TECHNICAL FIELD

The present disclosure relates to use of the alloy of UNS S31266 in at least one tube of an LNG vaporiser, and to an LNG vaporiser comprising a shell, at least one tube, and a tube sheet.

BACKGROUND

Combustible gases such as natural gas, bio gas, etc. may be liquefied in order to be transported from a source of gas, e.g. to overseas markets by means of liquefied gas tankers.

The term Liquefied Natural Gas, LNG, relates to such liquefied combustible gases. In order to utilise the gas, the LNG has to be vaporised. An LNG vaporiser comprises a heat exchanger, wherein the LNG is heated from its cryogenic storing temperature to the boiling point of the LNG. The LNG vaporiser may for instance comprise a shell and tube heat exchanger, wherein one of the LNG and a heating fluid, such as seawater, flows through the tube/s, and the other of the LNG and the heating fluid flows through the shell.

The alloys UNS S31254 and UNS N08367 have been used in tubes, and the alloy UNS N08367 has been in tube sheets, in LNG vaporisers. In use with seawater as a heating fluid, this combination of materials, although relatively corrosion resistant on their own, has caused crevice corrosion in the joints between the tubes and the tube sheets.

SUMMARY

It would be advantageous to achieve a Liquefied Natural Gas (LNG) vaporiser overcoming, or at least alleviating, the above mentioned drawback. In particular, it would be desirable to eliminate, or at least reduce crevice corrosion in an LNG vaporiser. To address these concerns, use of a material in an LNG vaporiser, and an LNG vaporiser, having the features defined in the independent claims are provided.

According to an aspect of the present disclosure, there is suggested a use of the alloy which has a composition within UNS S31266 in at least one tube of an LNG vaporiser, the LNG vaporiser comprising a shell and arranged therein the at least one tube extending at least partially through a tube sheet. By using the austenitic stainless steel alloy according to the standard UNS S31266 in the tube of the LNG vaporiser, crevice corrosion affecting the at least one tube is reduced, or even eliminated. As a result, the above mentioned drawback is at least alleviated. It has been proposed to weld the tubes of a prior art LNG vaporiser to both sides of the tube sheet in order to eliminate crevices between the tubes and tube sheets, and thus to eliminate crevice corrosion. It is problematic to achieve high quality weld joints on both sides of the tube sheet and this may cause fabrication defects, and since the tubes are subjected to the low temperature of the LNG, the weld joints are subjected to stress. Both of these concerns may again lead to problems with corrosion. It has been realised by the inventor that using the austenitic stainless steel material according to the standard UNS S31266 in the tubes of an LNG vaporiser essentially prevents crevice corrosion from affecting the tubes of an LNG vaporiser. Accordingly, crevices around the tubes need not be seal with welds from both sides of the tube sheet.

According to a further aspect of the present disclosure, there is provided an LNG vaporiser 2 comprising a shell, at least one tube, and a tube sheet. The at least one tube and the tube sheet are arranged inside the shell, and the at least one tube extends at least partially through the tube sheet. The at least one tube is made from an alloy according to the standard UNS S31266.

In line with the discussion above, the austenitic stainless steel material according to the standard UNS S31266 in the at least one tube of the LNG vaporiser reduces, or even eliminates crevice corrosion affecting the at least one tube.

Further features of, and advantages with, the invention will become apparent when studying the appended claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and/or embodiments of the invention, including its particular features and advantages, will be readily understood from the example embodiments discussed in the following detailed description and the accompanying drawing, in which:

Fig. 1 illustrates a cross section through an LNG vaporiser according to embodiments, and Fig. 2 illustrates a cross section through a tube and a tube sheet of an LNG vaporiser according to embodiments. DETAILED DESCRIPTION

Aspects and/or embodiments of the present disclosure will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity.

Fig. 1 illustrates a cross section through an LNG vaporiser 2 according to embodiments. An LNG vaporiser may also be referred to as a regasification unit. The LNG vaporiser 2 comprises a shell 4, at least one tube 6, and two tube sheets 8, one at each end portion of the shell 4. The at least one tube 6 and the tube sheet 8 are arranged inside the shell 4. The at least one tube 6 is made from the austenitic stainless steel material UNS S31266.

Herein the term UNS S31266 refers to an austenitic alloy having the following chemical composition in weight %:

Cr 23.0 - 25.0;

Ni 21.0 - 24.0;

Mo 5.20 - 6.20;

C <0.030;

Si < 1 .00;

Mn 2.00 - 4.00;

P < 0.040;

S < 0.020;

Cu 1 .00 - 2.50;

W 1 .50 - 2.50;

N 0.35 - 0.55;

balance Fe and unavoidable impurities.

According to some embodiments, the at least one tube 6 may be a seamless tube. In this manner a durable tube may be provided in the LNG vaporiser 2. According to one embodiment, the at least one tube 6 may be a welded tube.

