January 2018
The Fundamentals of Variable Area Flowmeters
The Fundamentals of Variable Area Flowmeters
By Walt Boyes
Most flowmeters have a fixed orifice. Whether they are differential pressure devices, velocity measuring devices or mass measuring devices, the cross-sectional area of the measurement zone stays the same. There is a whole class of flowmeters, though, where the cross-sectional area of the measurement zone changes with the flow rate. These are called variable area flowmeters, and there are several common types: the rotameter, the orifice-and-plug meter and the cylinder-and-piston type. We often don’t think of these devices, but if you look around, you will see them everywhere. They are used in machine shops, HVAC applications, industrial and municipal water treatment, wastewater treatment, hydraulics, clean rooms and air, gas, water, and other liquid services. They can be made from many different materials, and can be bought in many accuracy ranges, from “rough indication” to precision metering for laboratories, hospitals and specialty gases.
More next month.
This article originally appeared in Flow Control magazine (August 2002) at www.flowcontrolnetwork.com.
Gas Flow in Large Pipes
Gas Flow in Large Pipes
By David W Spitzer
The number of different flowmeters that can be applied to measure the flows of industrial liquids and gases can be overwhelming. Consider that there are more than a dozen potential flowmeter technologies (each of which has multiple variations and often many suppliers), so as many as 1000 distinct flowmeters could be purchased in some applications. However, some services and applications can preclude the use of most technologies — leaving only a few options.
Such is the case of the measurement of process gases in large pipes in steel mills and other heavy industrial processes. These pipes often have diameters of more than perhaps 60 inches, but they can certainly be somewhat smaller. Further, most piping configurations have limited straight run that cannot be altered. Just think about the cost and real estate involved with providing many diameters of 60-inch pipe to develop a good velocity profile for your flowmeter. Components in coke oven gas and blast furnace gas services can coat the flowmeter and plug impulse lines. In addition, the “pipes” may be rectangular and operate at low pressures (only a few inches of water column) that can limit the pressure drop across the flowmeter.
Quickly eliminated are most differential pressure, magnetic, Coriolis mass, open channel, oscillatory, positive displacement, turbine and variable area flowmeters. Insertion flowmeters can pose accuracy problems in many installations and thermal flowmeters can coat in some applications. Remaining technologies include some differential pressure, some insertion, target and ultrasonic flowmeters, most of which require special design considerations and/or maintenance to remain in service.
The application presented may seem common — measure gas flow in a pipe — but it can quickly evolve into a much more complicated problem as additional constraints are added.
This article originally appeared in Flow Control magazine (January 2017) at www.flowcontrolnetwork.com.
Flowmeters for HVAC
Flowmeters for HVAC
By David W Spitzer
Which of the following flowmeters could reasonably be applied to measure air flow in a heating, ventilation and air conditioning (HVAC) system?
- Pitot tube
- Orifice plate
- Thermal
- Magnetic
- Flume
Commentary
Magnetic flowmeters (Answer D) and flumes (Answer E) only measure liquid flow so they are not correct. Pitot tube flowmeters (Answer A), orifice plate flowmeters (Answer B) and thermal flowmeters (Answer C) could be applied in this service.
That said, the question is not clear because not all HVAC systems are created equal. Pitot tubes (Answer A) and thermal flowmeters (Answer C) typically exhibit negligible pressure drops and are often applied to air pipes and ducts operating at low pressure. In many such applications, insufficient pressure may be available to operate orifice plates (Answer B) that exhibit higher pressure drops. Further, even if sufficient pressure were available, the higher pressure drop across the flowmeter can increase energy consumption that can be saved when the air fan incorporates a variable speed drive. That said, orifice plate flowmeters are available for these applications (typically in round pipes).
Additional Complicating Factors
The HVAC industry is notoriously known for being sensitive to cost. As such, the initial cost of the flowmeters may override the cited technical evaluation.
This article originally appeared in Flow Control magazine (January 2017) at www.flowcontrolnetwork.com.
February 2018
The Fundamentals of Variable Area Flowmeters: Cylinder-and-Piston Meters
The Fundamentals of Variable Area Flowmeters: Cylinder-and-Piston Meters
By Walt Boyes
Probably the most common is the cylinder-and-piston variable area meter. These devices are used for many in-plant requirements, such as hydraulic fluid flow, cutting oil recirculation flow and other simple applications, where indication only is acceptable. Piston type variable area meters can be made to operate on high viscosity liquids, and even gases and compressed air. They will operate up to relatively high pressures and temperatures if the materials of construction are compatible.
The piston type meter has an open centered piston with a tapered metering cone. This cone rides in a sharp-edged annular orifice and the piston is spring-opposed. Velocity causes a pressure differential across the orifice, which moves the piston and the metering cone. A magnetic follower indicator tracks the movement of the metering cone externally. The use of the retention spring to hold the piston in the “zero” flow position when there is no velocity allows this type of meter to be installed inline in any orientation.
More next month.
This article originally appeared in Flow Control magazine (August 2002) at www.flowcontrolnetwork.com.
Use of the Word 'Large'
Use of the Word ‘Large’
By David W Spitzer
The measurement of gas flow in large pipes in a steel mill was discussed in a recent article. Although the article was targeted to a somewhat specific application, the general use of the word “large” begs the question of what the word “large” actually means. Many years ago, I helped start up a steel mill with a large 6,000 hp motor that sat on a two-meter-high pedestal (complete with a stairway and handrail). Some years later, I visited a wind tunnel with a motor rated at more than 100,000 hp. In between, I was led to what was described as a “large” motor that I could not find — until I was instructed to look down where I saw a 25 hp motor at my feet. What a disappointment! I almost tripped over the “large” motor and did not know it.
The word “large” is a relative term that often reflects the previous experience of the person using the word. Based on my experience in steel mills and water systems, the word “large” conjures images of pipes with diameters of more than about 60 inches, although I have worked with flowmeters as large as 180 inches. If I were to limit my experience to the chemical plant in which I worked, “large” might be about 8 inches or so. A person in the pharmaceutical industry may consider 1 inch to be “large.”
If you think that this will not happen to you, think again. We commonly use terms that are subjective and open to interpretation. For example, what does “accurate” mean? Conversely, terms that have specific meanings can also be used inappropriately. Has anyone ever told you they would call back in a minute?
I suggest being careful about what you say and wary of what you hear.
This article originally appeared in Flow Control magazine (February 2017) at www.flowcontrolnetwork.com.
