CN113551150A

CN113551150A - Low-temperature tank container

Info

Publication number: CN113551150A
Application number: CN202110807586.2A
Authority: CN
Inventors: 沈卫东; 海航; 周小翔; 陈来生; 蒋平安; 吴霄婷; 刘磊; 朱小林; 张亚萍; 余康; 张云凯
Original assignee: China International Marine Containers Group Co Ltd; CIMC Enric Investment Holdings Shenzhen Co Ltd; Nantong CIMC Energy Equipment Co Ltd
Current assignee: China International Marine Containers Group Co Ltd; CIMC Enric Investment Holdings Shenzhen Co Ltd; Nantong CIMC Energy Equipment Co Ltd
Priority date: 2021-07-16
Filing date: 2021-07-16
Publication date: 2021-10-26

Abstract

The invention provides a low-temperature tank container, which comprises a frame and a tank body horizontally arranged in the frame; the tank body is used for storing low-temperature liquid; the low temperature tank container further includes: a connecting pipe penetrating through the tank body and extending into the interior of the tank body so as to be communicated with the gas phase space of the tank body; the gasification heat exchange tube is fixedly arranged on the tank body; the gasification heat exchange tube extends along the axial direction of the tank body and does not exceed the outer side of the frame; the two ends of the gasification heat exchange pipe along the axial direction are respectively an inlet and an outlet, and the inlet is connected and communicated with the end part of the connecting pipe exposed out of the tank body; and the discharge pipeline system is communicated with the outlet of the gasification heat exchange pipe. The gasification heat exchange tube is located atmospheric environment, and the low temperature medium in the heat exchange tube that flows through can all participate in with the heat exchange of external atmospheric, and then the heat absorption and gasification be gaseous, and gaseous is discharged out by the pipe-line system that releases, and gaseous temperature is higher relatively can not produce the frost crack risk to adjacent tank and hull.

Description

Low-temperature tank container

Technical Field

The invention relates to the technical field of low-temperature containers, in particular to a low-temperature tank container.

Background

At present, the specific gravity of LNG (liquefied natural gas) in energy structures of various countries is continuously increased, and the transportation mode of the LNG is continuously innovated. Containerized shipping using LNG tankers has been attempted in the industry as a new mode of LNG logistics and commerce.

LNG tanks are transported as containers, and particularly in the case of sea transportation, there must be large-scale stacking and stockpiling, but the cryogenic nature (-162 ℃) of LNG determines that once a tank has LNG erupted, it can cause significant damage to adjacent tanks and the hull. The LNG tank generally adopts a vacuum insulation tank body, and when the vacuum of the tank is accidentally destroyed or the storage time of the tank exceeds the maintenance time of the tank, liquid LNG may be discharged from a discharge port, which is one of the bottlenecks that restrict the large-scale application of the tank at present.

The main technical solutions for the discharge of the liquid cryogenic medium that may be present from the discharge opening are as follows:

the first is to use a common pipe ventilation scheme. The discharge ports of all tanks are collected using low temperature resistant piping so that after cryogenic liquid is discharged, it can be collected and directed to a dedicated area. However, the scheme needs a whole recovery system, the investment is large, after the tank boxes are stacked, the workload for connecting a plurality of discharge pipes is huge, the interface of each pipeline is a leakage point, and no liquid leakage can be guaranteed. Meanwhile, the supervision and maintenance of the common-pipe ventilation are very troublesome.

The second is to adopt a scheme of a diversion trench and a safe distance. The stainless steel protection diversion trench is arranged at the discharge port of the tank box, and a safe distance is reserved between the tank box and the tank box, so that liquid can not be sprayed onto the adjacent tank box after being discharged. However, this greatly increases the stacking distance of the tanks, and lowers the stacking quantity of the tanks per unit area, which is very poor in economy.

Disclosure of Invention

The invention aims to provide a low-temperature tank container, which is used for preventing liquid low-temperature medium from being discharged from a tank body and has the advantages of low installation cost and high practicability.

