Steam Accumulator for EPS Plant

Steam Accumulator Working Principle

The working principle of a steam accumulator tank involves storing excess steam during periods of low demand and releasing it during periods of high demand. Here’s a step-by-step explanation of how it operates:

Steam Accumulator Components:

1. Pressure Vessel: A large, insulated cylindrical tank capable of withstanding high pressure.
2. Water: The vessel is partially filled with water.
3. Steam Inlet and Outlet: Pipes for introducing steam into and drawing steam from the accumulator.
4. Pressure and Temperature Controls: Devices to monitor and manage the pressure and temperature inside the vessel.

Phases of Operation:

1. Charging Phase:

• Excess Steam: When the boiler generates more steam than the process requires, the surplus steam is introduced into the accumulator.
• Condensation and Heating: The introduced steam condenses upon contact with the water in the accumulator, transferring its latent heat to the water. This process raises the temperature and pressure of the water in the vessel.
• Energy Storage: The energy from the excess steam is stored in the form of high-pressure, high-temperature water.

2. Storage Phase:

Maintaining Pressure: The insulated vessel keeps the water at high pressure and temperature until the steam is needed. The stored energy is in a stable state, with the water holding both sensible heat and latent heat from the condensed steam.

3. Discharging Phase:

• High Demand: When the steam demand exceeds the boiler’s capacity, the steam accumulator begins to release steam.
• Flashing: The high-pressure water in the accumulator flashes into steam as it is released. This occurs because the pressure drops as the steam exits, causing a portion of the water to rapidly vaporize.
• Steam Supply: The generated steam is then supplied to the process, supplementing the boiler output and meeting the increased demand.

Key Points:

• Pressure Regulation: The pressure inside the accumulator must be carefully regulated to ensure efficient storage and release of steam. Pressure and temperature controls help maintain optimal conditions.
• Heat Exchanger: Some steam accumulators may include internal heat exchangers to improve the efficiency of heat transfer during the charging and discharging phases.
• Safety Valves: Safety mechanisms are essential to prevent overpressure and ensure safe operation.

Benefits:

• Load Balancing: Smooths out fluctuations in steam demand, providing a consistent steam supply.
• Energy Efficiency: Utilizes surplus steam effectively, reducing waste and improving overall energy efficiency.
• System Stability: Helps maintain stable pressure and temperature conditions in the steam system, ensuring reliable operation of steam-dependent processes.

A steam accumulator works by storing energy in the form of high-pressure, high-temperature water during periods of low steam demand and releasing it as steam during periods of high demand. This process helps balance the load on the boiler, improves energy efficiency, and ensures a stable steam supply.

Steam Accumulator Sizing

Sizing a steam accumulator involves determining the required capacity to effectively store and release steam based on the specific needs of the steam system. The process considers factors such as steam demand, boiler capacity, and pressure requirements. Here’s a step-by-step guide to sizing a steam accumulator:

Step-by-Step Guide to Sizing a Steam Accumulator

Determine Steam Demand:

• Peak Demand (Q_peak): Identify the highest steam demand during peak usage periods.
• Average Demand (Q_avg): Calculate the average steam demand during normal operations.
• Duration of Peak Demand (t_peak): Measure the time duration for which the peak demand lasts.

Assess Boiler Capacity:

• Boiler Output (Q_boiler): Determine the steam generation capacity of the boiler.
• Available Excess Steam (Q_excess): Calculate the excess steam available during off-peak times. This is the difference between boiler output and average demand (Q_boiler – Q_avg).

Calculate Storage Requirements:

• Steam Deficit (Q_deficit): Calculate the steam deficit during peak periods as the difference between peak demand and boiler capacity (Q_peak – Q_boiler).
• Total Steam Deficit: Multiply the steam deficit by the duration of peak demand to get the total steam required (Q_deficit * t_peak).

Determine Accumulator Capacity:

• Accumulator Efficiency: Assume an efficiency factor for the accumulator (typically around 80-90%) to account for heat losses and other inefficiencies.
• Effective Steam Storage: Calculate the effective steam storage requirement by dividing the total steam deficit by the efficiency factor.

Volume Calculation:

• Pressure and Temperature: Establish the operating pressure and temperature of the steam accumulator. These parameters are crucial because the volume of water and steam changes with pressure and temperature.
• Volume of Water and Steam: Use steam tables or thermodynamic software to determine the specific volume of water and steam at the given pressure and temperature.
• Total Volume: The total volume of the accumulator tank is the sum of the volume required to store the water and the steam. Typically, a part of the accumulator is filled with water (usually around 50-75%), and the rest with steam.

Safety and Buffer Capacity:

• Safety Margin: Add a safety margin to the calculated volume to ensure reliable operation and accommodate unexpected demand fluctuations.
• Buffer Capacity: Ensure the accumulator has some buffer capacity to handle short-term variations and avoid overloading.

Example Calculation

Let’s consider a simplified example to illustrate the process:

Steam Demand:

• Peak Demand (Q_peak): 10,000 kg/h
• Average Demand (Q_avg): 7,000 kg/h
• Duration of Peak Demand (t_peak): 2 hours

Boiler Capacity:

• Boiler Output (Q_boiler): 8,000 kg/h

Calculate Storage Requirements:

• Steam Deficit (Q_deficit): 10,000 kg/h – 8,000 kg/h = 2,000 kg/h
• Total Steam Deficit: 2,000 kg/h * 2 h = 4,000 kg

Accumulator Efficiency:

• Assume 90% efficiency: Effective Steam Storage = 4,000 kg / 0.9 ≈ 4,444 kg

Volume Calculation:

• Operating Pressure: 10 bar
• Use steam tables to find specific volume at 10 bar (assuming water at 180°C and steam at 10 bar)
  • Specific volume of water (V_water): 0.001 m³/kg
  • Specific volume of steam (V_steam): 0.1 m³/kg
• Volume of Water: 4,444 kg * 0.001 m³/kg = 4.444 m³
• Volume of Steam: Assume 50% water, 50% steam: 4,444 kg * 0.1 m³/kg = 444.4 m³
• Total Volume = Volume of Water + Volume of Steam = 4.444 m³ + 444.4 m³ ≈ 448.8 m³

Safety and Buffer Capacity:

• Add 10% safety margin: 448.8 m³ * 1.1 ≈ 493.7 m³

Final Accumulator Size

The steam accumulator should have a total volume of approximately 494 m³ to manage the steam load in this example effectively.