How Full is a Horizontal Tank?
How Full is a Horizontal Tank?
By David W Spitzer
Approximately how full is a horizontal tank in which the differential pressure level transmitter measures 90 percent level?
- 100 percent
- 97 percent
- 90 percent
- 83 percent
- 80 percent
Commentary
Perhaps stating the obvious, a horizontal tank is a cylindrical tank resting on its side. Therefore the percentage of liquid fill will not be linear with the percentage of level. For example, a relatively small amount of material added to an empty tank will increase the level measurement more quickly than the amount of liquid fill would indicate. Similarly, less than 10 percent of additional capacity is available when the tank level is at 90 percent.
Therefore, Answers A, C, D and E are not correct. Answer B is most reasonable.
Additional Complicating Factors
This analysis assumes that the 90 percent level measurement corresponds to a 90 percent level in the tank. This would not necessarily be the case if level transmitter taps or diaphragms are not located tangent with the top and bottom of the tank, as is often the case because of physical constraints and the location of existing nozzles.
This article originally appeared in Flow Control magazine (February 2017) at www.flowcontrolnetwork.com.
March 2018
The Fundamentals of Variable Area Flowmeters: Orifice-and-Plug Meters
The Fundamentals of Variable Area Flowmeters: Orifice-and-Plug Meters
By Walt Boyes
Other types of variable area flowmeters must be installed vertically. They are commonly referred to as “rotameters,” but only one actually is. Most “armored rotameters” are actually operating under the orifice-and-tapered-plug principle. A tapered plug generally of a vee-shape, held in place by a vertical, center-mounted follower, moves up and down in a sharp-edged annular orifice as flow changes. Flow is always in an upward direction, and the height of the plug in relation to the orifice is proportional to the flow rate. The follower is connected (usually by magnetic coupling) to a local indicator, which may be either mechanical or electronic, or even a transmitter.
“Armored rotameters” can be used for steam and gases, but most often are used for liquids under high pressure and corrosive liquids under high pressure. They are often made from specialty alloys, such as Hastelloy, for this service. These orifice-and-plug meters can be made from a variety of opaque plastics as well, and use the follower to operate the mechanical or electronic indicator.
More next month.
This article originally appeared in Flow Control magazine (August 2002) at www.flowcontrolnetwork.com.
Steam Flowmeter Audit: Discovering the Problem
Steam Flowmeter Audit: Discovering the Problem
By David W Spitzer
About 10 years ago, I was asked to audit the flowmeters used to bill a chemical plant for its steam consumption. In general, hiring a consultant to audit the flowmeters illustrates the importance placed on having accurate measurements that ensure the multimillion-dollar annual steam bill is as accurate as possible.
This activity makes financial sense because hundreds of thousands of dollars of steam can flow through one flowmeter annually. A measurement error of one percent will cause the flowmeter to be in error by thousands of dollars per year. Spending a few thousand dollars to have the flowmeter installations, operation and maintenance audited can be money well-spent, especially when conflicts between parties can be prevented before they even start. That said, most people do not think this way, so I typically get involved when something is wrong or when the parties are fighting — or both.
In this case, something was wrong because the “As Found” quarterly calibration check on a flowmeter was almost always in error despite acceptable previous “As Left” calibrations. The usual fixes had already been implemented to include switching and then replacing the transmitter, yet the error continued. Nothing seemed to work. Recognizing that a problem existed and hiring a consultant to audit the system was suggested.
Keep in mind that measurements are implemented using a flow measurement system where the key word is “system.” While problems often lie where the symptoms occur, it is not uncommon to stumble across problems located in other part(s) of the flow measurement system. For example, the flowmeters may not have sufficient straight run, or the pipe diameter is not per specifications, or the flowmeter is coating, or the calculations are incorrect, or the input electronics are set up incorrectly, etc. Hundreds of problems can potentially exist in the flow measurement system.
More next month…
This article originally appeared in Flow Control magazine (March 2017) at www.flowcontrolnetwork.com.
Variable Speed Drive Configurations
Variable Speed Drive Configurations
By David W Spitzer
Which of the following configurations lend themselves to becoming potentially viable applications of variable speed drives?
- One pump feeding one user
- One pump feeding multiple users
- Two operating parallel pumps feeding one user
- Two operating parallel pumps feeding multiple users
Commentary
Installing a variable speed drive to operate one pump that feeds one user (Answer A) allows the variable speed drive to manipulate that flow as part of a feedback control loop. This configuration lends itself to being viable when the energy savings associated with operating with the variable speed drive sufficiently offsets the cost of the installation. Similarly, using two operating parallel pumps (Answer C) can be viable, although two variable speed drives will likely be required to operate the system.
Installing a variable speed drive to operate one pump that feeds multiple users (Answer B) allows the variable speed drive to manipulate the header pressure. This configuration may lend itself to being viable when header pressure reductions (as compared to the existing header pressure) result in energy savings that offset the cost of the installation. Similarly, using two operating parallel pumps (Answer D) can be viable while two variable speed drives will likely be required to operate the system.
Additional Complicating Factors
These configurations generally focus on energy savings. Making process improvements as a result of applying variable speed drives in other configurations can reduce operating costs significantly.
This article originally appeared in Flow Control magazine (March 2017) at www.flowcontrolnetwork.com.
April 2018
The Fundamentals of Variable Area Flowmeters: Rotameters (Part 1)
The Fundamentals of Variable Area Flowmeters: Rotameters (Part 1)
By Walt Boyes
The true “rotameter” differs from the orifice-and-plug variable area meter considerably. In a true rotameter, the walls of the measuring tube are tapered, and the float moves up and down in the tapered tube based on the flow through the rotameter. The float can be made in many shapes, from round to compound with sharp edges. Because the float is “free” inside the tube, it is very difficult to interface a true rotameter to a remote indicator or transmitter. However, most rotameters are made of glass, or a transparent or translucent plastic, and the indicator scale is usually painted, screened or overlaid onto the device itself. In some cases, the float has a captured follower like the orifice-and-plug meter, but still rides in a tapered tube. Unlike the tapered-plug design, however, the float rides up and down on the follower rod, rather than moving it.
