Claims:WE CLAIM
1. A waste heat recovery boiler (200) having arrangement of U-tubes designed for large temperature difference between inlet and outlet zones of a tubesheet, the waste heat recovery boiler (200) comprising:
a tubesheet (120) having a cylindrical shell (105) and a channel (130) fitted on either sides of a peripheral edge thereof forming closed volumes, the cylindrical shell (105) provided with a passage for boiler feed water there through and the channel (130) provided with a passage for gas;
a plurality of U-tubes (110) having two open ends fitted on holes of the tubesheet (120), each U-tube of the plurality of U-tubes (110) having: an upstream leg (110A) and a downstream leg (110B) joined with a 180° bend and defined with respect to a flow of gas there within; each upstream leg (110A) and the corresponding hole on the tubesheet having an inside surface near the gas inlet covered with a ferrule (160) optionally provided with a heat resistant layer (162) between the inside surface and the ferrule (160), wherein the open ends of the plurality of U-tubes (110) are fitted on the tubesheet (120) with at least one row of the upstream legs (110A) located adjacent to at least one row of the downstream legs (110B) in a repeating sequence;
a leg duct assembly (116) fitted within the channel (130) below the tubesheet (120), the leg duct assembly (116) having a plurality of duct segments (118) connected together with a first connecting means (114) to form upstream leg ducts (112A) and downstream leg ducts (112B) respectively covering the rows of holes corresponding to the upstream legs (110A) and the downstream legs (110B);
an inlet gas chamber (150) fitted within the channel (130) thereby forming an outer passage (136) within the channel (130), the inlet gas chamber (150) provided with a gas inlet (132) and connected below the leg duct assembly (116) with a third connecting means (155) for supplying hot gas thereto, wherein the inlet gas chamber (150) is formed by a plurality of segments (140) connected together with a second connecting means (144); and hot gas enters the upstream legs (110A) through the upstream leg ducts (112A);
a gas outlet (134) provided on the outer passage (136), wherein the cooled gas from the downstream legs (110B) enters the outer passage through the downstream leg ducts (112B).
2. The waste heat recovery boiler (200) as claimed in claim 1, wherein the heat resistant layer (162) covers a lower part (160y) of the ferrule (160).
3. The waste heat recovery boiler (200) as claimed in claim 1, wherein an upper part (160x) of the ferrule (160) inside the upstream leg (110A) extends beyond the shell side face of the tubesheet (120).
4. The waste heat recovery boiler (200) as claimed in claim 1, wherein the ferrule (160) is made of a nitriding resistant material.
5. The waste heat recovery boiler (200) as claimed in claim 1, wherein the heat resistant layer (162) is an insulation layer of a material selected from a refractory material, a ceramic material and a glass wool.
6. The waste heat recovery boiler (200) as claimed in claim 1, wherein the open ends of the plurality of U-tubes (110) are fitted on the tubesheet (120) with two rows of the upstream legs (110A) located adjacent to two rows of the downstream legs (110B) in a repeating sequence.
7. The waste heat recovery boiler (200) as claimed in claim 1, wherein an overlay (164) of corrosion resistant material is provided on the gas side surface of the tubesheet (120).
8. The waste heat recovery boiler (200) as claimed in claim 1, wherein the upstream leg ducts (110B) are made of a nitriding resistant material
9. The waste heat recovery boiler (200) as claimed in claim 1, wherein a cover (142) is removably fitted on the inlet gas chamber (150).
10. The waste heat recovery boiler (200) as claimed in claim 1, wherein the first connecting means (114), the second connecting means (144) and the third connecting means (155) are selected from a flange, a welding means, a brazing means, a bolting means, and a riveting means.
Dated this on 7th day of March, 2022

Ashwini Kelkar
(Agent for the applicant)
(IN/PA-2461)

, Description:WASTE HEAT RECOVERY BOILER
Field of the invention:
The present invention generally relates to a waste heat recovery boiler and more particularly it relates to a waste heat recovery boiler with U-tube arrangement designed to suit the large temperature difference between inlet and outlet zones of a tubesheet, such as in synloop boilers used in ammonia production plants.
