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Thread: Gas Pipeline Max velocity

  1. #1 Gas Pipeline Max velocity 
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    Hi Id like to know how to work out what the maximum amount of gas i can get down a pipeline
    The gas is methane. Its composition is roughly ch4 @ 59.5%, co2 @40%, 02 @ trace, h2o @0.5%, Nox & Sox = Trace, HCL = Trace. Its biogas and temperature is around 34 C entering the pipework and 6 C before use.
    The Pipework is various diameters but it has a bottleneck minimum of 6" I can say there are no bends in the pipework but its overall length is 100 meters, it has 3 diameters reducing from source 0 to 40 meters is 400mm 40 to 80 meters is 300mm and the final leg is 200mm.
    The pressure at the start of the system is around 15mBar / 150mm water gauge.
    The pressure varies across the system with diameter
    There may be modifications the the pipeline coming involving two 90 degree bends.
    Thanks for your help in advance.


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    Forum Isotope Bunbury's Avatar
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    What is the pipe discharging into? You can't calculate the flow unless you know the pressure difference from inlet to outlet. (You could if the flow was choked, but at 15mbar it won't be choked unless you're discharging to a vacuum. If that's the case you need to do a Fanno calculation.) It seems odd that you would start with a larger diameter and progressively reduce the diameter, because the gas density is the lowest at the outlet.

    It is more usual to know the flow rate that you need to make your process work, and then to calculate the pipe size to handle that flow with an acceptable pressure drop. Why are you doing it bass-ackward?


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    Quote Originally Posted by Bunbury
    What is the pipe discharging into? You can't calculate the flow unless you know the pressure difference from inlet to outlet. (You could if the flow was choked, but at 15mbar it won't be choked unless you're discharging to a vacuum. If that's the case you need to do a Fanno calculation.) It seems odd that you would start with a larger diameter and progressively reduce the diameter, because the gas density is the lowest at the outlet.

    It is more usual to know the flow rate that you need to make your process work, and then to calculate the pipe size to handle that flow with an acceptable pressure drop. Why are you doing it bass-ackward?

    15mbar if I am not mistaken is 0.217556616 pounds per square inch. That is reduced gas pressure.

    The reason you start big and end small is friction. If you ran a small pipe all the way, it could not supply a volume of gas to a device or devices at the end of the line.

    The same is true of duct work, at low pressures. You reduce the trunk as you go along.

    Also if this line feeds one or more devices you want the flow to evenly distribute itself along the run. By creating step downs in pipe diameter, the flow and pressure is more equalized.

    There are going to be flow dynamics as well as just pressure dynamics involved. Especially if there are more then one tap along the main line.

    We did this work at a Federal building. It is stepped down as it should be. It has drip legs as per the bosses orders. The drip leg can catch liquid, that might condensate on a cold morning or during a cold night. And then later the drip leg will evaporate back into the line. Each unit gets one. As the gas whips around the tee it tends to drop out any liquid or liquid vapor that might be in the gas, into the drip leg that is just a nipple and end cap. Keeping it from entering into the unit and causing a shut down, or flame out.







    If you are just tapping the end, you should get whatever a six inch pipe will flow at 0.217556616 pounds per square inch. As long as your regulator is able to handle the volume of a six inch pipe maintaining a 0.217556616 psi head pressure, while flowing at that pressure.

    100 meters is not that long of a run. For that size pipe. Sounds like very large pipe actually to feed a six inch pipe. However at those pressures probably very well thought out, for maximum flow.

    At this job in the pictures to bleed the line, that started out at about four inches. It took almost ten minutes to get gas to the device I was bleeding the line from. From a 1" union I believe. It is always a good idea to put a union in a gas line. If only to bleed the system with. All you could smell was the skunk oil. They had to wonder if I was blowing up the building. Ha-ha. It stunk so bad.