Rotameters are ubiquitous. You find them everywhere, from laboratories and hospitals, to chlorine and other disinfecting gas feeders in water and wastewater plants. Rotameters may be used to measure both gases and liquids, although air entrainment and solids in liquids cause major problems with readings and accuracy. Rotameters can be very accurate. Accuracy in a rotameter depends on the length of the tapered tube, and the accuracy with which the tube and the float are designed and made. A 20-cm rotameter will be significantly more accurate than a 100-mm rotameter with the same scale because you can see higher resolution.
More next month.
This article originally appeared in Flow Control magazine (August 2002) at www.flowcontrolnetwork.com.
Steam Flowmeter Audit: Investigate the Problem
Steam Flowmeter Audit: Investigate the Problem
By David W Spitzer
Last month I mentioned being asked to audit the flowmeters used to bill a chemical plant for its steam consumption. Satisfied with a review of the overall design, calibration and installation of the various steam flow measurement systems, I was able to focus on the symptom at hand: One of the differential pressure transmitters would be calibrated to be within its “As Left” specification only to be consistently out of its “As Found” specification during its next calibration approximately three months later.
Given the symptom, a more detailed investigation into the calibration technique was warranted. An interview with the technician revealed that the transmitter was taken out of service, removed from the line, transported to the shop where it sat for a few hours for its temperature to equilibrate to the shop temperature, calibrated with a dead weight tester, reinstalled in the line, zeroed at pressure and then rezeroed at pressure the next day after the transmitter temperature equilibrated to the outside ambient temperature (that could reach 20 degrees below zero Celsius in the winter).
This procedure appears to be correct from a measurement perspective but presents a problem with calibration. In particular, the “As Found” calibration is generally compared to the previous “As Left” calibration to determine if the transmitter has drifted since the last calibration. Note that in this installation the transmitter is rezeroed the next day. Therefore the next “As Found” calibration would be expected to be different than the previous “As Left” calibration, as it was for this transmitter.
More next month…
This article originally appeared in Flow Control magazine (April 2017) at www.flowcontrolnetwork.com.
Zeroing Differential Pressure Transmitter
Zeroing Differential Pressure Transmitter
By David W Spitzer
Should a differential pressure transmitter be zeroed after it is removed, calibrated in the shop and reinstalled in the same location? Why or why not?
Commentary
One might think that removing a differential pressure transmitter, calibrating it in the shop and then reinstalling it exactly where it was previously installed would be appropriate. However, a number of issues can present themselves in this activity, so the transmitter should be rezeroed after reinstallation.
Different stresses on the differential pressure transmitter connections can affect the performance of the transmitter, typically by causing a zero shift. These stresses can be (and often are) different prior to removal from the process, in the shop with test connections (where the zero may be adjusted in order to perform a calibration), and after reinstallation in the process. The latter can easily occur as a result of a different amount of torque being applied to the connection fittings during reinstallation. In addition, jostling the transmitter can cause problems.
Additional Complicating Factors
Similarly, many differential pressure transmitters are affected by whether the transmitter is oriented in the same plane in the shop as in the actual installation. In addition, operating conditions such as the ambient environment, operating pressure, operating temperature and presence of heat tracing can likewise affect the transmitter.
This article originally appeared in Flow Control magazine (April 2017) at www.flowcontrolnetwork.com.
May 2018
The Fundamentals of Variable Area Flowmeters: Rotameters (Part 2)
The Fundamentals of Variable Area Flowmeters: Rotameters (Part 2)
By Walt Boyes
Rotameters have received a new lease on life in the semiconductor and pharmaceuticals industries where ultra-clean and accurate flow measurement is required. Rotameters have been made from polysulfone, PFA and other high-purity materials, and used for specialty gas and chemical service in wet benches and wafer processing machines.
Because a rotameter can be made out of a variety of materials, they range in price from extremely inexpensive to quite costly. A rotameter bored out of a single block of acrylic, with a plastic float, for example, might cost less than $50. A rotameter made of borosilicate glass, with a borosilicate glass float, 25-cm long, with a very low flow range and a precision metering valve, could run several hundred dollars.
Remember the variable area meter type when you need a good, durable, inexpensive indicator for your next project.
This article originally appeared in Flow Control magazine (August 2002) at www.flowcontrolnetwork.com.
Steam Flowmeter Audit: A Calibration Solution
Steam Flowmeter Audit: A Calibration Solution
By David W Spitzer
In previous articles, I mentioned being asked to audit the flowmeters used to bill a chemical plant for its steam consumption. One of the differential pressure transmitters was within its “As Left” specification only to be consistently out of its “As Found” specification during its next calibration approximately three months later. This was determined to be caused by rezeroing the transmitter on the day after reinstallation — after the transmitter temperature was allowed to equilibrate with the outside ambient temperature. While one particular installation consistently exhibited this behavior, it is likely that the other installations were likewise affected, albeit not as much.
Understanding that the calibrations are performed correctly and that they can affect the “As Found” calibration, one approach to solve the problem would be to widen the “As Found” calibration tolerance. However, recognizing that it takes almost a full day to remove, equilibrate, calibrate and reinstall one transmitter, it might be more prudent to calibrate the transmitter in-situ, thereby eliminating transmitter removal, equilibration, re-installation, rezeroing the next day and the “As Found” shift. I suggest that the time necessary to calibrate a transmitter might be reduced to two hours instead of all day, and involve considerably less work.
This sounds good on paper, but the plant did not have a portable calibrator so the technician continued to calibrate these flowmeters in the shop. I suggest a day may come when one of the parties realizes that the transmitter shift generally benefits the other party. This tends to create acrimonious feelings between the parties, especially when the calibration shift cannot be used to make accurate corrections to the steam invoice. Understanding the problem, potentially undesirable consequences and unproductive legal expenses can be avoided by using appropriate calibration techniques.
Often you can lead a horse to water, but you cannot make it drink.
This article originally appeared in Flow Control magazine (May 2017) at www.flowcontrolnetwork.com.
Level Measurement for Oil/Water
Level Measurement for Oil/Water
By David W Spitzer
Which of the following technologies can be used to measure the level in a tank containing oil and water?
- Capacitance
- Differential pressure
- Float/Displacer
- Ultrasonic
- All of the above
Commentary
The contents of the tank will likely separate into two liquid phases where the oil will float on the water because its density is lower. All of these technologies can measure the level of the oil, so Answer E is correct.
Notwithstanding the answer to the stated question, the questioner should be interrogated as to whether he or she really wants to measure the level in the tank or the level of the oil-water interface — or both — understanding that people often get so engrossed in their work that they use shortcuts in their speech and writing.
If the intent is to measure the interface level, capacitance, differential pressure and float/displacer level technologies could be applicable. Ultrasonic level technology would not penetrate the liquid surface and hence measure the level — not the interface level.
Additional Complicating Factors
Many factors can complicate these measurements such as the presence of foam above the oil, agitation in the tank and a rag layer at the oil-water interface.
This article originally appeared in Flow Control magazine (May 2017) at www.flowcontrolnetwork.com.
June 2018
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right?
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right?
By Walt Boyes
If I were to compile a list of FAQs (frequently asked questions) from the “Flowmeter Help Desk” the single most common one would have to be, “My flowmeter reads wrong. How do I fix it?”
The Help Desk person takes a deep breath and starts through the checklist. Let’s go through an example. Doing this checklist yourself may save you the time of calling the Help Desk, and may enable you to solve your own problem.
First, exactly how is the flowmeter reading wrong? Does it not read at all? Is there an error message on the display? Checking the output, is there one? So, the first thing to do is to check the mechanical and electrical/electronic integrity of the flowmeter. Assuming it isn’t physically broken, and all of its systems appear to work, the next thing to look at is whether the reading on the meter corresponds to reality.
Here is the first area where people can get seriously confused. It happens all of the time. You are expecting a flow of a certain value. The flowmeter doesn’t read that flow. What is going on here?
More next month.
This article originally appeared in Flow Control magazine (July 2002) at www.flowcontrolnetwork.com.
Incinerator Combustion Air Flowmeter: Do Not Overlook Operating Constraints
Incinerator Combustion Air Flowmeter: Do Not Overlook Operating Constraints
By David W Spitzer
Flow measurement applications can be difficult because of physical properties, operating conditions, operating constraints or some combination of all three. For example, the fluid may be corrosive, abrasive and/or lethal. It may be operating near its vapor pressure or at an extremely high or low temperature or extremely high or low pressure. Selecting a flowmeter to measure even common fluids operating at nonextreme conditions can be challenging when operated under these conditions. That said, the operating constraints are often overlooked (at least initially).
Consider the measurement of combustion air to an incinerator. The ambient air was not corrosive, abrasive or lethal and operated at atmospheric pressure and ambient temperature. In other words, the air was benign and not operating at extreme conditions. Finding a flowmeter to measure the air flow should be easy, right?
Not so fast! The flowmeter must measure the combustion air flow over the entire firing rate of the incinerator — typically over a 10-to-1 turndown. However, this incinerator is different because one of the plant fumes is mixed with combustion air to provide better destruction, increase incinerator capacity and reduce energy consumption. Therefore, the flowmeter must control the air flow from about 30 to more than 500 flow units, so it would be appropriate for the flowmeter to measure from approximately 15 to 700 flow units, effectively resulting in a 45-to-1 turndown.
And by the way, it is possible for the fumes to flow backward through the combustion air fan to the atmosphere under certain conditions. This is not good, so the flowmeter must measure zero flow under reverse flow conditions (or utilize a separate flow switch that forces the flowmeter output to zero when reverse flow occurs).
We will select a flowmeter for this application next month.
This article originally appeared in Flow Control magazine (June 2017) at www.flowcontrolnetwork.com.
Flowmeters for Air Flow
Flowmeters for Air Flow
By David W Spitzer
Which of the following flowmeter technologies can be applied to measure the flow of air in a round pipe?
- Coriolis mass
- Differential pressure
- Magnetic
- Thermal
- Vortex shedding
Commentary
Magnetic flowmeters only measure the flow of liquids, so Answer C is not correct. The remaining technologies can be applied to measure the flow of air. Thermal flowmeters and some differential pressure flowmeters can measure the flow of air under most commonly encountered operating conditions. Coriolis mass and vortex shedding flowmeters can also measure the flow of air under some operating conditions. Answers A, B, D and E are correct.
Additional Complicating Factors
Vortex shedding flowmeters turn off when the Reynolds number and air velocity are below their respective operating constraints. In addition, vortex flowmeters will turn off when the air density is not sufficiently high to operate the sensing system. The pressure drop across Coriolis mass flowmeters in air service can become excessive unless the air pressure is sufficiently high.
Just because a flowmeter technology can measure the flow of air does not mean that the technology will measure the flow of air.
This article originally appeared in Flow Control magazine (June 2017) at www.flowcontrolnetwork.com.
July 2018
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Discrepancies)
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Discrepancies)
By Walt Boyes
At a flow lab, they run a known quantity of fluid (almost always water) through the metering element, and compare the reading of the meter with the actual throughput of the lab. The difference, if any, is the inaccuracy of the meter. Adjusting the meter’s calibration factor (sometimes known as “k-factor”) to account for this inaccuracy is the process of calibrating the meter.
The first thing people do is suspect that the flowmeter isn’t working. If you’ve already established that it is functioning mechanically and electronically, assume that it is reading correctly for the time being. You should assume this because it is very likely that the meter is working, and the results that you are expecting are not what you really have.
One of the first things I ask, when somebody brings me this sort of problem, is, “How do you know it isn’t reading correctly?” Often, the answer I get is, “We should have X flow rate, and the meter is reading higher (or lower) than that.” Ah, but how do you know you should have X flow rate? The answers break down to a few major categories:
- Because that’s what we’ve always had.
- The pump curves out of the book say that is what we should have.
- The engineer told me we should have that flow rate.
- I did a drawdown test, and we actually have another flow rate.
Notice here that only one of the response categories has real data in it. You can’t tell if a flowmeter is working poorly based on hearsay, or even historical record by itself. You can’t tell if a flowmeter is working poorly based on the design engineer’s statements. You can’t tell if a flowmeter is working poorly by looking at the flow with your Mark I Eyeball. You can suspect that something is wrong, if, for example, you know you should have a flow from a pump of about 350 gallons per minute (gpm), and the flowmeter says 1,100 gpm. You can suspect something is wrong if the flowmeter reads 400 gpm and the pump is off. You can suspect something is wrong if the reading on the flowmeter can’t pass the test of reasonableness. In other words, could the flowmeter be right? If it is possible that the flowmeter is right, it probably is.
More next month.
This article originally appeared in Flow Control magazine (July 2002) at www.flowcontrolnetwork.com.
