Digital Flow Meter: Piping Requirements for Various Technologies

Some people don’t know this, but almost all installation guidelines for various flow meter types and designs are largely based on flow meter piping requirements. In fact, it basically plays a significant role in choosing the right metering principle or technology behind the instrument, specific to the flow measurement application.

Regardless of its characteristics, when fluid flows through a channel, it moves pretty consistently with the peak velocity occurring near the middle of the pipe. When flow meter installation is not done correctly, or if the wrong digital flow meter technology is installed, flow measurement can become inconsistent and inaccurate. Improper flow meter configuration may cause one of two types of common fluid disturbances; flow profile distortion and swirl.

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Flow profile distortion normally happens whenever there is some form of obstruction inside the pipe, right where the flow meter is installed. A partially open valve, a flange gasket that’s poorly mounted, or any form of blockage could affect the flow profile of the fluid, effectively making flow meter readings inaccurate or inconsistent. A swirl, on the other hand, happens whenever the liquid flows through bends in the piping in separate planes. Compared to flow profile distortion, swirls are not easy to spot or correct. But both these fluid disturbances, occurring upstream and in the vicinity where the flow meter is installed, may cause inaccurate readings that can go beyond 50%.

Most reputable manufacturers of flow meters and other similar devices will recommend varying sections of straight pipe both downstream and upstream of the flow meter in order to maximize the efficiency of the instrument. Flow conditioners and flow straightening contraptions, such as tube bundles, internal tabs, and perforated plates can also be used to avoid long straight pipe lengths. Although these particular solutions can minimize the swirl effect, they don’t necessarily address the problem with flow profile distortion. Flow conditioners, however, do reduce swirl as well as minimize flow profile distortions.

Piping Specs for Various Digital Flow Meter Technologies

Ultrasonic Flow Meter Technology

There are two basic types of ultrasonic flow meters that utilize two different principles, Doppler and transit-time. Doppler flow meters work by measuring the frequency shifts of beam reflections off the tiny particles and bubbles generally present in almost any type of fluid. Transit-time flow meters, on the other hand, use ultrasonic beams that are moving with the flow and countering the flow; they measure the time difference between these. For clean fluid and viscous liquids where bubbles and tiny particles are rarely present, the Doppler principle is much less effective compared to transit-time.

Ultrasonic flow meter technology is very effective on many types of fluids, clean, viscous, or dirty, so long as the right type is used. However, this technology is rendered ineffective on slurry fluids or liquids that are acoustically absorbent. The highly absorbent nature of certain fluids can reduce the effect of beams and signals well below usable parameters.

As far as installation is concerned, ultrasonic flow meters should be installed upstream of flow obstacles like piping elbows, valves, or reducers. They should be installed in a location where flow profile distortions and swirls are minimal.

Magnetic Flow Meter Technology

Electromagnetic flow meters, otherwise known as magmeters, should always be installed in a location where it is constantly full of liquid. Avoiding the accumulation of solids, air, or other particles is crucial in the pipe configuration of magnetic flow meters. Any secondary phase like air should by pushed or carried out of the meter system by flow or by buoyancy at zero flow. Similarly, any solids or particles that get carried by the flow should be moved by flow or through gravity at zero flow.

Due to the likelihood of air and solids getting trapped inside the meter, it is ideal to install magnetic flow meters in vertically positioned pipelines with upward flow. Although, it is possible to install this kind of flow meter technology in a horizontal line as long as proper precautions are taken to avoid any secondary contaminants like air and solids affecting the output readings. It is also important to place the meter in a location where it can remain full even at zero flow. Otherwise, you may experience inconsistent or erratic readings due to the electrode being exposed to air.

Differential Pressure Flow Meter Technology

Differential pressure flow meters; often referred to as DP flow meters, measure flow rate by calculating the pressure drop along a flow element inside the piping. A standard DP flow meter usually consists of a flow element, like an orifice plate in the piping, and a differential pressure meter located nearby. Impulse lines, which are basically small tubes, placed on both sides of the flow section leads to the differential pressure meter for measuring purposes.

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One of the most basic piping recommendations agreed upon by most industry experts and professionals when it comes to differential pressure flow meters is to use static pressure and temperature measurements on top of the flow rate. This will help you get a much more accurate and consistent reading on fluid flow. Also, by installing a tee to connect to the correct DP impulse line, you can effectively reduce or minimize potential disturbances in the flow profile caused by the added pressure sensor.

If temperature measurement is available, make sure to install the thermowell—utilized for measuring temperature levels—at least 10 pipe diameters downstream of the DP meter’s flow element. This type of installation must be carried out as neatly as possible. Neatly trimmed gaskets and smooth ground welds and help make sure that there are no protrusions present that could disrupt the way the fluid flows and distort the flow profile of the liquid moving through.

Adhering to the recommendations of industry experts and making sure that piping requirements are met when installing flow meters can help you avoid headaches and stressful situations later on in the future. These will ensure successful applications that yield accurate and consistent outputs.

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