Flow rates and flow totals are two of the key types of measurements used in process facilities around the world. No matter the process industry, there will always be a wide variety of flow measurements that must be made for monitoring, control, and regulatory purposes. However, the appropriate selection and sizing of flowmeters are not something we can always take for granted.
Like any piece of equipment, instrumentation must be carefully selected so the measurements will be as accurate and repeatable as possible. Flowmeters are no exception. In industries where low-bid design and construction services are commonly used, it is even more important that the main aspects of properly sizing flowmeters are taken into account. In this article, we explain key considerations of sizing a flowmeter from the perspectives of accuracy, repeatability, and rangeability, as well as common installation pitfalls to avoid.
The first aspect to think about when sizing a flowmeter is the expected flow range. Whether a flow rate is expected to be a fairly consistent narrow range, or the rate is expected to vary widely has a substantial effect on how a flowmeter should be sized.
In general, when sizing a flowmeter, conditions to consider include:
A way to do this is by estimating a flow envelope using a table such as the sample shown in table 1.
After the flow envelope has been determined for a flowmeter, the next aspect to consider is how well a proposed flowmeter can measure the flow. “Accuracy” refers to how close the flowmeter reading is to the actual flow value. “Repeatability” (also known as precision) is how consistent readings are over time. Accuracy and repeatability are not the same. The difference between accuracy and repeatability is shown in figure 1.
Another way to understand the difference between accuracy and repeatability is to imagine two archers shooting at a conventional archery target. Suppose one archer hit the bull’s-eye consistently. Because he was always accurate, the shots were repeatable. Now imagine an archer who hit the target but consistently missed the bull’s-eye. Although the archer had good repeatability, he was not accurate. Good repeatability does not guarantee accuracy. If you do not see a proper accuracy statement on equipment, (i.e., there is only a repeatability statement), be cautious.
A flowmeter with good repeatability (but poor accuracy) can be adjusted to read more accurately. However, a flowmeter with poor repeatability cannot be adjusted, because there is no consistency in the readings.
“Rangeability” is a measure of how accurate and how repeatable the flowmeter will be over the expected range of flow readings. Some flowmeters are better at turndown (being able to read very low flow rates) than others. Thus, whenever an accuracy claim is being made for a flowmeter, it is important to look at the expected flow range accuracy that the vendor is guaranteeing.
Before looking at some of the more common flowmeter selection/sizing pitfalls, here is a more detailed discussion about defining accuracy and repeatability.
It pays to read the fine print when it comes to accuracy claims (or specifications) associated with instrumentation such as flowmeters. At lower flow rates, the accuracy often drops off significantly. For example, if an instrument has an accuracy claim of 0.5 percent of full scale, it is important to recognize that the actual accuracy diminishes as the operating conditions fall below the full-scale setting.
Another way of stating accuracy is to define it in terms of the reading, such as ±0.5 percent of the reading over a certain part of the flowmeter’s range. Depending on the range in which the flowmeter is used, this stated accuracy could be either negligible or a significant difference. On flowmeters used for billing or other revenue-related purposes, the meter’s accuracy can have a major financial impact.
Imagine that a paddle-wheel flowmeter claims to have an accuracy of ±0.5 percent. Suppose further that it is a percent of the full range, and the full range is 50 feet per second (ft/s). If the flow range where you will use it is 6 ft/s, which is common in wastewater treatment plants, the actual accuracy is much different than you might expect:
0.005 × 50 f/s = ±0.25 ft/s
If you apply this accuracy against a flow rate of 6 ft/s, the actual accuracy is:
±0.25 / 6 ft/s = ±0., or 4.17%
Comparing a magnetic flowmeter with an accuracy of 0.5 percent of reading to a doppler flowmeter with an accuracy of 0.5 percent of full range yields a similar result.
A common problem occurs when a city or municipality uses two different types of flowmeters. Imagine one meter is a highly accurate magnetic flowmeter located in a meter vault to monitor the plant’s effluent flow, and the other is a doppler meter monitoring the influent flow. Doppler flowmeters tend to decline in accuracy as the flow rate drops. Even highly accurate magnetic flowmeters have both extremely high and low reading limits under which they will not operate accurately.
