Solving Corrosive Flow Measurement

Virtual Reference for Magmeters

Traci: Obviously, a very necessary process and very prevalent in all of our lives. Let's go into the paper itself. You mentioned something called virtual reference for mag meters to eliminate leak paths while still allowing the meter to operate properly. That sounds a little like voodoo. Is it unique to KROHNE, and does it apply to all of your offerings?

Al: Yes, virtual reference is unique to KROHNE, and it is one of the grounding methodologies employed. I'll talk briefly about the operation of the mag meter to put everything in context.

A magmeter makes use of Faraday's law of electromagnetic induction. This law states that if you have a magnetic field and you insert a conductor into this magnetic field, you will generate or induce voltage. That's exactly how a mag meter works.

With a magmeter, the magnetic field is created by coils embedded in the housing of the meter. The conductor is actually the fluid itself, which is why one of the first questions asked when applying a mag meter is: Is the fluid conductive?

Once you've got the magnetic field created by the coils and the conductor — the fluid — moving through this magnetic field, it generates a voltage. This voltage is picked up by electrodes embedded in the body of the meter. They're wetted, so they protrude through the liner and actually come into contact with the fluid. There are two of them, usually installed in the horizontal plane, at about the 3 o'clock and 9 o'clock positions in the tube.

The last thing on a magmeter is you want to make sure you have a stable reference, which is usually provided by your grounding methodology. There are several grounding methodologies. Virtual reference is one; grounding rings are another.

Grounding rings are mounted upstream and downstream of the meter. As this conductive fluid makes its way toward the meter, there is a good chance that stray electrical noise from pumps, variable frequency drives or other components in the process area will get induced into the liquid. You want to make sure that by the time this fluid hits the magmeter, you've taken care of this stray electrical noise because it will interfere with the performance and accuracy of the magmeter.

As the fluid approaches and hits the grounding ring, the grounding ring dissipates all of this electrical noise, leaving just the fluid with no induced stray electrical noise as it goes past the magnetic field. It generates a voltage that's picked up by the electrodes, sent up to the electronics of the magmeter for processing, and that's how it calculates flow.

Another methodology for grounding is a grounding electrode, which is inserted into the magmeter. It's a wetted part, which means you drill a hole into the liner and introduce it, usually at about the 12 o'clock point in the tube. Being a wetted grounding methodology, you want to make sure it will stand up to the product itself. It needs to handle the chemical compatibility and survive any kind of attack from an aggressive fluid. And when you introduce it into the liner, you create another potential leak path. You have two potential leak points from the two electrodes, but adding a grounding electrode introduces another leak point.

Now we come to virtual reference. The virtual reference grounding methodology is a non-wetted grounding methodology. We install a circuit board into the electronics of the magmeter. The virtual reference provides a stable ground reference for the magmeter to make the measurement. The signals that come into the mag meter, or any stray noise that comes into the virtual reference circuit as a result of stray noise in the plant, are introduced at 180 degrees out of phase. So they cancel each other out, and the net result is you're left with just a signal from the flow measurement.

So you're looking at several grounding methodologies. Virtual reference is unique to KROHNE. A grounding electrode is a grounding methodology, or you can have grounding rings.

Some of the benefits of having virtual reference: It's not wetted, first of all. It could reduce cost. If you get into a situation where the grounding rings need to be made of some kind of exotic alloy instead of stainless steel, a lot of times the price of these grounding rings could be as much as the mag meter itself or even higher. So there's another benefit of having virtual reference.

Last but not least, when you get rid of the grounding rings, you eliminate at least two potential leak paths in the pipeline itself because now you've got the process pipe with a gasket meeting right to the meter instead of having a grounding ring in the middle on both sides. That's really how virtual reference operates and some of the benefits virtual reference grounding can bring to the measurement. (More information on virtual reference can be found here.)

Straight Truth on Coriolis Meters

Traci: Thank you for that. I want to move on now to Coriolis meters. In the paper, you make a point about abrasion and the impact on bent-tube meters. It's always been my impression that straight-tube meters are much more expensive and have limited performance capabilities compared with traditional bent-tube versions. Is that the case?

Al: From a price viewpoint, just like you have with bent-tube meters, you've got different models with straight-tube meters, and straight-tube meters are not necessarily expensive. They're fairly competitive. In fact, I would argue there are models out there that are more economical than an equivalent bent tube.

