JPS5947199B2 - Method for reducing pressure rise in pressure accumulation type low temperature liquefied gas carrier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing the rise in gas pressure in a pressure accumulation type tank of a low temperature liquefied gas carrier.

In ships carrying low-temperature liquefied gas such as LNG and LPG, boil-off gas is generated due to heat entering from outside the tank.

Therefore, LNG ships are equipped with a combustion processing device using a boiler, and LPG ships are equipped with a mechanical cooling device to treat the boil-off gas and control the pressure in the tank to a predetermined value.

However, for coastal low-temperature liquefied gas carriers, the installation of these boil-off gas treatment facilities has been judged to be economically disadvantageous, and there is a tendency to adopt the so-called pressure accumulation method.

In the case of this pressure accumulation method, the tank must be designed with a pressure commensurate with the gas pressure that increases during the voyage, which increases costs accordingly, and therefore it is important to suppress the increase in gas pressure as much as possible to ensure safety. It is important not only from an economic perspective, but also from an economic perspective.

By the way, the mechanism of gas pressure increase in a tank containing low-temperature liquefied gas has not yet been accurately elucidated, but according to literature, it is known that gas pressure is strongly (controlled) by the liquid surface temperature.

It is also known that a natural convection boundary layer generated by heat entering from the tank side wall rises to the liquid surface and forms a warm layer near the liquid surface.

Therefore, based on the above-mentioned reasons, the present invention uses a baffle plate to diffuse the natural convection boundary layer (heated liquid) generated inside the tank side wall to reduce the temperature rise on the liquid surface (and reduce the gas pressure inside the tank. This paper proposes a method to reduce the increase in

Hereinafter, one embodiment of the present invention will be described based on the drawings.

Here, the thickness δ of the natural convection boundary layer generated inside the tank side wall is determined as follows.

In the above formula, Here, physical property values of methane are used as LNG.

p −425K9/rrl Cp”=O-83&al/Kg・’Cν=1.18
7 XI 0-3m2・h λ-=0.16 zKcai/ m-h-'Cβ=3
．． 37XI O''-31/'C g= 1.27 X I O8m/h2 and side wall height x=15m, i degree difference θ7-θ.

== Substituting 1 'c into each formula above gives 425X0.83X1.187X10""Pr=
=2.5850.162 158X 1.27 X 108X3.37X10”
3X IGr= (1,187XlO-3)2 =1.0252X1015 1 Pr 15=0.6026 Gr +o= 0.03154 (1+0.494Pra)so =1.068,°, δ
=0.58X0.6026X0.03154X1.06
8X15 = 0.1766 Medium 0.2m As you can see from the above calculation, the thickness of the natural convection boundary layer is 0.2
It is of the order of m.

Therefore, as shown in FIGS. 1 and 2, the baffle plate 1 having a width d wider than the thickness of the natural convection boundary layer a is placed at a predetermined position on the inside of the tank side wall 2 in a substantially horizontal direction, that is, the liquefied gas A certain distance below the liquid level when fully loaded and on the tank side wall 2
If the flow of the convective boundary layer a generated by the heat entering from the tank side wall 2 is changed in the direction of the arrow and mixed with other liquids, the natural convection boundary layer a that reaches the liquid surface can be The temperature of the convective boundary layer becomes much larger than when there is no baffle plate 1.

Thus, as the temperature of the warm layer that collects on the liquid surface decreases, the increase in gas pressure within the tank is reduced.

Note that, as shown in FIG. 3, if the baffle plates 11 are alternately arranged in a plurality of stages so that the natural convection boundary layer a and other liquids mix well, the effect will be even greater.

In addition, as shown in FIG. 4, if the baffle plate 21 is quite wide (and its tip is bent downward), the natural convection boundary layer a will accumulate near the bottom surface of the baffle plate 21, and the liquid It is even more effective if you prevent it from reaching the surface.

As described above, according to the method of the present invention, the baffle plate diffuses the rising temperature of the liquid formed by heat entering from the side wall of the tank, thereby effectively reducing the rise in gas pressure within the tank. can be done.

[Brief explanation of drawings]

1 and 2 are cross-sectional views of one embodiment of the present invention, and FIGS. 3 and 4 are cross-sectional views showing a modification of the same. a... Delirious convection boundary layer, 1, 11.21... Baffle plate,
2...Tank side wall.

Claims (1)

[Claims] 1. A substantially horizontal baffle plate is provided at a certain distance below the liquefied gas-filled liquid level inside the side wall of the pressure storage tank of a low-temperature liquefied gas carrier, and the heated liquid rising along the tank side wall surface is diffused. A method for reducing pressure rise in a pressure accumulation type low-temperature liquefied gas ship, the method comprising reducing the rise in gas pressure in the tank.