Level and Steam Temperature Controls

The valve body size is frequently determined not only on the basis of the required valve capacity (C ), but also by considering the maximum acceptable flow velocity (approximately 30 ft/s [9 m/s]). 

Steam temperature is usually controlled with desuperheating valves installed between the first and second superheaters, similarly to the arrange- 

Three-element feedwater system, plus density compensation with drum pressure. 

Condensate preheater (economizer) temperature control is unique to HRSGs and is not used on conventional fired boilers. Its purpose is to (1) prevent external condensation on the tubes of the preheater, which could cause external corrosion and (2) to obtain the correct inlet temperature to the deaerator. 

The correct inlet temperature for the deaerator is obtained by mixing the hot condensate downstream of the preheater with cold condensate. This inlet temperature is maintained by a temperature controller around the preheater, which detects the mixed condensate s inlet temperature to the deaerator and is set at a little lower (15°F [8°C]) than the boiling temperature. 

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