What is the purpose of installing regular drains in superheated steam pipes?
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According to specification 5054, the steam should be in a superheated state; nevertheless, regular drain valves are still required to be installed in the main steam pipes. After checking various online sources, it was found that: 1. Heat is lost during steam transmission, causing the temperature to drop and water to condense. Personally, I think this claim might be incorrect; since the steam entering the turbine is required to be superheated, how is it possible for it to cool down to a saturated state? 2. Used to warm the pipes of the boiler when starting it up. Since a start-up drain is specifically provided for starting up the heating pipes, why is a regular drain needed as well? 3. In the event of a superheater failure, the bypass bypasses the superheater, allowing saturated steam to enter the main steam pipeline directly. I checked the boiler drawings we use regularly; there are no superheater bypasses, only temperature-reduction spray nozzles. I’m not sure if my understanding is correct; I hope to get an answer
Steam loses heat through radiation in the pipes, causing its temperature to drop rapidly. Insulation alone cannot ensure that the steam remains in a superheated state at all times; therefore, its temperature will inevitably drop to that of saturated steam, resulting in some condensation. To prevent water hammer, it is necessary to install a drain valve
If the superheat of the superheated steam is too high, it cannot be condensed; there is no need for frequent water drainage, and the specifications are incorrect
Taking our actual project as an example, the superheater outlet contains superheated steam at 4.0 MPA and 400°C; the saturation temperature of steam at 4.0 MPA is only 250°C. The distance from the main steam pipeline to the turbine inlet is at most 100 meters – how is it possible for so much heat to be lost, resulting in the steam becoming saturated? Moreover, the rated steam at the turbine inlet is 3.8 MPa and 390°C – how can saturated steam be present in that case?
Theoretically, there should be no condensate, but in practice, due to insufficient local insulation, condensation can occur in certain sections of the steam pipes. Additionally, at the expansion joints of these pipes, changes in flow direction lead to the accumulation of condensate; traps are used to prevent water hammer
There is still a bit of a difference between theory and practice; the insulation isn’t good enough, and there are faults
No matter how much overheating occurs, insulation is necessary on the outside! When the insulation effect does not meet expectations (or when 100% insulation is simply not possible), heat loss becomes a problem... Condensation will then gradually form... Therefore, drainage is still necessary