WATER DISTRIBUTION 2
This course is a detailed view of piping projects. It includes a spreadsheet that I recommend that you download now and open in a window adjacent to this window.
PARAMETERS
Water Distribution Piping consists of construction of pipes of various sizes with fittings (bends, etc.), and meters, etc. We need to find out what flows we will be dealing with, and then how those flows will perform in our proposed system. This spreadsheet employs the Hazen-Williams formula to determine, from pipe characteristics and flow of water, what headloss there will be.
The flows are found many ways, with local rules sometimes prevailing (the entity maintaining the system may have rules, and the local health department will have others). The effect of this flow on our system includes lengths of pipe and bends, etc..
This course focuses on small additions to existing systems. To design such an addition to the system, one usually will be able to obtain from the entity maintaining the system the pressure and flow from fire hydrants close to your connection. This will be your basis for the addition pressures. Hopefully you will be given a flow greater than that you need with a residual pressure at a hydrant that will provide you satisfactory results with reasonably sized piping.
We first will be looking at a moderately sized project that includes 100 acres of homes, 3 acres of commercial area, and a proposed hotel. Below is a sketch of the draft proposed water lines:
The sketch is not exhaustive, not all of the bends, etc. are shown, but you get the idea. Hopefully you will have found the data for the existing waterline, and you determine that the existing main can provide 1500 GPM with a residual of 50 PSI at the beginning of the project. Before we begin to fill in data, there are several concepts I would like to discuss: friction, momentum and headloss; and equivalents.
FRICTION LOSSES
As water moves through pipes, friction slows it down, this is called headloss and it can be measured in feet or pounds per square inch (psi). As water moves through bends, momentum is lost, resulting in a loss of energy. It also can be measured in feet or psi.
The energy lost in straight pipe is inversely related to the C factor. So, if DIP has a C factor of 110, water running through it looses more momentum then in a smoother pipe such as a PVC pipe with a factor of 140. The loss in pressure is thus more than the loss in pressure in PVC.
EQUIVALENT LENGTH OF PIPE
The loss of energy around a bend is represented in calculations as an equivalent length of pipe. In other words, if water is moving at 800 gpm through an 8-inch pipe, when it rounds a 90° bend, it will loose a certain amount of energy. The amount of energy lost in that bend in this case is the same amount of energy lost in 35 feet of straight PVC pipe: the “equivalent pipe length”. So, we can add up the bends’ equivalent pipe lengths, add in the straight pipe, and then we can solve one equation for headloss, rather than many for each of the losses.
I have included a calculator to solve for this which we will cover under CALCULATORS as shown below.
EQUIVALENT PIPES
In parallel pipes, or pipes from two different sources, it is not immediately known how much flow is coming from which side, even though you know what your design flow is. For instance, if you want to model a 1000 gpm fire flow, and your project will be tied to two existing pipelines, one in Pine Street, and one in Maple Street, you would not know initially how much of that 1000 gpm of water was coming from Pine Street, and how much is coming from Maple Street. If for example the pipe down to Pine Street was going to be an 8″ PVC line, and you wanted the pipe up to Maple Street to be a 12″ DIP pipe, how would you know how much water was coming from each?
Your first step is to convert the two pipelines to the same equivalent size and type of pipe. Lets say that there are two valves and a 90° bend in the line to Pine and it is 146 feet to the connection. Thus we have a total of 200 feet (which includes 54 equivalent feet of fittings and 146 feet of pipe) of 8″ PVC to Pine Street. For now lets use that as is, and convert the 100 feet (including equivalent lengths of fittings) of 12″ pipe to Maple. The friction loss in 100 feet of 12″ pipe at any given flow is much less than if you were using an 8″ pipe. In fact, the friction in 100 feet of 12″ pipe is the same as in 14 feet of 8″. So, to start, for every 100 feet of 12″, we would say that we could replace it with 14 feet of 8″ and get the same results.
In our case we have a double whammy. We have a different type of pipe: DIP, not PVC, so we are only part way there. We know that 14 feet of 8″ would be the equivalent, if they were both PVC, but the 12″ line is going to be DIP. How much 8″ PVC would we need? As the relationship is inverse, we would need 26 feet of 8″ PVC to mimic the original 100 feet of 12″ DIP pipe to Maple. This is what is called an equivalent pipe.
Again, I have included a calculator to solve for this which we will cover as shown below.
EQUIVALENT C
The two pipes to Pine and Maple have proportionate flows related to their equivalent C factor. To get an equivalent C in this spreadsheet, I have used an equivalent pipe of 6″ PVC. We need to reduce all runs to 6″ PVC to use the calculator. We have 200 feet of 8″ PVC going to Pine. We need to reduce that to an equivalent length of 6″. In this case that would be 49 feet of 6″ PVC. The equivalent C for this 49 feet of pipe is 335
When we convert the 100 feet of 12″ DIP (which we already have converted to 26 feet of 8″ PVC) going to Maple, that would be 6 feet of 6″ PVC. The 6 feet has an equivalent C of 1005.
Now we can determine the amount of the flow from each:
Flow1 = C2/C1+C2 times the Total Flow
In this case, the flow from Pine will be 250 gpm through the 12″ DIP and 750 gpm through the 8″ PVC from Maple.
Let’s go to the tab “HEADLOSS” to see how we can make this easier.
CALCULATORS
Click on the tab “HEADLOSS” and you should see the screen shown below. There are four calculators available here. This screen shows you what input and output to expect from each.
