How to switch the main feedwater circuit for firing the furnace

During the unit startup process, when the high-pressure heater bypass is switched to operate on the main circuit, before feeding water to the boiler, we can fill and drain the water in the main circuit bypass at the same time. After starting the electric pump, water can be fed to the boiler directly through the main circuit of the high-pressure heater. This approach avoids the need for switching between the bypass and the main circuit, thereby preventing incidents related to low feedwater flow rates. I highly recommend this method. If, after the electric pump starts, water is supplied to the boiler via the high-pressure heater bypass, it is best to make the switch before the boiler is ignited. This prevents the occurrence of a low feedwater flow rate, which could trigger an MFT and force the boiler to be purged and restarted, thereby affecting the startup timeline of the unit. When performing the operation of switching from the high-pressure heater bypass to the main circuit, it is necessary to strictly follow the regulations specified in the turbine procedures for filling and draining water from the main circuit of the high-pressure heater. This is especially important when there are maintenance tasks on the water inlet side of the high-pressure heater, or when the steam pump or electric pump is started up after maintenance – in such cases, it is essential to fill and drain water from the water inlet side of the high-pressure heater. Slightly open the feed water valve on the high-pressure heater side; once water is seen coming out of the drain valve on the water side at the highest level of the high-pressure heater, close that drain valve. When the pressure on the feed water side of the high-pressure heater equals the pressure in the main feed water pipe, close the feed water valve for the high-pressure heater. Check that the pressure on that side does not drop and that there is no leakage from the high-pressure heater. Open the electric valve at the high-temperature heater outlet, and then open the electric valve at the high-temperature heater inlet; pay attention to any changes in the water supply flow rate. There are no pressure measurement points on the high-pressure water side of our plant; it is recommended to install them. Before opening the electric valve at the high-pressure heater outlet, it is necessary to inform the boiler operator to prepare for potential accidents. At the same time, the output of the electric pump can be increased to raise the main feedwater flow rate, thereby reducing the likelihood of low feedwater flow. If a low feedwater flow rate actually occurs, the opening of the feedwater control valve should be increased immediately, as well as the opening of the electric pump’s scoop tube, in order to restore the feedwater flow rate as quickly as possible. During the boiler water filling process, since the deaerator is at no pressure, the pressure at the inlet of the electric pump is low. When increasing the output of the electric pump rapidly, it is necessary to ensure that the pressure at its inlet remains above 0.5 Mpa, in order to prevent the pump’s overload protection from being triggered and thus avoiding the escalation of an accident. However, sometimes a low feedwater flow rate can also occur when switching from the bypass circuit to the main circuit after adding water to the high-pressure heater; we must pay special attention to this.

Can’t the main feed water be switched over when the load is at 30%?