From a performance point of view, straight-tube meters are extremely accurate. You can get a straight-tube meter that's equipped with CT approval, or custody transfer approval. For custody transfer, you're basically transferring product from Company A to Company B, and there's an exchange of money. You're selling product, so this meter becomes a cash register. To get CT approval, it's not something a vendor certifies on their own. You need to submit your meter for testing. It goes through various tests to make sure the meter is highly precise and can actually be used in this application because even small measurement errors in a CT application can lead to significant financial discrepancies or negative impacts for either the person selling the product or buying the product.

So straight-tube meters are available in economical versions, but they're also available in versions that are extremely accurate.

There are also different benefits to a straight-tube meter. We talked about abrasion in the article. When you're dealing with a straight-tube meter, you're not having the fluid come in and then change direction several times from the time it comes into the meter to the time it goes out, because every time it makes a turn, you're basically wearing out the tube at those particular points.

Another benefit is if you're measuring product that's extremely viscous, imagine trying to push that product down a bent tube and then bring it back up and push it back out. There are definitely benefits to straight-tube meters. They're economical and highly accurate.

They're also very easy to install because the straight-tube meter has the same physical geometry as a pipe. It is round just like the pipe is. You install it, bolt it up and off you go. You don't have to make adjustments or take into consideration the belly part of the meter, which is typical for a bent-tube meter.

You also have less chance of buildup in a straight-tube meter because there's no change in direction. However, if the product is prone to buildup and it does build up even in a straight-tube meter, it's very easy to look in there and see that you have buildup. Then it's just as easy to clean out that meter because you can run a rag or a cleaning brush straight through. Once the meter is clean, you can verify that it is clean and free of buildup.

A straight-tube Coriolis meter will also have less of a pressure drop. I hope that in addition to answering your questions about pricing and accuracy, I've also given you a flavor of additional benefits that a straight-tube meter can provide. I would argue that the first type to look at is: Can I get the application done with a straight tube? Because there are lots of other benefits it can provide.

Entrained Gas Dilemma

Traci: OK. Let's talk a little about entrained gas. You mentioned that in the paper but don't go into much of the impact this can have on the measurement. Can we talk about that?

Al: Yes, absolutely. Before we talk about entrained gas, let's quickly revisit how a Coriolis meter works because that'll give us a clearer picture of the impact of entrained gas.

When you have a Coriolis meter, you have a tube that's typically excited or oscillated at a particular frequency, and this is done by a circuit called the driver circuit. You oscillate the tubes at a particular frequency, and then you have two sensors, or pickups, also installed on this measuring tube. These two sensors pick up the frequency that's generated by this driver circuit.

If there's no flow going through the meter, the time it takes for the frequencies to be received from these sensors is exactly the same, which means when you look at the received signals, they're superimposed on each other. As long as they're superimposed, we call these in phase — they're 100% superimposed, and this tells us there is no flow going through the meter.

As soon as you introduce flow in the meter, you will have a difference in time for when the signals are received from these sensors. When you receive these signals, they're out of phase once you have flow going through the Coriolis meter. The amount of out of phase, or the phase shift, is directly proportional to mass flow. The degree of phase shift relates to how much mass flow you have. If you've got a product flowing at a low rate, the phase shift will exist, but it'll be pretty small. The faster you flow, that phase shift will get larger and larger.

So with a Coriolis meter, you're dealing with the driver circuit and the sensors that pick up the frequency. The phase shift is directly proportional to mass flow. When you introduce a fluid into an empty pipe, you also change the frequency it's operating at, and this change in frequency is directly proportional to density. You make two measurements with a Coriolis meter: one is mass, and one is density. Then you can calculate volume from there.

Now let's get into the topic of entrained gas. When you talk about entrained gas — for example, air — it's extremely susceptible to pressure changes. When you have a Coriolis meter and it's operating or measuring a single-phase fluid, as long as the fluid is inert and has no gas entrained in there, the meter is extremely happy. Once the flow starts to go through there, I get a change in phase shift. I may get a little up and down depending on what's going on with the flow rate, and I'll have a change in frequency based on the density. But other than that, there's not a lot going on.

