Nitrogen-Blanketed Tank Entry During a Seal Change
You are the millwright lead on a turnaround at a food-processing plant. A 1,200-gallon stainless mixing tank needs its agitator mechanical seal replaced. The tank is 9 ft deep with a single 18-inch manway, and internal baffles crowd the working space. Operations has drained and rinsed the tank. A nitrogen line used for inerting is still connected through a closed manual valve. Pre-entry testing at the manway reads 20.9% oxygen, 0% LEL, and 0 ppm H2S. The entrant will wear a supplied-air respirator with a 5-minute escape bottle. The permit is written as "alternate procedures" because the manway is too small for a full-body rescue harness. Two entrants, one attendant, and an off-site rescue service 25 minutes away are listed.
1. Is the manway atmosphere test enough to authorize entry? Identify every control that must be in place before the first entrant crosses the plane, and explain why each one matters here.
Read the worked answer
No. A single point-in-time reading at the opening does not authorize entry.
1. Positive isolation of the nitrogen line. A closed manual valve is not isolation. The valve can leak by, be opened by mistake, or be bumped. Remove the line and blind it, or use a double block and bleed with the boundary locked out. Nitrogen is odorless and will pool in the low areas of the tank and displace oxygen without any warning to the senses.
2. Continuous monitoring inside the space. Test at the breathing zone at multiple depths, not just at the manway. Heavier-than-air gas and rust, rinse residue, or decay can stratify, so a 20.9% reading at the opening can sit above a deficient pocket.
3. Agitator energy isolation. The seal is on the agitator, so the drive motor must be locked out, the disconnect tagged, and the shaft blocked against rotation before anyone is inside with the shaft in front of them.
4. Rescue feasibility. A 25-minute off-site rescue is not a rescue plan for an IDLH-capable space. The "alternate procedures" designation must be justified by a documented hazard evaluation, and a retrieval system or a trained on-site rescue team must be standing by. If the space cannot support retrieval, entry should not proceed as planned.
5. Attendant and communication. A dedicated attendant who does not enter, with continuous voice or radio contact, and a means to summon help without leaving the post.
Bottom line: the permit is defective as written. Re-do the hazard evaluation, fix the nitrogen isolation and rescue plan, and re-test before entry.
2. Ten minutes into the job the entrant reports feeling lightheaded. The attendant's meter, still at the manway, reads 20.8% oxygen. State your actions in order and explain what the discrepancy tells you.
Read the worked answer
Actions, in order:
- Evacuate now. Order the entrant out by the fastest route. Do not send the attendant in, and do not let a second entrant go in to "help."
- Account for everyone at the opening and move them upwind and clear of the manway.
- Treat as an exposure, not a complaint. Get the entrant on fresh air, keep him seated, and have him medically evaluated.
- Do not re-enter until the cause is found and the permit is re-issued.
- Investigate with a calibrated meter inside the space at multiple depths and with the entrant's own personal monitor if one was worn.
What the discrepancy means: an opening reading of 20.8% only proves the air at the opening. If the entrant's breathing zone is lower and the space is stratified, or if nitrogen is leaking past the closed valve and pooling at the bottom, the manway meter will not see it. The reverse is also possible: the supplied-air source itself is the problem. Check for a compressor-supplied system with carbon monoxide carryover, a cross-connection with plant air or nitrogen, a kinked or disconnected hose, and the escape bottle status.
Lesson to record: one meter at the entry point is not monitoring. The entrant should have had continuous personal monitoring, and the nitrogen line should have been positively isolated before the first entry.