First, we are going to introduce the formulas.
Relationship between flow rate (LPM), flow speed (m/s), and pipe diameter.
With the below formula, we can understand the relationship between flow rate, flow velocity, and pipe diameter.
Flow rate = Pipe cross-sectional area X Flow speed
Pipe cross-sectional area could be obtained by the following formula.
Pipe cross-sectional area ＝ ID² / 4 Ｘπ (ID stands for pipe inner diameter, π stands for Pi which is 3.14)
Speaking of flow velocity, flow velocity is the speed of fluid flow, which is the distance the fluid moves in a unit time duration.
Here are the applicable flow velocity ranges of 3 different flow meters. Each flow meter uses a different theory of measurement.
Variable area flow meters：0.05~3.5 m/s
Paddle wheel flow meters of other brands：0.3~10 m/s
LORRIC AxleSense paddle wheel flow meters：0.15~10 m/s
Ultrasonic flow meters：0.1~20 m/s
Below is an example to explain the formula to calculate the flow rate.
How much is the flow rate when the fluid flows in a 2” pipe within 1 m/s flow speed?
The inner diameter of a 2” pipe is 51mm. Flow speed is 1 m/s. The below formula shows the way to obtain flow rate with these 2 data points.
0.051² / 4Ｘ3.1415Ｘ1= 0.0020427604 m³/s (1m³＝1000L, LPM stands for liters per minute.）
＝ 0.0020427604Ｘ1000Ｘ60 L/min(LPM)
=122.565624 L/min(LPM)
Pay attention to the following matters
1. Consistent Flow: Flow rate should be consistent throughout the pipe as long as there are no leaks or forks. Therefore, the flow meter should be installed where there is a stable flow stream locations.
2. At any given flow rate, flow velocity is inversely proportional to the t cross sectional area of the pipe. Smaller pipes will lead to higher flow speeds; larger pipes, will lead to slower flow speeds. Therefore, we do not suggest you use a small flow meter on a large pipe. It will be a waste of energy and the flow speed will be too fast to measure the flow rate.
3. Even for the same pipe size, the inner diameter will be different between US and Japan standard pipe. DN is defined as the outer diameter of a standard pipe. ID could be estimated as the inner diameter of a standard pipe.
4. If necessary, make any metric or imperial conversions when calculating. For example, 1000ml= 1l；1mm= 0.001m.