Differential Flowmeter Selection for Challenging Compressible Fluid Flow Applications

Summary

Differential pressure (DP) has long been the gold standard for compressible fluid flow measurement, including products such as air, natural gas, steam and other industrial gases. Of the various DP measuring techniques, the traditional orifice plate has been considered the standard for these measurements. Honed through decades of flow applications and research, orifice plate installations remain a leading means of flow measurement for many custody transfer applications.
 
However, a host of new DP primary element technologies have been introduced, each having distinct advantages in specific applications. DP transmitters themselves have also been re-engineered, offering multivariable measurement, dynamic compensation capabilities, bias elimination and exceptional accuracy. Suddenly, the flowmeter selection process went from choosing an orifice bore size and DP transmitter, to evaluating a wide array of primary element and transmitter options.
 
This article explains the options available to help end users better understand the tradeoffs involved in the DP flowmeter selection process. It also describes tools and available services that can be extremely helpful for choosing the best flow measurement components for a particular application.
 

DP Flow measurement

While the venturi flowmeter was patented in 1888, the concept was based on work performed by Sextus Julius Frontinus who was responsible for measuring Roman aqueduct water flows around 100 AD. Generally, the concept is simple: when a fluid is made to flow through a restriction in a pipe, the fluid velocity must speed up to allow the same flow to pass through a smaller area.
 
Since the net energy of the fluid remains the same, the fluid must trade energy of pressure into energy of velocity as it passes through the restriction. This creates a pressure drop across the restriction that can be measured and converted to a volumetric fluid flow measurement. For most DP producers, the DP measurement varies as the square of flow, so doubling the flow through a restriction increases the DP measurement by a factor of four.
 
Over the decades, a concentric orifice (Figure 1) has been the primary element of choice. Dozens of books and a century of research have resulted in very high levels of accuracy by fine tuning the design of the orifice itself, specifying details of how it must be installed in the piping system, and tweaking the rather complicated mathematics to minimize uncertainty.

• Fluid type
• Fluid temperature
• Fluid pressure
• Fluid density
• Fluid min/norm/max flow
• Fluid viscosity
• Reynolds number
• Pipe ID
• Pipe friction factors
• Orifice bore
• Weep/drain hole size
• Installation factors
• Compressibility factors
• C_d of primary element
• And many others

Variations in any of these parameters, particularly fluid pressure and temperature, can significantly impact the accuracy of the flow measurement. Variable process temperatures and pressures change the fluid’s density, as well as the viscosity and Reynolds number, affecting the measurement. That is why the most accurate DP flowmeters require a separate process temperature and pressure measurement, both used to compensate and correct the flow calculations as process conditions change.
 
While the concentric orifice is certainly the most common primary element, it has limitations making it a less-than-ideal choice for flow measurement, including:

• It requires a significant straight run of pipe upstream (25 diameters or more), as well as 10 diameters downstream, to minimize flow profile errors.
• It generates a high permanent pressure drop (about 66% of the measured DP), increasing line loss and wasting compression energy.
• It is susceptible to wear, which is problematic since measurement accuracy is dependent upon sharp edges of the orifice plate bore.

As a result, a wide range of alternate primary flow elements (Figure 2) has been introduced. Each of these designs has certain advantages, such as reduced pressure drop, better turndown, or reduced straight-pipe run requirements.

Primary element selection

Here is a quick list of the compressible fluid flow primary element designs, including advantages and disadvantages of each:
 
Integral Orifice: An integral orifice is best applied when small line sizes and very low flow rates must be measured accurately. There are a few integral orifice designs to suit various installation situations, with the most common being the standard straight run type (shown in Figure 2).
 
Rosemount Annubar: An Annubar uses a multiport averaging pitot tube. The sensing bar has small rectangular ports on the front and back, allowing the sensor to generate a DP correlated to flow. Annubars require less upstream/downstream straight runs than orifice plates and create far less permanent pressure drop. Due to its unique physical design, an Annubar maintains a flat discharge coefficient curve with respect to the Reynolds number, which creates a more repeatable measurement as flow varies. An Annubar is less expensive to install than other primary elements, especially on large size pipes. The limitation of applying Annubar designs is that they may not perform as well at very low flow rates.
 
Concentric orifice: This is the standard for most DP applications. While inexpensive, these orifice plates require significant straight run and can incur significant pressure drop in the line. The bore diameter impacts low flow turndown, length of required meter straight pipe run upstream, and permanent pressure loss. Smaller bores generate more permanent pressure loss but allow the meter to read lower flow rates, while requiring less upstream/downstream meter runs. Higher bore sizes have the opposite effect.
 
