US1902146A - Method of annealing vessels - Google Patents

Description

March 21, 1933. I R. STRESAU 1,902,146

METHOD OF ANNEALING VESSELS Filed Sept. 18, 1930 2 Sheets-Sheet l INVENTOR.

Richard Stresau A TTORNEY.

March 21, 1933. R. sTREsAu 1,902,

METHOD OF ANNEALING VESSELS Filed Sept. 18, 1930 2 Sheets-Sheet 2 INVENTQR.

Rich ard S 2 resau A TTORNE Y.

Patented Mar. 21, 1933 UNITED STATES PATENT OFFICE RICHARD STBESAU, OF WAUWATOSA, WISCONSIN, ASSIGNOR TO A. 0. SMITH CORPORA- 'IION, OF MILWAUKEE, WISCONSIN, A CORPORATION OF NEW YORK METHOD OF ANNEALING VESSELS Application filed September 18, 1980. Serial No. 482,657.

This invention relates to a method of annealing vessels.

In the manufacture of metal vessels and particularly welded vessels it becomes desirable to heat the completed structure to a temperature capable of relieving any local stresses that may be present therein. It has been found difiicult, particularly in vessels of relatively thin wall, to use a temperature high enough to accomplish the desired stress relief, within a commercially desirable length of time, without causing distortion and sagging of the walls of the vessel. It has been found additionally diificult to heat to a sufliciently high temperature to accomplish the result sought in a metal softening anneal without experiencing such distortion.

An object of this invention is to provide a method of annealing a metal vessel which will permit the use of a desirable stress relieving temperature and prevent detrimental dlstortion of the vessel during the appllcation of this temperature.

Another object of this invention is to provide a method of correcting localirregularities of shape in metal pressure vessels, thereb ll their use undero eratiiu condiy a owmg p h nealmg heat treatment is applied to the tions. substantially free from points of detri mental local stress intensification.

The proper removal of shape irregularities is important in vessels subject, in use, to either internal pressure, vacuum or external pressure. The term pressure vessel in this specification is therefore intended to mean a vessel intended to operate under any of these conditions.

In its broader aspect, therefore, the invention resides in a method of annealing pressure vessels while obtaining the shape thereof which is correct for subsequent use under working conditions.

Other aspects of the invention will be pointed out hereinafter with reference to the accompanying drawings in'which:

Figure 1 is a side elevation of a simple cylindrical vessel with elliptical heads.

Fig. 2 is an end elevation of the vessel shown in Fig. 1.

Fig. 3 is an enlarged section through Fig. 1

on line 3-3 and illustrates the action of certain forces tending to distort the vessel.

Fig. 4 is a section similar to Fig. 3 after the temperature has lowered the strength of the metal sufficiently to permit the distortion 'of the bottom.

Fig. 5 is a view similar to Fig. 3 showing bottom supports so placed as to assist in preventing distortion.

Fig. 6 is an enlarged view of the welded joint in Fig. 3.

Fig. 7 is a section of a thin walled vessel showing a modified form of support.

Fig. 8 is a section of a vessel to be heat treated sus ended by supports attached to the top of its s ell.

Inthe drawings, 1 is a sim 1e cylindrical vessel, 2, 3,4 and 5 are circumfiarential welds made in the construction of the vessel, 6 and 7 are longitudinal welds, 8 is a manway opening in thevessel, 9 is a reinforcing plate used to sustain the vessel wall at the manway against local distortion due to internal pressures, while in use, and 10 is a manway cover secured to the manway and suitable to sustain internal pressure.

In the practice of this-invention, an anfabricated vessel in which the vessel is subjected to internal pressure to assist in the prevention and elimination of irregularities therein.

The use of the term internal pressure in its. various modified forms refers to subjecting the inner wall of the vessel to a fluid pressure which is greater per unit area than the pressure to which the outside of the vessel is subjected. This internal pressure insures a tensioning of the walls of the vessel during its ap lication.

