Water Distribution

At the conclusion of this course, you will:

  • Be familiar with the basic information you need to build a water distribution system.
  • Be able to communicate intelligently with pump suppliers in order to get the best pump for your system.
  • Be able to decide whether to replace pumps, or build a parallel line if the existing one does not provide the water you need.

The water supply is in place; now you need to get that water moving! In the old days, artesian wells and springs made life simple. A pipe was run from the source to the kitchen, and voila, water! The water was left running year round to insure the pipe did not freeze, and the wife had clean water whenever she wanted it. That was yesterday; today it is slightly more complicated. The water is usually well below the water spigot, and we worry about the things that might have gotten into that water, or might get into it before it reaches the tap.

The first thing needed for water distribution is a pump to add energy to the water. The second thing usually needed is a storage facility. This could be a gravity water tank (on the surface or a tower) or a hydropneumatic tank.

The minimum treatment would be chlorine for well water, or chloramine for surface waters. Use of chloramines reduces the regulated cancer-causing compounds, such as trihalomethanes (THMs), which result from the combination of chlorine with organics found in surface water.

Lastly, you will need transmission and distribution lines.

Water, water everywhere, but not a drop to drink.

Once you have your water source, how are you going to get the water where you want it? The pump is the first item of our series. How much horsepower do you need and how do you go about it?

Lets use this for an example:

Pump System

The first task is to decide on the type of pump you need. If it is for a single family home, a small well with water a long way down, a jet pump may be in order. This pump consists of a centrifugal pump, which pumps water down the well through one pipe, and pushes more water up the second pipe in the well. The venturi effect is used to draw the water from the well. This pump is easily selected from those available given your well size and water depth.

Our example, however, will assume a larger well with water needs for a small community or subdivision of 100 homes. Our well is 6 inches in diameter with water at an elevation of 124 feet, at no flow. The pump discharge is 30 feet of 3 inch steel pipe. Our water storage is to be a hydropneumatic tank 500 feet from the well connected using a 6 inch PVC pipe.(See sketch)

Pump volume needed:The water need is calculated as follows: # homes x gpd x 10/1440
If we assume the water use of 250 gpd, our well needs to provide 174 gallons per minute.

Pump static head at rest:In this example, the water is at rest in the well at an elevation of 124 feet. The hydropneumatic tank is to be set at a water elevation of 144 feet at the low working pressure of 45 psi. Thus, the static head at low pressure would be 20 feet (144 minus 124) plus the pressure in the tank of 45 psi. 45 psi is converted to feet of head 45*144/62.43 or 104 feet. Thus the total is 104 plus 20 or 124 feet.

Hydropneumatic tank

Pump static head at flow:
Given the flow rate of 174 gpm, we need to determine what our drawdown elevation in the well to be. Assume, for this example, that it is determined by testing to be at elevation 114 feet. The hydropneumatic tank was set at a low water elevation of 144 feet at the low working pressure of 45 psi. The water tank needs to be sized at 25 times the flow from the well, thus it needs to be at least 4,350 gallons in size. This “25 times the flow from the well” is an arbitrary figure determined from experience.   If we use a typical horizontal 5,000 gallon tank and a high pressure of 65 psi, the high water in the tank will be approximately 145 feet. Thus the static head at high pressure would be 31 feet (145 minus 114) plus 65 psi. 65 psi is 65*144/62.43 or 150 feet. The total static head is 150 plus 31 or 181 feet.


Transmission piping friction:
The friction in the fittings can be determined from the following chart:


In our case we have a 3 inch 90º discharge, a 3 inch gate valve, an enlargement from 3 inch to 6 inch, some 6 inch bends, a 6 inch check valve, and a sudden enlargement from the 6 inch pipe into the tank. We can solve for the equivalent length of piping using the chart. In our example, the 3 inch 90º bend equates to 7 feet of pipe, the gate valve equates to 2 feet of pipe, and the enlargement equates to another 3 feet of pipe for a total of 12 feet to be added to the 30 feet of run, or 42 feet. The 6 inch equivalent lengths can be calculated the same way.

The friction in the piping can be determined from the following chart:

In our case we have 585 equivalent feet of PVC pipe (500 feet plus the fittings) plus 42 equivalent feet of 3 inch pipe in the well.