In order to solve the technical problems, the invention adopts the following technical scheme:

a low-temperature tank container comprises a frame and a tank body horizontally arranged in the frame; the tank body is used for storing cryogenic liquid; the low temperature tank container further includes: a connection pipe penetrating the tank and extending into the interior of the tank so as to communicate with the gas phase space of the tank; the gasification heat exchange tube is fixedly arranged on the tank body; the gasification heat exchange tube extends along the axial direction of the tank body and does not exceed the outer side of the frame; the two ends of the gasification heat exchange pipe along the axial direction are respectively provided with an inlet and an outlet, and the inlet is connected and communicated with the end part of the connecting pipe exposed out of the tank body; and the discharge pipeline system is communicated with the outlet of the gasification heat exchange pipe.

According to one embodiment of the invention, the outer wall of the gasification heat exchange tube is further provided with a plurality of circumferentially spaced fins.

According to one embodiment of the invention, the frame includes two oppositely spaced end bells; the gasification heat exchange tube is arranged at the upper end of the tank body and does not exceed the upper end surface of the end frame.

According to one embodiment of the invention, two ends of the gasification heat exchange pipe in the axial direction respectively extend to the inner sides of the two end frames.

According to one embodiment of the invention, there is only one vaporization heat exchange tube.

According to one embodiment of the invention, the number of the gasification heat exchange pipes is multiple and is divided into two groups with equal number; and the two groups of gasification heat exchange tubes are symmetrically arranged on the two transverse sides of the tank body.

According to one embodiment of the invention, the vaporizing heat exchange tubes are single-layer tubes.

According to one embodiment of the invention, the diameter of the gasification heat exchange tube is in the range of 150mm to 350 mm.

According to one embodiment of the invention, the cryogenic tank container further comprises a gas phase pipeline; the gas phase pipeline is communicated with the other end of the gasification heat exchange pipe along the axial direction.

According to one embodiment of the invention, the bleed line system comprises a header pipe connected to the vaporizing heat exchange tube and a plurality of bleed branch pipes separated from one end of the header pipe remote from the vaporizing heat exchange tube; and a safety valve is arranged on each discharge branch pipe.

According to the technical scheme, the low-temperature tank container provided by the invention at least has the following advantages and positive effects:

the low-temperature tank container is additionally provided with a gasification heat exchange pipe on the outer wall of the tank body. The gasification heat exchange tube is communicated with the gas phase space of the tank body through a connecting tube, and is equivalent to an additional gas phase space of the tank body. When the low-temperature liquid is sprayed out, the low-temperature liquid firstly absorbs heat and expands in the tank, so that the tank is filled with gas and the pressure is higher; due to the existence of the additional gas phase space, the low-temperature liquid medium or gaseous medium can enter the gasification heat exchange tube from the tank under the action of the pressure difference. The gasification heat exchange tube is positioned in the atmospheric environment, and the low-temperature medium can participate in heat exchange with the external atmosphere, so that heat is absorbed, and the low-temperature medium is rapidly gasified into gas in a non-low-temperature state. And the gasification heat exchange tube extends along the axial direction of the tank body to have a tube pass as long as possible, so that the low-temperature medium in the tube can perform sufficient heat exchange with the external atmosphere to be completely gasified. When the outlet of the gasification heat exchange tube is connected with a discharge pipeline system, the tank can only discharge gaseous natural gas, the temperature of the gaseous natural gas is relatively high, the risk of frost cracking of adjacent tanks and a ship body is avoided, the natural gas is lighter than air, the natural gas can quickly float and cannot gather, the safety performance is greatly improved, and the problem of low-temperature liquid discharge is fundamentally solved.

Meanwhile, the gasification heat exchange tube does not exceed the external dimension of the end frame after being installed on the tank body, so that interference on stacking and storage of a plurality of tank boxes is avoided, a safety interval is not required to be reserved between the tank boxes, and the economy is improved; and the gasification heat exchange pipe has simple structure and low installation and maintenance cost.

Drawings

Fig. 1 is an isometric view of a cryogenic tank container in an embodiment of the invention.

Fig. 2 is a front end view of the cryogenic tank container of fig. 1.

Fig. 3 is a side view of the cryogenic tank container of fig. 1.

Fig. 4 is a schematic connection diagram of a gasification heat exchange pipe and a bleed-off pipe system according to an embodiment of the present invention.

Fig. 5 is a flowchart illustrating the operation of the cryogenic tank container in an embodiment of the present invention.