Incinerator Combustion Air Flowmeter: Choosing the Right Technology
Incinerator Combustion Air Flowmeter: Choosing the Right Technology
By David W Spitzer
The physical properties, operating conditions and operating constraints of a combustion air flowmeter for an incinerator were discussed last month. Combustion air flow measurements were required for flows of 15 to 700 units and the flowmeter had to measure zero flow under reverse flow conditions lest the fumes overpower the combustion air fan and escape to the atmosphere.
Pipe size (24-inch) precluded the application of many flowmeter technologies in this application. Most full-bore flowmeters could not be used because of size and/or cost. Various types of insertion flowmeters could be applied. Full-bore and insertion differential pressure flowmeters might also be applicable.
Turndown flow requirements eliminate even more flowmeter technologies. For example, the effective 45-to-1 flow measurement turndown with a differential pressure flowmeter would result in a differential pressure turndown of 2025-to-1. Differential pressure transmitters can exhibit somewhat reasonable accuracy over approximately a 5-to-1 flow measurement turndown. Stated differently, the differential pressure generated at low combustion air flow rates would be too low for an industrial differential pressure transmitter to measure accurately.
Reverse flow requirements further eliminate flowmeter technologies. For example, an insertion vortex flowmeter could not be used because it registers reverse flow as if it were forward flow. Therefore, a flow switch would be needed to force the transmitter output to zero flow under reverse flow conditions.
In the end, an insertion turbine flowmeter with a quadrature amplifier (to sense reverse flow and force the output to zero) met all of the requirements for this application and enabled the incinerator to operate more efficiently for years. In addition, accuracy was reasonable because flowmeter performance was expressed as a percentage of flow rate (not full scale) over the applicable range.
In summary, selecting a flowmeter to measure a common fluid (air) operating at nonextreme conditions (atmospheric pressure and ambient temperature) can become a challenge because of the constraints of the process in which the flowmeter must operate.
This article originally appeared in Flow Control magazine (July 2017) at www.flowcontrolnetwork.com.
Sequencing Valves to Remove a Differential Pressure Transmitter
Sequencing Valves to Remove a Differential Pressure Transmitter
By David W Spitzer
An orifice plate flow element has shut-off valves located at each pressure tap. Its differential pressure transmitter has a three-valve manifold. The transmitter is to be removed for shop calibration. In what sequence should the valves be opened and closed to safely remove the transmitter and manifold?
- Close upstream and then downstream shut-off valves
- Close downstream and then upstream shut-off valves
- Close high and then low manifold valves then close shut-off valves
- Close low and then high manifold valves then close shut-off valves
- Open manifold bypass valve then close shut-off valves
Commentary
First, closing one of the shut-off valves or the low or high three-valve manifold valve will isolate one side of the transmitter while the other side is still connected to the process. If the process pressure changes when one of these valves is closed, a potentially large differential pressure can be created across the transmitter that can damage the transmitter, especially if there is a process upset that causes the line pressure to change dramatically (on only one side of the differential pressure diaphragm).
To avoid this possibility, it would be prudent to protect the transmitter by applying a “hydraulic jumper” across the transmitter by opening the manifold bypass valve. This will cause some fluid to flow from the high tap to the low tap and generate a relatively small differential pressure across the transmitter that can be tolerated.
Additional Complicating Factors
Opening the bypass first in high-pressure steam service will cause live steam to remove the liquid seal and flow through the transmitter. Not only can this damage the transmitter (because of the presence of live steam in the transmitter), but it also creates a potential hazard for the instrument technician. In this service, Answers A, B, C or D would be safer alternatives but Answers C and D would usually be even safer and more practical.
This article originally appeared in Flow Control magazine (July 2017) at www.flowcontrolnetwork.com.
August 2018
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Pump Curves)
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Pump Curves)
By Walt Boyes
One of the things people often do is compare the flowmeter’s reading to expected flows based on generic pump curves. This almost always gets people into trouble. There are some really good reasons for this. First, the pump curves are “generic,” that is, they are published for a whole family of pumps, not a specific pump installed in your installation. Your pump should perform fairly closely to the curve, the day it is installed. “Fairly close” of course needs to take into account problems with upstream and downstream piping, air entrainment and similar things that the pump curves do not take into account. After the pump has been in service for a while, it is likely that wear on the pump internals has changed the flow characteristics of the pump considerably. Usually, this means that the pump’s actual performance will vary from the generic pump curve quite a bit. So, if you call a flowmeter manufacturer’s Help Desk and tell them that you haven’t pulled maintenance on the pump for five years, and the flowmeter isn’t reading what the pump curves say, the manufacturer is going to tell you that the problem is in the pump. And they are probably right.
More next month.
This article originally appeared in Flow Control magazine (July 2002) at www.flowcontrolnetwork.com.
Incinerator Combustion Air Flowmeter: Calibrating the Instrument
Incinerator Combustion Air Flowmeter: Calibrating the Instrument
By David W Spitzer
The selection of an insertion turbine flowmeter to measure the combustion air flow in an incinerator was discussed previously. Other flow technologies might have been preferable from a maintenance perspective because we did not have any similar turbine flowmeters in the plant. However, the insertion turbine flowmeter was the only flowmeter found that would meet the requirements of the application. Therefore, once installed, we had to maintain this instrument and a similar flowmeter used to measure the natural gas flow to the incinerator.
Theoretically, calibrating the flowmeter involves verifying the performance of the flow element (turbine) and transmitter. Calibrating the transmitter is relatively straightforward. However, verifying the performance of the turbine is not easy because the flowmeter measurement should be compared to the passage of a known amount of fluid. We did not have a gas calibration facility to perform these calibrations, nor were we willing to incur the cost associated with sending them out for calibration.
Recognizing that turbines tend to slow down before they stop working, a surrogate method to check the operation of the turbine was to blow on the turbine and observe how quickly it stopped spinning. A new turbine will spin for quite some time, while a worn turbine will stop relatively quickly.
By experience, we found that the turbine in the combustion air service would slow down somewhat after six months of operation and require replacement after one year. Interestingly, the turbine in natural gas service spun almost like new after seven years of operation!
In these applications, the calibration interval was not so much dependent upon the measurement instrument itself but rather on the service in which it was installed.
This article originally appeared in Flow Control magazine (August 2017) at www.flowcontrolnetwork.com.
Estimate the Annual Cost of a Raw Material
Estimate the Annual Cost of a Raw Material
By David W Spitzer
What is the approximate annual cost of a raw material purchased for $2 per liter flows into a chemical process at 10 liters per minute?
