Level measurement technologies have become more versatile, robust and accurate thanks to recent developments. Differential pressure and radar level measurement provide examples
Level measurement applications within the chemical and petrochemical industries can be extremely challenging. Level instruments must provide repeatable, reliable and accurate measurement of hazardous materials over wide temperature and pressure ranges, and with complicating factors, such as steam, dust, foam, turbulence and condensation.
Solving the challenges of measuring the levels of fluids and solids in chemical processing applications is as old as the industry itself. Traditional methods, such as sight glasses and mechanical float switches or differential pressure (DP) level systems with wet- or dry-leg impulse piping, have all been used extensively and are still around today. But they can cause maintenance and reliability headaches, so many are getting replaced or augmented by more sophisticated methods.
DP level systems are reliable when applied properly, and well understood, so they continue to be commonly used. Advances in technology have solved many of the difficulties caused by traditional impulse-piping based systems. Radar is another option for easy and reliable level measurement that has also seen many advancements in technology to broaden overall capabilities. DP level and radar instruments are a mainstay with producers today because they can protect people, plants and products. This article looks at ways in which DP level and radar have advanced in recent years to deliver reliable performance for critical level measurements.
Making DP more robust
Using a DP reading to determine liquid depth is a proven and reliable practice (Figure 1). Because of this, DP level continues to be one of the most common methods used to make level measurements. Internal tank structures, active agitation and corrosive or viscous processes that may cause challenges for alternative level technologies are ideal applications for DP level technology, which delivers a repeatable, stable and accurate measurement provided it is installed correctly and product density characteristics are understood.
DP challenges. But using DP to measure fluid levels is not without its challenges. Most problems relate to the necessity of mounting the head space pressure tap some distance above the tank bottom, and then connecting it to a DP transmitter using impulse lines. Since those impulse lines are connected directly to the process, they are often a source of headaches for any kind of DP application, because they can plug from accumulated debris, fill with the wrong product (gas in liquid lines and vice versa), and are subject to freezing and other environmental effects.
Remote seal systems. Remote diaphragm seal systems that use capillary tubes are one solution to some of the DP level challenges. A capillary tube is small-diameter, oil-filled tube that is provided as part of a DP level remote-seal system. It connects the remote diaphragm seals back to the pressure transmitter, which eliminates the need for impulse piping. This solves the main problems associated with plugged impulse lines — condensation in a dry leg or evaporation in a wet — but it introduces new trade-offs.
Balanced systems. There are two ways to implement remote-seal capillary tube systems. One creates a balanced system using two capillaries that are even in length on both the high and low side to keep temperature effects from creating errors between the two readings. Unfortunately, with balanced systems, the difference in the height between the two measurements creates its own error. Also, having lots of extra capillary tubing, which often ends up coiled up by the transmitter, adds unnecessary cost.
Tuned assemblies. It was because of the drawbacks associated with balanced systems that the tuned assembly system was introduced. With this approach, the high-pressure side has a DP transmitter directly connected to the remote seal at the process connection and a single length of capillary tubing connected to the remote seal on the low-pressure side at the top of the tank. This creates an asymmetrical design that minimizes the overall fill-fluid volume within the system and allows the temperature effects from a vertical installation of capillary tubing to be compensated in the transmitter’s calculations. This reduces overall system errors and improves response time when compared to a balanced system. Tuned assemblies, however, do have limitations. The longer the length required for the low-pressure capillary tube to reach the top of the tank, the greater the error associated with it. Therefore, for tall vessels, another approach is needed.
Electronic remote sensor. The newest technology for DP level measurement is an electronic remote sensor (ERS) system (Figure 2). An ERS system replaces the capillary tube connection to the top of the tank with a remote sensor connected electrically. This allows the head-space pressure measurement to be sent digitally, eliminating the problematic impulse lines and capillary tubes. While this ERS technique may sound like an obvious solution, it needs to be implemented correctly to make sure other errors are not introduced. It is critical to have special signal processing between the two transmitters, so they are working in synch to take readings simultaneously and provide a single DP output that avoids errors caused by changing level or pressure. If instead two independent readings are taken and used to make a calculation in the distributed control system (DCS), this creates the added cost of requiring two connections back to the host. Still, other errors can creep in that compromise the accuracy of the DP measurement.
Thermal range expander. Another specialized technology available for DP level systems used above 400°F (200°C) is a thermal range expander using multiple fill fluids. This type of system uses two fill fluids — high- and low-temperature — separated by an internal diaphragm. A thermal-range expander system can connect to the tank via capillary, or it can be directly mounted to the remote seal, which is ideal for use with tuned assembly or ERS systems. The high-temperature fill fluid can be exposed to the full process temperature while a general-use fill fluid covers the rest of the system. Traditionally, high-temperature fill fluids cannot move fast enough to get a good response at ambient temperatures and would otherwise need to be heat traced to make sure they are always within their operating range. A thermal range expander eliminates the need for heat tracing because it allows both fill fluids to stay within their optimal operation temperature ranges.
