The HP and IP flange heating systems are brought into operation by the remote manual opening, from the central control room, of the motorised isolating valves. Each cylinder flange heating system then operates automatically and no further action should be required until the system is shut down by reclosing the motorised isolating valves. The system may be shut down when a load of 40% or more has been reached or, alternatively, when the cylinder barrel and flanges are heated to about 250°C.
The flow of steam is controlled by preset regulating valves and orifices. These valves, in each cylinder supply line, are sized in conjunction with their corresponding flange outlet orifice plates to give maximum pressure in the flanges. This pressure is approximately equal to the pressures on the inner surface of the respective outer cylinder under full-load conditions. As each cylinder flange heating supply is taken from after the governing valves, any change in the cylinder inlet steam pressure has a corresponding effect on both flange and cylinder steam pressures. As a result, rapid heating due to condensation heating is maintained at the same relative levels, thus enabling the difference in temperature between the casing and the flange to be maintained within acceptable limits.
Suitable steam distribution is achieved using balancing orifices installed and set during commissioning. Once the correct orifice size has been determined, it remains unaltered.
With the system as described above, the flange heating steam temperature is always higher than that of the steam within the cylinder casing. In some designs, however, the steam supply is taken from inside the relevant casing and hence its temperature is always equal to that of the steam within the casing.