Design Features of Rolled Joints and Tube Rolling in High-Pressure Boilers

Engineer L. D. Ginzburg-Shik

Materials for High-Pressure Boilers

The increase in the operating pressure of boilers over the past decade has caused a considerable increase in the wall thickness of drums, headers, and boiler tubes; this, in addition to significantly increasing the weight and cost of the boiler unit as a whole, gives rise to high thermal stresses in the walls of drums owing to the considerable temperature difference that develops in a thick wall.

The desire to reduce the wall thickness of drums, headers, and tubes of high-pressure boilers has led to their manufacture from steels with an increased carbon content or from low-alloy steels possessing higher strength characteristics.

At the same time as boiler pressure increased, the temperature of superheated steam also rose. As is known, creep-resistance conditions at steam temperatures \(\geq 450^\circ\mathrm{C}\) require the use of molybdenum steels, and at temperatures \(\geq 500^\circ\mathrm{C}\), chromium-molybdenum or other grades of low-alloy steels.

Methods of Connecting Tubes to Drums and Headers

The method of connecting boiler tubes to drums, screen headers, superheaters, and water economizers has also undergone a number of design changes in high-pressure boilers as compared with low- and medium-pressure boilers. The connection of tubes to headers is carried out both by domestic plants and by the majority of foreign plants by welding. In this case, nipples are welded to the headers, or the nipples are fastened by threads, followed by external seal welding with a light tight seam. Figure 1 shows a header with welded nipples, and Fig. 2 a header with threaded nipples and external seal welding.

After the nipples have been welded, the headers undergo the appropriate heat treatment at the plant.

The replacement of rolled joints in headers by welded joints is explained by the development in recent years of welding techniques for low-alloy steels, and also by the considerable increase in the wall thickness of high-pressure boiler tubes, the rolling of which—especially for small-diameter superheater and water-economizer tubes—is incomparably more difficult under field conditions than welding the tubes.

The problem of connecting tubes to the drums of high-pressure boilers is solved differently.

The presence of steam and water-transfer tubes connecting the main and separation drums of modern high-pressure boilers, with small transverse and longitudinal pitch, which does not provide sufficient space for welding operations, makes it impossible for the designer to solve the problem of joining these tubes to the drum by means of welding.

Fig. 1. Header with welded-on fittings

In addition, it should be borne in mind that welding the fittings entails the need for heat treatment of the drum in order to relieve stresses, which requires the installation of large heating furnaces at the plant.

Fig. 2. Header with threaded fittings and external welding

For these reasons, in high-pressure boilers the connection of water- and steam-transfer tubes to drums is carried out by rolling.

Design Factors Governing the Production of Reliable Rolled Joints

If, in medium-pressure boilers, the design stresses in the drum walls were relatively small, then in high-pressure boilers they exert a considerable influence on the operation of rolled joints, placing them under more severe conditions and requiring increased strength. In addition, the high steam pressure sharply increases the forces tending to pull out the tubes.

The temperature difference that arises during boiler operation between the walls of the tubes and the drum—especially during boiler firing-up and shutdown—also tends to reduce the tightness of rolled joints.

Because of the factors listed above, the rolled joints of high-pressure boilers, in order to operate reliably, must possess increased strength and tightness.

The principal conditions ensuring the production of reliable rolled joints are set forth below.

The Correct Relationship Between the Material of the Drum and the Tubes

As is known, the tightness of a rolled joint is created by the residual deformation of the tube metal, which during the rolling process causes elastic deformation of the tube seat.

After rolling of the tube is completed, the elastic deformation of the tube seat creates the necessary “contact pressure” between the seat and the body of the tube; the magnitude of this pressure determines the degree of tightness of the joint.

To ensure the tightness of the joint under operating conditions, the magnitude of the “contact pressure” must be sufficient to compensate for the deformations of the seat and tube that arise under the action of working pressures and temperatures.

Let us consider several cases of different relationships between the metal of the drum and that of the tubes:

1. The drum metal is softer than the tube metal. In this case, during rolling, the metal of the tube plate acquires residual deformations which, after rolling, cannot create the required “contact pressure” between the tube and the seat; under these conditions it is impossible to produce a high-quality joint, as has been confirmed by a number of investigations.

