Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
(OP)
Did a search and I like the one quote "there are as many rules of thumb as there are engineers".
I'm looking at a 10" gathering system (whole thing is 10") and trying to determine max capacity. It was suggested to me to use a figure of 0.5 #/100' as the design basis for the pipe to determine the maximum flow through that pipe. Works out to just over 26#/mile. Seems a bit high to me but I'm no expert.
What are your thoughts on this "rule of thumb"? I noticed (BigInch, I believe) suggested 15#/mile for 12" and smaller, and 5#/mile for 20" and larger. Sound good?
Thanks for your help!
-Matt
I'm looking at a 10" gathering system (whole thing is 10") and trying to determine max capacity. It was suggested to me to use a figure of 0.5 #/100' as the design basis for the pipe to determine the maximum flow through that pipe. Works out to just over 26#/mile. Seems a bit high to me but I'm no expert.
What are your thoughts on this "rule of thumb"? I noticed (BigInch, I believe) suggested 15#/mile for 12" and smaller, and 5#/mile for 20" and larger. Sound good?
Thanks for your help!
-Matt





RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
To answer your question, I believe people can communicate just fine when they are polite. Saying less is always better if you're emotional or short on patience, in my opinion.
-m
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
1) You might know what # meant, but please use units and not abbreviations or your shorthand. There are people from all over the world here and frankly # could have been anything.
2) capacity of gas in a pipeline is wholly dependent on pressure.
3) "not very wet" is extraordinarily vague. Basically you're in two or three phase flow and then all bets are off in terms of guess work. An undulating line with liquid will have a completely different capacity than a line which is flat or runs downhill and has a good gas flow to push the liquid along. The amount of liquid normally increases as water condenses as it cold along the line, sometimes matched by extra gas evolving from the HC fraction a the pressure decreases. Any one coming up with a guess for this type of flow without knowing the pressure and the liquid fraction in actual terms at the pressure and temperature of your system could be factors out.
4) Having a "pop" at contributors is frowned on in this site if you want people to respond in a technical manner.
5) So, info required – inlet and outlet pressure, liquid fraction at the outlet point (actual not stock tank, though CGR is better than nothing), length and profile of pipeline will be a good start point.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
Your statement that your gas has nothing heavier than nC11 bothers me a lot. My GPSA Engineering Field Data book only goes to nC10 which has a boiling point of 142°F. nC11 would be a liquid in any conceivable gas-flow scenario. The gas chromatagraphs that I've sent samples to over the years have had to have a special column added to get C6 (and sometimes to get C5), C11 would have to have pretty special handling.
Companies often have velocity and/or pressure drop targets/limits. I often see a limit of 100 ft/sec for gas flows. This number is pretty arbitrary, but very common. I also see numbers like 15 psi/mile for raw gas gathering and 5 psi/mile for processed gas (this "rule" often gets stated in terms of pipe size, but that is kind of a function of the pipe sizes common gathering vs. transmission, I tend to size 20-inch pipe in gas gathering at 15 psi/mile). The main reason for the difference is that the uncertainties in raw-gas gathering are significantly greater than with processed gas and our ability to deal with subtle changes is pretty limited so we tend to bull our way through with raw gas.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
Thank you for your kind response.
1) I've never seen "#" used to represent anything other than "pounds per square inch", so this is news to me. Thank you. (I am curious what other unit it could have represented?)
2) Correct. I assumed this is something that is done often but perhaps not. I have to evaluate the capacity of a gathering system. I know the delivery pressure, 1200 psig. The "actual" source pressure is somewhat irrelevant (would you agree?) when estimating maximum capacity of the system. I was given a rule of thumb to assume a 0.5 psi/hundred feet pressure drop, so knowing the length of the pipeline would give me an estimated design upstream pressure. To me, in my limited experience, 0.5 psi/hundred feet seemed high for a gas service, so I was asking for feedback as to what others thought. I searched through the forums and found some information but nothing from a similar situation. So I posted my question. [As an aside, I confirmed with my co-worker that the figure should have been 5 psig/mile, not the aforementioned figure. I need to confirm where he got that, however.)
