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BLD-NUCL84-03 · Lesson 5 · 22 clock hours · demo

Feedwater and Condensate Systems

Objective. Students will describe the flow path and function of feedwater heaters, deaerator, condensate pumps, and feedwater pumps, and explain how feedwater temperature and flow rate affect reactor power and steam generator level control.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Feedwater and Condensate Systems

The feedwater and condensate system forms a critical part of the secondary side of a nuclear power plant. Its job is to return water from the condenser back to the steam generator (or reactor vessel in a boiling-water reactor) at the correct temperature, pressure, and flow rate. Understanding this system is essential for any operator or plant technician.

The Basic Flow Path

After steam does work in the turbine, it exhausts into the condenser, where cooling water removes heat and the steam condenses back into liquid water, called condensate. From here the flow path is:

1. Condensate pumps pull water from the condenser hotwell and push it forward at relatively low pressure.

2. The condensate passes through a series of low-pressure (LP) feedwater heaters, where extraction steam from intermediate turbine stages heats the water in stages.

3. The water then enters the deaerator, a combined feedwater heater and dissolved-gas removal device.

4. Feedwater pumps (also called booster or main feedwater pumps) raise the water to high pressure so it can enter the steam generator or reactor vessel.

5. The high-pressure water passes through high-pressure (HP) feedwater heaters for final heating before entering the steam generator.

Low-Pressure and High-Pressure Feedwater Heaters

Feedwater heaters are shell-and-tube heat exchangers. Extraction steam from the turbine flows on the shell side; feedwater flows through the tubes. By extracting steam at progressively higher pressures and using it to heat the feedwater in stages, the plant recovers energy that would otherwise be lost in the condenser. This process, called regenerative feedwater heating, improves overall plant thermal efficiency.

LP heaters operate below deaerator pressure. HP heaters operate above deaerator pressure, between the feedwater pumps and the steam generator. Each heater stage has a drain path that routes the condensed extraction steam (called heater drain) either forward to the next heater (drain cascading) or back to the condenser. Bypass valves allow operators to take individual heaters out of service without tripping the unit, though doing so reduces efficiency and must be accounted for in power calculations.

The Deaerator

The deaerator serves two purposes: it heats feedwater to saturation temperature at its operating pressure, and it mechanically removes dissolved oxygen and carbon dioxide from the water. Dissolved oxygen causes corrosion in feedwater piping and steam generators, so deaeration is critical for plant chemistry control. The deaerator is typically a large horizontal or vertical vessel located high in the plant to provide adequate net positive suction head (NPSH) for the feedwater pumps below it.

Feedwater Pumps and Condensate Pumps

Condensate pumps are lower-pressure, lower-flow devices that move water from the condenser hotwell to the deaerator. They are typically motor-driven. Feedwater pumps operate at much higher pressures and are often turbine-driven (using auxiliary steam) or motor-driven. Speed control of feedwater pumps is the primary means operators use to regulate feedwater flow and, therefore, steam generator water level.

Effect on Reactor Power and Steam Generator Level

Feedwater temperature directly affects steam generator level and reactor power. If feedwater temperature drops (for example, after a heater trip), cooler water enters the steam generator. Cooler water contracts, causing a momentary shrink in indicated steam generator level, even though actual mass inventory has not changed immediately. Operators must recognize this as a normal transient and avoid overfeeding, which could cause high-level trips.

Feedwater flow rate controls steam generator level directly: increasing pump speed raises level; decreasing speed lowers it. During power operations, feedwater flow and steam flow must be balanced continuously. A sustained mismatch causes level transients that can lead to high-level or low-level reactor trips. Low steam generator level is an important alarm because inadequate water inventory can uncover steam generator tubes, leading to loss of heat sink — a serious safety concern.

Safety Considerations

Lockout/tagout (LOTO): Before performing maintenance on any pump, heater, or valve in this system, follow plant LOTO procedures in accordance with OSHA 29 CFR 1910.147 to prevent unexpected energization. Feedwater pumps operate at very high pressures and temperatures — an uncontrolled release is a serious burn and high-pressure injection hazard.

PPE: When working near feedwater or steam lines, wear appropriate thermal protection and face shields. High-pressure hot water and steam flashing to vapor can cause severe burns.

Radiation: On the secondary side of a pressurized-water reactor, activity levels are normally low, but plant chemistry upsets can concentrate activity. Follow radiological work permits and monitor dose continuously. NRRPT-level knowledge of radiological controls applies whenever dose rates are elevated.

