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Article # 0059








This article identifies common sources of air pollutants from maintenance activities typically conducted at bulk storage terminals.  The primary air emissions sources operated at terminals include storage tanks, product transfer operations, emission control devices such as vapor combustion units to control tank and loading losses, and fugitive emissions from minor process equipment such as leaks from pump seals, valves, and connectors.  Federal and state regulatory authorities have recently focused on other sources of emissions that are both predictable and quantifiable and have requested that these sources be completely identified, quantified, and authorized by available permitting mechanisms.  These sources have been identified as emissions resulting from Maintenance, Start-up, and/or Shut-down (MSS) activities that support the terminals operation.  This article focuses on maintenance type activities that appear to comprise the majority of MSS emission potential at a traditional bulk storage terminal.


Main Sources of Air Emissions from Maintenance Activity


Some of the main air emission sources resulting from bulk storage terminal maintenance activities are:


Planned Floating Roof Storage Tank Landings

Fixed roof storage tanks can incorporate internal floating roofs (IFR) that “float” on top of the liquid (on pontoons) reducing the vapor space and thus air emission potential of the tank.  Storage tanks are also constructed with external floating roofs (EFR) that actually lower and rise as liquid is withdrawn or the tank is filled.  Emissions from “normal” operation of these tanks are well documented in the technical literature and typically quantified and included in site construction and operating permits. 


When these tanks are withdrawn to a very low level the floating roof will eventually “land” on the tanks deck legs or wires such that the floating roof no longer rests on the liquid is further drawn below the leg height.  For example, pontoons may keep the floating roof only a foot above the liquid, but deck legs can extend 3 to 10 feet from the tank base so that the vapor space could increase from 1 foot to between 3 and 10 feet.  When roofs are landed additional emissions result as the vapor space is expanded from its normal volume.  This additional volume of vapor is not considered in the main calculation models (industry computer programs) and thus must be estimated separately at this time.  The equations for estimating these emissions can be found in EPA AP-42 Compilation of Air Pollutant Emission Factors, Chapter 7.  The terminal must record the landings and the necessary data (e.g., duration landed, leg set height, etc…) to estimate and report these emissions.  And the emissions must be authorized by an available permit mechanism such as the sites construction or operating permit.

Landings occur during several operating scenarios that include: 

Landings can also occur as a result of an “upset” or unforeseen condition (accidental or emergency landing).  The emissions from these events are not considered to be “maintenance” emissions as they were not planned.  These emissions are quantified as well and reported under “upset” or emission event regulations.


Planned Storage Tank Cleanings/Degassings

When tanks are required to be emptied completely they may be degassed and/or cleaned to accommodate a new service, scheduled maintenance, or inspection.  When a tank is completely emptied there is typically residual product on the floor which is termed a “heel”.  Some tanks can be drained dry, but still there is liquid clinging to the inside annular surface and some vapor in the entire space of the tank.   When vapor is to be removed from a tank the industry term for this process is “degassing” the tank.  There are many technical considerations and safety precautions involved in degassing a tank that are not the subject of this article.  The emissions that will result from degassing the tank must be quantified prior to the activity and authorized by an available permit mechanism.  Some maintenance activities may involve degassing, but not cleaning the tank.  In some instances both degassing and cleaning (typically by high pressure water spray) occur simultaneously.  Environmental regulations require control of the degassed emissions from high vapor pressure materials and/or large capacity tanks.


Planned Piping/Component Degassings/Purges

Process lines and/or process equipment may be degassed to switch out use of common piping or to isolate sections of pipe to perform maintenance.  For example, incompatible liquids stored in separate tanks may use common transfer lines by switching valves.  These lines may require “purging” or degassing before the piping “change-in-service” to avoid product cross-contamination or other problems.  Also, periodic maintenance to repair or replace piping, connections, valves, and pumps result in the “opening” of this equipment to the atmosphere and the release of residual vapor.  Note that pollutants released due to equipment failure are not considered to be maintenance emissions, but unforeseen “upset” emission events.  But the work performed (e.g., replacement of faulty valve) in response to such failures are maintenance activities whose emissions should be estimated and authorized as such.


Vacuum Truck Events

Terminals periodically utilize trucks to “pump” out hydrocarbon tainted wastes or sludge that collect in oily water separators, tank bottoms, and wastewater treatment equipment.  When the truck vessel is filled the vapor generated inside the vessel is displaced by the incoming liquid and released to the atmosphere as emissions.  Environmental regulations require control of these emissions in some cases.  Some vacuum trucks are equipped with a control device or they may be connected to a site vapor collection system that routes vapor to the site’s vapor control device. Purchase records, shipping manifests, work orders, and process knowledge can be used to estimate emissions from these operations.


Maintenance Material Usage

Terminals use a variety of cleaning and degreasing solvents, paint, and coatings that contain volatile organic compounds (VOC).  In addition, outside contractors may bring and use material on-site.  All materials must be evaluated and emissions quantified based on VOC content and usage rate.  Purchase records are typically good sources of data to estimate usage for material purchased and used by terminal operators.


Temporary and Portable Maintenance Facilities

Temporary and portable maintenance facilities can cover a wide range of equipment that are driven by engines or turbines that burn fuel resulting in combustion product emissions.  For example, companies that clean and degass tanks may bring a diesel-engine driven compressor on-site to drive high pressure spray equipment and might also combust the degassed vapor in the same engine or through a portable vapor combustion device releasing emissions to the atmosphere.  Also, as an example, outside contractors can set-up temporary maintenance facilities on-site to perform abrasive blasting of tanks to prepare them for painting; the blasting resulting in particulate emissions from the blast media and dislodged paint.   They may use engine-driven (combustion emissions) compressors to shoot the media and drive paint equipment.  And the application of paint will also result in the release of VOC to the atmosphere. 

It should be noted that material (i.e., type and amount of abrasive media, fuel use to drive equipment, and total volume of paint sprayed) used by contractors is not typically found in company purchase records and might need to be requested from the contractor.  The terminal could include in the work contract a requirement to supply information necessary to estimate emissions form the project.  An estimate of proposed materials and their usage rates should be presented by the contractor to the terminal prior to start of project and then actual materials and total usage rates should be recorded and provided at end of project.  The proposed information will allow an emission estimate to ensure that the propose project meets an available preconstruction permitting mechanism and the actual information will be used to calculate actual emissions that are required to be reported in state annual site-wide air emission inventories.




Facilities are required to estimate and quantify all sources of air emissions and evaluate these emissions compliance status under existing state and federal regulations.  Maintenance, Start-up, and/or Shut-down (MSS) emissions may not be currently represented in existing environmental permits since these emission types have not traditionally been seen as significant or quantifiable.   Each terminal should evaluate all sources of emissions from maintenance performed by terminal staff and that performed by contractors to ensure preauthorization permitting is in place and to consider the 24-hr reportable quantity status of all pollutant types prior to start of project.





Jack L. Bullard, P.E. is an Engineering Partner of Bullard Environmental Consulting, Inc. He has over 20 years experience in environmental engineering, compliance, and permitting. Jack has a Bachelors of Science Degree in Chemical Engineering from the University of Texas at Austin.


Jack L. Bullard, P.E. No. 83547




Article # 0059 

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