If t e vessel shell were thin enough to be practically without stiffness or were rfectly flexible, then the bottom thereo would assume a fiat position as 11 between planes 1212, Fig. 4. When-11 is straight, the forces 13 due to internal pressure must be transmitted undiminished through 11 to its support. tion 11 will then be such that when multi- The length of the straight por-- plied by the unit pressure inside the vessel, the

product will not be greater than the total weight of the vessel and fluid being supported on the surface under 11. When a gas of negligible weight is used, we can say PL is equal to or less than W, in which P= pressure per unit area applied internally, L flat portion 11 and WV= total weight of vessel per unit of length. Any stiffness of the shell giving resistance against deflection may decrease the flat horizontal distance L Examining Fig. 4, it will be noted that the internal pressure at flat portion 11 is sustained by the external supporting surface so that there is an unbalanced vertical force in the vessel. This acts on the inside top of the vessel and is efl'ective in helping tosupport the weight of the shell.

The fact that the shell hasthis effective support at the top explains why, with very moderate internal pressures, a very nearly true circumferential shape can be maintained without external confinement of the vessel during the annealing operation.

It is desirable to use a curved support at the bottom as shown at 1414 in Figs. 1, 2 and 5.

The total width of this curved support in thin vessels preferably should be equal to or greater than L as determined by PL =W above or A supported curved surface of a shell will act very similar to the flat surface 11 as in Fig.

temperature thereby avoiding any material permanent enlargement of the vessel being treated. If f=fiber stress at this yield point, then 2fd=P X D where D diameter of vessel in inches and d= the thickness of the shell in inches.

ll? D and L Ji l 1 2fd/D 2fd W is the weight of vessel per unit length, and it is equal to 11' D (1.71), in which '10 the weight of a cubic inch of the material in pounds, then The foregoing is true for very flexible walls and is a guide forthe selection of supports to minimize deflections. However, the stiffness of vessel walls effects the total deflections.

.der of the shell 1 rests upon this beam 16 at its outer extremities at 18 and 18.

Lines 19 and 19 represent this weight and line 20 represents the'force applied to support the vessel at line 15. The moment of bending on this beam is the where force 20 is W or the weight of the vessel per unit of length and distance 21 is L. This moment of bending is a fixed amount if the entire weight of this portion of the vessel is left free to rest on points 18-18 M is equal to page 562, Civil Eng. Handbook by Frye 1913, in which F=outer or greatest fiber stress, y=distance from center of the plate to the extreme fiber, I=the moment of inertia of the section.

where d =thickness of plate. Page 525, Frye,

b=unit width of sheet (one), and then and If all dimensions of the vessel should remain the same except the shell thickness, M would change directly as (1, because in which W changes proportionally to 03, thereby making F the extreme fiber stress vary inversely as the thickness of the plate. The above simple illustration is used to show that as the yield point is reached by raising the temperature, the thinner shells tend to yield earlier under their own weight than the thicker ones.

It has been suggested to support the weight of the upper part of the vessel by struts placed in a vertical position within the vessel, thereby relieving the points 18-18 of at least a portion of the weight of the vessel. This method would not be very satisfactory because it gives points of concentrated force which are detrimental to the vessel.

By applying internal pressure to the vessel during heating there are, in addition to the gravity forces 19 and 19 acting on the ends of the beam 16, tangential forces 22 and 22'. These forces are in a direction inclined upward, thereby assisting to prevent downward deflection of the beam. These forces, however, may not be fully efiective since there are radial forces, not shown, acting downward on beam 16, tending to depress it.

In Fig. 6 is shown an enlarged section through horizontal weld 6. This is shown as having been formed with an angle between the plate ends different than that of the average curvature 23. The tension forces in this sheet set up by internal pressure applied during annealing will tend to cause the sheet to take the average curvature position and exert an excess of tension at the region 24.

By applying an internal pressure large enough to approach the yield point of the heated shell, the point 24 can be made to yield under tension because it receives a tension greater than the average. It will be seen that in this way, local irregularities may be permanently corrected without expanding the other portions of the shell. The same can be shown with regard to irregularitles due to circumferential seams or other causes.

In very thin walled vessels slight lrregularities in the supports may cause Indentations during the heating period. It has been found desirable in such cases to gain the advantage of the curved support shown in 5 as well as a yielding distributed application of such support to prevent local deformations of the vessel. This has been accomplished by the application of bands 25 extending under the vessel and secured above as illustrated in Fig. 7. The bands25 are secured to bolts 26 and 26 which are secured to beams 27 by nuts 28 and 28'. Internal pressure is introduced into this vessel by a connection similar to that of Fig. 1.

