The use of LNG has become vastly increased in variety of engineering for recent decades. Due to the fact that working with LNG is dealing with cryogenic condition, as a consequence, the development of equipments for this field of engineering is also in high dynamic market. As a key component in every fluid systems, valve is an indispensable equipment. So do cryogenic valves, they even sometimes play utmost important role in LNG system where perfect sealing as well as heat transfer resistance have to be done.
Thus, studies on cryogenic valve and its heat transfer prevention method have always been being a necessary work. Among various methods of heat transfer prevention, this study focuses on the use of vacuum layer which is done by enclosing the whole valve inside a jacket. Moreover, this work puts interest in small size valve scoped for module / portable LNG carriers, therefore vacuum jacketed layer is an economic suitable approach. The use of vacuum is a good method to minimize the heat transfer because it can eliminate the effect of heat conduction and convection.
However, heat radiation is unavoidable. Thus, to answer the question that how well this method can work, the study was carried out with varied situation and setup. The vacuum pressure is set to 25%, 50%, 75% and 99% of the maximum relative vacuum pressure each time. It should be noted that 100% vacuum (perfect vacuum) is actually unavailable. The each different set of vacuum pressure represents the leak rate or quality of the vacuum layer.
Additionally, there are 3 alternatives of jacket size representing the thickness of vacuum layer. Vacuum layer thickness is defined based on non-dimensional expression of ratio between jacket's radius and valve's nominal pipe size (D), which results in three case, low thickness (2.0D), medium thickness (2.5D) and high thickness (3.0D). The valve is put in normal condition (ambient temperature) before LNG (at –163℃) flow in. By computational method, the temperature gradient within the vacuum space is visualized.
The results show that jacket size and vacuum percentage indeed have noticeable influence on the heat transfer preventing performance. It is resulted that at lack of vacuum condition (under 65%), the valve's thermal resistance effectiveness is poor regardless of jacket size, the larger size insulation jacket even somewhat has the lowest performance. For higher rate of vacuum, thermal transfer effect is gradually prevented, and it is seen that high thickness insulation layer (larger jacket size) is more effective than medium and low thickness case. At maximum achievable vacuum rate (99%), large size jacket still proves to be the best choice although all three cases are applicable since measured temperature at jacket's surface is maintained above zero.
Study results bring about a suitable design for small cryogenic valve where numerical method and experiment were done to find optimum size for its insulation jacket. Thus, 2.5D jacket radius is recommended as the most suitable design. In a practical condition, combining with extra multi-layer insulation material shall guarantee the most efficient heat transfer prevention for the valve.