The reference numerals are explained below:

1. a frame; 11. an end frame; 111. a front end frame; 112. a rear end frame; 2. a tank body; 21. an inner tank; 22. an outer tank; 3. a connecting pipe; 4. a gasification heat exchange pipe; 401. an inlet; 402. an outlet; 5. a bleed line system; 50. a header pipe; 51. a tapping branch pipe; 53. a safety valve; 6. a gas collection pipeline; 7. a gas phase pipeline.

Detailed Description

Exemplary embodiments that embody features and advantages of the invention are described in detail below in the specification. It is to be understood that the invention is capable of other embodiments and that various changes in form and details may be made therein without departing from the scope of the invention and the description and drawings are to be regarded as illustrative in nature and not as restrictive.

In the related art, for the existing solution that the liquid low-temperature medium is discharged from the discharge port, no matter the solution is a common pipe ventilation solution or a solution of a diversion trench and a safe distance, the consideration is given to how to deal with the angle after the liquid in the tank is discharged, and the problem that the solution is difficult to solve is existed.

In contrast, the low-temperature tank container provided by the embodiment is considered from the tank itself, so that only gaseous media can be discharged, and the problem of discharging low-temperature liquid can be effectively solved. That is to say, if the tank can discharge gaseous natural gas (not less than-40 ℃) instead of low-temperature LNG liquid, the risk of frost cracking of the adjacent tank and the ship body is avoided, and the problem is fundamentally solved.

The cryogenic tank container of this embodiment is particularly suited for marine transportation of LNG, but is of course also suited for conventional land transportation.

Referring to fig. 1 and 2 together, a specific structure of a cryogenic tank container according to the present embodiment is shown.

The low-temperature tank container mainly comprises a frame 1, a tank body 2 horizontally arranged in the frame 1, and a connecting pipe 3, a gasification heat exchange pipe 4 and a discharge pipeline system 5 which are arranged on the tank body 2.

The frame 1 is a hollow box structure and includes two end frames 11. The two end frames 11 are a front end frame 111 and a rear end frame 112.

The tank 2 is accommodated in the frame 1, and both axial ends of the tank 2 are respectively connected and fixed with the front end frame 111 and the rear end frame 112.

The can body 2 has a double-layer structure including an inner can 21 and an outer can 22 wrapping the inner can 21. The inner tank 21 is mainly used for storing a liquid medium in a cryogenic state such as LNG, liquefied ethylene, liquid oxygen, liquid nitrogen, liquid argon, etc., while the inner tank 21 is a main component that receives the pressure generated by vaporization of the liquid, and the inside of the inner tank 21 forms a liquid phase space and a gas phase space located above the liquid phase space. The outer vessel 22 serves to insulate and protect the inner vessel 21. A vacuum interlayer capable of being vacuumized is formed between the inner tank 21 and the outer tank 22, and the vacuum interlayer is filled with a heat insulation material to further reduce heat conduction to ensure the heat insulation effect of the inner tank 21.

Note that, the tank body 2 is used as a reference, and the direction toward the inner tank 21 is taken as the inner direction; correspondingly, in a direction away from the inner vessel 21.

Referring to fig. 3, the connection pipe 3 penetrates the outer vessel 22 and extends inward into the interior of the inner vessel 21, thereby communicating with the gas phase space of the inner vessel 21.

Preferably, the position where the connection pipe 3 penetrates the outer tank 22 is located at the top of the outer tank 22, and the connection pipe 3 is located near one side of the front end frame 111.

The connecting pipe 3 has a short extension length exposed at the end of the outer tank 22 to ensure that the vaporizing heat exchange pipe 4 connected thereto can be accommodated inside the frame 1.

The gasification heat exchange pipe 4 is fixedly arranged on the outer tank 22 by welding or support. The gasification heat exchange pipe 4 extends in the axial direction of the tank body 2 without exceeding the outside of the frame 1.

The two ends of the gasification heat exchange tube 4 along the axial direction are respectively an inlet 401 and an outlet 402, the inlet 401 is connected and communicated with the end part of the connecting tube 3 exposed out of the tank body 2, namely, the gasification heat exchange tube 4 is communicated with the gas phase space of the tank body 2. The gasification heat exchange pipe 4 has an outlet 402 near the rear end frame 112, and the outlet 402 is connected and communicated with the vent pipe system 5, so that the LNG gas inside the tank 2 can be safely discharged through the vent pipe system 5.

