Industrial boilers normally produce steam at high pressures and temperatures, since in this superheated condition it carries more energy and results in more efficient production of power in the turbine. This high pressure also allows the steam to be transported around the plant in smaller lines with less heat loss. For use in a process plant, both the pressure and the temperature have to be reduced.
A letdown station solves this problem. It comprises a pressure reducing valve and the temperature reducing apparatus. The solution is to use a valve with a special trim that is engineered to solve the complex requirement of both pressure reduction and noise control. A Mitech energy dissipating disk stack that fits into the company’s range of standard Globe valves is one of the best methods to achieve this. The pressure is dropped across many stages. The disk stack combines the concept of many restrictions in series (along each passageway) with multiple paths in parallel (several passageways in each disk and many disks in a stack). This enables the valve to handle low flow rates as well as the maximum flows. One or more diffuser plates may be installed downstream of the valve to create back-pressure to enable a smaller valve to be used.
But, reducing the pressure of the superheated steam does very little to reduce the temperature. To reduce the temperature of the steam, a de-superheater is utilised to add water. To vaporise the water a large amount of energy is required to overcome the latent heat vaporisation. The high temperature steam provides this. The result is an increased flow of steam at a reduced temperature. By varying the amount of water added, the temperature can be controlled.
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