This article describes the equipment and guidelines used for the detection of leaks. Air or Nitrogen can be used as the detection medium. This document should be used when new piping/equipment is put into service. This document should also be used to check for leaks on piping/equipment that has been opened up for inspection/repair before putting back into service.

Safety Considerations

Although Nitrogen is non-toxic and environmentally friendly, careful attention to safety must be taken in the planning and performance of a Nitrogen leak test. Minimum safety precautions must include:

  • Use of calibrated pressure gauges.
  • Use of correctly sized pressure relief devices.
  • Isolation of the pressure source while maintaining the test pressure prior to inspection.
  • Determination that all components subject to pressure, vacuum or heating (for drying purposes) are not subject to damage by these procedures.
  • Maintain awareness that Nitrogen is an asphyxiant and can collect in confined spaces.

Principle of Operation

The component to be tested is isolated, fitted with a pressure-measuring device and pressurized with a dry gas. Alternately, a vacuum can be applied to the component. A change in pressure over time indicates a leak in the component. Results can be excellent in small components, but as the size of the test object increases, the time required to obtain meaningful results becomes prohibitive. The necessary time for a test is calculated using the volume of the component, the maximum allowable leak rate, and the smallest incremental pressure change that can be observed on pressure measuring device being used.


Personnel performing the leak check should be thoroughly familiar with both the theoretical and practical aspects of this form of testing and with the operation of the instrument they are using.

Preparing the Vessels

Make sure the vessel that is going to be tested is empty and cleaned before the test.

  • Have operations mechanics close the vessel(s) and install correct gaskets as needed and apply proper torque.
  • Pressurize the vessel(s) with Air or Nitrogen up to the desired pressure. Where available, install a tree with a Check Valve to protect the air or nitrogen systems.
  • Close the air/nitrogen source and let the gas remain in the vessel for 30 minutes.
  • If the pressure does not hold, need to Bubble Test the flanges/welds to see where the gas is leaking out.