Background of the invention:
In process plants, the process gas is heated to higher temperatures. This heat is recovered and thereby the gas is cooled by transferring heat to the boiler feed water and converting the water into steam. Equipment called waste heat recovery boiler is used for this application. In these boilers, the hot process gas is passed through multiple tubes of the boiler and is cooled by indirect heat transfer to boiling water which surrounds the tubes from shell side. Large temperature gradient of gas (of the order of 110 ~120°C) between inlet and outlet zones of the tubesheet makes it impractical to adopt conventional U-tube layout for such heat exchangers. Hence design with non-conventional tube layout which is commonly referred to as fountain type layout is used for such applications.
In the ammonia production process, typically a mixture of natural gas and steam undergoes reforming reaction in primary reformer to generate reformed gas, primarily consisting of hydrogen, carbon monoxide and carbon dioxide. The reformed gas undergoes further conversion in secondary reformer and shift converters to increase the hydrogen content. Air is also added in the secondary reformer as a source of nitrogen. After removal of carbon dioxide, ammonia synthesis is performed in a reactor at high pressure and elevated temperature, where nitrogen and hydrogen react in the presence of an appropriate catalyst. Such a reactor is called an ammonia converter, which typically operates at a pressure of 150 bar to 250 bar and at a temperature of 430 to 460°C. The conversion of nitrogen and hydrogen to ammonia in the ammonia converter is only to the extent of 20-24%. The synthesis gas exiting from the ammonia converter thus contains a mixture of unconverted hydrogen, nitrogen and ammonia. In order to separate the ammonia and recycle the unconverted nitrogen and hydrogen to the ammonia converter, the synthesis gas is cooled in a series of heat exchangers located downstream of the ammonia converter known as the synthesis loop exchangers. Typically, the first heat exchanger in the synthesis loop is a waste heat boiler, which is commonly known as Synloop boiler. Synthesis gas is passed through multiple tubes of the Synloop boiler and is cooled to around 300°C to 350°C by indirect heat transfer to boiling water which surrounds the tubes from shell side. Boiler feed water enters at pressure of around 40 to 130 bar and around 250 to 320°C and the steam-water mixture exits the boiler at around 250°C to 320°C.
Figure 1 shows tubesheet and tube arrangement in existing synthesis loop boiler (100) of shell and tube type which comprises of a plurality of U-tubes (10) whose open ends are welded to a tubesheet (20). The peripheral edge of the tubesheet (20) is welded to a cylindrical shell (not shown) on one side, thus forming a closed vessel with path for the passage of the shell side fluid. The peripheral edge of the tubesheet on the other side is welded to another cylindrical shell known as channel. The channel is provided with a flat cover or dished end to form an enclosed volume through which the synthesis gas enters and exits the boiler. Hot synthesis gas enters through an inlet (30) on the channel and flows through a short length of pipe which is connected at one end to the opening on the channel and at the other end to a gas inlet compartment (50), which is bolted to the central portion of tubesheet (20). The gas inlet compartment (50) covers 50% of the tube holes on the tubesheet (20). The synthesis gas after flowing through the hot leg (10A) passes through the U-bends and is further cooled in the other straight leg of the tubes, hereinafter referred to as the cold leg (10B). The tubes are arranged in such a manner that all the hot legs (10A) of tubes are located at the central region of tubesheet (20) and all the cold legs (10B) are located in the annular region surrounding the central region of tubesheet (20). This non-conventional tube layout is commonly referred to as fountain type layout. This layout is different than conventional U-tube layout in which all the hot legs (10A) are placed in one half disc region of the tubesheet (20) and all cold legs (10B) are in other half disc region.
As is well known to those skilled in the art, large temperature gradient of gas (of the order of 110 ~120°C) between inlet and outlet zones of the tubesheet makes it impractical to adopt conventional U-tube layout for such heat exchangers. This temperature gradient for conventional U-tube layout would induce very high thermal stresses across two half-disc portions of the tubesheet separated by a gap which is called as pass lane. High thermal stresses could lead to failure of tubesheet and leaks at the tube-to-tubesheet joints limiting application of design with conventional U-tube layout for these heat exchangers. The fountain type tube layout in prior art thus reduces the thermal stresses within allowable limits and hence such design is generally used for these heat exchangers.
