US2294636A - Steam chest construction - Google Patents

Description

K. R.' STEARNS 2,294,636

STEAM CHEST CONSTRUCTION Filed Feb. 24, 1940 2 Sheets-Sheet 1 m. 9 Mm a mmwmm W n A 5 mmw m V L m-/%: 1

, a m; F M. w?

Sept; 1, 1942.

F K. a.

INVENTOR KENNETH R. STEnRNs.

ATTORNEY WITNESSES: WY RM pt- 1942. K. R. STEAE'QNS 2,294,636

STEAM CHEST CONSTRUCTION Filed Feb. 24', 1940 2 Sheets-Sheet 2 INVENTOR KENNETH RSTEHRNS.

' I BY ATTORN EY Patented Sept. 1, 1942 STEAM CHEST CONSTRUCTION Kenneth R. Stearns, Prospect Park,,l?a., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa.,:-a corporation of Pennsylvania.

Application February 24, 1940, Serial No. 320,569

3 Claims.

This invention relates to high-pressure vessels, more particularly to closures therefor, and has for an object the provision of a closure so'constructed and arranged as to provide minimum shear and bending stresses in the pressure vessel,

the closure and its bolts, and maximum sealing load at the joint between the vessel and the closure.

Another object of the invention is the provision of a cylindrical high-pressure vessel having a relatively longand narrow opening whose major axis is parallel to the longitudinal axis of the vessel together with a closure for such opening so constructed and arranged that the combined vessel and cover constitute a mechanically cylindrical structure.

Yet another object of the invention is the provision of a spherical high-pressure vessel having a circular cover so constructed and arranged that the combined vessel and cover constitute a mechanically spherical structure.

These and other objects are effected by my invention as will be apparent from the following description and claims taken in connection with the accompanyingdrawings, forming a part of this application, in which;

Fig. l is a longitudinal sectional view through a turbine cylindrical steam chest embodying the invention;

Fig. 2 is a stress diagram of an internally loaded cylinder;

Fig. 3 is a fragmentary transverse'sectional view of a turbine steam chest constructed in accordance with prior practice; and,

Fig. 4 is a similar view of a cylindrical steam I chest constructed in accordance with the present invention.

Referring now to the drawings more in detail, there is shown, at IU (Fig. 1), an elastic fluid turbine comprised byga stator I l and arotor I2. Motive fluid, preferably steam, is supplied to the turbine through passages I3 controlled by valve structures l4 housed in a steam chest to which the steam is supplied through the connections I6. I

Each valve structure includes an insert or sleeve l8 through which extends a steam' passage l9 controlled by a plug valve 20. Each plug valve has an upwardly projecting stem Zlpassing through an opening in .the lifting bar 22Nand carries at its upper end a nut' 23. The nuts are adjustable to control the relative order'and extent of opening of the various valves. The lifting bar 22 is connectedby rods to av conventional governor mechanism or linkage 25.-

The structure so far described may be taken as representative of present-day steam turbine practice and constitutes no part of the present invention.

Heretofore, it has been the practice to design turbine steam chests of the integral cast type with a cross-section similar to that shown in Fig. 3. The principle was simply to provide a circular opening of radius r, suflicient to conduct steam to the valves with reasonable losses, surrounded by a wall of thickness t, having a slot at the top wide enough to enable machining of the valve seat openings and long enough to pass the valve bar assembly. The risers A on each side of the slot were added to form a flange to accommodate the cover stud bolts B and to provide the inner and outer contact surfaces. The cover S was machined from plate, with clamps T at the edges to prevent the walls from spreading and protect the bolts against bending stresses.

The foregoing design, although wasteful of material, was generally satisfactory until applied to high-pressure and high-temperature machines, where it was found difiicult to maintain a steamtight joint at the inner contact surface after relaxation hadtaken place in the bolts, although the bolts had been madeample to care for the steam load on the cover.

