Gas Piping Sizing

[Dymalica]

I am little confused on how to size a gas pipe. I understand the length and BTU to get the size. But I don't get what chart to use. As I understand it, Low Pressure gas is <0.5psi and Medium is 0.5psi to 5 psi. If I want to run a medium pressure line from the meter and pressure regulator to the rooftop Gas-Electric A.C. units (350 ft) and then reduce the pressure to Low Pressure for the units at the units. I don't know what my inlet pressure on the line, might have to field verify. There are a couple tables is the California Plumbing Code 2007: 1. Inlet Pressure: Less than 2 psi, Pressure Drop: 0.3 in w.c., 2. Inlet Press: 2 psi, Press Drop: 1.0 psi, 3. Inlet pressure: 3psi, Press Drop: 2.0 psi. What exactly does inlet pressure and pressure drop mean. Pressure drop from after the meter to the unit? Which table should I use? Thank You for your help

 

[MintJulep]

First: Take some time to organize your thoughts and ask clear questions. There's a lot of stuff in your post that's irrelevant, and things that aren't even sentences.

Second: There's a key on the right side of your keyboard labeled "Enter" or "Return". Use it occasionally. It will make it easier for people to read your posts.

Now:

Any gas fired appliance needs a given flow rate of gas to deliver its rated output. In round numbers 1 cubic foot of gas contains 1000 BTU. If you have an appliance rated at 6000 BTU/hr you need to feed it 100 cfm. Any less and you get less than the rated output.

Also, for a burner to work correctly it needs a certain minimum pressure at its inlet. Any less and it won't work correctly because gas won't flow through the burner fast enough because there won't be enough pressure to overcome the burner's internal pressure drop.

Pressure drop is one of the things you asked about. Flow and pressure are inexorably linked to each other. If there is no pressure difference between two points then there will not be any flow.

Imagine a drinking straw in a glass of chocolate milk. With the straw just sitting in the glass there is no milk moving through the straw because the pressure on the milk's surface inside the straw and outside the straw are equal.

But suck on the straw and milk starts to flow. Why? Because you've lowered the pressure inside the straw.

Pressure difference = flow.

So one end of the straw is at a high pressure (the end in the glass), and the other end is at a low pressure (the end in your mouth). Where do you think the pressure changes from high to low? Not at any single location. Pressure changes continuously through the length of the straw.

If you are really thirsty and want to drink faster you suck harder. This makes more milk flow. The pressure outside the straw is the same as before, but the pressure at the mouth end is lower, so there is a greater pressure difference.

Larger pressure difference = more flow.

For any given straw, and any given drink there is a direct relationship between how hard you suck and how much liquid flows through the straw.

It works the other way too. If you have a mouthful of water and blow it out through a straw you will find that the harder you blow the faster water flows.

This all happens because of friction between the liquid and the wall of the straw. It takes force to overcome the friction, and the faster water flows the more energy is required to overcome the frictional force. The more liquid you try to move through a pipe the more flow energy is lost to overcoming friction. That energy needs to come from the driving force - the pressure difference. As the liquid moves through the pipe the pressure is continually decreased due to frictional losses. In a pipe with flow the pressure downstream must always be lower than the pressure upstream.

That's pressure drop.

For a given flow a small pipe will have a larger pressure drop than a big pipe.

So imagine that your appliance needs 100 cfm at 1 in wc inlet pressure, and that your utility provides you 5 in wc at the exit of your meter. You need to select your pipe diameter so that you don't have more than 4 inches of pressure drop at 100 cfm. One of your charts will cover this.

 

[PEDARRIN2]

You will want to run two calculations.

The first will be betweeen the two pressure regulators. The second will be after your second pressure regulator to the units.

Tally up all BTUH/CFH loads on the equipment. I typically do a line diagram listing the pipe segments and loads on each piece of equipment.

Determine the linear feet of pipe to be installed. Go from your first regulator to your second regulator. The second system would be from your second regulator to your most distant piece of equipment. Add ~25% for valves and fittings. That will be your equivalent length.

Determine the pressure you will be using at the beginning. A lot of times you will have a range. I typically use either the 2 psi or 5 psi table for medium pressure. The pressure drop will tell you what pressure you will get at the end. Larger pressure drop will equate to smaller piping from increased friction. So if you are trying to minimize the cost, use a slightly higher pressure drop.

