Hydraulic analysis of cooling water systems for power plants

Introduction

Electric power is often generated by conversion of water into high-pressure steam that drive turbines. Once water has gone through this cycle, it is cooled and condensed back to water and then reheated to drive the turbines again. The process of condensation requires a separate cooling water system to absorb heat. The most common systems are cooling towers (“closed-in”) and intake – outfall systems (“once-through”).

The industry faces challenges such as:

• Heat exchangers need to be designed to maximize the surface area of the wall between the two fluids, while minimizing the resistance to the fluid flow through the heat exchanger.
• Air can enter the system via the intake or during transient situations. Even a small amount of air has a negative effect on the throughput capacity. Sudden release of air bubbles can generate unacceptable dynamic forces and vibrations of structures
• Cooling water systems may face operational problems due to insufficient water treatment, sediment ingress and seaweed accumulation. This leads to scale, corrosion and biological growth. As a result of these problems, system life time, efficiency and safety can be reduced.

Deltares’ services

Deltares offers research and specialised consultancy on the following aspects of cooling water systems:

• Environmental modelling including thermal and brine recirculation, see also intake – outfall systems
• Hydraulic design and optimization of pump sumps, intake and outfall structures
• Accurate head loss calculations of arbitrary pipeline, intake and outfall system in WANDA
• Computation of dynamic hydraulic forces
• Air entrainment, air accumulation, air removal
• Performance optimization
• Hydraulic design of control systems for a main condensers and emergency condensers
• Surge analysis for exceptional and emergency situations such as:

1. Full pump trip,
2. Single pump trip,
3. Emergency shut-down

• Surge analysis for normal and maintenance operation such as:

1. Scheduled start-up,
2. Scheduled shut-down,
3. Filling,
4. Switch-over,
5. Back-wash

• Design of anti-surge protection such as:

1. Air valves and vacuum breakers,
2. Surge vessels,
3. Valve opening and closure pattern optimization.

• Commissioning program
• Trouble shooting
• Advanced Computational Fluid Dynamics (CFD) modelling

Example for modelling of a cooling water system in WANDA

In the example below, the cooling water system consists of two running pumps. Each supply line is equipped with a check valve and a discharge flow control valve. The common discharge header is connected to two condenser shells. At the downstream side, the water feeds a common discharge culvert which returns water into the sealpit. Small process piping is not shown here for simplicity reasons.

A full pump trip is simulated. The graph depict selected results: pressure and cavitation volume versus time.

Tools

WANDA has been developed by Deltares (WL | Delft Hydraulics) and is the state-of-the-art simulation tool that supports all hydraulic decisions during the lifetime of a cooling water system. WANDA enables the simulation of emergency transients and normal operations with complex control systems. WANDA supports system calibration, such that the hydraulic performance of existing systems can be monitored and further improved.

The CONTROL module in WANDA allows to implement and evaluate sophisticated operational concepts.

The newly developed module “WANDA HEAT” makes it possible to calculate heat transfer and to monitor the temperatures in a liquid system. Fluid properties such as density, viscosity, vapour pressure and specific heat vary as a consequence of the variation in temperature. The temperature is influenced by the various heat fluxes, caused by transport due to flow, heat loss to the surroundings, heat transferred by dedicated components (such as heat exchangers) and heat generation by friction. Unsteady flow conditions such as valve closure and pump trip (water hammer) can be simulated. WANDA HEAT can be used to support the hydraulic design of heat exchangers