Types of Storage Tanks
Oil storage tanks come in various sizes, shapes, materials, and types, and are employed from initial production to the distribution and refining of different petroleum oil products. Most modern industrial oil storage tanks are made of carbon steel, stainless steel, reinforced concrete, and/or plastic.
There are seven different types of storage tank vessels available to store volatile organic liquids (VOL). These include:
1. Fixed roof tanks – Employed for holding oil products in lower vol-umes; the oil stored is not exposed.
2. External floating roof tanks – Con-sists of an external floating roof
3. Internal floating roof tanks – Con-sists of an internal floating roof which falls or rises according to the level of oil in the tank.
4. Domed external floating roof tanks
– Enclosed by one more tank or has a containment dike surrounding the tank, acting as a catch system for leaks.
5. Horizontal tanks (or leg tank) -Equipped with molded-in legs to support the tank and band locators for tie down.
6. Pressure tanks – Designed to hold gases or liquids at a pressure sub-stantially different from the ambient pressure.
7. Variable vapor space tanks -Equipped with expandable vapor reservoirs to accommodate vapor volume fluctuations attributable to temperature and barometric pressure changes.
Tank Emission Calculations
The document, ‘Compilation of Air Pollutant Emission Factors (AP-42)’, was published by the U.S. Environmental Protection Agency (EPA) and enforced since 1972. Supplements are regularly issued to add new emission source categories and to update existing emission factors including state and local air pollution control programs, and industry needs.
An emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. The emission factors presented in AP-42 may be appropriate to use in several situations, such as making source specific emission estimates for areawide inventories for dispersion modeling, developing control strategies, screening sources for compliance purposes, establishing operating permit fees, and making permit applicability determinations.
The purpose of the report is to pro-vide background information to sup-port revisions to AP-42 Section 7.1, Organic Liquid Storage Tanks and American Petroleum Institute (API) documents 19.2 and 19.4. Further, the U.S. EPA has provided TankESP, a tank emissions calculation software product suite that uses the emission estimation procedures from Chapter 7 of U.S. EPA’s Compilation of Air Pollutant Emission Factors (AP-42) for volatile organic compound (VOC) emissions from storage tanks. The software can help users:
• Include all storage tank types
(internal, external, and domed external floating roof tanks, fixed-roof tanks, and heated tanks).
• Include Meteorological data for 250+ U.S. locations and can be customized to include data for any location worldwide.
• Estimate emissions from floating roof drain-dry and liquid-heel landings
• Includes all deck fittings and tank modifications found in AP-42; easi-ly compare how emissions will be affected by changes in operation and equipment.
• Report total VOC emissions and specifies emissions of each HAP
• Break down emissions by source (rim seal, withdrawal, deck fitting, deck seam, landing and filling losses).
• Be user friendly – simply enter tank data to generate detailed, thorough reports.
• Be compatible with any operating system (unlike TANKS 4.09d*).
• Export printable reports to Micro-soft Excel.
Leak Detection on Tanks
It is important that leak detection and repair (LDAR) training for tank emission calculation tools be provided to the technicians doing the monitoring, no matter how simple the tool – for example user-friendly optical gas imaging (OGI) cameras used currently in U.S. and Canada. It is known that OGI cameras are efficient in finding emissions, but training on OGI cam-eras and experience is still required. Understanding the limitations of the OGI camera are equally important, such as when to slow down, and how to use the camera effectively.
Detection and quantification technicians need to know not only what type of leak they are looking at, for scanning purposes, but also how to describe where leaks are, and have the ability communicate that to their clientele. Having a written protocol and training procedure is very im-portant for maintaining a high level of compliance with technicians.
While a survey is being conducted on a plant site, it is imperative to have maintenance personnel around to identify fugitive emission leaks, safety requirements, and further guide technicians on the equipment usage; for example flow rates, peak performance hours etc. The survey should prioritize repairs based on safety, large emission leaks, operational schedules, production requirements etc. This will help clients to not only reduce emissions, but comply with codes and regulations, save product, and earn back credits. The only way to prioritize the list is to detect and quantify the emissions, some vendors offer use of high-flow samplers, however the most effective method is using an OGI camera that can compare differences in volume and leak size. A very tiny leak can be worth next to nothing, when compared to a massive leak costing hundreds of thousands of dollars a year in lost revenue.
With hundreds of tank sites operating within a corporation, regional plant level managers should recognize the value of using an effective data management program over traditional spreadsheet tracking. Traditional tracking might work for a few facilities, but if you are trying to manage many facilities in different areas, and different stakeholders, it is important to have an effective database system that helps everyone involved follow the exact progress on repairs and data feedback.
As we all know that safety is of utmost important during an emission survey. If a technician discovers a potential-ly massive safety concern using the OGI camera, the technician should arrange a meeting with the relevant stakeholder on the hazard assessment system, as established by client corporate policies, and all stakeholders should be informed accordingly.
Under the federal methane regulations and most provincial regulations, a facility owner may apply for approval to use an alternative fugitive emissions monitoring plan (FEMP) for tanks, which involves the use of other technologies to modify the survey frequency or conduct a different type of survey. In Alberta, a few alternative FEMPs have been approved for use.
There are four main transportation methods for tank monitoring, including satellites, vehicles, drones, and people (Method 21). All these methods usually work in conjunction with a comprehensive survey. However, technicians can use both to reduce the frequency of surveys. While LDAR technologies provide macro detection of fugitive emissions from tanks, as they do not indicate the source, complete monitoring still re-quires a comprehensive OGI survey on-site. A macro system is useful to eliminate sites that do not have detectable emissions, thus reducing the number of facilities that require a comprehensive survey.
Drone and vehicle-based testing technologies are growing alternatives to the LDAR system as well. One new drone technology is a la-ser-based system that can detect methane across the path of the laser. When paired with OGI scanning, technicians can actually visualize the emissions.
The future for storage tank standards and practice is yet to be determined, as environmental initiatives continue to seep into every facet of the oil & gas process line. Companies and the public alike will continue to push for more sustainable practices as we aim to veer closer to net zero.
1. Air Emissions Modeling Advances for Oil and Gas Production Facilities. By Barry L. Burr and Adam M. Georgeson and Kha Mach.
2. Alberta Daily Oil Bulletin.
3. U. S. Environmental Protection Agency Office of Air Quality Planning and Standards Emission Factor and Inventory Group.
4. CFR Environmental.
5. ERA Environmental Management Solutions Inc.
6. AP 42 Chapter 7: Liquid Storage Tanks. https://www3.epa.gov/ttnchie1/ap42/c00s00.pdf.
7. Tank Emissions Calculation Software Tools. https://www.trinityconsultants.com/software/tanks/tankesp.
* TANKS is a Windows-based computer software program that estimates volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from fixed- and floating-roof storage tanks. The TANKS model was developed using a software that is now outdated.