Due to the particular characteristics of Liquefied Natural Gas (LNG) facilities, special care needs to be taken when working on insulation systems for LNG piping and equipment. The following are important design factors to consider when insulating LNG piping and equipment.
Any insulation system for below-ambient applications needs to be designed vapour tight. This is particularly true for insulation systems for applications with cryogenic operating temperatures. Water intrusion into the system will degrade performance of the insulation system and can lead to other problems.
Moisture intrusion and ice formation within the insulation system will reduce the system efficiency and increase operating costs. Ice formation can result from a failure in the insulation system or from insufficient insulation thickness; either way, ice adds weight. This added weight can lead to equipment damage if it exceeds the load capacities of supports or equipment.
The risk of fire is always a concern for LNG facilities, especially for off-shore platforms and Floating Production, Storage, and Offloading (FPSO) vessels. The combustibility of the insulation system will be a consideration in these cases.
Pipe load at supports
Supports are critical since they need to be well insulated to prevent ice formation and need to be able to accommodate system movement as the system is put in service and the pipe contracts.
Most of these LNG systems include long runs of piping. Even a short section of LNG line will contract up to an inch when put in service. This contraction must be addressed in the design of the insulation system. It is important that the specifier address each of these factors in the design of the insulation system.
Cellular glass insulation for LNG systems
Given the specific needs of LNG systems, cellular glass insulation can be a good option. There are several reasons cellular glass insulation can function well on an LNG system. Cellular glass insulation is foam of sodium silicate glass and the cell walls of the glass foam are impermeable; this is important when considering the aspect of water vapour intrusion into the system. Relative to density, cellular glass insulation has a high compressive strength. In addition, for applications that require high-load bearing materials, cellular glass insulation is available in a number of higher densities, high-load bearing grades.
Cellular glass insulation is 100% glass. Since it does not burn, cellular glass insulation helps increase the fire resistance of the overall insulation system. As mentioned previously, the piping on an LNG system will contract when put in service. Since the coefficient of thermal expansion for cellular glass insulation is close to that of steel, the cellular glass insulation will contract proportionally to the pipe and the differential between the two is easily addressed.
Uses of cellular glass in LNG systems
Cellular glass insulation is primarily used in an LNG terminal on piping and equipment as well as on tank bases. Cellular glass insulation can also be found on the outer sidewalls of double-walled tanks as “corner protection.” Corner protection insulates the outer wall against a possible leak in the primary containment tank. Cellular glass insulation will also sometimes be found in LNG spill containment pits to complement the pit’s fire-suppression systems.
Cellular glass as pipe insulation
When cellular glass insulation is used for LNG and other cryogenic pipe applications, the general approach is as follows:
- Double-layer construction.
- The inner layer of insulation is applied with the insulation joints unsealed.
- The outer layer insulation joints are sealed and the joints of the outer layer are offset from the joints of the inner layer.
- A vapour retarder is applied over the insulation.
- Finally, a metal jacket finish is applied.
All insulation systems have interruptions. Vapour stops are used to isolate the insulation system at the interruptions to ensure any break in the insulation system that might occur at the critical area will not compromise the entire system. Vapour stops are recommended at all pipe supports, protrusions, and insulation terminations.
As with the straight-run piping, fittings are also required to be double-layer construction. This poses a challenge for the insulation fabricator when trying to achieve offset joint configuration. In some cases box-type construction is used. This results in a void that is usually filled with inorganic loose fill insulation. Modern fabrication techniques have given rise to custom-fit valve covers that eliminate the void. These have become increasingly commonplace.
Pipe supports are another critical area on cryogenic pipe systems. Depending on the diameter of the line and support design, LNG lines may be supported directly on the cellular glass, on some type of insert, or directly on the pipe with a thermal break.
Installing cellular glass
Cellular glass insulation for LNG Vessels, exchangers, large diameter piping, and other equipment follow the same double-layer insulation approach. The primary difference is the securement of the inner layer is done with stainless steel banding rather than temporary securement with filament tape. Instead of half-sections used on smaller diameter piping, the cellular glass insulation is supplied as curved sidewall segments, head segments, or special shapes, depending on the geometry involved.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/11042017/cellular-glass-on-lng-systems/