2. The drum metal is considerably harder than the tube metal.

In this case, even if the tubes are over-rolled, the metal of the tube plate does not acquire elastic deformations of sufficient magnitude to create the necessary “contact pressure” between the tube and the seat. This is confirmed, in particular, by the experience of installing high-pressure boilers at one power plant, where the boiler drums were made of chromium-nickel-molybdenum steel with an ultimate strength of 74.5–80.0 kg/mm², while the tubes were made of carbon steel with an ultimate strength of 38.6–39.3 kg/mm².

From the examples given, it is clear how important the choice of the correct relationship between the drum metal and the tube metal is for producing a high-quality rolled joint.

On the basis of the conditions required for producing high-quality rolled joints, boiler-manufacturing plants have established a certain optimum relationship between the metal of the drum and the tubes:

in terms of hardness—the drum metal should be harder than the tube metal by 10–15 Brinell units;

in terms of yield strength—the yield strength of the drum metal should be higher than the yield strength of the tube metal (at 20°C) by 3–5 kg/mm².

Width of the Rolling Belt

Creating, by means of rolling, the required “contact pressure” with a rolling-belt width equal to the wall thickness of the drum, which in high-pressure boilers reaches 100 mm and more, proves practically impossible because of springing of the obliquely arranged rollers of the expander. Numerous investigations have shown that, in order to create the necessary tightness of the joint at pressures of 100–120 atm, a rolling-belt width of 40 mm is sufficient. Accordingly, most plants have adopted a rolling-belt width of 40 mm, irrespective of the drum wall thickness, and only in rare cases does the belt width reach 50 mm.

Grooves in Tube Seats and Their Influence

on the Strength and Tightness of the Joint

To increase the strength of rolled joints, one or two annular grooves are made in the tube seats. During rolling, the metal of the tube is squeezed out in the direction of the tube axis and fills the grooves. Experiments have shown that, when grooves are present, approximately 50% greater force is required to pull out the tube than with a smooth seat.

Fig. 3. Schematic representation of the formation of a bearing surface with 2 grooves

In addition to increasing strength, double grooves, by forming a transverse bearing surface around the circumference of the seat, increase the tightness of the joint (Fig. 3).

Design of Tube Seats

Figure 4 presents the designs of tube seats used by boiler-building plants.

As can be seen from Fig. 4, the rolling belt is located on the inner side of the drum.

Beyond the rolling belt, tube seats either have a cylindrical shape through the entire thickness of the drum wall (Fig. 4a, b, d), or their diameter increases conically (Fig. 4c).

The grooves have a rectangular, trapezoidal, or semicircular cross-section, with a depth of 0.8–1.0 mm and a width of 3–6 mm. With one groove, it is located at a distance of approximately \(1/3\) of the rolling-belt width, closer to the inner wall of the drum. With 2 grooves, they are arranged symmetrically with respect to the middle of the rolling belt (Fig. 4a and b).

In some designs of tube seats, a conical boring of the inner edge is provided for partial or complete sinking of the flare, in order to increase the tube’s resistance to being pulled out (Fig. 4b). The taper of the boring is usually made at an angle of 15°, corresponding to the taper of the flanging rollers. When a boring is present, the height of the flare should be counted from the beginning of the boring (plane AA).

Selection of the Optimum Degree of Tube Rolling

In low- and medium-pressure boilers, the degree of tube rolling was selected only as a function of the tube diameter. In high-pressure boilers, other factors also have an effect, and the proper selection of the degree of rolling depends on these factors, namely: the wall thickness of the tube, the presence of grooves, and the tube material.

Fig. 4. Designs of tube holes in drums of high-pressure boilers

a) \(P_{\text{work}} = 120\) atm; b) \(P_{\text{work}} = 125\) atm; c) \(P_{\text{work}} = 65\) atm; d) \(P_{\text{work}} = 128\) atm.

A number of boiler-building plants and erection organizations have developed empirical norms for optimum values of the degree of tube rolling, amounting to 18–25% of the tube wall thickness; the lower limit applies to tubes of smaller diameter, and the upper limit to tubes of larger diameter.

