US3885530A

US3885530A - Shield tube supports

Info

Publication number: US3885530A
Application number: US485523A
Authority: US
Inventors: John A Kivlen; Jr James E Massey; Jr James H Martin
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1974-07-03
Filing date: 1974-07-03
Publication date: 1975-05-27
Anticipated expiration: 1992-05-27
Also published as: GB1510697A; SE414827B; JPS519075A; JPS5847634B2; SE7505030L; FR2277315A1; FR2277315B1; DE2524106A1; AU8074975A; BE829586A; DE2524106C2; CA1017638A

Abstract

An improved support is disclosed for furnace shield tubes, which are located between the radiant and convection sections of industrial furnaces, and which, since they are exposed to radiant heat, are subject to significant weakening. In the preferred embodiment, a hollow pipe serves as a beam support for the shield tubes, the support being extensively insulated to minimize radiant heat input and being cooled by air passing inside it as induced by natural draft available from the furnace. Air from outside the furnace is drawn into the hollow support, cools it and passes out into the furnace stack above the convection section. The support beam is cooled by air while the supports on which the shield tubes rest are cooled by conduction, either into the air cooled support or into the shield tubes. Contact of the supports with radiant energy is avoided.

Description

United States Patent [1 1 Kivlen et al.

[4 1 May 27, 1975 SHIELD TUBE SUPPORTS [75] Inventors: John A. Kivlen, Denville, N.J.;

James E. Massey, Jr., Baton Rouge; James H. Martin, Jr., Zachary, both of La.

[73] Assignee: Exxon Research and Engineering Company, Linden, NJ.

[22] Filed: July 3, 1974 [21] Appl. No.: 485,523

3,378,064 4/1968 Benkert 122/510 X Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-Harold N. Wells 57 ABSTRACT An improved support is disclosed for furnace shield tubes, which are located between the radiant and convection sections of industrial furnaces, and which, since they are exposed to radiant heat, are subject to significant weakening. In the preferred embodiment, a hollow pipe serves as a beam support for the shield tubes, the support being extensively insulated to minimize radiant heat input and being cooled by air passing inside it as induced by natural draft available from the furnace. Air from outside the furnace is drawn into the hollow support, cools it and passes out into the furnace stack above the convection section. The support beam iscooled by air while the supports on which the shield tubes rest are cooled by conduction, either into the air cooled support or into the shield tubes. Contact of the supports with radiant energy is avoided.

10 Claims, 5 Drawing Figures SHIELD TUBE SUPPORTS BACKGROUND OF THE INVENTION Operating temperatures of industrial furnaces have continually increased as attempts are made to reduce the cost of construction and the efficiency of performance. Where very high temperatures are employed, for example, in steam cracking furnaces used for produc tion of olefins, high temperatures have created significant problems with regard to the support of shield tubes. These tubes are located between the radiant section of the furnace which receives heat directly from the burner flames and the convection section which receives no heat directly from the flames, but only by convective heat transfer from hot gases passing around the tubes. The shield tubes, lying between these other two groups of tubes are exposed on one side to radiant energy from the radiant section. In modern furnaces the temperature of support beams for shield tubes in such furnaces may exceed 2000F. At such temperatures these beam supports are susceptible to creep failure. The radiant section tubes are much hotter and so are usually supported from outside the furnace so their supports are not subjected to high temperatures. The convection tube supports are not unduly hot, but the shield tube supports are. After a period of operation the beam may gradually sag to the point that a complete failure occurs, causing damage to the furnace and necessitating an expensive shutdown. One approach to preventing such creep problems is to use materials which retain high strength at the high temperatures which are experienced. This is too costly to be a practical solution. Another alternative is to shield the support beam from the radiation which causes them to sag and eventually fail. However, at the high temperatures currently being used, such shielded beams may have a relatively short life owing to gradual creep. What is needed, but not heretofore available, is a support for shield tubes in such high temperature furnaces that will greatly extend the life of the supports and thereby avoid unexpected and expensive failures.

