Differential pressure transmitter in flow measurement
Oh, hi! Didn’t see you there. What’s new? Let’s talk about pressure transmitters and where we can use them! Sounds like the name of that movie, Fantastic Beasts and Where to Find Them. Sorry, watched it recently.
Pressure transmitters are really versatile, and we can use them to measure other things besides pressure. In fact, we used them a lot in measuring level and flow for more than a decade. However, new tech in flow and level measurement have begun to edge out the old pressure solutions. Still, they have their uses.
Basically, a differential pressure (DP) flow meter combines three essential elements. The first, the primary (kinda obvious) element, creates a change in pressure. The second, the transmitter, reads the pressure change and converts it to numbers we can read. Last but not least, we have the third element, a structure to send the pressure created by the primary element to the transmitter. That covers impulse lines, tubing, valves, and other mechanical bits.
We now have DP meters with primary elements, transmitters, and connections all integrated, which makes installing them easier.
Now, let’s dig into the details behind these devices.
Why should we care about this guy Bernoulli’s principles?
We can skip the huge history lesson, but knowing the basics won’t kill you and may even help, so pay attention! It won’t even hurt that much, I promise.
First, we need to start with fluid mechanics. Hang on, don’t run away yet! Our old reliable DP flow meter works on fluid mechanics, so you already know a little bit. We’ll just give you a little bit more.
So Bernoulli was a Swiss mathematician in the 1700s. Now in that age, they hadn’t invented smartphones or videogames yet. Otherwise, history might be different. Anyway, this math nerd just decides to study hydrodynamics for a while. He focused on the conservation of energy, and after a few years he developed this principle, and we named it after him. Score a point for the math nerds!
In a nutshell, the sum of all energies – static, potential, and kinetic – in a fluid running within a pipe is the same throughout the pipe. For a differential pressure meter, the sum of all these energies upstream equals the energies downstream. You can read static energy as pressure, potential energy as elevation, and kinetic energy as velocity. Still with me? Good!
Yep, we have another dude responsible for finding out something vital! Osborne Reynolds became famous for his study of flow. We use his Reynolds number to predict turbulence and find out how fluid behaves on different scales. Useful, huh?
And in instrumentation, we use the Reynolds number to scale out new flow meters, finding out its range and applicability. Even more useful, at least for us!
We’re done with principles here, so let’s go to the practical stuff.
You have to scale out and choose the right primary element for your process, so we’ll give you a brief overview of the most common, with pros and cons. Later, we may tackle more primary elements in another article. Keep an eye out for it!
Before we go over the types, we need to understand their effects in the pipe. Basically, a primary element creates a pressure drop by restricting the flow. The DP transmitter then measures the drop and uses Bernoulli’s equation to calculate the flow. Ready? The square root of the pressure drop across the restriction is proportional to the flow. Pretty easy, right? So let’s move on to the types, three in particular.
The most common solution to differential pressure flow measurement, orifice plates are easy to install and cover a wide range of applications, like gas, liquids, and steam. You also have different types of plates. For instance, the conditioning orifice plate can resolve irregular flow profiles. It has the advantage of working in short pipes with straight runs, usually only two diameters before and after the sensor.
Oh look, another thing named after an old dead guy! This one we don’t capitalize, though. No idea why. Anyway, the pitot tube goes across the pipe. Upstream you have the total pressure, downstream the static pressure, and the output is the pressure differential.
Yes, we have one more! The Venturi effect covers the drop in pressure when fluid goes through the restricted section of the pipe. A Venturi tube creates a lower pressure downstream. When you connect your DP transmitter to the upstream and downstream sides, you’ll get the pressure differential.
DP meter installation and concerns
The three main elements of a DP meter we mentioned earlier – primary element, transmitter, and structure – can cause problems as time marches on, such as mechanical misalignment, seal pot level changes, and others. To avoid these problems, we now have DP flow meters with primary elements already integrated with the transmitters.
Also, calibrating the transmitter is fairly easy. However, a primary element can go years without a calibration, because removing it requires stopping your process and taking apart your structure. Nobody likes doing that.
You don’t encounter these issues with flow devices like magmeters, Coriolis, and others.
We still use differential pressure transmitters in the instrumentation world because they still work. However, we have a lot of new tech out there that provides better accuracy and easy installation. These meters may one day go the way of the dodo.
Will you miss them as much as I will? Let us know in the comments!