Radar offers an incredibly sensitive measurement of surface level that can be nearly maintenance free. The potential for an accurate inventory measurement is dramatic but with this extreme capability comes some extraordinary application considerations. A checklist is offered of implementation details to help achieve the full capability of this measurement.
The biggest benefits are for an accurate accounting of raw material and product tank inventories. Additionally, a rate of level change measurement by simply sending the level through a deadtime block with a deadtime large enough to show an appreciable change in level to maximize the signal to noise ratio can provide a feed rate measurement for a constant composition that rivals a mass flow meter. In these raw material and product storage tanks the composition and hence the density is well defined.
An extremely accurate level measurement can enable material balance and composition control and analysis as discussed in "Advances in flow and level measurements enhance process knowledge, control". The use of radar on distillate receiver level can enable internal reflux control and the use of the preferred material balance control scheme when the reflux flow is high enough where distillate level manipulates reflux flow. The use of radar on reactor level can enable residence time control for conversion control as seen on slides 6-9 and 10-16 in ISA Automation Week 2011 tutorial ISA-AW-2011-Biological-and-Chemical-Reactor-Control-Opportunities.pdf
The principal consideration is associated with the fact that radar is a surface measurement. Vortexes, turbulence, and even just ripples in the surface can cause significant noise. A slanted surface can cause a "no return" signal and until recently a very high level could be misread as no level. Success depends upon a careful review of vessel operating conditions, mixing, and geometry. Maintenance can be minimal if changes to the process and vessel are reviewed as to their effect on the radar installation. At extremely low levels, surface abnormalities as the tank is being emptied creating something similar to what you see in a flushing toilet may necessitate the installation of a different technology as a backup.
The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for non-contacting and guide wave radar. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature. For more information see the Chemical Processing July 2011 article "Making the most of Radar" and the Control magazine February 2012 Control Talk Column "Radar Love". For a detailed understanding see Chapters 5 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries. Reliability and precision (noise, repeatability, resolution, and threshold sensitivity) are most important.
- Is the dielectric constant of the liquid too low for even guided wave radar?
- Is software available to improve signal strength and ignore false echos?
- If foam is present, do you want to detect surface of foam or surface of liquid?
- Is a stilling well needed to reduce turbulence and foam?
- Will the return signal be affected by gaps/holes in the stilling well?'
- Will tank bottom reflect signals causing false returns?
- Is the non-contacting beam or guided wave radar probe located away from vessel center, agitator, coils, and inlet streams?
- Is the path open enough for non-contacting radar?
- Is the nozzle large enough for the cone (horn) antenna preferred for non-contacting radar?
- Will the nozzle neck be too long interfering with the horn antenna?
- For tall tanks and low dielectric, is the antenna large enough to handle the range and dielectric?
- Is the antenna size matched to stilling well size?
- Is high frequency radar needed for the non-contacting beam to be narrow enough for a tall tank and to avoid vessel internals?
- Is high frequency radar needed for recessed antenna or full port valve in nozzle?
- Is there too much vapor, foam, or condensation for high frequency radar?
- Will highest level including foam and swell be sufficiently below the radar antenna?
- Is the fluid too viscous, sticky, abrasive, or corrosive for guided wave radar?
- Is the dielectric constant so low guided wave radar is needed?
- Is the signal to noise ratio so low guided wave radar is needed?
- Is the surface so slanted a reflected signal to a non-contact device is unlikely requiring the use of guided wave radar?
- Is the minimum clearance between guided-wave probe and vessel internals > 4 inches?
- Is the stilling well diameter > 4 inches for guided wave radar?
- Do coatings and deposits require the use of a single lead guided wave probe?
- Do obstructing objects require the use of coaxial guided wave probes?
- Does a low dielectric constant require the use of coaxial guided wave probes?
- Does a viscous non-coating fluid require twin guided wave probes?
- Is the vessel so tall, flexible guided wave probes are needed for level measurement range?
- Does the guided wave radar probe need to be anchored to vessel bottom to reduce sway?
- Does a DP need to be used for low level measurement due to erratic surface when vessel is nearly empty (e.g. voids and vortexes)?
- Any need for separate lightning arresters on top of the tank?
- Is tank properly grounded to minimize noise and transformer effect?
- If an electronic calibration simulation is prepared for installation, will it match actual conditions?
- Does the electronic housing allow removal of components for repairs while in service?
- Do electrical connections and enclosure meet electrical area classification and codes in plant?