3B1 Chapter 9: Boiler Procedures Study Guide

Core Principles of Boiler Procedures

Boilers must be operated strictly within the limitations of their specific design. Professional boiler procedures establish the essential framework for proper treatment and define the required steps for the entire lifecycle of the equipment, including commissioning, storage, operation, startup, shutdown, inspection, and cleaning.

Following these procedures, even for routine tasks, is a fundamental requirement of an operator’s due diligence. This ensures safe and efficient plant operation while protecting a significant capital investment.

Objective 1: Commissioning a New or Repaired Boiler

Before a new boiler or one that has undergone major repairs is placed into service, five critical steps must be completed to ensure the safety of personnel, the public, the environment, and the equipment itself.

1. External and Internal Inspection

External Inspection Tasks

Leak Testing: Running the forced draft (FD) fan to check for potential air or flue gas leaks.
Safety Systems: Ensuring safety and relief valve discharge outlets are piped to safe locations to prevent injury or property damage.
Instrumentation: Verifying that all controls and instruments are installed, operational, and calibrated for normal use.
Passage Inspection: Checking furnace and flue gas passages to ensure all debris, ladders, tools, and scaffolding have been removed.
Control Verification: Testing fans, dampers, and fuel-burning equipment before closing access openings to the furnace and breeches.

Internal Inspection Tasks

Clearance Verification: Ensuring no tools or materials are lodged in tubes or headers.
Straight Tubes: Sighting through tubes using a portable light.
Bent Tubes: Proving they are clear of obstruction by passing a wooden ball through them using compressed air.
Cleanliness: Ensuring headers and drums are clean so the boiler can be closed for hydrostatic testing.

2. Hydrostatic Testing

The purpose of the hydrostatic test is to prove the boiler is tight under internal pressure. This test is performed by the manufacturer or repair agency and must be witnessed by an authorized person, such as a provincial government inspector, an insurance company inspector, or a qualified employee of the purchasing company (subject to local jurisdiction laws).

Preparation for Hydrostatic Test

External Access: Boiler casing, insulation, and brickwork must be left off for visibility.
Drum Internals: These are usually left out during the test.
Pressure Relief Valves: Ideally removed and connections blinded to prevent sediment entry. If removal is not possible, they must be gagged or clamped closed.
Instrumentation: Waterside instrumentation on repaired boilers may need isolation to prevent damage.

Testing Procedure

Filling: All high-point vents must be open to release air.
Water Quality: Use clean water following ASME Section I, PG-99 rules.
Temperature Requirements: Water must not be less than ambient temperature and in no case less than $20^\circ C$ . Metal temperature must not exceed $50^\circ C$ during examination.
Primary Membrane Test Pressure (PG-99.1): Raise pressure gradually to no less than $1.5 \times$ the Maximum Allowable Working Pressure (MAWP) stamped on the boiler.
Examination Pressure (PG-99.2): The pressure may be reduced to the MAWP and maintained while the boiler is carefully examined for leaks.

Post-Test Activities

Drain the boiler and open vents.
Install drum internals and fit safety valves.
Inspect drums again for debris; prove tubes are clear if anything might have fallen in.
Replace manhole covers using new gaskets.

3. Drying Out the Refractory

Moisture must be removed slowly from new or repaired refractory and insulation to prevent cracking (the "Dry Out" process).

Prerequisites: Ensure low water level shutdowns/alarms are active and the gauge glass is in service (a temporary glass is used during boil-out due to caustic chemicals).
Filling: Fill with clean water to approximately $1/3$ of the gauge glass.
Conditions: Keep superheater drains fully open throughout the process to allow steam vapor circulation and tube cooling.
Operation: Run fans at minimum output with a low flame. The fire is maintained for several hours or days based on manufacturer guidelines for temperature rise per hour.

4. Boiling Out

This process removes oil, grease, and contaminants from internal waterside surfaces.

Chemical Mixture: Chemicals are added (typically through the steam drum manhole or via a chemical feed pump). A suggested treatment for every $45000\,dm^3$ ( $45000\,L$ ) includes: - $18\,kg$ trisodium phosphate ( $Na_3PO_4$ ) - $2.25\,kg$ caustic soda ( $NaOH$ ) - $2.25\,kg$ sodium silicate ( $Na_2SiO_3$ ) - $22.5\,kg$ soda ash ( $Na_2CO_3$ )
Procedure: Close drum vents when pressure reaches $170\,kPa$ . Raise pressure in stages to approximately half the design working pressure.
Maintenance: Operate blow-off valves every $2$ to $4$ hours to remove sludge/sediment. Add makeup water and chemicals as needed.
Completion: Burners are shut down when blow-off samples appear clear. Open drum vents at $170\,kPa$ to prevent a vacuum. Flush the cooled boiler with high-pressure water and inspect for remaining grease.

5. Testing Shutdowns and Safety Devices

Before full service, the following must be tested under low-firing/operating conditions:

Low Water Fuel Cutoff: Tested by closing the feedwater valve or opening the bottom blow-off to observe the fuel valve closure at the correct water level.
Fuel Pressure Shutdowns: Artificially create high and low fuel pressure conditions to verify.
Pressure Relief Valves: Test with minimum load by closing the outlet Non-Return Valve (NRV). Note the "pop" pressure and the closing pressure after opening the NRV.

Objective 2: Boiler Lay-up Methods

Boilers taken offline for extended periods must be cooled, drained, cleaned, and inspected.

1. Wet Storage (Standby)

Used for boilers needed on short notice. Not for reheaters or freezing environments.