EQUIVALENT LENGTH OF PIPE CALCULATOR
In this first case, to determine my “total equivalent lengths”, including the fittings, I clicked on the “Fitting” button and found out my equivalent lengths of all the fittings for each pipe run.
The rough length of an 8″ 90 degree bend is 38 feet, each valve is 8 feet, for my total above of 54. I did the same for the 12″ DIP line – I just changed the “8” in cell P55 to “12” and read the rough lengths for the fittings in DIP.
EQUIVALENT PIPES CALCULATOR
If you click on the button “Equivalent Pipe Length Calculator, you will get the following:
In the case in hand, (200 feet of 8″ that needs to be converted to 6″) I entered the 8″ in cell A57 and 200 feet in cell A60, then 6 in cell C57. The result can be read in cell C60. If the pipe is of a different type, fill in the length in cell B68 or B71. This is a quick and easy way to solve all equivalent pipes.
EQUIVALENT C CALCULATOR
Now that we have everything down to 6″ PVC, we can solve for the “equivalent C”.
If you click on the “Equivalent C” button you will get the following screen:
I had entered the equivalent pipe length of 6.4 for the run to Maple and found an equivalent C of 1005. I entered that as C2. I had entered the 49.3 feet of 6″ previously and read an equivalent C of 335. When that was put in for C1, and the flow of 1000 was entered, I found out that three times as much water would flow from Maple as would from Pine.
HEADLOSS CALCULATOR
Now that you know that 750 gpm is coming through the 12′, you can go to the “Headloss Calculator” to see what the resulting friction loss would be. Click on the “Headloss Calculator” button and this screen will come up:
Enter the known values and the headloss will be calculated.
PROJECT SPREADSHEET – SINGLE SOURCE
We will now model the following:
This is a sample project that is connected to one source of water supply. If you click on the “Single Source” tab, you will get the following screen. This tab incorporates all of the calculators shown earlier in the course, you just need to enter the data. As discussed before, you need a flow, and the parameters of the piping system to solve for the resultant pressure. We begin with the C’s of the pipes to be used:
FLOWS
There are three parts to this spreadsheet relating to flows. Part one is flows expected from acreages that will be using this pipe. These flows vary depending on the density of housing. Part two (see cells below and to the right) are “Building Flows” as determined from any large building. The third is fire flows as proscribed by the mechanical engineer, by the governing agencies, or as reasonable to assume.
First fill in the acres of each zoning anticipated (see above). This will provide a daily flow. I have used a factor of 2.5 times the average flow as my design parameter. You may change cells I8 to I18 to represent a different factor if desired.
Part two is filling out the Fixture Units portion of the spreadsheet (to the right of the pipe data) If the architect/mechanical engineer has counted them you are way ahead of the game. If not, and you know what the system should require, clear the cells of amounts and include the flow in cell C44.
Part three is obtaining the fire flows needed from the mechanical engineer or governing agency and fill in cell C42.
Now that we have the flows, we need to know how these flows are going to afect our system.
In this spreadsheet you can enter the number of fittings and straight pipe as shown below.
I have separated the 12″ pipe and fittings in Pipe 1, and we will enter the second run of 6″ pipe afterwards in Pipe 2.
After you enter the fittings, meters, bends, etc., it is time to enter your first trial size and material. The spreadsheet checks to see if “pvc” (capitalization ignored) is in the cell for the pipe material (Type). If it is not, then the calculations are performed using cell I3’s value as C. To the right of the size and material are parameters of the pipe selected.
Below the flow summary (see picture above) is the Length: the total length including the equivalent lengths of the bends, etc. (shown as 1041), the flow expected (1493), and the parameters of the pipe. Below that is the headloss in PSI for Pipe 1.
The second pipe is solved for in the same way:
At the end, if you enter the known pressure at the flow expected (50 PSI) and the elevation difference, you will find a resultant pressure in the system, This should be greater than 20 PSI to allow the users (fire department and others) useable pressure. If a fire department pumper truck were to try to pump 2000 gpm (for example) through a system that had a residual pressure of a negative pressure at that volume, they could flatten the piping system or pull water backwards from other customers. Neither one would be very healthy.
Following is a copy of the graph used for converting from Fixture Units as used by architects (as well as mechanical engineers and building officials) to GPM.
To the right of the graph is another version off the Equivalent length calculator (cell K51). This is provided for easy access:
PROJECT SPREADSHEET – TWO SOURCES
Next we will solve the two source problem – click on tab “Two Sources”. The problem solved is shown as follows:
You may note that there are four segments of piping leading in from each source to the common point. If you have more segments, just reduce several segments into one as we demonstrated (convert each to lengths of a common type and add the lengths) and enter as one of the four pipes. Another option is to enter the first four, record the Total LF (cell I46) then the next four, etc. multiple times until you have entered all, then use the results as your pipes.
Below the Equivalent pipe is your Equivalent C figured for you.
Next do the same for your second source:
Next is the common pipe:
The next step is to determine your total flow expected:
After you have entered the data for the piping and the flows, the spreadsheet calculates the proportionate flow from each of the sources. Given the initial pressure and the elevation difference, the residual pressure is solved for. This is a lot easier than working out all the parts!
PARALLEL LINES
Parallel lines are treated just like Two Sources. The piping in one leg of the parallel lines is entered in Piping System Source 1 (cell A1), and the second leg is entered in Piping System Source 2 (cell A52). The common piping system may or may not be needed. The results are found in cell G201 just like the Two Sources results were, (“Q Source 1” is the flow in leg one, “Q Source 2” is the flow in leg 2).
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