Nitrogen and Helium Leak Detection Services
Nitrogen/Helium Leak Detection in the Oil and Gas Industry
A combined nitrogen and helium test is now widely used in the commissioning of process plants, vessels, and piping systems in refineries and petrochemical plants. In addition to commissioning of entire process plants, the procedure is also being used to test individual components such as heat exchangers and small vessels/pipes. This detection method offers a number of advantages over more traditional methods:
- The oxygen content is reduced to avoid an explosive mixture when hydrocarbon gases are introduced.
- The system is dewatered and dried.
- The system is tested at its working pressure with a gas closely simulating “live” conditions; this also allows instruments to be calibrated or checked and operators to be trained if desired.
- Nitrogen and helium are inert, non-toxic, and perfectly safe.
- Detection using helium allows all leaks to be located and repaired, so that a nitrogen blanket can be left in the system indefinitely, ready for start-up.
- Costs savings – valuable production time is being saved by avoiding the use of “live” gas-detection devices and being able to repair leaks prior to start-up date.
It is mandatory that new constructions, piping, and vessels must be subjected to a hydrostatic test prior to commissioning. However, this test is designed to prove the integrity of the equipment and not necessarily for the detection of small leaks. Generally, once the system is fully installed, it will be subjected to a low-pressure “gross” leak test using compressed air. Unfortunately, these low-pressure tests use soapy water and have many drawbacks. The only way to ensure a tight system is to pressurise it to a pressure as close to operating pressure as possible, then use a detection system that will detect the small leaks that soapy water does not detect.
The nitrogen/helium procedure used for high-integrity process and gas systems will guarantee a leak-free start-up. We start by carefully pressurising the system in stages to its operating pressure (or lower) with 99% nitrogen and 1% helium. Once it is safely under pressure, it can be easily leak tested at operating conditions and the potential dangers associated with flammable gas escaping can be eliminated by testing the system before final commissioning.
Since the nitrogen introduced into the system is dry (its dewpoint is below -60 °C), any residual moisture in the system will be absorbed by the dry gas. At the same time, any air pockets in the system will be purged by the nitrogen, reducing potentially dangerous local concentrations of oxygen. In addition, any process equipment, such as compressors, instrumentation, etc. can actually be started and, therefore, tested prior to start-up. Furthermore, all instrumentation lines can be purged and appropriate instruments function can be tested and the instruments calibrated.
To enable detection of the leaks, all flanges are taped so that any gas being forced through gaskets or seals is collected in the space between the outer edge of the gasket and the tape. Gas is detected by puncturing the tape with the tip of a remote probe which, in turn, is connected to the handheld detector or portable helium mass spectrometer. Hence, any helium gas which has collected beneath the tape is sampled and detected.
The detectors are calibrated and can be adjusted so as to read the leak rate directly on the meter scale, giving a simple interpretation of any leak rates found. Therefore once one has been detected, it is quantified and can be repaired. Sometimes, this is just a matter of tightening a joint but it can also result in having to change a gasket. If these had gone undetected, they could have resulted in potential hazardous leaks or having to shut down the plant or system to repair after the plant has already gone online – an expensive exercise.