Issue Date: June 8, 2021

Preferred Utilities

Interstitial Vacuum Leak Monitoring System


Leak rate of 0.1 gph with PD=100.0% and PFA=0% “Test Procedures For Tightness Testing Using A
Vacuum Monitor On A Double- Walled Tank Interstice With Or Without The Addition Of A Liquid Sensor” -
Standard Test Procedures for Evaluating Release Detection Monitoring Methods: Volumetric and Non-volumetric Tank Tightness Testing - USEPA May 2019 section 4.5.3.

Operating Principle:
System uses vacuum generated by a vacuum pump to continuously maintain and monitor up to 4 zones.
The console also has a “learn” function that is used during setup to determine the time to establish the vacuum on the interstice to a specific level. The time to establish the vacuum during the learn process is stored by the console and is used to determine if there is a liquid or air leak whenever the vacuum needs to be re-established at any time.
System also has a float valve that is designed to detect the presence of liquid between the interstice and the vacuum pump.

Alarm Condition:
System alarms when there is a loss of vacuum in the interstice within a specific amount of time below a specific vacuum. The system will also alarm if the system cannot replenish the vacuum within 150% of the initial “learn” time. The time to alarm is determined based on the size of the interstice. The float valve located between the interstice and the vacuum pump shuts the flow of air from the vacuum pump to the interstice in the presence of liquid, therefore causing an alarm due to the system not being able to replenish the vacuum within 150% of “learn” time.

Double-walled piping or double-walled tank with an interstice up to 2,400 gallons (max volume to declare a tight test within 30 days), storing gasoline, gasohol, diesel, heating oil #2, kerosene, aviation fuel, motor oil, water. Storage of biodiesel blends B6-B20 meeting ASTM D7467 and biodiesel B100 meeting ASTM D6751 would also produce a system alarm if the system threshold is exceeded. Responses to these fuels were not determined but would be expected to be very similar to the system's response when storing diesel.

Interstitial Volume

Time To Alarm with a 0.1 gal/hr leak

Time to Declare a Tight Test








1 hour

1h 30m

10 2 hours 3h
20 4 hours 6h
50 10 hours 15h
100 20 hours 30h
200 40 hours 60h
500 100 hours 150h
1000 200 hours 300h
2000 400 hours 600h
2400* 480 hours 720h (30 days)

*maximum volume allowed in order to allow for a passing test within 30 days

Manufacturer's Specifications:
Alarm will activate when interstitial vacuum decreases by a specific amount during a specific amount of time, as well as the system being unable to replenish the vacuum within 150% of the “learn” time.
Volume of monitored interstitial space must not exceed 2,400 gallons to satisfy 30-day release detection requirement.

The system must be programmed by a factory trained technician or under the direction of the manufacturer. Maintenance schedule – Biannually - check solenoid, transmitter, and pump operation, test the alarm horn and strobe, inspect tubing, fittings, and any connections, check tubing for condensate, drain any liquid that is in the liquid stop valve.

Change the filter on the suction side of the vacuum pump that is in the control panel annually.

The system described herein was tested with a 5-gallon vessel to simulate an interstice. Float valve was tested with water, gasoline, and diesel. System tested with a 100% probability of leak detection of .1gph ullage leak and 0% probability of false alarm.  This system may not be compatible with all secondarily contained tanks and/or piping. Always consult with the tank and/or piping manufacturer and the manufacturer’s applicable recommended installation practices before installing this system, or damage may be caused to the tank or piping by its use.

Preferred Utilities

Evaluator: Ken Wilcox Associates, Inc.

31-35 South Street

1125 Valley Ridge Drive

Danbury, CT 06810

Grain Valley, MO 64029


Tel: (816)–443-2494

  Date of Evaluation: March 29, 2021