Managing Storage Tank Pressure and Overfill Prevention

Across the energy supply chain bulk petroleum storage terminals play an important role in managing supply and demand. A critical safety function is to prevent an overfill condition in each of the storage tanks. Another critical function is to manage the pressure inside the tank to avoid overpressure or vacuum conditions that could damage the structural integrity of a tank.

In this one-hour recorded webinar, Tank Pressure Control and Overfill Prevention, Emerson’s Michael Calaway and Magnus Johansson discuss the technologies and applicable standards to protect these tanks from overfill events and unsafe pressure conditions.

Emerson's Michael Calaway

Emerson's Magnus Johansson

After some introductory remarks, Michael opened his part of the webinar at 5:35 discussing tank pressure control for atmospheric or low-pressure storage tanks. He defined these as ones below 15 psig. Within refineries and chemical plants, many types of storage tanks exist including open-top tanks, fixed-roof tanks, and floating-roof tanks.

Floating-roof tanks may include external floating-roof tanks, internal floating-roof tanks and closed floating-roof tanks. Moving from open-top tanks toward closed floating-roof tanks improves safety and product loss controls. For this presentation, Michael focused on fixed-roof tanks.

The key for these types of tanks is to control the vapor space between the liquid level in the tank and the tank roof. The idea behind floating roof tanks is to eliminate or minimize this vapor space.

Common terms for tank pressure control are out-breathing and in-breathing. Out-breathing is when a tank releases pressure. This increased pressure can be caused by inflow into the tank or increasing temperatures inside the vapor space. In-breathing is when the vapor space cools and contracts or liquids are pumped out of the tank and the pressure control system compensates by adding to the vapor space.

API 2000 – Venting Atmospheric and Low-pressure Storage Tanks (adopting ISO280300) is the standard to look at for pressure management. This standard has gone through many editions and now at version 7. Changes from earlier editions may cause older tanks not to have enough vacuum capacity.

Poor pressure control can not only lead to tank damage in the most severe case, but also high nitrogen gas blanketing use, increased maintenance and oxygen increase, which may affect product quality.

Beginning at 12:50 of the recording, Michael describes tank pressure control equipment including emergency venting, tank blanking pressure regulators, pressure/vacuum relief valves (PVRVs), and vapor recovery systems.

At the 31:05 part of the recording, Magnus discusses tank overfill prevention. He discusses how to protect your facility, implementing to the relevant standards, overfill prevention in practice, and simplifying compliance to safety standards.

The control system for the terminal site provides the first layer of protection. Safety instrumented functions monitoring the level of the tank provide the second layer of protection. Beyond these layers, mitigation layers include secondary containment dikes and plant emergency response procedures. The tank gauging system exists at the first layer.

An independent overfill prevention system is at the second layer. If the site has a safety instrumented system managing all the safety instrumented functions, level measurements suitable for the risk reduction requirements are used.

Magnus discusses how radar technology can be used for these level measurements at the safety system layer. Radar technology is non-contacting with the liquid level, has no moving parts, is proven in use with a high mean time between failure, is highly accurate and has a long lifespan.

Watch the recorded webinar for more on the applicable standards such as API 2350 and IEC 61511, and radar technologies suitable for overfill prevention systems.

You can also learn more by visiting the Terminals & Logistics section of