In today’s economic environment, energy usage has become a huge concern, particularly on energy efficiency and using flow meters to guarantee accuracy in energy measurement. The upsurge in energy usage demands industries and end users to measure energy usage as accurately as possible. In chilled and heated water applications, the installation of a temperature and flow-measuring device is required. Linked to a BTU computer, the data enables end users to determine how energy is being used, where and how it is used and take steps towards improvement accordingly.
However, there are certain limiting factors, which can affect the accuracy of flow meter systems. The following is a discussion on various technologies that can be used to improve flow meter accuracy:
Temperature Sensory Accuracy
Let us take an application from matched-pair temperature-sensing technology. It is accurate to (+/-) 0.01% of span (0.02 F). The accuracy of both sensors combined is around 0.04 F. By a differential temperature of 10F, you can expect a +/-0.04% accuracy. Even in worst-case scenarios, this measure of accuracy is far better than some flow meter technologies.
Flow Computer Accuracy
Today’s advances in computer technology enable professionals to use flow computers to check calculations, reduce temperature errors by pinpointing errors in the differential span of applications and provide adjustments and linearization. With a compliance rate of OIML R75, flow computers are able to provide a reading accuracy of up to 0.1%, making them a highly accurate component in flow meter applications.
Flow Meter Accuracy
Optimal flow velocity has a standard range of accuracy of up to 0.5%, when using flow meter technology. However, this measure does not accurately indicate the numbers you will get when operating in the field. To achieve an accuracy of +/-0.5%, the flow meter technology will require a properly developed flow profile. This means sufficient downstream and straight-run pipe up is required.
Additionally, flow meter accuracy statements are usually associated with precise turndown values. They point out the lowest velocity at which the flow meter maintains its accuracy. For instance, if a flow meter has a max velocity of 20 GPM and performs exactly on a turn down of 10-1, then minimum velocity at which the flow meter will uphold the accuracy is 2 GPM. If the flow meter goes below this minimum velocity statement, it will arrive at a different accuracy specification, pointing to a fixed error that is not dependent on the flow velocity. The conclusion that can be drawn here is that as velocity continues to drop below the indicated turn down minimum, the flow meter becomes less precise.
Mechanical meters with moving parts such as turbines and paddle wheels have a set point at which the flow meter will stop functioning altogether. As these moving parts begin to break down, the point of inoperability will happen as higher velocities are reached.
Vortex meters also stop operating at low flow rates. When the velocity drops to a level where vortices are not created, the flow meter will not sense flow.
Magnetic flow meters are more successful at maintaining accurate reading during low flow velocities. They even deliver accurate measurements when the flow profile state is in less-than-ideal conditions. However, many magnetic flow meters can also battle to provide accurate measurements in low flow velocity conditions.
Devices that measure extremely low flow rates
Even if your industry uses the most accurate flow meter available in the market, it may not correctly measure the extended range that it actually operates in. In such cases, what technology can you use to measure flow rates beyond the flow meter’s threshold?
A flow meter has the following three operating categories:
The accuracy of flow measuring devices ranges around +/-5.0%, even though we are now aware that a flow meter is not the only factor in arriving at an error percentage.
In this category, flow meters have accuracies that range from +/- 5.0% to +/- 50.0%. This is a vast range and the reasons for determining the flow will be dependent upon the application and use of the data that is collected.
Error rates above +/- 50% classify devices as flow switches. This gives an on or off indication used to start and stop pumps so that they don’t dry up!
Devices in the above three categories offer useful and valid information, depending upon how it is applied. For the purposes of energy efficiency and usage, it is imperative to remain in flow measurement.
Reducing Energy Use in a Pumped Water System
Before the use of modulating flow valves and VFDs (variable frequency drive), most chilled and hot water systems would operate in a pump on or off mode with a parallel pumping system that was designed to meet the strains of the application. Since concerns about excess electricity use were minimal, the pumps would operate on performance curves, relying on as much electricity as required to maintain flow and meet upstream pressure decreases. To decrease this, VFD’s were implemented in the 80s to reduce energy expenses and provide sufficient flow to meet downstream demands. Then a new group of products were introduced to meet usage requirements while using reducing energy needs. Therefore it has become customary to use VFDs along with flow control diversion valves in order to optimize electricity demand needed to transport water. This impacts flow conditions in retrofitted and new applications.
Before the use of automated diverting valves and VFDs, the velocity of flow in pumps was 1-15ft./s. Today this velocity range is 0.1-15 ft./s. This is a decrease in flow velocity by a rate of 10. With knowledge of this extended flow system, it is important to keep in mind how flow meters operate at low flow rates.
Three flow meter technologies: full-bore magmeters, dual-electrode magmeters, and dual-element turbines, provide sufficient measurements in certain operating ranges. Dual –electrode insertion magmeters has operation accuracy in the extended range. It has a logarithmically declining accuracy in the lower end of the range. The full-bore magmeter is a traditional technology can also indicate flow at low velocities.
In light of the information presented here, it is important for end-users to keep in mind that flow meter accuracy can be improved in the low-velocity range provided certain technologies are used alongside.