- $1 million
- $2.5 million
- $5 million
- $7.5 million
- $10 million
Commentary
The annual cost of material valued at $1 per unit flowing at one unit per minute can be calculated as follows:
($1/unit) * (1 unit/minute) * (60 minute/hour) * (24 hours/day) * (365 days/year)
The calculated value is $525,600 per year, but this can be approximated as $500,000 per year and used as a rule of thumb.
Therefore, the approximate annual cost of this raw material is Answer E: $10 million (2 * 10 * $500,000).
Additional Complicating Factors
The value of the flowing material should be considered when selecting a flowmeter because more accurate measurement can often enable tighter control. In some applications, this can enable the plant to consume a lower amount of the raw material, making the process more efficient.
In general, measuring materials with large economic value can often justify the purchase of more accurate (and often more expensive) flowmeters.
This article originally appeared in Flow Control magazine (August 2017) at www.flowcontrolnetwork.com.
September 2018
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Checking Accuracy)
Flowmeter Quirks: Why Is My Flowmeter Not Reading Right? (Checking Accuracy)
By Walt Boyes
There are a few ways to see for certain if a flowmeter is working correctly in an installation. The first is to do a volumetric drawdown test. You must capture all of the flow through the meter and direct it to a tank, or a tank truck, or some other means of capturing a sizeable quantity of flow. Best, if possible, is to use a tank truck, and take it to a scale. Weigh the truck empty for tare, then weigh it full of fluid. Many people can’t do this test. It isn’t easy to do. If you have a sump, clearwell, wetwell or stationary tank that you know the volume of very accurately, you can do the same thing by level and calculator. If the value on the flowmeter reads within about five percent of the value you obtain, the flowmeter is working right.
The other way to check the accuracy of a flowmeter in an installation is to use another flowmeter installed as close as possible to the original. I recommend using a transit-time ultrasonic flowmeter where possible, because it is a non-invasive measurement method, and it is generally quite accurate. Sometimes other means are required, like a multiple port Pitot tube type meter, or other device. Remember, though, that this is not a “field calibration” method, this is a reality check. If the two devices agree within about five to 10 percent of reading, the original meter is probably correct.
If you’ve gone this far, you need to start looking at why you don’t have the flow you think you have. The flowmeter is sending you a message.
This article originally appeared in Flow Control magazine (July 2002) at www.flowcontrolnetwork.com.
Viscous Flow Measurement
Viscous Flow Measurement
By David W Spitzer
If you attend one of my flow measurement seminars, you will hear me say various times that the viscosity of a fluid is the ability of the fluid to flow over itself. Water has a viscosity of approximately 1 centipoise and flows easily over itself. More viscous fluids such as honey have higher viscosities. The actual viscosity of the fluid is dependent upon its temperature — where viscosity decreases (often dramatically) with higher temperature. Comparing the ability of cold and hot honey to flow illustrates this relationship.
The viscosity of Newtonian fluids is constant regardless of how the fluid is stressed. Conversely, non-Newtonian fluids exhibit different viscosities depending on how the fluid is stressed. Stated differently, these fluids may not flow under certain stress conditions but will flow under these same conditions when stressed differently.
For example, ketchup will not flow from its bottle until it is stressed (and then it may flow excessively). Similarly, some slurries can become more viscous when pumped at higher pressures. This can result in a strange phenomenon whereby providing additional pumping pressure can actually reduce the flow rate because the higher apparent fluid viscosity increases the pressure drop in the downstream piping system.
Most flowmeters are affected in some way by viscosity such as mechanically via slippage, hydraulically by affecting the pressure drop across the flowmeter or indirectly via Reynolds number. For the purposes of flow measurement, it is generally assumed that the fluid at hand is Newtonian even if it is not. This is because there are many unknowns with regard to the apparent fluid viscosity and its effect on flowmeter operation, so a nominal viscosity is selected for selection and sizing.
In summary, sometimes it is pragmatic to bury one’s head in the sand and assume a constant viscosity. However, it may be more practical to apply a flowmeter that is relatively unaffected by viscosity in these applications.
This article originally appeared in Flow Control magazine (September 2017) at www.flowcontrolnetwork.com.
Approximate the Viscosity of Honey
Approximate the Viscosity of Honey
By David W Spitzer
What is the approximate viscosity of honey in centipoise (cP)?
- 1 cP
- 10 cP
- 100 cP
- 1000 cP
- 10,000 cP
Commentary
A quick internet search shows that the viscosity of honey is 10,000 cP at a room temperature of 21.1ºC. More research reveals that blended flower honey has a viscosity of approximately 12,200 cP at the same temperature. The first viscosity appears to be a generic estimate whereas the second refers to a specific commercially available product.
Answer E appears to be correct.
Additional Complicating Factors
Not so fast! Viscosity can be described as the ability of a fluid to flow over itself, and it can be highly dependent on temperature. Honey in a jar will flow at room temperature, albeit slowly. However, the ability of the honey to flow after it is cooled in a refrigerator is greatly diminished. Conversely heating the honey to (say) 50ºC will greatly reduce its viscosity and improve its ability to flow. Higher water content will similarly reduce the viscosity of honey. Answer D may be possible in some applications.
Therefore, when someone asks for or cites the viscosity of a fluid, it is prudent to ask for the composition and temperature to which it applies.
This article originally appeared in Flow Control magazine (September 2017) at www.flowcontrolnetwork.com.
October 2018
Getting the Best from a Magnetic Flowmeter
Getting the Best from a Magnetic Flowmeter
By Walt Boyes
Magnetic flowmeters are widely used flow measurement devices for many kinds of conductive liquids. There are simple reasons for this. First, magnetic flowmeters are inherently extremely accurate. They measure the average of all of the velocities traveling through the pipe, from wall to wall, directly as a function of Faraday’s Law. Second, magnetic flowmeters are obstructionless, and have about the same pressure loss as an equivalent length of pipe. Third, they are remarkably maintenance free, even in cases of high corrosion or abrasion. Fourth, they can measure a wide range of fluids, from clean water to very dense slurries.
If I were going to build a plant somewhere in the world where rapid delivery of parts and replacement units was going to be a problem, magnetic flowmeters would definitely be in my arsenal. If I had to pick two liquid flowmeter types, and only two, to use at that remote location, magnetic flowmeters would be one choice, and I would be very torn about what the other type would be.
