40
Flow Level Pressure
The Importance of Overfill Safe Operation for Accurate Level Detection Using Guided Wave Radar
Magnetrol Tel: +32 52 45 11 11 • Email:
info@magnetrol.be
• Web:
www.magnetrol.com
Magnetrol® pioneered guided wave radar (GWR) by introducing the ECLIPSE® Model 705 two-wire, loop- powered transmitter for use in industrial liquid level applications. With unprecedented performance, GWR offered users a unique way to measure and control challenging media and process variations. Over the years, our engineers continued to drive GWR innovation with the release of the first high-temperature, high-pressure probe; the first patented steam probe; the first interface transmitter; and culminating in the incorporation of GWR into a patented Aurora® MLI chamber to offer true redundant measurement.
Although approvals like the German Federal Water Act (WHG) obtained from TUV, or the Flemish Regulation on Environment (VLAREM), certify Overfill Proof Protection, defined as the tested, reliable operation when the transmitter is used as overfill alarm, it is assumed in the analysis that the installation is designed in such a way that the vessel or side mounted cage will not physically overfill. In other words, transmitters can obtain overfill proof approval, without necessarily having the capability to measure level to the top of the probe. The only requirements are that the transmitters be installed properly and used within their defined measuring range.
However, there are practical applications where a guided wave radar (GWR) probe can be completely flooded with level all the way up to the process connection (face of the flange). Due to the physics of the technology, when this occurs, there can be adverse interaction between the desired level reflection and residual reflections at the top of the probe. This affected area at the top of a GWR probe is dependent not only on the probe itself, but also on the application and installation. Typical GWR probes have a transition zone (or possibly even a dead zone) at the top of the probe where interacting signals can either affect the linearity of the measurement or, more dramatically, result in a complete loss of signal.
While some manufacturers of GWR transmitters may use special algorithms to “infer” level measurement when this undesirable signal interaction occurs and the actual level signal is lost, the ECLIPSE®
Model 706 transmitter offers a unique solution by utilizing a concept called Overfill
Safe Operation. An overfill safe probe is defined by the fact that it has predictable and uniform characteristic impedance all the way down the entire length of the waveguide (probe). With a probe physically designed to be overfill safe, signal loss will not occur when level reaches the top of the probe. Inferring level measurement instead of actually measuring true product level always comes with some assumptions. But does making assumptions in an industrial process control environment make sense?
Magnetrol takes the stance that true level measurement should be the primary goal, and overfill safe probes are offered in a variety of coaxial and caged designs.
Coaxial Probes Figure 1
The coaxial probe is the most efficient of all GWR probe configurations and should be the first consideration in all applications. Analogous to the efficiency of coaxial cable, a coaxial probe allows almost unimpeded movement of the high frequency pulses throughout its length. The electromagnetic field that develops between the inner rod and outer tube is completely contained and uniform down the entire length of the probe. (Refer to Figure 1). This means that the coaxial probe is immune to any proximity effects from other objects in
Figure 2 the vessel and, therefore, can be used anywhere it can mechanically fit.
This unimpeded movement of pulses is critical to the concept of overfill safe probes. With this unimpeded movement and careful design of the upper probe seal assembly comes the fact that there is simply no adverse interaction of signals. With no adverse interaction of signals, the true level signal can always be accurately detected. As shown in Figure 2, the level signal is shown unaffected all the way up to the process flange.
The efficiency and overall sensitivity of a coaxial configuration yieldsrobust signal strength, even in extremely low dielectric
(εr ≥ 1.4) applications. The sensitivity of this “closed” design, however, also makes it more susceptible to measurement error in applications that can have coating and buildup. Even though the predictable coaxial signals allow for simpler configuration and commission, single element probes are becoming increasingly popular due to their immunity to coating and buildup.
Single Rod Probes
Single element GWR probes act quite differently than coaxial designs. With only one conductor, the pulses of energy develop between the single rod probe itself and the mounting nut or flange. In other words, the pulse propagates down and around the rod as it references its ground at the top of the tank.
Figure 3 shows the single element design and how the electromagnetic pulse effectively expands into a teardrop shape as it propagates away from the top of the tank (the inherent ground reference). This single element configuration (rod or cable) is the least efficient GWR probe style.
Because the design is “open,” it exhibits two strong tendencies:
• It is the most forgiving of coating and buildup.
• It is most affected by proximity issues. As these tendencies are
application/installation dependent, top of probe effects can only be described in general terms with respect to single rod probes.
To illustrate the effect, please refer to Figure 5 Figure 3
Figure 4
APRIL / MAY 2013 •
WWW.PETRO-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
