Residual Sludge and Ventilation Strategy in a Product Tank
A 20 m diameter, 14 m high tank previously held diesel and now contains approximately 40 cm of sludge and scale on the floor. The plan is to have two workers enter and shovel sludge to a vacuum hose. Ventilation is provided by a blower delivering air through the top manway, with exhaust through the bottom shell manway. Initial readings at the bottom manway are 20.5% oxygen, 0% LEL, 0 ppm H2S. The workers will be at floor level, roughly 14 m below the top manway. The blower has been running for 30 minutes. The entry supervisor notes readings 'look great' at the manway and wants to send both workers in together to speed the job. One worker asks whether readings at the manway are representative of the floor.
1. Assess the adequacy of the ventilation and monitoring plan. What is wrong with relying on bottom-manway readings?
Read the worked answer
Manway readings do not establish the atmosphere where the workers will actually be. The workers are at the floor, 14 m from the top manway and at a different elevation than the sampling point. Gases and vapors can stratify; heavier-than-air vapors or displaced oxygen-deficient air can accumulate at low points. Sludge disturbance (shoveling, vacuuming) can liberate trapped hydrocarbons and change the atmosphere during the task.
Requirements: test at the actual work location and at multiple depths before entry, and continue monitoring during work with personal monitors on each entrant. Thirty minutes of ventilation may be insufficient to turn over a tank this size; verify effective air changes rather than assuming. The blower/exhaust arrangement should be confirmed to sweep the work zone, not short-circuit between manways.
2. The supervisor wants both workers in simultaneously. Discuss the entry configuration and monitoring implications.
Read the worked answer
Two entrants increase the monitoring and rescue burden. Each entrant needs a personal gas monitor and reliable communication with the attendant. The attendant must be able to account for both and initiate rescue. Rescue provisions must be adequate for two people at floor level, 14 m below the top manway, which is a significant retrieval challenge.
Because sludge work liberates vapors, continuous monitoring with alarm setpoints is essential, and the entry should be halted if readings trend toward limits. The decision to use two entrants should be justified by task need and supported by adequate rescue capability, not merely by schedule. If rescue cannot reliably handle two entrants at that depth, entry should be sequenced.