Greetings from SoundWater Technologies! Today we discuss Straight Pipe Requirements and Accuracy.
Nearly all types and makes of flowmeters require some length of straight pipe upstream and downstream of the flowmeter itself. Why? Because the flowmeter doesn’t necessarily sense all parts of the fluid inside the pipe. For example, insertion flowmeters only detect the fluid immediately surrounding the probe head, i.e., they do not sense the fluid passing near the pipe wall (away from the probe head). In this case, if the fluid velocity was higher near the pipe wall and lower near the probe head, then the flowmeter would measure lower than the average flow rate; since it does not know about the fluid near the pipe wall.
Figure 1 below illustrates a few examples of the fluid flow immediately after typical piping situations: bends, contractions, expansions, and tee’s. Note that the fluid velocity may become irregular. All types of flowmeters are subject to flow profile (i.e., the shape of the flow as it passes by the flowmeter), although some types of meters are affected more than others. The degree that any flowmeter is affected by the flow profile has a LOT to do with how much of the fluid it “sees” (or senses).
Figure 1. This figure illustrates various types of irregular flow profiles. Not ideal for metering flow.
Flowmeter measurements are more accurate when the fluid flow profile is symmetric and steady. That is, the flow profile is regularly distributed throughout the pipe, and does not change with time. Figure 2 illustrates a good flow profile for any flowmeter, note that is is symmetric. The process of changing the flow from irregular to regular is called “conditioning”. For example, to change the flow profile from that shown in figure 1 to that shown in figure 2. There are several ways to condition flow, the most basic method is simply to add a length of straight pipe ahead of the flowmeter so the fluid has time stabilize and become symmetric
Figure 2. This figure illustrates a typical symmetrical flow profile. Good for metering flow.
Each piping system is unique, and some installations may require more straight pipe that others. Typically, flowmeter manufacturer’s specify a minimum of 10 to 15 diameters of straight pipe. This means that for a 4″ diameter pipe, you would need 10 x 4 = 40″ of straight pipe, or for a 2″ pipe you would need 10 x 2 = 20″ of straight pipe. However, for more severe flow distortions, you may require more.
How much straight pipe do you need? Well, first start with the manufacture’s typical minimum. For example, at SoundWater, we specify a minimum of 15 diameters for the Orcas portable ultrasonic flowmeter (i.e., 30″ of straight pipe for a 2″ pipe diameter). In most cases this should be sufficient. However, to do better, you may evaluate the flow profile yourself using your flowmeter. To do this, simply mount the flowmeter on each side of the pipe, top, bottom, and both sides, and taking a measurement at each location. Compare the measurements collected, and if the flow profile is symmetric the measurements should all be about the same. If the flow profile is irregular, then you may see differences in the measurements taken at each location around the pipe. With the Orcas flowmeter, this is very easy, since it only take seconds to mount it on any pipe, and it installs on the outside of the pipe (i.e., the meter senses through the pipe wall – no need to cut pipe!).
In some cases, there is just not enough pipe to accommodate the minimum straight pipe requirement. In this case, you should use the 2/3 rule. Install the flowmeter with 2/3 of the pipe upstream of the meter and 1/3 of the pipe downstream of the meter. Again, it is useful to evaluate the symmetry of the flow profile by taking measurements around the pipe as described above.
Finally, if you are as fanatic about flow metering as we are, then install a flow conditioner. This is hardware installed upstream of the straight pipe that helps to forcefully condition the flow profile. There are a handful of designs and manufacturers out there to accommodate most needs. This is a great way to assure you have a good measurement.
Feel free to contact us if you would like to discuss these details. We are here for you!
SoundWater Technologies