Case histories have shown that the plant appears to be generating wastewater, because the effluent is more than the influent, or something is evaporating the wastewater. We know in both cases that neither of these conditions really exists. What is happening is that the doppler meter is not matching the accuracy of the magnetic meter. The difference between 0.5 percent of 12 million gallons a day (Mgd) and 4.17 percent of 12 Mgd is substantial.
(. – 0.005) × 12 Mgd = 0.44 Mgd, or 305 gal/min
Matters are made even worse if the doppler meter is used for pacing chemical feed into the wastewater with the same inaccuracies; the results are either overdosing or underdosing. Water treatment plants have low, average daily, and high peak demand flows, and further, low and average daily flows occur more frequently. This demonstrates the importance of being cautious in choosing meter types for those flow variables.
Many types of flowmeters suffer in performance as the flows decrease and approach the lower end of their viable flow range. Therefore, pacing during low flow periods may be highly suspect. Chemicals are becoming more costly, analytical instruments for measuring the effects of these chemicals are becoming costly, and corrosion due to underdosing or overdosing wastewater can also be costly to equipment. All of these may contribute to effluent that is a danger to wildlife and, in extended cases, can be harmful to the health of people living in the area.
In many ways, repeatability is even more important than accuracy. If an instrument is consistently wrong (inaccurate, but repeatable), the instrument can be adjusted to read correctly. However, if an instrument is inconsistent with how it reads, no amount of calibration work can fix the poor readings it provides.
Today, many field instruments work on force-balance techniques (where a process reading is converted to a force that then impacts a force-based sensor) such as piezoelectric crystals, capacitance, and strain gauges. These work on the principle that if you put a force on an instrument, there should be no motion even though an electric signal is generated on the output of that instrument. There are still flow, level, and chemical measuring devices that do not work on the force-balance principle, and for these types, looking at the repeatability of that piece of equipment is still important. A steady widening of the repeatability is an indication that something is going wrong with the instrument.
While the accuracy of an instrument can be improved with calibration, repeatability is often something that the design of the instrument defines.
As previously noted, the rangeability of an instrument must be taken into consideration during the sizing and selection part of a plant design. It is important that installed flowmeters can read the various intended flow ranges specific to where they are installed. At a minimum, they must meet the needed accuracy/repeatability for each flow rate for the application.
One of the most common problems with a piece of instrumentation equipment is the exaggeration of its range. How many times have you heard that a meter can read flow rates at velocities of 1 to 100 ft/s, giving the impression that you can read flows accurately through that total velocity range?
What often goes unmentioned is that the particular meter’s accuracy has a 10:1 turndown ratio. This means that a meter sized to measure a range of 0 to 30 Mgd has a true accuracy over the full range of 3 to 30 Mgd. Below 3 Mgd, the meter accuracy diminishes.
Additionally, different types of meters have different turndown ratios over their full range. It is common for a Venturi tube, for example, to have two transmitters measuring the flow. This is because a Venturi tube with one transmitter measures accurately with a 6:1 turndown ratio over the full range. So, if we look at a range of 0 to 30 Mgd, the meter’s accuracy diminishes below 5 Mgd.
The range over which the instrument meets the stated linearity of uncertainty requirements is its “rangeability.” “Uncertainty” is the range of values within which the true values lie with a specified probability. Uncertainty of ±1 percent at 95 percent confidence means the instrument will give the user a range of ±1 percent for 95 readings out of 100.
Another common error occurs during equipment sizing. In the municipal wastewater sector, it is a common practice to assume that solids in wastewater will settle out around a velocity of 2 ft/s. A magnetic flowmeter reads accurately if the minimum velocity is above 2 ft/s, but below this, settling is likely to occur—and then who can say what the accuracy really is?
Typically, designers size plants to handle increased flow capacities for 20 years. For this reason, designers often oversize pipes for early lifecycle flow, and there is a corresponding settlement inside the pipe. This settling can also occur in the inner liner of the meters. Because these meters are velocity-sensing devices with an assumed constant cross section, they will give a false reading if the inner liner becomes coated with sludge.
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A solution may be to reduce the size of the meter to increase velocity by using a pipe reducer on the inlet side and a pipe expansion section on the discharge side of the meter. If possible, avoid connecting the reducer and expander directly to the meter. Manufacturers recommend that when you reduce the pipe, the flowmeter should be placed a minimum of six to 10 pipe diameters upstream from an elbow or valve and at least two pipe diameters downstream of a pipe elbow or valve. This provides a less distorted flow profile for the meter to read.
