Ventilator Valve

In case of load rejection or turbine trip, the turbine stop valve close immediately and the HP by-pass system takes over the steam flow.

To avoid overheating of the still rotating HP turbine, the turbine must be depressurized immediately. The turbine control system usually trigger the turbine ventilator valve to open and will evacuate the trapped steam to the condenser.


Method and apparatus to limit and control rotational loss heating such as occurs in a large steam turbine in the bypass mode of operation under no-load and low-load operating conditions. According to the invention, a portion of the high pressure bypass steam is admitted to the lower pressure sections of the turbine to provide motive fluid for driving the turbine while, simultaneously, a second portion of the high-pressure bypass steam is admitted to the high-pressure section of the turbine in a reverse-flow direction to pass backwards therethrough and limit the rotational loss heating.

The two flows may be proportioned to control rotational loss heating in both the high-pressure and lower pressure sections of the turbine. A reverse-flow valve and a ventilator valve are provided for routing the reverse-flow of steam


In the event of a load rejection, or following a trip after carrying load, the high pressure section of an opposed flow turbine such as that shown in Figure above may
overheat due to windage losses: such losses can occur
as a result of being allowed to spin in the high pressure, high temperature steam, bottled up between the main stop valve and the reheat stop valve.


As the turbine increases speed above the rated value in this high
density steam, rotational losses quickly raise the temperature of the buckets and related parts. The combination of overspeed (higher stress levels) and
increased temperature (lower strength) may cause
damage to these parts.
The design stress capability of materials typically
used for buckets and covers can decrease as much as
50 percent or more with a temperature increase from
700°F to only 1000°F. It is therefore apparent that in a very short time many parts and thin sections in the turbine could be experiencing some degree of distress if
there were no provision for ventilation.

To alleviate this problem, a ventilator valve is incorporated in the turbine piping arrangement, as shown
schematically in Figure above. When the ventilator valve
is automatically opened following a trip out, the high
pressure steam trapped in the reheater and high pressure turbine flows in a reverse direction through the
turbine to the condenser in response to the large pressure differential. When this happens, it is the relatively cooler steam from the reheater system that
maintains the high pressure turbine parts at reasonable temperatures.

The ventilator valve is of balanced design to allow the
use of a small operating mechanism. Once again, the
operator is an air piston which gets its signal from an
air valve on the speed relay. Thus, when the control
valves close, the ventilating valve opens, allowing
steam to flow to the condenser.

Figure above illustrates what happens to the flows and
temperatures in a high pressure turbine when the ventilator valve opens. Upon loss of load and trip out, the
trapped steam would be quickly heated to excessive
temperature if there were no ventilator valve. However, with a quick opening ventilator valve, the reverse
flow keeps the high pressure turbine last stage at essentially normal exhaust temperature. As friction
losses increase the temperature of the steam as it
progresses toward the first stage, protection is provided against both excessive heating and abnormal
cooling.

(sorry.. I don't remember where I put this from)