By Ricardo Pires
What is LNG?
LNG or Liquefied Natural Gas is natural gas that is refrigerated to reduce its volume and turns to a liquid at -259.6°F (-162°C). It is a clear, colorless, odorless, non-toxic and non-flammable liquid. At atmospheric pressure, the ratio of volumes of gas to the liquid state is about 620:1. This allows for a large storage capacity. LNG is not shipped or stored at high pressure. It is stored at -259.6°F at atmospheric pressure +1. Most of the LNG is Methane, but it can also be other gases such as Propane, Ethane, Butane, Pentane, Hexane or Heptane. When the LNG is vaporized into natural gas it can be used for power generation or gas distribution.
The LNG process
First the natural gas has to be extracted, then it usually goes into a pipeline from the resource to a LNG plant. Once the gas arrives at the plant, the liquefaction process begins: the gas is cooled to 259.6°F, which reduces its volume by 620 times. Then the LNG is transported from an LNG terminal to market areas. Once it arrives, it is stored, then re-gasified and connected to the local distribution.
Stages in the LNG Plant Life Cycle and applications
From the construction phase of the project, followed by the pre-commissioning and commissioning phases and finally the operation, temporary air and nitrogen are present throughout:
- In the Construction phase during the LNG module fabrication offsite and onsite, line blowing, pneumatic testing and nitrogen purging are common applications for air compressors and dryers, boosters and nitrogen generators. The same equipment needs to be used for roof pressurization, pneumatic testing and nitrogen purging during the tank construction onsite. During the LNG jetty/product loading facility construction, air compressors need to step in for drilling and/or pile driving.
- During the Pre-commissioning and commissioning, air compressors and dryers are used to provide temporary instrument air (and/or for instrument testing) and also for air blowing. To complete a high pressure leak testing-nitrogen spread, boosters and Nitrogen generators are added.
- When the plant is operating, there should be compressors in standby just in case they are needed. For painting, blasting and valve upgrades, compressed dry air is essential, so compressors and dryers are needed. For nitrogen purging, a booster and a nitrogen generator needs to be added to a compressor and a dryer.
Now let’s go into more detail regarding key rental air applications at a LNG facility.
Pneumatic tests are conducted to check the structural integrity of the vessel or piping when a hydrostatic test is not possible for various reasons. Generally it requires high quality air to be used for testing critical systems. Pneumatic testing is gaining popularity as systems are becoming more and more advanced and cannot tolerate water.
To allow proper operation of the facility and avoid any hydrate formation in the line or the system, all cryogenic piping and LNG tanks must be thoroughly dried out. For pipeline drying, oil free and dry air is required. LNG tank drying requires purging of wet air through the tank vents while replacing it with super-dry oil free air at high volumes.
Nitrogen, being an inert gas, is used to create an inert atmosphere for a LNG tank. The oxygen content in LNG tanks needs to be limited before commissioning and start-up, so that it does not react with the hydrocarbons and create explosive mixtures. To purge the LNG tank, the required acceptance criteria is of 8 percent or less oxygen content in all tank spaces: There is an option of using either liquid nitrogen or an on-site Nitrogen Generation system. The logistics of liquid nitrogen and managing huge quantities of it to a remote project site is always challenging, hence it is preferable to have on-site nitrogen using Membrane technology. A state of the art specialty rental air company can offer the complete Nitrogen Spread with the feed air compressors along with the Nitrogen Membrane Unit to produce nitrogen up to a purity of 99%. The key strengths of Nitrogen units vs. Liquid Nitrogen are:
- On-site Nitrogen Generation system, avoiding all logistics hassles of getting tankers at site.
- Cost Savings on transportation costs especially if the site is in a remote location.
- Cost savings on the overall project especially when the quantity of Nitrogen required is substantial.
- Can be used continuously without any stoppages.
- The operation is not affected by the external factors that drive the market (ex: shortage of liquid nitrogen).
- It is much safer than using liquid Nitrogen.
Nitrogen-helium leak testing
It is generally used to test vessels during pre-commissioning of any LNG facility. Typically systems are pressurized with a mixture of 1% Helium and 99% Nitrogen. The pressurization takes place in several steps up to normal operating pressure or up to 95% of the relief valve setting. The system is then tested and any leaks are reported using Helium as a tracer gas. Any leak is detected using a mass spectrometer device. As Helium has the smallest molecular size, even a very small leak can be detected.
A few final words, always remember to ask your compressed air and nitrogen rental supplier if they had experience with an LNG project: a world renowned company is always preferable and will avoid surprises and headaches.