Be certain you can afford to lose the pressure head when you reduce the meter size. Maximum velocities should not exceed 15 ft/s. By maintaining a minimum scouring effect inside the pipe, your sludge buildup inside pipes and any inline equipment will diminish, helping avoid measurement errors and costly maintenance downtime.
Some people ask for the accuracy of a certain flowmeter, level, or pressure-measuring device and, upon hearing a low number, think that everything involved with the flowmeter will be of the same accuracy. However, the meter accuracy is not the accuracy for the entire flow system. A mathematical equation known as the root mean square (RMS) correctly determines the accuracy of the complete system. Consider the case of a magnetic flowmeter that records flow locally, sending an analog signal to an operator’s workstation via a programmable logic controller (PLC).
One must look at each component’s accuracy:
Each component in the system has its own measurement errors and uncertainties that contribute to the overall accuracy of the complete system. In real cases, there could be more components attached to a control system.
To use the RMS method, first square each number, yielding 0., 0., 0., 0., and 0.. Second, add the numbers. Then find the square root of the sum. The entire system has an accuracy of approximately ±0., or ±0.813 percent instead of 0.5 percent. This accuracy equation works for any individual chemical, pressure, level, temperature, or flow loop.
Remember that no two flow measuring applications are the same. Each has its own unique flow characteristic, range, and accuracy requirements. It is always important to review the expected operating envelope to determine the normal and extended operating ranges to be measured before sizing any flowmeter.
Once the requirements are known, any accuracy, repeatability, or rangeability specifications for possible flowmeter products should always be carefully reviewed. Two nearly identical flowmeter products can have similar sounding accuracy and different details about how the accuracy is measured.
Regardless of the application, it is always recommended to carefully review the requirements and manufacturer literature regarding accuracy and to consider the range, repeatability, turndown ratio, installation requirements, and other aspects. These details make the difference between a well-specified flowmeter and a measurement system that has issues.
Also, depending on the application, it may be worthwhile to do more advanced simulations, to perform RMS error analysis, or to look at installed examples. Lastly, do not be afraid to make use of application engineering services offered by most leading flowmeter vendors.
Like many things in life, it is worthwhile to do some homework when it comes to sizing and selecting flowmeters. It always pays to do the background work up front to ensure that a flowmeter is able to measure the flow and meet the needs of the application.
By Marc Buttler
When considering new technology, there are many data points to consider before making a substantial investment. One of the most important factors to consider when looking at upgrading flow measurement tools is the calibration and measurement traceability of that equipment.
The calibration of the meter is just as important to your confidence in the measurements as the design, skill, and quality of materials used in manufacturing it. The calibration-related capabilities, accreditation, and experience of the manufacturer will often influence whether you can accept the manufacturer’s factory calibration, or if you need to seek a third-party calibration service provider at additional cost to achieve the measurement uncertainty and traceability you need to meet your goals and expectations. The best advice is to choose a manufacturer that has invested the necessary resources in building and maintaining the competence needed to provide superior calibrations along with their products.
Not all Coriolis flowmeters are created equal, and not all can meet the most exacting measurement standards. There are many industry applications where “good enough” is not an acceptable measurement. These types of applications, such as pharmaceutical production or custody transfer, for example, require a more precise measurement to satisfy stringent local and federal regulations. Even global standards can apply when a facility manufactures in one place but sells into another in our connected world.
Having a well-calibrated device affects the ultimate product a company produces as well. By producing a product within tight tolerances and unique specifications, a business builds a reputation for quality, but that can only be maintained if that business has the tools to do so. Before we delve deeper into calibration, let’s first look at what makes a Coriolis flowmeter such a reliable and valuable tool to different industries.
The first important element to note about a Coriolis flowmeter is the fact that it has no mechanical parts that can wear out, which increases the lifespan of a meter significantly. Another benefit of this, somewhat unique to Coriolis meters, is they do not need nearly as frequent recalibration as mechanical type meters. The measuring element has no cause to wear or change over time.
The operation of a Coriolis flowmeter is based on the principles of motion mechanics. As fluid moves through a vibrating tube, Coriolis force is generated as the fluid is forced to increase its angular momentum to move with the tube vibration as it approaches the point of peak-amplitude vibration. Conversely, angular momentum is lost when the fluid moves away from the point of peak amplitude as it approaches the exit point of the tube. The result is a twisting reaction of the flow tube during flowing conditions as it traverses each vibration cycle.
