Section 1:Identifyif the statement is TRUE (“T”)orFALSE (“F”).

1.The Feedheating system raises the pressure and the temperature of the condensate and returns it as feedwater to the steam generators. / T / F
2.The temperature of the incoming feedwater is raised to the saturation point in the Preheater that is an integral part of the Steam Generator. / T / F
3.The steam that leaves the steam drum and flows to the high pressure turbine has a moisture content of at least 1%. / T / F
4.The only purpose of taking Extraction Steam from the High and Low Pressure Turbines is to heat the feedwater in order to optimize the thermo-dynamic efficiency of the steam and feedwater cycle. / T / F
5.Safety Valves installed on top of the steam generators protect the steam system from over pressure. / T / F
6.In order to ensure that the turbine is protected from water carry-over, the turbine will be automatically tripped if the level in the steam generators goes too high. / T / F
7.The High Steam Generator Level alarm will result in a Turbine Runback so as to protect the Turbine from potential carry-over of water droplets that could damage the blades of the turbine. / T / F
8.The Low Steam Generator Level alarm will result in a Reactor Setback or Stepback (depending on station design) in anticipation of a turbine trip. / T / F
9.Control of steam generator level close to its set-point is achieved by using a “three element” control algorithm, which includes steam flow, feedwater flow and steam generator level measurements. / T / F
10.The automatic transfer of the large feedwater valve that is selected to control feedwater flow will take place whenever the Steam Generator Level activates the Low Level alarm. / T / F

Section 2: Identify the one correct statement in each group of four by inserting the appropriate letter on the line ______.

1.Which one of the following statements is correct?

  1. If steam pressure rises above predetermined limits, the Atmospheric and/or the Condenser Steam Discharge Valves are opened by the Steam Generator Pressure Control System.
  2. The quick closure of the Emergency Stop valves is sufficient to prevent the turbine from over-speeding.
  3. Safety Valves installed on top of the steam generators are the first line of defense protecting the steam system from overpressure.
  4. The main purpose of the Governor Valves is to control the speed of rotation of the turbine.

2.Which one of the following statements is correct?

  1. In the high pressure turbine the latent heat of the steam is converted to rotational energy, and in this process the moisture content of the steam increases to about 20%.
  2. The steam outlet from the HP Turbine stage passes to the Moisture Separator which removes the moisture in the steam and raises its temperature.
  3. In the Reheater steam taken directly from the Steam Generator is used to raise the pressure and temperature of the steam from the Moisture Separator to 230C and 900 kPa, which correspond to superheated conditions.
  4. All the steam that passes through the Low Pressure Turbine is exhausted to one Condenser at approximately 5 kPa(a), 35⁰C and 10% moisture content.

3.Which one of the following statements is correct?

  1. Under normal operating conditions the steam generator pressure will need to rise 70kPa above its setpoint before the CSDVs begin to open.
  2. Atmospheric Steam Discharge Valves are opened by the steam pressure control program in proportion to the pressure error, normally with an offset of 70 kPa in the steam pressure setpoint.
  3. There are 12 CSDVs in a typical CANDU unit, connected in such a way that all the valves discharge steam to one condenser shell.
  4. All CANDU units have Condenser Steam Discharge Valves that allow the steam to bypass the turbine and flow directly to the condenser when the turbine is unable to accept the steam flow.

4.Which one of the following statements is correct?

  1. The feedwater system raises the temperature of the feedwater flowing from the condenser to the steam generators by heating it in the Low Pressure and High Pressure Heaters.
  2. The Condensate Extraction Pump (CEP) pumps the water from the Condenser Hotwell through the Low Pressure Heaters to the Preheater.
  3. The LP feedheaters use extraction steam from the HP turbines as their heating medium; the extraction steam condenses in the shell of the heater, and is pumped back to the condenser hotwell.
  4. The temperature of the Condensate is raised to about 170C by the heat added in the Low Pressure Heaters, the Deaerator and the High Pressure Heaters.

5.Which one of the following statements is correct?

  1. The Steam Generator Feed Pumps take their suction from the Deaerator Storage Tank, and having raised the feedwater pressure to between 4 and 7 MPa, send the feedwater flows directly to the Preheaters of the respective Steam Generators.
  2. Depending on the station design, one or two auxiliary steam generator feed pump is provided to supply feedwater flow to supplement the pressure that can be provided by the main pumps.
  3. Connections to the Condensate system allow for recirculating flow when the steam generator feed pumps are operating but the level control valves are closed.
  4. The High Pressure Feedheaters use Extraction Steam from the High Pressure Turbine to raise the pressure of the feedwater above the pressure produced by the steam generator feed pumps.

Section 3.Provide short answers to the following questions:

3.1Steam Generator Level Control Valve Failure Modes

The steam generator level control valves that regulate the flow of feedwater into the steam generators have the failure modes given below, for the cases when the pressurized air that normally operates them is not available:

Explain the response of the unit and this choice of failure modes in each of the following cases:

a)the large (100%FP flow capacity) valves fail to the closed position

  1. the small (18%FP flow capacity) valves fail open
  1. the air supply to all the steam generator level control valves fails

3.2 Steam Generator Tube Break

  1. Explain how the CANDU station design helps to identify a pinhole tube leak inside the Steam Generator?
  1. What would be the major concerns in continuing to operate a unit under these conditions at or near full power?
  1. How would the identification and operating concerns change if the leak developed into a single tube break?
  1. What particular precautions would need to be followed while shutting down such a unit?

3.3 Pressure and Level Control

The following simplified diagrams illustrate the devices involved in pressure and level control. Assume that the unit is operating in ALTERNATE Mode.

(1) for the Heat Transport System in NORMAL Mode using the Pressurizer and

(2) for the Steam Generator.

Simplified CANDU Main Heat Transport System Simplified CANDU Steam Generator System

a)Similarities of Pressure Control between the two systems:

b)Difference in Pressure Control between the two systems:

c)Similarities of Level Control between the two systems:

d)Difference in Level Control between the two systems: