Types of waste-heat boilers in steam reforming plants
Given these harsher process conditions, i.e. high temperature, high pressure, and high gradients of both, construction and manufacturing are more challenging. Therein, a larger factor lies on the tube-to-tubesheet welding, i.e. the connection of each single tube-to-tubesheet. Conceptionally, two types can be distinguished by their operation mode: water-tube boilers, and fire-tube boilers. In water-tube boilers, the hot gas is on the shell side, which is outside the tubes and the cooling water is inside the tubes of the heat exchanger/ boiler. It is vice-versa for the fire-tube boiler . The main difference in manufacturing and operation: The fire-tube boiler is more complex to manufacture because of higher stresses in the tube-to-tubesheet connection, but has advantages in the operation as the water evaporation can be more adaptive to the load when on the shell side. Still, both types are demanding regarding the construction and manufacturing process.
A main criterion for safe operation: Material selection for tube-to-tubesheet connections
To withstand harsh process conditions it is important to select the proper material, especially the tube, tubesheet, and the welding filler material. The tube and tubesheet material thereby is chosen according to the gas and process demand and guided by regulations, for example, the American Petroleum Institute standards API-941. In so-called hydrogen service, such materials, especially the tubes and the plates, are susceptible to hydrogen embrittlement and bubble formation, which can lead to stress crack formation or even surface decarburization at high temperatures. Due to their high temperature properties, low-alloyed Cr-Mo carbon steels are suitable for use in waste-heat boilers.
Generally, in addition to the standardized material testing (hot tensile test and charpy impact test), a simulation heat treatment is carried out and a new metallurgical test is conducted.
The second criterion is the suitable welding filler material: the Cr- and Mo content of the filler material must meet the specifications of the base material.
Process development of automated tube-to-tubesheet welding connections for waste-heat recovery boilers in ammonia plants.
The prime goal in the process development and welding process is a 100% reproducibility, at the lowest error rate (zero error strategy). The tube-to-tubesheet welding is carried out with a fully automatic tungsten inert gas welding process (TIG orbital welding). In addition, hazards, such as hydrogen embrittlement or stress corrosion (SCC), for example, are minimized by defining optimal tube welding details, as determined by means of FEM analysis of the tube-to-tubesheet connections. The first step is to determine the boundary conditions, such as the operating data, materials used and welding seam geometry. The next step is to check the weldability and feasibility of the geometry using an automated TIG orbital welding head. A 100% guarantee of the welding head position for the welding of every tube is the most important step to ensure controlled, fully automated tube-to-tubesheet welding. Thus, the constructive design of a clamping and centering system for the orbital welding head is crucial. Once the welding geometry has been determined and the orbital clamps have been precisely positioned, test specimens for trial welds are prepared. For the tests, at least three to four test series with at least ten tube-to-tubesheet welds each are carried out. The metallurgical and metallographic evaluations of the test welds are used to determine the exact values for the feed speed of the filler metal, the current strength and the geometric adjustment of the orbital welding head.
This empirical determination of the welding parameters is decisive for the development of reliable tube-to-tubesheet welding for use in waste-heat boilers in ammonia plants. From a manufacturing point of view, care must be taken to avoid the formation of stress cracks. The majority of defects in tube welding stems from the mechanism of hydrogen-assisted SCC.
In order to reduce the risk of cracking, a metallographic examination and a chemical analysis of each of the work samples must be carried out on in order to rule out any welding defects in advance.
Practical experience on the part of ACE: Examples of the production of operationally reliable tube-to-tubesheet connections:
The highest priority when manufacturing a waste-heat boiler is the production of a safe tube-to-tubesheet connection. Besides optimizing the seam geometry for fully automated tube-to-tubesheet welding, additional requirements, such as repair options in the event of damage, a new design due to increased system performance, or the incorporation of specific customer requirements, also play a significant role. Experience has shown that the most important step in this process is the successful evaluation of the test welds produced in order to actually use the tube-to-tubesheet welding on the apparatus. The following examples reflect the proven designs of waste-heat boilers, namely the water-tube boiler and the fire-tube boiler.