Online communities continue to grow and prosper in our world of process automation because they provide a peer-to-peer environment for problem solving. The DeltaV LinkedIn group is one that has had a steady growth in membership. In this group, a question was recently posed:
Good day everyone. i was just wondering what are your thought/experience with using Hydro-static tank gauging systems and Radar for custody transfer. We are in a dilema on which we should use
When I saw that message, I sent a note off to the Rosemount Tank Gauging team for their thoughts. I received a note back and posted:
Whether to use radar or HTG systems depends on what parameter you need to measure. Do you need mass measurements or level measurements? HTG systems are based on head pressure readings where the level reading is dependent upon the Specific gravity of the fluid and the head pressure. Density of the fluid can be measured and this allows compensation for any density changes of the fluid. If you are trying to measure the mass of the fluid, this is the way to go. However, if you are interested in the actual level, the radar is a better option. Radar will measure the distance to the surface and is unaffected by changes in fluid properties. It will provide a much more accurate level reading, but will not provide a mass reading.
Jonas Berge offered additional guidance:
I would like to add that while radar tank gauging systems in the past required lots of cable and flame proof / explosion proof conduits for each of the radar, multi-point temperature / bottom water transmitter, top and bottom pressure transmitters, and field displays, this is a thing of the past. You can now deploy a two-wire intrinsically safe bus-based tank gauging system: http://www.rosemount-tg.com/raptor
It is an excellent example of how bus technology make projects easier and operation better.
I turned to Emerson’s Carl-Johan Roos, on the Rosemount Tank Radar team for more on the Raptor tank gauging system. The development of this system was five years in the making. The Raptor development team worked with more than 250 customers to identify shortcomings with existing tank gauging systems. Carl-Johan enumerated these areas of pain including:
- Risk of overfill
- Being locked in to one supplier
- Problems with interoperability
- Worry about investing for the future
- Compliance to future rules and regulations
- Ability to deliver accurate storage information at high availability to their customers
- High maintenance and installation cost
The key advancements were to base communications on the industry standard Foundation fieldbus digital protocol and IEC 62591 WirelessHART protocol. The wired devices are two-wire and are suitable for use in process safety applications up to SIL 3. Also, more diagnostics and improved access to device information are available with these digital protocol-based devices.
Components in the Raptor system include level measurement, temperature & water interface, pressure measurement,
wireless, communication & accessories, and the TankMaster operator interface. Carl-Johan noted that the Rosemount 5900S non-contacting radar became the first of this class of level measurement device to receive certification for SIL 3 safety applications.
Carl-Johan shared a specific example of how the Rosemount 5900S with a 2-in-1 radar level gauge can address the risk of overfill pain. A single unit can use one of the level measurements for the actual level and the second as an overfill measurement. The onboard diagnostics provide continuous proof testing between the two measurements and alert the operators if the delta between the two signals exceeds a defined threshold. Here is an example of a chemical terminal tank overfill protection application highlighting its challenges, solution, and results.
Back to the original question posed above as to whether to use a hydrostatic system based on mass, or a radar-based system based on volume, the Raptor system provides for a combination of the two in a hybrid system. Hence, radar is used for level, and pressure is used for on-line density measurement. The output is both net standard volume, and mass. An advantage of this system configuration compared to a traditional hydrostatic system is that it keeps track of the level to help you prevent overfills.