Accordingly, there exists a need to provide waste heat recovery boiler of improved design that will work with conventional U-tube layout by overcoming design related challenges in the waste heat boilers of prior art.
UK patent application GB 2089951 teaches a heat exchanger for the generation of steam in an ammonia synthesis plant comprising conventional U-tube layout. In this, the two ends of U tubes are connected to the tubesheet and so disposed one beside other in the tubesheet that only one gas entry end of the tube (hot leg) is always arranged each time beside a gas exit end of the tube (cold leg) in the resulting rows. While this prior art may provide a partial solution, it is limited as briefly described below.
The solution disclosed in UK patent application GB 2089951 helps to achieve uniform tubesheet temperature however results in to congested tube layout and complex channel internal arrangement. In particular, to access the channel side face of this heat exchanger for maintenance and tube plugging will need removal of multiple complex components inside the channel which is very difficult and time consuming. This results into longer downtime of plant during maintenance and loss of production.
Objects of the invention:
An object of the present invention is to provide a waste heat recovery boiler that will reduce the thermal stresses induced due to higher thermal gradients across two half-disc portions of a tubesheet.
Another object of the present invention is to provide a waste heat recovery boiler for ammonia production plants that works with conventional ‘U’ type tube layout.
Yet another object of the present invention is to provide a waste heat recovery boiler with lower tubesheet temperature, lower tubesheet thickness and weight.
Still another object of the present invention is to provide a waste heat recovery boiler with space for better blow-down arrangement for significantly reducing settling of solid deposits.
Still another object of the present invention is to provide a waste heat recovery boiler with a gas inlet and outlet arrangement for conventional ‘U’ type configuration which will enable easy access of channel side of the equipment and convenient maintenance of tubesheet from channel side.
Still another object of the present invention is to provide a waste heat recovery boiler having easy access to specific local region of channel side face of tubesheet to reduce plant downtime and loss of production during maintenance.
Yet another objective of this invention is to provide a waste heat recovery boiler with smaller Outer Tubular Limit (OTL) as a result of conventional ‘U’ type layout with reduced inner diameter and thickness of shell and channel.
Summary of the invention
The present invention discloses a waste heat recovery boiler such as synthesis loop boiler for ammonia production plants for cooling the hot gas with a boiler feed water. The waste heat recovery boiler comprises a tubesheet, a plurality of U-tubes, a leg duct assembly, an inlet gas chamber and an outer passage. The tubesheet is having a cylindrical shell and a channel fitted on either sides of a peripheral edge thereof forming closed volumes. The cylindrical shell is provided with a passage for boiler feed water there through. The plurality of U-tubes is having two open ends fitted on the holes of the tubesheet. Each U-tube of the plurality of U-tubes is having: an upstream leg and a downstream leg defined with respect to a flow of hot gas there within and joined with a 180° bend. Inside surface of each upstream leg near the gas inlet and the inside surface of the corresponding hole in the tubesheet is covered with a ferrule and a heat resistant layer. The ferrule is made up of a suitable material including but not limited to Inconel material to avoid nitriding. A heat resistant layer is provided between the ferrule and inside of the leg surface and the inside surface of corresponding hole in the tubesheet. The open ends of the plurality of U-tubes are fitted on the tubesheet with at least one row of the upstream legs located adjacent to at least one rows of the downstream legs in a repeating sequence. In a preferred embodiment, two rows of the upstream legs are located adjacent to two rows of the downstream legs in a repeating sequence. The leg duct assembly is fitted within the channel. The leg duct assembly is fitted within the channel below the tubesheet. The leg duct assembly is formed with a plurality of duct segments connected together with a first connecting means to form upstream leg ducts and downstream leg ducts respectively covering the rows of holes corresponding to the upstream legs and the downstream legs. The inlet gas chamber is fitted within the channel thereby forming an outer passage within the channel. The inlet gas chamber is provided with a gas inlet and connected to the leg duct assembly with a third connecting means for supplying hot gas thereto wherein the inlet gas chamber is formed by a plurality of segments connected together with a second connecting means. A gas outlet is provided on the outer passage.
