13.4.2 Steam-to-steam reheaters - part 3
The tube length or support has to be arranged to ensure that the natural frequency of the tube bundles is clear of machine frequency, low harmonics of machine frequency, flow-induced Karman vortex shedding frequencies and acoustic standing wave frequencies.
Tube diameters are normally chosen to ensure that the tube natural frequencies are clear of aeroelastic frequencies, but tube first-mode natural frequencies have been found close to acoustic standing wave frequencies. In an attempt to avoid this, horizontal support plates adjacent to and within the tube element are sometimes split and wedged apart with spacer bars to brace the tubes against the tube support plates. The plates must be fitted with care to ensure that no damage occurs to the tube fins on assembly. By adjusting the spacing of these plates, the vibrational characteristics of the tube can be arranged to be dear of damaging vibrations, whether mechanically or flow-induced.
On-load detection of tube leakage can be determined by isolating each tube element and comparing the internal 'settle-out' pressure with the prevailing shell-side pressure.
On Canadian units, in the event of any tube leakage, the particular tube element is isolated until it can be replaced by a spare element at a convenient outage: the leaking element tubes are then plugged. Tube elements are removed by taking off the shell cover, cutting the tube-side steam supply and drain pipes (live or bled), and lifting out the tube element. The replacement of a tube element is simply the reverse of element removal.
A problem with vertical reheaters is that the maximum height of the crane hook has to be sufficient to lift the heating elements out vertically. To reduce the amount of cutting during dismantling, the live and bled-steam pipework normally enters the shell through the side wall rather than through the shell cover. This pipework from the shell wall to the element inlet headers, and the drain pipework from the bottom headers through the domed base, has to be sufficiently flexible to permit relative thermal expansion of pipework, heater elements and shell. Occasionally, expansion bends are incorporated to provide the required flexibility.
Manholes are normally provided for inspection of the reheater internals and the vessel is generally rigidly supported on its own foundation from the basement floor. Shell-side relief is normally accommodated on vertical reheaters by a combination of bursting diaphragms and pressure relief valves in the hot reheat lines between the reheater and the interceptor valves.