Completely Filled: Filled with softened, deaerated feedwater. Add oxygen scavengers (Sodium Sulfite) and maintain pH at a minimum of $10.0$ using caustic soda. Keep water pressure slightly above atmospheric (using a head tank if necessary).
Partially Filled: Fill to normal level and maintain a nitrogen blanket in the space above it at approximately $35\,kPa$ .

2. Dry Storage (Extended Service Outage)

Preferred for long outages or freezing conditions.

Procedure: Drain completely and dry with a small flame until drum vent vapors show no moisture. Stack flue gas should not exceed $200^\circ C$ .
Maintenance: Place moisture-absorbing material (silica gel) on trays in drums. Blank off all connections and maintain a nitrogen blanket at $35\,kPa$ . Tag the boiler for nitrogen use.

Objective 3: Boiler Shutdown and Inspection

Shutdown Procedure

Reduce firing to minimum and switch controls to manual.
The NRV will close if connected to a common header; isolate and lock the NRV and the header valve.
Open the drain between the NRV and the header valve.
Isolate fuel supply, lock out draft fans, feedwater pump motor breakers, and feedwater supply valves.
Cool slowly with dampers shut. Open drum vents at $170\,kPa$ .
Do not drain until cool enough to prevent sludge baking. Lock out blowdown valves after draining and open manholes.

Cleaning and Inspection Focus

Fireside Cleaning: Wash with a hot alkaline solution to prevent acidic corrosion from sulfur-containing deposits.
Waterside Inspection: Check for corrosion (pitting from dissolved oxygen), cracking in ligaments between tube holes, and loose drum internals. Analyze sludge to evaluate water treatment effectiveness.
Furnace-Side Inspection: Look for bulges/blisters (overheating), refractory deterioration, fire tube end leaks, and burner carbon buildup.

Objective 4: Waterside Cleaning Methods

1. Mechanical Cleaning

Involves power-driven tools (water, air, steam, or electric).

Firetube Boilers: Use a vibrating hammer or "rattler" inside tubes to dislodge scale on the outside.
Watertube Boilers: Use rotary wire brushes, scrapers, cutters, or drill heads to remove scale from the inside of tubes.
Limitations: Costly for large boilers and may not ensure 100% cleanliness.

2. Chemical Cleaning

Faster, cheaper, and more thorough. Requires replacing brass/bronze fittings with steel and providing venting for acid vapors.

Circulation Method

Superheater is filled with demineralized water and pressurized with nitrogen.
Boiler is filled with demineralized water and circulated at $95^\circ C$ using a pump and steam injection.
Chemicals are added and monitored every half-hour. After completion, neutralize with Ammonia and Hydrazine for two hours at $95^\circ C$ .

Soak Method

Boiler is preheated to between $77^\circ C$ and $82^\circ C$ using light firing, then drained.
Superheater is backfilled with condensate to protect from chemical vapors.
Solvent is added and the boiler soaks for $4$ to $8$ hours under atmospheric pressure (vents open).
Once drained under nitrogen pressure ( $35\,kPa$ ), flush with demineralized water (repeat if pH is $< 5$ ).
Neutralize by boiling out with soda ash ( $10\,kg$ per $100\,kg$ water) for $4$ to $6$ hours.
Finish with a $0.5\%$ sodium nitrite solution to prevent rusting.

Objective 5: Routine Operation and Task Log

Water Level: Maintain correct level. Blow down gauge glass and water column connections regularly to ensure they are clear.
Feed Pumps: Monitor discharge/suction pressures and bearing temperatures. Alternate between standby and service pumps to equalize run time.
Combustion: Monitor automatic controls; be prepared for manual operation. Conduct regular flue gas analysis and visual flame observation.
Ash Removal: For coal-fired boilers, remove ash at least once per shift. Personnel must wear goggles, gloves, and protective clothing.
Logs: Perform a general plant inspection every $2$ hours and record all occurrences in a log book.

Objective 6: Boiler Explosions

1. Furnace Explosions

Occur when combustible gas accumulations ignite.

Causes: Inadequate purging, admitting fuel before ignition source is established, flame-out (loss of fuel/air or water in fuel), or lighting one burner from another.
Prevention: Instruct staff on lighting procedures, maintain igniters/pilots, limit ignition trials, and purge furnace with at least $50\%$ fan capacity for $5$ minutes between attempts.

2. Pressure Explosions

Occur when a pressure part fails.

Causes: Overstressing from rapid heating, overheating due to low water, scale/oil buildup, or corrosion.
Prevention: Regular safety valve testing, following startup procedures, adhering to water treatment guidelines, and proper soot blower positioning.

Objective 7: Soot Blowers

Soot and ash act as insulators and can obstruct gas passages, increasing draft fan power requirements.

Retractable Soot Blowers

Used in high-temperature zones (furnace walls, secondary superheaters).
Withdrawn when not in use to prevent heat damage.
Utilize a single nozzle tip directing superheated steam or dry compressed air with a cleaning radius of approx $1.5\,m$ .

Fixed/Non-Retractable Soot Blowers

Economical; used in lower temperature zones where high-mass energy is not required.
Nozzle sizes range from $6\,mm$ to $10\,mm$ .

Operating Procedures

Ensure boiler load is at least $50\%$ for sufficient flue gas flow.
Maintain slightly negative furnace pressure.
Ensure blowing medium (steam or air) is dry to avoid tube erosion.
Operate sequentially from the bottom of the furnace toward the rear.
Confirm blowers are advanced before triggering the steam valve and withdrawn immediately after use.