Bulk liquid storage tanks are found in many industrial processes including refineries, tank terminals, pipeline terminals, petrochemical and LNG plants, and aviation fuel depots to name a few. There are two predominant requirements from a process safety and control standpoint—inventory management and overfill protection.
Emerson’s Ulf Johannesson, a member of the Rosemount Tank Gauging team, presented, SIL 3 Certified 2-in-1 Radar gauge redefines Tank Gauging best practices, at the 2012 Emerson Exchange conference this past October. He opened noting that the tank gauging system is the most critical layer to avoid overfills. And, level is a critical process variable to measure for the safety of a tank farm.
Traditional gauging and overfill protection has been performed with level switches connected as inputs to safety PLCs, which in turn is connected to the safety valve to complete the safety instrumented function (a.k.a. safety loop). Overall safety of the tank farm is a driving force behind replacing the switches with radar level gauges that provide continuous measurement. Most switches provide a signal only when they change state, require manual proof testing, and usually do not provide remote communications or diagnostics beyond the state change.
The continuous level process variable coming from a radar gauge can provide multiple alarm levels, remote proof testing, and change in level or delta measurements.
Ulf noted that several standards exist for the use of radar gauges in process safety applications. The global IEC 61508 (for instrumentation suppliers) and IEC 61511 (for process manufacturers) encompass the entire safety lifecycle and provide a path for safety-certified devices, based on their failure statistics. These devices provide users with a quantitative risk reduction factor by defining requirements for different safety integrity levels (SIL).
The TÜV/DIBt WHG (Wasser Haushalts Gesetz) is another standard applying to water-protection approval. It allows a single device for both level and overfill measurements. Although it has German approval, it has been used in other locations due to a lack of alternatives. The American Petroleum institute has the API2350 standard, Overfill Protection for Storage Tanks in Petroleum Facilities (described in detail in 10-2420).
Ulf noted that the Raptor Tank Gauging System has attained certification for use in up to SIL 3 applications. The system includes 5900S non-contacting radar level gauges, 2240S multi-input temperature transmitters, and a 2410 tank hub which powers and collects data from the field units. Here’s an architecture picture from the presentation:
One issue present in some tanks such as those with a spherical design is that they only have a single opening at the top. Ulf described a 2-in-1 radar level gauge that contains two independent and continuous level measurements, which can occur through the same tank opening. A common use for these 2-in-1 transmitters is to have one measurement for level and the second for overfill measurement.
Another use is for redundant measurements, and a third use is to have an onboard spare. These 2-in-1 and 3-in-1 measurements came from the marine industry for measurement of level, high, and high-high level alarms on crude oil tankers, bitumen and sulfur tankers, petrochemical carriers, and floating production storage loading and offloading (FPSO) vessels.
The 2-in-1 5900S is rated for use in SIL 2 and SIL 3 overfill prevention system applications. Proof testing is built in since the two radar gauges continuously monitor one another. These real-time diagnostics extend the required manual proof testing intervals and reduce the need for plant personnel to enter these hazardous areas to perform these tests.
Ulf noted that the device has been certified for use with an SIS/SIL certificate, TÜV WHG, and API 2350 Category 2 & 3.
He closed his presentation pointing to a couple of documents for further reading: The Engineer’s Guide to Level Measurement, Level Engineer’s Guide to Refining, and The Complete Guide to API 2350 Overfill Protection. Check them out if tank farms are a part of your process.
Update: I fixed the link to the The Complete Guide to API 2350 Overfill Protection guide.