2.3 System configuration part 2
Considering the functions of the feedwater isolating valves under normal conditions of operation, water flows from the boiler feed pumps through the heaters to the boiler. The by-pass is in the closed position, as the spring-loaded valve is set such that it does not commence to open until a heater bank is isolated on the water side. Assuming that the feed flow is to be proportioned so that 60% flows through the remaining active bank and 40% through the by-pass, the redistribution for a typical pair of HP heaters is shown in Fig 3.20.
Representative friction losses for each heater are used to demonstrate how the head loss across the spring-loaded by-pass valve is determined. Should the pressure loss across the by-pass valve be larger than needed, the flow through the active bank will increase to balance the pressure loss, which leads to excessive steam flows. The larger steam flows could lead to damage of the external tube surface from greater than design steam velocities within the heater. The topic of steam-flow-induced damage to heater tubes is discussed in Section 6.3 of this chapter. There is also the possibility of erosion damage to the internal surface of the tubes from high velocity feedwater due to the excessive flow through the active bank.
The spring-loaded by-pass system is the simplest by-pass system in current use; but in the past, difficulties in setting and maintenance of the by-pass valves led to the use of alternative designs of by-pass system. Systems using motorised parallel slide valves for both HP heater isolation and by-pass have been provided. This requires a complicated control sequence to be provided, using many electrical relays and valve limit switches. A typical isolation sequence would be:
- Open by-pass valve.
- When by-pass valve is fully open, allow sequence to start closing the isolating valves. (A system which allows by-pass and heater isolating valves to act in antiphase has also been used.)
- Close isolating valves.
Opening the isolating valves is the reverse of the closing sequence:
- Open isolating valves.
- When isolating valves are fully open, allow sequence to start closing the by-pass valve.
- Shut by-pass valve.
In the event of sequence malfunction, it is possible to have both isolating and by-pass valves shut at the same time. To allow an alternative path to the boiler, an emergency by-pass spring-loaded valve is sometimes provided to act if all normal routes to the boiler are isolated. The system as described above is expensive to provide and maintain, and so the simple spring-loaded by-pass system has been adopted as current practice with improved methods of setting and mainlining the spring-loaded by-pass valves.
Another principle in current use is to utilise the energy in the high pressure feedwater to close the eater isolating valves. The use of feedwater energy medium-actuated valves is not an original idea but the method by which it is achieved is new. There were two reasons why medium-actuated valves were considered: reliability and speed of actuation.
Motor-operated parallel slide valves of the size required for HP heater isolation duty have a minimum closing time of about 20 seconds. The rate of flooding of certain designs of vertical HP heater for the 500 MW units, assuming a double-ended tube failure and blocked drains, is about 25 mm/s. With this rate of flooding, the time for the water to rise from the normal working level (NWL) to the bottom of the bled-steam connection is about 8 s. To provide a margin, a factor of 2 is used to allow for the valve speed of closure increasing due to operational factors and uncertainties in calculation of flooding rates. To meet these needs, valves with a closure time of about 4 s are required; however, the flooding time of vertical HP heaters can be extended by making the steam pipework loop vertically to a suitable level before leading it to the turbine. The shell volume is then available to contain the flooding water up to the level of the top of the bled-steam pipe loop, as shown in Fig 3.27.