It has been shown in the prior art that air can be used to cool tube supports. For example, U.S. Pat. Nos. 1,622,303; 2,270,863; 2,355,800; 2,355,892; 2,557,569; 2,348,181; and 3,212,480 disclose that this can be done, although it is also indicated by U.S. Pat. No. 3,212,480 that air cooling may not be sufficient and that steam addition may be required. Although these patents illustrate the attempts to cool radiant section tubes, which are exposed to very high temperature, the supports used for radiant tubes are structurally different from those used for the shield and convection section tubes, where a severe bending load on the supports is typical. The present invention applies the broad principle of air cooling to a specific structure which has been found to successfully support shield tubes at temperatures around 2200F., which has not been possible with prior art shielded tube supports.

SUMMARY OF THE INVENTION The solid beam which is typical of the prior art shield tube supports is replaced in the present invention with a hollow structure, usually a piece of commercially available high strength pipe, which serves as a beam support. Such pipe would not be sufficiently strong to resist the high temperatures unless it is cooled. Cooling is provided by opening one end of the hollow support beam to the atmosphere outside the furnace through which relatively cool air can enter. The air is induced into the hollow support beam by the pressure differential which is available between the atmosphere and the furnace stack. Accordingly, the outlet end of the support beam is connected to piping which is passed upwardly and into the furnace stack above the convection section. The furnace stack, being at a negative pressure, creates a natural draft which induces sufficient air to cool the support beam so that it may resist the high temperatures which exist in the furnace. In order to avoid substantial heating of the support, a light weight insulation is placed completely about the support beam structure.

The shield tubes. themselves are supported by T- shaped supports having the bottom of the T welded to the support beam and the cross bar of the T serving as the support for the tube. Insulation is placed around these T-shaped support pieces so that they receive a minimal amount of heat from radiation or from convection and they are cooled both by air passing through the support beam and by process fluid passing through the inside of theshield tubes. This combination of cooling and insulation prevents failures from high temperature creep. The second row of shield tubes, less exposed to radiant heat than the first row, are supported on Y- shaped supports which rest directly on the cross bar of the T-shaped supports for the first row of shielded tubes. These Y-shaped supports, being shielded by the air-cooled support, are less exposed to'radiant heat and mainly to convection heat. They are capable of supporting the second row of shield tubes without additional insulation.

With a novel support beam and the supports attached to it, the temperature of the structure is substantially below that which would otherwise occur were no cooling provided. This temperature is sufficiently low so that the furnace can operate for long periods without excessive creep and failure of the supports.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation view of a typical vertical furnace to which the present invention has been applied.

FIG. 2 is an enlarged cross section taken substantially along line 22 of FIG. 1.

FIG. 3 is a sectional view taken substantially along line 33 of FIG. 2.

FIG. 4 is a cut-away prospective view.

FIG. 5 illustrates a non-cooled shielded support beam of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an elevation view of a vertical furnace 10 such as is often used in refineries or chemical plants and in particular, used for steam cracking of hydrocarbons to produce olefins. Heat produced by burning of fuels occurs principally by large burners 12 at the floor of the furnace 10, although at times smaller burners spaced along the sidewalls may also be used. In either instance, a significant amount of heat is transferred in the radiant section 14 which may be defined generally as the area of the furnace in which the tubes are within the line of sight of the flames produced by the burners. The radiant section 14 is the hottest part of the furnace 10 and the tubes located there: receive the highest heat density. Also, the process temperatures are the highest in this area. It should be understood that furnace tubes are positioned vertically in the radiant section and support from outside it, but not being particularly pertinent to the present invention, they are not shown in FIG. I.

Hot gases leaving the radiant section 14 pass upwardly into the convection section 16 of the furnace where heat is transferred to the convection section tubes 18 by convective heat transfer in much the same fashion as in a shell and tube heat exchanger. After giving up the optimum amount of heat to incoming feeds or to other extraneous streams, the furnace gas is passed upwardly past the damper 20 and out furnace stack 22 above, which is not shown in this illustration.