The hot gas received through the gas inlet enters the upstream legs through an inlet gas chamber and the upstream leg ducts of the leg duct assembly and cools down while passing through the plurality of U-tubes. The cooled gas exiting from the tubesheet through the downstream leg ducts passes through the outer passage and flows out through the gas outlet. With the unique arrangement of plurality of U-tubes, the portion of tubesheet close to the upstream legs (hot legs) gets cooled by adjacent portion close to the downstream legs (cold legs). This results into majority of the tubesheet section attaining much lower temperature than tubesheet in prior art. The waste heat recovery boiler of the present invention is preferably used in the ammonia production plants for cooling the hot synthesis gas exiting from the ammonia converter.
Brief description of the drawings:
The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein
Figure 1 shows a schematic view of tubesheet and tube arrangement in a waste heat recovery boiler in accordance with the prior art;
Figure 2 shows a schematic view of tubesheet and tube arrangement in the waste heat recovery boiler, in accordance with an embodiment of the present invention;
Figure 3 shows a schematic view of a tubesheet in the waste heat recovery boiler, in accordance with an embodiment of the present invention;
Figure 4 shows a schematic view of a leg duct assembly in the waste heat recovery boiler, in accordance with an embodiment of the present invention;
Figure 5 shows a schematic view of one of the ferrule arrangements in the waste heat recovery boiler, in accordance with the present invention; and
Figure 6 shows a schematic view of channel internals with unique gas inlet design in the waste heat recovery boiler, in accordance with the present invention.
Detailed description of the embodiments:
The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior art techniques and approaches are overcome by the present invention described in the present embodiments.
The present invention provides a waste heat recovery boiler that is preferably suited for ammonia production plants. The waste heat recovery boiler with improved design works with conventional U-tube layout by overcoming thermal stress related problems. Unique features of the present invention reduce the thermal stress induced due to higher thermal gradients across two half-disc portions of the tubesheet and also provides a number of other benefits.
The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description and in the table below.
Table:
Ref No: Component Ref No: Component
110 U tube 136 Outer passage
110A Upstream leg 134 Gas outlet
110B Downstream leg 140 Segment
112A Upstream leg duct 142 Chamber cover
112B Downstream leg duct 144 Second connecting means
114 First connecting means 150 Inlet gas chamber
116 Leg duct assembly 155 Third connecting means
118 Duct segments 160 Ferrule
120 Tubesheet 160x Ferrule upper part
122 Stud 160y Ferrule lower part
124 ‘C’ slots 162 Refractory paper
130 Channel 164 Tubesheet overlay
132 Gas inlet
Referring to the figures 2 to 6, a waste heat recovery boiler (200) with reduced thermal stress (herein after referred to as “boiler (200)” throughout the document) in accordance with the present invention is shown. The waste heat recovery boiler (200) comprises a plurality of U-tubes (110), a tubesheet (120), a leg duct assembly (116) and an inlet gas chamber (150).
The tubesheet (120) is having a cylindrical shell (105) and a channel (130) fitted on either sides of a peripheral edge thereof. The cylindrical shell (105) fitted on the tubesheet (120) forms a closed vessel that is provided with a passage (not shown) for boiler feed water there through. The peripheral edge of the tubesheet (120) on the other side is welded to the channel (130) forming a closed volume through which the gas enters and exits the boiler
The plurality of U-tubes (110) having two open ends is fitted on the tubesheet (120). Each U-tube of the plurality of U-tubes (110) is having an upstream leg (110A) and a downstream leg (110B) with respect to a flow of hot gas there within, joined with a 180° bend (not numbered). The open ends of the plurality of U-tubes are fitted on the tubesheet (120) with at least one row of the upstream legs (110A) located adjacent to at least one row of the downstream legs (110B) in a repeating sequence. The number of rows of the upstream legs (110A) located adjacent to the number of rows of the downstream legs (110B) can vary up to one fourth the number of total rows on tubesheet, depending upon the application. In a preferred embodiment, two rows of the upstream legs (110A) are located adjacent to two rows of the downstream legs (110B) in a repeating sequence, as shown in figure 3.
