Method for manufacturing a compound pipe

A method of preparing a compound pipe, the wall of which comprises an inner layer of a first material and an outer layer of a second material. Powders of the two materials are isostatically hot pressed to form a cylindrical tubular blank in which the first material forms an inner layer and the second material forms an outer layer, the blank after heating to a suitable temperature being extruded over a mandrel.

The invention relates to a method for manufacturing a compound pipe the 
wall of which comprises an inner layer of a first material and an outer 
layer of a second material. 
In furnaces for pressurized fluidized bed combustion (PFBC), water pipes 
for generating steam or hot water are provided in the bed. These pipes are 
exposed to great wear (erosion) due to the fact that during the operation 
of the furnace the pipes are continuously blasted with the particulate 
material forming the bed. The pipes which are under high pressure must be 
made of a pressure vessel material, i.e. a material having great 
toughness, and such material is not at the same time a wear resistant 
material. Therefore, it is customary to provide on the pipes a wear 
resistant (brittle) surface layer consisting of a metal or metal alloy 
other than that from which the pipes are made, or of a metal oxide, and 
according to known technique this layer is sprayed onto the pipes 
consisting of pressure vessel material. 
The purpose of the invention is to provide the wear layer on the outside of 
the pipes or on the inside thereof already at the manufacture of the 
pipes. There is a demand for such pipes also for transport of bed material 
or ash between different fluidized beds or between a bed and a receiving 
vessel for bed material or ash. It is the intention to provide by the 
invention a uniform layer of the wear material, which is integrated with 
the pressure vessel material such that a long life will be imparted to the 
pipes. 
According to the invention said purpose is achieved by the method referred 
to above having obtained the features of claim 1.

A casing 10 for isostatic hot pressing, forming an annular cavity which is 
divided by means of a partition 11 into two co-axial compartments, one 
inner compartment and one outer compartment, the inner compartment is 
filled with a first powder 12 and the outer compartment is filled with a 
second powder 13. The powder 12 can consist of a pressure vessel material, 
for example an austenic stainless steel or a nickel based alloy, the 
powder 13 then consisting of a metal or metal alloy which has 
distinguished wear resistance, for example the material described in 
SE-C-8500773-0. The two materials can also be reversed in manufacturing a 
transport pipe of the kind referred to above. When choosing a tube 
material the yield strength of the materials should be taken into account 
as will be discussed in more detail below. 
When the casing has been filled and evacuated, isostatic hot pressing takes 
place at the stage indicated at 14 in the drawing, and it is obtained by 
this step a tubular blank 15, the wall of which comprises an inner layer 
of pressure vessel material and an outer layer of wear resistant material. 
In the following stage 16 this blank is preheated in an oven in order that 
the blank then in stage 17 shall be coated with glass powder 18 on the 
outside thereof and the blank at this coating shall have such a high 
temperature that the glass powder will melt and form a glass layer on the 
outside of the blank. 
A following stage 19 comprises induction heating of the blank to a suitable 
temperature for extrusion of the blank and this temperature should be of 
the order of 1000.degree.-1100.degree. C. In this connection it should be 
mentioned that the two materials should be chosen such that the materials 
at the necessary extrusion temperature have substantially the same yield 
strength. 
When the blank thus prepared has reached the necessary temperature it is 
supplied to an extruder 20 to be extruded over a mandrel 21 providing a 
compound pipe 22, FIG. 2, having an inner layer 23 of pressure vessel 
material and an outer layer 24 of resistant material. During the extrusion 
the glass layer applied functions as a lubricant.