wrong) that in the OP’s case, thetemperature of the two points is identical – they’re both in the same temperature-controlled building, for instance.
Not necessarily so.
Pressure drop in the pipes as well of length betwee points of interest can affect pressure and hence volume/temperatue.
A thermometer well or a sensor should have been installed in the mail line at point ‘x’ but a thermometer stuck on the outside of the pipe with modeling clay would be fairly close.
Once T-x is known P-x can be calculated followed by converting to STP conditions.
If the fluid velocity is less than about Mach 0.3, the compressible fluid may be treated as incompressible for the purpose of applying Bernoulli’s equation.
You didn’t mention if the pressure indicated was static, dynamic or stagnation pressure?