SPECIAL TOPIC: EMISSION CONTROL IN TANKS
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 commenced construction, modification or reconstruction after 6th 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 need to breathe out when these events occur:
Figure 1: It does not take much pressure or vacuum to damage or even destroy a typical storage tank.
Figure 2: Tank pressure controls require a variety of devices that operate to keep the tank pressure neutral( green zone). Pressure protection is provided in layers, with each device coming into operation as pressure or vacuum changes in the tank vapour space.
• 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 minimizes vapour / product loss and environmental emissions.
Unusual events, such as extreme weather, may overwhelm the capability of the first protection of layers and activate a tank pressure / vacuum vent into operation( Figure 2, Item 3). This vent pulls in air to break the vacuum, or it vents air to the atmosphere to relieve pressure.
Regulations and codes require tanks to be protected with emergency vents to handle scenarios such as a nearby fire. In some cases, the tank pressure / vacuum vent( Figure 2, Item 3) can serve this purpose. However, many tanks will require a very large emergency vent( Figure 2, Item 4) to relieve the large vapour load created by this type of fire. Some tanks also include a thief hatch( Figure 2, Item 5) to allow manual level measurement and / or sampling of tank contents.
Pressure, vacuum and emission equipment design
Requirements will vary by tank and application, but a hazardous storage tank will usually require several different devices to provide pressure and / or vacuum relief. Each of these devices and their key selection criteria are discussed below:
Tank blanketing regulators
Tank blanketing regulators sense tank pressure and add an inert gas, usually nitrogen, to break the vacuum. As a good practice, tank blanketing regulators are set at a few inches of water, maintaining a slight positive pressure and keeping oxygen from entering the tank. Blanketing regulators face the challenge of accepting full-pressure nitrogen( often 80 PSI or higher) and reducing it to maintain a
APRIL 2025 • FUGITIVE EMISSIONS JOURNAL 13