Radar level measurement
While DP level measurement starts at the bottom, radar works from the top down. It measures the amount of space inside the tank between the top of the vessel and the surface of the medium, which can be liquid, solid or powder. Radar is relatively new as level technologies go, but because of its advantages in reliability and low maintenance, the chemical process industries (CPI) have widely adopted it, especially for control applications.
Radar takes two forms: guided-wave radar (GWR) and non-contact radar. Both work by sending out a low-energy microwave signal towards the product surface, where it is reflected. Using the reflection, the distance to the surface is calculated and then translated into the level measurement. In non-contacting radar, the signal travels freely through the air or vapor space on its journey to the surface. With GWR, a metal probe extends down through the air or vapor space and into the process medium. If the medium has a low-dielectric or low-reflective property, a portion of the pulse continues down the probe past the surface, which can allow for detection and measurement of an interface level in addition to the basic level measurement. With older units there were limitations on how thin interface layers could be, with a minimum top layer of around 10 in., but advancements have been made to allow for accurate measurements of layers as thin as about 2 in.
Another key advantage of radar technology is that no compensation is necessary for changes in density, dielectric constant or conductivity of the medium to measure the level accurately.
GWR technology. With GWR, the probe helps concentrate the signal, which gives it the advantage of seeing beneath the initial level to measure an interface, but also can help with measurements on a turbulent surface. Moreover, when faced with turbulence combined with a low dielectric constant medium, which causes a low signal return, a method called probe-end projection is able to use the location of the end of the probe to calculate the actual liquid surface.
GWR devices are easy to install and can be used in chemical and petrochemical tanks of all sizes, including those with side connections. GWR transmitters have been developed that are able to send their data via WirelessHART and are driven by an internal power module. The ability to install these without any power or signal cables is a significant advance, substantially reducing the installation cost.
The main drawback of GWR is the probe itself, making its use impractical if there are moving agitators or other equipment inside the tank. Some situations allow for material to coat and build-up on the probe, but the effects can be managed by using a transmitter capable of providing enough power to allow for a single lead probe, thereby reducing the opportunity for buildup, or by using diagnostics, such as signal quality metrics, to monitor the signal and determine when buildup is severe enough to affect accuracy so it can be cleaned off before causing problems.
If the dielectric constant of the vapor in the headspace changes significantly, such as happens when the space is filled with saturated steam, it can affect the accuracy of the level reading, but there are advanced GWR transmitters on the market that can use dynamic vapor compensation to correct themselves (Figure 3).
Advances in safety for GWR also include the ability to use a target on the probe to perform high-level proof testing without the need to raise the liquid level. This provides a safer and simpler way to do a full proof test.
Non-contacting radar transmitters. Non-contacting radar level transmitters provide continuous level measurements, but without touching the process medium. Some models use a microwave pulse, while others send a frequency-modulated continuous wave (FMCW) to perform the measurement. With pulse radar, the same time-of-flight technique used by GWR determines distance. With FMCW, the transmitter sends a continuous signal sweep with a constantly changing frequency (Figure 4). The frequency of the reflected signal is compared with the frequency of the signal transmitted at that moment, and the difference between these frequencies is proportional to the distance from the radar to the surface, providing the level measurement.
Until recently, pulse radar transmitters were the only units suitable for two-wire installations, due to the higher power consumption of FMCW, but advances in FMCW transmitter electronics have solved this problem. This is a major advance for many users because the FMCW transmitters actually provide much stronger signals due to the amount of information allowed by the stream of signals, as opposed to single pulses. Non-contacting radar transmitters have key advantages over GWR. Since there is no probe, they are easier to isolate from a corrosive process using a polymer process seal antenna to cover the metallic components, or where the transmitter has to be mounted on a valve so it can be closed off from the process when necessary.
Some newer non-contacting radar transmitters provide an internal data historian and diagnostic routines that can help diagnose issues with the process and increase the safety capability of the devices. Simple proof-test methods can help reduce dangerous undetected failures, making them safer and simpler to use for safety instrumented system (SIS) or safety instrumented function (SIF) applications. Those simple proof tests can be performed remotely, which not only makes proof testing easier, but it saves time, minimizes exposure of workers to hazards in the field and avoids shutdowns of the process, as it can be done while the vessel is still in operation.
Expanding on the topic of safety for level measurements, vibrating-fork level switches provide a major advancement from float switches and can provide an additional layer of protection for overfill prevention and run-dry protection on pumps. Some advanced vibrating-fork level switches employ on-line diagnostics and remote proof testing, which make them excellent complements to the continuous measurements provided by radar and DP level.
Lydia Miller is a senior product engineer for Rosemount level measurement products at Emerson Automation Solutions (6021 Innovation Blvd., Shakopee, MN 55379; Phone: 800-999-9307; Email: firstname.lastname@example.org). Prior to this position, she was a senior marketing engineer at Emerson’s Chanhassen, Minn. office. Before joining Emerson in 2011, Miller worked in engineering sales at Innovent Air Handling Equipment and at North Atlantic Technologies. She holds a B.S. in mechanical engineering from the University of Minnesota (Minneapolis).
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