Alarm response discipline: When a low steam generator level alarm annunciates, use the written alarm response card. Do not take action from memory alone. Identify which feedwater pump is controlling level, verify flow instrumentation, and escalate per emergency operating procedures if level cannot be restored.

Key terms8

CondensateLiquid water formed when exhaust steam from the turbine is cooled and condensed in the condenser; the starting point of the feedwater flow path.
Feedwater heaterA shell-and-tube heat exchanger that uses extraction steam from the turbine to preheat feedwater, improving plant thermal efficiency through regenerative heating.
DeaeratorA vessel that heats feedwater to saturation temperature and mechanically removes dissolved oxygen and carbon dioxide to prevent corrosion in feedwater and steam generator systems.
Extraction steamSteam taken from intermediate stages of the turbine and routed to feedwater heaters; using it for heating rather than exhausting it to the condenser recovers energy and raises efficiency.
Shrink and swellApparent changes in steam generator water level caused by changes in temperature or pressure, not actual changes in water mass; shrink occurs when cooler feedwater enters, swell when pressure drops suddenly.
Net Positive Suction Head (NPSH)The margin of pressure above the vapor pressure of the liquid at the pump inlet; adequate NPSH prevents cavitation, which can damage pump impellers and reduce flow.
Heater drainThe condensed extraction steam that collects on the shell side of a feedwater heater and is routed either forward to the next lower-pressure heater or back to the condenser.
Regenerative feedwater heatingThe thermodynamic cycle practice of using turbine extraction steam to heat feedwater before it enters the steam generator, recovering heat that would otherwise be rejected in the condenser.

