Method of determining diametrical growth of reformer tubes

Abstract

The present invention relates to modifying a reformer tube to facilitate regular tube inspections to assess the service life and subsequently predict its remaining life accurately.

Claims

1. A method for determining diametrical growth in a reformer tube comprising: providing a reformer tube having at least one machined, circumferential reference marking or groove on the inside of the reformer tube located in a region of low reformer tube creep growth; measuring a first set of inside diameters at a time t.sub.1 along a predefined length of the reformer tube that contains the circumferential reference marking(s) as well as at least a portion of the tube that while in operation gets heated to a temperature greater than 1300 F.; measuring a second set of inside diameters along the predefined length of the reformer tube after operating the reformer tube for an additional period of time, t, where t.sub.2=t.sub.1+t; aligning the second set of diameter measurements using the circumferential reference marking and a second available reference point to ensure that the axial locations of the entire length for the measurements from time t.sub.1 are the same as the axial locations of the entire length of the measurements from time t.sub.2, and calculating creep strain along the length of the tube by dividing the difference in diameter measurements at times t.sub.1 and t.sub.2, by the diameter measurement at time t.sub.1, for each location along the length of the tube, thereby determining diametrical growth in a reformer tube.

2. A method for determining diametrical growth in a reformer tube comprising: providing a reformer tube having a circumferential reference marking on the outside of the reformer tube achieved through welding at least one metallic ring on the outer surface of the reformer tube; measuring a first set of outside diameters at a time t.sub.1 along a predefined length of the reformer tube that contains the circumferential reference marking(s) as well as at least a portion of the tube that while in operation gets heated to a temperature greater than 1300 F.; measuring a second set of outside diameters along the predefined length of the reformer tube after operating the reformer tube for an additional period of time, t, where t.sub.2=t.sub.1+t; aligning the second set of diameter measurements using the circumferential reference marking and a second available reference point to ensure that the axial locations of the entire length for the measurements from time t.sub.1 are the same as the axial locations of the entire length of the measurements from time t.sub.2; and calculating creep strain along the length of the tube by dividing the difference in diameter measurements at times t.sub.1 and t.sub.2, by the diameter measurement at time t.sub.1, for each location along the length of the tube, thereby determining diametrical growth in a reformer tube.

3. A reformer tube according to claim 1.

4. The reformer tube according to claim 3, wherein the marking of the depth of the circumferential reference marking(s) is larger than the detection limit of the diametrical inspection device.

5. The marking of claim 1, wherein the circumferential reference marking has a predetermined depth necessary to retain the design minimum sound wall thickness.

6. The method of claim 1, wherein a diametrical growth measurement is performed through a technique selected from either laser profilometry or stylus based measurements.

7. The method of claim 1, wherein the diameter growth is determined by automated computational algorithms.

8. The method of claim 1, wherein the diametrical growth in a reformer tube is measured though the use of a second available reference point, wherein said second reference point is a weld mark.

9. The method of claim 2, wherein a diametrical growth measurement is performed through a technique selected from either laser profilometry or stylus based measurements.

10. The method of claim 2, wherein the diametrical growth in a reformer tube is measured though the use of a second available reference point, wherein said second reference point is a weld mark.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The objects and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figures wherein like numbers denote same features throughout, wherein:

(2) FIG. 1 is a depiction of the current practice where two sets of Laser profilometry data collected during inspections, separated by time period t=t.sub.2-t.sub.1.

(3) FIG. 2 is a depiction of a set of inspection data where one of the weld points cannot be clearly resolved by the inspection technology due to manufacturing differences and high creep.

(4) FIG. 3 is a simplified schematic of a reformer tube assembly constituted by 3 alloy segments, a conical reducer and an outlet pigtail wherein a circumferential marking has been provided on the inside surface of the reformer tube;

(5) FIG. 4 is a simplified schematic of a reformer tube assembly constituted by 3 alloy segments, a conical reducer and an outlet pigtail wherein a circumferential marking has been provided on the outside of the reformer tube;

(6) FIG. 5 is a depiction of the inspection data with the proposed reference mark in a region of low creep.

DETAILED DESCRIPTION OF THE INVENTION

(7) For illustrative purposes the problem that the present invention addresses will be described with reference to a reactor comprising a tube or plurality of tubes made by centrifugal castings welded together to the desired length.