Of course, I am referring to full-bore magnetic flowmeters, whether of the spool-piece design or the wafer design. Insertion magnetic flowmeters of the typical design, with a single point-sensor, have about the same accuracy and repeatability as an insertion paddlewheel meter. Of course, there are always exceptions to the rule because there are a few manufacturers who produce profiling insertion magnetic flowmeters, and they can be quite accurate, even coming close to full-bore meters in performance in good hydraulic conditions.
More next month.
This article originally appeared in Flow Control magazine (June 2002) at www.flowcontrolnetwork.com.
Control Valve Plugging
Control Valve Plugging
By David W Spitzer
Often it helps to walk around and observe people at work. I once observed an operator get up, walk around his desk, put a controller in manual, open the control valve fully for about 30 seconds and then return it to its previous position before putting the controller back into automatic. This was so strange that I asked why he did it.
The operator explained that impurities in the raw material tend to plug that valve over time and cause problems. In response, the operators open the valve fully for a few seconds every shift to clear the valve of the impurities and reduce the chance of plugging. This solution apparently worked quite well.
However, this is a relatively poor solution from an engineer’s perspective because the operator is repeatedly distracted from the process by a detail that adds little value to the operation. A better solution would be to install a valve that did not accumulate the impurities, mitigating the need for this activity.
Investigation revealed that the control valve was a globe valve. It was theorized that the impurities collected within the valve until it was fully opened (manually), at which time they would flow downstream. A segmented ball control valve was deemed to be superior in this application because if buildup occurred, the ball valve would automatically open a bit more and allow the impurities to flow downstream.
This may sound like the ending to a fairy tale, but after a segmented ball valve was installed, the control loop operated happily ever after.
This article originally appeared in Flow Control magazine (October 2017) at www.flowcontrolnetwork.com.
Flowmeter Accuracy Statement
Flowmeter Accuracy Statement
By David W Spitzer
A flowmeter is calibrated to its full scale of 200 flow units but is capable of up to 400 flow units. The flowmeter can measure a flow of 100 flow units within ±1 unit and can measure 200 flow units within ±2 units. What is the accuracy statement associated with this flowmeter?
- ± 1 percent of rate
- ± 1 percent of full scale
- ± 1 percent of calibrated span
- ± 1 percent of meter capacity
Commentary
The full scale and calibrated span are both 200 flow units. The accuracy at 100 flow units expressed as a percentage of these parameters would be ±1/200 or 0.5 percent of full scale or calibrated span. Answers B and C are not correct.
At 100 flow units, the accuracy expressed as a percentage of meter capacity (400 flow units) would be ±1/400 or 0.25 percent of meter capacity. Answer D is not correct.
Similarly, the accuracy expressed as a percentage of the flow rates (100 and 200 flow units) is ±1/100 (and ±2/200) or 1 percent of rate which confirms that Answer A is correct.
Additional Complicating Factors
Note that “±1 percent” is not a complete specification. Be sure to ask, “Percentage of what?”
This article originally appeared in Flow Control magazine (October 2017) at www.flowcontrolnetwork.com.
November 2018
Getting the Best from a Magnetic Flowmeter (General Guidelines – Part 1)
Getting the Best from a Magnetic Flowmeter (General Guidelines – Part 1)
By Walt Boyes
Nevertheless, like any other flowmeter, full-bore magnetic flowmeters aren’t magical, and there are quite a few ways to install and operate one that will give you the worst results. Here are some don’ts for installing and operating magnetic flowmeters. These are general guidelines and one should consult with the manufacturer to discuss the particular operating parameters of a given meter. In other words, these aren’t written in stone — talk to your vendor!
- Don’t use a magnetic flowmeter on a non-conductive fluid. The minimum conductivity is generally between 5 and 20 µS. You should not approach those minimum values for normal use. Give yourself a safety factor. If you are measuring deionized water at about 6 µS, you might want to look at using another type of meter, like a transit-time ultrasonic meter or a turbine meter. This is very important since the minimum conductivity requirement can increase with the distance between the primary (the flow tube) and the converter (the transmitter). So, if you are building a water treatment plant, and you want to use a magnetic flowmeter on a deionized water line, don’t run the cable 300 feet to where it is convenient to install the transmitter.
- Don’t use a magnetic flowmeter on fluids with a lot of entrained air. Air or other gases may collect in the meter, especially if it is installed at a high point in the line (such as an inverted U-tube). While you may think you have forced the meter to remain full, there is likely to be a sizeable void caused by air or other gases coming out of solution and staying in the meter body. This will seriously impair the accuracy of the meter.
- Don’t use a magnetic flowmeter on a self-dewatering slurry. Some slurries, such as calcium carbonate or lime, have the property that at relatively high densities (three to four g/cc slurry gravity) they will dewater, that is, the solids and the liquid will separate. The velocities of the solids and the liquids can become quite different, and since the liquid is conductive and the solids are not, the meter will be in substantial error.
More next month.
This article originally appeared in Flow Control magazine (June 2002) at www.flowcontrolnetwork.com.
Pumping System Efficiency
Pumping System Efficiency
By David W Spitzer
The overall efficiency of a pump system can be defined as the ratio of the hydraulic output of the pump to the electrical energy input to its motor. It can be used to help identify opportunities for potential energy savings, process improvement and maintenance planning.
Overall pump efficiency can be calculated using measurements of motor power, pump inlet pressure, pump outlet pressure and pump flow. These measurements are usually not available in most plants and adding them permanently can be expensive. However, it is relatively easy to measure the motor power, pump inlet pressure and pump outlet pressure on a temporary basis by connecting electrical instruments and installing pressure measurement instruments on important pumps.
Installing flow measurement instruments can be more complicated to implement because fluid properties, operating conditions and materials of construction need to be considered for each desired measurement. Using a portable ultrasonic flowmeter is a viable option for some applications. However, these flowmeters have limitations and can produce inaccurate measurements at times.
Installing a full-bore flowmeter can be expensive, although the benefit of doing so sometimes outweighs the costs involved. Pumping configurations often consist of one operating pump that pumps a fluid from one location to one other location, sometimes by throttling a control valve to control, for example, the level in a tank. In these applications, installing a variable speed drive to operate the pump and eliminate the control valve can often generate sufficient energy savings to pay for a new flowmeter that can be used in a cascade control strategy to stabilize operation in addition to being used to calculate the efficiency of the pump system.
It is quite common to take an indirect approach to justify improvements.
This article originally appeared in Flow Control magazine (November 2017) at www.flowcontrolnetwork.com.
