8.3 Construction of low pressure heaters
The construction of vertical and horizontal LP heaters is very similar. The following descriptions are for horizontal heaters but any significant points of dissimilarity between horizontal and vertical heaters are included.
Figures 3.60 and 3.61 show arrangements of two typical U-tube two-flow LP heaters as manufactured by NEIP and GEC, respectively.
The maximum head that the condenser extraction pump can generate occurs at the no flow condition and is sometimes called the 'closed valve head'. The LP heaters are designed on the feedwater side to withstand the extraction pump 'closed valve head'. On a typical 660 MW unit this is a pressure of about 40 bar. Because of this modest feed water pressure (compared with HP heaters), LP heaters can be designed to take full feed flow without the resultant tube bundle diameter leading to an unacceptably thick tubeplate.
The general form of the LP heater is similar to HP heaters but, because the feedwater side pressure is modest, the water header can be of cylindrical design with a dished end. The shell side pressures are also modest, so again the shell is cylindrical in section with a welded dished end. A fixed and a sliding foot is provided to support the heater. The shell, tubeplate and water header are all made of mild steel.
An all-welded construction is used and it is accepted that in the unlikely event of access being required to the heater internals, the shell will have to be removed by cutting close to the back of the tubeplate. The tubes are roller-expanded into the tubeplate.
The tubes of LP heaters may be of 70/30 brass or stainless steel as dictated by steam temperature or boiler feedwater chemistry requirements. Brass may be used in LP heaters where the steam temperature is not greater than 150°C. Above this temperature stainless steel is used. For AGR feed systems, where full flow polishing plant is needed, copper-bearing materials should not be employed downstream of the polishing plant: so stainless steel is used for all LP heater tubes, including the drain cooler and TMEC.
Until 1976, LP heaters were designed to comply with BS1500; heaters specified since that date have been constructed to BS5500. The heaters shown in Figs 3.60 and 3.61 are constructed to comply with BS5500 . The methods used to determine the physical size of an LP heater to fulfil a particular duty, as defined by feed cycle needs, is very similar to that illustrated for HP heaters in Section 6 of this chapter. Using the same order of calculation as for HP heaters, the details are shown in the next section.