Worked examples4

1. A simplified enthalpy worksheet gives feedwater entering the steam generator at Case A: 420 °F with an enthalpy of 397 BTU/lbm, and at Case B: 350 °F with an enthalpy of 321 BTU/lbm. The steam generator produces steam with an enthalpy of 1,190 BTU/lbm. The feedwater flow rate is 1,000 lbm/hr in both cases. Calculate the heat added in the steam generator for each case and the difference.
  1. Step 1 – Identify the heat added formula: Q = ṁ × (h_steam − h_feedwater), where ṁ is mass flow rate, h_steam is steam enthalpy, and h_feedwater is entering feedwater enthalpy.
  2. Step 2 – Case A: Q_A = 1,000 lbm/hr × (1,190 − 397) BTU/lbm = 1,000 × 793 = 793,000 BTU/hr.
  3. Step 3 – Case B: Q_B = 1,000 lbm/hr × (1,190 − 321) BTU/lbm = 1,000 × 869 = 869,000 BTU/hr.
  4. Step 4 – Difference: Q_B − Q_A = 869,000 − 793,000 = 76,000 BTU/hr more heat must be added to the steam generator when feedwater is cooler (Case B). This means the reactor must supply more power for the same steam output, illustrating why feedwater heating improves efficiency.
Answer: Case A requires 793,000 BTU/hr; Case B requires 869,000 BTU/hr. Cooler feedwater forces the reactor to supply an additional 76,000 BTU/hr for the same steam production, confirming that regenerative feedwater heating reduces the heat burden on the reactor and improves thermal efficiency.
2. During a heater bypass exercise, the operator notes that bypassing the final HP heater drops feedwater temperature from 420 °F (enthalpy 397 BTU/lbm) to 350 °F (enthalpy 321 BTU/lbm). If full-power feedwater flow is 500,000 lbm/hr, what is the additional heat load placed on the steam generator by this bypass?
  1. Step 1 – Calculate the enthalpy difference per lbm: Δh = 397 − 321 = 76 BTU/lbm.
  2. Step 2 – Multiply by the mass flow rate: Additional heat load = 500,000 lbm/hr × 76 BTU/lbm = 38,000,000 BTU/hr.
  3. Step 3 – Convert to a more recognizable unit if desired: 38,000,000 BTU/hr ÷ 3,412 BTU/hr per kW ≈ 11,138 kW ≈ 11.1 MW of additional thermal load the reactor must supply.
Answer: Bypassing the HP heater imposes an additional 38,000,000 BTU/hr (approximately 11.1 MW thermal) on the steam generator, which the reactor must compensate for if steam output is to remain constant.
3. A condensate pump delivers water to the deaerator. The deaerator is located 40 feet above the pump inlet. If the water density is approximately 62.4 lbm/ft³, what is the static head pressure (in psi) the pump must overcome just to lift the water to the deaerator level? (Use: P = ρ × g × h, and 1 psi = 144 lbf/ft²)
  1. Step 1 – Calculate pressure in lbf/ft²: P = 62.4 lbm/ft³ × 40 ft = 2,496 lbf/ft² (using standard gravity, where lbm numerically equals lbf under standard conditions for this type of simplified head calculation).
  2. Step 2 – Convert to psi: P = 2,496 lbf/ft² ÷ 144 in²/ft² = 17.3 psi.
  3. Step 3 – Interpret: The condensate pump must produce at least 17.3 psi of discharge pressure just to overcome the static elevation difference, before accounting for friction losses or the deaerator's operating pressure.
Answer: The static head pressure to lift condensate 40 feet to the deaerator level is approximately 17.3 psi. Actual pump discharge pressure must be higher to also overcome pipe friction and the deaerator operating pressure.
4. An operator notices the steam generator level drops by 4 inches immediately after feedwater temperature decreases due to an LP heater trip. No feedwater flow change has occurred. Explain in a step-by-step analysis whether the operator should immediately increase feedwater pump speed to compensate.
  1. Step 1 – Identify the phenomenon: A sudden decrease in feedwater temperature causes the water already in the steam generator to contract slightly and reduces the amount of steam bubble fraction in the two-phase mixture. This is a 'shrink' transient — indicated level drops without an actual loss of water mass.
  2. Step 2 – Check actual mass balance: If feedwater flow equals steam flow (verified by instrumentation), actual inventory is balanced. The level drop is an instrument-indicated effect of the temperature change, not a real loss of inventory.
  3. Step 3 – Determine correct response: The operator should NOT immediately increase pump speed. Overcorrecting by adding feedwater will actually overfill the steam generator as the thermal transient stabilizes, potentially causing a high-level trip.
  4. Step 4 – Correct action: Monitor level trend, verify feedwater and steam flow rates are matched, follow the alarm response card, and allow level to recover naturally as the system reaches a new thermal equilibrium. Escalate only if level continues to fall despite balanced flows.
Answer: The operator should NOT increase feedwater pump speed immediately. The level drop is a 'shrink' artifact caused by thermal contraction and reduced void fraction, not a true loss of inventory. Overcorrecting risks a high-level trip. The correct response is to verify flow balance, monitor the trend, and follow the written alarm response procedure.
In class: Watch an instructor walkthrough of a generic feedwater system P&ID on the projector, then individually mark up a printed copy identifying each feedwater heater stage (low-pressure and high-pressure), the deaerator, drain paths, and bypass valves. · Calculate the heat transfer improvement achieved by feedwater heating using a simplified enthalpy worksheet, comparing feedwater entering the steam generator at two different temperatures to understand the efficiency benefit. · Role-play as an operator responding to a low steam generator level alarm: use a generic alarm response card to list immediate checks, identify which feedwater pump controls to verify, and state when to escalate to emergency procedures.
Materials: Generic feedwater system P&ID, enthalpy worksheet, steam tables, alarm response card template, projector
Check: Students correctly identify all labeled components on the marked-up P&ID and complete the enthalpy worksheet within 10% of the instructor answer key; pass mark 80% combined.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NUCP Uniform Curriculum — Plant Systems
1. What is the primary purpose of regenerative feedwater heating in a nuclear power plant?
NUCP Uniform Curriculum — Plant Systems
2. A deaerator is placed in the feedwater flow path for which two reasons?
NRC Reactor Operator License — Thermodynamics and Plant Systems
3. When an HP feedwater heater is bypassed at full power, what is the most likely immediate effect on reactor operation?
NRC Non-Licensed Operator Prep — Alarm Response and Procedures
4. A steam generator level alarm annunciates LOW. According to alarm response discipline, what should the operator do first?
NRC Reactor Operator License — Reactor Plant Transients
5. What phenomenon causes an apparent drop in steam generator indicated level when cooler feedwater suddenly enters the steam generator, even though no water has actually left the system?
OSHA 10 — Control of Hazardous Energy
6. Before a maintenance technician removes a feedwater pump for repair, which safety procedure must be completed to prevent unexpected energization of the pump?
NUCP Uniform Curriculum — Plant Systems
7. In the feedwater system, heater drain from a high-pressure feedwater heater can be routed in two ways. Which option correctly describes both?
NRRPT Radiation Protection — Radiological Work Control
8. A radiation protection technician finds elevated dose rates near a secondary-side feedwater line following a plant chemistry upset. What is the correct initial action?
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