Failure to identify and prevent the overfilling of tanks and vessels containing hazardous, flammable and even explosive materials can have catastrophic consequences, as high-profile incidents such as the Texas City Refinery fire in 2005 illustrate. Because of the nature of the materials typically involved in the process and bulk liquid storage industries, overfills can result in loss of life and devastating damage to assets and the environment. The cost of such incidents can reach billions of dollars, while legal consequences and bad publicity can seriously undermine a company’s reputation.
It is essential, therefore, for companies in these industries to implement robust overfill prevention systems (OPS), which minimize risk and comply with relevant safety standards. The explosion and subsequent fire at the Texas City Refinery resulted from tank overfilling and a lack of overfill prevention technology. This disaster, and other similar incidents, led to the introduction of new standards and safety guidelines, which are being widely adopted. API 2350 and IEC 61511 are the two key global standards for overfill prevention. IEC 61511 provides best safety practices for the implementation of a modern OPS within the process industry. API 2350 provides minimum requirements to comply with modern best practices in the specific application of nonpressurized, above-ground large petroleum storage tanks.
Layers of protection
To minimize the risk of overfills, it is common practice to implement a series of independent layers of protection. The Basic Process Control System (BPCS), which monitors and controls production processes, forms the primary protection layer. Then comes the safety layer (the OPS), which must remain separate and independent of the BPCS to provide redundancy. If the BPCS fails or is not working correctly, the OPS should prevent an overfill from occurring. The third layer is passive protection, which involves containment with dikes or concrete walls. In the unlikely event of all these layers failing to prevent or contain an overfill, the emergency response layer is there, which involves alerting the emergency services.
MOPS and AOPS
OPS can be manual (MOPS) or automatic (AOPS). MOPS typically involve a level sensor or switch transmitting an alarm to an operator, notifying them of what actions should be taken to prevent an overfill. MOPS are regarded as easier to implement and less complex and costly than AOPS. However, the potential for human error limits their risk reduction factor, so there is a strong trend toward replacing them with AOPS, which consist of a level sensor, a logic solver and a final control element in the form of actuated valve technology. The significant benefits of AOPS include higher risk reduction factors, shorter response times and reduced operator workloads.
It is important to understand, though, that no "one size fits all" solution exists for overfill prevention. Each application has its specific challenges, so it is vital to select the appropriate technologies for each. These technologies include:
Electromechanical float and displacer switches — These switches have moving parts, which require frequent and costly maintenance, are affected by mechanical vibration and turbulence, and can give false readings. They are used for point level, interface and density applications, in which the buoyancy of the displacer in the liquid is the primary measurement principle. However, they are increasingly being replaced by more reliable electronic technologies that offer greater diagnostics and lower life cycle costs.
Vibrating fork switches — In this point level technology, a piezoelectric crystal oscillates two prongs at their natural frequency, with the frequency then varying as they are immersed in the medium. Frequency changes are detected by the electronics, enabling the presence or absence of liquids to be detected. These switches are highly reliable because they have no moving parts to wear or stick, making them less prone to failure than other technologies. They are also virtually unaffected by flow, turbulence, bubbles, foam, vibration and changing density.
Guided wave radar (GWR) — In GWR technology, low-energy microwave pulses are guided down a probe, which is submerged into the medium. The level can be measured when the microwaves are reflected from the surface to the transmitter. An interface can also be detected since a proportion of the pulse continues down the probe. GWR transmitters are easy to install, and no compensation is necessary for changes in density, dielectric or conductivity. Changes in pressure, temperature and most vapor space conditions do not affect accuracy. The technology is unaffected by high turbulence or vibrations, buildup has practically no effect, and maintenance requirements are low.
Noncontacting radar — Noncontacting radar transmitters can measure using either pulse or Frequency Modulated Continuous Wave (FMCW) techniques. With pulse radar, microwaves are emitted toward the surface and reflected to the sensor, with the level being directly proportional to the time from signal transmission to reception. With FMCW, the radar transmits a continuous signal sweep with a constantly changing frequency. The difference between the frequency of the reflected signal and the frequency of the signal transmitted at that moment is proportional to the distance from the radar to the surface, which enables the level to be measured. This easy-to-install technology is unaffected by density, viscosity and conductivity, little affected by coating and vapors, and requires little maintenance.
Health monitoring and proof-testing
The latest vibrating fork switches, GWR and noncontacting radars feature powerful built-in diagnostics and can perform partial proof tests remotely, therefore providing significant advantages over older, mechanical technologies. Monitoring the health of devices ensures they will perform correctly in OPS, while the ability to perform remote partial proof tests saves time and increases worker efficiency.
One noncontacting radar level gauge offers a unique two-in-one solution when delivered with two separate and independent electrical units within one housing. When connected with the cables separated in different cable trays, this option enables a single gauge to be used for BPCS and separate OPS purposes in compliance with IEC 61511 and API 2350.
Overfill prevention measures are employed in three general application types — process vessels and storage tanks within process applications, and storage tanks deployed within the bulk liquid handling industry. Each presents specific challenges for overfill protection technology.
Process vessels — These vessels are where a specific industrial process, or part of an overall process, takes place, and the vessel’s shape, size and design must be considered when selecting overfill prevention technology. On cone-shaped tanks, for example, level sensors are top-mounted, so GWR transmitters, noncontacting radar transmitters and vibrating fork switches are suitable options. In vessels with internal restrictions, such as agitators, heat exchangers and other structures that require the use of a separate chamber to perform the level measurement, GWR transmitters are recommended.
Vibrating fork switches are ideal when a side-mounted solution is required. In distillation columns, where chambers are required, GWR transmitters are commonly used for the AOPS. Because blending tanks contain agitators, this places restrictions on sensors that protrude into the tank, so a top-mounted, noncontacting radar is a good choice of sensor. In boiler drum applications, SIL 3 GWR transmitters are required for AOPS — usually with triple redundancy — because they are unaffected by changes in process conditions.
Storage tanks — Tank monitoring systems for multiple small- or medium-sized vessels, or a smaller tank farm of five to 20 tanks, require an automated system to provide level monitoring, but not necessarily control. Tank monitoring would typically involve gross volume calculations, but not fiscal measurements. In such applications, overfill technology selection would be determined by the tank type, the number of available openings and the liquids contained.
Bulk liquid applications — An automatic tank gauging (ATG) system is typically used as the BPCS in these applications to measure level and calculate inventory. These systems require technology with high levels of accuracy since a small inaccuracy in level measurement can equate to thousands of gallons of volume uncertainty, which would be extremely costly. The AOPS typically consists of a noncontacting radar transmitter, a logic solver and an actuator. Alternatives would be a vibrating fork switch or a GWR transmitter. In bulk storage tanks with floating roofs, the best practice is to measure through a still pipe, which requires high-precision, noncontacting radar for accuracy.
Support and guidance
Because level measurement technology selection is application-dependent, it is vital to understand the specific challenges each one presents. Automation suppliers that offer a broad portfolio of level measurement solutions are perfectly placed to provide support and guidance on selecting, installing and implementing the most suitable technology.
Per Skogberg is an engineer at Emerson Automation Solutions. He has expertise in radar level measurement and tank gauging systems, first and foremost for tank farms, refineries and bulk liquid storage of petroleum products. He can be found on LinkedIn. To learn more about overfill prevention, visit emerson.com/overfillprevention.