Below is part of the table of optimal degrees of rolling for various diameters and wall thicknesses of tubes (Table 1).

Table 1

For the preliminary selection of the degree of tube rolling

However, these data must not be followed blindly, since they are valid only for the given conditions and materials. Therefore, before beginning rolling work on a high-pressure boiler, it is mandatory to carry out rolling of 5–10 trial samples of tubes of each diameter in the body of the drum or header at different degrees of rolling, close to those indicated in Table 1 for the given diameter and wall thickness of the tube. Then the rolled tube ends are knocked out and inspected. The sample in which a more complete filling of the groove with the tube metal and a clearer “imprint” of the socket on the tube have been obtained at the minimum degree of rolling is considered the best, and its degree of rolling is adopted for subsequent rolling work.

Objective methods for monitoring the degree of rolling and the rolling tool

In view of the presence, in high-pressure boilers, of a number of factors affecting the degree of rolling, one cannot rely here solely on the skill of the roller operator, according to which he completes the rolling of the tube.

In addition, an increase in the wall thickness of tubes requires a sharp increase in torque during tube rolling. The need to obtain high-quality rolled joints and the drive toward mechanization of the labor-intensive process of tube rolling have created the necessity of establishing objective monitoring of the degree of rolling.

There are several methods of objective monitoring of the degree of rolling:

1. By the displacement of the cone relative to the body of the expander.
2. By the longitudinal elongation of the tube, measured with an indicator.
3. By torque.

In domestic installation practice for high-pressure boilers, the first inspection method, used in the installation of high-pressure boilers at a number of domestic power plants, has become the most widespread.

To measure the displacement of the cone relative to the body, the cone of the expander is provided with a vernier.

The rolling process is divided, as usual, into two separate operations:

a) fastening of the tube (setting by rolling),
b) flaring of the tube and flanging.

In accordance with this, two types of expanders are used: an oblique fastening expander with a vernier and an oblique flanging expander with a vernier.

Note. At present, the Centroenergomontazh trust is carrying out experimental work to establish the required contact pressure for ensuring a tight and strong joint, the longitudinal elongation of the tube, and the torque.

The oblique fastening expander (Fig. 5) is a cover-type expander with the cover located on the lower side of the body. The number of rollers is 3; the taper of the cone is \(1/18\), that of the rollers \(1/30\); the angle of inclination of the rollers is \(1 \tfrac{1}{2}^{\circ}\).

As can be seen from Fig. 5, the end of cone 3 has a thread; a ruler with a scale (vernier) is fixed on the cone: one division of the scale is equal to 1.8 mm, which corresponds to a tube expansion of 0.1 mm.

To reduce friction, the expander is provided with thrust ball bearing 5, thrust ring 7, and two setting nuts 8.

Oblique fastening expander with vernier for a drum.

Fig. 5

1—body, 2—cover, 3—cone, 4—expander roller, 5—thrust bearing,
6—body nut, 7—thrust ring, 8—setting nuts, 9—vernier

During the rolling process the cone moves, together with the setting nuts, relative to the expander body. Consequently, the difference between the readings on the ruler scale at the end and at the beginning of rolling represents the longitudinal displacement of the cone relative to the body, which determines the magnitude of the degree of tube flaring.

It is also possible to limit the specified degree of flaring by setting the distance nuts 8, according to the scale, at the corresponding distance from the end face of body cover 6, and rolling the tube until the nuts come up against the latter.

The oblique flanging expander (Fig. 6) has two flanging and shortened rollers and one long roller. Its remaining parts do not differ in any way from those of the oblique fastening expander.

Because of the considerable wall thickness of high-pressure-boiler tubes and the use of alloy steels, the cones and rollers are made of steels of grades EI-10 or ShKh-15.

Recently, experimental rollers have been made of carbon steel grade ST-5 with case-hardening of the surface in order to increase the ability of the roller shank to resist bending, to which it is subjected

Oblique beading roller with vernier for a drum.