In some types of furnaces the radiant section and convection section are clearly separated from one another and there are no tubes in the convection section bank which see a portion of the radiant heat. In the furnace configuration of FIG. 1, this is not the case. The first two rows of convection section tubes do see some radiant energy from the burners 12 in the radiant section 14. These tubes are called shield tubes 22 since they shield the remainder of the convection section 16 from the radiant energy from the radiant section 14 of the furnace 10. As will be appreciated, the lower two rows of tubes 22 receive substantially higher heat density than the remainder of the convection section 16 and, in a furnace which operates at very high temperatures, the shield tubes 22 and their supports will be subjected to very severe temperatures. By way of illustration, the process temperature of hydrocarbons leaving the shield tube section may be of the order of 1250F. If no heat is removed, temperatures in this region may reach about 2,200 to 2,250F. At these temperatures, most metallic materials have very little strength. As a result, support of the shield tubes becomes a major problem and a limiting factor in the life of the furnace. It should be understood that the support beam rests on the furnace structure and must carry a bending load different from that of radiant tube supports.

In order to avoid these excessively high temperatures and to permit reasonable life of the shield tube supports, the present invention provides cooling induced by natural draft as will be discussed in more detail later. Turning briefly to FIG. 5, a prior art configuration is illustrated which has been heretofore used with some success. A typical shield tube 22 is supported by an I- shaped beam 24 which, if no heat were removed, could reach temperatures of 2,200F. and above, at which point the beam 24 would become exceedingly weak and its ability to support the shield tubes very limited even when high temperature metals are used, such as HK-40. Failures may occur in a very short time from creep under these high temperatures. The prior art method illustrated in FIG. 5 protected the lower portion of the shield support beam 24 with insulation 26 and protected the insulation 26 by an overwrap of a metallic material 28, typically I-IK-40. While this protection will reduce the temperature of the lowest portion of the shield beam, at the same time, the upper portion of the shield beam 24 is cooled by heat losses into the hydrocarbons in tubes 22 which, it will be recalled, may operate at about l,200F., thus providing a means of limited cooling of the shield beam. Such a technique has been found to reduce the temperature of the shield beam in the order of 50-150F., which, although seemingly small compared to the 2,000 operating temperatures, has significant effect upon the creep strength of the beam and, accordingly, upon its life. As furnace designs push to ever higher and higher temperatures, such a design has only limited application.

A solution of this problem has been found to provide additional cooling. Whenever a furnace is operating, there will generally be a negative pressure inside it caused by the relatively low density of hot combustion gases compared to the outside air. In fact, most furnaces induce the air needed for combustion by this negative pressure, i.e., natural draft. A portion of the natural draft may be used to induce a stream of air selectively into a novel shield tube support beam, cooling it to provide a substantial decrease in temperature and an increase in strength. Although, as has previously been mentioned, air cooling has been provided in the prior art, it has not been used for shield tube support beams in the manner of the present invention. Its principal use in previous applications has been in radiant section service. In fact, some of the prior art indicates that its use there has been relatively unsuccessful. In the present invention, the general principle has been specifically adapted to shield sections and has proved to be an effective method of cooling and strengthening the shield tube support.

In FIG. 2, a sectional view of the shield tube support is seen which, when combined with the views of FIGS. 3 and 4, illustrate the specific structure which has been found to give a substantial reduction of shield tube support temperature. The two rows of shield tubes 22a and I: lie in staggered fashion, one above the other, both resting upon the shield tube support beam. Instead of an I-shaped beam support of the prior art (FIG. 5), a hollow tube, typically a high strength HK-4O (25Cr/2ONi) steel may be used to support the shield tubes. In a typical furnace of 40-60 feet in length, five to seven number of intermediate supports will be required in order to support the shield tubes adequately. Air passing through the interior of the support significantly lowers its temperature and increases its strength. The importance of cooling is illustrated by the following data for a typical I-IK-4O material. The allowable stress is:

2500 psi at l800F.

