Article excerpt from the August 2017 issue of Modern Casting by Geoffrey D. Korff and Travis B. Stewart.
Steam turbines are built using a variety of castings ranging from standard class 25 gray iron to 400 series stainless steels. A typical single-stage turbine can contain up to 21 castings of different variations for the base design, while a typical multi-stage turbine can contain 30 castings or more. This does not include any castings that would be used for add-on equipment, such as trip and throttle valves and aftermarket governors or for other equipment on the equipment train.
The casting material is determined based on the temperature, pressure, environment, and steam conditions for the turbine application. In many cases, casting materials are interchangeable depending on these factors. For example, iron castings may be used in some applications for their thermal properties and ductility/dampening, while stainless steels may be used in other applications where it is needed for corrosion resistance. Material selection is important not only for the operation of the unit, but also for servicing the unit. Depending on the operating temperatures, there may be requirements for J-Factor calculations to avoid embrittlement in the time vs. temperature designs. Charpy impact testing may be required for low temperature service. For turbines, this is mostly environment-based and not dependent on operating temperatures.
The design of turbine castings has changed substantially since the 1970s, when everything was typically made from iron. Those castings were big, bulky, and over-engineered to build-in high safety factors. Contemporary designs are more precisely engineered with tighter specifications and closely calculated safety factors validated by FMEA models. Turbomachinery manufacturer Elliott Group (Jeannette, Pennsylvania) has seen an increase in requirements such as radiography, ultrasonic testing and requirements for 3.2 level certifications. This caused Elliott to look at new ways to validate the process of producing its castings so they can be ordered without the requirements up front but still meet them on the back side.
One example is a cast diaphragm that is created by casting ductile iron around a series of stainless steel vanes.
The vanes direct and redirect the flow-off steam passing through a multi-stage steam turbine. These diaphragms can be used to generate a large amount of power or to increase efficiency. The current design of the part has been in existence for over 35 years and has undergone only minor changes since its conception date. The chemistry of the iron has been the only adjustment made to the actual part.
The primary reason for changing the chemistry was the natural transfers that occur between the iron and stainless steel. These transfers cause a lack of fusion between the dissimilar materials and create large hard spots that hamper machining. Because the pouring temperature plays a large role in the transfer, Elliott Group worked to define a lower pouring temperature and used the vanes as a natural chill. The modified chemistry, along with a controlled pouring temperature, eliminated the lack of fusion and hard spots, giving the casting a uniform surface that is easily machined.
Designing a Sand Core Solution
The process for making the basic shape of the cast diaphragm is relatively simple. Bringing the core to life is a different story.
When the turbine diaphragm design was first conceived and produced, the core was difficult to make, requiring a burdensome manual process. The initial design used a corebox with a spacer that was indexed to create the vane spacing. The spacer was built in to a handle that was mounted in the center of the corebox. As each vane was placed, the handle was moved and secured into a hole with a dowel that was incorporated into the handle. The sand was packed by hand. Using this method to make the core required the foundry to use oil sand due to the long production process.
The biggest downfall to the corebox method was the required total area calculation was inconsistent and virtually impossible to achieve. Hand-packed sand meant there was nothing to maintain the spacing of the vanes after the spacer was indexed. Depending on how hard the core maker packed the sand, the vanes could easily shift both in position and angle. Since the surface area was always well over the defined limits, the vanes had to be bent back into position after the casting layout was completed. This caused more problems than the simple distortion of the vane throat area. Because it takes two diaphragm halves to create a full rotation, the castings have to be laid out to define the split.
The first vanes on each half must be machined to marry up when assembled. With the issue of the vanes moving during coremaking, this was often very difficult to achieve. Surface machining was also often an issue because the surface was defined by the height of the highest vane and vane height tolerance exceptions were common. If the vane was too high, the material around it would peel away, causing the part to be scrapped. Another downfall to the corebox method was the rotation was not interchangeable. The corebox was only good for either clockwise or counter clockwise rotation. As demand increased and new throat sizes were needed, it became apparent Elliott needed new ways to develop the cores in a more efficient and consistent manner.
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