Alloy pipes and methods of making same

Details Alloy pipes and methods of making same Alloy pipes and methods of making same

1999-08-17

2001-06-26

Brinson, Patrick (Department: 3752)

Pipes and tubular conduits

Structure

C138S121000

Reexamination Certificate

active

06250340

ABSTRACT:

This invention relates to alloy pipes or tubes and to methods of manufacturing such tubes. In particular, the invention is concerned with tubes destined for the chemical processing industry and especially for petrochemical processing.
The world relies upon petrochemical plant to supply essential materials such as plastics, fertilisers and other chemical products that are part of modern life. The demand for increased output and efficiency, with reduced costs and pollution, means that plant design and operation requires constant attention and improvement.
Central or key areas of a plant, which can benefit from such improvements, are the main fired furnaces, for example, “Steam Cracker” furnaces, which utilise the pyrolysis cracking process to produce ethylene, and primary “Steam Reformer” furnaces, which produce hydrogen, possibly for subsequent conversion to ammonia, methanol etc.
These processes consume large amounts of energy (fuel and heat) and also expose the furnace materials, particularly the process tubes, to some of the most damaging environments in industry. Therefore, selection of the most advanced alloys is highly desirable in order to achieve the required improvements in service life, efficiency and performance.
Pyrolysis and reforming furnace technology is dependent upon one common element, the efficiency of the furnace tubes. Fired heaters must operate continuously at extremely high temperatures, for substantial periods of time. Typically a cracking furnace will operate in the range of 1050-1120° C., with an expected life of, for example, five to seven years. A steam reforming furnace, with an expected life of say 12 years, may typically operate with tube temperatures in the range 900-1000° C. By selecting the best alloy for the furnace tubes, several efficiency gains may be achieved with one decision.
The range of high temperature alloys required for fired heaters used within the petrochemical industry can sometimes cause confusion since the majority are not included in international specifications such as “ASTM” and “ASME”. In addition, these alloys are frequently known by their proprietary name. In reality there are a small family of alloys currently suitable for selection for fired heater tubes. Within this family, the major properties are influenced by the amount of important alloying elements present, and these can be summarised as follows:
NICKEL: Gives a stable austenitic structure, which contributes to both hot strength and good ductility. Nickel is also the principal element which reduces carburisation of the alloy and corrosion rates at high temperature.
CHROMIUM: Provides resistance to high temperature carburisation by the formation of an adherent tube surface film rich in chromium oxide. Chromium also contributes to high temperature strength through the formation of carbides.
CARBON: Is an austenitic stabiliser and by far the most important element for controlling hot strength and creep resistance. Carbon forms both primary interdendritic carbides, and, during service, precipitates the essential secondary carbides in the matrix, which reduce creep damage.
NIOBIUM: Improves the stability of the carbides improving creep strength and also improving weldability.
SILICON: Like chromium, silicon contributes to the formation of oxide films, which increase carburisation resistance.
There is a range of alloys available having various quantities of the above ingredients and, as a result, having varying qualities. However, typically, such alloys comprise the following constituents in the proportion indicated, the balance comprising iron:
Element
% by weight
Carbon
0.1-0.5
Chromium
20-35
Nickel
20-45
Niobium
0-2
Silicon
0-2
Tungsten
0-5
additions
0-1
The present invention is primarily concerned with such alloys which are hereby defined as “creep resistant alloys” in view of their resistance to creep. However, there are other materials such as cast superalloys, nickel based intermetallics and possibly even iron aluminides which may be suitable materials and hence are to be considered within the scope of the term “creep resistant alloys”.
An alternative definition of a “creep resistant alloy”, also within the ambit of the present invention, is to refer to its 100,000 hour mean stress rupture value, which is a value of most use to a furnace designer when selecting alloys and the dimensions to be employed. Thus in another aspect the invention is concerned with alloys which have a value of more than 6 MPa at 1000° C. in this test, and preferably greater than 10 MPa.
Since the excellent high temperature properties, high carbon content and cast grain size of carbon steel based creep resistant alloys reduce the plasticity and malleability of these alloys, such creep resistant alloys cannot be readily forged, and casting is essentially the only manufacturing method which can be used to produce the required tubes and fittings. The same is true of other creep resistant alloys. Consequently it is only with cast creep resistant alloys that the present invention is concerned.
The two major furnace types (steam cracking—ethylene, and steam reforming—hydrogen) each present different problems and each can be considered separately in order to show how optimum alloy selection can provide significant benefits to operators and designers.
1. Steam Cracker Furnaces
Pyrolysis furnaces basically consist of an insulated box containing tubular coils that enter and exit through the furnace wall, providing a flowpath through the furnace. Since the required pyrolysis reactions (conversion of steam and hydrocarbons into ethylene) are endothermic, burners located on the side and/or bottom of the furnace heat the outside surface of the coils. Whilst a few furnaces use horizontal coils, the great majority use coils mounted vertically.
Feedstock (such as naphtha, liquid propane gas or ethane) is mixed with steam and passed into the furnace under pressure (around 2-5 Kg/cm
2
). Generally an upper, convection section pre-heats the incoming feedstock by convection of heat from the burners in a lower radiant zone. Once through the convection section the feedstock enters a pyrolysis or radiant section. This main section consists of tubes which are typically 12-14 metres long and diameters ranging from 50-100 mm. In the majority of designs these tubes are arranged vertically. They are heated by the furnace burners to 950-1150° C., transferring sufficient heat through the tubes to the feedstock to break it down into ethylene. During operation, a coke layer will build up inside the hotter sections of the coils, and this coke must be removed by burning the coke away with de-coke operations performed at required intervals. The coil alloy itself will become carburised when in contact with the deposited coke and with the feedstock during high temperature operations, and this process will become more severe with time, dramatically affecting alloy properties. The production of ethylene is therefore one of the most aggressive environments to which alloys can be exposed.
Optimum performance is required in various areas to resist several possible damage mechanisms, which include carburisation, thermal cycling, creep damage and coke build-up. Carburisation will reduce alloy properties such as ductility, creep strength, and weldability. Resistance to carburisation can be improved by generating a protective coating on the internal surface of the coils, and using tubes with a higher alloy content. Silicon and chromium contribute to developing a protective oxide film, although this may be damaged during de-coke operations or thermal cycles and should be restored where possible. Increasing the nickel content is an effective method of inhibiting carbon absorption.
2. Steam Reformer (catalyst) Furnaces
For the reformer furnace designer or operator the problems are slightly different, but similar efficiency benefits are possible through optimum alloy selection. As with pyrolysis furnaces, several different designs of furnace exist, but the demands on the reformer tube alloy are the same. Primary steam reforming furnaces a

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