Flow meters in water supply

In today's business environment, we are always faced with the bottom line.

Energy use and other overhead costs naturally become an issue. As the focus on energy efficiency increases, so does our reliance on flow measurement equipment for process energy measurement.

Society-wide demand for energy conservation requires end users and industry to measure energy use. For heating and chilled water applications, this requires the installation of flow and temperature measurement equipment tied to a BTU computer. End users can use this information to evaluate where and how energy is being consumed and then take steps to improve efficiency.

Temperature Sensor Accuracy Extreme low-end flow rates are becoming increasingly important in certain applications as a result of increased energy efficiency. While technologies like variable frequency drives (VFDs) and flow regulating valves allow end users to optimize flow in their hot and cold water systems, do we really understand how these lower flow rates affect flow meters? Let's consider an application (hot/cold water) and its flow measurement components (a paired temperature sensor, flow computer and flow meter). What is the biggest limiting factor on system accuracy?

Let's consider an application with matched pair temperature sensing technology that is accurate to +/- 0.01% of range (typically 200 F) or 0.02F. The combined accuracy of the two sensors is approximately 0.04F. At a temperature difference of 10 F, the accuracy is approximately +/- 0.4%. Even in the worst case scenario, this accuracy is better than some widely used flow meter technologies, as you will see in the following paragraphs. (Note: Temperature sensor accuracy decreases as temperature decreases.)

Flow Transmitter Accuracy
Today's advanced technology enables flow computers to perform calculations, provide linearization adjustments, and even further reduce temperature errors by zeroing out the error within the application's operating differential range. Compliant with OIML R75 standards, the flow computer can then provide an accuracy of +/- 0.1% of reading, making it a fairly accurate component in the flow meter applications considered here.

flow meter accuracy
Now let's consider flow meter technology. Within the standard range of optimal flow rates, you can expect the flow meter technologies we will consider here to be accurate to +/- 0.5% of the rate or reading. However, this specification does not reflect the actual operating range of the instrument in the field or the requirements for straight pipes. In order to achieve +/- 0.5% accuracy, most flow meter technologies under consideration require a fully developed flow curve, which usually means they require sufficient straight pipe upstream and downstream.

Additionally, flow meter accuracy claims are often associated with specific turndown values. This represents the lowest speed at which the meter can maintain its accuracy statement. For example, if the meter's maximum speed is 20 ft./s and it performs accurately at a 10:1 range ratio, you can divide 20 by 10 to find the minimum speed at which the meter will maintain its accuracy of 2 ft./s. Below this minimum velocity, the flow meter will have another accuracy specification indicating a fixed error independent of the actual flow rate. This means that as the speed drops further and further below the regulated minimum, the meter will become increasingly inaccurate.

Mechanical gauges that use moving parts, such as turbines (typically +/- 0.5% of reading on a 20-to-1 scale) and paddle wheels (typically +/- 0.75% of reading on a 10-to-1 scale), will have a point at which the gauge stops working altogether. This point of inoperability will occur at higher and higher speeds as moving parts scale and wear over time.

Vortex flow meter (typically +/- 0.8% of reading at a 25:1 turndown ratio) will also stop operating at low flow. When the speed is reduced to the point where eddy currents are no longer created, the meter will no longer sense flow.

Magnetic flow meters (typically +/- 0.5% of reading at a 30:1 turndown ratio) maintain accuracy at low flow rates and can provide accurate measurements even under less than ideal flow curve conditions. That said, most magnetometers will also struggle to provide accurate measurements at low flow rates.

Measure very low flow rates
Even if you had the most accurate flow meter on the market, would the range of flow rates it was designed to measure align within the extended range your system is operating at? If not, what happens below the technology threshold?

The operating range of a flow meter can be placed into one of three categories, which are defined by the relative error expressed as a percentage of the reading or measurement.

Flow Measurement: The accuracy standard for flow measurement equipment is +/- 5.0%. While we now know that the flow meter itself is not the only factor in this error percentage, for the sake of clarity and rounding we will stick to +/- 5.0%.
 

Flow Indication: This category includes flow meters with an accuracy range of +/- 5.0% to worst case +/- 50.0%. Of course, the range is large, but the reason for detecting traffic will depend on the application and end use of the data collected.
 

Flow Switch: Any error rate above +/- 50% classifies the device as a flow switch. Essentially this provides an on/off indication which in many cases is used to start/stop pumps so that they do not run dry.

 
Depending on the application, devices from all three categories can provide valid and useful information. However, maintaining flow measurement categories is critical when collecting data for the purpose of determining energy use and efficiency. That said, here we'll show how some flow meter technologies will be forced to maintain +/- 5.0% accuracy when faced with the low flow rates common in today's pumping systems.

Reduce energy consumption in pumping systems

Figure 1. When flow rates are set below 1 ft./s, it is important to consider how this will affect the accuracy of the flow meter.

Before the use of VFDs and regulated flow valves, most hot/chilled water systems operated in pump on/off mode and used parallel pump systems to meet application needs. There is little concern about excessive power usage, so the pump will run its performance curve to draw the current needed to meet the upstream pressure drop and maintain flow. To alleviate this situation, VFDs were introduced in the 1980s to reduce energy costs and provide sufficient flow to meet downstream demand.

Subsequently, a new generation of products is created to meet point-of-use requirements while reducing the energy used to meet these needs. Therefore, VFDs and flow control reversing valves are often employed to optimize the power requirements required to move water. This, in turn, affects flow conditions in new and retrofit applications.

Before the use of VFDs and automatic reversing valves, pumps operating in on/off mode had a speed range of 1–15 ft./s. But today, the velocity range is 0.1-15 feet/second, or a 10-fold reduction in flow rate.

Therefore, when exposed to this extended flow range, it is important to consider how the flow meter responds when operating at low flow rates.  

Performance issues
To illustrate the answer to this question, we will examine three popular flow meter technologies (dual-element turbine, dual-electrode insertion electromagnetic flow meter, and full-bore electromagnetic flow meter). These meters are compared in Figure 1, showing a percentage error that remains within +/- 5.0% of the rate or reading. It will be interesting to see how each gauge responds as it moves downward over an extended speed range in this application scenario.

As you can see, the meters gradually lose accuracy at the lower end of the operating range, with some struggling to provide adequate measurements over half of the new extended operating range. Although the two-electrode insertion electromagnetic flow meter still has operating accuracy specifications over the extended range, its logarithmic accuracy decreases making it a flow indicator at the lower end of the new range.

Even traditional full-bore electromagnetic flow meter become low-end flow indicators, making a bold statement about how difficult it is to make accurate flow measurements at low-end speeds when using VFDs and flow control reversing valves to optimize the power requirements required for water transfer.

Based on the information provided here, it is important for end users using VFDs and diverter valves for hot/cold water applications to consider how this installation will affect flow meter performance.