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componenTs

Figure 11a: Open position Figure 11b: Control position Figure 11c: Closed position

Turbines in nuclear power plants are operated with saturated
steam or only very slightly superheated steam.
“This leads to condensate in the turbine housing and pipelines
during operation that must constantly be removed

The opening sequence of a GESTRA ZK 313, rature sensor, which consists of bimetallic plates.
shown in Figure 10 a-c, illustrates the way the
double seat functions. The outer valve plug ini- Phase 1:
tially rises from the valve seat, while the inner val- When the system starts up and upon the for-
ve plug only follows according to a specified lift mation of cold condensate and air, the bimetallic
and releases the flow. At the moment of opening steel plates lie flat upon each other. The opera-
or closing, the flow speed at the main seat is ting pressure acts in the direction of opening, so
zero, which prevents blasting wear. The double that the valve is opened fully, see Figure 11a.
seat has proven itself even at differential pres-
sures of over 500 bar, which can occur in recir- Phase 2:
culation control valves, for example. Here too, As the temperature of the condensate rises,
all internal components, including the seat, are the bimetallic plates deflect and pull the nozzle
replaceable or allow for reworking with subse- needle to the closed position. The operating
quent grinding-in. pressure opposes this thermal process. This
As described above, the design ensures a long state of equilibrium between the temperature-
service life and tight closing. Chrome-plated ste- induced closing and pressure-induced opening
els such as 1.4112, which are partially hardened forces determines the outlet cross section.
in the seat area, have proven themselves as seat
material. Phase 3:
Just below saturation temperature (boiling tem-
Steam traps perature), the bimetallic plates are deflected to
such an extent that the nozzle needle is in the
Steam traps are medium-controlled control val- closed position and the steam trap is closed.
ves that open upon the formation of condensa- The column of these plates acts like a spring,
te and close after the drainage process before whose characteristic is adjusted to that of the
steam passes through them. There are several saturated steam line. This way, thermal and
different steam trap systems with varying fun- spring properties are coordinated in such a way
ctional principles. Power plants generally use that the opening and closing temperatures are
steam traps with a float or bimetallic regulator. always just below boiling point. This prevents a
The latter are used more often and their functio- loss of steam, even in the hot steam lines, as the
nal principle is described in more detail below. steam trap always closes below the respective
saturated steam temperature. These steam trap
Functional principle of the BK-type bimetal- types can therefore also be used for drainage in
lic regulator supercritical steam lines.
The way that a modern bimetallic regulator fun-
ctions can be divided into three phases. The
opening and closing process is controlled by the
interaction of the nozzle needle and the tempe-

64 64 Impiantistica Italiana - Maggio-Giugno 2019