Pump System Efficiency
Pump System Efficiency
By David W Spitzer
What is the efficiency of a pump system that pumps 100 gallons per minute of water at a pressure of 100 feet of water column using a 7.5 horsepower motor?
Commentary
This question is woefully deficient of information needed to calculate pump efficiency. Nonetheless, we can examine different approaches to reaching a solution.
A quick way to estimate pumping efficiency is to confirm the pump is operating on its pump curve, read the efficiency of the pump at this operating condition from the pump curve, and then multiply it by an estimated motor efficiency. Note that the pump curve (if available) reflects a new pump which may not be an accurate representation of an existing pump that may be worn.
The the pump system efficiency is the ratio of the hydraulic energy produced to the electrical energy consumed. The hydraulic energy produced can be calculated as:
100 gpm * 100 feet * 1.00 specific gravity / 3960 = 2.53 horsepower
The electrical energy consumed at this operating condition can be measured with a portable instrument that measures the electrical energy consumed by the motor. The efficiency of the pump system can then be calculated.
Additional Complicating Factors
The hydraulic energy calculation assumes that the pump suction pressure is atmospheric. Taking the static head of the water in the tank (above the pump suction) and any pressure that may be in the tank above the water will complicate this calculation.
This article originally appeared in Flow Control magazine (November 2017) at www.flowcontrolnetwork.com.
December 2018
Getting the Best from a Magnetic Flowmeter (General Guidelines – Part 2)
Getting the Best from a Magnetic Flowmeter (General Guidelines – Part 2)
By Walt Boyes
Continuing the list of guidelines…
- Don’t use a garden-variety magnetic flowmeter in very rapid batching operations. Because of the time necessary to make a reading, the average magnetic flowmeter may not be fast enough to handle the step-changes in flow of a batch and fill operation. There are some magnetic flowmeters on the market that have faster response times than others, so check the specifications to see if one will work in your batching operation.
- Don’t use a magnetic flowmeter that is very much oversized for the flow rate. Very often, because of the exigencies of bureaucracy, we are forced to design plants for expansion, and run them under current conditions. This is especially true in the water and wastewater industries. However, any flowmeter’s performance degrades as you approach the lower end of its measuring range, and magnetic flowmeters are no different. If you must use an oversized meter, be sure that the pipe will remain full, that air or other gases will not become entrapped and check your supplier’s specifications because there is considerable variation in performance at the low end between magnetic flowmeter models.
- Don’t use a magnetic flowmeter on a spiraling flow. Passing the fluid through two 90o elbows at right angles to each other can easily create these flows. Since the magnetic flowmeter’s raw signal output is the sum of all of the velocity vectors in the magnetic field, and in a spiraling flow some of those velocity vectors have a negative sign, the output can be in error by as much as 40 percent. Magnetic flowmeters actually need very little straight run. A common specification is three to five diameters upstream of the electrode plane. Since the electrode plane is near the horizontal center of the meter body, if the meter body is long enough, the required straight run may actually be inside the meter body.
The reductions in the cost of full-bore meters have made it possible to use them competitively with mechanical meters in water distribution and in industrial water treatment, as well as in wastewater treatment, mining, dredging, and the chemical industry. They are even used in agricultural irrigation.
This article originally appeared in Flow Control magazine (June 2002) at www.flowcontrolnetwork.com.
Audit Flowmeters To Be Sure You Get What You Pay For
Audit Flowmeters To Be Sure You Get What You Pay For
By David W Spitzer
Sometimes it makes sense to make sure you get what you pay for, especially when large quantities of material and/or energy change hands. The problem is that by the time flowmeters (aka, the cash register) are rigorously audited, the various parties are often locked in a legal battle that makes resolving the technical problems much more difficult. Let me give you a couple instances of how companies got into this position with the understanding that I cannot divulge information about specific legal cases.
A new sewage district contracted to have an existing water treatment plant accept its sewage. Residents in the new district noticed their sewage bills were much larger than other residents already served by the water treatment plant. The problem was traced to expensive surcharges for high flows during wet weather events. Flowmeter performance was not seriously considered during the initial legal battle and the district was ordered to pay the invoices with the surcharges. By the time I became involved, residents in the new district did not even shop in the town where the sewage treatment plant was located. My audit revealed significant flow measurement and calibration issues that were painfully resolved over time at considerable ongoing expense. These issues likely would have been resolved quickly and more economically had I been involved prior to or during the original legal proceeding.
In another case, two companies were locked in a multimillion-dollar legal battle when (as part of its strategy) one company claimed the steam flowmeters used for billing purposes were not accurate. The primary reasons for this claim were that the flowmeters were approximately 50 years old and had not been calibrated for about seven years. The case was settled shortly after I audited the steam flowmeters and their measurements in conjunction with the steam generation process — finding them to be surprisingly accurate despite their age and calibration history.
Repeating my opening statement, sometimes it makes sense to make sure you are getting what you pay for.
This article originally appeared in Flow Control magazine (December 2017) at www.flowcontrolnetwork.com.
Flowmeters for High Viscosity Liquids
Flowmeters for High Viscosity Liquids
By David W Spitzer
Which of the following flowmeters can be applied to high-viscosity liquids?
- Coriolis mass
- Differential pressure
- Magnetic
- Positive displacement
- Vortex shedding
Commentary
The question is vague in the sense that “high viscosity” can mean different things to different people in different industries. All of these technologies are potentially applicable when dealing with a liquid viscosity of 20 centipoise (cP) (which could be termed “high” in some industries).
Additional Complicating Factors
Defining “high viscosity” as more than (say) 1000 cP reveals an entirely different set of considerations. Positive displacement flowmeters that continually entrap liquid are often used in these applications because they exhibit low slippage (unmeasured liquid) at high viscosity. However, they are mechanical and tend to exhibit higher pressure drop with increasing viscosity.
Coriolis mass flowmeters can measure high-viscosity liquids; However, performance can sometimes be compromised if the pressure drop across the flowmeter limits accurate flow measurement to the lower part of the flow range.
Magnetic flowmeters can also measure electrically conductive liquids that exhibit high viscosity. As a practical matter, magnetic flowmeters are not often applied to high-viscosity liquids because the majority of such applications involve hydrocarbons that are not sufficiently conductive.
Differential pressure and vortex shedding flowmeters are generally not applied to high viscosity liquids because of their Reynolds number constraints.
This article originally appeared in Flow Control magazine (December 2017) at www.flowcontrolnetwork.com.