1050 psi at 2000F.

300 psi at 2l00F.

This material melts at 2270F. and its operating temperature without protection and cooling is 22002250F. Cooling only a few hundred degrees is important. In order for cooling air to reduce the temperature substantially as required, the hollow support beam 30 is completely encased in a light weight insulation 32, for example, a kaowool, which serves to reduce the temperature of the beam and to protect against creep failures. Performance of this insulation is critical to the success of the invention and, accordingly, in many applications, it will be desirable to provide a high temperature alarm to sense the temperature of the air leaving the support beam and warn of insulation failure. Contrary to the earlier art, it will be noted that the insulation 32 covers a major portion of the T-shaped support 34 which is welded to the support beam 30 and which accepts the weight of the lowest row of shield tubes 22a, and indirectly supports the upper row of shield tubes 22!) through a Y-shaped support 36 which can be more clearly seen in FIG. 3.

In FIG. 3 the Y-shaped support 36 is shown welded directly to the T-shaped member 34, which is in turn directly welded to the air cooled support beam 30. It will be noted that no insulation is provided for the Y- shaped support 36 and it might be thought that it would fail from overheating. In fact, however, it has been found that no insulation is required, since the heat removed from the Y-shaped support 36 through the T- shaped member 34 to the air-cooled beam 30 is very substantial. By way of illustration, it has been found by direct measurement that the temperature of the Y- shaped support may well be 300 lower than the 2200 which was typically experienced in this area without cooling. At this temperature the Y-shaped support without any further insulation is sufficiently strong to support the second row of shield tubes 22b. It will be appreciated that the supports lose heat in two ways,

first and principally, by direct conduction to the aircooled support beam 30 and secondly, and less importantly, to the process fluid in the shield tubes 22. It has been found that approximately 80 percent of the heat which is removed from the supports flows directly into the air-cooled support beam 30, with the remainder flowing into the shield tubes 22. The substantial reduction in temperature is sufficient to greatly increase the strength of the material and, accordingly, to prevent premature failure due to creep of the tube supports.

Although a T-shaped member with a wide extension is preferred embodiment, other shapes could also be used as were deemed to be mechanically suitable, as long as they followed the general teachings of the invention.

It has been found by direct measurement that the amount of air induced by natural draft through the hollow support beam is sufficient to cause a significant reduction in temperature of the supports as has been noted. The air temperature increase across the aircooled beam itself has been found to be of the order of 150 to 200F.

FIG. 4, in a perspective view illustrates the elements of the invention which have been previously described showing in a partially cut-away view the air-cooled support beam 30, its insulation 32, and the tube supports 34 and 36.

The foregoing description of the preferred embodiments is by way of illustration of the invention only and is not intended to limit the scope thereof which is defined by the claims which follow.

What is claimed is:

l. A method of supporting furnace tubes exposed to high temperatures at which metal support beams for said tubes are subject to creep failure comprising:

a. providing an air passageway through each of said support beams;

b. admitting cool ambient air to one end of said passageways of (a) and flowing said air therethrough thereby increasing its temperature;

0. removing said heated air of (b) from the passageway of (a) and discharging it into the furnace at a location having negative pressure relative to the pressure of said ambient air;

d. insulating each of said tube support beams to limit heat input to said supports sufficient to allow the operating temperature of said support beams to be reduced by air passing through said passageways of (a) to below the temperature at which creep failure occurs, l

e. supporting said furnace tubes on pairs of metal supports welded to said beams, one of said supports being within said insulation of (d) and supporting a tube immediately adjacent said insulation and the second of said supports supporting a second tube disposed further away from said passageway and being welded to said first support and outside said insulation of (d).