Certain types of cylindrical vessels are adapted to be suspended from lugs or fittings secured along one side of the cylindrical shell as indicated by a section shown in Fig. 8. Lugs 29-29 are secured by welding or other meansto the vessel along one side. These are secured by bolts 30 and 30' and nuts 31 and 31' to beams 32.- Internal pressure is applied through a connection similar to that shown in Fig. 1.

\Vithout internal pressure, sagging, due to the lowering of the strength of the vessel shell by the application of heat during annealing, would cause the vertical diameter 33 to become greater than the horizon diameter 34.

Upon the application of internal pressure to the vessel, the unit pressure being the same in all directions, the total horizontal pressure on the larger diameter 33 would be reater than that on diameter 34. The diference of these total pressures would be available to swing the shell sides outwardly rotating thein about the supporting points 2929 as fulcrums and thereby lifting them backtoward the truly circular form. This modification operates on the principal of unequal diameters. However, this difierence can in practice be reduced to values that are entirely within the requirements usually set upon such vessels.

If the lugs 29-29 indicated are well distributed or are welded frequently over an extended area in such a .way as to avoid serious local stress due to the weight of the vessel, there is considerable advantage in thus supporting thin vessels for annealing. The upper surface is then substantially retained in true shape and the lower one has a tendency to depend in a naturally nearly true shape. The addition of internal pressure then makes it entirely practicable to maintain a proper shape during the time that a weak- I ening of the vessel wall occurs due to raised temperature. The above method applies equally well to a spherical shape.

It will be appreciated that when a given internal pressure has been predetermined from the above factors it is important to maintain this pressure accuratel The procedure of operation may be as ollowsz The sealed vessel is placed in position in the heating chamber and properly supported. An air control connection 35 is preferably attached tothe manway cover 10 of the vessel and communicates with the interior of the vessel. This connection may consist of a pipe with a valve having a port communicating with the atmosphere. This pipe may also be connected with a source of air pressure by an-- other valve. A pressure gauge is provided to indicate the internal pressure.

The vessel is initially full of air at atmospheric pressure, and as the temperature of the vessel is raised this air expands and the pressure in'the vessel increases above the initial atmospheric ressure. When the desired predetermine pressure is reached, the valve connecting the pipe to the atmosphere is opened occasionally to expel enough air to limit the pressure to the desired value until the final temperature is reached.

In place of starting with atmospheric pressure, the valve connecting with the source of air pressure may have been opened, long enough to obtain the predetermined pressure initially.

The proper annealin time is allowed, then the vessel is cooled. he pressure'is maintained by the admission of air frequently in desired amounts from the source of air pressure untilthe proper metal strength is regained by the cooling of the metal. It is desirable to have a very careful and accurate control of pressure during both the rise and fall of temperature. For this purpose, pressure diaphragm actuated relays have been supplied to control the openin and closing of the two valves described a ove, thereby providing a Very sensitive control of the pressure.

The predetermined pressure for supporting the walls of the vessel is usually much lower than that necessary to equalize the irregularities in the walls. However, irregularities in the vessel wall may be equalized after the full stress relieving temperature is reached by manually opening the pressure valve and running the gauge indication up to the predetermined amount just under the amount necessary to reach the yield point of that portion of the shell material which has the predetermined desired shape, maintaining this pressure for ashort time, and then lowering the same by opening the valve to the atmosphere until the shell supporting pressure is reached. The latter pressure is then maintained as before.

It is also possible to apply the supporting pressure only during the cooling period of the treatment, the vessel being first heated to the maximum temperature or nearly so and pressure being then applied to correct any irregularities in shape and to maintain the required shape of the vessel during the cooling period in which the metal regains its strength.

For the purposes of this invention, the expression referring to the applicationof internal pressure during the period of elevated temperature is intended to mean that the pressure may he applied either during the entire or only a portion of the period of elevated temperature.

While the drawings and descriptions have been with reference to cylindrical vessels, the use of internal pressure to support structures during annealin may be applied to various shapes of vesse s.