(8) The present invention provides a reformer tube having a circumferential reference marking to facilitate regular tube inspections to assess the service life and subsequently predict remaining service life accurately. This facilitation pertains to alignment of the current inspection data with one or more of previous inspection data. The present invention proposes circumferentially marking a specific region along the length of the tube that is relatively less susceptible to thermal stresses and serves either as an initial starting point or a generic reference point for aligning inspection data at a later time.

(9) The modification consists of circumferential markings on either the inner or outer surface of the reformer tube or both. The marking can either appear as a groove due to removal of metal (invasive) on either the inner surface or as a circumferential metallic ring (non-invasive) on the outer surface. The markings can be made either during the process of manufacturing the tubes, or after the manufacturing process before the assemblies are put into service. The inner and/or outer markings should be made in a region of low creep growth that is where the operating wall temperatures of the reformer tubes are below 1300 F. Inner marking can be made near the top of the tube such that after installing the tube in the furnace, the inner marking will be above the furnace ceiling. Outer marking must be made such that after installing the reformer tube in the furnace, the marking is below the ceiling of the furnace, since the crawler cannot move beyond the ceiling of the furnace box. The marking should not violate the minimum required soundness of the tube wall. The dimensions of any marking should be resolved by the inspection methodology such as laser profilometry. Alternatively, diametrical growth measurement is performed through a stylus-based measurement technique.

(10) The inner wall of each of the newly cast tubes are rough. They are subsequently machined to remove surface roughness, and accumulated impurities on the inner surfaces during the manufacturing process. Machining is performed using a lathe in a pull boring process. In the pull boring process, a bar is inserted inside the tube all the way through its length. After emerging at the opposite end, the bar is fit with a single-point or multi-point pull boring head. The bar is engaged in the tool drive mechanism and drawn back through the inner surface of the reactor tube. The tooling head holds a carbide insert of the appropriate grade and style suitable for the tube material. This insert functions to enlarge the inner diameter slightly as it is pulled into the tube. Coolant pumped around the boring bar channels flows up and out through the hollow center to aid metal removal and protect the surface. The inner diameter marking can also be made using the pull boring process. The outside diameter markings can be achieved by welding a ring made of a similar Ni based alloy on the external surface of the reformer tube.

(11) FIG. 3 is a simplified schematic of a reformer tube 100 inside a simplified furnace box 200. The reformer tube is made of several alloy segments 10, 20 and 30 welded at points 54-1 and 54-2. The top of the tube is closed by a flange 70. The process gas enters the tube through an inlet pigtail 60. The bottom alloy segment 30 is welded at 52 to a conical reducer 40 which is subsequently welded to an outlet pigtail 45. Catalyst support plate 50 supports the catalyst inside the tube. The internal surface of the tube is accessed by removing flange 70 and diameter measurements are recorded by lowering a crawler with a laser probe and receiver down to the catalyst support plate 50.

(12) In one exemplary embodiment of the invention, a circumferential reference marking is provided on the inner surface of the top alloy segment 10. Several details of the reference marking are shown as an inset in FIG. 3. The circumferential reference is made at a predetermined distance XID from flange 70. The distance XID can be such that the circumferential reference maybe located either outside of the furnace box 200 or inside it, depending on the marking desired, as further discussed herein. The circumferential reference marking is a machined groove made around the entire inner circumference of the reformer tube. The groove has a depth t.sub.g (60-3) and length l.sub.g (60-4). The dimensions of the groove should be such that it is resolvable by the inspection device such as a laser profilometry probe. The length of the groove l.sub.g must be greater than the pitch of the crawler. In all cases the depth t.sub.g (60-3) must be small enough such that it does not compromise the service life of the reformer tube. Thus, for a reformer tube with nominal thickness denoted by t.sub.n (60-1), the thickness of the residual wall t.sub.R (60-2) where t.sub.R=t.sub.nt.sub.g, must always be greater than the minimum sound wall thickness (MSW). This is the minimum allowable thickness of the reformer tube to meet the design service life at the design operating conditions. Thus t.sub.RMSW.