Calculations based on the theory of beams on elastic foundations showed thatthe stiffening effect of the end Walls of the steam chest on the flanges of the side walls becomes negligible in the central region of the chest, and it was, therefore, possible to take out at the center a crosssectional slice of unit length as a free body and calculate the statically indeterminate clamp force F, inner contact force Q, and outer contact force R, under an internal pressure p, for any bolt load P, under elastic conditions, by equating (1.),the rotations of the cover and of the wall at the contact surface and (2) their horizontal deflections at the clamp surface. With these forces known, the deflectionsv and stresses at any point can be determined. In the designs studied, it Was found that a bolt stress on the order of three times that needed to hold the cover against steam load Was required to obtain contact at the innercontact surface, because of the tendency of the wall inner contact surface to rotate downward away from the cover. With a F. temperature'gradient through the walls and cover due to a sudden loss of superheat, the bolt pull would again be more than doubled to maintain contact at the inner contact surface. Furthermore, with no temperature gradient, a bending stress (tension outside) of six times the hoop stress due to internal pressure was found at sections CC. The ensuing relaxation of the Wall near CC would necessitate a still greater pull from the bolts to keep force Q positive. Measurements on a steam chest which had been in service for about six months showed a permanent lowering of .023 in. at point D, Fig. 3, presumably mostly due to creep of the outer fibers at CC.

Elastic calculations showed that by reducing the thickness L of the portion of the cover between the inner contact surfaces by one half, to make the cover more flexible and enable it to follow up the wall, the bolt pull needed to maintain a plus Q was brought down to about the same order as that needed to hold the cover against the steam load on its under surface. However, the bending stress at CC rose to ten times the hoop stress, indicating a creep rate so high as to make the life of the structure short. The thick, rigid cover is an essential safeguard to this design, which has large offsets from a mechanically cylindrical form, even though the surface presented to the confined fluid appears roughly circular.

The solution of the problem proposed here consists of making the vessel mechanically, as well as apparently, cylindrical. It is proposed to eliminate the offsets, and thereby unnecessary bending stresses, and so save material and bolting. Consider the forces acting in a thick-wa1led cylinder under internal pressure. See Strength of Materials, S. Timoshenko, vol. II, pages 528 to 533. Take a unit length of a thick circular cylinder subject to internal pressure as in Fig. 2.

Suppose it is clamped along a radius at AA A and cut radially through at CD without allowing the internal pressure p to escape. Due to symmetry, there is no radial shear. We know that it has a tangential holding force 111 with a distribution similar to that shown. Let Tm be the radius to the centroid of the tangential forces and 1' the internal radius.

The horizontal component due to p on parts A-B is pr sin acting on the moment arm rsind about A. Total moment about A due to 29 is which reduces to M i=p'r1m (1 cos 6) in clockwise direction.

Now if the tangential forces which existed on the surface C-D before it was out are replaced by a single force p'r equal to their sum and acting on their centroid at B, this force will give a counterclockwise moment about A of M z prrm (l-cos 0) which exactly balances M1 and leaves the stress distribution on section A'-A exactly as it was before the cylinder was cut, provided the length A-B is sufficient to allow the application of St. Venants Theory. The clamp may be removed, since it is not loaded. Since the points A and B were chosen at random, we conclude that there is no bending moment in the wall of a circular cylinder subject to uniform internal pressure (aside from the non-uniformity of the tangential stress distribution, sometimes loosely considered to be the result of an equivalent bending moment superimposed on a uniform tensile stress).

Referring to Fig. 3, it will be seen that the supporting forces acting on the wall are the clamp force F and a portion of P. Their resultant acts through their intersection with an eccentricity E with the centroid of the tangential stress in the wall; this eccentricity being in a direction such as to unload the inner contact surface and load the outer, opposite to what is desired. A large part of the bolting effort is wasted in overcoming this eccentric moment.

It is proposed to so proportion the steam chest or other vessel and cover that the tangential holding force lies (a) along the center of gravity of the tangential bursting stresses, in order to eliminate any important tendency toward rotation or load transfer at the joint, or (b) so that it lies inside the centroid enough to give load transfer to the inner contact if desired to give the joint more margin against temperature gradients caused by sudden lowering of the steam temperature.

This may be accomplished by the arrangement of Fig. 4, where there is shown the steam chest I 5 provided with the cover 26 held in place by the bolts 2'1.

The steam chest has a first contact surface 28 cooperating with the inner and outer first contact surfaces 29 and 353, respectively, of the cover. The chest also has second contact surfaces 3| cooperating with second contact surfaces 32 of the cover, said second contact surfaces being in planes substantially perpendicular to the first contact surfaces.

