How to Test an Underground Tank for Leaks Without Emptying or Cleaning It

An underground tank can be leak tested without draining the product or cleaning the vessel. A vacuum of up to 250 mbar is drawn in the full tank, and two sensors, one in the liquid and one in the vapor zone, listen for the ultrasound of air drawn in through a perforation. That is how SDT340 TankTest works: preparation takes minutes, and the result is confirmed by a number on screen and a sound in the headphones.

Why classic tank tests do not scale

Traditional methods require emptying the tank, cleaning it and waiting for the temperature to stabilize. Preparation takes longer than the measurement itself, and the client pays for every hour of downtime. While mandatory periodic inspections were rare, this was tolerable. When many sites have to be inspected, preparation becomes the main cost.

The manufacturer, SDT Ultrasound Solutions of Belgium, describes exactly this problem: requirements are tightening and older methods cannot keep up with the volume of work. TankTest is built around it.

How the acoustic vacuum method works

1. Two sensors go into the tank through the manhole: the red one in the liquid, the black one above the product.
2. A vacuum pump draws up to 250 mbar.
3. When the vacuum exceeds the hydrostatic pressure of the liquid column, air starts to enter through any perforation.
4. The ingress produces an ultrasonic signal. The instrument amplifies it and shows the level in dBuV.
5. The operator confirms by ear: bubbling means a leak in the liquid, hissing means a leak in the vapor zone.

The method does not enlarge the defect, because the wall is under vacuum, not excess pressure. It does not depend on product temperature and finds perforations that do not yet leak.

How to tell a leak from interference

Condition	Liquid sensor	Vapor-zone sensor	Headphone sound
Tank is tight	Near baseline	Near baseline	No signal
Leak below product level	Above baseline	Near baseline	Bubbling
Leak in vapor zone	Near baseline	Above baseline	Hissing
Outside noise	Short spike on both	Short spike on both	Unlike a leak

A leak is indicated by a level more than 2 dBuV above baseline, confirmed by ear. Two sensors plus sound give a double check, so a random spike is not mistaken for a leak.

How TankTest differs from hydraulic testing

Parameter	SDT340 TankTest	Hydraulic method
Preparation	No draining or cleaning	Water filling, cleaning
Temperature stabilization	Not required	Must wait
Temperature effect	None	Distorts the result
What is tested	Tank and piping in one cycle	Usually separately
Consumables	None	Water and its disposal
Effect on the defect	Vacuum does not enlarge it	Load can enlarge it

How the operator runs the test

In Guided Mode the instrument leads the operator through the mandatory steps: pick the tank from the database, enter diameter and liquid height, place the sensors, create the vacuum, collect data from the black and red sensors, save. Settings are predefined, so the test runs the same way for different operators. Before work, the system is verified on the built-in leak simulator.

What report the client gets

Data moves over USB to SDT TankTest Reporter, which plots each sensor and produces a PDF report with customer and operator details, company logo and comments. Reports are numbered automatically and results cannot be edited. If a result is ever questioned, the document protects both the contractor and the client.

Which tanks the method suits

The system covers tanks per EN 12285 from 1,500 to 100,000 L. For the largest vessels the vacuum is limited below 250 mbar:

Capacity	Maximum vacuum
1,500 to 60,000 L	250 mbar
80,000 L	200 mbar
100,000 L	150 mbar

Tanks with flat ends or flat partitions, aboveground tanks with a flat bottom, box-shaped tanks and polyethylene tanks do not suit the method. Three configurations are possible: tank only, tank with piping, or only the pipes connected to a double-wall tank.

What to consider before adopting it

• Regulation. The method is recognized in several European countries and the USA, but in Kazakhstan the mandatory inspections and permitted methods are set by industry and customer documents. Check them with your regulator before launching the service.
• Safety. The SDT340 is not available in an explosion-proof version, so work in hazardous areas follows the site's procedures and work permits.
• Training. The manufacturer trains users in TankTest: theory, equipment setup on the tank, tests on tight and leaking tanks, simulated leaks, reporting. KEG TRK arranges training at your site or in our training center.
• Economics. The benefit grows with the number of sites: less preparation per tank means more inspections per day for one crew.

For specifications, scope of supply and a quotation request see SDT340 TankTest. The base instrument is described on the SDT340 page. For pressure-free tightness checks of rooms, hatches and cabins, see TIGHTChecker.

The diagram and instrument screens come from the manufacturer's materials and carry English labels.