I typically never go above a 10% pressure drop. Past that you get into gray areas, although technically you can go up to ~40% pressure drop with not much problem.

After you know your equivalent lengths, go to the appropriate table and to the pipe length that first exceeds your length, i.e. if you have 150 feet equivalent length, go to the value of 160 or 175 or 200.

Go across the table until you reach the first load that exceeds your load. Look over/up on the table to determine the pipe length to use.

Continue this approach for the entire system, subtracting loads as you progress along the system. Stay in the same length column/row as you decrease your load and that will tell you the pipe sizes for your system.

This is a conservative approach to gas pipe sizing. The tables are naturally conservative and this approach adds a little more.

If you want other methods, check the ASPE Data Books or the ASHRAE sources.

 

[Dymalica]

Lets say I have a gas regulator that regulates at 2psi, how far can this pipe go? Is there a loss/pipe length for natural gas?

 

[PEDARRIN2]

The length will be dependent upon what pressure drop you can accept and the capacity of the piece of equipment you are serving.

The tables are based upon a total pressure loss over the entire system, not per a unit length. So if your regulator drops your pressure to 2 psi at the outlet and you want only a 10% pressure drop over the entire system, you look on the table for 2 psi and 10% pressure drop for the load and equivalent length of piping and this will determine what pipe diameter you would use.

 

[KiwiMace]

Try the Fuel Gas Code, not the plumbing code.

 

[PEDARRIN2]

I thought, in the past, in some jurisdictions, the gas sizing tables are in the plumbing code since a lot of the time, it is the plumber who does the gas piping installation.

The information is usually extracted from a gas code but put in the plumbing code because that might be all the plumber has on hand.

 

[KiwiMace]

I looked through a copy of the Ohio version of the IPC I had at hand, and there was naff all on gas. I generally use NFPA 54 for low pressure and a spreadsheet I have used in the past for anything service or higher pressure.

 

[PEDARRIN2]

The Ohio Plumbing Code doesn't have anything on gas pipe installation. I believe it used to prior to joining the International Codes.

The International Fuel Gas Code is where it is found in those codes.

I haven't used the Uniform Plumbing Code in awhile, but I think it still includes gas piping in an appendix.

NFPA 54 and the International Fuel Gas Code are similar. I tend to use the latter, although the most recent edition has removed some, in my opinion, very good tables. I also use a little slide rule that experience has shown gives the same results.

Since I have an inherent distrust of just taking numbers from tables at face value all the time, I also use some equations, from time to time, that come from the IFGC and also from Crane Technical Paper to back up the results I get from the tables.

 

[vzeos]

Dymalica:
Goto thread378-216048 and download the calculator I posted on 2 May 08 21:30. This program may prove helpful.

 

[DuctPipeMan]

Using the pipe sizing tables as instructed by the code books can lead to vastly oversized piping in some instances. I agree that if you use that procedure, it will work.

The code procedure does not seem to allow the deisgner to consider pressure losses for individual components in the systems. By using the same length column for ALL pieces in the system, the available pressure will not be used to its full advantage. If all i need is 3" wc at a unit, why do I oversize the piping so that with 7" wc inlet, there will be 6.3" at the unit.

I see that a lot when a majority of the load happens very close to the meter (ie a boiler room with kitchen on far end of the building) I have seen systems where a 2" pipe is sufficient, but the code calls for a 6" pipe. That is not being responsible.

If someone know of a reason for this I would like to know.

 

[PEDARRIN2]

If you size the pipe so you start with 7" and get down to 3" at your piece of equipment, what happens if you have to add load and you need 5". You now have too small a pipe.

Also, with such a large pressure drop compared to starting pressure, you go beyond the 40% pressure drop threshold that the tables and equations typically use for compressible gases. After that, you have to start using different equations and throw out the tables.

The codes do allow you to size differently. If you go to the appendix - at least in the International Code, I believe they show 3 different methods to sizing the pipe. Using the tables and using the same distance is the most conservative and if you have a large load drop near the source, you probably should use a different approach and get smaller piping.