When the gasification heat exchange tube 4 is communicated with the gas phase space of the tank body 2, the gasification heat exchange tube 4 is equivalent to an additional gas phase space of the tank body 2. When the LNG liquid is sprayed out, firstly, the LNG in the tank absorbs heat and expands, so that the tank is filled with the LNG, and the pressure is increased; due to the extra gas phase space, the low-temperature liquid LNG or gaseous LNG enters the vaporizing heat exchange tube 4 from the tank under the action of the pressure difference. Because the heat exchange tube 4 is located in the atmosphere, the low-temperature LNG medium (such as-150 ℃ LNG liquid, LNG gas, and a mixture of both) inside the tube can participate in heat exchange with the outside atmosphere, thereby absorbing heat and rapidly gasifying the LNG medium into gas in a non-low-temperature state. The LNG gas temperature is above-40 ℃, the risk of frost cracking of adjacent tanks and ship bodies is avoided, the natural gas density is lighter than that of air, the natural gas can rapidly float and cannot gather, and the safety performance is improved.

In addition, the additional gas phase space is arranged, so that the gas phase space of the tank body 2 is increased; under the prerequisite that gas phase space need occupy whole volume in jar 6% -8%, the jar body 2 of the same volume can have more spaces to store LNG liquid, can reach the maximize of tank transportation load capacity, has promoted the transportation economy of tank.

In this embodiment, the diameter size of the heat exchange tube 4 is big, and it is greater than the diameter size of the connecting tube 3, because heat transfer area is big, just can guarantee to fully gasify via cryogenic liquid LNG or gaseous LNG in self pipeline. On the premise that the diameter of the gasification heat exchange tube 4 meets the heat exchange requirement, the gasification heat exchange tube 4 is not beyond the outer side of the frame 1 after being installed on the tank body 2, and the stacking of the tank box is not interfered.

Therefore, referring to fig. 2, the vaporizing heat exchange tube 4 is preferably installed at the upper end of the can body 2 without exceeding the upper end surface of the end frame 11, i.e., the vaporizing heat exchange tube 4 is located in the upper portion of the can body 2 and the space defined by the two end frames 11, making full use of the marginal space of the can box.

Further, the vaporizing heat exchange tubes 4 extend along both ends of the tank 2 in the axial direction and are close to the inner sides of the front and rear end frames 11, respectively. The axial length of the gasification heat exchange tube 4 and the axial length of the tank body 2 are almost nearly equal.

In the case that the vaporizing heat exchange tube 4 has a sufficiently long tube pass, the low-temperature medium in the tube can be sufficiently heat-exchanged with the external atmosphere during the process of flowing from the inlet 401 to the outlet 402 of the vaporizing heat exchange tube 4 to be completely vaporized.

In the present embodiment, the vaporizing heat exchange tube 4 is a single-layer metal tube. In other words, the tube wall of the gasification heat exchange tube 4 is thin, the heat conduction resistance is small, the heat transfer coefficient is large, and the complete heating and gasification of the liquid LNG are facilitated.

The diameter size range of the gasification heat exchange tube 4 is 150 mm-350 mm. The diameter of the gasification heat exchange tube 4 can be changed adaptively according to the size of the tank and the design size of the tank body 2, so that the requirements that the heat exchange area of the pipeline is large enough and the pipeline does not exceed the outer side of the frame 1 are met.

And, a plurality of fins arranged at intervals in the circumferential direction are also arranged on the outer wall of the gasification heat exchange tube 4. Due to the arrangement of the fins, the heat exchange area of the gasification heat exchange tube 4 is further increased, the heat exchange efficiency is improved, and the liquid LNG can be quickly gasified by absorbing heat.

In the present embodiment, it is preferable that the number of the vaporization heat exchange tubes 4 of the above-described design is two as shown in fig. 1. The two gasification heat exchange tubes 4 are arranged on the two lateral sides of the tank body 2 in a bilateral symmetry manner; therefore, the existence of the two gasification heat exchange tubes 4 is equivalent to the addition of two additional gas phase spaces on the tank body 2, the gas phase space of the tank can be effectively increased by 10-30%, and the economy of the tank is greatly improved.

