1.7 Boiler feed pump turbine governors
On CEGB 660 MW turbine-generator systems, it is normal practice to use an auxiliary back-pressure turbine to drive the main feed pump. The feed pump is required to maintain the boiler drum level during steam raising prior to start-up of the main turbine, and also over the full load range of the main turbine.
The precise arrangements for achieving feed flow control are outside the scope of this chapter, but the part played by the boiler feed pump turbine (BFPT) is to drive the pump at a speed such as to maintain a constant pressure drop across the feed regulating valves which follow the pump. The feed flow may then be controlled by setting the feed valve position. This requires a variable-speed turbine whose speed or steam valve position demand may be set by the feed control system.
The normal exhaust route for the BFPT is to the IP/LP crossover pipe of the main turbine, although at low loads it will be routed through to the condenser, automatic changeover from one exhaust to the other being sensed by a load-dependent parameter of the main turbine — usually crossover pressure.
The steam supply to the BFPT may be derived from one or other of two sources, or from a mixture of the two. Normally, when the main turbine is on-load, the second-stage blading of the BFPT is supplied with steam from the main turbine HP exhaust via bled-steam emergency and governing valves. Either one pair, or two pairs in parallel, of these valves are fitted. Because of non-availability of steam from the main turbine HP exhaust prior to start-up and when the main turbine is operating at low loads, the first-stage blading of the BFPT may be supplied with boiler steam through a single pair of live steam emergency and governing valves. In the event of non-availability of the bled-steam supply at full-load on the main turbine, the live steam valves will supply sufficient steam to enable full-load to be maintained.
The nominal maximum power rating of such a boiler feed pump turbine is about 15 MW and the possibility of a load rejection leading to a potential overspeed must be considered. The loss of load could occur as a result of loss of feed pump suction. If this occurred with the turbine speed and power output close to the full-load value, an extremely rapid acceleration would result due to the low rotor inertia. In the unlikely event that the turbine did overspeed to destruction, the strength of the casing would contain the disaster and protect personnel. However, it is normal practice to follow main turbine practice and fit the BFPT with a three-channel electronic governor, coupled with separate over-speed protection.
The turbine governor is a modular arrangement using many of the same elements applied to the main turbine. Thus, speed-sensing and on-load testing facilities will be similar in principle. The emergency valves upstream of the governing valves provide protection shutdowns of the BFPT in the event of failure of more than one governor channel or in the event of a mechanical trip condition (e.g., loss of lubricating oil) being sensed. The valve relay systems use similar servo-valves to those fitted to the main turbine and the following stages of hydraulic operation are often similar in principle to those of the main turbine. It has sometimes been possible to omit the pilot stage associated with the main turbine valve relays and to drive the power piston direct with the servo-valve. Referring back to Fig 2.24, this would be equivalent to considering the relay to comprise only the servo-valve and primary ram. In all cases, the hydraulic fluid is supplied from the main turbine system. Live and bled-steam governing valves are controlled in parallel, with an offset being applied to the live steam valves so that they only begin to open when the bled-steam valves are fully open on a speed loop droop of about 5%. Any additional speed error, requiring further steam supply, progressively opens the live steam valves until they are also fully open.
During start-up of the main turbine, the steam is supplied via the live steam valve. A common means of ensuring this automatically is to superimpose on the bled-steam channel a limited maximum opening characteristic as a function of bled-steam pressure, as shown in Fig 2.32. This will keep the bled-steam valves closed until the HP exhaust pressure has risen to the value at which it can sustain the feed flow demand via the BFPT.
These features and others, engineered by techniques similar to those of the main turbine governor, enable convenient interfaces to be provided for automatic feed control systems and automatic run-up.