Boiler Operator Interview Questions

Technical Questions

Here are common technical questions asked in boiler operator interviews, along with what the interviewer is looking for:

Water Treatment

• Q: What is TDS and why is it important?: Total Dissolved Solids. If TDS is too high, it causes foaming, priming, and carryover. Controlled by blowdown.
• Q: How do you test for dissolved oxygen?: Using a colorimetric test or a dissolved oxygen meter on feedwater. Should be below 0.02 ppm.
• Q: What happens if boiler water pH is too low?: Acidic water causes corrosion of tubes and drums. pH should be 10.5–12.0.
• Q: What is the purpose of bottom blowdown vs surface blowdown?: Bottom removes sludge from the lowest point. Surface removes dissolved solids concentrated at the water surface.

Safety

• Q: How often do you test the low-water cutoff?: Daily. Blow down the float column and verify the burner shuts down.
• Q: What would you do if the water level drops below the gauge glass?: Shut down the burner immediately. Do NOT add cold feedwater to an overheated boiler — it can flash to steam and rupture the boiler. Allow it to cool, investigate the cause, and do not restart until the problem is fixed.
• Q: What is a safety relief valve's function?: To release excess pressure if all other controls fail, preventing a catastrophic boiler explosion.
• Q: What is water hammer and how do you prevent it?: Water hammer is the impact of condensate being driven by steam. Prevent by ensuring traps are working, opening steam valves slowly, and warming lines gradually.

Combustion

• Q: What is the ideal O₂ reading in flue gas?: 2–4% for gas, 4–6% for oil. Too low = incomplete combustion (CO). Too high = wasted heat up the stack.
• Q: What does a rising stack temperature indicate?: Fouled tubes, scale on the water side, or poor combustion — investigate and clean or adjust.
• Q: What is the warm-up procedure for a cold boiler?: Fire at low rate, warm slowly (1 bar per 15 minutes), vent air, check expansion, bring to pressure gradually over several hours.

Tips for the Interview

• Know your water treatment: This is the #1 technical topic. Be able to explain TDS, blowdown, pH, hardness, and oxygen.
• Emphasise safety: Every answer should include the safety aspect. Boiler operators are safety-critical.
• Know the logbook: Be familiar with what's recorded and why — the logbook is a legal document.
• Be honest: If you don't know, say so. Boiler operation is not a place to fake knowledge.