JP4773780B2 - Heat transfer tube for LNG vaporizer and LNG vaporizer using the same - Google Patents

Description

  The present invention relates to a heat transfer tube for an LNG (liquefied natural gas) vaporizer having an excellent anticorrosion effect, and an LNG vaporizer using the heat transfer tube.

  Liquefied natural gas (hereinafter referred to as LNG) is usually transported or stored in a liquid at a low temperature and high pressure, and is vaporized before being used. For this vaporization, a vaporizer called an open rack vaporizer (hereinafter referred to as ORV) capable of vaporizing a large amount of LNG is used. FIG. 1 shows an example of this ORV. The ORV is a kind of heat exchanger that heats and vaporizes LNG by heat exchange with seawater (see, for example, Patent Document 1). Seawater is accumulated in the trough 8 from the seawater header 6 through the watering nozzle 7, overflows from the side edge of the trough 8, and hangs down while wetting the outer surface of the panel 3 formed by arranging the heat transfer tubes 3a in a curtain shape. . On the other hand, the LNG is introduced into the LNG manifold 1 and sent to the lower header 2 through which the LNG connected to the lower portion of the panel 3 circulates. The LNG is heated by heat exchange with the seawater and air is passed through each heat transfer tube 3 a of the panel 3. The natural gas (NG) that has risen and vaporized is discharged from the upper headers 4 and 4 to the NG manifold 5.

  As a material of the heat transfer tube 3a forming the panel 3, an aluminum alloy is usually used from the viewpoints of good thermal conductivity and easy processing into a complicated shape required for the panel 3. . Aluminum alloys are easily corroded when immersed in seawater, and once corroded, the corroded portion is eroded intensively and is susceptible to pitting corrosion. For this reason, with respect to aluminum alloys used for applications such as being immersed in seawater, anticorrosion treatment has been actively studied, and at present, anticorrosion treatment using sacrificial anticorrosive action has become mainstream. In Patent Document 1, the material of the panel 3 (heat transfer tube 3a) is attached to the lower header 2 in which the LNG immersed in the seawater pond in which the drooping seawater accumulated while wetting the outer surface of the panel 3 is wet with the LNG vaporizer. A metal, such as zinc (Zn), that is more susceptible to corrosion than an aluminum alloy, that is, a metal or alloy bulk (not shown) that has a high tendency to ionize, is electrically connected to form a sacrificial anode. An anticorrosion treatment method is disclosed in which the surfaces of the lower header 2 and the panel 3 serving as counter electrodes are anticorrosive by being consumed. However, in the LNG vaporizer, since the seawater overflowing from the side edge of the trough 8 directly hits the surface of the heat transfer tube 3a constituting the panel 3, even if the sacrificial anode is provided, corrosion caused by so-called erosion / corrosion is caused. Occurrence is inevitable. For this reason, the surface of the heat transfer tube 3a is made of the material so that the seawater is not in direct contact and the corrosion of the heat transfer tube surface is prevented by the anticorrosion action even when the coating alloy is locally peeled off. It is desirable to coat an alloy having a higher ionization tendency than an aluminum alloy (hereinafter referred to as a coating alloy). Conventionally, an Al—Zn alloy is well known as an alloy having such sacrificial anticorrosive action, and an Al-2% Zn alloy, an Al-15% Zn alloy, or the like is often used. Corrosion is effectively prevented by spraying this coating alloy to form a film on the surface of the heat transfer tube.

In order to further improve the corrosion resistance of the film formed on the surface of the heat transfer tube, for example, in Patent Document 2, the surface of the heat transfer tube (extruded tube material) of aluminum or aluminum alloy is electrochemically sacrificed as a first layer. In order to coat Zn acting as a layer and prevent evaporation of Zn due to brazing at the time of manufacturing a heat exchanger, Al or Al-Ca, Al-Zn-Ca-based Al alloy or the like is formed on the first layer. An aluminum heat exchanger tube with improved corrosion resistance by thermal spraying is disclosed. In Patent Document 3, an Al—Zn alloy layer is formed on the surface of the heat transfer tube, and further, an Al—Zn alloy layer containing one or more elements selected from In, Sn, Hg, and Cd on the surface thereof. A heat transfer tube made of an Al alloy is disclosed which has a high corrosion resistance by forming the structure. On the other hand, Patent Document 4 discloses a fin tube (fin type heat transfer tube) for an ORV type vaporizer in which an Al—Zn alloy material is clad on a surface of an Al alloy base material tube to form a thick sacrificial anode coating. ing.
JP-A-9-178391 Japanese Patent Laid-Open No. 1-114698 Japanese Patent Publication No.7-1157 Japanese Patent Laid-Open No. 5-16496