2. A supoort structure for furnace tubes exposed to high temperatures in a furnace having radiant and convection sections and where metal support beams for said tubes are subject to creep failure comprising:

a. an elongated air-cooled support beam having a passageway therethrough with one end of said passageway open to cool ambient air outside the furnace and the other end communicating with a location within the furnace having a negative pressure relative to the pressure of said ambient air;

b. insulation completely surrounding said support beam of (a) where said beam is exposed to high temperatures within the furnace, said insulation having sufficient resistance to heat transfer to permit the support beam to operate below the temperature at which creep failure occurs;

0. at least one pair of metal supports attached to said support beam of (a) for holding an associated pair of furnace tubes, the first of said supports being welded to said beam and completely covered by said insulation of (b) for supporting the first of said furnace tubes, the second! of said supports being welded to said first support for supporting said second furnace tube further away from said support beam of (a).

3. The support structure of claim 2 wherein said sec ond support of (c) is insulated against heat input from the furnace.

4. The support structure of claim 2 wherein said first support of (c) is T-shaped and attached at the base of the T to said support beam.

5. The support structure of claim 2 wherein said second support of (c) is Y-shaped and attached at the base of the Y to the crossbar of said T-shaped support,

6. The support structure of claim 2 wherein said support beam of (a) comprises a hollow cylindrical tube fabricated of a metal having high temperature creep resistance.

7. The support structure of claim 2 wherein said furnace tubes are horizontal shield tubes disposed between said radiant and convection sections.

8. The support structure of claim 7 wherein said shield tubes are supported by a plurality of said support beams extending normal to the axes of said furnace tubes and supported near the ends of said beams by the furnace structure, whereby the: support beams are subject to bending load by said shield tubes.

9. The support structure of claim 2 wherein said furnace location of negative pressure is subsequent to the convection section.

10. The method of claim 1 further comprising sensing the temperature of the air exiting said passageway of (a) as indicative of the heat limiting ability of said insulation.

Claims

1. A method of supporting furnace tubes exposed to high temperatures at which metal support beams for said tubes are subject to creep failure comprising: a. providing an air passageway through each of said support beams; b. admitting cool ambient air to one end of said passageways of (a) and flowing said air therethrough thereby increasing its temperature; c. removing said heated air of (b) from the passageway of (a) and discharging it into the furnace at a location having negative pressure relative to the pressure of said ambient air; d. insulating each of said tube support beams to limit heat input to said supports sufficient to allow the operating temperature of said support beams to be reduced by air passing through said passageways of (a) to below the temperature at which creep failure occurs, e. supporting said furnace tubes on pairs of metal supports welded to said beams, one of said supports being within said insulation of (d) and supporting a tube immediately adjacent said insulation and the second of said supports supporting a second tube disposed further away from said passageway and being welded to said first support and outside said insulation of (d).

2. A support structure for furnace tubes exposed to high temperatures in a furnace having radiant and convection sections and where metal support beams for said tubes are subject to creep failure comprising: a. an elongated air-cooled support beam having a passageway therethrough with one end of said passageway open to cool ambient air outside the furnace and the other end communicating with a location within the furnace having a negative pressure relative to the pressure of said ambient air; b. insulation completely surrounding said support beam of (a) where said beam is exposed to high temperatures within the furnace, said insulation having sufficient resistance to heat transfer to permit the support beam to operate below the temperature at which creep failure occurs; c. at least one pair of metal supports attached to said support beam of (a) for holding an associated pair of furnaCe tubes, the first of said supports being welded to said beam and completely covered by said insulation of (b) for supporting the first of said furnace tubes, the second of said supports being welded to said first support for supporting said second furnace tube further away from said support beam of (a).

3. The support structure of claim 2 wherein said second support of (c) is insulated against heat input from the furnace.

5. The support structure of claim 2 wherein said second support of (c) is Y-shaped and attached at the base of the Y to the crossbar of said T-shaped support.

8. The support structure of claim 7 wherein said shield tubes are supported by a plurality of said support beams extending normal to the axes of said furnace tubes and supported near the ends of said beams by the furnace structure, whereby the support beams are subject to bending load by said shield tubes.