Moving and storing hydrocarbons and other volatile fluids in tank farms and bulk liquid storage facilities require well-designed and functioning measurement and safety systems.In a Tank Storage magazine article, The Critical First Line Of Defence Against Overfills, Emerson’s Johan Sandberg describes how the root cause of overfill incidents can be avoided. He opens with a sobering statistic:
…insurance data shows that one in every 3,300 filling operations results in an overspill.
Avoiding overspill requires:
…multiple layers of overfill protection, such as an automatic overfill prevention system, and while these back-up measures are undoubtedly vital, it should be remembered that the tank gauging system provides the critical first layer of overfill prevention.
Johan cited an accident in Puerto Rico, which was analyzed and documented in a U.S. Chemical Safety Board video. The accident investigation determined that the site had an:
…unreliable system for monitoring and controlling the level of gasoline inside the storage tanks, and this had set the stage for the incident. The float and tape measuring devices used… were prone to mechanical failure, were poorly maintained, and were frequently not working on multiple tanks at the same time. Cable breakages often disabled the electronic transmitters that sent tank level measurements to the control room.
The CSB investigation noted:
…good engineering practice would have called for at least one additional layer of protection, to detect and alert operators to the danger of an overfill, even if the primary system for measuring the tank level fails, and to shut off or divert the flow into another tank when the level is critically high. Had this been in place and properly maintained, the accident most likely would not have occurred.
Johan explains that the first layer of defense is the basic control system including a distributed control system (DCS), tank gauging system and inventory software. Different protection layers to prevent or mitigate include the second layer:
…an automatic overfill prevention system, and the third level is a layer of secondary overfill containment, for example dikes. In a worst case scenario, the top level is the emergency response layer, namely alerting the fire brigade and other emergency services.
Technologies such as the Rosemount Tank Gauging system:
…provide precise tank gauging through the use of a radar level gauge, a tank hub to collect the level measurements, and an inventory management system that enables operators to observe and check the level measurements…
He describes 2-in-1 technology, which:
…enables a single radar device to contain two separate and independent electrical units within its transmitter head. These can act as one primary and one back-up level gauge, or one level gauge plus a high level alarm. This system offers enhanced reliability as the gauges are always in operation, have no moving parts and do not make contact with the liquid in the tanks.
And, where the infrastructure in tank farms and bulk liquid storage facilities don’t include existing wiring or capacity to add additional wired measurements, wireless radar level gauges can be added.
The automatic overfill prevention system is always wired, but using wireless and wired communication in combination provides two independent data paths to the host/DCS. The use of wireless for tank gauging data means that the existing field cabling (which may have limited availability) can be used for other purposes, for example when operators need to get both tank gauging data and a high level alarm signal back to the control room but only have one set of wiring available to the tank.
Read the article for more as Johan describes the need for tank farm assessments to make sure the site fulfills the requirements outlined in the applicable safety standards and regulations. Here is additional information on overfill prevention and level measurement.