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    William McCormick
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    William,
    Thanks for your reply,

    Ill try and explain a bit more about the problem I have -

    The system is set up to supply two gas engines running big alternators producing electricity. At the same time as electric production they heat water that is used to heat 2 anaerobic digesters fed with waste to around 35C which is its optimal for biogas production. The digesters are both large covered tanks with 5000m3 capacity and 10m Diameter. The pressure in the system is produced by the digestion process as the gas is released from the sludge mixture held within.
    The roofs of the digesters are floating type so they rise and fall to collect gas from the system.

    The engines are sized at 600kWe each and theyre about 34% electrically efficient. The biogas has a CV of 35.8mj/m3 so need to supply the whole system with

    600kW*2 =1200kW/34 = 1%
    *100
    3529kW = Total Energy Input Requirment
    converts to MJ (/3.2) = 11294MJ
    divided by cv of 1 m3 of biogas (35.8MJ) = 315m3 / hr

    I have a new boiler on the system using the gas aswell and when the boiler is running the engines are starved of gas. I know they are because their output goes down and bounces around.

    So I suppose what im trying to work out is what happens if I fluctuate the pressure at the top of the system or if I start to draw more than can be supplied down the pipeline.

    I assume there must be a maximum that can be drawn down a leg at a certain diameter and pressure. A bit like if you try and breath only through your mouth with a straw and you get out of breath because you cant get enough down the straw.

    Thanks again.
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    Quote Originally Posted by mathlover
    William,
    Thanks for your reply,

    Ill try and explain a bit more about the problem I have -

    The system is set up to supply two gas engines running big alternators producing electricity. At the same time as electric production they heat water that is used to heat 2 anaerobic digesters fed with waste to around 35C which is its optimal for biogas production. The digesters are both large covered tanks with 5000m3 capacity and 10m Diameter. The pressure in the system is produced by the digestion process as the gas is released from the sludge mixture held within.
    The roofs of the digesters are floating type so they rise and fall to collect gas from the system.

    The engines are sized at 600kWe each and theyre about 34% electrically efficient. The biogas has a CV of 35.8mj/m3 so need to supply the whole system with

    600kW*2 =1200kW/34 = 1%
    *100
    3529kW = Total Energy Input Requirment
    converts to MJ (/3.2) = 11294MJ
    divided by cv of 1 m3 of biogas (35.8MJ) = 315m3 / hr

    I have a new boiler on the system using the gas aswell and when the boiler is running the engines are starved of gas. I know they are because their output goes down and bounces around.

    So I suppose what im trying to work out is what happens if I fluctuate the pressure at the top of the system or if I start to draw more than can be supplied down the pipeline.

    I assume there must be a maximum that can be drawn down a leg at a certain diameter and pressure. A bit like if you try and breath only through your mouth with a straw and you get out of breath because you cant get enough down the straw.

    Thanks again.
    The fact that you are getting low pressure, would indicate that, it is probably a pressure problem.


    Disregard this below in red, it was based on minutes of flow not hours of flow.

    I will be honest you are actually supposed to have a 10 inch pipe for that amount of cfm you want to push over that distance.
    Even though today you do not see that kind of stuff built anymore. Almost everything is undersized today.
    When I reread your post I saw that you have about 220 feet of six inch pipe. That is a long run. I thought you had about 100 feet of six inch pipe. The velocity in a ten inch pipe 220 feet long will be up around 7,000 feet per minute.

    You want to pump 3,390 cubic feet a minute. You have 6.022 inches of static pressure to do it.
    That is where the losses in flow will occur in that six inch pipe. Could you just run a second 6 inch pipe from the larger pipe, with a 300mm x 200mm x 200mm Tee. If you do have your 6 inches of static pressure at the next larger sized pipe.

    If you only had the trouble after adding on the burner which is located at the source, I am assuming. Could you create a weight for your tanks to increase the pressure, when the boiler is running, to bring it back to 6 inches of static pressure? After you get the system running just sandbag the tanks?



