Cooper, R.E.; P.T. Dowell Schlumberger Indonesia (Indonesia)
A combined nitrogen purge and helium leak-detection system has been developed to safely pressure test and commission oil and gas process equipment. The system is able to detect leaks of a much lower magnitude than conventional leak detection systems and it offers several advantages over conventional hydrostatic tasting. Inert nitrogen gas (used an the pressurizing medium) also safely purges the process system of oxygen and flammable gases, while helium in trace amounts provides a means of detecting leaks as low an 0.03 m3/yr. A mass spectrometer is used to detect extremely low levels of the helium detector gas.
Once the process equipment has been certified as leak free, a low pressure nitrogen blanklet can be left in the system. Gas process equipment also can be operated under simulated working conditions using nitrogen. This has proved beneficial for instrument calibration and training purposes.
The application of internal pressure to a closed system causes stress to be applied to individual components of the system. Those components may experience elastic deformation which, in turn, can result in leaks. Unfortunately, there is no current way of establishing the pressure integrity of a system without actually applying pressure.
For this reason, oil and gas handling systems are pressure tested prior to being placed into operation.
If a system is designed to operate with natural gas at high pressure, it is desirable to prove the system's integrity as closely as possible to its designed operating parameters before admitting a flammable material. Should a leak develop or a component fail, it is preferable that this occurs with an inert gas rather than with a highly flammable material such as methane, with the consequent risk of explosion. The use of nitrogen as a pressure test medium allows a follow-up test under circumstances which simulate actual operating conditions.
Experience from North Sea platforms shows that certain items which have previously been hydrostatically tested with success can fail under a leak detection test with nitrogen and helium. One explanation of this is that helium, due to smaller molecular structure and lower viscosity, penetrates the system's joints or porosity more than liquid thus causing leakage.