Choosing the right differential pressure flow meter for an application can be challenging. This article outlines the considerations and trade-offs in selecting the optimal technology.
Selecting the right differential pressure flow meter is a balancing act—one that involves defining the purpose of the measurement, specifying the application parameters, understanding the environmental and safety needs, and prioritizing the selection criteria to determine the optimal flow technology.
Purpose of Measurement
Understanding the purpose of the measurement is the first step in the selection process. What will be done with the flow information? How critical is the measurement? What decisions will the user make with the information? In general, flow information is used for either the controlling or monitoring of processes. There are many reasons to measure flow, some more critical than others. They can range from batching, continuous blending, custody transfer and mass balance, to inventory control, governmental regulation compliance and safety.
Flow Application
Of the many flow meters available for measuring fluid flow, the type of flow meter used often depends on the phase of the fluid—liquid, gas or steam—and the conditions under which the fluid is measured. These conditions include line size, flow rate, process pressure and temperature, ambient pressure and temperature, and chemical properties. For example, insertion flowmeters are more frequently used in steam, large line sizes and in situations where process shutdown is not a possibility.
Wet gas applications, where the presence of liquid in a gas stream can pose significant challenges, are another example. Technologies like conditioning orifice plates eliminate damming and provide predictable and correctable results when the moisture content is known. Inline metering technologies are the best practice for small line sizes as the possibility of errors caused by installation are reduced.
The flow application example referenced in the remainder of the article is detailed in Figure 1 and compares four different flow meter technologies, as shown in Figure 2.
Figure 1. Flow data for application example
Figure 2. Differential pressure flowmeter technologies for comparison
Flow meter Capability
The next step in the selection process is to determine how well the flow meter meets the given application. Flow meter capability is, among other things, dependent on the following parameters: installed performance, transmitter performance, permanent pressure loss and straight pipe requirements. These performance parameters should be prioritized based on the measurement purpose.
Installed Performance
When considering installed performance, be sure to understand the performance of the flow meter over the entire flow range. It is important to note that the installed performance is not equal to reference accuracy because it takes in account environmental effects, including line pressure variability, temperature fluctuation and drift/stability. Additionally, transmitters specified as a percent reading perform better at low flow ranges.
When a flow meter is specified as percent of span the error at low flow rates gets magnified.
For example, if the transmitter is spanned for 0 to 100 inH2O, a reference accuracy of ±1 percent of span reflects a 1 inH2O error across the full flow range. If the transmitter is reading 5 inH2O, this represents a 20 percent error. Alternately, a transmitter specified to be 1 percent of reading would have a .05 inH20 of error if it were reading 5 inH2O.
Another key consideration is that coefficients, such as the discharge coefficient of the primary element, the gas expansion factor of the liquid source and the thermal expansion of the construction material, vary with flow. If accuracy is sized at a specific flow point, then there is risk that errors will occur away from that sizing point. It is therefore recommended that accuracy is sized over the full range of flow to dynamically correct for these variables. Other factors may also contribute to measurement variation.
Errors in flow measurements are also attributed to errors in density measurement. For uncompensated velocity—a volumetric measurement that does not take into account the fluid's density properties, such as a turbine meter—the percent error in flow is optimal. While an uncompensated differential pressure measurement results in reduced percent error, it is still not optimal. A multivariable measuring device compensates for both pressure and temperature variation, and consequently produces the lowest flow error at <0.1 percent. Figure 3 compares the total system performance of the various flow meter technologies. It is evident that (B) and (C) rate higher in accuracy through a wider flow range.
Figure 3. Total system performance of differential pressure flowmeter installations
Permanent Pressure Loss (PPL)
Whenever a piece of equipment is added to a flow system, pressure is lost. This pressure loss makes the pump or compressor work harder to generate the same flows in the system. In the case of flow meters, a loss is incurred because a piece of straight pipe would not have as much loss as the flow meter. This pressure loss varies by DP flow meter. Starting with the orifice plates (highest PPL) and sorted in descending order of permanent pressure loss are orifice/nozzle, wedge, V-cone, venturi and Averaging Pitot Tube.
A properly sized orifice plate with a beta ratio of 0.6 will typically lose 40 percent of the sensed DP to permanent pressure loss, whereas an Averaging Pitot Tube has a blockage range between 15 and 20 percent. Lowering permanent pressure loss will reduce pumping or compressing costs, increase capacity and minimize the size requirements for the compressor, pump or boiler. For this article's example, the permanent pressure loss for each flow meter is shown in Figure 4.
Figure 4. Permanent pressure loss for DP flowmeters
Straight Pipe Run Requirements
Traditional orifice plate flow meters require long straight pipe lengths to meet specifications. Minimizing piping requirements improve performance and lower installed cost. This presents a challenge as most plants are not designed with sufficient straight pipe, making it difficult to engineer and add flow measurements. Technologies are available that require shorter straight run, thus eliminating
