By Keith Reid
The release of gasoline vapor has long-been considered to be an environmental and health hazard by USEPA. For example, the Clean Air Act of the 1990s required Stage II vapor recovery systems to prevent vapor releases during the fueling process in ozone non-attainment areas. Equipment was installed on the dispensing side to suck the vapors back into the storage tank. Automakers also followed suit with onboard vapor recovery systems (ORVR).
The ORVR systems eventually became ubiquitous as older cars left the road. At the same time, ORVR systems were having issues with certain dispenser vapor recovery technologies. In 2012 USEPA gave the states the option to discontinue Stage II if the states could prove it would not have a negative impact on air quality. This has generally proceeded with some holdouts such as California and select non-attainment areas in other states.
Which brings us to Stage I vapor recovery, which is still active nationally. This process covers dropping fuel into storage tanks from tanker trucks. Vapor is recovered using a closed-loop system where the vapor displaced in the storage tank as fuel is added is cycled into the truck’s tank and then removed back at the fuel terminal. This process is considered effective; however, there are some areas where the system can fail.
Vapor and Pressure
Fuel tanks and the supporting fueling infrastructure are designed to handle a moderate range of higher and lower pressures. This is safeguarded by the pressure vent (P/V) relief valve on the vent riser. This valve provides resistance to both the intake and exhaust of outside air to help balance the system and should seldom activate. However, various conditions can lead to notably higher pressures and lead to vapor releases.
Tank pressure shifts back and forth during the fueling process. While fuel is being sold the tanks tends to be negative. It can be neutral when the amount of fuel being sold balances the amount of expansion. And it can go positive when the pumping slacks allowing the volume of air in the tank to absorb hydrocarbons.
“Gasoline evaporating to vapor-phase gas is a big volume—520-to-1,” said Ted Tiberi, founder of Arid Systems, which manufacturers vapor recovery systems. Arid makes equipment that captures vapor and returns it to the tank. “In an effort to resaturate itself it builds pressure and then the hydrocarbons escape.”
For example, vapor releases can potentially occur at high volume sites during the “cool down” period—perhaps two to four hours– after a period of heavy activity, according to Greg Young, president of Vaporless Manufacturing, Inc. Vaporless manufactures mechanical line leak detectors and electronic line leak detection for a range of fueling operations. “You’re pulling a lot of air into the tank and as the volume of customers drop off after peak periods that air sits and begins to saturate with hydrocarbon, pops the P/V valve and vents.” he said.
Similarly, temperature fluctuations can come into play with Stage 1 during the fuel drop.
“During different parts of the delivery we see pressure all over the place,” said Tiberi. “We think it is not only the hardware being used, but the differential temperatures. Is the sun out? Is it beating on the top of the tanker truck? It’s 3 p.m. in Arizona where the tanker is heating up but the vapor coming from underground is cool. Or, it’s 3 a.m. where you have the relatively cooler tanker versus the vapors coming out of the underground tank. There’s a whole host of dynamics at play.”
As noted earlier, even some Stage II equipment can cause issues with ORVR, as the two competing technologies (ORVR drawing vapors into the car while Stage II tries to draw the vapor into the tank) resulting in air being drawn back into the underground storage instead of vapor.
Driver Delivery Issues
A more significant concern is delivery drivers not following proper Stage 1 procedures, either through poor training, inattention or the desire to speed up the delivery.
For example, hooking up the vapor line to the storage tank first instead of the tanker releases the negative pressure in the tank and allows a significant quantity of air to enter the system.
Zane Miller, fueling system consultant and owner of Testing LLC, observed 75 bulk deliveries (25 in metro Atlanta, 25 in metro Kansas City and 25 in metro St. Louis) between Dec. 24, 2018, and Jan. 1, 2019, as part of research into the issue. He noted that in each market only one out of the 25 deliveries observed were being made in a condition considered out of compliance due to driver errors in configuring drop and stage 1 vapor recovery connections (though not all resulted in a vapor discharge). The concerning issue was typically where a driver would vent negative pressure when connecting the Stage 1 vapor recovery hose to the tank before connecting to the delivery truck.
He noted that adding to the bypass of Stage 1 problem observed in a number of fuel delivery events, was the delivering tanker compartments being intentionally opened to atmosphere allowing increased, out of dynamic balance fuel flows (drops). This can create pressure issues that not only lead to significant vapor discharge but can strain pressure limits throughout the system.
This was also established in the study Vent Pipe Emissions from Storage Tanks at Gas Stations: Implications For Setback Distances from Columbia University researchers Markus Hilpert and Bernat Adria-Mora, along with Johns Hopkins’ Ana Maria Ruleb. This study looked at vent pipe emissions with an eye on health concerns and setback distances from sensitive populations. Ted Tiberic, also quoted in this article, provided technology solutions to assist the study.
Although limited to two high-volume sites, the Hilpert study did find indications of excessive discharges that were likely related to non-compliant bulk fuel deliveries in addition to possible influences from temperature fluctuations and Stage II systems (installed at both sites) conflicting with automobiles’ ORVR systems. As the study noted: “Based on observations and interpretation of time series of the tank pressure data, it is likely that the peak vent emissions were partly due to non-compliant bulk fuel drops where the Stage I vapor recovery system either was not correctly hooked up by the delivery driver or to hardware problems with piping and/or valves.”
In some cases, of course, it can also be faulty Stage 1 equipment on the truck or at the site.
Solutions to this problem can start with delivery driver education. Placing signage accessible to the driver about the proper procedures and other educational efforts can educate and encourage proper unloading and attention to detail. Vaporless Manufacturing also makes a system that monitors tank pressure to track potential delivery events, and sounds a visual and audio alarm to alert a driver that something is not right with the delivery. It can similarly track pressurization patterns that might indicate vapor leaks in the system.
A What Does this Mean to the Site Operator?
Generally, most Stage 1 systems are regulated simply by showing that operational, approved equipment is installed. However, according to those interviewed for the article, regulators in a handful of states including California and Colorado are showing an enhanced interest in the issue, such as a review of vent riser setbacks.
The issue can involve more than just a vapor discharge through the vent riser. A vapor-porous system under excessive pressurization system can lead to vapor leaking into the air well short of the riser (such as through the sump), or into the soil. Both can lead to significant problems for the tank operator including unwanted attention from regulators, expensive environmental remediation and complaints from customers, employees and neighbors. In some cases that can also promote acidification inside the sump and corrosion issues, according to Young.
If the leak is through a fitting into the ground, the vapor can travel through the soil until it makes its way to the surface or into ground water. Young noted that this was the driver for the MTBE issue in the late 1990s. When California began exploring MTBE contamination it was not from a liquid release–they tracked it to the vapor plume that traveled through the soil and reached the water table.
There is also a financial angle to be considered. Even with a fully functional Stage 1 system liquid gallons in the form of vapor end up going back to the terminal. Excessive vapor loss though a riser or other sources through over pressurization only adds to that volume. While gasoline is relatively cheap these days, for high volume sites those gallons can add up. Losses could be between the range of two and five gallons per 1000 gallons pumped.
So, even though this is not necessarily a front burner issue in the industry, and not one that would necessarily be driven by the actions of the site operator, it is worth keeping an eye on to make sure it stays at the back burner.