Referring to fig. 4, when installing, the two gasification heat exchange tubes 4 can be independently arranged, and are respectively connected through a vertically arranged gas collecting pipeline 6 and share a discharge pipeline system 5, and the two are not affected with each other, so that the safety performance is high. Of course, in other embodiments, the first vaporizing heat exchange tube 4 can be connected and communicated with the second vaporizing heat exchange tube 4 through a pipeline to realize series connection; and the second gasification heat exchange tube 4 is connected with a discharge pipeline system 5 through a gas collecting pipeline 6 to discharge LNG gas, so that the number and the length of pipelines are saved.

In other embodiments, the number of the vaporizing heat exchange tubes 4 may be only one, and a single vaporizing heat exchange tube 4 may also satisfy the vaporizing requirement of the liquid LNG. Of course, according to the use requirements of different working conditions, the number of the gasification heat exchange tubes 4 can also be multiple (preferably, double), and the gasification heat exchange tubes are divided into two groups with equal number; two groups of gasification heat exchange tubes 4 are symmetrically arranged on two lateral sides of the tank body 2.

In the present embodiment, the bleed line system 5 is provided at the bottom of the tank 2, integrating a plurality of lines, valves and meters.

Referring to fig. 5, fig. 5 is a flowchart illustrating the operation of the low temperature tank container in this embodiment.

The bleed line system 5 includes a header pipe 50 connected to the vaporizing heat exchange tube 4 and a plurality of bleed branch tubes 51 separated from one end of the header pipe 50 remote from the vaporizing heat exchange tube 4; a relief valve 53 is provided in each of the relief branch pipes 51. The safety valve 53 is a normally closed structure as an automatic pressure relief device driven by static pressure of medium before the valve, and can automatically discharge medium inside when the pressure exceeds the self-set opening pressure, so as to ensure the safety of the tank body 2.

When the gasification heat exchange tube works normally, the gasification heat exchange tube 4 is communicated with the gas phase space of the tank body 2 through the connecting tube 3, and the pressure of the two is equal; the BOG gas is present inside the vaporizing heat exchange tube 4.

When the tank box is damaged due to accidental collision, the LNG is shaken to break the stable state, the LNG in the tank absorbs heat and expands, the tank is filled with gaseous LNG, the pressure in the tank rises, and a pressure difference is generated. Under the action of the pressure difference, the low-temperature liquid LNG or gaseous LNG enters the low-pressure vaporizing heat exchange tube 4 from the tank. Because the gasification heat exchange tube 4 is located in the atmospheric environment, the low-temperature LNG medium in the tube can participate in heat exchange with the outside atmosphere, and further absorbs heat and is gasified into gas in a non-low-temperature state.

During the gasification process, the gas in the tube is increased, and the pressure of the gasification heat exchange tube 4 is gradually increased. And when the pressure on the safety valve 53 communicated with the gasification heat exchange tube 4 is higher than the self opening pressure, the valve port is automatically opened to finish the discharge of the LNG gas.

The cryogenic tank container also comprises a gas phase pipeline 7. The gas phase pipeline 7 is connected and communicated with the other end (i.e. the outlet 402) of the gasification heat exchange pipe 4 along the axial direction. The gas phase pipeline 7 is provided with an emergency cut-off valve and a stop valve at intervals.

The purpose is that the gasification heat exchange tube 4 is equivalent to an additional gas phase space of the tank body 2, and the gasification heat exchange tube 4 can naturally realize the gas supply function through the gas phase pipeline 7. In addition, due to the vaporization heat exchange tube, when the tank box discharges liquid, gas can be reversely conveyed to the tank body 2, so that the self-pressurization of the tank body 2 is realized, and the pressurization efficiency is improved.

In the present embodiment, the arrangement of the vaporizing heat exchange tube 4 is applicable to a variety of cryogenic liquid tank cases. Certainly, the gasification heat exchange tube 4 is not limited to a tank box, can also be popularized and applied to all low-temperature movable containers and fixed pressure containers, and has a wide application range.

