i want to support downcomers in which the system has no fixed point at all?

downcomers connecting from waterdrum to furnace headers, waterdrum supported from steamdrum through the tube bank.

steamdrum was supported with u clamps.

furnace headers supported on top by rod hangers.

plz help me how to solve this problem

Would you please post a sketch reflecting what you explained above.

Water drum, steam drum and tubes between these drums, and the furnace are the saturation temperature of the steam for the drum operating pressure. So, you know the operating temperature. Tubes between drums may have some flexibility but it was already expended by the addition of water drum weight during the construction before downcomer attachment, ignore the water weight although it is substantial for the simple analysis.

Simple method:

Calculate the thermal expansion from the support location to the steam drum to the centreline of the water drum. From the centreline to the connection of the downcomer you can create a nozzle connection by going through the CII procedure, and model the downcomer(s). Assign the thermal expansion of the centreline of water drum. In case you have the downcomer(s) eccentrically located to the drum/furnace centreline you'd better make model to suit that.

I do not know what type of boiler you are working on; sometimes you need to model the thermal expansion of the furnace at the other end of the downcomer(s).

In reality the drum system, downcomers and the furnace are at the same operating temperature. If the system support points are at the same elevation (normally this is expected) vertically and horizontally, there will not be an expected stress due to vertical and horizontal expansion.

However, I would consider that there is not any thermal expansion between water drum and the furnace in horizontal (in two directions), but consider the downcomer thermal expansion at saturating temperature. This is conservative but in reality this can happen due to the boiler support arrangement and connectivity of the steam drum to the furnace. This will introduce bending at the connection(s) of the downcomer(s) in addition to the self- weight force.

I do not want to go into the details, and I guess you can figure out the rest.

I believe you will have some more question in your mind. You need to solve those with the boiler mechanical/structural design engineer in the office.

Hope this helps,

Ibrahim Demir