(13) FIG. 4 depicts another exemplary embodiment of the invention where a circumferential reference is provided on the outer surface of the top alloy segment 10. Several details of the reference marking are shown as an inset. The circumferential reference is made at a predetermined distance XOD from the top flange 70. The distance XOD must be such that the circumferential reference is located inside of the furnace box 200 such that it is accessible by the inspection device. The thickness of the protrusion is denoted by t.sub.p (60-7) and the length of the protrusion is ted by l.sub.p (60-6). The length of the protrusion l.sub.p must be greater than the pitch of the crawler. The outer diameter measurements are performed by attaching a crawler on the outer surface of the reformer tubes from the weld point 52. The tubes are accessed from inside of the furnace box 200. The crawler moves up the length of the tube to the roof of the furnace box 200.

(14) As an example of a circumferential reference mark being made on the inner surface of the reformer tube, and with reference to FIG. 3, the minimum sound wall thickness (MSW) for a reformer tube is specified to be 0.465 in. During manufacturing, the sound wall usually provided by the vendor is at least 15% higher than the minimum sound wall thickness specified. Thus, the nominal thickness of the wall t.sub.n (60-1) is about 0.535 in. In such a case the circumferential reference mark is made on the inside of the reformer tube at distance XID (about 2 ft. from flange 70) by prolonging the pull-boring process such that the depth reference mark t.sub.g (60-3) is 0.05 in. The thickness of the residual wall t.sub.R (60-2) is, therefore, 0.485 in. which is greater than the specified minimum sound wall thickness. The length of the groove l.sub.g (60-4) should be greater than the pitch of the inspection device which is defined herein as the minimum distance along the length of the tube, the internal diameter inspection device can traverse to gather information.

(15) As another example, and with reference to FIG. 4 for the reference mark being made on the outer surface of the reformer tube at a distance XOD (about 3 ft. from flange 70), a circular ring of thickness t.sub.p (60-7) is welded on the surface. As an example the thickness t.sub.p= in. and the length of the protrusion l.sub.p (60-6) should be greater than the pitch of the inspection device on the outside.

(16) The distinct circumferential reference marking proposed in the current invention serves the purpose of providing a distinct mark that would be used to process inspection data using automated analytic algorithms. While practicing the proposed invention, and as depicted in FIG. 5, additional peaks Z1 and Z2 would be observed corresponding to circumferential reference marking in the inspection traces for times t.sub.1 and t.sub.2 respectively. The algorithm described above is used to align the inspection data such that Z1 and Z2 correspond to the same point along the length of the reformer tube. Based on the linear transformation algorithm, the data points in the two sets of inspection data at times t.sub.1 and t.sub.2 are aligned. Based on the data and the design of the tube assembly, more than one set of linear transformations maybe necessary. The process is much more reliable compared to the current best practice for two reasons. First, the markings are provided solely for aligning the inspection data. Thus, it can aid in calibrating the inspection device. Second, the markings are made in a region of low creep growth near the top flange of the tube assembly where the skin temperature of the tube does not exceed 1300 F. Additional reference marks may be made for the purpose of validating the linear transformation.

(17) This procedure would ensure the use of proper diameter measurements from the inspection at t.sub.1 to be compared to the diameter measurements at time t.sub.2. The presence of the proposed circumferential reference marking in both the inspection data sets separated in time facilitates the identification of a clear reference mark not exposed to high temperatures inducing creep, for comparing the two sets of inspections for calculating metrics such as strain and the corresponding strain rate at axial locations along the length of the tube.

(18) A proper remaining life estimate can have far reaching economic benefits. Installation of one reformer tube costs about $25,000 given the current cost of nickel. Additionally, installations are performed offline which entails shutdown of the entire reformer leading to losses from production. With the help of better estimates of remnant life, it is possible to schedule maintenance and harvest reformer tubes during scheduled turnaround times. This will also prevent shutdowns due to unprecedented tube failures arising out of incorrectly analyzed tubes leading to incorrect estimates of tube life. The current invention proposes a methodology of providing a reference marking prior to installation, to facilitate accurate diameter measurements along the length of reformer tubes to obtain accurate metrics to estimate remaining service lives at specific time intervals during operation.

(19) Although various embodiments have been shown and described, the present disclosure is not so limited and will be understood to include all such modifications and variations as would be apparent to one skilled in the art.