I claim:

1. The method of treating a fabricated pressure vessel to relieve accumulated stresses and correct imperfections of the stress receiving shape, which comprises sealing said vessel for receiving internal fluid pressure, heating the vessel to a temperature suficient to eflect the desired stress relief, maintaining an internal fluid pressure of a predetermined amount suflicient to support the weight of the walls against deformation during the period of elevated temperature, raising the pressure for a limited time to a value suflicient to impose upon the walls of the vessel a fiber stress near the yield point of the material, and allowing the vessel to cool while -value of the metal, maintaining said internal pressure near the higher value for a. portion of the time of elevated temperature to correct local irregularities of shape, and then allowing the vessel to cool.

3. The method of relieving the stresses in a fabricated cylindrical vessel which comprises heating the unconfined vessel to a stress relieving temperature, and applying.

an internal fluid pressure to the vessel during the period of elevated temperature while supporting said vessel at circumferentially distributed points some of which lie in longitudinal lines laterally spaced a distance equal to or greater than WD 2 f inches, W being equal to the weight of one cubic inch of the material of the vessel, D being equal to the internal diameter of the vessel in inches and f being equal to the pounds per square inch in stress at the yield point of the material for the highest temperature employed.

4. The method of relieving the fabrication stresses in a cylindrical metal "essel which comprises resting said vessel upon supports contacting their surfaces laterally over a portion of the circumference, heating the vessel to a stress relieving temperature and applying an internal fluid pressure to the vessel during the part of the period of elevated temperature in which the yield point .of the metal of the vessel walls is efi'ectively reduced.

5. The method of relieving the stresses in a cylindrical metal vessel which comprises resting the vessel in a suspended flexible band support, heating the vessel to a stress relieving temperature, and applying an internal fluid pressure to the vessel during at least a portion of the period of elevated temperature.

6. The method of relieving the stresses in a metal vessel which comprises resting the ves sel in a suspended metal band support, heating the vessel to a stress relieving temperature, and applying an internal fluid pressure to the vessel during a portion of the period of elevated temperature.

7. The method of relieving the stresses in a cylindrical metal vessel which comprises suspending the vessel horizontally by means of laterally spaced lugs attached to the upper surface thereof, heating the Vessel to a stress relieving temperature, and maintaining an internal pressure during at least a part of the period of elevated temperature.

8. The method of relieving the stresses in a metal vessel which comprises suspending the Vessel horizontally by means oflaterally spaced lugs attached to the upper surface thereof, heating the vessel to a stress relieving temperature, and maintaining an internal fluid pressure during a part of the period of elevated temperature.

9. The method of supporting a fabricated pressure Vessel during the process of annealing, which comprises supplying an in ternal fluid pressure to the vessel, and heating the vessel to an annealing temperature to release internal stresses locked in the metal walls of the vessel, the internal pressure supplied being suflicient to support the vessel when heated to maintain the fabricated shape.

10. The method of supporting a fabricated pressure vessel during the process of annealing, which comprises supplying an internal fluid pressure to the vessel, heating the vessel to an annealing temperature to release stresses locked in the vessel wall, and controlling the internal pressure to compensate for changes in temperature to maintain the vessel in its fabricated shape.

11. The method of supporting a fabricated vessel during the process of annealing, which comprises supplying an internal fluid pressure to the vessel, heating the vessel to an annealing temperature to release stresses locked in the wall of the vessel, maintaining the internal pressure at a value sufficient to support the Wall when heated, and raising the pressure to remove irregularities formed in the vessel during the process of fabrication.

12. In the method of fabricating pressure vessels, the step of welding plates to erect a vessel of the desired shape, applying an internal fluid pressure to the vessel, heating the vessel to an annealing temperature to release stresses locked in the wall of thevessel raising the internal pressure to a predetermined value to remove irregularities that may be in the vessel after fabrication.

14." The method of supporting the walls of a fabricated pressure vessel during the process of annealing which comprises heating the vessel to an annealing temperature to release stresses in the metal walls, and supplying andmaintaining in the vessel a fluid pressure sufiicient to support the metal wall when it reaches a temperature at which it cannot support its own weight.

In witness whereof I have signed my name at Milwaukee, Wisconsin, this 13th day of September, 1930.

RICHARD STRESAU.

during the welding of the plates, and maintemperature to release stresses locked in thevessel Wall, maintaining the internal fluid pressure in the vessel afterit is heated, and

Images