Gas Station Manhole Covers: What’s Underneath?

Gas stations are everywhere and serve the primary function of supplying drivers with fuel for transportation on an everyday basis. Yet, when you pull into a gas station, and your car bumps back and forth, do you ever stop to think – what’s underneath?

​​Those bumps beneath you are manhole covers. Although each manhole cover may feel the same as you drive over it with your vehicle, they serve different purposes. Manhole covers in gas stations are primarily used to access and protect underground tank systems.
Let’s dive into the underground world of gas stations to further explain the function beneath each manhole cover that gets on your nerves when you pull in to gas up.

Before jumping into the functions of manhole covers in gas stations, you should know some basic information about where the fuel comes from to fill up your vehicle.

Where Does The Fuel Come From?

Although this question may sound simple, not many people know where their vehicles’ fuel comes from. The answer lies in the intricate system of the underground storage tanks. Fuel is delivered to the gas station by a tanker truck, which is transferred from the truck tank into the underground tank through spill containment manholes, also known as spill buckets. We will talk more about these later.

How Many Tanks Are Down There?

Typically, four fuel options are available at most gas stations: regular/unleaded, mid-grade, premium/super, and sometimes diesel. Each fuel type flows through separate pipes from different underground fuel tanks containing different types of fuel. For example, the picture above shows a gas station with three underground fuel tanks.

Next time you visit your local gas station, take a look at the manhole covers and see if you notice a pattern of covers in a straight line, which would indicate the presence of multiple underground tanks. How many tanks are at your gas station?

Why Are There So Many Different Manhole Covers?

The grade access for each tank system usually consists of four or five different manhole covers. These might include spill buckets (mentioned above), vapor recovery, tank sumps, tank monitors, and monitor wells. Each of these serves a different function for the tank below. Let’s look at some of them.

Spill Bucket Manhole Covers

Spill bucket manhole covers serve as the gateway between the tanker truck fill hose and the underground fuel tank fill pipe.

You can easily recognize these covers because they are raised above grade. They are raised to shed rainwater, which helps keep water out of the bucket. They are usually 11” in diameter, ductile iron, and may be painted a color. The different colors help the fuel delivery driver identify which tank is for which fuel. For example, white with a black cross may indicate regular or unleaded fuel.

This American Petroleum Institute color chart indicates what the different colors mean:

Tank Sump Manhole Cover

The tank sump manhole cover is the access to the tank sump. The tank sump houses the pump that sucks the fuel from the tanks, through the pipes, into the fuel dispensers.

Tank sump manhole covers are usually 36” – 48” in diameter. These are larger than the average manhole cover because they allow a person to climb down to the tank sump inside for repairs. They are also durable enough to withstand the weight of gas trucks and various vehicles daily.

Tank Monitor Manhole Cover

Tank monitor manhole covers allow access to monitor the interstitial space. What is the interstitial space? Most tanks are double-walled, so if the primary tank springs a leak, the fuel will stay contained in the secondary tank. The space in between the tanks is called the interstitial space. Although underground storage tanks are durable, rust accumulation can create holes in the fuel tank over time, leading to leaks. Monitoring this helps protect the environment from fuel contamination.

The cover is usually 18” with big, bold letters that say “Tank Monitor.” This helps differentiate between the spill buckets and tank monitor.

Monitor Well Manhole Cover

Monitor well covers are typically in the four corners surrounding the tanks, ranging from 5” to 18” in diameter. These are easily identifiable by a triangle that says “DO NOT FILL.” Monitor well covers provide an access point for inspections to take samples of groundwater or soil to test for contamination.

Next time you stop for fuel and drive over a manhole cover at the gas station and feel that bump, maybe you will have a new appreciation for the infrastructure beneath your feet and the manhole covers that protect them.  Do you need to replace a cover at your gas station? Contact us today or shop Replacement Manhole Covers at