  However, the lower and lower headers 2 of the ORV panel 3 are cooled to below freezing point because LNG, which is a natural gas in liquid state, circulates. In the state of contact with seawater overflowing and dripping in such a low temperature region of ORV, it is difficult to form an oxide film on the aluminum alloy surface of the heat transfer tube base material, and the electrode potential of the heat transfer tube base material made of Al alloy is patented. There is a possibility that the electrode potential of the Al—Zn alloy film described in Literatures 1 to 4 is lower, the sacrificial anticorrosive action of the Al—Zn alloy film is not exhibited, and the heat transfer tube base material is not protected. For example, depending on the environmental conditions such as when the seawater temperature is high or when the cooling load due to LNG circulation of the panel is large, the aluminum alloy heat transfer tube base material is pulled to the high potential of the Al-Zn alloy film, and the heat transfer tube mother There is a risk of galvanic corrosion of the material.

  The film formed on the surface of the heat transfer tube is required to have durability in addition to anticorrosion. Even if the sacrificial anticorrosive ability for the Al alloy heat transfer tube base material is excellent, if the corrosion rate is high and the durability as a film is inferior, the heat transfer tube base material is eventually damaged. In the LNG vaporizer, as described above, since the seawater overflowing from the side edge of the trough 8 directly hits the surface of the heat transfer tube 3a constituting the panel 3, measures against erosion and corrosion are also necessary.

  The present invention has been made in view of such problems, and the problem is that even if it is arranged on the lower panel side or the lower header where the oxide film is difficult to be formed on the surface because the cooling load is large, the Al alloy An object of the present invention is to provide an LNG vaporizer heat transfer tube excellent in the effect of preventing corrosion damage on the base metal surface and durability, and an LNG vaporizer using the heat transfer tube.

  In order to solve the above problems, the present invention employs the following configuration.

That is, the heat transfer tube for an LNG vaporizer according to the present invention has a sacrificial anticorrosive coating on the outer surface, in which LNG flows and seawater is supplied to the outer surface, and the seawater and the LNG exchange heat to vaporize LNG. A heat transfer tube for an LNG vaporizer made of an Al alloy in which the sacrificial anticorrosive coating contains 0.3 to 3.0 mass% of Mn, 0.3 to 5.0 mass% of Mg, and the balance There Ri Do Al and inevitable impurities, and wherein the electrode potential than Al alloy of the heat transfer tube is lower Al alloy coating. Moreover, it is preferable that the sacrificial anticorrosive coating is a thermal spray coating formed by thermal spraying.

  As described above, since the natural electrode potential of Zn is higher than that of the base metal alloy under an environmental condition in which an oxide film is not easily formed on the surface of the aluminum alloy base material of the heat transfer tube, the Al-Zn sprayed coating is more preferable. The potential becomes higher than the base metal alloy of the heat transfer tube or the lower header, and the sacrificial anticorrosive effect cannot be obtained. For this reason, in order to exert a sacrificial anticorrosive action even under an environmental condition where it is difficult to form an oxide film on the surface of the aluminum alloy, a film of a metal having a lower thermodynamic potential than Al, for example, by thermal spraying is formed. There is a need. As such a metal, Mg is the most suitable, and if it is an alloy film containing Mg and is a "base" film than the Al alloy base material used as the material of the heat transfer tube and the lower header, a sacrificial anticorrosive film Can be applied as well. In addition to Mg, there are Hf (hafnium), Ti (titanium), and Be (beryllium) as thermodynamically lower metals than Al. Among these, Ti and Be oxide films are stronger than Al oxide films, and even if these metals are thermodynamically “base” metals than Al, the environment in which the LNG vaporizer is operated When considered, the metal is substantially more noble than Al. Moreover, the metal containing Hf and Ti has remarkably poor drawability, and it is difficult to process it into a thermal spray material used for flame spraying as a film forming means. Therefore, Hf and Ti cannot be applied to the sacrificial anticorrosive film. On the other hand, since Be is toxic, there is a risk of film formation work and marine pollution during ORV operation, and it is a very expensive material, so it is not suitable as a sacrificial anticorrosive film.

On the other hand, as an erosion countermeasure, it is effective to add an element that strengthens the parent phase by dissolving in Al with an Al alloy base material of a heat transfer tube, and when this element precipitates as a compound, It is necessary not to make the electrode potential of the alloy film nobler than the electrode potential of the Al alloy base material of the heat transfer tube. Examples of the reinforcing element include elements such as Zn, Nb, Mn, Zr, and Ti. Nb, Zr, and Ti are very expensive elements in addition to forming a stronger oxide film than Al. Since it is difficult to alloy with Al, it is unsuitable as an additive element. Therefore, as an erosion countermeasure, it is preferable to use Zn and / or Mn as additive elements. Zn and / or Mn are preferably dissolved in the Al alloy matrix, but depending on the amount of addition, etc., a compound with Mg such as Zn—Mg, Mn—Mg, or Zn—Mn—Mg may be present. If generated, a lower electrode potential than the Al alloy base material is maintained. In the present invention, Mn is adopted as an additive element.