We first solve for the loss in an 8 inch steel pipe for the given lengths (see the nomograph). The nomograph shows 0.7 feet of headloss given the 585 feet of 8 inch steel pipe with a flow of 174 gpm. This is converted to 6 inch by multiplying by 4.06 (see green circle in the middle of the image) and then to PVC by multiplying by 0.71 (light blue on left). The result is 2 feet of headloss.

We next solve for the loss in the 42 feet of pipe. This is .05 feet. This is converted to 3 inch by multiplying by 120, or 6 feet of headloss.



Total Head at Flow
The total head seen by the pumps is the static head of 187 feet plus the friction loss of 2 feet in the six inch pipe and 6 feet in the 3 inch pipe for a total of 192 feet.

Pump Horsepower

System curve:The head pressure seen by the pump will vary considerably. As the pressure in the tank gets low, the switch will energize the pump motor. At that point, the total head will be the static head of 20 feet as previously calculated. As water begins to flow and the pressure increases in the hydro tank, the head will increase. At the top end of your desired flow, the total dynamic head will be 192 feet!

You can approximate the horsepower needed from a graph such as the following: (This chart had to be divided into two, one half below the other, so that it was readable. Please excuse this awkward display)


The chart shows that we need a 10 HP pump. This step is not necessary but it provides a good check.

The following is a sketch showing a pump curve (an available 1150 rpm pump) and a system curve plotted in dashed lines.

As the flow increases, the head increases. As the head increases, the flow provided by the pump decreases. Thus there is only one point where the curves match. The pump will stabilize at that point. The trick is to find a pump and set of impellers which produce the flow you need, at the head you need, using the least horsepower.

If you sketch your system curve on several curves, you can see which impeller will get the volume you need; you can add impeller bowls to increase your head if the impeller you want will not give adequate head. If you find an impeller that gives 180 gpm, provides 100 feet of head, and requires 5 horsepower, you can add a second bowl. This will provide the 180 gpm, 200 feet of head and will require 10 horsepower, just what we need! Eventually, given enough available pump impellers, you will be able to identify the best solution for your needs.

One item to check, however, is that, upon starting, the static head does not over rev the pump and exceed the horsepower prior to the flow catching up. In some cases, a gate valve may be needed to be partially closed to provide the initial head to prevent damage from cavitation.

Distribution System

Now that we have decided on our pump and have our supply connected to our hydropneumatic tank, we need to distribute the water to our customers.

Following are some guidelines I have created:

Table 1 above is a quick and easy way to calculate the needed PVC pipe equivalent lengths of some common distribution piping fittings. In our previous example, if we decided to install a turbine meter, we can add 84 feet to the equivalent length of 585 that we previously had. Then we could go back and check to see if that would require a different impeller or more horsepower.

Table 2 provides a quick way to solve for equivalent lengths of different size piping. In our example, if we were to add the turbine meter and the result was not satisfactory (we went off the curve for our impeller maybe), we could quickly check to see what would happen if we changed the 6 inch pipe to an 8 inch pipe to see if that could be a solution. The 8 inch pipe would have one fourth (100/406) the head requirement. The six inch equivalent length was 585 feet, the 8 inch would be only 100/405*585 or 144 feet, more than enough savings (585-144 or 441 feet) to accommodate the 84 feet needed for the meter!

In designing a distribution system, the following are guidelines which I us

Number
of lots
Size of
PVC pipe
2768 inch
1616
694
413
182
91 1/2
41


This does not include fire flows. Below is a way to go back and check for adequate systems to handle a fire flow:



For a very simple system where there is one source, and a direct line to the fire hydrant. The amount of head needed can be solved for using the formula or by using the nomograph provided earlier in this course. The above provides for a more complicated system;, for example, two wells and a distribution system between them. Again, the amount of head needed can be either solved for using the formula, or by using the nomograph provided earlier in this course.

In summary, the piping and elevations need to be preliminarily designed between the supply and the storage facility. Then the system can be analyzed. If the system is not optimum, it is easy to place equivalent piping in and re do the calculations to provide a cost effective solution. Upon design of the supply, the distribution system can be designed using the guidelines. Upon completion of the layout, the system needs to be checked for extraordinary flows such as fire flows

The piping and elevations need to be preliminarily designed between the supply and the storage facility. This can be done using the provided tables and nomographs. Then the system can be analyzed, the horsepower can be estimated, and a pump can be selected from those available. If the system is not optimum, it is easy to place equivalent piping in and re-do the calculations to provide a cost effective solution.

The distribution system can be designed using the guidelines provided. Upon completion of the layout, the system needs to be checked for extraordinary flows such as fire flows.