As soon as you introduce air into the meter, as I said earlier, air or gas is extremely susceptible to pressure. You could have a bubble entrained in the fluid that's the size of a marble. If the pressure being imposed on this fluid and air bubble is high and then the pressure decreases, you'll very quickly get a significant change in the size of the bubble. It could go from the size of a marble to the size of a baseball or even larger, and every time it does that, it's displacing fluid.

Instead of having a Coriolis meter that was single phase, plump full of fluid, you now have a Coriolis meter that's got a two-phase flow condition, where you've got a mixture of fluid and gas. When you get into these types of changes and because of the impact of pressure on gas being so profound, the Coriolis meter circuits need to react very quickly to still make the measurement. A lot of times, without the capability of entrained gas, without the entrained gas algorithm, a meter will have difficulties, and often the meter will stop measuring.

If you're going to install a Coriolis meter in a process, my recommendation would be to get one with entrained gas management. Every vendor offers this capability. If you do get into an entrained gas regime where the meter is struggling to keep up, with the entrained gas measurement algorithm, the meter will continue to measure, albeit at a lower accuracy. It will still continue to measure. You won't have issues about the meter getting saturated and going through a startup mode. At the same time, if you can take the signal out of the meter — a two-phase flow signal — the operator or operations can be notified that something has changed inside the process because there's air inside a line where there typically should not be air. When you look at a process pipe taking product from one process to another or one reactor to another, it's supposed to be a closed system. You're not supposed to have air in there.

If you start getting air, the air that's getting entrained into the fluid is coming from a source that could be compromised. It could be a pump that's maybe cavitating. You could have seals on a valve that are starting to fail, allowing air to seep into the line. All of this can give you additional visibility to what's going on in the process and prevent you from having a premature shutdown where the pump cavitates, basically burns itself out, and now you have a plant shutdown because you've got to replace the pump.

Whereas if you had the entrained gas management algorithm, the meter would notify you that it's operating in an entrained gas regime. Very quickly, maintenance personnel or somebody can get involved and start looking upstream of the meter for the source of that air.

As I said earlier, all vendors offer entrained gas algorithms, and it is a good feature to have because it provides you with predictive diagnostics. (More information on entrained gas can be found here.)

Joe: If I can add something: You've talked almost exclusively about a liquid with entrained gas, but a mass meter is equally adaptable to a gas application or a liquid. Because it measures density along the way as well, it can also diagnose a situation where a liquid is entrained in a gas where there shouldn't be any liquid. So that's not to be overlooked — mass meters are extremely adaptable to all sorts of different applications, whether it's viscous fluids or even light gases. It's pretty cool.

I'd also want to make one other mention, a little plug for KROHNE here. While EGM is available from all vendors, one of the unique things KROHNE brings to market is that this is a standard feature in every one of our versions of meters. It's adaptable in straight tubes or bent tubes and any size meter, regardless of the materials. That's not something every vendor might be able to boast about, but it's something we're very proud of at KROHNE.

Variable Area or Rotameter?

Traci: My next question is: Are variable area meters the same as rotameters?

Al: Yes, they are. Rotameter is really a brand name. It's a VA meter manufactured by a company, but the brand name for that VA meter is Rotameter. A very similar comparison could be tissues. When you use tissues to blow your nose or to wipe something off, a lot of times people will say, "Hey, pass me a Kleenex," even though the box itself might be manufactured by a vendor that doesn't have the name Kleenex on it. Kleenex is a brand name for a tissue, and the same thing with a rotameter. It's a brand name for a VA meter.

In essence, a VA meter, or variable area meter, is an ISO-recognized flow measurement technique. VA meters are usually fairly simplistic. They're used for visual indication, but now they can also be equipped with additional capabilities. You can take a 4-20 signal back, you can put alarm contacts on there to indicate maybe low flow or high flow, and you can have different types of liner materials or float materials to be able to withstand different types of fluids or to be compatible with the different types of fluids or gases that you're actually flowing through these meters. Believe it or not, there are VA meters out there now that are even still capable.

SIL Levels

Traci: Good visual there. Right as you mentioned Kleenex and tissues, my nose started twitching. The power of suggestion.

Can you remind us about the various safety integrity levels and what they represent?

