Comprehensive Industrial Water Systems and Heat Exchange Operations

Fire Water and Initial Screening Systems

• Fire water represents a unique utility system because it typically only requires screening for solid removal.
• While other systems require control of algae, bacteria, and scale, fire water does not prioritize these because they do not interfere with the immediate goal of extinguishing a fire.
• Screening is essential for fire water systems to protect the pumps from damage.
• Screens are the first physical barrier in the water treatment sequence and are designed to catch large debris such as sticks, stones, or chihuahuas.

Boiler Feedwater Treatment Sequence

• The complete progression from raw water to boiler-ready water follows a specific sequence intended to remove varying levels of contaminants:
  1. Raw Water Intake
  2. Screening (Debris Removal)
  3. Clarifier (Gravity-based Settling)
  4. Gravity Sand Filter (Fine Particle Removal)
  5. Demineralization (Removal of Dissolved Inorganic Salts)
  6. Deaerator (Removal of Dissolved Gases like oxygen and carbon dioxide)
  7. Boiler Feed Water Discharge

Operational Safety: Interlocks

• Interlocks are safety systems that automatically prevent unsafe conditions or shut down equipment in response to a malfunction. They link specific parameters or conditions to equipment actions.
• High Limit Switch Interlocks: These trigger when a parameter exceeds a set safety threshold.
  • Example: A blow dryer that shuts off when a temperature limit is reached. The device will not restart until the temperature drops below the set limit, regardless of resets.
  • Example: Vibration sensors on a compressor. If vibration exceeds the high limit, the compressor will trip (shut down) to prevent structural damage.
• Permissive Interlock: A condition that must be met before a process can begin.
  • Example: A car that will not start unless the driver’s foot is on the brake or clutch.
• Co-requisite Interlock: A dependency where two conditions must occur simultaneously for a process to continue.
  • Example: A dishwasher that stops instantly if the door is opened during a cycle and cannot be started while the door is open.

Fire Water Distribution and Sprinkler Mechanics

• Jockey Pumps: These are specialized fire water pumps used to maintain pressure on the fire water supply header.
• Supply Header: This line must remain continuously pressurized and filled with water to ensure immediate distribution to fire cannons and sprinklers.
• Jockey Pump Interlocks: These pumps are interlocked with the header pressure. If pressure drops below a setpoint, the pumps kick on automatically. Multiple pumps may start sequentially as pressure continues to drop.
• Backup Systems: Fire water pumps are often equipped with diesel engines or generators to ensure operation during power outages.
• Sprinkler Heads:
  • Most heads feature a small glass bulb filled with a glycerin-based liquid and a small air bubble for expansion.
  • Heat causes the liquid to expand and shatter the glass, allowing pressurized water to flow through the deflector.
  • Deflector: A small metal plate at the end of the sprinkler body that creates a specific spray pattern to maximize water coverage.
  • Pendant Sprinkler Head: The most common type, installed hanging down from ceiling pipes.
  • Glass Bulb Sizes: Common sizes include the $5\,\text{mm}$ diameter for standard response elements.
• Fire Cannons and Cooling: Systems may be used to spray water on nearby tanks that are not yet on fire to prevent the heat from igniting their contents.

Clarifier Operation and Chemical Treatment

• Clarifiers use the principle of sedimentation to separate contaminants from liquid. Large tanks provide the necessary residence (retention) time to slow water down and allow particles to settle.
• Coagulation: The addition of chemicals known as coagulants to cause small, suspended particles (like algae) to clump together.
• Flocculation: The process where coagulated particles settle at the bottom of the vessel as sludge.
• Mechanical Components:
  • Energy Dissipating Inlet/Feed Well: Used to slow incoming water (called MLSS or Mixed Liquor Suspended Solids).
  • Scraper Mechanism: Arms that rotate slowly to collect sludge at the bottom and direct it to a centrally located removal pipe.
  • Overflow Weirs: Located at the top of the tank where the clarified (clear) water is released.
  • Rotating Skimmer: Removes fats, oils, and grease from the water surface.
• Sludge Disposal:
  • RAS (Return Activated Sludge): Sludge that is recycled back into the process.
  • WAS (Waste Activated Sludge): Sludge that is removed as waste.
• Lamellar Clarifiers/Tube Settlers: Use inclined plates inside the tank to increase the settling surface area and efficiency.
• State Point Analysis (SPA): A method for evaluating clarifier operation using a diagram that tracks:
  • Solid Concentration on the x-axis.
  • Solid Flux Rate ($\text{mass/hour/area}$) on the y-axis.
  • The intersection of overflow and underflow curves must stay below the settling curve for effective operation.