The inside surface of each upstream leg (110A) near the gas inlet and the inside surface of the corresponding hole of the tubesheet on which the upstream leg (110A) is fitted, is covered with a ferrule (160). A heat resistant layer (162) is provided between the ferrule (160) and the upstream leg inner surface. The heat resistant layer (162) is an insulation layer of a material selected from a refractory material, a ceramic material and a glass wool. In an embodiment, the heat resistant layer (162) covers a lower part (160y) of the ferrule (160), majority of which is inside the tubesheet, wherein the lower part (160y) is having a step provided on the outer diameter thereof and an upper part (160x) of the ferrule (160) is expanded inside the U-tube and extended beyond the shell side surface of the tubesheet (120). In an embodiment, the ferrule is made of any material selected from a corrosion resistant material and a nitriding resistant material including but not limited to Inconel material. In a preferred embodiment, the ferrule (160) is made of any material selected from high nickel alloy and stainless steel. The ferrule made up of nitriding resistant material is specifically used for the application of the boiler (200) in ammonia plant to avoid nitriding due to ammonia present in the Synthesis Gas. The heat resistant layer (162) reduces the heat transfer from hot gas flowing through the upstream legs (110A) to the tubesheet (120) thereby helping in reduction in heat transfer to the tubesheet (120) and reduction in the thermal gradient in tubesheet (120), while the upper part (160x) without any wrapping of refractory paper transfers the heat from hot gas inside the tube to the shell side fluid.
In an embodiment, the upper part (160x) and the lower part (160y) of the ferrule (160) are fabricated separately and joined together with suitable means. In another embodiment, the ferrule (160) is machined from a single piece. Depending upon the application, the ferrule (160) is wrapped with the heat resistant layer (162) along the complete length without any expansion, or the ferrule (160) is expanded throughout the length without the heat resistant layer (162). An overlay (164) or cladding layer is provided on the gas side surface of the tubesheet (120). In an embodiment, the overlay (164) is made of a suitable material to protect the tubesheet against corrosion and nitriding and also to facilitate fixing of ferrule (160) with the tubesheet (120).
The leg duct assembly (116) is fitted within the channel (130) below the tubesheet (120). The leg duct assembly (116) is formed by connecting a plurality of duct segments (118) together with a first connecting means (114). The duct segments are connected together to form upstream leg ducts (112A) and downstream leg ducts (112B) i.e. separate flow passages respectively for hot gas entering the upstream legs (110A) and cooled gas coming from the downstream legs (110B). Each upstream leg duct (112A) is fitted to cover the adjacent rows of holes in the tubesheet (120) corresponding to the adjacent upstream legs (110A) and each downstream leg duct (112B) is fitted to cover the adjacent rows of holes in the tubesheet (120) corresponding to the adjacent downstream legs (110B). In an embodiment, the duct segments (118) are made up of a suitable material including but not limited to Inconel material to avoid nitriding.
The inlet gas chamber (150) is fitted within the channel (130) and provided with a gas inlet (132). The inlet gas chamber (150) is connected below the leg duct assembly (116) with a third connecting means (155). The inlet gas chamber (150) is formed by a plurality of segments (140) connected together with a second connecting means (144). The inlet gas chamber (150) is having diameter less than that of the channel (130) thereby forming an outer passage (136) within the channel (130) through which the gas passes and exits through the gas outlet (134). A chamber cover (142) is removably fitted on the inlet gas chamber (150) to provide easy access to the internal region.
In an embodiment, the first connecting means (114), the second connecting means (144) and the third connecting means (155) are selected from a flange, a welding means, a brazing means, a bolting means, and a riveting means. In a preferred embodiment, the first connecting means (114), the second connecting means (144) and the third connecting means (155) are flanges of suitable dimensions. The connecting parts of the inlet gas chamber (150) and the leg duct assembly (116) are fastened together with bolted joints or any other suitable method.
The segmentation of the leg duct assembly (116) and the inlet gas chamber (150) facilitates insertion of internal components and also provides easy access of particular region of tubesheet (120) or other regions inside the channel (130) by convenient handling and removal of segments during maintenance.
