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How To Measure Water Head And Flow In Hydropower Projects

November 18, 2020

Measuring Head and Flow

HEAD and FLOW are the two most important facts you need to know about your site. You simply cannot move forward without these measurements. Your Head and Flow will determine everything about your hydro system–pipeline size, turbine type, rotational speed, generator size – everything. Even rough cost estimates will be impossible until you’ve measured Head and Flow. Also keep in mind that accuracy is important. Unless your measurements are accurate, you could end up with a hydro system designed to the wrong specs, producing less power at a higher cost-per-watt than would otherwise be possible.

Measuring Head

HEAD is pressure, created by the difference in elevation between the intake of your pipeline, and your water turbine. Head can be measured as vertical distance (feet or meters) or as pressure (pounds per square inch, newtons per square meter, etc.). Regardless of the size of your stream, higher HEAD will produce greater pressure – and therefore power – at the turbine.

The following conversions may be helpful:

1 vertical foot = 0.433 pounds per square inch (psi) pressure

1 psi = 2.31 vertical feet

Accuracy is critical when measuring HEAD. It not only affects power, but also determines the type of turbine to use (such as a Francis or Pelton design), as well as the hydrodynamic design of the turbine buckets or blades. An altimeter can be useful in estimating Head for preliminary site evaluation, but should not be used for the final measurement. It is quite common for low-cost barometric altimeters to reflect errors of 150 feet or more, even when calibrated. GPS altimeters are often even less accurate.

There are two accurate methods for measuring HEAD: direct distance measurement, and water pressure.

Direct Distance Measurement

You can use a surveyor’s transit, a contractor's level on a tripod, or a level taped to a straight board to measure head. You will also require a pole with graduated measurements. (A measuring tape affixed to a 20' section of PVC pipe works well.) Direct measurement requires an assistant. Make a series of vertical measurements using the transit level and the vertical measuring pole. Make sure each transit setup is exactly level, and ensure the measuring pole is vertical. Keep detailed notes at each step, and then add up the series of measurements (A,B,C,D,etc.) to find total HEAD.

Water Pressure Measurement If the distance is short enough, you can use one or more garden hoses to measure Head. This method relies on the constant that each vertical foot of HEAD creates 0.433 psi of water pressure. (10 vertical feet would create 4.33 psi.) By measuring the pressure in the hose, you can calculate the elevation change of your system. Run the hose (or hoses) from your proposed intake site to your proposed turbine location. If you attach multiple hoses together, ensure each connection is tight and leak-free. Attach an accurate pressure meter to the bottom end of the hose and completely fill the hose with water. Make sure there are no high spots in the hose that could trap air. If necessary, you can measure total HEAD over longer distances by moving the hose(s) and taking multiple readings. Keep in mind, however, that there is less than a half-psi difference for every vertical foot. Except for very steep hillsides, even a hundred foot hose may drop only a few vertical feet. The chance for error significantly increases with a series of low-Head readings. Use the longest possible hose, along with a highly accurate pressure meter, to measure HEAD.The pressure meter must be graduated so that measurements are taken in the middle of the pressure gauge's range. Don't use a 0 - 800 PSI gauge to measure 5 -15 PSI pressure. Select instead a 0 - 30 PSI gauge.

Gross Head vs. Net Head

By recording these actual measurements, you have determined Gross Head. As described later in Computing Net Head, however, the effective Head at the nozzle is actually lower when water begins to flow, due to pipeline (penstock) friction. A properly designed pipeline will yield a Net Head of about 85%-90% of the Gross Head you measured.


Measuring Flow

Stream levels change through the seasons, so it is important to measure FLOW at various times of the year. If this is not possible, attempt to determine various annual flows by discussing the stream with a neighbor, or finding government geological survey flow data for your stream or a nearby larger stream. Also keep in mind that fish, birds, plants and other living things rely on your stream for survival.

Especially during low water seasons, avoid using all the water for your hydro system.

FLOW is typically expressed as some volume of water per second or minute. Common examples are gallons or liters per second (or minute), and cubic feet or cubic meters per second (or minute): Each can be easily converted to another, as follows:

1 cubic foot = 7.481 gallons

1 cubic meter = 35.31 cubic feet

1 cubic meter = 1,000 liters

There are three popular methods for measuring FLOW: using a Container, Float, or Weir. Each will be described in detail below. Once again, accuracy is important to ensure correct system design and optimum power generation.

Method 1: Measuring Time to Fill Container The Container Fill method works only for very small systems. Build a temporary dam that forces all the water to flow through a single outlet pipe, Using a bucket or larger container of a known volume0.625 gps / 7.481 = 0.0835 cubic feet per second (cfs).

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Method 2: Measuring with a Float

The Float method is useful for large streams if you can locate a section about 10 feet long where the stream is fairly consistent in width and depth.

STEP 1: Measure the average depth of the stream. Select a board able to span the width of the stream and mark it at one-foot intervals. Lay the board across the stream, and measure the stream depth at each one-foot interval. To compute the average depth, add all of your measurements together and divide by the number of measurements you made.

STEP 2: Compute the area of the cross section you just measured. Multiply the average depth you just computed by the width of the stream. For example, a 6-foot wide stream with an average depth of 1.5 feet would yield a cross section area of 9 square feet.

STEP 3: Measure the Speed. A good way to measure speed is to mark off about a 10-foot length of the stream that includes the point where you measured the cross section. Remember, you only want to know the speed of the water where measured the cross section, so the shorter the length of stream you measure, the better. Using a weighted float that can be clearly seen (an orange works well), place it in the stream well upstream of your measurement area, and then use a stopwatch to time how long it takes to cover thelength of your measurement section (e.g. 10 feet). The stream speed probably varies across its width, so record the times for various locations and average them. With these time and distance measurements, you can now compute the water speed.

For example, let’s assume it took 5 seconds for your float to travel 10 feet:

10 feet / 5 seconds = 2 feet per second, or

2 feet per second x 60 = 120 feet per minute

You can then compute FLOW by multiplying the feet traveled by the cross section area. Using our cross section area and speed examples:

120 feet per minute x 9 square feet = 1,080 cubic feet per minute (cfm) FLOW

STEP 4: Correct for Friction. Because the stream bed creates friction against the moving water, the bottom of the stream tends to move a little slower than the top.This means actual flow is a little less than what we computed. By multiplying our result by 0.83, we get a closer approximation of actual flow:

1,080 CFM x 0.83 = 896.4 cfm (cubic feet per minute), or 896.4 CFM / 60 = 14.94 cfs (cubic feet per second)

Method 3: Measuring with a Weir.

A Weir is perhaps the most accurate way to measure small and medium sized streams. All the water is directed through an area that is exactly rectangular, making it very easy to measure the height and width of the water to compute FLOW. A Weir is a temporary dam with a rectangular slot, or Gate. The bottom of the Gate should be exactly level, and the width of the Gate should allow all the water to pass through without spilling over the top of the dam. A narrower Gate will increase the depth of the water as it passes through, making it easier to measure.

It’s important to note that your depth measurement is not taken at the gate itself because the water depth distorts as it moves through the gate. Instead, insert a stake well upstream of the Weir gate and make the top of the stake exactly level with the bottom of the Wier gate.Measure the depth of the water from the top of the stake.