Conditioning orifice: There are a few different designs for this type, but in each case a conditioning orifice uses multiple holes to create the pressure drop. The design is much less sensitive to flow profile, so it requires significantly reduced upstream/downstream straight pipe runs. Pressure drop is equivalent to a similarly sized concentric bore orifice.
 
Flow venturis: This DP element is expensive compared to the designs listed above and has limited turndown, but it has an extremely low permanent pressure loss. A flow venturi is often utilized when pressure loss and accuracy are primary concerns.
 

Transmitter selection

Once a primary element has been selected, the user must choose a flow transmitter. Historically this has been rather simple, with a default DP transmitter chosen to suit the range of expected DP values encountered at different flow rates.
 
The challenge for nearly all DP flowmeters is the square root relationship between DP and flow. If the flow is reduced by a factor of 10, the resulting DP will be reduced by a factor of 100. At low flow rates, DP values can become extremely small, so the measurement error rises exponentially. This is why the turndown on DP flowmeters is typically limited to 8:1, or even 5:1.

For measurements across a wide range of flows, a common but expensive solution is to install two DP transmitters connected to a single orifice plate. One is ranged to handle the high flows and the other is ranged much lower to measure the lower DP value created under low flow conditions.
 
Another challenge for DP flow measurement is handling the variability of process pressure and temperature on the flow reading. If the process pressure and/or temperature changes, it will affect the flow calculation and introduce error. The typical solution is to install separate pressure and temperature transmitters to measure the fluid conditions, and to then use these values to compensate the flow reading. This solution works well, but the material and installation costs can be quite high.
 
Fortunately, new advances in technology allow both these challenges to be easily resolved with a single, less expensive solution.

 
Advanced transmitter options

Advances in transmitter technology allow a single transmitter sensor to handle a very broad range of DP measurements, while significantly reducing the measurement error at very low values.

This enables a single transmitter to read both high and very low ranges, nearly doubling the DP measurement turndown to 14:1 or better.
 
The same sensor platform also enables this single transmitter to read both DP and process pressure, and an external temperature sensor can be easily added. The resulting transmitter takes the place of three, measuring DP, pressure and temperature within a single device.

Because the measurements are incorporated into a single device, the transmitter continuously compensates the flow calculation for temperature and pressure 24 times a second, calculating fluid flow with significantly higher accuracy. Like all DP flow measurement technologies, an Annubar creates a DP value that can be used to calculate uncompensated flow, but the temperature and pressure measurements can be used to calculate fully compensated mass flow when varying process densities are present.
 

Tools for DP measurement

Given the wide variety of options, choosing the best combination of primary element and transmitter for a particular application can be a daunting task. Fortunately, there are free online tools available that take the user through a series of questions to define the application, with the answers used to provide a range of suitable solutions, including the pros and cons of each.

Such tools allow users to quickly narrow their options for a primary flow element and transmitter that will satisfy their measurement requirements and meet their budget.
 

The benefits of mass flow measurement

Careful selection of the primary element and multivariable transmitter allow users to address challenges that created measurement problems in the past.
 
For example, a university in the northeastern U.S. uses steam to heat buildings across the campus. During the winter, steam flows are high and easy to measure. During the summer, flows drop dramatically, making flow readings highly inaccurate. University personnel also wished to add some new measurement points, but lacked the straight pipe runs needed for a typical orifice plate.
 
The solution employed conditioning orifice plates paired with multivariable, ultra-low flow transmitters. The conditioning orifice required only two diameters of straight pipe upstream and downstream, and the multivariable, ultra-low flow transmitters could handle the dynamic range and provide very high accurate steam flow measurement year-round.
 
A company in Asia sells specialty gases to clients at a multitude of sites. The application involved relatively low and variable flow rates that had to be measured with very high accuracy for custody transfer. In this case, the best solution involved integral flow orifice assemblies paired with multivariable, ultra-low flow transmitters. This combination provided highly accurate measurement across a range of flows, and it was purchased as a single, ready-to-install unit, reducing installation costs at each site.
 

Conclusion

The range of solutions for DP measurement has exploded in recent years and advanced far beyond the simple concentric orifice and basic DP transmitter. While the array of primary element and transmitter options may be overwhelming, the challenge of flowmeter sizing and selection can be mastered. Armed with online selection tools and an understanding of their process needs, users can quickly evaluate the options and choose the right combination for their specific application. If questions remain, automation providers have a staff of trained professionals who can address your needs.