The at least one tube 6 and the tube sheet 8 are arranged inside the shell 4. The LNG vaporiser 2 comprises an inlet header 13 and an outlet header 15. The at least one tube 6 is fluidly connected to the inlet and outlet headers 13, 15. The LNG vaporiser 2 comprises a gas inlet 10 fluidly connected to the inlet header 13 and a gas outlet 12 fluidly connected to the outlet header 15. The LNG vaporiser 2 comprises a heating fluid inlet 14 fluidly connected to a heating fluid space 17 inside the shell 4 and a heating fluid outlet 16 fluidly connected to the heating fluid space 17. The tube sheets 8 delimit the heating fluid space 17 within the shell 4 from an inlet header 13 of the LNG and an outlet header 15 of vaporised LNG.

In these embodiments, the LNG vaporiser 2 comprises a number of tubes 6, wherein each of the number of tubes 6 extends at least partially through the tube sheet 8, and wherein each of the number of tubes 6 is made from UNS S31266. According to some embodiments, each of the tubes 6 of the number of tubes 6 is a seamless tube. In this manner, durable tubes 6 may be provided in the LNG vaporiser 2.

The LNG vaporiser 2 forms a shell and tube heat exchanger and during operation of the LNG vaporiser 2, LNG is provided to the gas inlet 10 and conducted through the inlet header 13 and the at least one tube 6 to the outlet header 15 and the gas outlet 12. A heating fluid, such as seawater, is conducted through the heating fluid space of the shell 4, from the heating fluid inlet 14 to the heating fluid outlet 16. As the LNG and the heating fluid pass through the LNG vaporiser 2, heat is transferred from the heating fluid in the heating fluid space 17 to the LNG in the at least one tube 6, vaporising the LNG. Accordingly, vaporised LNG flows of out of the gas outlet 12. In use of the LNG vaporiser 2, the LNG vaporiser 2 may be positioned at an angle such that LNG in liquid form is held in the inlet header 13 and partially in the at least one tube 6, and only vaporised LNG in the at least one tube 6 reaches the outlet header 15 and the gas outlet 12. However, a vertical or horizontal orientation of the LNG vaporiser 2 may alternatively be used.

Due to the use of the austenitic stainless steel material UNS S31266 in the tubes 6, seawater may be used as a heating fluid in the LNG vaporiser 2 without crevice corrosion affecting the tubes 6, or at least with crevice corrosion affecting the tubes 6 only to a limited extent. The term seawater relates to water from an ocean, which may have a salinity of e.g. 3.5 %.

According to alternative embodiments, the heating fluid may flow through the tubes 6 of the LNG vaporiser 2, and the LNG may flow through the shell 4.

Many different kinds of shell and tube heat exchangers may be utilised as LNG vaporisers. In Fig. 1 one of many different kinds of LNG vaporisers encompassed by the appended claims is shown. Mentioned purely as examples, the LNG vaporiser may comprise at least one tube extending in the shape of a helical coil inside the shell, the LNG vaporiser may comprise at least one U-shaped tube and only one tube sheet, the inlet and outlet headers being fluidly connected to respective ends of the U-shaped tube. Flow directing baffles may be arranged inside the shell in order to direct the flow of heating fluid along a particular path, e.g. an undulating path, from the heating fluid inlet 14 to the heating fluid outlet 16.

Fig. 2 illustrates a cross section through an end portion of a tube 6 and a portion of a tube sheet 8 of an LNG vaporiser according to embodiments. The LNG vaporiser comprises a large number of such tubes 6 arranged at the tube sheet 8, of which only one tube 6 is shown.

The tube 6 extends at least partially through the tube sheet 8. In these embodiments, the tube 6 extends through the entire tube sheet 8. The tube sheet 8 is provided with a through hole 18 and the end portion of the tube 6 extends in, and through, the through hole 18. The tube sheet 8 is provided with the through hole 18, wherein the at least one tube 6 is fastened to the tube sheet 8 via an expanded portion 20 of the at least one tube 6 engaging with the tube sheet 8. In these embodiments, a recess 22 is provided in the tube sheet 8 in connection with the through hole 18. The expanded portion 20 engages with the recess 22. Suitably, the recess 22 extends circumferentially along a perimeter of the through hole 18 and the expanded portion 20 forms a ring-shaped bulge around the tube 6. Thus, a fluid tight seal between the tube 6 and the tube sheet 8 may be provided. The expanded portion 20 may be produced by applying pneumatic or hydraulic pressure to an inside of the end portion of the tube 6 within a region of the through hole 18, or by roller expansion. Since the at least one tube 6 is made from the austenitic stainless steel material

