Letdown stations from PSV Mitech
March 2013
Valves, Actuators & Pump Control
Steam is generated in industrial boilers at high pressures and temperatures. This superheated condition results in the efficient production of power in the turbine and the high pressure allows the steam to be transported around the plant in smaller lines with less heat loss. However, in a process plant the pressure and temperature of the steam have to be reduced. This can be achieved with a letdown station, which has a pressure reducing valve and a desuperheater.
The letdown station has a valve with a special trim that meets the complex requirements of both pressure reduction and noise control. This can be achieved with a PSV Mitech energy dissipating disc stack that fits into its range of standard globe valves. The pressure is dropped across many stages. The disc stack has restrictions situated in series along each passageway with multiple paths in parallel. There are several passageways in each disc and many discs in a stack. This enables the valve to handle low flow rates as well as 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.
To reduce the temperature of the steam a desuperheater is used to add water to the superheated steam. By varying the amount of water added, the temperature can be controlled. The secret of success for any desuperheater is to ensure that the water atomises into a fine spray so that the steam can absorb the water droplets. If this does not happen the result is poor temperature control and cracked pipework downstream. PSV Mitech’s steam atomising desuperheaters use a source of high pressure steam. A small part of the total steam flow is bled off from the high pressure system and fed to a nozzle arrangement where low pressure water is injected into the high velocity steam flow. The velocity of the steam assists with the atomisation of the water and the result is a unit capable of a 30:1 turndown.
For more information contact Greg Walker, PSV Mitech, +27 (0)11 657 6000, [email protected], www.mitech.co.za
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