Gravity Sand Filtration and Backwashing

• Gravity sand filters utilize sand beds held in place by gravity to catch small particles that bypassed the clarifier.
• Backwashing (Backflushing): The process of reversing water flow through the filter to clear accumulated debris from the sand.
• Instrumentation: Operators monitor the Differential Pressure ($\Delta P$) to determine when a filter is clogged. As debris builds up, the upstream pressure rises and the downstream pressure drops, causing an increase in $\Delta P$.
• Backwash Valving Sequence:
  1. Close the inlet and the clean water outlet.
  2. Open the backwash drain and the backwash supply line to reverse flow.
  3. Rinse the trapped debris out through the drain.

Cooling Tower Mechanics and Maintenance

• Purpose: Cooling towers remove heat from water through evaporation. They are used in conjunction with heat exchangers to cool process fluids.
• Fill: Material inside the tower that breaks water streams into small droplets to maximize surface area contact with air.
• Basin: The bottom area that holds cold water. It is a breeding ground for algae and bacteria if the water becomes stagnant.
• Legionnaires' Disease: Caused by Legionella bacteria which thrive in warm, stagnant water. This is a primary safety concern for cooling tower operators.
• Blowdown: The intentional removal of water from the tower to control the concentration of total dissolved solids (TDS), scale, and prevent stagnation.
• Slipsstream Filter: A small line (often filtering $5\%$ of flow) where makeup water, biocides, acid, and caustic (for pH control) are injected to maintain water quality.
• Environmental Factors: High atmospheric humidity reduces cooling tower efficiency because the air is already saturated and cannot absorb as much evaporated moisture.

Heat Exchangers: Principles and Fouling

• Heat Transfer Without Mixing: Heat exchangers transfer thermal energy between two fluids (e.g., process fluid and cooling water) without permitting them to mix.
• Condenser: A specific type of heat exchanger that cools a vapor until it reaches its dew point (the temperature where vapor turns to liquid).
• Normal Operations:
  • Low Differential Pressure ($\Delta P$) across the exchangers (minimal restriction).
  • High Temperature Difference ($\Delta T$) across the exchangers (maximum heat transfer efficiency).
• Fouling: The buildup of scale, algae, or dirt on the surface of the exchanger tubes.
  • Fouling acts as insulation, which decreases heat transfer efficiency ($\Delta T$ drops).
  • Fouling restricts flow, which increases $\Delta P$ (Inlet pressure rises, Outlet pressure drops).
• Backflushing Exchangers: To clear fouling, operators reverse the flow by utilizing the pressure from the return header.
  • Step 1: Open the backflush drain line on the inlet side.
  • Step 2: Close the inlet isolation block valve.
  • Step 3: High pressure from the return header forces water backwards through the tubes to the drain.

Questions & Discussion

Q: What do you have to do in order to turn your vehicle on?A: You have to have your foot on the brake or the clutch. This is a permissive interlock.

Q: How often is the blowdown line open?A: The frequency is dependent on laboratory samples testing for pH, conductivity, and biocides. In regions like Louisiana, it is often kept open nearly continuously.

Q: What happens if it is super humid outside?A: The cooling tower is not as efficient because the air is saturated and cannot absorb more moisture to facilitate evaporative cooling.

Q: If the process gets too hot in a cooling system, what does the valve do?A: The temperature control valve (TCV) will open to allow more cold water through the exchanger.

Q: How can you tell which side is the shell side and which is the tube side on an exchanger?A: You look for the channel head and the bolt-up points. The tube inlet and outlet are always in the channel head.

Q: What do we look at to see if the cooling tower is working efficiently?A: Compare the temperature of the water entering the tower ($TI-101$) with the temperature of the water leaving the tower ($TI-102$). The leaving water should be significantly colder.

Q: What instrumentation tells us it is time to change a filter?A: An increase in differential pressure at $PDI-100$ indicates the filter element is clogged.