Pressure-Based Level Measurements Keep Getting Better and Better

While Radar and Ultrasonics Are Getting Most of the Attention, Automation Vendors Are Quietly Improving Their Pressure-Based Level Measurement Instrumentation

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Due to the number of variables involved―including the physical characteristics of the fill fluid, capillary length and diameter, and the mechanical design of the diaphragm seal system itself―Emerson offers a software tool that simplifies the design of a fully compensated, or “tuned” DP seal system. This toolkit provides the capability to calculate numerous potential compensated seal systems for any given application condition quickly and easily.

“Electronic” remote seals

For specialized applications or in situations where the installation or maintenance of hydraulic systems may be an issue, both Emerson and Endress+Hauser are promoting a pressure-based level measurement approach that replaces a DP transmitter and its associated wet legs, dry legs or long capillaries with separate pressure transmitters that are made into a multivariable system using fieldbus communications. Emerson calls this approach, “electronic remote seals,” since it replaces the conventional hydraulic systems with digital communications.

According to Emerson’s Graupmann, “Electronic remote seals can improve measurement performance by eliminating dry/wet legs or capillaries. Dry/wet leg installations can introduce measurement uncertainty if there is any variation in these references caused by condensation in a dry leg or evaporation in a wet leg. Both lead to a shift in the DP zero reference. Capillary systems have head temperature effects caused by the change in density of the column of fill fluid in the capillary. This column is eliminated in an electronic remote seal system where the sensors are direct-mounted to the vessel fittings. Capillary systems also have seal temperature effects related to the fill-fluid volume change. These temperature effects are minimized by direct-mount seal systems.”

Graupman continues, “Electronic remote seals can also help reduce maintenance costs. Keeping the wet/dry leg reference stable is key to measurement performance. This typically requires extensive maintenance. Periodic draining or filling of legs, insulation and heat tracing maintenance costs can all be minimized by eliminating the wet/dry leg installation practice.

“Installed cost is a little more difficult to compare as you need to compare the instrumentation cost of the different installation approaches. The electronic remote seal system has two pressure sensors, so it will more expensive than a DP transmitter with two capillaries most of the time. But to be fair, you also need to consider the other installation costs like heat tracing that might be required for a particular transmitter installation.”
The Ziemann Group is the world’s largest manufacturer and supplier of beer brewing plants. At the company’s pilot brewing plant in Ludwigsburg, Germany, liquid level in the closed vessels of the fermentation and storage tanks is monitored using two Siemens’ digital pressure transmitters equipped with stainless steel housings and sanitary flush mounted diaphragms. One pressure transmitter, mounted at the bottom of the vessel, measures the hydrostatic pressure plus the superposed overpressure. The second pressure transmitter, mounted toward the top of the vessel, determines the pressure above the liquid level. The liquid level then is calculated in the control system using both measured values.

However, in the pilot plant’s open brewing vessels, a single Siemens pressure transmitter is used to measure the hydrostatic pressure of the liquid column at the bottom of the vessel to determine the liquid level.

With a measuring range up to 150 °C, these Siemens pressure transmitters are well-suited to sterilization-in-place, without showing any drift effects.

Multivariable level transmitters

Multivariable transmitters offer yet another approach to obtaining highly accurate pressure-based level measurements in either open or closed vessels.

Bristol Babcock introduced the first multivariable in 1992. Since that time, Emerson (Rosemount), Honeywell, ABB, Invensys (Foxboro), and Yokogawa have entered the multivariable market. Multivariable transmitters measure more than one process variable, typically pressure, differential pressure and temperature and can use these measurements to calculate fluid flow, mass flow, density or level.

According to Flow Research’s Yoder, “Much of the growth in the pressure transmitter market is due to growth in the multivariable transmitter market. The market for multivariable DP transmitters for flow has more than doubled in recent years.”

Foxboro introduced the first dedicated multivariable level transmitter in 2004. This transmitter uses pressure, differential pressure and temperature measurements, plus on-board physical property tables, to compensate for density changes in the tank liquid and vapor and density changes in both vapor or liquid in external dry or wet legs. According to the company, this makes the transmitter ideally suited for critical applications, such as boiler drum level and for other high-performance applications where fluid densities are significantly affected by variations in pressure and temperature.

“For these types of applications, the conventional pressure-based level measurement approach was to install three separate transmitters―gauge pressure, differential pressure and temperature―and run these wires back to the control system where the level calculations were made,” said Pat Cupo, pressure and temperature product manager at Invensys Process Systems. “Now, a single Foxboro multivariable level transmitter can handle all three measurements, compensate for density changes and make the level measurement calculation right in the transmitter. We believe that these result in better level measurements, reduce installation and wiring costs, and reduce the bandwidth load on the control system. What’s more, some level applications, boiler-drum level, for example, are not all science and still require quite a bit of art to properly locate and configure the instruments. With multivariable transmitters, there’s only one instrument to deal with, rather than three. This helps to keep things a lot simpler.”

Cupo continues, “That said, many end users tend to treat multivariable transmitters as ordinary DP transmitters, and that’s clearly not the case. For example, if you relocate the transmitter, it’s important to update the transmitter’s database so that it can make the appropriate compensations. Safety is another consideration, particularly with high-pressure, high-temperature boiler-drum level measurements where the wet reference leg can go dry due to steam boil-off. This would result in incorrect drum-level measurements that can cause bad things to happen. We’re working to eliminate this type of problem through improved documentation and configuration procedures. This is critical, since we’re finding that as ‘old school’ maintenance technicians retire, their knowledge and experience often retires with them. This is a huge issue, particularly since the new generation of maintenance technicians are often more familiar with information technology than with instrumentation technology.”

However, despite the obvious advantages, multivariable transmitters have not yet achieved the market acceptance for level measurement applications as they have for flow measurement applications.

According to Emerson’s Graupmann, “Multivariable transmitters continue to be associated mainly with flow measurements, and to date, have gained only limited acceptance for level measurements. Multivariable transmitters are used when pressure and temperature level compensation is required. Multivariable transmitters can save on installation costs by combining three transmitters into one. Acceptance levels depend on a user’s willingness to adopt new installation practices based on resulting operating or maintenance improvements.”

The continuous investments being made in DP level measurement technology by automation vendors, combined with the widespread acceptance across the user community, mean that pressure-based level measurements will remain a viable approach well into the future.

Paul Miller is a Control contributing editor.

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  • <p>it valuable information that easy to understand and help for calibration of the level measurement instrumentations.</p>

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  • <p>Generally, this is a good summary article, but it fails to include Hydrostatic Tank Gauging, HTG. HTG provides density compensated level measurement for pressurized storage tanks based on actually measuring the density with two pressure taps placed in the liquid level at a known distance apart. This requires a multivariable transmitter with 2 differential pressure inputs: dP vapor space to bottom tap and dP between a mid-tank density tap and the bottom tap. There is a good explanation here: page 86 of this book "Instrumentation Fundamentals for Process Control" By Douglas O de Sa, CRC Press, 2001. Most instrument companies offer this multivariable transmitter. The theory is that the dP between the midpoint tap and the bottom tap, both of which are submerged in the tank liquid, is proportional to liquid density represented by the difference in head between the two taps. You would use a HTG on tanks with a pressurized vapor space and varying liquid density. You do not need to know the temperature correction for liquid density, although most of these transmitters also provide an RTD temperature input as well in case there is no midpoint pressure tap.</p>

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