If the Mn content is less than 0.3%, the solid solution strengthening is insufficient, and the required erosion resistance characteristics cannot be obtained. If it exceeds 3.0% by mass, the strengthening effect of the Al alloy matrix is saturated. However, it may segregate in the Al alloy film and adversely affect the erosion resistance. In addition, when the Mg content is less than 0.3%, Mg is almost completely dissolved in the Al matrix regardless of the film forming conditions, so the electrode potential of the Al alloy film is the electrode of the Al alloy heat transfer tube matrix. Compared to the potential, the effect of making it sufficiently low cannot be obtained. In addition, if the Mg content exceeds 5% by mass, the electrode potential of the Al alloy film becomes lower than necessary depending on the usage environment, and there is a risk that the amount of Mg eluted increases and the corrosion rate becomes excessive, It is not preferable.

The LNG vaporizer according to the present invention includes a panel in which a plurality of heat transfer tubes on which the sacrificial anticorrosion film is formed are arranged in a curtain shape, and an upper header and an LNG for discharging vaporized gas connected to the upper and lower portions of the panel, respectively. Heat exchange between seawater flown along the surface of the panel from the upper part of the panel unit and LNG flowing from the lower header side to the upper header side in the heat transfer pipe, comprising a panel unit comprising a lower header for supply Thus, the LNG vaporizer is configured to vaporize LNG. Moreover, it is preferable that the sacrificial anticorrosive coating is a thermal spray coating formed by thermal spraying.

Moreover, it is preferable that the LNG vaporizer which concerns on this invention is a LNG vaporizer in which the sacrificial anticorrosive film was formed in the part located in the said panel lower part in a heat exchanger tube.

As described above, in this type of LNG vaporizer, since LNG is in a liquid state in the portion of the heat transfer tube located at the lower part of the panel, it is cooled to below freezing point. In the contact state, an oxide film is hardly formed on the aluminum alloy surface of the heat transfer tube base material. For this reason, if the above-mentioned Al alloy film is coated on the heat transfer tube at least in a portion of the heat transfer tube located at the lower part of the panel , a good corrosion prevention effect with excellent durability can be obtained.

In this invention, Mn is added to the surface of the heat transfer tube for LNG vaporizer as a solid solution strengthening element of the heat transfer tube base material in order to improve the erosion resistance, and Mg is a metal having a thermodynamically lower potential than Al. Since the Al alloy film containing the LNG vaporizer is formed, the outer surface of the heat transfer tube on the lower side of the panel of the LNG vaporizer that is susceptible to corrosion damage is in an environment where it is difficult to form an oxide film by contact with seawater in a low temperature region. The sacrificial anticorrosive effect with excellent erosion resistance and good durability can be obtained on the outer surface of the lower header. Thereby, the corrosion damage of the heat transfer tube is avoided, and the operation efficiency and the service life of the LNG vaporizer are improved.

  Embodiments of the present invention will be described below with reference to FIG.

FIG. 1 shows an LNG vaporizer in which a heat transfer tube according to the embodiment is incorporated. A panel 3 in which a plurality of heat transfer tubes 3a are arranged in a curtain shape, and a lower header connected to the upper and lower portions of the panel 3, respectively. And a plurality of panel units U made of an Al alloy (for example, an Al—Mn alloy such as A3203, an Al—Mg alloy such as A5083, and an Al—Mg—Si alloy such as A6063) including the upper header 4 and the upper header 4. They are arranged in parallel. The lower header 2 and the upper header 4 are connected to a lower LNG manifold 1 and an upper NG manifold 5 , respectively. Above each panel unit U of the panel units U, troughs 8 are provided for flowing down seawater as a heat source for vaporizing LNG. LNG is sent from the lower manifold 1 to the lower header 2 and vaporizes by exchanging heat with the seawater in the process of ascending through the heat transfer pipes 3a of the panel 3, and from the upper header through the upper NG manifold 5 to the gas line. (Not shown).

The outer surface of each of the heat transfer tube 3a and the lower header 2 has an Al content of 0.3 to 5% by mass, preferably 2 to 4% by mass, and 0.3 to 3% by mass of Mn. The -Mn-Mg alloy film is formed to a film thickness in the range of 100 to 1000 µm by thermal spraying. In order to improve the adhesion of the Al-Mn-Mg alloy film to the heat transfer tube 3a and the lower header 2, that is, the Al alloy base material by the thermal spraying process, a fine blasting agent is used as a pretreatment for the film formation by the thermal spraying process. The outer surface of the Al alloy base material is subjected to blast roughening to adjust the unevenness at the interface between the thermal spray coating and the Al alloy base material. The unevenness of the interface can be given by machining instead of blasting. The heat transfer tube 3a is not necessarily coated with the Al alloy film on the entire surface of the heat transfer tube 3a, but may be at least about 1 m below the panel 3. In addition, after forming the sprayed film, it is desirable to perform a sealing process that is excellent in the permeability to the Al—Mn— Mg alloy film, for example, applying a polymer epoxy resin to the surface of the sprayed film at least once. In addition, before or after this sealing treatment, it is more desirable to perform machining such as grinder grinding or shot peening in order to remove pore defects existing in the surface layer of the sprayed coating.