Fig. 6

1—body, 2—cover, 3—cone, 4—rolling roller, 5—thrust bearing,
6—body nut, 7—thrust ring, 8—setting nuts, 9—vernier, 10—shortened roller, 11—beading roller

as a result of its oblique position relative to the rolling cone. The first operating trials showed good results.

Procedure for Carrying Out Rolling Work

Because tubes may have deviations in outside diameter from the nominal size within the tolerances established by MPTU 2579—1950, before installing the tubes, measurements are made of the diameter of each tube hole and of the inside and outside diameters of the tube ends. At the same time, a layout of the holes is prepared for each drum, with numbering of the rows and of the holes within each row, as well as a log of rolling work (Table 2).

To facilitate measurement of the diameters of holes and the inside diameters of tubes, special measuring devices are used, permitting measurements with an accuracy of up to ±0.1 mm.

Having the measurement data for the tubes and holes, columns 1, 2, 3, 4, 5, 6, 7, and 11 of the rolling-work log are filled in. The data for column 7 are obtained as the difference between the data in columns 4 and 5. The data for column 11 are obtained as the sum of the data in column 7 and the specified degree of rolling expansion in millimeters.

To grip the end of a tube, a fastening roller is inserted into it up to the stop of the support ring against the body of the drum. The roller is set in rotation until the clearance between the tube and the hole has been taken up and the tube ceases to wobble; at this moment the reading on the vernier scale is recorded, and the tube is secured by rolling it additionally by 3–4 divisions on the scale; this gives the tube an expansion of 0.3–0.4 mm.

Using the measuring device, the inside diameter of the tube after gripping is measured and entered in column 8 of the log.

Journal of tube-rolling operations in the boiler drum

Table 2

At the same time, column 9 is filled in—the excess grip. Before tube rolling is carried out, the worker is given the amount of the required expansion of each end of the tube, taking into account the excess grip made, as indicated in column 9 of the log. This required expansion value is calculated by subtracting from the total expansion of the tube (column 11) the sum of the values entered in columns 7 and 9 of the log; the resulting difference is entered in column 10.

For rolling, the flanging expander with the cone is inserted into the end of the tube so that the flanging roller enters the tube by 1–2 mm, and the expander cone is turned by hand until tight; then a reading is taken on the vernier scale and the tube is rolled by the amount of expansion indicated in column 10 of the log, checking it by the divisions of the vernier scale.

After rolling is completed, the inside diameter of the tube is measured with a gauge and entered in column 12 of the log. The figures in column 12 must be equal to the sum of columns 6 and 11.

The requirements for cleaning the ends of the tubes and the drum seats are the same as in medium-pressure boilers. Immediately before insertion, it is recommended to wipe the tube ends and seats with a rag moistened with a degreasing liquid (dichloroethane or trichloroethane). The diametral clearance of the tube in the seat must be not less than 0.5 mm and not more than 1 mm for all tube diameters.

The tubes must comply with MPTU 2579—1950.

Technical Documentation

Rolling of tubes in high-pressure boilers must be accompanied by the following technical documentation:

a) developed diagrams of each drum, indicating the rows of seats and the numbers of the seats in the row; on the developed diagrams, the seats and tubes that deviate from the standards both in dimensions and in the presence of deep scratches, etc., must be marked;

b) sketches of the tube seats of the drums, indicating their nominal dimensions;

c) data from chemical analysis and the mechanical properties of the tubes being rolled;

d) a report on the results of rolling trial specimens and technological tube tests, indicating the selected degree of rolling;

e) a log of rolling operations, separately for each drum.

Conclusions

1. The production of reliable rolled joints in high-pressure boilers (up to 120 atm) should be considered mastered.

2. The principal condition for obtaining high-quality rolled joints is the correct selection and objective, careful control of the degree of rolling of each joint.

3. The use of expanders with a vernier ensures precise control of the degree of rolling both with manual and with machine drive.

4. Further experimental work is necessary on the selection of steel for the cones and rollers of small-diameter expanders, with the aim of increasing their service life and developing a simpler objective method for controlling the degree of rolling, since the method described is very complicated, as it requires making numerous measurements of the tubes.