UST Monitoring Systems – How They Work and Why Every Tank Owner Needs Them
What is a UST Monitoring System?
A UST monitoring system is a release detection system that tracks fuel levels within an underground or aboveground storage tank over a period of time to see if the tank is leaking. It will also provide measurements of the fuel level, volume and temperature, water level and volume, and high and low fuel level warnings. Many systems are capable of monitoring double-wall tanks and lines, pressurized piping and providing remote communication. Tank monitoring systems are also referred to as fuel management systems, automatic tank gauge, and leak detection systems.
Why do Tank Owners need UST Monitoring Systems?
There are three main reasons tank owners require a tank monitoring system:
- Regulatory Requirements
- Product Management
- Leak Detection
Automatic Tank Gauges (ATGs) are important to most tank owners because they help them meet state regulations that require tank and line tightness testing. ATGs also provide accurate product levels at a glance. With an ATG you won’t have to manually stick a tank to measure the tank volume. Knowing the tank volume will help provide accurate fuel inventories. Accurate fuel inventories make for easy reconciliation of product use or sales. Most importantly, they eliminate manual measurements and calculation of monthly inventory by providing detailed reports of product delivered and product dispensed.
How do Tank Monitoring Systems work?
All tank monitoring systems have at least a magnetostrictive probe, interstitial or annular sensors, and sump leak detection sensors.
A magnetostrictive probe is permanently mounted in a 4-inch opening at the tank top. Two floats on the probe are used to measure fuel and water. A pulse is sent to the float and is measured back at the head of the probe. The magnetostrictive probe is capable of measuring fuel and water to 1/1000th of an inch. It also adjusts for temperature changes in the fluid. This is important because you get different product readings as the tank expands and contracts with surface temperature changes. This can be a significant volume difference in a large storage tank.
A flexible probe is used in aboveground tanks larger than 20 feet tall. Flexible probes are capable of monitoring tanks up to 70 feet. The flexible probe consists of a weight, water float, and product float.
Annular or interstitial space sensors are installed between the tank’s inner and outer walls. There are three types of annular space monitoring.
- Brine of glycol filled annular
- Steel tank annular
- Fiberglass dry annular
A dual float reservoir switch is used to measure the rise and fall of brine or glycol. During the tank operation, a change in level will indicate a leak because the brine or glycol will either enter through a hole in the tank or be displaced by leaking fuel. Float sensors are used in steel tank annular spaces.
A float or optical switch is used in fiberglass dry annular space. The presence of fluid in the annular triggers both types of switches and causes the automatic tank gauge to enter an alarm state.
Sumps are an important component of leak detection and prevention in both aboveground and underground storage tank systems. A sump is a liquid-tight container made of fiberglass or polyethylene. Dispensers have a sump located below the dispenser that will contain leaks. Underground tanks also have a sump located at the tank top. Piping between the tank and dispenser is double-walled, and a leak in the primary wall will travel to either the underground tank sump or dispenser sump. A float sensor installed in either space will trigger an alarm and notify the owner of a leak.
UST Leak Detection
Most underground fiberglass and steel tanks are cylindrical. ATG probes are installed around a rib of a fiberglass tank and set on the bottom of the rib. The ribs are interconnected via small air pockets so a leak on one end will migrate to the other end. Steel tanks typically utilize a float sensor located in a tube that connects to the bottom of the secondary containment. Any leaks in the secondary of a steel tank will trigger the sensor in the monitoring tube.
ATG’s are also used to monitor tank sumps, transition sumps, and dispenser sumps. Continuous monitoring of the sumps ensures that no fuel leaves the tank without being accounted for. Sensors are sometimes used in groundwater monitoring wells to check for fuel on water or for vapor monitoring.
These same sensors work for aboveground storage tanks to monitor for product level, water, or loss of product.
Sensor Types - Discriminating and Non-discriminating
Discriminating sensors detect the presence of liquid or fuel and differentiate between the two. They incorporate ultrasonic technology to detect water and conductive elastomer technology to detect hydrocarbons. Alarms can be programmed to notify when low-level liquid, high-level liquid, or fuel are detected.
A non-discriminating sensor is used to detect liquid in a containment sump. Placed at the base of a sump on an outside wall, sensors utilize a float/magnetic reed switch to detect the presence of liquid of 1.84” or higher. These sensors are compatible with most fuel types and do not distinguish between water or fuel.
Each sensor is connected to the ATG through an intrinsically safe set of wires. These wires must be incapable of producing heat or spark that would ignite fuel vapors. Intrinsically safe devices are low voltage circuits that protect the sensor wiring from producing a short and igniting fuel in the tank or sumps. ATGs are constantly monitoring the sensors installed at the site. These systems are also capable of shutting down dispensers, reducing the flow of in-tank pumps, and notifying the owner/operator that a problem has been detected. Methods of notification include text, email, fax, or external polling systems like a facility security alarm. ATGs are initially programmed by the tank service company to provide alerts at specific thresholds. Owners can specify a product level in inches or gallons that trigger a notification to a fuel delivery company. When the delivery is made the ATG measures the new level and provides a delivery report of inches and volume gained. In gasoline service stations, this volume is compared against sales at each pump, to reconcile the difference in the volume gained versus the fuel that was dispensed at the time of delivery.
ATGs are capable of monitoring levels and providing leak alarms but they also provide control of pumps. In the case of a generator where the fuel is supplied from a 110-gallon day tank or base tank, if the fuel level drops in the tank, the ATG can energize a transfer pump that will refill the day or base tank from a larger underground or aboveground tank.
Fuel dispensers have meters that measure the amount of fuel being pumped. ATG’s can indicate that a meter may be out of calibration if the inventory does not match the dispensed volume.
Oil Water Separator System
ATGs can be configured to monitor the oil level that accumulates in an oil-water separator system (OWSS). The OWSS collects rainwater and separates out oil and other hydrocarbons before discharging clean water to storm drains. An ATG is used to notify the owner or waste management company to remove the accumulated oil when it reaches a certain thickness or depth in the tank.
Leak Detection Recertification
ATGs are capable of performing a static or statistical test that meets federal tank testing requirements. A static test is done when the tank is not accepting deliveries or dispensing product. The test monitors the tank for level changes over a 2-hour period. A statistical test is performed while the tank is still in service. It ignores the dispensing cycles and captures the levels in between those cycles to create enough data as captured in a static test.
- Monthly test - 50% capacity at .2 gallon/hour detected leak rate
- Yearly test - 90% capacity at .1 gallon/hour detected leak rate
In order to ensure the ATG is capable of testing the .1 gallon/hour rate, the system needs to be calibrated and certified yearly to the manufacturer’s standard. The New Hampshire Department of Environmental Services developed this ATG test form to document the required annual testing of ATG equipment. Massachusetts does not provide a test form so CommTank technicians use a form created in-house to document the recertification procedure. The technician performing the test must be certified by the manufacturer and be trained in the performance of the test.
Example of a tank monitor inspection report:
ATG System Manufacturers
The following is a list of electronic monitoring system manufacturers whose equipment can be found in USTs and ASTs at facilities throughout New England.
- The Veeder-Root Company
- Omtec Mfg., Inc.
- Franklin Fueling Systems
- Preferred Utilities
- Pneumercator Inc.