Method of manufacturing glass-lined metal tubes

A method of manufacturing glass-lined tube. An undercoat of glass is fused onto the surfaces of nozzle-shaped flange-pieces, from their outer peripheral edges to their molded and curved inner faces, welding flange components then welded to either end of the metal tube to form a complete flange at each end. The inner faces of the weld zones are then shaped by grinding and polishing. A glass tube that is greater in length than the metal tube is fitted loosely into the latter tube. The glass tube is supported at either end within the wells of conical caps which are fitted around the flanges by air-permeable spacer-rings, with the ends of the glass tube extending beyond the outward faces of the flange joints and with the glass tube being securely positioned at the center line of the metal tube. The entire assembly is heated so that the glass tube will soften and expand under the influence of internal pressure, thus providing a continuous covering of the inner surface of the metal tube extending to the surface of the flange junction.

BACKGROUND OF THE INVENTION 
The invention concerns a method of manufacturing glass-lined metal tubes in 
which the inserted glass tube is formed into a continuous fused coating 
over the entire inner surface of the metal tube and over the surfaces of 
flanged joints. 
Glass-lined metal tubes are widely used in many fields of the chemical 
engineering industry for the transfer of corrosive fluids and for heat 
transfer applications owing to their combined characteristics of high 
resistance to corrosion inherent in the glass and the mechanical strength 
offered by the metal tube. Further advantages are seen in the lack of 
scaling as a result of the smoothness of the glass lining and the strength 
of the tubes under conditions of heat. 
Glass-lined metal tubes at present in general use fall into two main 
categories as far as the method adopted for the glass lining is concerned. 
The first method is one whereby the inner surface of the metal tube and 
the faces of flange joints are coated with a glazing agent which is then 
dried. The glazing agent is then calcined at high temperature to cause it 
to adhere to the surface of the metal. The process steps are repeated thus 
providing a multi-layer coating of agglutinated glass on the metal base. 
In the second method, a glass tube whose outer diameter is slightly 
smaller than the inner diameter of the metal tube is inserted into the 
latter, softened by heating and allowed to expand by normal air pressure 
so that it will become attached to the inner wall of the metal tube. 
Modifications of this method are described in U.S. Pat. Nos. 2,986,847 and 
3,235,290. 
The first of the above methods is a similar process to that which has been 
adopted for providing glass linings for containment vessels and it may 
also be used not only for straight tube sections but for valves, joints of 
various configurations, and components for pipe systems in which the 
diameters of the pipes and flanges are dissimilar. Because the shape of 
the flange faces is the same as the nozzle components of the glass lined 
containment vessels and because the glass coating is continuous up to the 
faces of the flange junctions, these types of glass-lined components are 
extensively used as accessories in such containment facilities. 
However, when this method is adopted for the manufacture of small diameter 
and long lengths of glass-lined metal tube, it is difficult to ensure 
total covering of the interior of straight pipe sections and there is also 
some difficulty in inspection and rectification. The process is also 
expensive in that several repetitions of the glazing and calcination steps 
are necessary to ensure that all pinholes have been eliminated. 
With the second method, on the other hand, the glass-lining process can be 
completed by a single heating stage in which the inserted glass tube is 
expanded and attached against the inner wall of the metal tube. There is 
no risk of the occurrence of pinholes in the glass lining in straight 
sections and there is also virtually no danger of damage occurring to the 
glass lining in straight sections when the tube is in use. The 
disadvantage, however, lies in the fact that there is a difficulty in 
extending the adhesion of the inserted glass tube to the flange faces and 
it is therefore the normal practice in this case for the lining to extend 
no further than the inner face of the flange. It is for that reason that 
many of the breakdowns in such tube systems caused by corrosion have, up 
to the present time, occurred at the flange joints. 
The type of glass-lined metal tube most commonly used at the present time 
is produced by the glass tube insertion method illustrated in FIG. 1 
below. In this case, a glass ring (3a), having a triangular cross-section, 
is inserted into an annular recess on the inner surface of the junction 
face (2) of the flange (1) so that, when it is heated and becomes soft, it 
will be fused with the end of the glass tube (5) which has been inserted 
into the metal tube (4). In this form of glass-lined metal tube, the glass 
material at the tube junctions will extend over only about one-half of the 
width of the flange-junction surface (2) so that, when the tubes are in 
service, permeation of corrosive fluid between those end faces (2) and 
gaskets will accelerate corrosion in the metallic parts of the periphery 
of the joints and will cause the glass ring (3a) to "float". This 
phenomenon is most marked in those locations where repeated heating and 
cooling makes it difficult to maintain the junction pressure at the flange 
faces and also in those locations where there are flanged junctions with, 
for example, glass-lined containment vessels in which there are curved 
sections in which the inner faces of the flanges have been molded or 
otherwise shaped. It is especially a problem in the case of joints with 
flanges of the type found in glass-lined containment vessels that, even if 
the curved inner faces of the flanges are precisely lined up with the 
above-mentioned glass rings (3a), and even when special gaskets are used, 
because of the virtual impossibility of ensuring a continuous surface of 
glass material at the joint area, this problem will not be entirely 
eliminated. 