There are a few things to consider about calibration best practices before choosing a Coriolis flowmeter. Primarily those are the traceability of the reference standard that will be used for the meter calibration and the calibration procedures that apply to a specific meter model.
There are two essential elements to traceability:
It is essential to a facility that its most important tools for flow process control (i.e., Coriolis flowmeters) meet local, state, and federal regulations, as well as their own internal quality standards. Having a rigorous measurement traceability to international standards helps ensure flow measurement equipment can meet those standards. In the case of mass flow measurement, such as those a Coriolis meter can provide, traceability is the chain of measurement standards going back all the way to the International Mass Standard as determined by the Planck constant. (Previously, the standard was set by the International Bureau of Weights and Measurements, but the world has recently adopted a natural standard defined by Planck’s constant for greater consistency and accuracy).
Traceability documents the paths by which everything ties back to a single central starting point. In addition, traceability allows a facility to understand and apply the appropriate level of uncertainty to its measurements. Finding the right balance between the cost of uncertainty and the benefits of product quality and process efficiency can be achieved by understanding the uncertainty as documented in a traceability fact sheet or website for the meter under consideration. In all instances, the level of precision and refinement of a product will depend on the level of uncertainty found in the meter. Figure 1 illustrates a chain of traceability, which is an essential tool in achieving traceability accreditation.
The traceability of a calibration reference standard obviously matters, but how is calibration against a set standard achieved? The calibration process requires a strictly controlled procedure and an environment where a device can be put through a series of reproducible tests.
When calibrating a Coriolis flowmeter, there are two different preferred methods that can be used to reach optimal calibration. There are other methods available for other types of flowmeters that are sometimes used in applications that do not require the meter to handle flow rate transitions as well as a Coriolis meter can.
When the calibration batch begins and ends at a no-flow condition, it is a static start/finish method of gravimetric calibration. A weigh scale is used as a reference in this method, and the scale will have been calibrated using traceable mass standards. In this instance, the test fluid is water. The water is measured through the unit under test (UUT) and collected in the tank. At the end of the test, the tank is weighed again and compared to the total mass measured by the UUT.
In an application where it is necessary to obtain a better uncertainty, the mass shown on the scale will be corrected for the effect of buoyancy acting on the water in the tank, as well as for the effect of buoyancy on any immersed pipe in the setup. Fluid pressure and temperature are measured upstream and downstream of the UUT. For static start/finish, the ambient pressure, temperature, and humidity are measured during each test. Figure 2 represents graphically how this method works.
The transfer standard method (TSM) of calibration is a dynamic start-finish method where a batch starts and ends at a steady flow. Using water as the test fluid, the calibration is performed in closed conduits. Water passes through both the reference meter (RM) and the unit under test. In this example, two reference meters are used that are known as the master meters and are known good meters initially calibrated on an ISO/IEC accredited primary gravimetric flow stand following the static start/finish method. The traceability associated with this method is maintained annually using global reference meters for comparison testing.
With the TSM method the mass total from the UUT is compared to the mass total from the RM using pulse counters. Pulse counters are triggered on and off for both the UUT and RM at the same time. Upstream and downstream measurement of fluid temperature and pressure are done to ensure consistency and reproducibility. Figure 3 illustrates the TSM calibration process.
When considering calibration methods and traceability standards, it is critical to understand the cost of reaching each incremental level of improvement to the uncertainty and how tight of an uncertainty might be justified given the impact on the process where the Coriolis flowmeter is used. Determining uncertainty levels of a meter is done by the combined uncertainty of each of the traceable steps leading to the calibration of the meter. Uncertainty increases in small increments, often negligible to the results of the process, with each step further down the traceability chain.
What happens when a Coriolis flowmeter leaves the laboratory and is no longer in the controlled environment where it was calibrated? Secondary effects then come into play and add more uncertainty to the performance of the meter in situ in a process.
Calibration uncertainty and the effects of process on the flowmeter combined will determine the final uncertainty of the measurement. This is why it is important to understand both the uncertainty of the calibration and the impact and magnitude of secondary effects of a process in order to know the appropriate level of calibration uncertainty that is needed—or is acceptable—to ensure a quality final product.
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