A flexible solution for the future

LNG facility insulation thermal design

From a thermal design standpoint, cryogenic insulation systems are charged with two main tasks: the first is to limit heat gain to the LNG and the second is to control condensation on the insulation surface. It is evident why minimising heat gain is important to the efficient and safe operation of liquefaction and import facilities. In most every case, the LNG contained by the piping and vessels of the facility is at its boiling point given its operating pressure. Every W/Btu of heat that leaks through the insulation converts directly into the formation of boil-off gas (BOG). Certainly, facilities are designed to handle a defined amount of BOG, but what happens if that becomes too much? What are the consequences of a facility facing higher-than-designed-for BOG?

A secondary thermal design criteria is to control or reduce condensation that can form on the outer surface of the insulation jacket when its temperature falls below the ambient dewpoint. While this may appear to be a secondary design consideration, excessive surface condensation can cause significant operational problems at start up and as facilities age. Continued excessive condensation can run off the insulated surfaces and create slipping hazards. Consistent and excessive surface condensation can also cause mould and other bio growth on the insulation jacket, giving the appearance of an ill-maintained facility (Figure 1). Over time, that same consistent condensation run off can cause significant corrosion to supporting structures and decking, giving rise to repair and reliability concerns.

Appropriate thickness tables are based on the operating temperature of the asset and, more importantly, should be an accurate representation of the ambient weather conditions where the facility is located. Typical insulation specifications may contain separate design thickness tables prescribing thickness of insulation by pipe size and process temperature for each criterion. It is also common to have a single table for thermal insulation that is a combination of the greater calculated thickness for any given pipe size and process temperature of the two design criteria. These combination thickness tables are often referred to as condensation control tables with a heat gain backstop. When the facility exists in a low relative humidity environment, a combination table is dominated by heat gain criteria; in high relative humidity environments, insulation thickness is dominated by condensation control criteria.

Industry trends in design

As a trend, insulation design criteria are becoming increasingly more stringent. This trend is being driven by several macro-economic factors. For one, countries or groups of countries are adopting stricter energy codes, as is the case with the Mexican NOMs that are now being integrated into upcoming project specifications. Since heat gain calculations are essentially an economic thickness calculation, some owners and EPCs are betting the cost of energy will continue to increase in the future and more stringent design will offset higher initial capital costs. The last element factoring into increasing thickness designs is the consideration around lowering LNG production carbon intensity. This can take the form of straightforward reduction in carbon emissions due to less heat gain and the carbon cost thereof, and the possible need for su-per-insulating designs which enable deep decarbonisation of the liquefaction process – more will be discussed later in the article. A recent example: a major gas importer in Southeast Asia completed a new LNG import facility. The specified insulation resistances on the new facility were, in some cases, 28% greater than the design used for a sister facility that was commissioned 12 years earlier (Figure 2).

Real-life performance and insulation system degradation

As the key transitional fuel source, there is high interest and public pressure to reduce the carbon intensity of LNG production, shipping, and distribution, by as much as is practical. When considering insulation’s role in this effort, one should consider the performance of the insulation system as designed (on day one of plant operations) and also how that insulation system’s performance may degrade over time.

Performance degradation in cold insulation systems is well documented and, like hot systems, is by and large caused by water ingress. For cryogenic insulation, the failure cycle begins with a breach of the all-important vapour barrier system. Once the vapour barrier is breached, the differential in relative humidity between atmosphere at cryogenic and ambient temperatures literally pulls the water vapour into the system. That vapour draw will not stop until every void is filled with ice or water. Many of these failures occur in locations where mechanical stresses in a system build up, or in features meant to absorb the differential thermal expansion between the cryogenic piping and the relatively warm outer layers of insulation (i.e. contraction joints) (Figure 3). Additionally, many of these vapour barrier failures can be attributed directly to using insulation materials that are ridged in nature. When a failure is gross it is easy to see (Figure 4), but often this vapour ingress can be subtle, i.e. jacketing that sweats more than it did at start up, or greater usage of the BOG compressors. Even subtle changes like these can cause significant reduction in the insulation system performance (Figure 5).

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