By Paul Miller
“Reliable measurements enable our plant to operate within limits, improving the safety and reliability of the plant, while maximizing profit,” says Greg Breaux, instrumentation specialist at the Motiva Enterprises Norco Refinery, located just outside New Orleans.
“DP [differential pressure] technology is perhaps the most simple and reliable technology for measuring level as well as flow. It is usually the right technology in applications where the specific gravity of the process does not change more than the desired accuracy of the measurement,” said Breaux. “DP technology is simple to understand, and simple to design and install. The basic technology has been around so long that everyone understands it, even when you add the new advancements, such as remote seal diaphragms, Foundation Fieldbus, etc.”
“Simple, reliable,” and “well-understood” are three expressions commonly used to describe pressure-based level measurement approaches. This reflects the fact that, while radar, ultrasonics and other new level measurement technologies are rapidly gaining acceptance, the majority of continuous level measurements in today’s process plants continue to be made using pressure or differential pressure transmitters.
Still Popular after All These Years
“Of the nearly two million DP transmitters sold every year, approximately 25% are used for level measurement applications. This represents a sizable number of transmitters,” commented Wil Chin, research director at ARC Advisory Group, Dedham, Mass. According to ARC estimates, two-thirds of the overall automation market is for existing plants, where replacement-in-kind makes a lot more economic sense than introducing new technology that would require different installation practices and additional training. DP transmitters do a very decent job for most level measurement applications and offer some benefits not always available with other approaches. For example, many DP transmitters are mounted with isolation manifolds that make it relatively quick and easy to replace the entire instrument, if necessary, without having to shut down the process.”
According to Jesse Yoder at Flow Research, Wakefield, Mass., “Installed base is the single biggest factor that has sustained growth in the pressure transmitter market. Many companies have invested very heavily in pressure transmitter technology and are not likely to abandon this investment. Changing technologies often requires changing suppliers and has additional start-up and educational costs. Many end users will choose to stay with their pressure transmitters unless they have a particular problem with them or are required to change technologies by regulations or because they need to move to a higher performance level. Even in this case, technology improvements by pressure transmitter suppliers may persuade end users to stick with pressure transmitter technology.”
In fact, most automation vendors continue to make incremental improvements to their pressure-based level measurement instrumentation families, both in terms of process interface capabilities and improved on-board intelligence, diagnostics and fieldbus communications. In recent years, several vendors have introduced either entirely new pressure-based level measurements products, such as multivariable level transmitters, or new approaches, such as asymmetrical, “tuned” diaphragm seal systems. These are just some of the advancements that are helping to improve measurement performance and reduce life-cycle costs. The recent proliferation of wireless technology in industrial facilities also provides new opportunities for accessing diagnostic information and new level monitoring points, such as redundant level measurements or remote monitoring of offsite tanks.
“Due to the infinite flexibility of process connections, operating limits and system configurations, DP level can be used in almost any liquid level application,” commented Jeff Graupmann, DP level marketing manager at Emerson Process Management. “Pressure has been used to measure level for more than forty years, so it is well-established and trusted for continuous measurements ranging from simple monitoring to advanced control applications. Differential pressure is a simple reliable technology, which can be used for level, pressure and flow measurements, so it is well-understood by everyone in a plant. Recent technological advancements include new practices, such as direct mounting and advanced diagnostics. Digital sensors can also be linked together to create multivariable systems.”
The basic principle behind hydrostatic pressure-based liquid level measurements is fairly simple. Pressure and DP transmitters measure level based on the principle that pressure is proportional to the level of liquid multiplied by the specific gravity (the ratio of the fluid’s density to the density of water). Another way of looking at it is that level equals the hydrostatic head (pressure) measurement divided by the density of the liquid.
In open (vented) tanks or vessels, the DP transmitter is mounted at the bottom of the vessel. The high side or the sensor measures the hydrostatic pressure exerted by the fluid in the vessel. The low (reference) side senses atmospheric pressure.
In sealed (pressurized) tanks or vessels, a diaphragm mounted on the vessel at a point above the liquid is connected to the low side of the sensor to provide a reference leg. With conventional analog transmitters, the pressure difference between the high and low sides is converted into a pressure-proportionate output signal. With intelligent transmitters, the pressure difference is converted directly into engineering units.