The hot gas enters through the gas inlet (132) and flows into the upstream legs (110A) through the inlet gas chamber (150) and the leg duct assembly (116). The cooled gas exiting from the tubesheet (120) passes through the outer passage (136) and flows out through the gas outlet (134).
Referring to figure 2, the flow of hot gas is arranged in a unique manner which results in two rows of upstream legs (110A) being located adjacent to two rows of downstream legs (110B). Figure 2, 3, and 4 illustrate the embodiment of the invention with an example in which the plurality of U-tubes (110) are arranged in a total of eight rows. The inlet gas chamber (150) inside the channel (130) is constructed in a unique manner such that hot Gas enters the upstream legs of tubes in rows 110A-1, 110A-2, 110A-3 and 110A-4 through the upstream leg ducts (112A). The hot gas after getting cooled by heat exchange with the shell side fluid exits the tubes through the downstream legs (110B) in rows 110B-1, 110B-2, 110B-3 and 110B-4. The tubes in rows 110A-1, 110A-2, 110A-3 and 110A-4 are designated as hot legs, while tubes in rows 110B-1, 110B-2, 110B-3 and 110B-4 are designated as cold legs based on the relative temperature of gas in those tube legs. Thus, in the example, two rows of upstream legs/ hot legs (110A) of tubes are located adjacent to two rows of downstream legs/ cold legs (110B) i.e. hot legs in rows 110A-1, 110A-2 are located adjacent to cold legs in rows 110B-1, 110B-2. Similarly, hot legs in rows 110A-3, 110A-4 are located adjacent to cold legs in rows 110B-3 and 110B-4. Two rows of hot legs and two rows of cold legs are arranged adjacent to each other in this example. It is understood here that a configuration with any number of rows of hot legs/ upstream legs (110A) can be arranged alongside rows of cold legs/ downstream legs (110B) based on specific case.
With the unique arrangement of plurality of U-tubes (110), the portion of tubesheet (120) close to the hot rows gets cooled by adjacent portion close to cold rows. This results into majority of the tubesheet (120) section attaining much lower temperature than tubesheet (120) in prior art.
The boiler (200) is specifically for ammonia production plants for cooling the hot synthesis gas exiting from the ammonia converter. However, it is understood here that the boiler (200) can be suitably used in other applications related to waste heat recovery. In case of the boiler (200) in ammonia plant, the hot gas inside the tubesheet region beyond ferrule region cools down to typically below 380°C - the temperature range causing nitriding of tube, before it comes in direct contact with the U-tube. In this way, the ferrule (160) serves two purposes, i.e. acts as heat barrier while inside the tubesheet region and conducts the heat in the region beyond the shell side face of tubesheet (120). The unique arrangement of plurality of U-tubes (110) results into majority of the tubesheet (120) section attaining much lower temperature than tubesheet (120) in prior art. The unique arrangement of the leg duct assembly (116) and the gas inlet chamber (150) provides access of tubesheet (120) from channel side for ease of maintenance.
Advantages of the invention:
• Innovative design of boiler (200) along with unique arrangement of ‘U’ tubes (110) results into majority of the tubesheet section attaining much lower temperature, leading to reduction in mean metal temperature of tubesheet and reduction of the thickness and weight of tubesheet (120)
• The boiler (200) enables the use of conventional U-tube layout arrangement.
• The unique arrangement of channel internals including the leg duct assembly (116) and the gas inlet chamber (150) provides convenient access of tubesheet (120) from channel side for ease of maintenance.
• The ferrule arrangement ensures that the temperature of inlet gas is below threshold and thus eliminates the possibility of nitriding of tube.
• Change in tube layout from fountain type to conventional ‘U’ type reduces the outer tubular limit and inner diameter of the shell and the channel hence resulting in reduction of overall weight of the shell and the channel making the boiler (200) compact.
• The pass lane between inlet and outlet side of tube layout on the tubesheet (120) facilitate space for easy and effective blow down and thus significantly reduces the possibility of solid deposit on shell side face of tubesheet and related failures.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the scope of the claims of the present invention.