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    William McCormick
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    When the boiler is running you are starving the gas engines of fuel, so it looks like your system is not limited by the piping, it is limited by the rate of production of fuel gas. On the face of it it seems to me you need to perform a detailed mass and energy balance before you start tinkering with pipe sizes.
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    Quote Originally Posted by Bunbury
    When the boiler is running you are starving the gas engines of fuel, so it looks like your system is not limited by the piping, it is limited by the rate of production of fuel gas. On the face of it it seems to me you need to perform a detailed mass and energy balance before you start tinkering with pipe sizes.

    If it is a close call, it could very well be a slight drop in pressure.

    Disregard information in red, it was based on cubic feet a minute not hours.

    It is definitely a close call. In actuality the last length of pipe is supposed to be 10" pipe. But most common calculations are based on a 35 percent over engineering value. That is why it worked originally.
    A lot of times the devices that are listed as consuming a certain quantity of fuel, are overrated as well. Or are rated at 100-135 percent actual running rpm's.

    Unfortunately many engineers know this, and under engineer everything. Today you are almost never praised for an awesome standard job, 35 percent over what is necessary. You are actually and usually praised for getting it done, quickly and cheaply and still getting a check.

    If his tanks are floating, he probably has enough gas, however if you tap the tank near the tank. You will cause a drop in pressure.
    If it was working before and this slight extra drain of the tank is screwing it up. There must be a pressure drop at the tank. I would think you could offset that with more weight on the tanks.

    You could even build a water tank on top of the tank.

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    William McCormick
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    Forum Isotope Bunbury's Avatar
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    Unfortunately many engineers know this, and under engineer everything.
    Engineers sit down with the customer and define what the customer really wants. They do not intentionally under engineer anything.

    Today you are almost never praised for an awesome standard job, 35 percent over what is necessary. You are actually and usually praised for getting it done, quickly and cheaply and still getting a check.
    If you built a plant that performed 35% over what is required you would never work for that client again, because you have wasted their money.

    From looking at mathlover's posts it looks as if an engineer has not been involved, and the plant is unlikely to perform properly. It may not be too late to correct the problem, but a hit and miss approach is not the way to solve it. For example, if you increase the weight on the floating roof tank you increase the pressure on the digester, do you know how increased pressure affects the reactions in the digester? Will this increase or decrease biogas production? This is a biochemical engineering problem, not a plumbing problem. The system has to be engineered first before you start trying this and trying that. You can waste an awful lot of time and money in field trial and error.
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    Quote Originally Posted by Bunbury
    Unfortunately many engineers know this, and under engineer everything.
    Engineers sit down with the customer and define what the customer really wants. They do not intentionally under engineer anything.

    Today you are almost never praised for an awesome standard job, 35 percent over what is necessary. You are actually and usually praised for getting it done, quickly and cheaply and still getting a check.
    If you built a plant that performed 35% over what is required you would never work for that client again, because you have wasted their money.

    From looking at mathlover's posts it looks as if an engineer has not been involved, and the plant is unlikely to perform properly. It may not be too late to correct the problem, but a hit and miss approach is not the way to solve it. For example, if you increase the weight on the floating roof tank you increase the pressure on the digester, do you know how increased pressure affects the reactions in the digester? Will this increase or decrease biogas production? This is a biochemical engineering problem, not a plumbing problem. The system has to be engineered first before you start trying this and trying that. You can waste an awful lot of time and money in field trial and error.
    That is not true about the digester, having the pressure increased upon it. Unless the opening in the digester to the main trunk is too small to supply the main trunk.

    Something that I just noticed. I used my memory from reading the post, Ha-ha. I was calculating for a flow based on cubic meters a minute. I did not catch the hours of flow till now.

    So that would change the 3,390 a minute to 56.5 cubic feet a minute. I only calculate in cubic feet a minute. So I naturally think in feet per minute.

