A. Main Feed Pump Controls
1. Main feed pump constant header pressure control system (Figure 1) - The control system consists of a hydraulic system which uses lube oil as a control medium, and a pneumatic system which utilizes air as a control medium. A pneumatic bellows, which actuates the turbine control system in response to pneumatic signal from the feedwater system, is mounted on each turbine. The pneumatic control components for the three main feed pumps are located in a control panel in each fireroom, with automatic/manual stations on the combustion control console.
2. Component identification and function:
a. Feedwater header pressure controller:
(1) The feedwater header pressure controller is a Hagan master sender.
(2) The beam, which is affected by the proportional band bellows, reset bellows and high pressure bellows, actuates the escapement valve.
(3) The high pressure bellows senses feedwater header pressure and exerts a force on the beam.
(4) The set point spring opposes the force exerted by the high pressure bellows and adjusts output pressure to maintain setpoint.
b. Signal range modifier - a Hagan totalizer with a spring attached at chamber three. The range modifier sends an air signal of 15 psig when the output is 0 psig. As the input increases, the output increases such that, when the input reaches 60 psig, the output is 47.8 psig. The range modifier in this application establishes compatibility between the normal 0‑60 psig range and that required by the feed pump.
c. Two-way automatic/manual control station - allows the operator to select automatic or manual operation only. There is no means of modifying the automatic signal as with the four way stations installed on the forced draft blowers. One is installed for each main feed pump. Output goes to teh bellows on the main feed pump turbine control operator.
3. Effects of increase in demand:
a. The Three Element Feedwater Control system opens the feed control valve, feeding the boiler in response to the increased demand.
b. Feed header pressure decreases.
c. Feed header pressure controller output decreases.
d. Signal range modifier decrease input gives decrease output.
e. Automatic/manual transfer station passes signal through in automatic.
f. Decrease input to main feed pump causes increase in revolutions per minute (rpms).
g. Increased RPM causes rise in feed header pressure.
4. Effects of a decrease in demand are opposite of an increase in demand.
5. Mechanical regulator constant pressure system
a. In a case where an automatic system that uses air pressure is not utilized, a mechanical regulating valve is installed to control the speed of the main feed pumps. This valve is a steam control valve that regulates the amount of steam to the turbine of the main feed pump thus increasing or decreasing the system pressure.
b. A pressure sensing line provides a path between the common discharge line of the feed system and a sensing diaphram on the regulating valve. Working system pressure acts on the valves diaphram and positions the steam valve to regulate the MFP(s) speed. Instead of using control air pressure, this system operates in response to hydralic pressure from the feed system.
FEED HEADER PRESSURE CONTROL SYSTEM
Figure 1