Al: Yeah. Safety integrity level, or SIL as it's referred to in the industry — we all like TLAs, or three-letter acronyms — what you're doing is assessing the risk of a hazard. The way you assess this risk is made up of frequency times the severity for the potential of this hazard.

I'll give you an example. If I have a tank that contains acid and when we do a hazard analysis on this tank, we determine that this tank will overflow one time per year possibly and the acid will spill on the ground, the risk mitigation technique for that could be putting a berm or a barrier around it to make sure the acid doesn't flow anywhere else.

By the same token, if I look at maybe the same type of tank with acid that's installed in another part of the plant, but because of the volume or the size of this tank, when it does overflow — and it may overflow also one time a year — it could come into contact with other chemicals stored nearby and create an explosion.

It's the frequency and the severity you're looking at. The risk mitigation technique for the acid tank in the first example, where I'll have a spill on the ground, I can contain that very quickly with some kind of a barrier or a containment pad, a concrete pad or something to collect this acid.

Whereas in the other case, where I could have an explosion, even though the frequency is still the same — it's still only one time per year — the effect of the impact is extremely high. In this latter case, you would probably be looking at a SIL level of SIL 4.

When we look at the SIL levels, they start out at SIL 1 and go up to SIL 4. The higher the number, the higher the consequence if an incident occurs. If it's a SIL 1 application, that could be something like this first example with the acid tank overflowing, and we're able to contain that spill. The risk mitigation technique was to build a concrete pad or some kind of containment vessel that would catch this spill, and we're done with it.

In the other case where we cause an explosion, that's more than likely going to be a SIL 4. The higher the SIL number, the higher the consequence if an incident occurs.

A lot of times when you do get into a SIL 4-type result, you're not going to instrument that with safety instruments. You're probably going to go back to the drawing board and look at a different risk mitigation technique. Maybe that could be: Let's move the chemicals we're storing there somewhere else, or maybe let's relocate the tank.

Ideally, the higher the SIL level, the higher the impact is to the environment, but also the risk mitigation technique is going to have to be extremely robust. What I mean by that is in that second example, I may have a continuous level device to measure the level in the tank. I may have a point-level switch at a high level that when it comes into contact with this switch, it starts sounding off an alarm. Then I may have a second level switch that when it comes into contact with that, I now disable the pump. There may be a third level switch where I basically shut power off to the entire area or deplete oxygen in the area to prevent a runaway explosion or something like that.

In a nutshell, SIL is a hazard mitigation technique, and the hazard mitigation technique is based on the frequency and severity of a potential hazard. SIL 1 is the minimal amount of risk mitigation; SIL 4 is the highest. But when you do get into SIL 4-type applications, a lot of times you're going back to the drawing board because the consequence of that hazard, if it were to occur, could be extremely devastating.

Traci: Well, Al and Joe, you've given so much information here. I want to toss out one last question, and that's: Is there anything you want to add to bring it on home?

Joe: Well, if I might interject here, I just want to remind the audience that KROHNE's been around for quite some time serving the chemical industry since its inception back in 1921. We started supplying variable area flow meters to the chemical industry. That was our target audience back then. So we know the chemical industry quite well, and conversely, the chemical industry knows KROHNE quite well because they keep bringing us challenges for new measurement technologies. We've had a codependence. They bring us the challenges; we come up with new instruments.

All I can say to our audience here is if you're struggling to find some instrumentation for flow or level measurement in your chemical process application, please reach out to us. We've got a lot of solutions. As Al mentioned, we know the applications quite well. In the show notes, you'll also see a link that points to the application stories and a page that's dedicated to the chemical industry and how we serve it. So please reach out to us. Our email is quite simply [email protected], and we'll jump on your questions.

Al: Yeah, and Traci, we're a 104-year-old company, and at 104 years old, we're just slightly older than Joe is, so...

Joe: I think this predates disco by a couple of years, but nevertheless.

Traci: Oh boy. Well, gentlemen, I appreciate the conversation and the laughs, your thoughtful answers and for participating in our Solutions Spotlight edition of Chemical Processing's Distilled Podcast. For our listeners, you can subscribe to this free podcast via your favorite platform. You can also visit chemicalprocessing.com for more tools and resources aimed at helping you achieve success. On behalf of Joe and Al and the folks at KROHNE who sponsored this episode, thanks for tuning in. Thanks again, gentlemen.