    That would be a different story, he will get a .02 static pressure drop, for each 100 feet of six inch pipe. For a total of 0.044 inches of water static pressure drop. In the length of pipe. That is probably not his problem. And that is based on air flow, not gas flow.

    I would say the problem is back at the digester.

    I also miscalculated the higher flow rate that I mistook for minutes instead of hours. I was looking at it based on a 100 foot run of ten inch pipe, with almost no back pressure just flow.

    Having an engineer come is great.

    But if he is really good, he is going to have to learn your system or scrap it.

    Either way, it is better to understand it yourself. So you can quickly explain it to him, with relevance. He is going to come up with all these questions and ideas himself.
    I hired engineers, myself. I would not even think of not having a second opinion from someone that goes around examining that kind of apparatus all day.

    However my engineer was very honest, and really just offered up what he felt confident with. Everything else he left up to my judgment. As long as it seemed like it was adequate or more then adequate.



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    William McCormick
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    Quote Originally Posted by Bunbury
    Unfortunately many engineers know this, and under engineer everything.
    Engineers sit down with the customer and define what the customer really wants. They do not intentionally under engineer anything.

    Today you are almost never praised for an awesome standard job, 35 percent over what is necessary. You are actually and usually praised for getting it done, quickly and cheaply and still getting a check.
    If you built a plant that performed 35% over what is required you would never work for that client again, because you have wasted their money.
    Engineers intentionally under engineer things, based on the customers wants.

    You will in fact work for the fellow again if you over engineer it by at least 35 percent. In fact if you do not allow for the 35 percent standard engineering leeway. You will find that your customer is out of business in a few years. Or who the customer sold the facility to is out of business.

    Look at the Empire State building. Hit by a plane, on fire, sways less then two inches total, in a 110 mile an hour wind. That was engineered at 35 percent over what it was designed to do. But still not considered earth quake proof.

    When you look at what happened to the earth quake proof buildings in California, you realize they did not over engineer that stuff by 35 percent. And some of the natural oversights, age, and building material variables played into their collapse.

    This fellow with the breeder, is a typical tale. He wants to add on another device. But cannot because there is no leeway built into the system. I have seen mega disasters because of stuff being precisely engineered with no leeway. The whole site becomes worthless, because it was done so cheap and so close to just working. That you cannot change anything. Even an oversight by the builder or engineer. So they often screw themselves.


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    William McCormick
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  12. #11  
    Forum Isotope Bunbury's Avatar
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    Quote Originally Posted by William McCormick
    Engineers intentionally under engineer things, based on the customers wants.
    An engineer who did that would have his engineering license revoked. I'm done with this discussion.
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    Quote Originally Posted by Bunbury
    Quote Originally Posted by William McCormick
    Engineers intentionally under engineer things, based on the customers wants.
    An engineer who did that would have his engineering license revoked. I'm done with this discussion.
    Yea, right. Look around at buildings today, and cranes and hoisting equipment. There is some mighty poor engineering going on.

    Do I think that they sit there in their own mind going lets see what we can cheapen up and make a little illegal, to keep Mr. Shallow pockets happy? NO!

    But they are sent back to the drawing board, turned down on what would be legitimate. So many times that they start racking their brains to try and do the impossible. So much so that after a whole month of overtime nights of working numbers they make a mistake and say "Wow that is more then enough"!

    Meanwhile four months later after the job is going up, someone catches it, the job should be torn down. But now it is too late. So they submit some papers, the town does not want to see the whole building come down either. They are supposed to start collecting taxes on it. So building goes up wrong.

    Today a lot of inspectors do not really know this cement construction, because they cannot. Cement is not a structural engineering material. By definition. It is an aggregate. So now they are making decisions on a structure that should never have been allowed. How are they going to condemn it? It could never have gotten a permit.

    That is where the engineers are. I know. They are between a rock and a hard place. I hope I did not make a pun. Ha-ha.

    Take a look at this site. The city should ask for Federal Aid to tear all these types of buildings down, and build some real American buildings.




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    William McCormick
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