UNS S31266 and according to some embodiments also the tube sheet 8, see below, crevice corrosion may be avoided or at least reduced to a significant extent in the crevice or crevices formed in the through hole 18, between the at least one tube 6 and the tube sheet 8. The through hole 18 may be drilled and reamed in the tube sheet 8, and the tube sheet 8 may be machined to form the ring-shaped recess 22. In the illustrated embodiments, two ring shaped recesses are shown. Additionally, or alternatively, the end portion of the tube 6 may be provided with an expanded portion adjacent to the tube sheet 8 on one or both sides of the through hole 18. The use of at least one expanded portion 20 of the at least one tube 6 to engage the tube sheet 8 of the LNG vaporiser 2 may eliminate the need for welding the at least one tube 6 to the tube sheet 8. According to some embodiments, the at least one tube 6 may be welded to the tube sheet 8 on one side of the tube sheet 8, suitably on the header side of the tube sheet 8. In such embodiments, a circumferential weld 24 may extend around the tube 6. The weld 24 may seal the tube 6 against the tube sheet 8. Providing a weld at only one side of the tube sheet 8 permits thermal contraction and expansion of the tube 6 in the region of the tube sheet 8 thus, avoiding stress in the weld 24.

The tube 6 will not be affected by crevice corrosion, at least not to any significant extent, if heating fluid should penetrate into the crevice between the tube 6 and the tube sheet 8. Namely, as mentioned above, the tube 6 is made from the austenitic stainless steel material UNS S31266. The use of an alloy according to the standard UNS S31266 in the at least one tube 6 of the LNG vaporiser 2 eliminates, or at least considerably reduces, the risk of crevice corrosion affecting the at least one tube 6.

Mentioned purely as an example, the tubes may have a diameter of 20 mm with a wall thickness of 1.5 mm. The tube sheet may have a thickness of 125 mm. Lengths and numbers of tubes as well as the diameter of the shell is selected depending on the required LNG vaporising capacity of the relevant LNG vaporiser. For instance, the diameter of the shell may be within a range of 1 - 3 m and the length of the shell may be within a range of 3 - 15 m.

According to some embodiments, the tube sheet 8 may be made from the alloy according to the standard UNS S31266. In such embodiments, the use of an alloy according to UNS S31266 in the tube sheet 8 of the LNG vaporiser 2 also eliminates, or at least considerably reduces, the risk of crevice corrosion affecting the tube sheet 8. Thus, an LNG vaporiser for use with seawater as a heating fluid, having low, or even no, risk of crevice corrosion between the tubes 6 and the tube sheet 8 may be provided without any additional sealing between the tubes 6 and the tube sheet 8.

According to some embodiments, the tube sheet 8 may be made from an alloy according to UNS N06022, a nickel alloy. Since UNS N06022 has high corrosion resistance, comparable to that of UNS S31266, although at a higher price, in such embodiments, the risk of crevice corrosion affecting the tube sheet 8 may also be eliminated, or at least considerably reduced. Thus, an LNG vaporiser for use with seawater as a heating fluid, having low, or even no, risk of crevice corrosion between the tube/s 6 and the tube sheet 8 may be provided without any additional sealing between the tube/s 6 and the tube sheet 8.

Herein the term UNS N06022 refers to a material having the following chemical composition in weight %:

C < 0.010;

Cr 22 - 22.5;

Co < 2.5;

Fe 2.0 - 6-0;

Mn < 0.50;

Mo 12.5 - 14.5;

P < 0.02;

Si < 0.08;

S < 0.02;

W 2.5 - 3.5;

V < 0.35;

balance Ni and unavoidable impurities. According to some embodiments, the tube sheet 8 may be made from an alloy according to UNS N10276, a different nickel alloy having somewhat lower corrosion resistance than UNS N06022. However, for some applications of LNG vaporisers, e.g. depending on the seawater used as a heating fluid, the corrosion resistance may be sufficient to at least reduce the risk of crevice corrosion affecting the tube sheet 8. Thus, an LNG vaporiser having low risk of crevice corrosion between the tube/s 6 and the tube sheet 8 may be provided without any additional sealing between the tube/s 6 and the tube sheet 8.

Herein the term UNS N 10276 refers to a material having the following chemical composition in weight %:

C < 0.010;

Si < 0.08;

Fe 4.0 - 7.0;

Cr 14.5 - 16.5;

Mo 15.0 - 17.0;

Mn < 1 .0;

P < 0.040;

S < 0.030; Co < 2.5;

V < 0.35;

balance Ni and unavoidable impurities. The invention also relates to the use of UNS S31266 in an LNG vaporiser as discussed above with reference to Figs. 1 and 2.

According to an aspect, there is suggested a use of UNS S31266 in at least one tube 6 of an LNG vaporiser 2, the LNG vaporiser 2 comprising a shell 4 and arranged therein the at least one tube 6 extending at least partially through a tube sheet 8.

According to embodiments, the use may encompass use of UNS S31266 in the tube sheet 8.

According to embodiments of the use, seawater is used as a heating fluid for vaporising LNG.

It is to be understood that the foregoing is illustrative of various example embodiments and that the invention is defined only by the appended claims. A person skilled in the art will realize that the example embodiments may be modified, and that different features of the example embodiments may be combined to create embodiments other than those described herein, without departing from the scope of the invention, as defined by the appended claims.