Bulk liquid storage tanks often contain fluids that are hazardous, flammable and/or explosive. These tanks require not only a control system to control levels but and overfill prevention system (OPS) as layer of protection beyond the control system. This basic process control system (BPCS) is commonly referred to as an automatic tank gauging (ATG) system and the OPS is the safety instrumented system (SIS).
A Fluid Handling Pro article, Technology Selection for Automatic Tank Gauging and Overfill Prevention Systems explores these systems and the relevant standards to apply to ensure safe operations.
IEC 61511 Functional safety – Safety instrumented systems for the process industry sector and API 2350 Overfill Protection for Storage Tanks in Petroleum Facilities are key standards to follow.
IEC 61511 sets out best safety practices for implementing a modern OPS in the process industry. In addition, API 2350 addresses the minimum requirements to comply with modern best practices in the specific application of non-pressurized above-ground large petroleum storage tanks.
While manual overflow prevention practices have been traditionally used:
…the risk reduction factor of manual systems is limited and there is a strong trend towards replacing them with automatic systems, which can achieve higher risk reduction factors, shorter response times, and a reduced workload for operators.
Level switches have historically been used in overfill prevention applications, but these can be difficult to test and verify that they are working correctly. Non-contacting radar gauges provide high:
…reliability, availability, and safety. Since no regular maintenance or recalibration is required, the availability is close to 100 percent during a device’s long lifespan.
Many believe the technologies used in OPS and ATG systems be diverse—not the same technology. This is known as diverse separation.
However, technology diversification is not a requirement of either IEC 61511 or API 2350.
Identical separation is:
…using the same technology for both the BPCS and SIS whereas diverse separation would mean using different technologies from the same or different manufacturer.
Diverse separation reduces:
…the probability of systematic faults affecting multiple channels at the same time and/or from the same cause and hence reduces correlated failure of multiple channels.
However, diverse separation also:
…introduces extra complexity and increases the likelihood of human error, as personnel need to learn about installing, configuring, and proof-testing two different technologies rather than just one.
Non-contacting radar technology, such as the Rosemount 5900S 2-in-1 radar level gauge includes two radar gauges in one housing for independent level and overfill measurement. It is SIL 2 and SIL 3 certified according to IEC 61508 and enables API 2350 compliant solutions.
The most obvious benefit of this configuration is that it requires only a single tank opening. This allows for cost-efficient safety upgrades of existing tanks by replacing a single existing ATG or OPS sensor with two continuous level measurements with a minimum of tank modifications.
Read the article for more on common misconceptions in LPG and LNG tank applications, safety certifications, and maintenance & proof-testing considerations.
Visit the Rosemount 5900S Radar Level Gauge on Emerson.com for more these non-contacting radar gauges and their use in automatic tank gauging and overfill prevention systems. You can also connect and interact with other tank gauging and level measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.