In conclusion, the low-temperature tank container provided by the invention at least has the following advantages and positive effects:

the low-temperature tank container is additionally provided with a gasification heat exchange tube 4 on the outer wall of the tank body 2. The gasification heat exchange tube 4 is communicated with the gas phase space of the tank body 2 through a connecting tube 3, and equivalently, the gasification heat exchange tube 4 becomes an additional gas phase space of the tank body 2. The extra gas phase space needs to satisfy the heat exchange area that tank liquid gasification needs on the one hand, and on the other hand can be calculated into the gas phase space of tank, has increased the transportation economic nature of tank.

Specifically, the gasification heat exchange tube 4 is used as a single-layer tube pipeline communicated with the tank 21 in the tank box and can be used as a liquid heat exchange tube to heat and gasify a low-temperature medium, so that the discharged medium can only be a gaseous medium. In actual use, when the LNG liquid is sprayed out, the LNG in the tank absorbs heat and expands, so that the tank is filled with the LNG, and the pressure is high; due to the extra gas phase space, the low-temperature liquid LNG or gaseous LNG enters the vaporizing heat exchange tube 4 from the tank under the action of the pressure difference. The gasification heat exchange tube 4 is located in the atmospheric environment, and the low-temperature LNG medium can participate in heat exchange with the outside atmosphere, so that heat is absorbed, and the LNG medium is rapidly gasified into gas in a non-low-temperature state. Gaseous natural gas (more than or equal to-40 ℃) can not generate frost crack risk to adjacent tanks and ship bodies, and the density of the natural gas is lighter than that of air, so that the natural gas can quickly float and can not generate aggregation, the safety performance is greatly improved, and the problem of discharging low-temperature LNG liquid is fundamentally solved.

Meanwhile, the gasification heat exchange tube 4 utilizes the edge space of the tank box during installation, does not exceed the external dimension of the end frame 11, does not interfere the stacking and storage of a plurality of tank boxes, does not need to consider the safety space between the tank boxes, and improves the economy; and the gasification heat exchange tube 4 has simple structure and low installation and maintenance cost.

While the present invention has been described with reference to several exemplary embodiments, it is understood that the terminology used is intended to be in the nature of words of description and illustration, rather than of limitation. As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.

Claims

1. A low-temperature tank container comprises a frame and a tank body horizontally arranged in the frame; the tank body is used for storing cryogenic liquid; characterized in that, the low temperature tank container still includes:

a connection pipe penetrating the tank and extending into the interior of the tank so as to communicate with the gas phase space of the tank;

the gasification heat exchange tube is fixedly arranged on the tank body; the gasification heat exchange tube extends along the axial direction of the tank body and does not exceed the outer side of the frame; the two ends of the gasification heat exchange pipe along the axial direction are respectively provided with an inlet and an outlet, and the inlet is connected and communicated with the end part of the connecting pipe exposed out of the tank body;

and the discharge pipeline system is communicated with the outlet of the gasification heat exchange pipe.

2. The cryogenic tank container of claim 1 wherein the outer wall of the vaporizing heat exchange tube is further provided with a plurality of circumferentially spaced fins.

3. The cryogenic tank container of claim 1, wherein the frame comprises two oppositely spaced end bells; the gasification heat exchange tube is arranged at the upper end of the tank body and does not exceed the upper end surface of the end frame.

4. The cryogenic tank container according to claim 3, wherein both ends of the vaporizing heat exchange pipe in the axial direction extend to the inner sides of both the end frames, respectively.

5. The cryogenic tank container of claim 3, wherein there is only one vaporization heat exchange tube.

6. The cryogenic tank container according to claim 3, wherein the vaporizing heat exchange tubes are plural in number and divided into two groups equal in number; and the two groups of gasification heat exchange tubes are symmetrically arranged on the two transverse sides of the tank body.

7. The cryogenic tank container of claim 1, wherein the vaporizing heat exchange tubes are single-layered tubes.

8. The cryogenic tank container of claim 1, wherein the vaporizing heat exchange tube has a diameter size ranging from 150mm to 350 mm.

9. The cryogenic tank container of claim 1, further comprising a gas phase pipeline; the gas phase pipeline is communicated with the other end of the gasification heat exchange pipe along the axial direction.

10. The cryogenic tank container of claim 1, wherein the bleed line system comprises a header pipe connected to the vaporizing heat exchange tubes and a plurality of bleed branch pipes branching off from an end of the header pipe remote from the vaporizing heat exchange tubes; and a safety valve is arranged on each discharge branch pipe.