  In order to simulate the environment near the panel 3 of the LNG vaporizer (ORV) and the lower header 2 (see FIG. 1), first, a disk made of aluminum alloy A5083 having a diameter of 16 mm and a thickness of 4 mm was prepared. A thermal spray coating of various compositions shown in Table 1 was formed on the surface of one region at a thickness of 300 μm with the straight line passing through the boundary as a test material without any particular treatment after the thermal spraying. And the thermal spray coating which cooled the disk back surface of the said test material to 20 degreeC below freezing by making a Peltier device closely_contact | adhere to the other area | region back surface which has not performed thermal spraying processing of this test material is formed into a film. The surface of the other region was exposed to 30 ° C. commercial artificial seawater (“Marine Art High” manufactured by Tomita Pharmaceutical Co., Ltd.) at a flow rate of 1 m / s for 20 hours at a temperature of 20 ° C. below the freezing point. The amount of indentation in the disk substrate and the amount of indentation in the sprayed coating were measured with a surface roughness meter. The measurement results are shown in Table 1.

From Table 1, in the case of the conventional Al-Zn-based thermal spray coating (NO.1, NO.2), the amount of indentation of the thermal spray coating is as small as 1-2 μm, while the amount of indentation of the disk substrate is as large as about 8 μm, It can be seen that the sacrificial anticorrosive effect of the thermal spray coating is not sufficiently exhibited in the seawater exposure environment. In addition, in the test piece of NO.4 where the alloying elements of Mn and Mg are outside the scope of the present invention, the amount of indentation of the spray coating is slightly less than the amount of indentation of the Al alloy base material (circular substrate), and sacrificial corrosion protection of the spray coating It cannot be said that the effect is demonstrated. On the other hand, in the thermal spray coating having the alloy composition of the present invention, the indentation amount of the thermal spray coating is about 5 to 10 μm, which is larger than the indentation amount (4.5 μm or less) of the Al alloy base material, and has a sacrificial anticorrosive effect. Since it is expressed and the amount of dents in the sprayed coating is relatively small, it can be seen that the durability of the sprayed coating is maintained well . In addition, specimen material No. 17 is an alloy element of Mn , Mg is outside the scope of the present invention, and apparently sacrificial anticorrosive effect is observed, but as described above, the alloy element content increases. There are problems such as segregation in the thermal spray coating and excessive corrosion rate, which is not suitable for practical use.

It is a perspective view of an LNG vaporizer.

DESCRIPTION OF SYMBOLS 1 ... LNG manifold 2 ... Lower header 3 ... Panel 3a ... Heat transfer pipe 4 ... Upper header 5 ... NG manifold 6 ... Seawater header 7 ... Sprinkling nozzle 8 ...・ Trough U ・ ・ ・ Panel unit

Claims (5)

LNG vaporizer heat transfer tube made of Al alloy in which sacrificial anticorrosive coating is formed on the outer surface, in which LNG circulates and seawater is supplied to the outer surface, and the seawater and the LNG exchange heat to vaporize LNG a is, the sacrificial corrosion prevention coating, the Mn 0.3 to 3.0 wt%, the Mg containing 0.3 to 5.0 wt%, Ri Do the balance of Al and unavoidable impurities, wherein the heat transfer tube A heat transfer tube for an LNG vaporizer, characterized in that the electrode potential is lower than that of the Al alloy . The heat transfer tube for an LNG vaporizer according to claim 1, wherein the sacrificial anticorrosive coating is a thermal spray coating formed by thermal spraying. A panel in which a plurality of heat transfer tubes formed with the sacrificial anticorrosive coating according to claim 1 are arranged in a curtain shape, an upper header for discharging vaporized gas connected to an upper part and a lower part of the panel, and a lower header for supplying LNG LNG is vaporized by heat exchange between seawater flown down from the top of the panel unit along the surface of the panel and LNG flowing through the heat transfer tube from the lower header side to the upper header side. An NG vaporizer designed to be used. The LNG vaporizer according to claim 3, wherein the sacrificial anticorrosive coating is a thermal spray coating formed by thermal spraying. The LNG vaporizer according to claim 4, wherein the sacrificial anticorrosive film is formed on a portion of the heat transfer tube located at a lower portion of the panel.

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