3) What I meant by "not very wet" was that nothing would fall out naturally and that it was a gas flow. Under compression, based on a gas analysis, we do get some liquids falling out which would be collected in your standard 400 bbl oil tanks. To me, a "dry" gas would not drop liquids under "normal" compression (anything under 1440 MAOP). But under normal circumstances you should not have two-phase flow in the pipe.
4) To be honest, it looked like he was having a "pop" at me, no? Regardless, he asked a simple question which I answered. Part of the learning experience is finding out what information others expect when posting a question, so taking a moment to politely point someone in the right direction is always appropriate, and professional. You did that, and I am grateful!
5) As I mentioned, the inlet pressure is estimated based on the assumption of some pressure drop "X" through the pipe, given the required delivery pressure. Was that assumption of pressure drop "OK"? I do know now that the producer is required to have a certain pressure at the wellhead/custody transfer point, which is 100 psig. That point by itself might make this whole discussion moot?
Follow up questions
Q1) What is meant by "profile of pipeline"? For this example let's assume there's 80 miles of buried 10" schedule 40 pipe. There are various branches from various wells along the way, so it is not a straight shot from a source to the delivery point.
Q2) What is CGR? I googled "pipeline natural gas cgr" and got "California Gas Report", so I'm not sure.
Thank you again for your helpful response!
-m
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
An extended analysis was provided which included components to C11. I quickly simulated a few miles of straight pipeline with that composition and indeed, you are correct, there are liquids present. Thank you for pointing that out.
Earlier today I was given the information that the minimum pressure required is 100 psig at the custody transfer point at the wellhead, so I think maybe my original question was unnecessary with this new information because I know the source pressure (100 psig), the required discharge pressure (1000 psig), the size and length of the pipelines. What I was trying to figure out was what the max capacity of the gathering system. If they want to expand, add new wells, could the system handle it? Perhaps they'll have to add compression but for now I'll leave that off the table.
I may be going about this the wrong way, of course, so advice is appreciated.
Thinking out loud, how does one evaluate a gathering system? You would want to know the capacity of the system, with the constraints of the pressure rating to which it was built. I am of the understanding that there are contracts and the gas changes hands a few times before it gets to the end user. So in order to determine the capability of a gas gathering system, one needs to know the composition of the gas, the minimum supply pressure from the wellhead, and the delivery pressure. After that the pipe size could be calculated, or the capacity of the pipe could be determined given the size and length.
Am I thinking about this correctly?
Thanks again for you all's help.
-m
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
Hmmm, this is getting more complex
1) I've seen it used for number, class rating (#600) and of course it is now used in Twitter.
2) Perhaps we have a different notion of what a "gathering" system is. To me it's the upstream system used to gather fluids from well heads and transport them to some sort of processing plant. As such the presusre depends almost wholly on what pressure the wells can give out. I've seen systems run at 5 to 10 bar and systems designed for 150 bar. The amount of gas you'll get in both of those will vary a lot(!) as will the pressure drop. I normally like to base a rough calcualtion on about 15 to 20 m/sec actual velocity at the outlet point / pressure. From there you can get the standard flowrate based on pressure and ID of the pipe. You can get much higher veleocities, but it doesn't get you much more flow.
3) Ok - As noted above. my definition of gathering system is a bit different from yours. I feel as if you connect directly to a wellhead, you will get liquids, condensed water at the very least. you seem to be somewhere in the middle...
4) No more comment required
5) Err yes. If you have an inlet of 100 psig and an outlet of 1000 or more you will need compression
CGR stands for Condensate to Gas Ratio, normally bbls/MMscf. This is an indication of how much liquid is anticiapted out of a wellhead / gathering system / field
Profile is the vertical profile of the pipeline, i.e. elevation versus distance or chainage. You need it for liquid and two phase lines, not so much for single phase gas.
I don't think you're far out to zdas04 with your description of how to evaluate a gatheirng system.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Gathering system evaluation - Pressure Drop and Velocity Rules of Thumb
I suggest that a well thought out, well labelled, dimensioned hand sketch could get everyone on the same page.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.