Furthermore, with this type of glass-lined metal tube, it will be necessary 
to grind off any projecting glass material from the flange faces (2) after 
cooling and the labor involved will increase in proportion to the size of 
the tube's diameter, with the added problem that there will be a risk of 
residual cracking of the glass material at those surfaces. 
FIG. 2 illustrates an approach to the solution of the foregoing problems at 
the flange faces of this type of glass-lined metal tube. After applying a 
coating (3b) of either ceramic or glass material, by glazing and 
calcination, from the end face of the flange (1) to the tube interior, the 
inserted glass tube (5) is heated and softened so that its end will 
overlap the coating (3b) and, at the same time, adhere to the inner wall 
of the metal tube (4). With this approach, however, if the durability of 
the flange coating (3b) in a corrosive environment is to match the 
durability of the glass tube (5) that has been inserted in the metal 
sleeve, it will be necessary to repeat the glazing and calcination process 
of the flange coating (3b) at least four or five times in order to obtain 
the correct finish. This will lead to high costs. Furthermore, irregular 
breaking in the vicinity of curved portions on the inner sides of flange 
faces during cooling will expose the broken ends to corrosive fluids 
during service and there will be a risk of glass fragments being carried 
into the fluid. 
The object of the embodied invention is to sustain the advantages of the 
glass tube insertion method and, at the same time, to overcome the 
problems listed above in connection with the flange faces. 
SUMMARY OF THE INVENTION 
Briefly, the process consists of manufacturing glass-lined metal tubes 
where it is possible to obtain a continuous coating of glass material. 
Flanges, onto which an undercoating of glass has been fused to extend from 
the face of the flange to the curved inner surface, are welded at both 
ends of the metal tube. A conical cap is fixed in position at each flange 
by means of a ventilation spacer surrounding the outer face of the flange. 
The caps support both ends of a inserted glass tube and holds it at the 
center line of the metal tube. The whole assembly is heated to allow the 
inserted glass tube to soften and expand. The process requires fewer steps 
necessary to ensure that the glass coating extends fully from the inner 
wall of the metal tube to the outer face of the flange.

DETAILED DESCRIPTION OF THE INVENTION 
The following detailed description of the embodied invention is given in 
conjunction with the example of its preferred embodiment as illustrated in 
the FIG. 3. Nozzle shaped flanges (1) are subjected to sand-blasting or 
similar surface treatment. Then a single thin undercoating of glass (3) is 
applied, by glazing and calcination, to extend from the face of each 
flange (2) to the curved inner face of the flange (1). A flange is then 
welded to each end of the steel or other metal tube (4). By 
surface-finishing treatment by grinders or similar means, the inner 
surfaces of a welded region is smoothed and the metal tube (4) is made 
ready for insertion of the glass tube. 
In the next step, a glass tube (5), whose outer diameter is slightly 
smaller than the inner diameter of the metal tube (4) is inserted into the 
latter. The ends of the glass tube (5) are sealed and are of such a length 
as to project beyond the flange faces (2) at each end of the metal tube 
(4). Each end of the glass tube is seated in a central recess of a conical 
cap (7) against a section of padding material (6). A split flange (9), 
which encloses a spacer (8) encircling the outer perimeter of the flange 
face (2), is placed in position and secured to the flange assembly (1) by 
means of a nut and bolt (10). The above-mentioned padding material (6) is 
intended to regulate the space between the glass tube (5) and the cap (7) 
and to prevent damage to the glass tube (5) when the nut and bolt (10) are 
tightened. The role of the central recess of the cap (7) is both to hold 
the glass tube (5) at the center line of the metal tube (4) and to prevent 
abnormal expansion at the end of the glass tube (5) during the heating and 
softening steps. The above-mentioned spacer (8) is of an air-permeable and 
heat-resistant fiber material whose thickness can be adjusted. 
When the foregoing assembly has been completed, it is placed in a furnace 
and heated at a prescribed rate to a temperature at which the glass tube 
(5) will become soft. The temperature is maintained at this level for a 
fixed period. 