The internal radius r is chosen, first, to give the passage area wanted, and second, to give a height inside at the center equal to length of the #1 valve plus allowances for lift and clearance thereof. The thickness of the wall is next made adequate for the radius, internal pressure and the material properties under working conditions, remembering that this design practically eliminates bending stresses in the wall. The width of opening is laid out, tentative width of the inner and outer contact faces and bolt diameters chosen and the values c=distance centerline of chest to centerline of inner contact face a distance centerline of inner contact to centerline of bolts b distance centerlin of bolts to centerline of outer contact face established, allowing for chamfers, fillets, counterbores and clearances, a trial value of h, the height of the joint above the center of the cylin der, is laid out and a radius drawn through the intersection of the inner contact centerlin and the joint, the assumption being that the internal pressure penetrates to, but not beyond, this point. If this point is very much off the circle of radius r, corrections will have to be made in the calculations. The radial line just drawn makes an angle 0 with the vertical centerline of the chest.

Now if each slice of'unit length of the wall cut out by two planes perpendicular to the longitudinJa-l axis were held by a tangential force of pr at the point A on the centroid (the vector pr in Fig. 4) it would exhibit no tendency to rotate due to the application of the internal pressure, as has been seen. The two holding forces available are the bolt pull, P, assumed to act vertically at the centerline of the bolts, and the hook or clamp force, pr cos 0, acting horizontally through the center of the clamp contact surface. Friction forces cannot be depended upon, because of vibration and temperature differences, but they will not greatly alter the lines of action or the magnitude of the clamp and bolt forces. The resultant of the clamp force and the part of the bolt force needed to resist tangential pull due to pressure p will act through their intersection at' point B. This resultant force pr through B is parallel and equal to pr, but may be found. to have a moment arm or eccentricity E about point A on the centroid. By varying the distance it any desired value of E may be had.

If Q and R are the loads per unit length on the inner and outer contact surfaces, the forces exerted by the cover on the wall are related as follows, assuming the wall and cover to be rigid bodies held by pure tangential forces at C and These values are approximate, due to elastic and plastic deformations of the clamp in service, second-order bending moments set up by variations in section modulus along the circumference, initial machinery or casting inaccuracies, et'c.

Enough metal may now be added inside the wall to form a flange to take the threaded ends of the studs and transmit the contact pressure along the bolt pitch, and similarly extra metal to form a combined flange and hook may be added beyond the outer circle in the cover.

The same principle is applicable to a spherical Wessel manhole or cover of any diameter less than that of the vessel. Fig. 4 could be thought of as showing the strainer cover on a spherical throttle valve body; in this application, considerable economy of material over that required for the usual blind flange type of cover could be efiected.

While I have shown my invention in but one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

What Ilclaim is:

1. A high-pressure vessel having a curved wall defining a hollow body of generally circular crosssection having substantially uniform inner and outer diameters, said wall having an opening of less diameter than the vessel, a cover for said opening, said FCOVEI and vessel having first contacting surfaces defining a first joint and having second contacting surfaces penpendicular to said first contacting surfaces and defining a second joint, and a plurality of bolts positioned within the confines of said first contacting surfaces and securing the cover in closing relation to the opening, the points of intersection of the longitudinal axes of the bolts with a plane normal to and intersecting the second contacting surfaces lying approximately within the mean diameter of the vessel and the inner portion of the first contacting surfaces also lying within the mean diameter of the vessel.

2. A high-pressure vessel having a curved wall defining a hollow cylindrical body having substantially uniform inner and outer diameters, said vessel having a relatively long and narrow opening whose major axis is parallel to the longitudinal axis of the vessel and whose width is less than the inner diameter of said vessel, a cover for said opening, said cover and vessel having first contact surfaces defining a joint lying in a plane parallel to the longitudinal axis of said vessel and havingse'cond contact surfaces lying in planes substantially perpendicular to said first contact surfaces, and a plurality of bolts extending perpendicular to and within the confines of said first contact surfaces and securing said cover in closing relation to said opening, the points of intersection of the longitudinal axes of the bolts with a plane normal to and intersecting the second contact surfaces lying approximately within the mean diameter of the vessel and. the inner portion of the first contacting surfaces also lying within the mean diameter of the \vessel.

3. Structure as specified in claim 2, wherein the cover has a minimum thickness approximately equal to the minimum thickness of the pressure vessel walls.

KENNETH R. STEARNS.

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