Alan Hiller, Mirage Machines
The process of converting natural gas into liquid form enables more efficient transport and storage but is widely held as hazardous and expensive, especially with the need for expensive cryogenic tanks used to store the liquid at extremely low temperatures. Due to the flammable nature of the gas involved, the commissioning of an LNG plant demands stringent safety measures and precautions.
With the stakes so high, ensuring the integrity of all LNG pipe flange joints is absolutely critical. The high-risk nature of the process and the need for multiple layers of pipe insulation materials make pipe maintenance very difficult once the plant is operational. This means a “right first-time approach” with zero defects at the point of commissioning should be the primary goal.
Best practice when designing an LNG plant suggests keeping the number of flange joints to a minimum, but inevitably there will be many areas where there is no other alternative. Provided best practice and relevant standards are adhered to, the risk of leakage from flange joints can be kept fully under control.
Let’s first look at the different types of systems used in an LNG plant. These fall into four categories, each with varying characteristics:
- Main process systems — low- and high-pressure LNG systems on the plant and the loading/unloading systems between the ship and the storage tanks
- Auxiliary process systems — hydrocarbon drain systems, plant fuel, cooling systems
- Utility systems — vary greatly according to the type of plant
- Fire protection systems — spraying system, water curtains, foam generation
In each of the four categories, there are many steps that can be taken to reduce risk of leakage.
Check flanges for damage before installation
Even a small amount of damage to a flange face can cause a joint to fail. Typical types of damage include:
- Scratches — often caused by items such as a wire brush or chisel, sometimes seen in transit and from the removal of protective coatings.
- Gouges — created by a dull object dragging across the flange face, such as a screwdriver.
- Pits — small rounded areas of material loss caused by corrosion.
- Dents — caused during the installation and commissioning phases through impact with equipment such as cables, rigging and positioning of mating flanges.
Pay particular attention to scratches running across the ridges, as these create a path for potential leaks.
Correct flange surface finish, gasket type, bolt tightening
A portable flange facing machine can be used either at the prefabrication location or on-site to ensure the required finish is achieved. Referring to the ASME B16.5 standard for flange finishes is recommended, plus ASME 16.20 and 16.21 standards for gasket surface finishes.
Gaskets used in the main process systems will need to be capable of maintaining a seal at temperatures as low as minus 162 degrees C. These are often made from compound materials such as “semi-metal” gaskets, which include a hoop containing expanded graphite or polytetrafluoroethylene filler.
You should never use gaskets not rated for the pressure to be applied, even if the gasket is temporary for testing purposes. ASME Standards and Certification (PCC-1-2010) advise temporary gaskets be used with caution only and gaskets not be reused.
As you would expect, bolt tightening has a huge impact on the integrity of the flange joint and is a highly specialized area. It is vital to adhere to relevant industry standards such as ASME PCC-1, which not only covers the equipment and process specification but also defines the requirements for training and qualification of bolted joint assembly personnel.
Ongoing maintenance
A strong focus on “zero defects” should minimize the need for flange joints to be dismantled for maintenance. However, should this be necessary, there is a wide range of flange facing machines and pipe cutting tools available, which can be used easily on-site.
For more information, visit www.miragesubsea.com or call (281) 859-1234.