Both the flange (1) and the cap (7) have a skin thickness that is greater 
than the walls of the metal tube (4), so that the transmission of heat in 
those components will be slower, with the result that those parts of the 
glass tube (5) within the metal tube (4) will soften quicker than the 
glass outside of the tube. The expansion of the air trapped in the tube 
will force the glass into close contact with the inner surface of the 
metal tube (4) expelling the air between the glass tube (5) and the metal 
tube (4) through the air-permeable spacer (8) encircling the outer 
periphery of the flange where it is discharged to the atmosphere, thus 
avoiding any entrapment of air between the glass tube (5) and the metal 
tube (4) at their cohesion points. When both ends of the glass tube (5) 
are at the softening temperature, the parts of the glass tube (5) in those 
areas will fold and bend in conformity with the conical face of the cap 
(7) which is facing towards the flange face (2) and also with the curved 
surface on the inner side of the flange face (2). The result will be that 
those parts of the tube will expand in such a manner as to be drawn into 
the spacer (8) around the outer face of the flanges and that those parts 
of the tube that extend as far as the spacer (8) will undergo a 
deceleration in expansion at the same time as the remaining parts are 
continuing to expand, thus ensuring that there is no variation in the skin 
thickness of the coating of glass as a result of irregularity in the 
expansion of the glass tube (5) overall. Furthermore, there will be no 
interruption in the glass coating in the area extending to the flange face 
(2) and, in this manner it has been found possible to achieve continuity 
in the glass coating from the interior surfaces of the metal tube (4) to 
the flange faces (2). Owing to the fact that the previously applied 
undercoating of glass at the latter parts will become fused and integrated 
with the glass tube (5) and the integrated body of glass will then become 
securely adhered to the metallic base, there will be no risk of breaking 
or chipping the glass coating at the flange faces (2) when the tube 
assembly is subjected to cooling. Those parts of the glass tube (5) which 
project beyond the flange faces (2) will be fractured by natural process 
at the line of the peripheries of the flange faces (2) when the assembly 
is cooled, giving a smooth surface at the break. The absence of glass 
splinters or sharp projections obviates the necessity for grinding or 
similar finishing work after cooling. 
The foregoing example of the preferred embodiment of the invention has 
described a coating process in which a glass tube (5), sealed at each end, 
has been employed. It is, however, possible to use glass tubes that are 
open at one or at both ends and to provide an air intake aperature at the 
center recess of a cap at one end of the assembly. The open ends of the 
glass tube are sealed at the recesses of the caps. When heating is applied 
and the temperature of the glass tube has reached the softening point, 
compressed air can be passed to the interior of the glass tube through the 
air intake aperture in the cap. 
The above description has shown that, in the process embodying the 
invention, the separate stage of applying an undercoating of glass to the 
flange faces can be regarded as a means of reducing the number of 
production steps for glass-lined metal tubes and also of providing a 
satisfactory surface finish to the joint between the flanges and the metal 
tube after welding. Furthermore, the subsequent coating by the glass tube 
enables the entire protection process to be carried out in a single 
heating cycle. In addition, because the undercoating of glass is fused to 
the metal by a glazing and calcination process, there will be no 
separation of that undercoating when the glass tube is inserted neither 
will there be any formation of air bubbles in that area during the heating 
and coating process that follows, so that the glass tube and the 
undercoating will become fused together and both will be securely fixed to 
the metal of the tube. Moreover, there will be no breaking or chipping of 
the glass undercoating when the assembly is cooled. Further heating 
processes will be made in conditions where both ends of the glass tube are 
supported by the conical caps that are placed so as to face towards the 
flange faces, to enable uniform expansion of the tube to take place 
towards the spacer adjoining the outer face of the flange at each end, 
thus resulting in a positive coating over the entire flange face area. 
Post-cooling finishing operations, by grinding or similar means, are not 
required due to the fact that those parts of the glass tube that extend 
beyond the flange faces will be broken off, by natural process, in the 
cooling stage, leaving a smooth surface of glass around the outer 
peripheries of the flange faces. 
Glass-lined metal tubes that have been produced by the method and procedure 
as described above will be such that the glass tube will extend over the 
entire inner surface of the metal tube up to and including its flange 
faces. The adhesion of the glass to the metal will be secure and there 
will, therefore, be no danger of corrosion damage occurring at the flanges 
due to permeation of corrosive liquids at the joints between tube 
sections. The process is also suitable for application to glass-lined 
containment vessels and their fittings, owing to the flange faces being of 
the same configuration as the nozzle sections of those vessels. 
EXAMPLES OF PRACTICAL APPLICATION 
(1) Resistance to Thermal Impact 
Experimental samples of assemblies as illustrated in FIGS. 1 and 2, with 
nominal diameters of 50 mm each, together with an experimental sample of 
an assembly produced by the process embodied in the invention, with a 
nominal diameter of 80 mm, were each first held for 1 hour at the 
prescribed heating temperature and then flash-cooled by immersion in water 
at 30.degree.+0.5.degree. C. The temperature differences, T, at which 
glass fracturing first occured in each case (i.e., the difference between 
the maintained heating temperature of the sample and the temperature of 
the cooling water) were as follows: 
1. Experimental sample from FIG. 1: T=90.degree. C.; 
2. Experimental sample from FIG. 2: T=140.degree. C.; 
3. Experimental sample by invented process: T=180.degree. C.; 
(2) Resistance to Hot Water 
The same experimental samples were used and each was sealed at the ends 
with blanking covers to provide short sections of tube containing pure 
water (ion-exchange water). The samples were then placed in a 
constant-temperature drying oven for 96 hours at 120.degree. C., after 
which the condition of the glass surface was examined. 
1. Experimental sample from FIG. 1: Needle-like glass fragments observed in 
the liquid. 
2. Experimental sample from FIG. 2: Flaked fragments of glass observed in 
the fluid after 24 hours. 
3. Experimental sample by claimed process: No change or abnormality. 
In none of the above cases was any abnormality in the surface of the glass 
detectable with the naked eye.