Emission Control
Hazardous storage tank pressure protection and emissions control
This article discusses the essential equipment necessary to safeguard lowpressure storage tanks from pressure change events , addressing the challenges posed by new standards , narrow operating ranges and the need for precise system component coordination . It also provides valuable insights into effective tank protection strategies , focussing on operational safety and minimised environmental impact .
By Omar Cruz , Emerson
About the author Omar Cruz has been working for Emerson for five years and is currently a marketing manager for Tank Products . He holds a BS in Mechanical Engineering , as well as dual master ’ s degrees in Innovation & Entrepreneurship and Business Administration , all from the University of Texas in Dallas .
While storage tanks can safely store thousands of barrels of product , they are sensitive to overpressure or vacuum conditions , both of which can lead to product loss or excess emissions . Tank ruptures or implosions can result when the tank ’ s design limits are exceeded . One straightforward solution is to install large vents on a tank , allowing it to release pressure or relieve the vacuum freely . This approach works well for tanks storing non-hazardous materials but poses significant safety and environmental risks for tanks containing volatile organic compounds . These tanks need a more sophisticated array of protection devices to ensure product quality , operational safety and minimal environmental impact . Tank protection solutions might include a combination of tank blanketing regulators , pressure vacuum relief valves , gauge hatches , emergency vents and other equipment .
Meeting new emission requirements
There is a wide range of storage tank designs , but this article concentrates on low-pressure storage tanks holding liquids that are stable at room temperature . These tanks usually have a fixed roof construction that creates a vapour space between the stored liquid and the top of the tank . High-pressure and cryogenic tanks are beyond the scope of this article . With these types of tanks , regulating the internal pressure within vacuum and pressure design limits is critical . Many storage tanks can sustain a vacuum of only a few inches of water , and the maximum pressure may be one PSI or less . Operation even slightly beyond design pressure or vacuum can result in immediate and catastrophic damage ( Figure 1 ).
Figure 1 : It does not take much pressure or vacuum to damage or even destroy a typical storage tank .
To avoid damage , the vapour space in the tank needs to be carefully regulated as ambient or operating conditions change . There are also a host of new regulation requirements released by the Environmental Protection Agency . Title 40 / Chapter I / Subchapter C / Part 60 / Subpart OOOOb / c ( more commonly referred to as Quad O ) mandates have been broadened to encompass storage tanks that commence construction , modification or reconstruction after 6 th December 2022 and emit over six tons of volatile organic compounds annually . These tanks must reduce their emissions by a minimum of 95 percent .
Pressure control design
It may not be obvious , but tanks naturally and continuously breathe in and out due to various operating modes and environmental effects . To design a proper tank protection system , these effects must be fully understood . Tanks tend to develop a vacuum and need to breathe in when the following events occur :
• Material is pumped out of the tank , so the volume of the liquid must be replaced with vapour .
• Ambient air temperature drops due to weather changes , causing the vapour in the tank to contract .
Tanks tend to develop excess pressure and needs to breathe out when these events occur :
• Material is pumped into the tank , so the volume of liquid increases , displacing the vapour space .
• A rise in ambient air temperature may result in the evaporation of the product into the vapour space , leading to an increase in vapour pressure .
• Fires in the area , for example a pool fire near the tank , will boil the contents and create an enormous vapour load that must be vented .
• Other overpressure scenarios depend on the tank contents and specific equipment around the tank .
Tank pressure control systems require a collection of equipment to maintain the tank pressure at a slightly positive level , often less than 10 ” WC . A typical tank pressure control system ( Figure 2 ) usually includes a number of pressure control devices working together to provide layers of protection . Ideally , the tank pressure is maintained in the green zone by adding nitrogen through a blanketing regulator ( Figure 2 , Item 1 ) or by venting excess pressure to a scrubber / vapour recovery / flare system ( Figure 2 , Item 2 ). This arrangement keeps oxygen out of the tank , avoiding an explosive mixture in the vapour space , and it minimises vapour / product loss and environmental emissions .
46 Valve World March 2025 www . valve-world . net